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Photochemistry of tetrahydro-1, 4-naphthoquinones in the solid state 1977

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PHOTOCHEMISTRY OF TETRAHYDRO-1,4- . NAPHTHOQUINONES IN THE SOLID STATE by ALICE AFI DZAKPASU B.Sc, Stanford University, 1969 M.Sc., University of California, Los Angeles, 1971 Lecturer in Chemistry, University of Cape Coast, Ghana, 1971-74 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA Ap r i l , 1977 <£) Alice A f i Dzakpasu, 1977 In presenting th is thes is in p a r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary shal l make it f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho la r ly purposes may be granted by the Head of my Department or by h is representat ives . It is understood that copying or pub l ica t ion of th is thes is for f inanc ia l gain sha l l not be allowed without my wri t ten permission. Department of Chemistry The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 i i To Professor John I. Brauman who taught and introduced me to the fascinating world of organic Chemistry. - i i i - Abstract Previous investigations involving the behaviour of c i s - 4a,5,8,8a-tetrahydro-l,4-naphthoquinone and i t s derivatives in solution under UV irradiation raised the question of what role, i f any, the ground state conformations of these molecules play in a) the type of photochemical behaviour the substrate exhibits and b) the product distribution in cases where more than one product i s formed. In addition, i t has never been established experimentally just what the geometric requirements for the various reactions encountered in this series are. Such requirements usually provide insights into the geometry of the transition state i t s e l f . In the solid state, the i n i t i a l conformation of molecules of any given substrate can be accurately determined by single crystal X-ray diffraction methods. Furthermore, since the crystal l a t t i c e usually resists any gross changes in conformation during the course of a reaction, i t follows that most reactions i n the crystalline state w i l l occur from the ground state conformation of the substrate. By studying these reactions i n the solid state and correlating the results with the X-ray data, i t was hoped that the questions raised as well as others e.g. concentration effects could be answered. - iv - Eleven substrates were chosen for the investigation. They were a l l prepared by the Diels Alder addition of a diene to a quinone. By slow cry s t a l l i z a t i o n a l l substrates with the exception of 6,7- dimethyl-4a3,5,8,8a3-tetrahydro-l,4-naphthoquinone, 1_ and 2,3,4aB,5a,8a,8a3-hexamethyl-4a3,5,8,8a3-tetrahydro-l,4-naphthoquinone, 10, gave crystals suitable for single crystal X-ray structure determination. Relevant X-ray data of nine of these substrates and five of the solid state photoproducts are reported. In the sol i d state, 5a,8a-dimethyl-4a8,5,8,8a3-tetrahydro- 1,4-naphthoquinone, _1, 6,7-dimethyl-4a3,5,8,8ag-tetrahydro-l,4- naphthoquinone, 2_, and the parent compound 4a3,5,8,8a3-tetrahydro- 1,4-naphthoquinone, _3, did not undergo the photochemical intramolecular hydrogen abstraction they undergo in solution. Instead, they dimerized stereospecifically to their respective centrosymmetric dimers when irradiated with UV light below their respective eutectic temperatures. This i s rationalized i n terms of a parallel alignment of the C(2), C(3) double bonds of adjacent monomers and short intermolecular centre-to-centre separation (<4.040A) of these double bonds. Five of the substrates, namely, 6,7-diphenyl-4a3,5,8,8a3- tetrahydro-1,4-naphthoquinone, 4_, 2,3-dimethyl-l, 4-4a3, 9a3~tetrahydro- 9,10-anthraquinone, 6̂  4a3,8a3-dicyano-6,7-dimethyl-4a3,5,8,8a3- tetrahydro-1,4-naphthoquinone, 1_, 2,3,4a3,6,7,8a3-hexamethyl- 4a3,5,8,8a3-tetrahydro-l,4-naphthoquinone, 9_ and 2,3,4a3,5S,83,8a3- - V - hexamethyl-4a8,5,8,8ae-tetrahydro-l,4-naphthoquinone, 11 have intermolecular double bond contacts exceeding 4 . l l and did not undergo dimerization in the sol i d state. Instead, upon UV irradiation, they formed t r i c y c l i c enone alcohols derived from intramolecular abstraction by oxygen of the C(8) hydrogen which l i e s in the plane of the C(1)=0(1) carbonyl group. Substrates 9_ and 11, i n addition, each gave a t r i c y c l i c diketone resulting from the abstraction of one of the C5 hydrogens by C2. The geometric requirements, the geometries of the transition states and intermediates and the factors governing the modes of closure of the biradical intermediates in both of these "hydrogen abstraction reactions and their implications for other hydrogen abstractions such as the Norrish Type II are discussed. One substrate, namely, 4a3,8aB-dicyano-5a,8a-dimethyl- 4a3,5,8,8aB-tetrahydro-l,4-naphthoquinone, j8, which lacks short intermolecular double bond contacts and has i t s C(8) hydrogen out of plane and far removed from the C(1)=0(1) group neither dimerized nor gave hydrogen abstraction products when irradiated in the solid state. Instead, i t gave an oxetane resulting from an intramolecular [^2+^2] addition of C(1)=0(1) to the C(6)=C(7) double bond. ' The intermolecular o double bond contact here was 3.20A. Lastly, 2,3,6,7-tetramethyl-4aB,5,8,8a6-tetrahydro-l,4- naphthoquinone, 5_, and 2,3,4agj5a,8a,8a3-hexamethyl-4ag,5,8,8ag- tetrahydro-l,4-naphthoquinone, 10, failed to react when irradiated in the solid state. The reason for this i s not clear but the possibility of deexci- tation via excimer formation and subsequent dissociation i s raised. - vi - Table of Contents Page INTRODUCTION 1 1. General 1 X-ray Diffraction Methods 2 Defects i n Crystals and Their Effects on Chemical Reactivity 3 Energy Transfer in Organic Solids 17 2. Photochemistry of Tetrahydro-1,4-naphthoquinone and Its Derivatives 19 Biradical vs Charge-Transfer Mechanisms 26 Characteristics of the 3- and y-Hydrogen Abstraction Reactions in the Tetrahydro-1,4-naphthoquinone Series: Substituent Effects 34 Solvent Effects 37 Quantum Yield and Quenching Studies 39 Stereoelectronic Requirements 40 Other Reactions 41 3. Objectives of Present Research 41 RESULTS AND DISCUSSION 47 Preparation of Substrates 47 1. Intermolecular [ 2 + 2] Dimerization: TT IT •'• 5a,8a-Dimethyl-4aS,5,8,8ag-tetrahydro-l,4- naphthoquinone, 1 54 6,7-Dimethyl-4aB,5,8,8ag-tetrahydro-l,4-naphthoquinone, 2 60 4ag,5,8,8a8-Tetrahydro-l,4-naphthoquinone, 3_ 64 - v i i - Page Irradiation of 2 i n Solution 80 Reactive State and Mechanism for Photodimerization 81 2. Intramolecular Hydrogen Abstraction: 6,7-Diphenyl-4aB,5,8,8aB-tetrahydro-l,4-naphthoquinone, 4 85 2.3- Dimethyl-l,4,4ag,9ag-tetrahydro-9,10-anthraquinone, 6 96 6,7-Dimethyl-4ag,8ag-dicyano-4ag,5,8,8ag-tetrahydro- 1.4- naphthoquinone, ]_ 99 2,3,4ag,6,7,8ag-Hexamethyl-4ag,5,8,8ag-tetrahydro- 1,4-naphthoquinone, 107 2,3,4ag,5 B,8g,8aB~Hexamethyl-4aB,5,8,8ag-tetrahydro- 1,4-naphthoquinone, 11 114 The Geometry of the Transition State for B- and y - Hydrogen Abstractions 121 3. Intramolecular Oxetane Formation: 4ag,8ag-Dicyano-5a,8a-dimethyl-4aB,5,8,8aB-tetrahydro- 1,4-naphthoquinone, 8̂  134 4. Unreactive Substrates:-. 2,3,6,7-Tetramethyl-4aB,5,8,8aB-tetrahydro-l,4- naphthoquinone, _5 143 2,3,4aB,5a,8a,8aB-Hexamethyl-4aB,5,8,8aB-tetrahydro- 1,4-naphthoquinone, 10 149 EXPERIMENTAL 156 BIBLIOGRAPHY ; 211 APPENDIX 221 - v i i i - L i s t of Tables Table # Caption Page I UV Absorptions 25 II Product Ratios Obtained in Benzene and tert-Butyl Alcohol 38 III Substrates and Reaction Types Observed in Solution 45 IV Product Yields for the 1 IA Conversion 59 V Product Yields for the 2_ -> 2A Conversion 63 VI Product Yields for the _3 -»• 3A Conversion 64 VII Product Yields for the /4(g) -*• 4A + 4B Conversion 88 VIII Product Yields for the 4solution 4A + 4B Conversion .. 91 IX Specific Rotation of Solutions of Crystals of 7A 105 X Product Ratios and Combined Yields for the 9_ -»• 9A + 9B Conversion in the Solid State 108 XI Product Ratios for the 9̂  -> 9A + 9B Conversion i n Solution 109 XII Product Yields and Ratios for the 11 -> 11A + 11B Conversion 115 XIII Effects of Structure on the McLafferty Rearrangement ... 123 XIV Interatomic Distances, A, and Approach Angles for Hydrogen Abstraction 129 XV Intramolecular C(1)=0(1)•C(6)=C(7) Bond Contacts and Orientation 139 XVI UV Absorption Spectra of Substrates 1-11 235 - ix - Li s t of Schemes Scheme // Page 1 12 2 13 3 14 4 15 5 21 6 23 7 29 8 32 9 33 10 35 11 36 12 40 13 87 14 95 15 98 16 102 17 111 18 113 19 119 20 ... 131 21 147 22 149 - x - List of Figures Figure ii Caption Page 1 Vacancy; I n t e r s t i t i a l Atom 4 2 Model of a simple cubic l a t t i c e and dislocations 5 3 Voids and Diffusion 8 4 Ground state, n,ir* and T T , T T * states 28 5 Operational Definition of the Angle T 42 6 Uncorrected Endothermic Transition for 1 + IA 52 Eutectic Mixture 7 Stereo diagram of an adjacent pair of molecules of 5a,8a-dimethyl-4a3,5,8,8ag-tetrahydro-1,4- naphthoquinone, 1 55 8 Stereo diagram of dimer IA in an orientation analogous to that of the monomer, 1̂  57 9 Stereo diagram of dimer 2A 62 10 Infrared spectra of KBr pellets containing 4-5% by weight of dimer 3A (Top) and dimer 3B (Bottom) 67 11 Stereo diagram of the 3B molecule 69 12 Part of the absorption spectrum of benzene 73 13 Stereo diagram of the contents of the unit c e l l for compound j} 76 14 Stereo diagram of molecule type A and i t s nearest neighbour related by a simple c e l l translation 77 15 Stereo diagram of a type B molecule and i t s nearest neighbour related by a centre of symmetry, X 77 16 Stereo diagram of compound 4̂  86 17 (a) A 100 MHz PMR Spectrum of 2,3-Diphenyltricyclo- [5.3.0.05»10]deca-2-ene-6,9-dione, 4B 90 (b) A 100 MHz PMR Spectrum of 2,3-Dimethyltricyclo- [5.3.0.05»10]deca-2-ene-6,9-dione, 2C 90 - xi - Figure it Caption Page 18 Stereo diagram of compound 6_ 98 19 Stereo diagram of compound ]_ 100 20 Stereo diagram of enone-alcohol, 7A 104 21 Neighbouring ene-dione systems of a pair of molecules of 1_ . 106 22 Stereo diagram of substrate 9_ 110 23 Stereo diagram of substrate 11 118 24 (a) Ground state geometry of substrate 9_ 132 (b) The proposed transition state geometry for the 9̂  9B conversion .132 25 Stereo diagram of substrate J3 136 26 Approach geometry of the C(1)=0 and C(6)=C(7) ir bonds... 136 27 Stereo diagram of oxetane 8A 137 28 Stereo diagram of substrate 5_ 143 29 Two adjacent molecules of 5_ within a crystallographic c e l l 145 30 The reaction cavity before reaction 152 31 Apparatus for irradiations i n the solid state 165,166 32 Apparatus for low temperature irradiations i n solutions 170 33 A 60 MHz PMR spectrum of 5a,8a-dimethyl-4aB,5,8,8ag- tetrahydro-1,4-naphthoquinone, _1 221 34 Fourier transform 100 MHz PMR spectrum of 5,8,15,18- tetramethylpentacyclo[10.8.0.0 2» 1 1.0 4» 9.0 1 4 > 1 9]eicosa- 6,16-dien-3,10,13,20-tetrone, IA 221 35 A 60 MHz PMR spectrum of 6,7-dimethyl-4ag,5,8,8ag- tetrahydro-1,4-naphthoquinone, 2_ 222 36 Fourier transform 100 MHz PMR spectrum of 6,7,16,17- tetramethylpentacyclo[10.8.0.0 2» 1 1.0*» 9.0 1 4» 1 9]eicosa- 6,16-dien-3,10,13,20-tetrone, 2A 222 - x i i - Figure // Caption Page 37 A 60 MHz PMR spectrum of 4ag,5,8,8ag-tetrahydro-l,4- naphthoquinone, 3̂  223 38 Fourier transform 100 MHz PMR spectrum of pentacyclo- [10.8.0.02 > 1 1.0 4» 9.0 1 4' 1 9]eicosa-6,16-diene-3,10,13,20- tetrone, 3A 223 39 A 60 MHz PMR spectrum of 6,7-diphenyl-4ag,5,8,8ag- tetrahydro-1,4-naphthoquinone, h_ 224 40 A 100 MHz PMR Spectrum of l-hydroxy-7,8-diphenyltri- cyclo[5.3.0.05»10]deca-2,8-dien-4-one, 4A 224 41 A 60 MHz PMR spectrum of 2,3,6,7-tetramethyl- 4ag,5,8,8ag-tetrahydro-l,4-naphthoquinone, 5_ 225 42 A 60 MHz PMR spectrum of 2,3-dimethyl-l,4,4ag,9ag- tetrahydro-9,10-anthraquinone, 6̂  226 43 A 100 MHz PMR spectrum of l-hydroxy-2,3-benzo-7,8- dimethyltricyclo[5.3.0.0 5> l u]deca-8-ene-4-one, 6A 226 44 A 60 MHz PMR spectrum of 4ag,8ag-dicyano-6,7-dimethyl- 4ag,5,8,8ag-tetrahydro-l,4-naphthoquinone, ]_ 227 45 A 100 MHz PMR spectrum of l-hydroxy-5,10-dicyano-7,8- dimethyltricyclo[5.3.0.05»l°]deca-2,8-dien-4-one, 7A ... 227 46 A 60 MHz PMR spectrum of 4a3,8ag-dicyano-5a,8a- dimethyl-4a$,5,8,8aB-tetrahydro-1,4-naphthoquinone, J3 .. 228 47 A 100 MHz PMR spectrum of 5,10-dicyano-6,9-dimethyl- 11-oxatetracyclo [ 6 • 2.1.0-*-»?. 0^»10] undec-2-ene-4-one, 8A , 228 48 A 60 MHz PMR spectrum of 2,3,4ag,6,7,8aB-hexamethyl- 4ag,5,8,8ag-tetrahydro-l,4-naphthoquinone, 9_ 229 49 A 100 MHz PMR spectrum of l-hydroxy-2,3,5,7,8,10- hexamethyltricyclo[5.3.0.0^ >10]deca-2,8-dien-4-one, 9A 229 50 A 100 MHz PMR spectrum of 2,3,5,7,8,10-hexamethyl- tricyclo[6.2.0.05»10]deca-2-en-6,9-dione, 9B 230 51 A 60 MHz PMR spectrum of 2,3,4ag,5a,8a,8ag-hexamethyl- 4ag,5,8,8ag-tetrahydro-l,4-naphthoquinone, 231 - x i i i - Figure ff Caption Page 52 A 60 MHz PMR spectrum of 2,3,4ae,5g,8g,8ag-hexamethyl- 4ag,5,8,8ag-tetrahydro-l,4-naphthoquinone, 11 232 53 A 100 MHz PMR spectrum of l-hydroxy-2,3,5,6,9,10- hexamethyltricyclo[5.3.0.0^ »1°]deca-2,8-dien-4-one, 11A 232 54 A 100 MHz PMR spectrum of 1,4,5,7,8,10-hexamethyltri- cyclo[6.2.0.05»l°]deca-2-en-6,9-dione, 11B; (a) 1000 Hz sweep width; (b) 250 Hz sweep width of the 6.5-5.256 region with amplitude magnification of xlO; (c) 250 Hz sweep width of the 3.05-0.86 region 233 55 Fourier transform 100 MHz PMR spectrum of CDCI3 from Merck Sharp & Dohme 234 - xiv - Acknowledgement My s i n c e r e thanks to my re s e a r c h d i r e c t o r , Dr. J.R. S c h e f f e r , f o r arousing and s u s t a i n i n g my i n t e r e s t i n t h i s area of research through many i n v a l u a b l e d i s c u s s i o n s and f o r h i s h e l p f u l suggestions d u r i n g the p r e p a r a t i o n of t h i s manuscript. I am g r e a t l y indebted to Dr. James T r o t t e r and Dr. Simon E.V. P h i l l i p s who c a r r i e d out a l l the X-ray s t r u c t u r e determinations reported i n t h i s work. I t r e a l l y was a p l e a s u r e c o l l a b o r a t i n g w i t h such a f i n e , zealous team. I a l s o thank Dr. Steve J . R e t t i g who, w i t h Dr. T r o t t e r , helped me w i t h some of the c a l c u l a t i o n s i n v o l v e d . I am a l s o indebted to other members of the teaching s t a f f of t h i s department e s p e c i a l l y Dr. R.E. Pincock and Dr. E.A. Ogryzlo f o r t h e i r h e l p f u l suggestions d u r i n g the i n i t i a l stages of t h i s r e s e a r c h and f o r being so generous w i t h some of t h e i r instruments. My thanks to the mechanical team of the t e c h n i c a l s t a f f of t h i s department e s p e c i a l l y Mr. B. Po w e l l and Mr. M. Symonds who helped design and b u i l d the photochemical r e a c t o r which was used f o r the s o l i d s t a t e r e a c t i o n s . I would a l s o l i k e to thank my former and present colleagues of l a b o r a t o r y 346 f o r t h e i r f r i e n d l i n e s s and co o p e r a t i o n . - XV - My thanks to the Canadian Commonwealth Scholarship and Fellowship Association for their award (1974-77) and to the University of Cape Coast,Ghana for granting me study leave from my teaching duties to undertake this project. Last, but not least, my special thanks to Louise Hon who typed this manuscript. - 1 - Introduction General v Organic solids, being generally low melting and complex as compared to metals, are more prone to crystal imperfections than the latter. It i s , therefore, not surprising that the Journal of Solid State Chemistry is exclusively inorganic. While this attitude of solid state chemists and physicists i s , perhaps, understandable, the aloofness of organic chemists themselves from this area of research is surprising since most organic compounds are solids. For, although i t is true that limited diffusion of reactants in the solid state places restrictions on the type of reactions which can occur, i t i s , nevertheless, also true that many unimolecular and polymerization reactions occur via mechanisms which require no assistance from solvents. The elimination of the use of solvents from such systems is not only beneficial in terms of cost but also in minimizing secondary reactions and in reducing the number of factors which need to be considered in proposing mechanisms. Organic solid state reactions have occasionally appeared in the literature since 1880 but the correlation of reactivity with structure of organic compounds became possible only after improved methods in - 2 - X-ray crystallography became available. X-ray Diffraction Methods^ When X-rays are passed through matter, some of the rays are scattered or diffracted, some are absorbed. X-ray crystal structure analysis relies on the recording and analyses of accurate values of the intensities of the diffracted rays. The intensity determinations can be made on a single crystal or on a fine powder composed of small grains. The powder method has serious drawbacks the greatest of which is the inherent lack of resolving power which causes reflections originating from different points in the crystal and which are not symmetrically related to f a l l together or coincide as a single spot. This makes the interpretation of intensity data from this method more d i f f i c u l t . Powder diffraction methods are, therefore, unsuitable for crystals having large c e l l s and are hardly ever used for complex structure analyses. Organic crystals are invariably investigated by the single-crystal method. In this method, a single crystal measuring 0.1 - 1.0 mm on a side i s mounted on a goniometer head and several photographs are taken at varying angles using Weissenberg and precession cameras. From these photographs the dimensions of the unit c e l l and also the space group to which the crystal structure belongs are determined. Next, dif f r a c t i o n intensities are recorded using an automatic diffractometer. Using these intensities, the structure factors are derived and their respective phases determined by s t a t i s t i c a l methods. These latter quantities allow for the calculation of electron - 3 - density maps from which the positions and nature of the atoms may be determined. Because of the uncertainties involved In pinpointing an atom from electron density maps, the resulting structure i s only approximate. This approximate structure is subsequently used to calculate intensities which are then compared with the observed intensities. Usually, the structure i s deemed satisfactory only when the average percent difference between the observed and calculated intensities, the R factor, f a l l s below 10%. Otherwise, the structure i s successively refined using a least-squares refinement method u n t i l the R factor is minimal and satisfactory. A l l computations involved in X-ray structure determinations are carried out using a d i g i t a l computer. Thus used, this method yields the structure, conformation, stereochemistry, intramolecular bond distances, the packing arrangement in the crystal as well as the intermolecular geometries and distances. The next step in establishing structure-reactivity correlations i s to study the reactions or the lack thereof of the crystalline compound. Since, a l l real crystals contain imperfections and since defects in crystals have been shown to play an important role in chemical 2 reactivity , a brief discussion of these defects i s desirable. 2—16 Defects in Crystals and Their Effects on Chemical Reactivity Imperfections in non-metallic solids, of which organic solids are an example, are complex and diverse and have not been as systematically and thoroughly studied as those found in metals. Furthermore, the study - A - of crystal defects has been the specialty of' s o l i d state physicists. The result has been a mathematical and highly technical treatment of the subject. Only a qualitative and simplistic discussion of crystal defects w i l l be attempted in this text. Possible defects in solids can be c l a s s i f i e d into four categories; A. Zero-dimensional or point defects e.g. vacancies and i n t e r s t i t i a l s . ooooooo o o o o o o ooooooo o o o o o o ooo ooo o o o o o o o ooooooo o o o^o o o ooooooo o o o o o o (a) (b) Figure 1. (a) Vacancy; (b) I n t e r s t i t i a l atom A vacancy is formed by the absence of an atom from an atomic site while an i n t e r s t i t i a l is what results when an atom i s present at a non-atomic si t e . These two defects are i l l u s t r a t e d in Figure 1. B. One-dimensional or l i n e defects commonly referred to as dislocations. A dislocation is a boundary between two parts of a crystal which are displaced with respect to one another. There are two types of dislocations viz.,edge and screw dislocations. The f i r s t type i s exemplified in Figure 2b in which an extra half plane of atoms has been inserted into the top half of the crystal l a t t i c e giving rise to a severe distortion of the atomic layers in the vicinity"'of the dislocation. An edge dislocation may begin and end anywhere in the crystal and so may not < a> (b) Figure 2. Model of a simple cubic l a t t i c e ; (a) the perfect c r y s t a l , (b) a view of an edge d i s l o c a t i o n caused by the i n s e r t i o n of an extra h a l f plane of atoms, ABCD. always be pe r c e p t i b l e e x t e r n a l l y . A screw d i s l o c a t i o n , on the other hand, manifests i t s e l f on the c r y s t a l surface. Figure 2c i l l u s t r a t e s a screw d i s l o c a t i o n caused by the displacement of the two faces of the ABCD plane Figure 2. (c) a screw d i s l o c a t i o n ; (d) growth s p i r a l on the face of an n - p a r a f f i n (C36H74) c r y s t a l r e s u l t i n g from a screw d i s l o c a t i o n (reproduced from reference 6). - 6 - in the directions shown. When such a s l i p occurs during crystallization, i t usually gives r i s e to a growth spiral which is not uncommon in long chain hydrocarbons grown from solution by cooling. Figure '2d is a photograph of one such crystal reproduced from reference 6. C. Two dimensional or surface defects include boundary dislocations of one kind or another. We shall concern ourselves with three kinds of boundary imperfections, viz., defects of surface atoms, phase boundary dislocations and grain boundary dislocations. The atoms in a single crystal are held together by a cohesive force which may be of the metallic, ionic, covalent, or van der Waals type. Regardless of which type of cohesive force i s involved, the atoms on the surface of the crystal experience only a fraction of the total force f e l t by the atoms in the interior of the crystal. This, plus the fact that they are exposed to an environment different from the environment of their interior counter- parts, makes surface atoms different from those of the bulk crystal. They are usually distorted and have properties unlike those of the bulk crystal. This deviation i s classified as a defect inasmuch as i t may be the i n i t i a t o r or inhibitor of a physical or chemical process. The second type of boundary defect is caused by the presence of one or more phases within the crystal e.g. the presence of an impurity or the formation of product(s) may give rise to new phases on the surface or inside of the crystal. The reactant-product interface may then . serve as a d e f e c t s i t e . Lastly, grain boundaries may act as defect - 7 - sites. Most solids c r y s t a l l i z e out, not as single crystals, but as aggregates of crystals. Any two grains represent two single crystals and a misorientation between them constitutes a grain boundary dislocation. The atoms at such sites are distorted. In addition, such locations are the sites of other defects such as i n t e r s t i t i a l s and holes. D. Three-dimensional or volume defects refer to voids and inclusions. In the perfect l a t t i c e , a l l atoms are in minimum energy positions and only vibrational motion about these positions occurs. When the atoms are displaced from their minimum energy positions, as i s the case at the site of dislocations, then the atoms are subjected to forces that tend to move them back to their equilibrium positions. The movement of atoms in the v i c i n i t y of the dislocation to correct the defect often results in the formation of clusters of vacancies or voids. These channels of empty space and vacancies, in general, provide a mechanism of diffusion in solids. Figure 3 i s a diagramatic representation of movement i n the l a t t i c e f a c i l i t a t e d by a void. As can be seen from this i l l u s t r a t i o n , the departure of the designated atom from an atomic site to the site of the void leaves behind a vacancy. This mechanism of diffusion, therefore, results in the creation of a new defect s i t e . The multiplication of defect sites, in this fashion, can be an effective method of propagating reactions permissible only at defect sites. A second example of volume defect i s the presence of impurities or inclusions, in general. An inclusion in the crystal l a t t i c e can be an impurity, an entrapped molecule of the solvent of crystallization or in the case of - 8 - o o o o o o o o o o o o o o o o o o o o o o o o o o o oo# ooo o o o o o o o o o o o o o o o o o o ooooooooo o o o o o o o o o (a) (b) Figure 3. (a) a void formed from a cluster of vacancies; (b) the diffusion of an atom ( © ) to a new atomic site and the creation of a vacancy at the original atomic site. solid state reactions, a product molecule. There are two major effects of these inclusions namely (i) they, generally, lower the melting point of the host and ( i i ) they may i n i t i a t e or terminate a physical and/or chemical process within the crystal. pf the above discussion applies equally well to polyatomic molecules such as organic solids, the sheer bulk and complexity of these molecules give rise to other considerations which w i l l now be discussed. such as metals and those of large organic entities i s the rate of diffusion through the sol i d . As mentioned ear l i e r , one mechanism of diffusion in solids entails the movement of an atom from an atomic site to an empty space (a void or vacancy). The energy required to move a For simplicity in i l l u s t r a t i o n s , only monatomic molecules have so far been used in defining structural faults. Although, a l l One of the major differences between solids of small molecules - 9 - s m a l l atom or i o n from one l o c a t i o n to another i s q u i t e s m a l l compared t o t h a t necessary to e f f e c t the movement of a complex molecule such as an o r g a n i c one. The c a l c u l a t e d c o e f f i c i e n t of d i f f u s i o n i n a s i n g l e c r y s t a l of anthracene i s of the order of 10 ^ cm^sec ^ at 40° below i t s m e l t i n g p o i n t and the a c t i v a t i o n energy f o r d i f f u s i o n i s c a l c u l a t e d to be 42 k c a l per mole . D i f f u s i o n i n organic s o l i d s can t h e r e f o r e be assumed to be n e g l i g i b l e . One m a n i f e s t a t i o n of t h i s , i s the i n a b i l i t y of r a d i c a l s formed during decomposition of o r g a n i c c r y s t a l s to combine to pr o d u c t s , a phenomenon termed a cage e f f e c t ^ ' ^ . Not only are mole- c u l e s confined to the same l o c a t i o n i n a c r y s t a l but other molecular motions such as tumbling seem to be absent as w e l l . Proof of t h i s comes from the p e r s i s t e n c e , over p e r i o d s up to months, of the anisotropy ( e l e c t r o n s p i n - n u c l e a r s p i n magnetic i n t e r a c t i o n ) e x e m p l i f i e d by proton h y p e r f i n e s p l i t t i n g s i n the e l e c t r o n s p i n resonance s p e c t r a of v a r i o u s 6 8 o r g a n i c compounds which have been exposed to i o n i z i n g r a d i a t i o n ' . R e s t r i c t i o n s such as t h i s on molecular movements form the b a s i s of the topochemical p o s t u l a t e which w i l l be d i s c u s s e d l a t e r . A second f e a t u r e of complex molecules such as organic compounds i s t h e i r p o t e n t i a l to c r y s t a l l i z e put. i n more than one polymorphic, form. Polymorphism may be a s s o c i a t e d w i t h v a r i a t i o n s i n conformation of the molecules and/or with.the packing arrangement of molecules i n the c r y s t a l l a t t i c e . Polymorphs of the same substance d i f f e r not only i n t h e i r p h y s i c a l p r o p e r t i e s but may r e a c t d i f f e r e n t l y . Although, t h i s m u l t i p l i c i t y of form i s not a defect as such, the contamination of one - 10 - crystal form by the presence of a second form i s undesirable in the study of such solids. There are two types of polymorphism namely enantio- tropic and monotropic. When each of two polymorphs i s stable at a given temperature range and pressure, then the two are said to be enantiotropic. This means that a substance A w i l l exist in polymorphic modification A^ at T° and a pressure of P^. At T° and T^, i t i s transformed into polymorphic modification k^. In the second type of polymorphism, one of the pair i s unstable at a l l temperatures below the melting point. Such a pair i s said to be monotropic. This latter form of polymorphism presents less problem in organic reactions than the former since the metastable form can only be prepared by quenching the melt and i s , therefore, unlikely to form during crystallization or reaction. Enantiotropic forms, on the otherhand, may interfere in reactions in the following manner: (i) where one form interconverts to the other during the course of a reaction, i t becomes d i f f i c u l t to determine from which modification the reaction i s occurring. One such compound i s cis-decahydronaphthalene which undergoes enantiotropic change at about 14° below i t s melting point"*"^; ( i i ) when a reaction sample containing predominantly one polymorph i s contaminated by a second polymorph. Although, i t i s possible to check single crystals grown for X-ray crystallographic purposes, the sensitivity of this method i s only about 5%. Furthermore, checking bulk samples used in organic s o l i d state reactions i s impractical. Fortunately, this type of contamination can be treated much like defects. : Since there are very few of the unwanted forms as compared to the bulk of the crystals, - l i - the properties and reactivity of the. crystal w i l l approach that.of _. _ the pure form. As mentioned earlier, organic solids are generally lower melting and more prone to contamination than the inorganic ones. One consequence of this i s that, unless reaction temperatures are chosen so as to be many degrees below the melting point, an apparent solid-phase reaction may actually be occurring in a molten region of the crystal. Moreover, as reaction proceeds, product molecules w i l l further depress the melting point of the host sample so that not only should reaction temperatures be below the melting point of the starting material but must be below the eutectic temperature of the mixture comprising reactant and product(s). We have so far defined some of the possible defects which can be present in a crystalline solid. Experimentally, the defects most often implicated in organic solid reactions are dislocations of one kind or another. To understand their role in such reactions, an examination of the stages involved in transforming reactant molecules to product within the crystal l a t t i c e of the reactant w i l l be useful. To be pertinent, the discussion w i l l be limited to photochemical transformations in the solid state. Let R denote a reactant molecule in the crystal l a t t i c e . Following the absorption of light by R, i t is promoted to the f i r s t 1 * excited singlet state, R . It may then (a) react to give product, P, or (b) deactivate (i) radiatively (fluoresce) or ( i i ) . non-radiatively to the - 12 - ground state, or (c) intersystem-cross into the excited t r i p l e t manifold, R , from which i t can (i) react, ( i i ) transfer i t s excitation energy to a second molecule, ( i i i ) deactivate radiatively (phosphoresce) or non-radiatively to the ground state or (d) transfer i t s excitation energy to another molecule R". This entire sequence of events, schematically represented in Scheme 1, i s crystal structure dependent. The only processes which may directly or eventually lead to"product formation are (a) and (d) Scheme 1 p. R from the excited singlet manifold and (c) (i) and ( i i ) from the excited t r i p l e t state. Attention w i l l , therefore, be focused on the effects of structural imperfections on these processes. In a reaction in which the absorption of energy leads directly to reaction i.e. energy transfer does not occur, molecules situated at or near l a t t i c e defects play no significant role since their concentration i s negligible compared to the concentration of molecules in ordered arrays. The photodimerization 11 12 of trans-cinnamic acid and i t s derivatives ' affords an example of this - 13 - type of reaction. In one such reaction, Cohen and coworkers^ irradiated mixed crystals of p-methoxycinnamic acid and p-methylcinnamic acid with both f i l t e r e d and unfiltered l i g h t . With f i l t e r e d l i g h t , only p-methoxy- cinnamic acid monomers absorbed the li g h t . Analyses of the resulting products showed them to be the homodimer formed by the combination of an excited p-methoxycinnamic acid with a non-excited p-methoxycinnamic acid together with the heterodimer formed by the reaction of excited p-methoxycinnamic acid with a non-excited p-methylcinnamic acid. Hardly any homodimer of p-methylcinnamic acid was formed. When unfiltered light was used, however, the products comprised both homodimers and also the heterodimer. These results, summarised in Scheme 2 below, show that only molecules which i n i t i a l l y absorb the irradiation react i.e. no 12 energy transfer occurs in this system. Schmidt and coworkers have Scheme 2 For f i l t e r e d light For unfiltered light A hv »* A hv * hv * A ^ A A > A B > B ic ic ic A + A > A 2 A + A —> A 2 B + B —> B 2 A* + B > AB A* + B » AB B* + A —> AB A = p-methoxycinnamic acid; B = p-methylcinnamic acid studied the crystal structures and dimerization reactions of these and other cinnamic acids and in a l l cases found only the products predicted - 14 - from the orientation of the monomers in the crystal l a t t i c e . Thus, the a-type crystal (Scheme 3. below) in which monomer pairs are related by a centre of symmetry give the centrosymmetric dimer. The second type of crystal, the 3-type, in which monomer pairs are related by simple translation along a crystallographic axis dimerize to the mirrorsymmetric dimer. And l a s t l y , the y-type crystal in which the double bonds of adjacent monomers are offset in such a way that they do not overlap and Scheme 3 A r — — — B m x — ~ — A r edge-on view of carboxylic acid pair \mu - C O O H X = centre of symmetry o a-type. Separation of double bonds - 4A ' Ar — =z-^mm—x. hv -=—:Ar C O O H X : O O H B-type. Separation of double bonds - 3.8-4.1A A r — — - , i I I - = — A r — A r hv Ar Ar C O O H 00H Y-type. Separation of double bonds - 4.8-5.2A A r — = r — • A r — = - — : — Ar • — = — A r hv -> No Reaction - 15 - o the distance between them i s 4.8A or greater are found to be photo- chemically inert. In contrast to these photodimerizations which are topochemically controlled, there are solid state reactions which are best understood in terms of the geometry of molecules situated at or near defective sites such as dislocations. The opportunity for molecules at dislocations to control reactivity arises when steric or other factors make reaction in the.perfect parts of the crystal energetically unfavorable and that at dislocations comparatively desirable and a mechanism exists for excitation energy to reach such defective sites. An example of this i s provided by the photodimerization of 9-substituted anthracenes in the'solid s t a t e ^ " ^ ' ^ . As in the cinnamic acids, there are three crystal forms, a, g and y. The a type i s predicted on the basis of the monomer arrangements in the crystal to give the centrosymmetric dimer and i t does, Scheme 4 ; the Y-type i s photoinert as expected but the g-type which on the basis of the crystal structure should give the mirrorsymmetric dimer reacts to give the centrosymmetric dimer. The formation of this non- topochemical dimer i s now f a i r l y well understood. Dimerization here involves bonding the C9, CIO positions of one monomer to the CIO, C9 Scheme 4 arrangement - 16 - g-type mirrorsymmetric dimer, centrosymmetric dimer, not formed formed Y-type No Reaction - 17 - p o s i t i o n s , . r e s p e c t i v e l y , of i t s nearest neighbour. When any of these p o s i t i o n s i s s u b s t i t u t e d , the 3-arrangement becomes e n e r g e t i c a l l y unfavourable f o r d i m e r i z a t i o n presumably because of s t e r i c hindrance. So t h a t one would expect such c r y s t a l s to be p h o t o i n e r t . This i s found t r u e f o r some of the 9 - s u b s t i t u t e d and 9 , 1 0 - d i s u b s t i t u t e d anthracenes"^"^ But i n others l i n e a r d e f e c t s i . e . d i s l o c a t i o n s have made p o s s i b l e the h e a d - t o - t a i l approach of two monomers so that r e a c t i o n begins only a t these d e f e c t i v e s i t e s and i s propagated through m u l t i p l i c a t i o n of the def e c t as r e a c t i o n proceeds. In these anthracene-derived compounds, i t has a l s o been shown t h a t the i n i t i a l a b s o r p t i o n of i r r a d i a t i o n need not be by the molecules at these d e f e c t i v e s i t e s i n order to have the 13 r e a c t i o n to occur. Both i m p u r i t i e s and d i s p l a c e d molecules at d e f e c t i v e 14 s i t e s have been shown to act as e f f e c t i v e e x c i t o n traps i n anthracene c r y s t a l s . So t h a t energy absorbed by the b u l k o f 9 - s u b s t i t u t e d anthracene c r y s t a l s i s passed on from one p a i r o f molecules to the next u n t i l i t reaches a d e f e c t i v e s i t e where monomer p a i r arrangements fav o r r e a c t i o n . The formation of the unexpected dimer i n such cases i s , t h e r e f o r e , not a v i o l a t i o n of the topochemical p o s t u l a t e s i n c e the r e a c t i o n i s not o c c u r r i n g w i t h i n the " p e r f e c t " regions of the c r y s t a l l a t t i c e but i s due to f a v o r a b l e r e a c t i o n c o n d i t i o n s at d i s l o c a t i o n s i n the c r y s t a l as suggested and shown by Thomas and Williams^""*. 17-21 Energy T r a n s f e r i n Organic S o l i d s : I t w i l l be u s e f u l to d e f i n e the term " e x c i t o n " as i t i s used i n the photochemical l i t e r a t u r e on o r g a n i c s o l i d s . In s o l i d s , e x c i t a t i o n - 18 - energy absorbed by a chromophore i n a molecule may be immediately passed on to a neighboring molecule i n the same c r y s t a l . Thus, the e x c i t a t i o n energy can be thought of as being shared between the molecules w i t h i n t h a t p a r t i c u l a r c r y s t a l . This d e l o c a l i z e d e x c i t a t i o n energy i s termed an e x c i t o n . The m i g r a t i o n of an e x c i t o n from molecule to 18 19 molecule which has been v a r i o u s l y d e s c r i b e d as t u n n e l l i n g , a hop , a j u m p ^ and a random-walk"^'^ 3 by d i f f e r e n t i n v e s t i g a t o r s occurs through long range i n t e r a c t i o n between traps i n c o n t r a s t to the mechanism o f t r i p l e t - t r i p l e t energy t r a n s f e r i n s o l u t i o n which occurs l a r g e l y through molecular c o l l i s i o n s . As mentioned e a r l i e r , energy t r a n s f e r occurs i n some systems and not i n others depending on the types and magnitude of the i n t e r a c t i o n between the donor and r e c i p i e n t molecules. In cases where the p r o b a b i l i t y of t r a n s f e r i s l a r g e (>10^ sec "*") i t i s estimated t h a t the jump time, T, f o r a m i g r a t i n g e x c i t o n can be as s h o r t as or s h o r t e r 91 -13 than l a t t i c e r e l a x a t i o n times^-1- (10 sec) so that i n these cases the * e x c i t e d s t a t e molecule S and i t s surrounding u n e x c i t e d molecules S Q have the same geometry.~ . This keeps the p r o b a b i l i t y of t r a n s f e r h i g h and promotes e x t e n s i v e e x c i t o n m i g r a t i o n . When, on the other hand, l a t t i c e r e l a x a t i o n precedes the t r a n s f e r , c o n s i d e r a b l e energy i s expended t h e r m a l l y and e x c i t o n m i g r a t i o n i s c u r t a i l e d . Examples of s o l i d systems which have been shown to e x h i b i t s u b s t a n t i a l e x c i t o n 17 18 20 m i g r a t i o n are those of benzene , naphthalene , and anthracene Se v e r a l important c o n c l u s i o n s can be drawn from these works: - 19 - (i) Since the average number of jumps or transfers i s x/T where x is the lifetime of the exciton and T i s the time within which a single 20a transfer occurs , and since a t r i p l e t exciton i s longer-lived than a singlet exciton, i t follows that migration of a t r i p l e t exciton through a crystal i s more extensive than that of a singlet exciton. Nieman and Robinson'''''' estimated that in pure organic crystals, ̂ 10^" t r i p l e t energy 4 6 migrations are possible per radiative lifetime as compared to 10 - 10 for the lowest singlet. ( i i ) The more extensive wandering of the t r i p l e t increases the probability of i t s annihilation at defective sites and also through t r i p l e t - t r i p l e t quenching. This i s borne out by the fact that many organic crystals having long-lived t r i p l e t states do not phosphoresce. ( i i i ) Structural faults and/or chemical impurities can act as exciton traps to terminate the transfer process. These energy sinks may then become the sites of physical processes such as phosphorescence and/or the reaction centres within the crystal. 2. Photochemistry of Tetrahydro-1,4-naphthoquinone and Its Derivatives 22 Diels and Alder reported the formation of a polymeric material of unknown molecular weight when they exposed crystals of the parent compound, 4a,5,8,8a-tetrahydro-l,4-naphthoquinone, 3_, to sunlight. 23 Over three decades later, Cookson and coworkers reinvestigated this and similar reactions. In both ethyl acetate solution and as crystalline material, UV irradiation through Pyrex of compound 3. was reported to - 20 - g i v e mainly t a r . Although, t h e i r attempts at c h a r a c t e r i s a t i o n met w i t h l i t t l e success, they t e n t a t i v e l y assigned i t the dimeric s t r u c t u r e , I I , of unknown st e r e o c h e m i s t r y , (eq. 1 ) . h u eq. 24 More r e c e n t l y , Scheffer and coworkers have shown th a t s e l e c t i v e i r r a d i a t i o n , (X > 340 nm) of 3. and i t s d e r i v a t i v e s i n s o l u t i o n gave t r i c y c l i c and t e t r a c y c l i c products i n moderate to q u a n t i t a t i v e y i e l d s . T h e i r r e s u l t s are summarized i n Scheme 5 . The type 8 process has only 25 r e c e n t l y appeared i n the l i t e r a t u r e and f o r m a l l y i n v o l v e s the a b s t r a c t i o n of a hydrogen v i a a five-membered t r a n s i t i o n s t a t e . On the other hand, the a b s t r a c t i o n of a Y -hydrogen by a c a r bonyl oxygen through a s i x - membered c y c l i c t r a n s i t i o n s t a t e i s commonplace and has been wi d e l y 26 r e f e r r e d to as the N o r r i s h Type I I process . The t h i r d r e a c t i o n type, c denoted as y to d i f f e r e n t i a t e i t from the p r e vious Y-process i n v o l v e s the a b s t r a c t i o n of a Y -hydrogen by carbon 2 of the ene-dione system. This process i s analogous to t h a t r e p o r t e d i n the photochemistry of some 27 s u b s t i t u t e d cyclopentenones and cyclohexenones . The f o u r t h r e a c t i o n type i . e . , i n t r a m o l e c u l a r oxetane formation, i s f o r m a l l y a 2+2 c y c l o a d d i t i o n of a c a r b o n y l group to an o l e f i n i c double bond. Such a r e a c t i o n i s w e l l 28 documented i n the l i t e r a t u r e . The l a s t r e a c t i o n type l e a d i n g to a caged - 21 - - 22 - - 23 - structure (Scheme 5 ) is also an intramolecular 2+2 cycloaddition. This time, however, the addition i s between the two o l e f i n i c double 29 bonds. This reaction i s also not without precedent . There are two simple mechanisms which can explain the formation of the various products arising from hydrogen abstraction. One of these is the biradical mechanism shown in Scheme . 5 and the other is the charge-transfer or ion pair mechanism shown in Scheme 6 . The latter mechanism involves the Scheme 6 Type 6 etc.  - 25 - H t ransfer ••etc- transfer of an electron from the ol e f i n i c ir system to the excited ene- dione chromophore followed by the abstraction of a proton by the oxygen. Electron reorganization in the resulting dipolar intermediate would then give rise to the same biradical intermediate formed from the direct abstraction of a hydrogen atom by the oxygen. The biradical pathway follows from that generally accepted for the Norrish Type II and related processes. On the other hand, charge transfer bands have been detected in the UV absorption spectra of some p-benzoquinone derivatives similar 24 to those studied by Scheffer and coworkers . For example, compound IV (Table 1) has an absorption at 307 nm with a higher extinction Table 1 UV absorptions, X n m (e) TH-TT* (allowed) charge-transfer Tr->-rr* (forbidden) n->-rr* 255(15,500) 355(800) 435(26) III 257(14,500) 307(260) 385(520) 448(50) IV : - 26 - coefficient than i t s n,ir* absorption. There was no corresponding absorption in the 300-350 nm region, however, for compound III. Cookson 30 and coworkers ascribed the 307 nm absorption to a charge-transfer state arising from overlap of the remote double bond with the carbonyl of the p-benzoquinone chromophore. This is not unreasonable since p-quinones are good electron acceptors and may accept an electron from the remote double bond i f the geometry of the molecule allows for overlap of the two chromophores. So that, although the biradical mechanism accounts for the products observed in the investigations of 24 Scheffer and coworkers , the possibility of a charge-transfer mechanism needs to be explored. Biradical vs Charge-transfer Mechanisms: Throughout this discussion i t w i l l be important to bear in mind that both of these mechanistic pathways involve a biradical inter- mediate. Such an intermediate is necessary to explain the different products formed in these hydrogen abstraction reactions. So that in considering these two pathways, one is solely concerned with the step(s) leading to the formation of the biradical. The f i r s t event i n the chain is surely the absorption of light. This promotes themolecule from i t s ground state energy level to an excited state. What is the nature of this excited state? As w i l l be shown later, l i k e the Norrish Type II.reaction, the reactions of these tetra- hydronaphthoquinones proceed.from both, the singlet and.triplet excited states. - 27 - Since d e a c t i v a t i o n from h i g h e r e x c i t a t i o n l e v e l s i s q u i t e r a p i d i n 12 13 -1 s o l u t i o n ( k - 10 -10 sec ) , one assumes t h a t only f i r s t e x c i t e d s t a t e s are i n v o l v e d . In the N o r r i s h r e a c t i o n , I t Is found that i n a c y c l i c ketones the percentage of s i n g l e t r e a c t i o n depends on the s t r e n g t h of the yC-H bond - as the bond becomes weaker, the p r o p o r t i o n of r e a c t i o n o c c u r r i n g 26a from the s i n g l e t s t a t e i n c r e a s e s . On the.other hand, ketones w i t h h i g h i n t e r s y s t e m c r o s s i n g e f f i c i e n c y e.g. aromatic ketones, may be expected to r e a c t mostly from t h e i r t r i p l e t s t a t e s . A second q u e s t i o n to be s e t t l e d about the nature of the e x c i t e d s t a t e ( s ) i n v o l v e d i n these r e a c t i o n s has to do w i t h the f a c t t h a t i n some ketones there are two l o w - l y i n g t r i p l e t s t a t e s namely, 3 n , 7 T * and^rr . T r * . T r i p l e t l i f e t i m e s obtained from phosphorescence s t u d i e s show t h a t , of the two t r i p l e t s , 3 the n,ir* i s the s h o r t e r l i v e d , i t s l i f e t i m e being g e n e r a l l y of the order of 10 ^ to 10 ^ s e c . ^ ' 3 . Using these l i f e t i m e s i t has been e s t a b l i s h e d t h a t ketones w i t h l o w - l y i n g pure n,ir* t r i p l e t s undergo the N o r r i 3 h Type I I r e a c t i o n w h i l e those w i t h pure I T , I T * lowest t r i p l e t s a r e , g e n e r a l l y , u n r e a c t i v e . The r e a c t i v i t i e s of these two t r i p l e t s 26b i n i n t r a m o l e c u l a r hydrogen a b s t r a c t i o n r e a c t i o n s have been estimated to d i f f e r by at l e a s t a f a c t o r of 10*\ This i s not s u r p r i s i n g , i f , as i s g e n e r a l l y assumed, the r e a c t i o n i n v o l v e s the a b s t r a c t i o n of a hydrogen atom by an e l e c t r o n - d e f i c i e n t oxygen. For the n , T T * s t a t e has a c o n f i g u r a t i o n i n which an e l e c t r o n d e f i c i e n c y has been created at the oxygen due to the promotion of one of i t s non-bonding p a i r of e l e c t r o n s i n t o an a n t i b o n d i n g (ir*) o r b i t a l . The n,Tr* e x c i t e d s t a t e of - 28 - carbonyl compounds, therefore, closely resembles and acts like an 26b alkoxy radical . The T T , T T * state, on the other hand, has an electron- rich rather than an electron deficient oxygen as depicted in Figure 4 making i t less reactive in Type II reactions. Unfortunately, some >: = 0 : ' J£>— O ^J3.—O: ground state n , T r * state T T , T T * Figure 4: 3 3 ketones react via excited states which are neither pure n,TT* nor T r , T r * , thus making the above generalization incomplete. Investigators d i f f e r in their interpretations of results in this area. To assume that only 3 n , T T * states undergo the hydrogen abstraction reaction would lead to the inevitable conclusion that ketones having their Tr,Tr* states below 3 their n ,Tr* react because (A) there i s vibronic mixing of the two states, thus conferring some n , T r * character on the excited state which reacts or, alternatively, that (B) the T T , T T * thermally equilibrates with the n , T r * and i t i s this latter t r i p l e t which reacts. Since neither of these two p o s s i b i l i t i e s shown in Scheme 7 seems satisfactory 31 in explaining a l l of the available experimental data , the earlier 26a view that either or both mechanisms may operate in a given ketone seems the safest interpretation to date. Whatever the nature of the excited state in these reactions, i t is ultimately tied i n with the question of whether or not the next - 29 - Scheme 7 26b,31 T T . T f * Tf , T f * n ,TT* Tf , TT* n,TT » \ \ X \ mixed, ̂ "mostly n , T r * product(s) n , T f * n«Tf * T f , TT* \ mixed, mostly ^ T f , T f * product(s) Phenyl ketones Naphthyl or biphenyl ketones Tf , T f * n ,TT* n , T r * J L L J U T* V product(s) - 30 - event i n the sequence i n v o l v e s the a b s t r a c t i o n of a hydrogen atom or the t r a n s f e r of an e l e c t r o n because, as mentioned e a r l i e r , the e l e c t r o n d e n s i t y at the oxygen d i f f e r s markedly f o r the n,rr* and T T , T T * s t a t e s . The c h a r g e - t r a n s f e r (CT) s t a t e resembles the TT , I T * s t a t e i n i t s e l e c t r o n d i s t r i b u t i o n . The oxygen i s n u c l e o p h i l i c and subsequently can only be r e a c t i v e i n hydrogen a b s t r a c t i o n r e a c t i o n s i f the t r a n s f e r i n v o l v e s 32 a proton r a t h e r than a hydrogen atom. P o r t e r and Suppan have s t u d i e d the s p e c t r a l and photochemical behaviour of a v a r i e t y of s u b s t i t u t e d benzophenones. T h e i r r e s u l t s showed that d e r i v a t i v e s having charge- t r a n s f e r lowest t r i p l e t s were even l e s s r e a c t i v e i n hydrogen a b s t r a c t i o n r e a c t i o n s than s u b s t r a t e s having pure TV , T T * lowest t r i p l e t s . Thus the order of r e a c t i v i t y of these three e x c i t e d s t a t e s i s n , T r * > T T , T T * >CT*. This order r e f l e c t s the order of e l e c t r o p h i l i c i t y of the oxygen i n these three s t a t e s thus s t r o n g l y i m p l y i n g that i t i s an e l e c t r o n d e f i c i e n t oxygen which a b s t r a c t s a hydrogen. This has r e c e n t l y been borne out by the photochemical behaviour of 3 - v i n y l phenyl 33 ketones . The i n t e r a c t i o n of the Y-6.double bond of ketones V_ and VI w i t h the e x c i t e d carbonyl chromophore i n t h e i r photochemical t r a n s - formations was evidenced by the i s o m e r i z a t i o n about the double bond i n VI. Comparison o f t h e i r UV s p e c t r a w i t h that of valerophenone showed t h a t the V VI V I I - 31 - interaction i s not a ground state phenomenon. Since ketones have t r i p l e t energies which f a l l below those of olefins, an energy transfer which excites the olefin partner i n a Franck-Condon fashion has to be endo- thermic. Such endothermic energy transfers are now believed to involve 33 34 charge transfer (CT) complexes ' . In V where the potential exists for y-hydrogen abstraction, i t was found that i t competed poorly with t r i p l e t decay via intramolecular quenching by the double bond. The latter process was favored 100:1 over abstraction. Furthermore, the total quantum yield of Type II products from compound V was only 1% of the value reported for compound VII for which the reactive state was clearly n , T f * . These results would seem to indicate that the abstraction of a y-hydrogen via a charge-transfer excited state occurs very i n e f f i c i e n t l y , i f at a l l . One example where a charge-transfer mechanism has been suggested is the photochemical hydrogen abstraction process in small ring nitrogen heterocycles studied by Padwa and coworkers^"*3'*3. The transformation, an example of which i s given in equation 2, was found to have, generally, low quantum yie l d (f0.02) in spite of the fact that the n ,Tf* t r i p l e t responsible for the transformation was too reactive to be quenched e f f i c i e n t l y . The usual explanation for such low quantum yields in certain hydrogen abstraction reactions has been the back eq. (2) tert-Bu - 32 - transfer of the abstracted hydrogen. The kinetic data on this particular system, however, showed that the abstraction of the hydrogen was irreversible. A charge-transfer mechanism (Scheme 8) as proposed by the authors^"*a,k provides an explanation which i s consistent with both the low quantum efficiency and the high t r i p l e t reactivity. The donation R =ter t -Bu - 33 - of an electron by the heteroatom is thought to occur at a rate which exceeds diffusion-controlled rates thus making the t r i p l e t unquenchable. The low quantum efficiency for product formation i s explained by competitive back-transfer of the electron in the radical ion pair accompanied by deactivation to the original ketone. This mechanism in which hydrogen abstraction involves electron transfer followed by proton transfer has, so far, been convincingly demonstrated only in amino systems. For example, photoreduction of carbonyl compounds by amines are well documented and have been shown in a few cases to involve a charge-transfer (CT) complex formed by the interaction of the electron 35 donor with the acceptor . In some cases, such complexes have been detected by their emission spectra and/or by their e.s.r. spectra. Nevertheless, the fact that the oxygen analog of amine VIII i.e. compound 9 IX reacts by a different mechanism (Scheme 9) should caution against the extension of this mechanism to other related systems. Since the CT Scheme ,9 IX mechanism relies on both the a v a i l a b i l i t y of electrons on the donor atom or group and the ease of reducing the acceptor, both the ionization - 34 - potential (IP) and the nature of the excited carbonyl chromophore w i l l determine whether or not this mechanism is feasible in any given system. Thus, the difference in the behaviour of substrates VIII and IX, for example, is l i k e l y due to the higher IP of the non-bonding electrons on oxygen relative to those on nitrogen as Padwa and Eisenhardt have 2 5b suggested . In the tetrahydro-1,4-naphthoquinone series, the possi- b i l i t y exists for a CT complex formation preceding a biradical inter- mediate since cyclohexene and i t s methyl substituted derivatives have IP's 36 ranging from 8.3 to 8.9 eV which are comparable to the reported IP 37 values for some primary and secondary aliphatic amines which are known to photoreduce carbonyl compounds by a CT mechanism. Characteristics of the 8- and y-Hydrogen Abstraction Reactions i n the Tetrahydro-1,4-riaphthoquinone Series: Substituent Effects There are two effects caused by substitution: (i) in two substrates where both the ene-dione double bond and the c bridgehead positions bear methyl groups, both g- and y -hydrogen abstraction products were isolated, the latter process being-absent i n a l l other substrates so far studied. The abstraction of a hydrogen by enone carbon has been reported in the literature as resulting from a 3 27 38 TTjir* state ' . This leads one to conclude that in the tetrahydro- naphthoquinone series, only when both the bridgehead positions and the ene-dione double bond are substituted with alkyl groups does the TT ,TT* - 35 - t r i p l e t energy level become sufficiently lowered to favor reaction from i t . This i s a tenable supposition in the light of the findings of 39 other workers that alkyl substitution on an enone double bond causes 3 a lowering of the ( T r , T r * ) energy level ( i i ) the second effect caused by substitution seems to involve the modes of closure of the intermediate biradical in the 3-hydrogen abstraction reactions. As shown in the generalized scheme (Scheme 10) for such a process, there are four modes of closure open to such a Scheme 10 XIII XIV - 36 - biradical. The formation of a strained cyclopropane ring in a t r i c y c l i c structure i s on steric grounds alone unfavorable and so the complete absence of product type XII i n the series i s understandable. Product XIII. formed by bonding C-l to C-6 i s formed by a l l substrates except the parent compound i.e. R=R'=R"=H. In some cases, i t i s the exclusive 24 product from a $-hydrogen abstraction process. Scheffer and coworkers have explained the product types encountered i n this series by proposing that the reactions are conformationally controlled. In this proposal, the assumption is made that these tetrahydro-1,4-naphthoquinones exist in solution mainly in the twist conformation A, Scheme 11, and that the Scheme 11 XV m i diradical intermediate XI, when formed at f i r s t has this same confor- mation. From this conformation, the only feasible form of ring closure i s between carbons 1 and 6. The other possible bonding centres are either too far apart or would lead to the s t e r i c a l l y unfavored cyclopropyl derivative. Thus, biradical intermediates which are immobilised, say, by bulky bridgehead substituents collapse only to enone-alcohol product XIII. On the other hand, biradicals which are not restricted to this conformation can adopt other conformations such as 1$ and from which C3 to C8 and C3 to C6 bonding modes are possible giving, ultimately, products XIV and _XV, respectively. Solvent Effects The one general solvent effect observed in this series i s shown in Table II. It would seem, therefore, that ring closure of biradical intermediate XI occurs preferentially between carbons 3 and 24a 6 in benzene. In a hydroxylic solvent, i t has been suggested that hydrogen bonding between biradical and solvent might lead to localization of the electrons at carbons 3 and 8 (XIV1). Closure between these centres - 38 - Table II. Product Ratios Obtained ln Benzene (unparentheslsed) and tert-Butyl Alcohol would then explain the preferential formation of product type XIV i n tert-butyl alcohol. The alternative structure, XIV", for such a XIV XIV" - 39 - solvated species would be a 1,3 biradical which can only collapse to the ster i c a l l y unfavored cyclopropyl derivative. Quantum Yield and Quenching Studies 24b The substrate chosen for study was compound 9̂  which had been 24a shown to give t r i c y c l i c products 9A and j)B_ in combined yields of 80% The quantum yields for appearances of enone-alcohol, 9A and diketone 9B 24b were 0.066 and 0.089 respectively . These low quantum yields may reflect, among other p o s s i b i l i t i e s , reaction from an inherently less reactive state than a pure n,ir* excited state or reversible hydrogen 24b transfer. Quenching studies showed that product 9B arising from Y-hydrogen abstraction by carbon i s formed from a t r i p l e t excited state. The formation of 9A, a g-hydrogen abstraction product, could not be quenched, however, showing that this process occurs from a very short-lived t r i p l e t or a singlet state, a result in accord with the finding of Agosta and 25d Cormier in the g-hydrogen abstraction reaction of a-methylene ketones. As mentioned earl i e r , hydrogen abstraction by enone carbon i s associated with a T r , T r * state. - 40 - Stereoelectronic Requirements Both 8- and y -hydrogen abstraction reactions seem to require that the hydrogen to be abstracted be trans to the bridgehead hydrogens or substituents. Thus, compound _11 undergoes both processes while no 24b hydrogen abstraction occurs on irradiating 10 . This i s best understood by examining the possible conformations which these substrates can adopt in solution. These are shown in Scheme 12 where C5 and C8 hydrogens cis to the bridgehead substituents are shown by X and those trans are represented by Y. It i s only i n conformations A and B, that the cis hydrogen (X) comes within less than 3.5A of the oxygen, and - 41 - i t i s in these same conformations that the C-H bond to be broken is ortho- gonal to the adjacent ir-system so that the developing radical centre i s 24a denied stabilization through d e r e a l i z a t i o n . The abstraction of only hydrogens in position Y i s also in agreement with the observation 40 made by Turro and coworkers that only the y-hydrogen which is in the plane of the carbonyl group (i.e. the position of the localized half- vacant n orbi t a l of the n -T f * state) i s abstracted. Other Reactions The preceding discussions have been concerned with only hydrogen abstraction processes. Intramolecular cycloaddition reactions leading to oxetane and cage products have been found to occur only in substrates which have no abstractable hydrogen which leads one to conclude that these reactions compete poorly with intramolecular hydrogen abstraction in tetrahydro-l,4-naphthoquinones. 3. Objectives of Present Research During the course of the previous discussion the possibility of the variation in product types being due to the conformational mobility or the lack thereof of the biradical intermediate was raised. Most important, there seems no doubt as in other hydrogen abstraction reactions, reactivity in this series also requires that the substrates f u l f i l l some geometric requirements. What are these require- ments? Is the partitioning of the biradical conformationally controlled? 22 23 In the f i r s t published works ' on the parent compound, extensive - 42 - polymerization was observed both in the solid and .in solution. Since 24a the later work of Scheffer and coworkers was carried out using selective irradiation, the question of whether the change from intermolecular to intra molecular reaction was due to the i n t r i n s i c properties of two different excited states needs to be answered. Djerassi and coworkers, i n a 41 series of papers , have established that the Y-hydrogen abstraction observed in the mass spectrometer when carbonyl compounds are bombarded with electrons i.e. the McLafferty rearrangement, does not occur i f the o Y-hydrogen is further than 1.8A from the abstracting oxygen. Recent 42 work by Henion and Kingston has revealed that, contrary to earlier 43 -calculations , the hydrogen to be abstracted need not be in the plane of the carbonyl group. Specifically they found that, the angle x between the itinerant hydrogen atom and the plane of the carbonyl group (F i g u r e -5) has an operational l i m i t , namely, 80°>x°^0°. Although, Figure 5. Operational Definition of the Angle x. the Norrish Type II reaction i s analogous to the McLafferty rearrangement, one would not expect i t to have the same geometric requirements as the McLafferty rearrangement because, unlike the latter, the Type II reaction occurs from electronically-excited states and "the geometry of this state need not be the same as those of. ground state or vibrationally- i excited molecules. - 43 - Not surprisingly, Lewis and coworkers found that compound XVI, in which the geometry of closest approach of the y-hydrogen to the o oxygen does not allow these two atoms to come any closer than 2.2A (as measured by Dreiding models), s t i l l undergoes the Norrish Type II etc. XVI reaction. Over what distances can these hydrogen abstractions occur then? In photochemical hydrogen abstraction, i s the hydrogen 43 required to l i e in the plane of the carbonyl group as earl i e r proposed . ? As mentioned earlier, most compounds which are conformationally mobile in f l u i d media, c r y s t a l l i z e out in just one conformation which can be accurately determined by X-ray diffraction methods. Also obtainable from the X-ray data are useful parameters such as bond distances, angles and intermolecular distances. So, the i n i t i a l geometry of a starting material can be accurately determined provided i t i s crystalline. Furthermore, due to the restrictions of atomic and molecular movements imposed by the crystal l a t t i c e , most solid reactions are topochemically 12 45 controlled ' i.e. they occur with minimum atomic and molecular move- ment. This means that a substrate, S, w i l l react within the crystal l a t t i c e without gross changes in conformation. It also means that an - 44 - intermediate such as the one postulated in the photochemical hydrogen abstraction reactions of tetrahydro-l,4-naphthoquinones would, most l i k e l y , have the same conformation as the substrate and close to product(s) from that conformation. Thus, the study of these reactions in the solid state, when used in conjunction with the X-ray structural data, promises to shed light on the geometric requirements for these reactions. To do this, eleven compounds were chosen for study. Their structures are shown below. 9 10 11 - 45 - These substrates, taken as a whole, ex h i b i t e d 8, Y, Y , oxetane, and 24 cage compound formation reactions i n s o l u t i o n as the table below shows. Table I I I Reaction Type Observed i n Solution Substrate 1 •X Oxetane Cage Compound 1 Yes No No No No 2. Yes No No No No 2 No Yes No No No A -. - - - - 5. Yes No No No No 6. Yes No No No No _7_ Yes No No No No No No No Yes No _9 Yes No Yes No No 10 No ' No No Yes Yes 11 Yes No Yes No No Although compounds^ and 10 do not undergo hydrogen ab s t r a c t i o n r e a c t i o n s , i t was of i n t e r e s t to f i n d out i f t h e i r c y c l o a d d i t i o n reactions also occur from the same conformation as the hydrogen a b s t r a c t i o n reactions, of the other com- 24c pounds. In p a r t i c u l a r , i t has been suggested that cage compound formation might be occurring from the same conformation as the hydrogen a b s t r a c t i o n - 46 - reactions, since an alternative conformation such as C which brings the two n-systems closer together is disfavored by bridgehead methyl eclipsing. - 47 - Results and Discussion Preparation of Substrates Without exception a l l of the substrates investigated were made by the Diels-Alder addition of a quinone to a diene. Most of the quinones and dienes used were readily available from commercial sources. Below i s a summary of the synthetic schemes used to obtain substrates 1-11. 56,83-Dimethyl-4ag ,5,8,8ag-tetrahydro-l, 4-naphthoquinone, 1_. ... ref. 95 6,7-Dimethyl-4aB ,5,8,8ag-tetrahydro-1,4-naphthoquinone, 2_. • • ref. 96 4ag,5 ,8,8aB-Tetrahydro-l,4-naphthoquinone, 3_. ... ref. 55 - 48 - 6,7-Diphenyl-4a3,5,8,8a8-tetrahydro-1,4-naphthoquinone, j4. o o FSJH2 2 2H n C a H s C - C C 6 r i ^ - ' g ^ C 6 H 5 C S C C 6 H 5 N N I i NH; NH. 9 2CH.SCH 3 3 2NaH 0 2CH SCH" 3 2 C6HC—CC H C H ^ 6 8 - 51 - 2,3,4aB ,6,7,8af3-Hexamethyl-4ag,5,8,8a$-tetrahydro-1,4-naphthoquinone, 9_. .. . ref. 102 9 2,3,4aB,5a,8a,8a8-Hexamethyl-4aB,5,8,8aB~tetrahydro-l,4-naphthoquinone, 10 ... ref. 24b 2,3,4aB,5B,80,8aB-Hexamethyl-4aB,5,8,8aB-tetrahydro-l,4-naphthoquinone, 11 ... ref. 24b 11 - 52 - A l l substrates were meticulously purified by two or more r e c r y s t a l l i - zations before use. The melting points and spectral data of a l l were in good agreement with the literature values. The NMR spectra of these compounds and of the products obtained by irradiating them in the solid state are given i n the Appendix. Unless otherwise stated, the apparatus for the solid state reactions was a specially designed photochemical reactor which allowed for evacuation of the reaction chamber to <0.05 torr. Reaction temperatures were chosen so as to be below the eutectic temperature of the reaction mixture comprising product(s) and starting material. . The eutectic temperature was determined by d i f f e r e n t i a l scanning calorimetry using varying compositions of crude reaction mixture and starting material. One such scan i s shown in Figure 6. Uncorrected eutectic 320 325 330 335 K Figure 6. Uncorrected Endothermic Transition for 1 + IA Eutectic Mixture. • <. - 53 - Reaction temperatures were maintained within f±5° by a circulating coolant from an Ultra Kryomat. The temperature at the reaction site was monitored by a copper-constantine thermocouple and read on a d i g i t a l millivoltmeter. Details on the apparatus and method of use are given i n the Experimental section. - 54 - 1. Intermolecular [̂ 2 + ^2] Dimerization 5a,8a-Dimethyl-4a 3,5,8,8a3-tetrahydro-1,4-naphthoquinone, _1 s. i _ Large yellow crystals of 1 were obtained by crystallization from a solvent mixture comprising petroleum ether and diethyl ether. A large, well-formed crystal of 1 measuring 0.40 x 0.70 x 1.0 mm was used for X-ray data collection. The X-ray structure determinations of this and other compounds discussed in this manuscript were carried out by Dr. James Trotter and Dr. Simon E.V. Phi l l i p s of this department and have been published as a series of papers in Acta CrystallographicaJi. Reference for the individual structures are given with the crystal data. Unit c e l l and intensity data on 1 were measured using a Datex automated G.E. XRD 6 diffractometer with Cu Ka radiation and the 6-26 scan technique. Accurate unit c e l l constants were obtained by a least-squares refinement method using 26 values of 18 manually 46 centred reflections. The structure was solved by direct methods using intensity data from 1557 independent reflections and refined by the full-matrix least-squares procedure to an R value of 0.048. - 55 - Crystal Data C12 H14°2' m o n o c l i n i c > space group P2^/c with a=7.189(1), b=22.241(4), c=6.843(1) A, 8=106.51° and Z=4. The molecules occur in pairs related by centres of symmetry, X (Figure 7). Figure 7. Stereo diagram of an adjacent pair of molecules of 5a,8a- dimethyl-4aB,5,8,8a8-tetrahydro-l,4-naphthoquinone, 1̂, with 50% probability vibration ellipsoids for non-hydrogen atoms. The centre of symmetry is indicated by X. Ellipsoids are shaded for one molecule and unshaded for i t s centre of symmetry related neighbour for c l a r i t y . - 56 - Irradiation of crystals of 1^ i n vacuo at temperatures below the eutectic temperature (52.5°) through a Corning glass f i l t e r transmitting A >_ 340 nm gave only one product. The latter crystallized from a solvent mixture comprising chloroform and hexane as small, colorless, sparkling plates. The product correctly analyzed for the dimer C2^H2g0^. In the infrared, the crystallized and the crude product had identical spectra. Both had carbonyl absorptions at 5.82 and 5.90 u. The nuclear magnetic resonance spectrum had a singlet at 63.47 which was indicative of cyclobutane ring protons. For comparison, the cyclobutane ring protons of compounds XVII and XVIII have been reported 48 49 at 63.90 and 62.90 respectively ' . The lack of coupling between the 0 0 0 o XVII cyclobutane ring protons i s due to the symmetry of the molecule. In order to determine the stereochemistry of the dimer, one of the crystals measuring 0.40 x 0.25 x 0.10 mm was used for X-ray structure determination. Cell constants were determined and refined using 20 values from 17 reflections. Intensity data were collected as for the monomer from 1458 independent reflections. The R value - 57 - after refinements was 0.050. Crystal D a t a 4 7 a C24 H28°4' m o n o c l i n i c » space group Vl^/c with a=ll.393(1), b=8.029(1), c=10.771(5) A, 8=91.04(1)° and Z=2. The molecule i s centrosymmetric and has a planar four-membered ring (Figure 8). •I i Figure 8. Stereo diagram of dimer IA i n an orientation analogous to that of the monomer 1. - 58 - The formation of 5,8,15,18-tetramethylpentacyclo- [10.8.0.0 2' 1 1.0 4• 9.0 1 4' 1 9]eicosa-6,16-dien-3,10,13,20-tetrone, IA, from the monomer 1^ formally involves bond formation between carbons 3 and 2' and between carbons 2 and 3' of adjacent monomer pairs related by a centre of symmetry. The reaction i s depicted diagrammatically below: 1 7 With the aid of a stereo viewer, i t can be seen from the arrangement of monomer pairs (Figure 7) that the C3, C2 double bond of one partner i s aligned p a r a l l e l to the C2', C3' double bond of i t s adjacent partner. Such an arrangement coupled with a short centre-to^centre double o bond contact of 4.040A between these two ir systems allows for good overlap of the p orbitals. The separation, d, between the two double c - 59 - bonds is within the limit 4.2 > d > 3.5 A established experimentally by Schmidt and coworkers^^*^ f o r photodimerizations i n the solid state. As Table IV shows, the chemical yield for the 1 ->- LA conversion was nearly quantitative both at low and high conversions of the monomer. Table IV. Product Yields for the 1 -»• IA Conversion Reaction Temperature % Conversion % Yield Room Temperature (uncontrolled) 17 91 -1.2° to -0.7° 37 87 -1.2° to -0.5° 56 94 -2.0° to -1.3° 79 96 -1.5° to -1.0° 79 96 -3.0° to -2.0° 100 89 In contrast to the above solid state dimerization, compound 1 undergoes intramolecular yhydrogen abstraction when irradiated i n 24a solution . The product of this reaction, 1B_, arises from the abstraction of one of the C9 hydrogens by 0(1) followed by bonding between carbons 3 and 9 and subsequent ketonization. Although the nearness of one of the C(9) hydrogens to 0(1) in the crystal (H' # , ,0 o distance = 2.38 A) makes the abstraction conceivable in the solid state, the subsequent collapse of the biradical intermediate to 1B_ i s - 60 - not l i k e l y to be permissible within the crystal l a t t i c e since C3 and C9 are quite remote from each other, i n the crystal of the substrate. h v solution (1) 3,9 bonding (2) keronization 6,7-Dimethyl-4aB. 5.8 .8aB - tetrahydro-1,4-naphthoquinone, 2 A second example of photodimerization:in the solid state was provided when crystals of compound 2_ were irradiated at long wavelength (A >_ 340 nm) below the eutectic temperature (109.5°). The sole product of the reaction, 2A, crystallized from acetonitrile solution as colorless, sparkling plates. The infrared spectrum of these crystals was identical to that of the crude product. Product 2A correctly analyzed for the dimer C24H28^4' I t : had carbonyl absorption at 5.85 y in the infrared. In the nuclear - 61 - magnetic resonance spectrum, there were no signals i n the 66.0 - 7.0 region indicating the absence of conjugated v i n y l hydrogens. A singlet sited at 63.64 was assigned to cyclobutane ring protons by analogy to previous cited examples. The positions of a l l other protons were comparable to the positions of the corresponding protons of monomer 2_.< The ultimate proof of the assigned structure came from X-ray crystal structure determination. Data collection was carried out on a piece of crystal measuring 0.07 x 0.25 x 0.75 mm which was cut from a larger crystal. Cell parameters were determined and refined from 10 reflections. The structure was solved as for compound JL using intensity data of 1443 independent reflections. The f i n a l R value was 0.065. 47b Crystal Data ^24^28^4' m o n o c l i n i c » space group P2^/c with a=15.247(1), b=6.2776(6), c=10.1949(7) A, 3=93.19(1)° and Z=2. - 62 - As the stereo diagram of the dimer shows, Figure 9, this dimer like IA has a planar cyclobutane ring and i s centrosymmetric. Figure 9. Stereo diagram of 2A. Non-hydrogen atoms are shaded for one of the monomer units of the union and open for the other for c l a r i t y . - 63 - As in the case of monomer JL, the 2_ 2A conversion i s quantitative both at low and high conversions (Table V). Table V. Product Yields for the 2 2k Conversion Reaction Temperature % Conversion % Yield Ambient Ambient Ambient 25 40 61 99 97 96 In contrast to i t s dimerization i n the solid state, compound 2_ undergoes intramolecular $-hydrogen abstraction to give photoproducts 2JB, 2C_ and 2D in solution. The relative yields of 24a these products were found to be solvent dependent bonding - 64 - 4ag ,5 ,8, 8a8-Tetrahydro-1,4-naphthoquinone, 3_ The parent compound of this series of tetrahydro-1,4- naphthoquinones, _3, provided a further example of photodimerization in the solid state. Irradiation of crystals of 3̂  at long wavelength, A >_ 340 nm, below the eutectic temperature (49.4°) afforded photodimer 3A in quantitative yield (Table VI). h v solid state -SA. Table VI. Product Yields for the 3 -> 3A Conversion Reaction Temperature Ambient Ambient Ambient Ambient " 0.0 - 4.3° % Conversion 16 19 36 77 94 % Yield 86 80 100 75 92 - 65 - Dimer 3A crystallized from chloroform solution as colorless flakes, the infrared spectrum of which was identical to that of the crude product and also to that of a photolyzed KBr pellet containing 3_. It correctly analyzed for the dimer ^20^20^4" Since the crystals obtained from chloroform were unsuitable for X-ray structure determination, the dimeric structure 3A was deduced from spectroscopic data. In the infrared, cyclobutane ring vibrations are not as well defined as are those of cyclopropanes"*^. Derfer and coworkers"*^" reported that seven substituted cyclobutanes which they investigated absorbed i n -1 52 the 920-910 cm region. Reid and Sack followed up with a report that when a l l the cyclobutane ring carbons are substituted, the absorp- tions are shifted to the 888-868 cm region. More recently, Dekker and 53 -1 coworkers reported absorptions in the 1000-850 cm region which they attributed to the cyclobutane ring vibrations in the 920-850 cm region which may be taken as indicative of the presence of a four- membered ring. The carbonyl chromophore absorbed at 5.85u. In the Raman, cyclobutanes are reported"*^ to have a charac- t e r i s t i c C-C stretch at 933 cm ̂ . Dimer 3A had an absorption at 935 cm ̂ in the Raman. The nuclear magnetic resonance spectrum was informative. A singlet at 63.64 was assigned to the cyclobutane ring protons. This i s in good agreement with 6 values of 3.47 and 3.64 for the cyclo- butane protons of IA and 2A respectively. The other assignments - 66 - which are given below compare w e l l with the absorptions of corresponding protons i n the monomer. 3_ _3A_ Absorptions Proton type Monomer _3_ Dimer ^A a 6.57(s) 3.64(s) b 5.63(m) 5.70(m) c 3.15(m) 3.20(m) d 2.28(m) 2.30(m) A l l the spectroscopic data summarised above and the elemental analysis l e d to the i d e n t i f i c a t i o n of the dimer as pentacyclo[10.8.0.0 2 * 1 1.0 4' 9.0 1 4' 1 9]eicosa-6,16-dien-3,10,13,20- tetrone, 3A of as yet unassigned stereochemistry. In attempts to obtain c r y s t a l s of 3A which are s u i t a b l e for X-ray structure determination, 3A was c r y s t a l l i z e d from a number of solvents i n c l u d i n g g l a c i a l a c e t i c a c i d and a c e t o n i t r i l e . The i n f r a r e d spectrum of c r y s t a l s obtained from g l a c i a l a c e t i c a c i d was i d e n t i c a l to that obtained by c r y s t a l l i z i n g from 30 40 5.0 MICRONS 6.0 . 7.0 8.0 9.0 10 12 16 20 30 40 1 I 1 i I i i l i 1 1 1—i 1 1—r-1 WAVENUMBER (cm"1) Figure 10. Infrared spectra of KBr pellets containing 4-5% by weight of Dimer 3A (Top) and Dimer 3B (Bottom). Ordinates have been shifted for c l a r i t y . - 68 ~ acetonitrile. These crystals are judged to be identical and w i l l be subsequently referred to as 3B. As Figure 10 shows the infrared spectrum of 3B_ greatly resembles that of the i n i t i a l dimer 3A. Nevertheless, the two spectra are not identical. The absorption spectra of 3A and 3_B crystals in the Raman were also different. The characteristic cyclobutane C-C stretch i n 3B was at 931 cm \ The nuclear magnetic resonance spectrum of 3B was however, identical to that of 3A. The 3B_ crystals correctly analyzed for the dimer C^H^O^ One of the 3B crystals obtained as colorless, sparkling l i t t l e rods and measuring 0.50 x 0.30 x 0.30 mm was used for X-ray structure determination. Accurate c e l l constants were obtained from 28 values of 15 manually centred reflections and refined by the least-squares refinement method. Intensity data from 1488 independent 47b reflections were collected and treated as described . The R value after refinements was 0.037. 47b Crystal Data C20H20°4'' m o n o c l i n i c » space group ?2^/c with a=ll.7302(5) , b=6.4142(2), c=10.9331(5) A, 8=114.624(3)° and Z=2. Figure 11 i s a stereo diagram of dimer 3B. - 69 - Figure 11. Stereo diagram of the 3B molecule. Non-hydrogen atoms are shaded for one of the monomer units and open for the other monomer unit for c l a r i t y . The v i t a l question i s whether or not the centrosymmetric dimer 3B i s the original product of the photodimerization of crystals 53 54 of 3_. ' Because of reports ' of syn->-anti isomerizations of cyclo- butanes and other small rings by acidic reagents such as sulfuric and phosphoric acids, i t was suspected, at f i r s t , that 3A and 3B might be stereoisomers. If one allows that no isomerization about the decalin ring junction occurs during the dimerization, then the number of - 70 - possible stereoisomers of the dimer i s only two - one of these, the "syn" isomer, arises when monomer pairs are oriented i n a mirror- symmetric fashion and the other, the "anti" isomer arises from a mirrorsymmetric pair - 71 - centrosymmetric pair of monomer units. The assumption made above i s ju s t i f i e d by the previously described dimerizations (1 -*• IA and 2_ -*• 2A) in which configurations about the decalin bridgeheads are main- tained during the reaction and also because such isomerizations proceed via the enolate ions and require basic catalysts"'"'. Since 3B_ crystals have been shown by the X-ray structure to be the "anti" isomer, attempts were made to establish whether or not the original dimer 3A was the "syn" isomer. The method of choice was the joint use of infrared and Raman spectroscopy as described by Ziffer and Levin"'*' for use in differentiating dimers having centres of symmetry, C^, from their non-centrosymmetric isomers. In this scheme, for a given sample, absorptions i n the Raman and infrared which occur at the same frequency, ±5 cm \ are termed coincidences. For a molecule possessing a symmetry element, the infrared and Raman absorptions are, mutually exclusive i.e. a vibration which i s infrared active i s Raman inactive and vice-versa. So, for a and a non-C^ isomeric pair, there w i l l be, in general, more coincidences for the non-C^ member than for the centrosymmetric isomer. For example, Ziffer and Levin"^ found that the head-to-head dimer of cyclopentenone which lacks a centre of symmetry had 84% of i t s Raman lines coinciding within ±5 cm ^ with i t s infrared lines while the centrosymmetric head-to-tail dimer had only 25%. As molecular size increases, however, the number of "accidental" coincidences also increases so that the differentiation by this method - 72 - becomes less clear-cut. . For example, the non-C^ and isomers of the dimer of 1-indenone had 67% and 38%, respectively, of their Raman lines coinciding with their infrared lines. Although, the 29% difference here i s not as startling as i n the previous example, i t was thought sufficient for a preliminary differentiation i f indeed 3A and 3B are mirrorsymmetric and centrosymmetric, respectively. Dimer 3A had, by this analysis, 45% of i t s Raman lines coinciding with i t s infrared lines while the centrosymmetric dimer 3B had 39%. The 6% difference here i s really too small to ascribe to mirror- symmetric, centrosymmetric configurational isomers. On the other hand, i f one assumes that dimer 3A i s also centrosymmetric, then one i s l e f t to explain the differences i n the infrared spectra of 3A and 3B. Given the supposition that 3A and are identical, the differences between the two infrared spectra can be due to contamina- tion of the two samples by different impurities and/or different intermolecular coupling effects"* 7. The f i r s t of these p o s s i b i l i t i e s i s unlikely since different batches of crystals obtained from the same solvent from different sources had identical spectra. Furthermore, the infrared spectra of these cr y s t a l l i z i n g solvents revealed that the crystals were not contaminated by any of them. This leaves one with the likelihood that the infrared spectral differences merely reflect different orientations of the individual molecules within the crystal. There i s ample documentation of these crystal packing - 73 - effects i n the infrared absorption spectra of solids . To prove that the absorption differences are due to intermolecular effects, one would normally need to record the infrared spectrum of a solution or melt where such effects are absent or less pronounced. Unfortunately, the inso l u b i l i t y of 3A and 3B crystals in addition to their thermal i n s t a b i l i t i e s near their melting points led to the fai l u r e of these methods. Nevertheless, examples of pronounced intermolecular effects in solid state infrared absorptions make the supposition of 3A being identical to a tenable one. For example, the spectra of gaseous and solid benzene (Figure 12) taken from reference 57 i l l u s t r a t e the sort of differences which can arise from intermolecular interactions. The gaseous state represents the "free" _ l l I l 1 — i 1 — 1 8 0 0 1 6 0 0 1 4 0 0 I E 0 0 1 0 0 0 8 0 0 Wove number Figure 12. Part of the absorption spectrum of benzene: i s gas and i s solid. Ordinates are shifted for c l a r i t y . - 74 - state in which the observed absorptions are purely those of the isolated molecule. For the crystal, there are absorptions arising from coupling between the vibrations of adjacent molecules i n addition to those due to intramolecular vibrations. The inter- molecular component of the absorption spectrum depends on the relative orientations of molecules within the crystal and hence, such factors as degree of c r y s t a l l i n i t y w i l l affect the spectrum. This is manifest i n the infrared spectra of polymer samples in which differing degrees of c r y s t a l l i n i t y can be achieved by crys t a l l i z a t i o n . The effects of crystallization on the spectra of such samples have been reported"^ Dows and o t h e r s " * 9 r e p o r t other incidences of differing infrared absorptions in the solid state which i l l u s t r a t e intermolecular coupling effects. In one of such examples, Dows 59 and coworkers report that ammonium azide solid obtained by sublimation represents a disordered crystal which becomes ordered on ' warming. The infrared spectra of the disordered and ordered crystals were similar but different. Crystals of 3A and 3B may, therefore, represent different crystal modifications of the same compound and the differences i n the infrared spectra of these two crystals dispersed i n KBr matrices may merely be due to different intermolecular interactions within each of these two crystals. Owing to the negligible so l u b i l i t y of the dimer i n chloroform, i t i s l i k e l y that the crystals formed from i t form quickly and are disordered. This i s especially true since - 75 - the infrared spectrum of these crystals i s identical to that of the crude product and there are indications from the packing diagram of the monomer (below) that the dimer molecules as they f i r s t form are at best only semi-oriented. The monomer, 3_, crystallized from a solvent mixture of petroleum ether and diethyl ether as well-formed, yellow needles. Owing to the i n s t a b i l i t y of the crystal in the X-ray beam, the collection of data for the structure determination required two pieces of crystal measuring 0.80 x 0.30 x 0.20 mm each which had been 47c cut from larger crystals. The X-ray data were obtained and ' N treated as i n previous cases. Accurate unit c e l l constants were obtained by least-squares refinement of 29 values of 16 manually centered reflections. The intensity data were obtained from 2496 independent reflections and refined by the full-matrix least-squares procedure. The f i n a l R value was 0.060. 47c Crystal Data ^20^20^2' m o n o c H n i c » space group P2^/c with a=5.266(1), b=24.267(5), c=14.506(4) A, 8=114.50(2)° and Z=8. The packing diagram of monomer 3_ (Figure 13) revealed two crystallographically independent types of molecules in the unit c e l l . These have been labelled A and B, respectively, in Figure 13. As can be seen from this diagram, layers of the A type alternate with those of the B type. Molecules of the A type form an axial repeat - 76 - B A B A 8 F i g u r e 13. Stereo diagram of the contents of the u n i t c e l l f o r compound 3. p a t t e r n , i . e . , each molecule i s r e l a t e d t p i t s nearest neighbour o by a c e l l t r a n s l a t i o n of 5.266 A. This l a t t e r d i s t a n c e , t h e r e f o r e , a l s o measures the i n t e r m o l e c u l a r d i s t a n c e between the mid-points of the C2, C3 and C2', C3' double bonds. Furthermore, as can be seen from the stereo diagram of adjacent molecules of the A type (Fig u r e 14), there i s no o v e r l a p of these two TT systems. On the other hand, molecules of the B type occur i n p a i r s r e l a t e d by a centre of symmetry denoted by X i n F i g u r e 15. I n t h i s arrangement, the C3, C2 and C2', C3' double bonds of adjacent molecules are p a r a l l e l but o f f s e t w i t h the C2 of one molecule d i r e c t l y above the C2' of the other. The C2 to C2' contact i s the s h o r t e s t i n t e r m o l e c u l a r o d i s t a n c e and measures 3.351 A; The c e n t r e - t o - c e n t r e s e p a r a t i o n of the - 77 - Figure 15. Stereo diagram of a type B molecule (shaded e l l i p s o i d s ) and i t s nearest neighbour (unshaded e l l i p s o i d s ) r e l a t e d by a centre of symmetry, X. - 78 - two double bonds Is 3.755 A i n this case. This i s also the separation between C2 and C3' and between C3 and C2'. With the aid of a stereo viewer, the overlap of the two double bonds can be appreciated. The arrangement of molecules of the B type are analogous to that found i n crystals of monomer 1 which as previously shown, dimerized to the centrosymmetric dimer IA. By analogy to the 1^ -*• IA conversion, the centrosymmetric dimer 3_B results from a topochemically controlled dimerization i n the B stack with C2, C3 of one monomer linking up with C3', C2', respectively of i t s nearest neighbour (Figure 15). The centre-to- c'entre separation, d, of the two double bonds involved i n this o dimerization i s 3.755 A which i s shorter than the corresponding separation i n monomer .1 crystals and should make dimerization i n o this stack relatively more f a c i l e . Also this distance, d=3.755 A, o f a l l s within the range 4.2>d> 3.5A experimentally established for similar photodimerizations in the solid s t a t e ^ " ^ ' ^ . o On the basis of the large separation of 5.266 A between the two double bonds of adjacent pairs of molecules of the A type, no dimerization i s expected to occur i n this stack. However, as shown by monomer conversions i n excess of 50% (Table VI) this i s not the case. Dimerization, most l i k e l y , originates i n the B stack giving r i s e to centrosymmetric dimers intercalated into the l a t t i c e of the monomer. The boundaries between the reacted and unreacted regions of the crystal are akin to dislocation sites since molecules - 79 - here may have different orientations from those located elsewhere in the crystal l a t t i c e . These boundaries may be sites for further reaction in the crystal. The lack of formation of the mirrorsymmetric ("syn") dimer when molecules in the A stack eventually react may merely reflect an energetically unfavorable process of forming a steri c a l l y crowded dimer. As mentioned earlier i n the Introduction, Cookson and 23 coworkers reported the formation of a dimer of 3̂  of unknown stereo- chemistry during their photolysis of _3. They reported that f a c i l e internal oxidation-reduction occurred when they tried to dehydro- genate the dimer to the dimer of naphthoquinone. They speculated that this might indicate that the dimer had a syn-configuration about the cyclobutane ring. Since the authors irradiated ethyl acetate solutions of 3_ rather than the pure crystals, i t cannot be said with any certainty that their dimer and the one reported i n this work have the same stereochemistry nor i s i t certain that their 'dimer' i s not actually a mixture of stereoisomeric dimers of _3 since such dimerizations in solution often produce a mixture of syn-, 62 63 and anti-configurated cyclobutane derivatives ' 64 An attempt was made recently by Dekker and coworkers to assign the stereochemistry about the cyclobutane rings of dimers of J3- and 1_ by comparing their rates of cleavage with those of naphthoquinone dimers of known stereochemistry using zinc and zinc chloride reagent. Their results were interpreted as indicative of a syn-configuration - 80 - about the cyclobutane ring for both dimers, but as the authors themselves cautioned, such a conclusion would be risky since l i t t l e i s known about the mechanism of the ring cleavage reaction by zinc, and zinc chloride and about the effects of the aromatic ring on one hand and the olefinic bond on the other. As the X-ray crystal structure shows (Figure 9) the dimer of monomer 2_ is centrosymmetric and, therefore, has the an t i - and not the syn-configuration about the cyclobutane ring. This does not rule out the poss i b i l i t y of the syn dimer being formed in the irradiation of 1_ i n solution and since i t i s not clear from Dekker's report whether the dimers were prepared by irradiating solutions or crystals of 3_ and 2_ respectively, one cannot say whether the dimers they isolated were the same as the ones reported here. Irradiation of 3_ in Solution In the solution photochemistry of 3, the presence of a 24a polymeric material i n the reaction mixture was indicated . This, no doubt, par t i a l l y accounted for the low yi e l d of 10%, of the identified products. It was, therefore of interest to find out i f this unidentified polymeric material contained dimer(s) of _3. To do this, benzene solutions of ,3 were irradiated. The amount of solid deposited during the reaction varied from run to run but never exceeded 49% of total recovered material. The infrared spectrum of the crude solid deposit showed both OH and C=0 absorptions. The presence of the OH group was further confirmed by NMR. Mass spectral - 81 - analysis of the solid showed peaks up to 495 as compared to 324 for the dimer of 3_. These spectral characteristics coupled with the so l u b i l i t y of the solid in CHCl^ and acetone show that whatever the structure of this "polymeric" material, i t contains very l i t t l e of the dimer of 3_. Preparative GLC of the benzene soluble portion of the 24a reaction mixture confirmed earlier results . These are summarised in the equation below. hv solution 3D major Reactive State and Mechanism for the Photodimerization Reactions Although no mechanistic investigations have been carried out for the photodimerizations of substrates JL, 2_ and J^, analogy can be drawn from numerous examples of such reactions both i n the solid ^ 61 . . . . 62.63 state and in solution For example, the dimerization of 2-cyclopentenone i n 62 solution has been shown by quenching studies to be t r i p l e t derived So are a number of photodimerizations of a,$-unsaturated cyclic 62 63 ketones ' . Nevertheless, a number of photodimerizations both i n the solid state and in solution are known to occur from the singlet - 82 - solution ../Ij 61 65 state ' . The solution photodimerization of coumarin, equations 2 and 3 below, has been shown to occur from either the singlet or the tr i p l e t depending on the reaction conditions^"*. Very interestingly, EtOH singlet-derived EtOH (C 6H 5) 2CO (major) triplet-derived YVA | (trace) --(2) •(3) - 83 - thymine photodimerizes from the t r i p l e t i n solution but the equivalent solid state reaction occurs from the singlet state****. This and Schmidt's generalization that the majority of solid-state photo-, dimerizations occurs from the f i r s t excited singlet*'"'" would seem to indicate that solid state photodimerization proceeds via a reactive state which i s different from that which obtains in solution. Given the narrow energy separation between the n , T r * singlet, the n , T f * and the TT,TT* t r i p l e t s i n some of these a,g-unsaturated ketones, i t might very well be that no generalization can -be made about the nature of the reactive excited species involved i n the photodimerizations of JL, 2 and _3. Whatever the nature of the excited species, there i s general agreement that the reaction involves the union of an excited monomer and a ground state monomer. This has been experimentally demonstrated 61 62 for both solid state and f l u i d media photodimerizations. Although the two new bonds to be formed i n this reaction have been shown to 62 63 occur i n a stepwise fashion for dimerizations i n solution ' , the concerted [ 2 + 2 1 mechanism cannot be ruled out for reactions TT S Tf S occurring from the singlet manifold for which no spin pairing process is necessary before ring closure. The stereospecificity of the solid state reactions of substrates 1_, 2̂  and 3̂  in no way identifies the reaction mechanism, however, since the formation of a biradical which is frozen in the conformation of i t s monomer units by the crystal l a t t i c e could s t i l l give a stereospecific product. - 84 - In conclusion, the topochemical principle has been well demonstrated by the UV induced dimerization of 4a8,5,8,8aB-tetrahydro- 1,4-naphthoquinone and two of i t s dimethyl derivatives. The stereo- chemistry of each of the dimers has been unambiguously determined for the f i r s t time using single crystal X-ray diffraction methods. - 85 - 2. Intramolecular Hydrogen Abstraction 6,7-Diphenyl-4aB ,5,8 ,'8a g - tetrahydro-1,4-naphthoquinone, A. Compound 4̂  crystallized out of a solvent mixture of acetone and hexane as well-formed yellow needles. One of these crystals 4 measuring 0.47 x 0.43 x 0.70 mm was used for X-ray structure determination. The method and data treatments are the same as for previously reported structure determinations. Unit c e l l parameters were refined from 26 values of 19 reflections. Intensity data were collected from 3365 independent reflections. The R value after refinements was 0.053. Crystal Data^ 7: C22 H18°2' m o n o c l i n i c > space group P2/c with a=27.092(4), b=6.527(2), c=22.112(3) A, g=120.562(9)° and Z=8. The conformation of the molecule is twisted such that the bridgehead hydrogens are staggered with a torsion angle of 62°. Bond lengths and angles are normal; A stereo diagram of the molecule i s shown in Figure 16. Intermolecular distances mostly correspond to van der Waals contacts except one notable distance, namely, the distance between 0(2) - 86 - of one unit and the H(3) of the nearest molecule. This distance i s 2.39 A compared to a normal van der Waals contact of about 2.6 A Irradiation of 4̂  both i n KBr and as the pure solid i n vacuo below the eutectic temperature (151.3°) gave the enone-alcohol 4A expected from the abstraction of a B-hydrogen by oxygen (Scheme 13) . As the stereo diagram i n Figure 16 shows, one of the oxygens, namely, 0(1) is located near and coplanar with one of the C8 hydrogens, H(8B). The abstraction most l i k e l y involves these two atoms. The interatomic - 87 - Scheme 13 2.46A solid state distance involved i n the abstraction i s 2.46 A. The diradical 4/ resulting from B-hydrogen abstraction i s most l i k e l y frozen i n the conformation of i t s precursor. In that conformation, radical centres o o - 1 and 6 are only 3.51 A apart as compared to distances of >4 A for C3, C6 and C3, C8 separations, respectively. Closure of b iradica l 4_' between centres 1 and 6 gives the observed enone alcohol product 4A. The la t ter was easily ident i f iable by i t s infrared and NMR spectra. The carbonyl absorption of the six-membered r ing enone came at 5.97u (cf. 5.92 and 5.95y for 4). The OH absorbed broadly but moderately at 2.92y. In the NMR of a l l previously encountered enone alcohols of this general structure, the CIO bridgehead proton absorbed in the range - 88 - 3.24 - 2.87 6***. This has become diagnostic for the enone alcohol structure. In 4A, this methine appeared as a doublet, J=3 Hz, at 3.456. The shift downfield i s l i k e l y due to the deshielding effect of the neighbouring aromatic ring. The hydroxyl proton resonated as a singlet at 2.736 and was easily exchanged for deuterium when D̂ O was added. In experimental runs i n which conversion of starting material to product(s) was carried to >17%, a second product, j4B was also isolated. That this product arose from a secondary photolysis of the primary photoproduct was borne out by the following experimental facts: (i) at low conversions, 4A was the sole product isolated; ( i i ) the formation of j4B led to diminished yields of 4A (Table VII) ; ( i i i ) during the photolysis of KBr pellets of 4_, depletion of 4_ Table VII. Product Yields for the 4^c) + 4A + 4B Conversion Reaction Temperature % Conversion % Yields 4A 4B -10.4 to -9.8° 17 67 None 18.5 to 22.0 66 47 8 17.3 to 18.5 >90 33 15 and formation of 4A preceded the appearance of 4B as shown by infrared spectra recorded at 10 minute intervals during the irradiation; (iv) 4A dispersed in KBr or as the pure solid was converted to 4B under - 89 - reaction conditions identical to those used for the k_ -»• 4A transformation. Photoproduct 4B correctly analyzed for C22 H18°2 a n d w a s characterised as 2,3-diphenyltricyclo [5.3.0.0"* deca-2-ene-6,9-dione on the basis of i t s infrared, NMR and mass spectra. 4B It had a carbonyl absorption at 5.73u which i s characteristic of the infrared absorption of compounds of similar structure having 24 the five-membered ring ketone . In the NMR spectrum the following assignments were made: 67.23 - 6.87 (m, 10H, aromatic), 3.23 (m, 2H, C7 and CIO methines) , 3.07 (nr, IH, C5 methine) , 2.83 (m, 3H, CI and C4 protons), c a l c d 6 9 2.57 (dd, J=20 and 5 Hz IH, C8 exo), c a l c d 6 9 2.20 (dd, J=20 and 1.5 Hz, C8 endo). The assignment of the C8 exo and endo protons was based on the disappearance of the 62.57 resonance upon deuterium exchange in basic deuterium oxide and the concomitant collapse of the doublet of doublets at calcd. 62.20 to a broad singlet at 62.21. The f a c i l e exchange of this exo hydrogen i n bicyclo[2.2.1]- heptanone systems has been reported in the l i t e r a t u r e ^ . The NMR absorption pattern of compound 4B i s very similar to that of the 24a previously reported analog compound 2_C (Figures 17a and 17b). For - 90 - Figure 17b. A 100 MHz FMR Spectrum of 2,3-Dimethyltricyclo[5.3.0.0 * ]- deca-2-ene-6,9-dione, 2C. - 91 - example, the exo and endo protons at C8 each resonated as a doublet of doublets at calcd. 62.47 and 2.19 i n 2C. In photoproduct 4B these protons also resonated each as a doublet of doublets at calcd. 62.57 and 2.20, respectively. Since the photochemical behaviour of substrate 4_ has, hitherto, not been studied i n solution, photolyses of i t i n benzene were also carried out to ascertain i f i t s reactivity in solution differed from the solid state photoreactivity. These results are summarised in equation 4 below. Table VIII. Product Yields for the 4(solution^ ^ 4 A + 4 B Conversion Duration of Irradiation Product Ratios 4A:4B 3.0 hours 3:1 3.1 " 3:2 3.9 " 1:2 - 92 - Unlike the solid state reaction, product 4B i s formed concomitantly with 4A as determined by following the reaction at 0.1 hour intervals using infrared spectroscopy. As i n the solid state reaction, photoproduct 4A was again found to be photolabile-as shown by (i) the varying ratio of 4A: 4B_ as the reaction proceeds (Table VIII); ( i i ) independent photolysis of 4A under the same reaction conditions as for the k_ -*- 4A conversion gave 4B and a compound having 5.68 and 5.80 y carbonyl stretches i n the infrared. Although this latter compound was not investigated further, i t probably has the assigned 24 structure 4C since previous investigations of the photochemistry of these tetrahydro-1,4-naphthoquinones i n solution have shown that only photoproducts of this structure absorb at ̂ 5.70 and 5.80 u. Examples of this are given below: 4C 3D 2D C=0 ,M 5.68, 5.80 5.68, 5.80 5.69, 5.81 - 93 - The formation of ene-diones 4B and 4C from enone alcohol 4A i s not 24a novel. Scheffer and coworkers reported the conversion of the dimethyl enone-alcohol 2JJ to the ene-dione 2C_ i n tert-butyl alcohol and 2JB to 2D in benzene (equation 6 and 7) . The conversion of an enone-alcohol to both ene-diones in benzene was also later reported (equation 8). \ - 94 - 5 0 % 50% The photochemical formation of 4B_ from 4A is formally a disallowed • ~ . 72 [3,3] suprafacial-suprafacial sigmatrppic rearrangement and i s , . therefore, l i k e l y , a non-concerted reaction. On the other hand, the 4A 4C_ conversion i s a [1,3] suprafacial sigmatropic shift which i s photochemically allowed. It can, therefore, but need not be, concerted. In both the solid state and in solution, photoproduct 4B was photostable under the reaction conditions. Turning now to the primary reaction, namely, the 4A + 4B reaction i n benzene, analogy i s drawn to earlier systems studied i n our laboratory i n which both product types were formed as primary 24 71 products ' . The formation of both of these products can be explained by postulating the same diradical intermediate 4_' which was used earlier to explain the formation of 4A from h_ in the solid state. As can be seen from Scheme 14, photoproducts 4A and 4B probably arise from different conformers of the biradical intermediate 4/. The reason behind this stipulation comes from the X-ray data as well as the solid state results. Photoproduct 4B can only form from biradical 4/ of unspecified conformation i f bond formation occurs between C3 and C8. This i s not l i k e l y to happen from conformer 4'A - 95 " Scheme 14 since these centres are greater than 4A apart. On the other hand, Cl o and C6 are only 3.51A apart in.substrate 4_. Assuming that the conformation in which the molecules c r y s t a l l i z e i s also the preferred conformation in solution, i t can be seen that the biradical inter- mediate may form and close to the enone-alcohol from this conformation i.e. no gross conformational changes are mandatory for the 4̂  -*• 4A conversion. The 4̂  -*• 4B conversion most l i k e l y requires a conformational change. Since conformational mobility i s possible in f l u i d media, a - 9 6 - conformer such as 4 ' B is possible in solution and not i n the solid state and the now nearness of the C3 and C8 centres would allow product 4B_ to be formed. The observation that photoproduct 4B i s the minor product of the primary reaction i n solution i s most li k e l y a reflection of the conformational change necessary to allow for i t s formation. 2 , 3-Dimethyl-l » 4 - 4 a B > 9 a B r-tetrahydro - 9 ,10-anthraquinone,, 6_ Compound 6_ crystallised from acetone solution as colorless rods. Irradiation of 6_ i n the solid state below the eutectic temperature ( 1 2 6 . 0 ° ) gave enone-alcohol 6A (equation 9 ) in 60% hv solid state "(9) isolated yield. The structure of 6A follows ultimately from comparison of i t s spectral data with those of authentic 6A prepared - 97 - using the procedure of Scheffer and coworkers^""1. The conversion of j> to 6A involves the abstraction of a g-hydrogen by one of the oxygens of an excited carbonyl chromophore followed by C9 to C3 bonding. The transformation i s analogous to the h_ ->• 4A conversion. A small piece of crystal 6_ measuring 1.0 x 0.80 x 0.30 mm was cut from a larger crystal and used for the determination of the 73 X-ray structure . Unit c e l l constants were refined by least-squares from the observed 29 values of 17 reflections. Intensity data from 1313 independent reflections were used for the structure determination. The R value after full-matrix refinements was 0.055. 73 Crystal Data ^16^16^2' o r t norhombic, space group Pna2^ with a=15.643(2), b=5.160(1), c=15.568(2) A, 8=90° and Z=4. The structure of the molecule i s shown in Figure 18. The hydrogen abstracted i n the 6_ -»- 6A conversion i s one of the Cl hydrogens. The interatomic separation between this hydrogen and the abstracting atom 0(1) i s 2.57 A. The abstraction would give ri s e to a biradical 6/ (Scheme 15). Electron d e r e a l i z a t i o n into the aromatic ring i s expected to aid the stabilization of 6/. However, no bond formation involving the aromatic ring carbon atoms i s expected since this w i l l result i n loss of the stabilization energy associated with the aromatic ring. This leaves only two possible modes of ring closure for biradical, 6/, namely bonding between C9 and  - 99 - CI and between C9 and C3. As already mentioned in the Introduction, the formation of a cyclopropane ring in these already r i g i d polycyclic systems i s unlikely and has to date not been observed. Bond formation between C3 and C9 gives rise to the observed product 6A. In substrate 6̂  the interatomic separation between C3 and C9 is only 3.46 A so here as i n the k_ -* 4A transformation, the biradical can form and collapse to the observed product without any gross changes in conformation. Not surprisingly, irradiation of degassed solutions of 6_ also gave 6A as the only product because as pointed out, the biradical intermediate 6/ can only close between C3 and C9. 6,7-Dimethyl-4ag,8ag-dicyano-4ag,5,8,8ag-tetrahydro-1,4-naphthoquinone, 1 Large, sparkling yellow crystals of _7 were obtained by crystallization from a solvent mixture of acetone and light petroleum ether. One crystal measuring 0.50 x 0.40 x 0.40 mm was used for X-ray structure determination. Unit c e l l constants were obtained by least-squares refinement of the observed 26 values of 23 reflections. - 100 - Intensity data from 2445 independent reflections were used to solve the structure. The f i n a l R value after refinements was 0.055. 74 Crystal Data C14 H12 N2°2' m o n o c H n i c » space group P2^/c with a=8.717(5), b=12.464(2), c=12.783(5) A, 8=117.87(3)°, and Z=4. A stereo view of the molecule is given in Figure 19. Figure 19. Stereo diagram of compound ]_. - 101 - Irradiation of ]_, either as a solid dispersed in a KBr matrix or as the pure solid in vacuo below the eutectic temperature (118.5°), gave the enone-alcohol 7A in 92% isolated average yield. Photoproduct 7A_ was conclusively identified by comparing i t s physical and spectral characteristics with those of authentic JA. prepared by 24b the method of Scheffer and coworkers The formation of l_k formally involves the abstraction of one of the C8 methylene hydrogens, Hg, by 0(1) and collapse of the resulting biradical through bond formation between carbons 1 and 6. o The Hg'-'O(l) distance in the crystal i s 2.58 A. The separation of o Cl and C6 i n the crystal i s 3.38 A. So here as i n the two previous examples, the formation of the biradical intermediate in this reaction and i t s subsequent collapse to enone-alcohol does not require any conformational changes. The only other possible but unobserved bonding modes are between C3 and C6 and between C3 and C8. The separations o between these carbons i n the crystal are 3.86 and >4 A, respectively. Not only are these distances longer than the C l , C6 separation i n the crystal but the p-orbitals at these centres are directed away from - 102 - Scheme 16 each other so that overlap between them cannot be achieved. One might expect that l i k e many of the substrates i n this series of tetrahydro-l,4-naphthoquinones, substrate ]_ i n solution might give the ene-diones _7JL and 7C. Irradiation of degassed benzene solutions of 1_ gave 74% yield of enone-alcohol _7A and 7% yield of a compound melting at 156 - 158°. The latter compound had the following spectral data: In the infrared i t had cyano stretching frequencies at 4.44 and 4.46 u and a carbonyl stretch at 5.90y. The single C=0 stretch at 5.90y ruled out both ene-diones T&_ and _7C_. In the nuclear magnetic resonance spectrum, i t had two doublets, J^g=10Hz, sited at 67.44 and 6.41, respectively, which integrated for IH each. The - 103 - rest of the spectrum had 62.56 (s,2H), 2.45 (d,J=15 Hz, 1H), 1.72 (d, J=15 Hz, IH), 1.56 (s, 3H) and 1.38 (s, 3H). Its mass spectrum showed a parent peak at m/e=240. This product, 7D i s tentatively assigned the oxetane structure below: The absence of ene-diones 7JB and _7C_ has been ascribed to the prevention of conformational changes even in solution as a result of bridgehead substitution. As an attempt to see i f any similarities exist between the geometries of a substrate and i t s solid state photoproduct, the X-ray structure of enone-alcohol 7A was also determined''"'. A stereo view of the molecule i s shown in Figure 20. As can be seen from comparing this diagram with that of the starting crystal _7, the 1_ 7A conversion i s attended by the conversion of the favorable half-chair conformation of the C5-C10 ring to a boat form. The conformation of the quinone ring remains essentially the same. During the X-ray structure determination, there were indications that enone-alcohol 7A might be optically active. The molecule i t s e l f i s c h i r a l , but one would expect that in a biradical - 104 - Figure 20. Stereo diagram of enone-alcohol, 7A. ring closure, both levorotatory and dextrorotatory molecules should form in equal proportions giving" racemic 7A. If crystals of 7A were opt ical ly active, i t meant that a racemic mixture had been formed during c rys ta l l i za t ion , an uncommon occurrence in organic chemistry. When single crystals as well as clusters of crystals of 7A were used in optical rotation determinations, the results shown in Table IX were obtained. - 105 - Table IX. Specific Rotation of Solutions of Crystals of IL Description of Crystal 25.5° Single +3.0° Single +2.5° Single -67.7° Clusters -14.5° The fact that some of the single crystals were levorotatory and others dextro- rotatory means that crystallization followed..by mechanical separation had effected resolution of the racemic photoproduct. ..The fact that the specific rotation of the levorotatory single crystal i s not equal to that of the dextrorotatory crystal i s an indication that the resolution i s only p a r t i a l . Nevertheless, such a spontaneous resolution through crystallization i s rare. The h i s t o r i c a l example of this phenomenon 76a was reported by Louis Pasteur who allowed a solution of sodium ammonium tartrate to crystallize by slow evaporation below 27° and was subsequently able to separate the two enantiomers by mechanically picking them apart. Since then, very few examples of this occurrence have been reported. Among these are the spontaneous crystallization of active material from d l solutions of (i) 3,3-diethyl-5-methyl- 2,4-diketopiperidine 7^, ( i i ) narcotine 7^ C, ( i i i ) laudanosine 7* 3 0 and (iv) methyl-ethyl-allyl-anilinium iodide 7 < 3 <*. The crystal packing diagram revealed that compound _7 represents the crossover from inter to intra-molecular reactivity in - 106 - this series of tetrahydro-l,4-naphthoquinones. Figure 21 shows portions of adjacent molecules within a crystallographic c e l l . The TT overlap of the ene-dione double bonds which i s necessary for dimerization i s not achieved due to a slight t i l t of the C2, C3 plane Figure 21. Neighbouring ene-dione systems of a pair of molecules of 1_ viewed perpendicular to the C ( l ) , C(2), C(3) plane. of the molecule above the plane of the paper relative to the C21, C3' plane of the molecule below i t (Figure 21). The intermolecular contacts are also much longer than the corresponding distances for O substrates 1, 2 and 3. For example the C2 to C2' distance i s 4.37A. Because of the t i l t of the C2, C3 plane of one of the molecules of the pair mentioned previously, the C2—C2' distance does not represent the shortest intermolecular contact. The latter turns out to be the C2 to C3' distance of 4.09& which s t i l l exceeds the sum of the van der 68 Waal's r a d i i of the two atoms by 0.691.. The centre-to-centre 107 - separation of the double bonds i s 4.1A, and so not only are the orbitals not-aligned for intermolecular overlap but the limit of double bond separation necessary for photodimerization i n the crystalline state^ 2' 3 has also been reached. Consequently, no photochemical dimerization was observed when crystals of 1_ were irradiated. 2,3,4ag,6,7,8ag-Hexamethyl-4aB, 5,8.8aB-tetrahydro-1,4-naphthoquinone, £ Large, pale yellow crystals of 9̂  were obtained by cr y s t a l l i z i n g from petroleum ether. Smaller but better-formed crystals for crystallographic purposes were obtained by crystallizing from acetone. Crystals obtained from either solvent reacted i n identical fashion - 108 - both in KBr and as the pure solid. Thus, one may assume that the crystal form is not changed as. a result of change of crystallizing solvent. UV irradiation of 9̂  below the eutectic temperature (-16.0°) led to the formation of enone-alcohol 9A and the ene-dione 9B_ i n • the ratio 2:3. This ratio was invariant below the eutectic at varying conversions (Table X) as one would expect from a reaction i n Table X. Product Ratios and Combined Yields for the 9 ->• 9A + 9B Conversion, i n the Solid State Reaction Combined Ratio Temperature % Conversion % Recovery GLC Yields 9A:9B -34.3° to -33.5° 41 91 77% 2:3 -34.7° to -32.5° 54 94 88% 2:3 -33.5° to -30.3° 68 63 46% 2:3 -25.6° to -23.1° 77 71 62% 2:3 -25.3° to -23.4° 88 78 75% 2:3 which both products are primary products. Yields are quite high provided the recovery of reaction mixture from the reactor i s high. The structures of 9A and 9_B were confirmed by comparing their physical and spectral data with those of 9A and 9B_ obtained previously and 24 reported in the literature . As mentioned in the Introduction, 9A and 9JS are thought to derive from different excited states, enone - 109 - alcohol 9A from an n , T f * singlet and ene-dione 91J from a T r , T r * t r i p l e t . This being the case, i t was of interest to repeat the solution photochemistry, which has hitherto been done only at ambient temperature, at temperatures below the eutectic to see i f the ratio of j?A:j)B was the same for the solution photolysate as for the solid state reaction at s i m i l a r temperatures. Table XI. summarizes results of the conversion _9 -»• 9A + j)B in anhydrous ether below the eutectic. In terms of percentages, this ratio differs Table XI. Product Ratios for the 9 -»• 9A + 9B Conversion in Solution 9 e t!* e r > 9A + 9B — hv — — Reaction Ratio Temperature % Conversion 9A:9B -33.0° to -31.5° 21 1:2 -31.5° to -29.0° 25 1:2 -32.5° to -31.5° 52 1:2 -31.5° to -29.5° 63 1:2 from the solid state products ratio by only 7%, a difference too small to warrant explaining especially in view of the fact that product 24 ratios in solution may be solvent dependent The X-ray structure of _9 was solved by direct methods as described^. The crystal dimensions were 0.15 x 0.20 x 0.20 mm. Unit c e l l constants were obtained by least-squares refinement of the observed - 110 - 20 values of 15 reflections. The intensity data of 2786 independent reflections were used i n the structure determination. The R value after refinements was 0.088. Crystal Data 7 7 C16 H22°2' m o n o c l i n i c » space group P2 1/c with a=7.312(3), b-11.540(4), c=16.674(3) A, 8=92.26(3)° and Z=4. Figure 22 gives a stereo view of the molecule. - 111 - Scheme 17 hu solid state 9A 9B Photoproduct 9A i s the enone-alcohol product arising from abstraction of Hg, one of the C8 hydrogens, by 0(1) followed by bond formation between Cl and C6 radical centres (Scheme 17). The Hg to - 1.12 - 0(1) distance i s 2.47A and the CI to C6 distance i s 3.35A. The unobserved bonding modes of biradical 9'A, namely C3 to C6 and C3 to o C8 were both >4A i n the crystal of 9_. The ene-dione product, 9B, formally arises from the abstraction of one of the C5 hydrogens, namely, H^, by C2 giving biradical 9'B which then closes by bonding C3 to C5. The distance between the abstractable hydrogen and the abstracting carbon, namely HA"'*C2 i s 2.89A i n the starting material 9_. As i n previous cases, i f there are no gross changes i n conformation attending the reaction, one could approximate interatomic separations i n the biradical intermediate to the corresponding interatomic distances in the substrate. The C3, C5 separation i s thus only 3.17A. Not only i s the interatomic separation favorable but the geometry of the molecule has the p orbitals at these centres directed towards each other thus promoting interaction and ultimately bond formation (Scheme 17). As in the case of biradical 9'A, the unobserved bonding mode in biradical 9'B, namely-_C3-tO--C7-,- would have .involved.-bringing- together .carbon • o centres which are >4A in the crystal of the substrate to closer proximity, a process which i s l i k e l y to entail conformational changes impermissible by the l a t t i c e of the host. The 3:2 ratio for photoproducts 913 and 9A does not reflect the relative rates for y- vs 3-hydrogen abstraction. As the. kinetic scheme below suggests, the rates of formation of products 9A and 9B_ depend on a number of factors. For example, the relative population of the - 113 _ Scheme 18 n ,Tr* and T T , T T * states, the rates of decay of these two excited states and the rates of biradical closure relative to reverse hydrogen transfer w i l l a l l be reflected i n the overall rates of formation of these two products. One interesting revelation from the crystallographic data i s that the hydrogen which i s abstracted by enone carbon i s quite distinct from the hydrogen which i s abstracted by oxygen, i.e. the g-carbon of the enone system and the oxygen do not compete for the same hydrogen in this reaction, a fact which i s not evident from the chemical structure of 9_ because of the symmetry of the molecule. Another interesting point to be noted i s that from the X-ray crystal structure, % can, i n principle, be abstracted by either C2 - 114 - or C3,being 2.89A from C2 and 2.80A away from C3. Abstraction by C3 would have given biradical 9X which can close to S>C, an unobserved photoproduct, by bonding centres, C2 and C5. Such an abstraction would have proceeded via a five-membered transition state as opposed to the six-membered transition state that gives rise to the observed photoproduct 9B_. That 9̂C_ i s not formed may mean that the well-known order 1,5 > 1,6 >> 1,4 for rates of hydrogen abstraction i n acyclic systems holds for this r i g i d system as well, the preference here being a 1,5 over a 1,4 hydrogen abstraction. 2,3,4aB,56,8g,8aB-Hexamethyl-4aB ,5,8,8aB-tetrahydro-l,4-naphthoquinone. 11 This adduct was studied in spite of the dismal yields (<5%) of i t s preparation because i t is the only substrate other than 9̂  which has been reported to date to undergo y~hydrogen abstraction by enone 24b carbon in solution . The study of i t s structure and reactivity in the solid state was desirable i f for no other reason than to ensure that the solid state reactivity of 9̂  was not an isolated case and thus provide a firmer ground for any conclusions which may be drawn from this reaction. - 115 - Compound 11 was obtained as pale yellow rods from petroleum ether as the crystallizing solvent. It reacted in KBr and as the pure solid below the eutectic temperature (60.5°) to give enone-alcohol 11A and ene-dione 11B in a combined isolated yield of 85%. Of the two photoproducts the ene-dione 11B was favored 2:1 over the enone-alcohol 11A. The 11A:11B ratio was shown to be invariant at varying conversions of the starting material (Table XII) which i s to be expected in a reaction in which both products are primary products and are photostable Table XII. Product Yields and Ratios for the 11 ->- 11A + 11B Conversion Reaction Temperature. % Conversion Combined Yields 11A;11B 11A + 11B -32.4° to -31.7° 100% 85%. 1:2 -31.6° to -27.3° 100% 86% 1:2 -32.1° to -31.5° 56% 81% 1:2 -32.0° to -30.9° 57% 95% 1:2 -31.9° to -31.6° 29% 93% 1:2 - 1 1 6 - under the reaction conditions. That these products are photostable was substantiated by irradiating a KBr pellet containing these two primary products for one hour. The recorded infrared spectrum showed neither new peaks nor changes in peak intensities. Pure crystals of 11A were also irradiated in a KBr matrix for 2 hours. The infrared spectrum of the irradiated pellet was identical to that of pure 11A. The structural assignments for the two products, 11A and 11B, were made on the basis of their spectral data which were identical to those reported for compounds of identical structure isolated from the photolysate of compound 11^ i n benzene by Scheffer and 24b coworkers The formation of these two products i s analogous to the 9_ -* 9A + j)B conversion discussed earlier. The yields and product ratios are comparable for the two cases. By analogy to the 9. 9A + 9B transformation, photoproduct 11A most, l i k e l y arises from the ^n,7T* state while the ene-dione photoproduct 11B most probably forms from the 3 T T , T T * state. As mentioned in the Introduction, methyl substitution on the ene-dione double bond and the bridgehead positions seems to be 3 required for the observation of the T T , T T * derived yhydrogen abstraction by carbon in this series of tetrahydro-1,4-naphthoquinones. It i s understandable that the substitution of electron donating groups such 3 as alkyl groups on the ene-dione double bond should lower the TT , T T * energy level since the promotion of a TT electron to a T T* level leaves the carbon(s) of the TT system electron deficient. It i s , however, clear - 117 - that substitution on the ene-dione double bond alone i s not sufficient for the observation of y-hydrogen abstraction by carbon since substrate 24a 5_ does not undergo y-hydrogen abstraction by carbon in solution . It has not yet been determined whether methyl substitution on the bridge- 3 head positions alone suffices for the observation of this T f , T f * reaction. The substrate necessary to decide this i s compound 12_ which cannot be obtained by the Diels-Alder addition of 2,3-dimethylbenzoquinone to 2,3-dimethylbutadiene (which gives exclusively 5_) . Substrate 12 has not been studied thus far because of this synthetic problem. Substrate LL not only has the same substitution pattern as 9̂ 97 but the X-ray structure also shows that i t has the prerequisite geometry for both g-hydrogen abstraction by oxygen and y-hydrogen abstraction by carbon. The crystal structure was determined using a crystal measuring 0.30 x 0.30 x 0.70mm. Unit c e l l and intensity data were measured on a Datex-automated G.E. XRD 6 diffTactometer with Cu technique. Unit c e l l parameters were obtained by a least-squares refinement of the observed 26 values of 16 reflections. The structure was solved using intensity data of 2051 independent reflections. Unlike the previously described X-ray structures, the structure of 1.1 - 118 - could not be solved by direct methods . It was solved by a symbolic 79 addition and tangent refinement procedure . The f i n a l R value was 0.070. 79 Crystal Data C16 H22°2' m o n o c l i n i c > space group C2/c with a=24.930(7), b=7.795(3), c=14.472(5) A, 6 = 101.13(3)° and Z=8. Like a l l the tetrahydro-1,4-naphthoquinones, the molecule i s twisted such that the bridgehead groups are staggered. Figure 23 i s a stereo diagram of the molecule. I Figure 23. Stereo diagram of substrate 11. _ 119 . - 120 - As in previous cases, the enone-alcohol product 11A arises from the abstraction of the C(8) hydrogen by 0(1) followed by bond formation between carbons 1 and 6 (Scheme 19). It can be seen from the stereo diagram (Figure 23) that the C(8) hydrogen i s i n the plane of the C(1)=0(1) group. The H to 0 separation in the crystal i s 2.26A o which i s a reduction of 0.21A over the corresponding distance in substrate 9. It i s , i n fact the shortest H to 0 distance observed — P in this series of tetrahydro-l,4-naphthoquinones. This should f a c i l i t a t e g-hydrogen abstraction. Furthermore, the C(l) to C(6) o separation i s 3.33A which i s short enough for a van der Waals inter- action. (The sum of the van der Waal's contact radius for the two carbons i s 3.40A based on r ^ values compiled by Bondi .) Thus the biradical intermediate 11'A (Scheme 19) can bond between C(l) and C(6) to form stable product 11A without prior conformational changes. The formation of ene-dione 11B arises from the abstraction of the C5 hydrogen by carbon 2 followed by ring closure between C3 and C5. The C5 hydrogen i s almost equidistant from C2 and C3, being 2.70& from C2 and 2 . 66A away from C3. The situation i s completely o analogous to the case of molecule _9 where this'hydrogen i s 2.89A and o 2.80A, respectively from C2 and C3. The relatively shorter hydrogen to carbon separations in 11 should result in a comparatively faster abstraction process. The comparatively shorter irradiation times for the complete conversion of 11. to products may be a manifestation of the nearness of both the C(8) hydrogen and the C(5) hydrogen to the - 121 - abstracting atoms. As in the case of substrate j?, the abstraction of the C(5) hydrogen by C(3) through a five-membered transition state . i s not observed. Photoproduct 11B arises from the abstraction of the C(5) hydrogen by C(2). The resulting biradical l l ' B then closes i t s radical centres by bonding C(3) to C(5). The C(3) to C(5) separation o i n the crystal i s a short 3.17A and so here, as i n a l l previous cases, the biradical intermediate can close to product from the conformation in which i t i s formed, i.e. no conformational changes are required for the conversion of substrate 11 to stable photoproducts. The Geometry of the Transition State for B- and y -Hydrogen Abstractions 43 Ever since the publication by McLafferty and coworkers , of the molecular orbital calculations on the transition state geometry for y-hydrogen abstraction in 2-pentanone, a number of investigators have probed the geometric requirements not only for the McLafferty o rearrangement but also for the photochemical Norrish Type II reaction. In the original paper cited, Boer, Shannon and McLafferty examined a number of conformations for the six-membered c y c l i c transition state including non-planar ones and found that the energy barrier to abstraction has a minimum when the abstractable hydrogen i s in the plane of the carbonyl group. They cited two geometries which have this arrangement:- one of these, shown below, has a l l the ring atoms of the six-membered transition state in the same plane with a favorable H -0 o distance of 1.1A but unfavorable eclipsing of the methylene hydrogens; - 122 - the second geometry, a non-planar one which has a l l the hydrogens staggered, i s shown below. Its calculated energy was roughly the same as that of the planar one previously discussed. In this latter conformation a l l six atoms of the six-membered cyclic transition state except C are i h the same plane. With normal bond distances and p angles assumed, McLafferty and coworkers found the H -0 distance to o o be a considerably longer, 1.81A, compared to the 1.1A for the planar conformation. Since the McLafferty rearrangement had earlier been shown to occur only when the H -0 distance was 1.8A or less ' , 43 McLafferty and coworkers concluded that the transition state geometry - 1 2 3 - for y-hydrogen abstraction was most l i k e l y planar. What happens, then, in r i g i d systems where planarity cannot be achieved? Does the rearrangement s t i l l occur? To answer these questions, Henion and 42 Kingston investigated a bicyclononanone, -two bicyclodecanones and a bicycloundecanone. Using Dreiding molecular models, they measured the distance Ĥ -0 and the angle T which the approaching hydrogen makes with the plane of the carbonyl group at the position of i t s closest approach to the oxygen. ~ They then investigated their mass spectra. Their results are summarised in Table XIII. Table XIII. Effects of Structure on the McLafferty Rearrangement o Ketone H to 0 distance, A T ° Occurrence of the —Y McLafferty Rearrangement - 124 " 43 In the earlier calculations the transition state geometry in which T=45° has an estimated energy which i s 76.3 kcal above that of the planar transition state geometry. Table XIII clearly shows that while a planar six-membered ring may be the most favorable geometry energetically for flexible molecules, i t i s not by any means a sine qua non for the occurrence of the McLafferty rearrangement. Turning now to photochemical hydrogen abstractions, i t i s important to note that both the Norrish Type II reaction and the McLafferty rearrangement require an electron-deficient carbonyl system. In the McLafferty rearrangement, this i s attained by electron removal during ionization while the photochemical system relies on the promotion of an electron from an n orbital into an antibonding T f * o r b i t a l . The net effect of electron promotion i s to leave an electron- deficient oxygen. It i s , therefore, not surprising that the photo- chemical abstraction reaction has a few characteristics in common with the thermal McLafferty rearrangement. For example, for both reactions y-hydrogen abstraction i s more f a c i l e for secondary hydrogens than for primary hydrogens. However, there are some differences worth noting when discussing the geometries of the activated complex for these reactions. 2 In the ground state, the carbonyl carbon i s sp hybridized o and the C=0 bond length i s approximately 1.2A. In the excited state, the carbonyl carbon of formaldehyde and presumably other saturated 3 ketones and aldehydes i s sp hybridized and the C=0 bond i s longer than i t i s in the ground state by about 0. In addition, the C=0 bond - 125 - of the excited molecule i s out of plane of the C l ^ group by 27° and 35° 8 l a f o r the s i n g l e t and t r i p l e t n , T f * states r e s p e c t i v e l y . O v e r a l l , the geometric change i s one of a planar ground s t a t e molecule becoming pyramidal i n the excited s t a t e . Such i s , however, not true f o r conjugated carbonyl compounds as revealed by the spectroscopic 81 studies of a number of i n v e s t i g a t o r s . There are two important 81 consequences of conjugation. F i r s t , i t has been e s t a b l i s h e d that conjugation s t a b i l i z e s the planar geometry r e l a t i v e to the pyramidal one. For example, the analyses of the n , i r * absorption spectra of 81c 81d propenal and propynal have shown that the C-C-0 angle changes by only 3° and 5°, r e s p e c t i v e l y , from the ground state values. This 81a i s i n sharp contrast to the reported change of 20-27° f o r formaldehyde ' The second e f f e c t of conjugation i s that e x c i t a t i o n i s no longer 81a confined to the carbonyl group and both the C=C and C=0 bonds i n propenal undergo bond lengthening although the l a t t e r ' s increase i s more pronounced. For propenal and propynal the C=0 bond lengthens °81c °81d by 0.08A and 0.095A r e s p e c t i v e l y as a r e s u l t of n , T r * e x c i t a t i o n . In summary, a,B-unsaturated carbonyl chromophores are expected to r e t a i n t h e i r planar configuration upon n , T f * e x c i t a t i o n i n contrast to t h e i r saturated analogs which are planar i n t h e i r ground state but pyramidal i n the excited s t a t e . This d i f f e r e n c e i n geometry has to be borne i n mind when discussing t r a n s i t i o n state geometries f o r photochemical hydrogen ab s t r a c t i o n reactions. There are i n d i c a t i o n s that although the Norrish II rea c t i o n and the McLafferty rearrangement both involve the - 126 - a b s t r a c t i o n of a y-hydrogen v i a a six-membered t r a n s i t i o n s t a t e , the two t r a n s i t i o n s t a t e s have d i f f e r e n t geometries. For example, Lewis 44 and coworkers found that both endo- and exo-2-benzoylnorbornane undergo the N o r r i s h I I photoelimination r e a c t i o n . Their molecular models revealed that the abstractable hydrogen, H^, comes with i n 1.7A of the carbonyl oxygen i n the endo isomer. The corresponding c l o s e s t approach to 0 distance i n the exo Isomer was, however, a o r e l a t i v e l y long 2.2A. Although t h i s greater to 0 distance was r e f l e c t e d by the ^600 f o l d d i f f e r e n c e i n the rates of hydrogen abstra c t i o n f o r the two substrates (equation 10 and 11), the f a c t that the exo- k = 7.0 x 10 sec - - - - (11) k = 1.2 x 10 7sec 1 - 127 - isomer reacted at a l l shows that for photochemical hydrogen abstraction o reactions the H -0 distance i s not limited to the 1.8A maximum esta-Y blished for the McLafferty rearrangement. Perhaps a better guide for the to 0 distance requirement should be the sum of the van der . Waals r a d i i , r w , of the two atoms involved, namely, hydrogen and oxygen. Van der Waals r a d i i d i f f e r widely depending on the method of their calculation. For instance, the van der Waals r a d i i , r, , b o calculated by Pauling s approximation for H and 0 are 1.06 and 1.42 A, 68 respectively . Use of these values would give the maximum H-0 o 82 contact distance of 2.5A reported by Winnik and coworkers . However, 68 83 as pointed out by Bondi and Edward the "best" values for the van der Waals r a d i i of various elements which are compatible with X-ray crystallographic data and are best suited for volume calculations are those designated r . If one depicts the interaction between the two atoms involved in the abstraction reaction as two colli d i n g spheres of rad i i r w ( 0 ) and * 00 respectively, then the contact distance between the two atoms w i l l be the sum of these two r a d i i . Using r (0) and w r (H) values from the recent calculations of Bondi yields a sum of w o 2.72A. This value represents an upper limit on the distance require- ments for the abstraction of a hydrogen by oxygen. The lower li m i t 84 w i l l be represented by the length of the OH bond i t s e l f . Thus, for the observation of hydrogen abstraction by oxygen, the H to 0 separation, d, must be such that 2.72A >̂  d > 0.96&. This condition i s calculated for ground state atoms but should1 hold for n,Tr* state hydrogen - 128 - abstractions by oxygen. The abstraction involves the h a l f - f i l l e d n orbital of oxygen which i s included in the ground state atomic radius, r . The analogous distance requirement, d, for hydrogen abstraction w o o - 68 by carbon w i l l be 2.90A >_ d > 1.07A using the sum of the r w for 84 C and H for the upper limit and the length of the C-H bond as the lower limit. The second parameter which has been used to define the geometry of the transition state i s the angle x defined as the angle that the itinerant H makes with the plane of the carbonyl group. This angle i s shown diagramatically below. The H to 0 distances and the x values for the substrates found to undergo 3-hydrogen abstraction are tabulated in Table XIV below. Also given in this table are the H to C Y distances and x' values for the substrates for which y-hydrogen - 129 - abstraction by carbon was observed. For 3-hydrogen abstraction, a l l the Ĥ ,0 interatomic separations were below the stipulated maximum o Table XIV. Interatomic Distances, A, and Approach Angles for Hydrogen Abstraction - 130 - separation. The distance requirement i s , t h e r e f o r e , w e l l met. Also as the x values i n d i c a t e , i n a l l cases the hydrogen to be abstracted i s also i n the plane of the carbonyl group,- the out of plane angle being <8° i n every case. Substrate 9_ a f f o r d s the i d e a l case, with T=0° i . e . the hydrogen l i e s exactly i n the plane of the carbonyl group. Using the X-ray data f o r t h i s s u b s t r a t e 7 7 , some important information can be glimpsed about the geometry of the t r a n s i t i o n state. The.transformation leading to the b i r a d i c a l precursor of enone-alcohol 9A and a l l enone-alcohols, i n general, most l i k e l y involves the abs t r a c t i o n of one of the C8 hydrogens by the h a l f - f i l l e d n - o r b i t a l of 0(1). The hydrogen which i s abstracted i s the one which i s c l o s e s t to and i n the plane of the C(1)=0(1) group. This i s evident from the lack of enone-alcohol formation i n i r r a d i a t e d c r y s t a l s of 10 i n which the C(8) hydrogen i s remote and out of plane with the C(l)= 0(1) group. In substrate 9_, atoms 0(1), C ( l ) , C(8a) and the hydrogen to be abstracted a l l l i e i n the same plane. The C(8) atom i s ca l c u l a t e d to be only 16° above the plane of these four atoms. The bond angles (Scheme 20) are also i n e x c e l l e n t agreement with the 2 3 expected values of 120° and 109.5° f o r sp and sp hybridized carbon, r e s p e c t i v e l y . Since the angle f o r planar cyclopentane i s 108°, there w i l l be some angle s t r a i n at the t r a n s i t i o n s t a t e i f , as i s expected, 2 C( l ) remains sp hybridized. Although, a planar cyclopentane-like t r a n s i t i o n state f o r substrate 9_ has undesirable e c l i p s i n g of i t s C(8a) bridgehead methyl and the unabstracted C(8) hydrogen, i t must be - 131 - Scheme 20 transition state borne in mind that this strain i s present in the substrate rather than introduced at the transition state. Turning now to the ybydrogen abstraction by carbon, the Ĥ ,,C separation, d, for substrates 9_ and 11 o are 2.89 and 2.66 A, respectively. Both distances satisfy the inter- atomic separation requirement stipulated for this reaction. Unlike the case of 3-hydrogen abstraction, the approach angle T deviates significantly from zero. As Table XIV shows, the hydrogen approaches the plane of the C2, C3 double bond at angles of 50° and 52° respectively for substrates _9 and 11_. Interestingly, this i s the same as the approach angle, x, estimated from molecular models by Henion and 42 Kingston for the McLafferty rearrangement of bicyclo[5.2.1]decan-10-one - 132 - and bicyclo[5.3.1]undecan-ll-one. The only transition state geometry compatible with the X-ray structure of substrates 9_ and 11 turns out to be a boat (Figure 24) not a chair-like cyclohexane ring 78 as suggested by Wagner and coworkers for acyclic substrates. Since the boat-like conformation i s forced on the transition state, no doubt, conversion. by the decalin ring structure of 9_ and 11, the possibility of a chair- like transition state for mobile systems cannot be ruled out. It does mean however that the conformation of the transition state for these hydrogen abstraction reactions i s not required to be a strain- free chair. - 133 - The abstraction of a y-H by carbon i n substrates 9̂  and 11 most li k e l y involves the h a l f - f i l l e d TT orbit a l of C(2). As mentioned 24b earlier, quenching studies have shown the reaction to involve a 3 t r i p l e t excited state, most l i k e l y a T f , T f * . In contrast to the B-hydrogen abstraction discussed earlier, the hydrogen abstracted does not l i e i n theplane of the reactive chromophore, namely the C(2)=C(3) TT system. Rather, i t approaches C(2) from above the C(2), C(3) plane at an angle of 52°. The suggested six-membered transition state i s i n a boat conformation. - 134 - 3. Intramolecular Oxetane Formation 4a B,8ag-Dicyano-5ct,8a-dimethyl-4ag,5,8,8aB-tetrahydro-l,4- naphthoquinone, 8. Well-formed, pale yellow crystals of were obtained by cr y s t a l l i z i n g from acetone-hexane. Irradiation of 8_ both in a KBr matrix and as the pure solid below the eutectic temperature (123.5°) led to the formation of 5,10-dicyano-6,9-dimethyl-ll-oxatetracyclo- [6.2.1.0.^'70^,^^]undec-2-ene-4-one 8A as the sole product. Irradia- te tion of solutions of j5 also give oxetane 8A as the sole photoproduct Ultimate proof of the product structure came from comparing i t s spectra with those of authentic 8A prepared by photolysis of 8̂  in benzene. The X-ray structure of 8A prepared by this latter method has been determined. The 8 -> 8A conversion is an intramolecular [ 2 + 2 1 — TT S TT S addition of a carbonyl functionality to an o l e f i n i c moiety. Recent studies on this type of intramolecular photocyclization show that the reaction can occur from the n , T r * singlet as well as from the T T , T T * - 135 - tri p l e t depending on the intersystem crossing efficiency of the 28 substrate . The i n i t i a l interaction of the excited carbonyl chromo- phore with the isolated double bond i s believed to yield an exciplex which may eventually yield an oxetane. In order for the carbonyl group to interact with the olefinic Tf system, the two TT systems must be close enough and aligned so as to have interaction between their 85 p-orbitals. Schmidt and Rabinovich found, for example, that short o intermolecular C=C"''*C=C contacts of 3.49 and 3.62 A in the two stacks of 2,5-dimethyl-p-benzoquinone and a paral l e l arrangement of the two Tf-systems favored oxetane formation upon irradiation. 87 X-ray structure determination was carried out on a single crystal of 8 measuring 0.50 x 0.30 x 0.30 mm. Unit-cell parameters were refined by least squares from the observed 26 values of 14 46 reflections. The structure was solved by direct methods using intensity data of 2419 independent reflections. The f i n a l R value was 0.046. 87 Crystal Data C14 H12 N2°2' o r t n o r n o m b i c > s P a c e group Pbcn with a=15.915(6), b=11.525(4), c=13.157(5) A, g=90° and Z=8. A stereo diagram of the molecule i s shown in Figure 25. The formation of oxetane 8A arises from a [ 2 + 2] cyclo- Tf Tf addition between C(1)=0(1) and C(6)=C(7). The centre-to-centre o separation, d, of these two Tf systems is only 3.20A which i s shorter than the intermolecular C=0 to C=C contacts encountered by Schmidt - 136 - Figure 25. Stereo diagram of substrate J5. 85 and Rabinovich . But un l i k e t h e i r systems, the intramolecular OO and G=C bonds involved i n the J3 -»• 8A conversion are not p a r a l l e l . The o r i e n t a t i o n of these two double bonds can be appreciated by viewing Figure 25 through a stereo viewer. The d i r e c t i o n of the p - o r b i t a l s are along the normals to the C(1)=0(1) and C(6)=C(7) planes. The angle between these normals i s 99°. This near-orthogonal arrangement of the two TT systems which i s diagrammatically represented below (Figure 26) promotes i n t e r a c t i o n and u l t i m a t e l y bond formation. Figure 26. Approach geometry of the C(1)=0 and C(6)=C(7) TT bonds. - 137 - The addition involves the bonding of C(l) to C(6) and 0(1) to C(7). o These distances are 3.37 and 3.20 A respectively in the crystal. o — Using 1.70 and 1.52 A for the mean van der Waals r a d i i , r w , for C and 68 0 respectively , the carbon to carbon van der Waal's contact radius o o i s 3.40A and that of carbon to oxygen 3.22A. These distances are almost exactly the distances separating the potential bonding centres i n substrate _8. Because of the favorable alignment of the two double bonds and the short interatomic contacts the 8̂  -> 8A conversion can occur without major conformational changes. As the X-ray structure of 8A (Figure 27) shows, the reaction i s attended by - 138 " very l i t t l e change in the quinone ring and only shifts i n the positions of C(6) and C(7) of the cyclohexene portion of the molecule. The X-ray data on photoproduct 8A were obtained and treated as in previous structure determination. The dimensions of the crystal used in the determination were 0.70 x 0.50 x 0.20 mm. Unit-cell parameters were obtained by least-squares refinement of the observed 29 values of 15 reflections. The structure was solved from treatment of intensity data from 1433 independent reflections. The f i n a l R value was 0.048. 86 .Crystal Data ^14^12N2^2' o r t h ° r h o m b i - c > space group P2^2^2^ with a=18.701(6), b=7.274(2), c=9.005(2) A, 8=90° and Z=4. The geometry of the molecule closely resembles that of i t s precursor 8̂. The oxetane ring i s very distorted with a l l internal torsion angles greater than 30°. Intramolecular oxetane formation in these tetrahydro-1,4- naphthoquinones seems to be a less preferred reaction than the cyclo- butane forming dimerization and the hydrogen abstraction reactions because a l l the substrates studied have roughly the same orientation of the C(1)=0(1) and C(6)=C(7) double bonds as does 8 and in addition have short contacts between these bonds (Table XV) and yet oxetane formation did not compete with either the dimerization or the hydrogen 88 abstraction reactions. Arnold has noted a similar trend earlier. - 139 - Table XV. Intramolecular Bond Contacts and Orientation Substrate Intramolecular C(1)=0(1);...C(6)=C(7) contact, A (centre to centre) Angle between normals to the C=0 and C=C planes (angle between the p orbital) a U . = o - 3.37 92.4° 06. ] 3.35 (Type A) 3.40 (Type B) 90.3°(Type A) 88.5°(Type B) DO), 3.45 87.1° 3.41 89.5° 3.39 89.8° M 0 IN 0 3.26 97.3° a c c i ( M 0 3.20 99.2° 3.29 95.1° 3.38 91.8° - 140 - He finds that, in general, whenever a hydrogen i s easi ly accessible to an excited carbonyl group, hydrogen abstraction becomes so important as to exclude oxetane formation. Substrates 8_ and 1̂  are two substrates which one might expect to undergo the Norrish Type II reaction since they have abstractable y-hydrogens. Substrate 1 did undergo the Type II reaction in solution but not in the sol id state because favorable intermolecular contacts in the crystal l ine state permitted a much preferred dimeriza- tion to occur. With unfavorable intermolecular contacts, substrate j3 might be expected to undergo the Type II reaction, especially o since the y-hydrogen to oxygen distance i s only 2.41A which compares o favorably with H^O distances of 2.58A or less for the observance of hydrogen abstractions discussed earl ier . However, i n order for a substrate to give stable products ar is ing from hydrogen abstraction, i t has to be able to not only abstract the hydrogen but also close i t s radical centres. For substrate 8̂  this means forming a bond between carbons 3 and 9 in the b i radica l 8'B without gross changes o in conformation. These centres are >4A apart i n the crystal of 8_. - 141 " Thus, Norrish Type II products cannot form from the conformations in which these tetrahydro-l,4-naphthoquinones c r y s t a l l i z e . The formation of the Type II product 1B_ during the solution photochemistry of 1 must then be seen as a manifestation of biradical IB's a b i l i t y to change conformation to one which permits ring closure between carbon centres 3 and 9. For biradical 8'B the change from i t s original conformation to one such as shown below> which permits C3 to C9 bonding, requires that the bridgehead cyano groups be moved, f i r s t , towards each other, then be eclipsed and f i n a l l y past each other. The f i n a l conformation i s , probably, as good as the i n i t i a l one but the process of attaining i t i s energetically unfavorable - 142 - because of dipole-dipole repulsion between the cyano groups. So the difference i n reactivity between jL_ and 8̂  i n solution i s most l i k e l y due to the a b i l i t y of biradical l'B and not 8'B to change conformation. - 1 4 3 - 4. Unreactive Substrates 2,3,6,7-Tetramethyl-4a8, 5,8,836 -tetrahydro-1,4-naphthoquinone, Jj Colorless needles of this compound were obtained by crystal- l i z i n g from petroleum-ether. One of these crystals measuring 0.25 x 0.10 x 0.10 mm was used for X-ray structure determination. Unit c e l l parameters were obtained by least-squares refinement of the observed 20 values of 19 reflections. The structure was solved using intensity data of 2442 independent reflections. The f i n a l R value was 0.072. 89 Crystal Data ^14^18^2' m o n o c l i n i c > space group P2^/c with a=5.245(2), b=29.452(7), c=8.278(5) A, 3=106.44(4)° and Z=4. A stereo view of the molecule i s shown in Figure 28. Figure 28. Stereo diagram of substrate 5_. - 144 - Irradiation of crystals of 5_ either i n a KBr matrix or as the pure solid in vacuo led to no detectable reaction. In solution, however, this substrate i s reported to give photoproducts 5A and 5_B fsolid state * N o r e a c t i o n requirements to undergo 8-hydrogen abstraction. The 0(1) to Hg o o distance i s 2.42A and the C(l) to C(6) separation i s 3.49A. So, one would expect 0(1) to abstract Hg upon UV irradiation with subsequent bond formation between C(l) and C(6) to give enone-alcohol 5A. As i n previous examples, photoproduct 5Ji i s not expected to form in the irradiation of 5_ in the solid state. The inertness of 5_ to irradiation i s particularly surprising since compound 4_ in which the Hg to 0(1) distance i s 2.46A and the C(l) to C(6) separation was - 145 - o 3.51A d i d react to give the expected enone-alcohol. These i n t e r - atomic separations are almost exactly equal to the Hg to 0(1) and C(l) to C(6) separations i n substrate 5_. A p o s s i b l e explanation fo r _5_'s inertness resides i n the c r y s t a l packing. Figure 29 shows two adjacent molecules within a c r y s t a l l o g r a p h i c c e l l . The C(1)=0(1) !• ' 1 Figure 29. Two adjacent molecules of 5_ wit h i n a c r y s t a l l o g r a p h i c c e l l . carbonyl group of one molecule l i e s above the C(4)-0(2) carbonyl group of a second molecule. The angle between the normals to the planes of the two carbonyl groups i s only 16.4°. So they are almost i n a p a r a l l e l o r i e n t a t i o n and the two TT systems should i n t e r a c t provided they are close enough to each other. The centre-to-centre o double bond separation f o r these two carbonyl groups i s 3.65A which i s c e r t a i n to promote a strong i n t e r a c t i o n between these two chromophores. Of the two carbonyl groups only C(1)=0(1) i s conformationally capable - 146 - of B-hydrogen abstraction upon irradiation. From a crystallographic viewpoint, there are two possible reasons for the photostability of 5_ in the solid state but not in solution. The short intermolecular separation of the two carbonyl groups may be giving rise to (I) exciton migration and self-quenching and/or ( i i ) an interaction between an excited molecule and an unexcited neighbour giving rise to an excimer which subsequently decays and dissociates to two ground state molecules. In connection with the f i r s t of these two possibi- l i t i e s , i t has already been mentioned in the Introduction that both singlet and t r i p l e t migrations are known to occur in crystals and the wandering of the t r i p l e t i s usually more extensive than that of the singlet partly as a result of the longer lifetime of the t r i p l e t state. During migration, the excitation energy i s frequently scattered by l a t t i c e vibrations u n t i l i t decays optically or thermally. In addition, migration also promotes the interaction between two excitons. The interaction can be between two singlet excitons or two t r i p l e t excitons, the latter interaction i s more frequently encountered than the former again because the t r i p l e t state lives longer, wanders more extensively and, therefore, has more probability of interacting with another t r i p l e t exciton than the singlet. During this intermolecular exciton interaction, one member of the pair comes off with a higher excitation energy than i t i n i t i a l l y had and the other molecule i s deexcited to the ground-state. The electronic energy of the excited partner i s usually less than the combined energies of the two excited states since some of the electronic energy - 1 4 7 - i s lost to the l a t t i c e as heat during the interaction. The process 90 is known as singlet-singlet annihilation when the excitons involved 90 are singlets and referred to as t r i p l e t - t r i p l e t annihilation when two tri p l e t s are involved. The 'new' excitons, and T.. (Scheme 21) cascade down to the lowest singlet which may continue i t s migration S± + S q + heat (singlet-singlet annihilation) J <10" 6sec S D + hv S + heat o Scheme 21 2 s r - 148 - t i l l another a n n i h i l a t i o n sets i n or u n t i l i t thermally or o p t i c a l l y loses i t s e x c i t a t i o n energy. In the quenching of i r r a d i a t e d 21 c r y s t a l s of 5_, h o s t - s e n s i t i z a t i o n i n which the l a t t i c e rather than impurities play the major r o l e of t r a n s m i t t i n g the e x c i t a t i o n energy from one part of the c r y s t a l to another, i s most l i k e l y to be the key d e a c t i v a t i o n pathway. Nevertheless, the p o s s i b i l i t i e s of d i s l o c a t i o n s and impurities also p l a y i n g a r o l e cannot be ruled out. The second possible major path f o r d e a c t i v a t i o n , namely, excimer or a-bond formation and decay requires an a s s o c i a t i o n between one excited molecule and one unexcited molecule. The i n t e r a c t i o n , may or may not e n t a i l a bond formation between the two partners. In the case where the i n t e r a c t i o n does not e n t a i l bond formation, the excited species w i l l resemble an intermolecular exciplex. Since the intermolecular C(1)=0(1) to C(4)=0(2) distance of 3.65A i n 5_ i s o almost equal to the 3.62A contact of C=0 to C=C which permitted 85 oxetane formation i n 2,5-dimethyl-p-benzoquinone , the formation of a 1,3-dioxetane as a t r a n s i e n t intermediate during the i r r a d i a t i o n of c r y s t a l s of 5_ i s at l e a s t conceivable. To date, no 1,3-dioxetanes have been reported i n the l i t e r a t u r e . Nonetheless, i t s formation as a transient intermediate followed by f a c i l e decomposition to two carbonyl fragments would c l o s e l y p a r a l l e l the behaviour of known * 1,2-dioxetanes. So that, whatever the nature of the excimer, (S^S^) , i t r e a d i l y decays thermally or photochemically to two ground state molecules. This second mode of d e a c t i v a t i o n of excited molecules of 149 - 5_ in the solid state i s summarised in Scheme 22 where SQ denotes the substrate 5_ i n i t s ground state and S^ denotes _5 in i t s f i r s t excited state. Scheme 22 s Q s , a _ [ S q S i ] . excimer 2S Q + hv' 2S Q + heat 2,3,4ag,5a,8a,8ag-^Hexamethyl-4ag,5,8,8ag-tetrahydro-l,4-naphtho- quinone. 10 This adduct crystallized out of hexane as low melting rods (mp 52-54°). The X-ray structure of this molecule i s not yet available. Nevertheless, i t s behaviour under UV irradiation i n the solid state 24b was studied because i t was reported to give photolabile oxetane 1QA and the caged compound 10B when irradiated in solution. Given the conformation i n which a l l the tetrahydronaphthoquinones so far - 150 - studied c r y s t a l l i z e , one does not expect 10B to be formed i n a l a t t i c e - c o n t r o l l e d s o l i d s tate r e a c t i o n , but 10A might be expected to form by analogy to the {$ 8A conversion p r e v i o u s l y described. I r r a d i a t i o n of 10 i n a KBr matrix gave no r e a c t i o n i n up to 2 hours of i r r a d i a t i o n . In the i n f r a r e d spectrum recorded a f t e r 6 hours of i r r a d i a t i o n , broadening i n the 5.67-5.78 y region was observed. A f t e r 17 hours two new carbonyl absorptions at 5.70 and 5.76 y became prominent. These absorptions could be due to the caged 24b isomer, the carbonyl absorptions of which have been reported at 5.68 and 5.75 y. Oxetane 10A could also be present but undetected i n the i n f r a r e d since i t s carbonyl absorption at 6.04y coincides with the absorption of r e s i d u a l 10 at 6.00y. The long induction period f o r the appearance of these product(s) and the fac t that the KBr p e l l e t was i r r a d i a t e d at room temperature with no p r o v i s i o n f o r preventing the p e l l e t from heating up strongly suggested that the re a c t i o n was occurring from the melt and not from the s o l i d . To further substantiate t h i s , pure c r y s t a l s of 10_ were i r r a d i a t e d i n vacuo at lower and c o n t r o l l e d temperatures to ensure that i t stayed s o l i d . I r r a d i a t i o n at 0.03 t o r r at temperatures between -9.9° and -9.1° f o r 14 hours gave 95% of recovered m a t e r i a l which was shown by th i n l a y e r chromatography, i n f r a r e d and NMR to be i d e n t i c a l to the solid state h v N o reaction 10 - 151 - starting material 10. Another t r i a l run at 0.03 torr and sl i g h t l y higher temperatures (-1.4° to -0.5°) for 13 hours also led to no reaction. As mentioned earlier, the 4a,5,8,8a-cis-tetrahydro-l,4- naphthoquinones which have thus far been studied seem to prefer cyclobutane ring forming dimerization and hydrogen abstraction to intramolecular oxetane formation. But as has been shown by the 8̂  -* 8A conversion, when none of these preferred reactions i s possible, oxetane formation can be observed. The formation of oxetane 10A in solution but not in the solid state may mean that excited 1(3 crystals have some efficient means of deactivating to ground state which the molecules i n solution do not have. But without the X-ray structure, i t cannot be ascertained whether this i s a crystal packing effect as in the case of substrate 5_ or some other conditions peculiar to the crystalline state. In summary, the four reaction types encountered during UV i r - radiation of a number of tetrahydro-1,4-naphthoquinones in the solid state can be rationalised in terms of the principle of least motion also known as the topochemical principle"^ '^~>. Cohen"^3 has recently elaborated on this principle. He defined a "reaction cavity" as the space occupied by a reactant molecule or molecules as the case may be. The size and shape of this cavity i s determined by the structure and geometry of the molecules of the substrate(s). The topochemical principle i s then interpreted to mean that a l a t t i c e - controlled transformation proceeds with minimal distortion of the - 152 - surface of the reaction cavity. Within-this interpretation, extrusions from the cavity as well as the creation of voids within the cavity as a result of a reaction are energetically unfavorable. Figure 30 i s a diagramatic representation of a favorable and unfavorable solid state transformation. As reaction proceeds a point i s Figure 30. The reaction cavity before reaction ( f u l l line) and in the transition state (broken line) for energetically a) favorable process, b) unfavorable process. reached when the restrictions imposed on the conformation of product molecules is relaxed and the latter then cr y s t a l l i z e out in their own conformation. But u n t i l this stage i s reached, products which have shapes most resembling those of reactant molecules or alterna- tively those with readily deformable structures w i l l be easily accommodated by the host l a t t i c e . The ideal solid state transformation w i l l be one which yields a product which i s crystallographically~ isomorphous with the starting material. Such reactions should - 153 - proceed essentially to completion without any distortion of the 91 crystal l a t t i c e . Wegner and coworkers reported one such solid state reaction. In the reactions presented in this publication, i t has been shown consistently from the X-ray data of the substrates that the transition state could be reached with hardly any change in the conformation of the substrate. Furthermore, i t has been shown that only stable products which can be formed without major changes in the substrates' conformations do form in the solid state reactions. This conformational control has permitted the estimation of certain useful reaction parameters;- a) The cyclobutane ring forming [̂ 2̂  + 2̂ ] dimerization has been shown to occur from a centre-to-centre double bond o separation, d, of 4.04A or less and the two double bonds approach each other in a par a l l e l orientation which allows for maximum overlap of their p-orbitals. The situation here i s completely analogous to 12 59 the dimerization of a-cinnamic acid crystals ' b) Intramolecular oxetane formation has been observed at o a centre-to-centre C=0 to C=C separation, d, of 3.20A. The approach of the two double bonds i s such that the p-orbitals are almost at right angles. This near-orthogonal approach i s also a good geometry for overlap. The distance separating the two double bonds i s short compared to the corresponding separation i n intermolecular oxetane 85 forming reactions reported by Schmidt and coworkers . The short - 154.- contact would tend to promote a stronger interaction and ultimately ring closure. c) The abstraction of a hydrogen $ to a carbonyl group by the carbonyl oxygen has been shown to occur from a distance ranging from 2.26A to 2.58A. d) The abstraction of a y-hydrogen by the carbon of an o o excited enone system occurred from distances of 2.66A and 2.89A respectively in two substrates. The longer separation between the abstractable hydrogen and the abstracting atom here as compared to (c) above i s a reflection of the larger van der Waals volume of carbon relative to that of oxygen. In both types of hydrogen abstraction, the 0 to H and C to H separations were less than or equal to the sum of the van der Waals r a d i i of the two atoms involved in the abstraction. The retention of gross conformation during the abstractions has also made i t possible to define the geometry of the transition state more precisely than has hitherto been.possible. The abstraction of a hydrogen 8 to an excited carbonyl group by the carbonyl oxygen has been shown to occur through a planar five-membered transition state. The corresponding abstraction of y~hydrogen by carbon of an excited enone chromophore has a non-planar six-membered cyclic transition state which i s , however, not in the torsion-free chair 43 conformation proposed by McLafferty and coworkers but rather in a boat-shaped conformation. - 155 - i Overall, intermolecular effects have been found to play an important role in the photochemical behaviour of crystals of the tetrahydro-l,4-naphthoquinones studied only when intermolecular contact between unsaturated centres in adjacent molecules i s o <4.04A. When this distance is exceeded, intramolecular processes predominate. Lastly, the pos s i b i l i t y of dislocations and/or defects playing a role in some of these systems cannot be overlooked especially in the 3^ -> J3B conversion and in explaining the behaviour of substrates 5_ and IJ) in the solid state. After the commencement of the investigations presented here, a report on gold-catalysed rearrangement of strained small ring 92 hydrocarbons appeared in the literature . There is no indication, however, that the gold metal surface on which the crystals of 1-11 were studied catalysed any of the reactions observed. This conclusion is based on the fact that a l l the reactions studied in the reactor were also studied in KBr matrices. The reactivity of each compound by both methods was identical. - 1 5 6 - EXPERIMENTAL General E u t e c t i c Temperature Eutectic temperatures were determined by d i f f e r e n t i a l thermal analyses (DTA) using a Perkin Elmer DSC-IB. In each case, the following mixtures were analysed u n t i l a sharp melting t r a n s i t i o n was observed: i ) crude r e a c t i o n mixture, i i ) varying compositions of crude rea c t i o n mixture and added s t a r t i n g m a t e r i a l . Each e u t e c t i c mixture was ground into a f i n e powder. In c e r t a i n cases, thorough mixing was accomplished by d i s s o l v i n g the mixture i n acetone or chloroform, removing the solvent and drying the sample i n vacuo. Sample s i z e s used ranged from 5 to 10 milligrams. The sample was placed i n an aluminum planchette and covered with an aluminum l i d . Both of these accessories are a v a i l a b l e as Perkin Elmer Sample Pan K i t No. 219-0041. The l i d was f i r m l y pressed against the sample without crumpling or creasing the l i d . Scanning f o r e u t e c t i c t r a n s i t i o n s above ambient temperature was c a r r i e d out at a slope s e t t i n g of 510, a d i f f e r e n t i a l temperature s e t t i n g of 485, an average temperature s e t t i n g of 522 and a scanning rate of 10°/min. Low temperature t r a n s i t i o n s were determined at slope s e t t i n g 500, d i f f e r e n t i a l temperature s e t t i n g 491, average - 157 - temperature setting 430 and a scanning rate of 5°/min. A transition was recorded as the f i r s t departure from base line i.e. at the, beginning of a peak. Low temperature transitions were corrected using a calibration graph obtained by scanning for the melting transitions of n-octane (m.p. -56.8°) and ethylene glycol (m.p. -11.5°). A l l other temperature read-outs were corrected using a calibration graph obtained by scanning melting point standard samples. Each calibration was carried out at the appropriate instrumental settings cited above. Melting point (m.p.) A l l melting points except those indicated by asterisk(s) were taken on a Fischer-Johns hot stage melting apparatus and are uncorrected. Melting point* (m.p.*) The melting point of a compound found to melt at about ambient temperature was determined by sealing the sample in a capillary tube, then repeatedly freezing i t in l i q u i d nitrogen and thawing i t t i l l the walls of the tube became evenly coated with the solid. The capillary tube was then immersed in a stirred dry ice-acetone mixture which was consequently allowed to warm to room temperature. An alcohol thermometer which had been calibrated against a copper-constantan thermocouple in the region 20° to -40° was used to note the temperature of melting. The reported m.p.* was corrected using the calibration graph. - 158 - Melting point** (m.p.**) The melting points of compounds which decompose during normal melting point determinations were c a r r i e d out by the DTA method. Instrumental s e t t i n g s reported f o r e u t e c t i c temperature determinations above ambient temperature were used. Infrared A l l i n f r a r e d spectra were recorded on a Perkin-Elmer 457 spectrophotometer. For s o l i d s , KBr p e l l e t s containing 1-2 mg of the sample per 250 mg of KBr were made using a Perkin-Elmer Potassium Bromide Evacuable Die 186-0002 and a Carver Laboratory Press Model B. The pressing load was generally 20,000 l b s per square inch. Infrared spectra of l i q u i d s were recorded neat. Nuclear Magnetic Resonance (NMR) Spectra of s t a r t i n g materials 1-11 and q u a n t i t a t i v e NMR analyses were taken on a Varian T-60 instrument by the author. Spectra of photoproducts 1A-11B were recorded on e i t h e r a Varian HA-100 or XL-100 by the departmental spectroscopists, Mr. W.B. Lee and Dr. 0. Chan. In a l l cases, tetramethylsilane (TMS) was added as an i n t e r n a l standard. 93 Quantitative NMR An a l y s i s This was the method of choice f o r the determination of the extent of r e a c t i o n f o r compounds which d i d not come o f f the GLC columns t r i e d . - 159 - An aliquot, usually 0.3 ml, of a standard solution containing 228.3 mg of CH3N02 per 10.0 ml of CVCl^ solution was added to an accurately weighed reaction mixture using a 2 ml pipette graduated in 0.01 ml. Sample sizes used ranged from 32.5 to 101.0 mg to ensure good signal to noise ratio. The mixture was shaken well to dissolve. The solution was transferred into an NMR tube. The sample v i a l was washed with a few drops of CDCl^ and the washings added to the contents of the NMR tube. The resonance used in each analysis was well resolved and removed f rom other resonances. The chosen resonance used in each such analysis i s indicated in the Experimental. This chosen resonance and that of the internal standard were electronically integrated 4 times. of magnetic nuclei , the sample's chosen resonance strength, x, i s given by Since the H signal strength i s proportional to the number 93 x °= and that of the internal standard's resonance i s given by b x w, s a s _ _M b ' M„ whence y w - 160 - and w s b M where w = milligrams of residual starting material present i n s the NMR mixture a g = no. of protons in the integrated resonance of the starting material b = no. of protons in the integrated resonance of the internal standard (=3 for CH3N02) M = molecular weight of the internal standard (=61.04) w = milligrams of internal standard i n the NMR sample x = average electronic integration of the compound's resonance y = average electronic integration of the internal standard's resonance where W = weight of the reaction mixture used in the analysis. In the case of the reaction of compound 6_, where none of the resonances of the starting material were well resolved from the product resonances, a well resolved resonance of the product, molecular weight of starting material The extent of reaction = % conversion = W-w8 W" - 161 - 6A, was used instead. In this case, the substitution, milligrams of product 6A i n the NMR sample = W-ws was made into equation (1). This equality was valid in this case because the consumption of 6̂  resulted in the production of only 6A. Mass Spectra A l l mass spectra reported i n this work are low resolution spectra obtained on a direct inle t Varian Atlas MAT CH 4-B or AEI MS-9 at 70eV and operated by the departmental analysts, Dr. G. Eigendorf, Mr. G.D. Gunn and Mr. J. Nip. Elemental Analyses Elemental analyses were performed by the departmental microanalyst, Mr. P. Borda. Thin Layer Chromatography (TLC) Thin layer chromatography carried out for analytical purposes was done on strips of aluminum-backed s i l i c a gel 60F-254 (thickness 0.2 to 0.25 mm) available from E. Merck. The developing solvent used in each instance i s indicated in parentheses elsewhere in the Experimental. Preparative TLC was carried out on 20x20 cm glass plates coated with s i l i c a gel GF-254 (thickness 0.7 to 0.8 mm) which had been dried in a i r and then at 45-50° overnight. - 162 - Column Chromatography Two types of adsorbents were used: s i l i c a g e l 60 ( p a r t i c l e s i z e l e s s than 0.063 mm) from E. Merck and Woelm's n e u t r a l aluminum oxide a c t i v i t y grade 1. The adsorbent and eluent used i n each instance are i n d i c a t e d i n parentheses. Gas L i q u i d Chromatography (GLC) GLC separations were c a r r i e d out on e i t h e r Varian Aerograph 90-P or a Varian Autoprep A-700 instruments connected to Honeywell E l e c t r o n i k 15 s t r i p chart recorders. The c a r r i e r gas i n a l l cases was helium. The column s i z e and packing m a t e r i a l as w e l l as the column temperature and helium flow rate through the column are indicated i n parentheses where ap p l i c a b l e . Q u antitative GLC Analysis Peak areas used i n c a l c u l a t i o n s represent averages of peak areas measured f o r four or more i n j e c t i o n s . Detector Response Factor, k: The detector response f a c t o r , k, f o r a compound, X, was determined by analysing a s o l u t i o n containing an accurately weighed amount of X and an a l i q u o t of a standard stock s o l u t i o n of an i n t e r n a l standard. Peak areas were c a l c u l a t e d using the formula Area = (peak width at half-height) x (height) . The detector response factor i s then given by _ 163 _ K = • — w s Ax where wx = weight of compound x ws = weight of internal standard A s = average peak area of internal standard A x = average peak area of compound x Weight Calculations: The analysis of a reaction mixture consisted of adding an aliquot of the same internal standard solution as that used for the detector response factor determination, to a weighed amount of the reaction mixture. The resulting solution was then analysed by GLC. The weight of a component, ŵ , in the solution i s given by the equation A i w± = w sk ±— (2) A s where ws = weight of the internal standard used k-ĵ  = the detector response factor for component i Aj[ = average peak area for component i A s = average peak area for the internal standard Using equation (2) the weight of residual starting material in a reaction mixture was calculated. As i n the case of the NMR analysis W - ws % conversion = — — W where W = weight of the reaction mixture used i n the analysis * and ws = weight of residual starting material in the mixture . - 164 - Solvents A l l solvents were d i s t i l l e d through a f r a c t i o n a t i n g column. Unless otherwise i n d i c a t e d , the petroleum ether used was the f r a c t i o n b o i l i n g >68°. O p t i c a l Rotations, a, f o r Enone-alcohol TA were taken on a Perkin- Elmer 141 Polarimeter operated at the sodium D l i n e (589 nm). Single c r y s t a l s f o r t h i s purpose were grown from acetone-petroleum ether and checked under a p o l a r i z i n g microscope to determine which c r y s t a l s were s i n g l e and which were twinned or c l u s t e r e d . C r y s t a l s i z e s ranged from 2.4 to 8.7 mg. A s o l u t i o n of known concentration was made up by d i s s o l v i n g a weighted s i n g l e c r y s t a l i n d i s t i l l e d acetone to make 1.0 ml of s o l u t i o n . A one-decimeter c e l l holding ^0.8 ml of s o l u t i o n was f i l l e d with pure acetone and used to zero the instrument. The c e l l was then f i l l e d with the sample s o l u t i o n and the r o t a t i o n , a , read. For each sample, four readings were made, and the average reading used i n the c a l c u l a t i o n below. The temperature was also noted, a was also determined f o r a c l u s t e r of c r y s t a l s (11.9 mg). t° S p e c i f i c r o t a t i o n s , [a ] , reported were c a l c u l a t e d from the equation below: [a] a 1-c where a the observed r o t a t i o n at A=589 nm and t=25.5° 1 the c e l l length i n decimeters (=1) c the concentration of the s o l u t i o n i n grams per ml of s o l u t i o n . Figure 31. Apparatus for Irradiations in the Solid State. (a) Part A; (b) Part A being inserted into Part B. Figure 31(c). The Assembled Apparatus for I r r a d i a t i o n s i n the Solid State. - 167 - The reactor consisted of two main par t s , A and B (Figure 31 above). Part A consisted of a double-walled c y l i n d r i c a l brass drum soldered onto a f l a t - f a c e half-drum. An i n l e t and o u t l e t provided f o r the c i r c u l a t i o n of coolant through t h i s u n i t . At the f l a t surface of the half-drum was a gold-plated c i r c u l a r groove where c r y s t a l s to be i r r a d i a t e d were grown. The temperature at the reaction s i t e was measured by a copper-constantan thermocouple one j u n c t i o n of which was soldered onto the edge of the golden groove. An aluminum f o i l roof was positioned over t h i s j u n c t i o n to s h i e l d i t from d i r e c t r a d i a t i o n . The second j u n c t i o n of the thermocouple served as a reference j u n c t i o n and was kept i n an i c e - s l u r r y during operations. Part B was e s s e n t i a l l y a brass casing f o r part A and had the following p r o v i s i o n s : a Pyrex window which allowed for the i r r a d i a t i o n of the sample e x t e r n a l l y ; a groove with an 0-ring to make the system a i r - t i g h t a f t e r the i n s e r t i o n of part A i n t o B; and a vacuum takeoff to an oil-pump f o r the evacuation of the reactor. The r e s t of the apparatus consisted of an U l t r a Kryomat K-80 DW f o r the c o o l i n g and c i r c u l a t i o n of coolant through the r e a c t o r , a d i g i t a l - m i c r o v o l t m e t e r ( D M V ) the reading of which i s 94 c o n v e r t i b l e to C using a copper-constantan c a l i b r a t i o n table , an a i r - c o o l e d glass f i l t e r (Corning 7380 f o r X >_ 340 nm or Corning 3850 f o r X >_ 355 nm) p o s i t i o n e d between the Pyrex window and the lamp and f i n a l l y a water-cooled 450-Watt Hanovia lamp connected to a power source. In a t y p i c a l run, c r y s t a l s of the compound to be studied were weighed and d i s s o l v e d i n 3-5 ml of the solvent(s) of c r y s t a l l i - z ation. Using a disposable p i p e t t e , the s o l u t i o n was slowly dropped - 168 - on the groove and allowed to s l o w l y evaporate and c r y s t a l l i z e o v e r n i g h t . A c o v e r i n g of aluminum f o i l p r o t e c t e d the sample from l i g h t and dust p a r t i c l e s d u r i n g t h i s stage. j The Kryomat was set a t the temperature chosen f o r a r e a c t i o n . The coolant was 50% (V/V) aqueous ethylene g l y c o l f o r temperatures down to -28°and CHCl-j f o r lower temperatures. While the c o o l a n t was being cooled to the d e s i r e d temperature, the f l o o r of the r e a c t o r was l i n e d w i t h f i l t e r paper. T h i s served to catch any s o l i d s which might f l a k e o f f from the groove d u r i n g the r e a c t i o n . P a r t A was then i n s e r t e d i n t o B and the two were secured together by screws. The i n l e t and o u t l e t l e a d s of p a r t A were secured to the a p p r o p r i a t e l e a d s of the Kryomat. The r e f e r e n c e j u n c t i o n of the thermocouple was i n s e r t e d i n t o a dewar of i c e - s l u r r y and the two t e r m i n a l s connected to the DMV. The r e a c t o r was then evacuated and he l d under vacuum f o r a minimum of 15 minutes before the c i r c u l a t i o n of the c o o l a n t through the r e a c t o r was.begun. A f t e r the DMV reading i n d i c a t e d the d e s i r e d temperature, a f u r t h e r 15 minutes were allowed f o r the system to e q u i l i b r a t e and s t a b i l i z e . Meanwhile, the f i l t e r and lamp were a l i g n e d w i t h the Pyrex window. The lamp was p o s i t i o n e d about 5" from the Pyrex window which, i n t u r n , was about 4" from the sample. Water c i r c u l a t i o n through the lamp j a c k e t was begun at t h i s time. A steady j e t of a i r was d i r e c t e d onto the g l a s s f i l t e r to prevent i t from o verheating and c r a c k i n g . A r e f l e c t o r was p l a c e d behind the lamp to minimize d i s s i p a t i o n of the l i g h t to - 169 - the environment, and the lamp was switched on. The DMV reading, the pressure reading of the McLeod gauge attached to the vacuum l i n e , and the time of switching on the lamp were recorded. For reactions last ing 2 hours or less , the reaction temperature was recorded at 10-15 minute intervals. . Temperatures of reactions last ing longer than 2 hours were recorded at longer time intervals . The entire temperature range of any run is indicated in the appropriate section of the Experimental Section. The vacuum attained is .also given. The longest continuous run was 13.00 hours. Irradiations last ing longer than this were carried out discontinuously over a period not exceeding 3 days. At the end of a run, the cooling was stopped and the system allowed to reach room temperature before the vacuum was released to prevent condensation of moisture on the reaction mixture. Part A was taken out and l a id horizontally. Using disposable pipettes and either acetone or CHCl^, the sample was washed off the groove. The solvent was removed and the sample dried in vacuo. The % recovery of material from the reactor i s indicated for each run i n the appropriate section of the Experimental. Apparatus and Procedure for Irradiations in Benzene Photolyses carried out in benzene at room temperature were done using a conventional external i r radiat ion apparatus. Interposed between the water-cooled 450-Watt Hanovia lamp and the solution was an - 1 7 0 - air-cooled Corning glass f i l t e r which was either No. 7380 transmitting X > 340 nm or No. 3850 transmitting X > 355 nm. A l l solutions were thoroughly deoxygenated for about 0.5 hour prior to irradiation and photolysed under Argon of <5 ppm oxygen content available from Canadian Liquid Air. Apparatus and Procedure for Low Temperature Irradiations i n Solution Figure 32. Apparatus for Low Temperature Irradiations i n Solution. - 171 - The apparatus consisted of a double-walled Pyrex vessel with an inlet and outlet from the outer wall to allow for circulation of coolant. . The apparatus also had an outlet with a Teflon stopcock from i t s innerwall for easy sampling of the reaction mixture at intervals during the course of a reaction. Solutions were deoxygenated and photolysed under L grade high purity nitrogen which was f i r s t passed through a tower of Drierite. Gas inlet and exit into the solution were through hypodermic needles inserted through an otherwise airtight capplug stopper equipped with an alcohol thermometer precalibrated against a copper-constantan thermocouple. • Solutions were magnetically stirred. Moisture condensation and i t s subsequent freezing on the apparatus were mimimized by directing a fast current of a i r over the sides of the reactor. Irradiations were discontinuous but were completed within two days. As in the case of a l l other modes of irradiation, the light source was a 450-Watt Hanovia lamp, and an air-cooled f i l t e r (Corning No. 7380 for X >_ 340 nm and No. 3850 for X >̂  355 nm) was placed i n between lamp and reactor. Wavelength of Irradiations Compounds 1̂  through 8̂  and substrate 10_ were studied at X >_ 340 nm. Compounds 9_ and 11̂  were irradiated at A > 355 nm. These are the same wavelengths at which these compounds have been studied in'solution in earl i e r work^. - 172 - P r e p a r a t i o n of 5a,8a-Dimethyl-4aB,5,8,8ag-tetrahydro-l,4-naphtho- 95 . quinone , 1, A s l u r r y of 3.04 g (0.028 mole) of p-benzoquinone i n 5.29 g of trans,trans-2,4-hexadiene was heated under r e f l u x at 62° w i t h s t i r r i n g u n t i l a l l of the quinone had d i s s o l v e d . Heating was continued f o r a f u r t h e r 15 minutes. The mixture was then l e f t s t i r r i n g a t room temperature f o r 3 hours. Removal of the excess diene i n vacuo l e f t 5.03 g (94%) of a b r i g h t y e l l o w s o l i d . C r y s t a l l i z a t i o n twice from ether-petroleum e t h e r gave y e l l o w rods -of 1. Mp 55-56° ( l i t . 9 5 58-59.5°). Ir - . (KBr), 5.92, 5.98 (C=0), 6.24 ( c o n j . C=C) y. NMR (CC1 4) 6, 6.62 (S, 2H, C2 and C3 v i n y l ) , 5.53 ( s , 2H, C6 and C7 v i n y l ) , 3.25 (m, 2H, 4ag and 8ag methines), 2.50 (m, 2H, C5 and C8 methines), 1.12 (d, J=7 Hz, 6H, m e t h y l s ) . I r r a d i a t i o n of Adduct 1_ i n KBr I r r a d i a t i o n of a 0.5% KBr p e l l e t of 1^ f o r 1.5 hours l e d to complete disappearance of the 6.24y a b s o r p t i o n of 1_. The i n f r a r e d i n a d d i t i o n , showed new C=0 a b s o r p t i o n s at 5.82 and 5.90 p. I r r a d i a t i o n of 1 i n the S o l i d State A s o l u t i o n of 114.9 mg of 1^ i n petroleum ether was s l o w l y evaporated on the r e a c t o r ' s groove and l e f t o v e r n i g h t . The - 1 7 3 - r e s u l t i n g c r y s t a l s were i r r a d i a t e d at 0.03 t o r r between -2.0° and -1.5° f o r 1 hour. The recovery was 98%. The r e s u l t i n g s o l i d was washed t h r i c e w i t h petroleum ether and the washings s t r i p p e d o f s o l v e n t and d r i e d i n vacuo to g i v e 43.2 mg of s o l i d which was shown by TLC, NMR and i r to be r e s i d u a l s t a r t i n g m a t e r i a l !L. The i n s o l u b l e product was d r i e d i n vacuo. This gave 67.9 mg (95% y i e l d ) of IA. The e x t e n t . o f r e a c t i o n c a l c u l a t e d from weight of unrecovered 1_ was 62%. Y i e l d s of IA a t other conversions are t a b u l a t e d i n the t e x t . An i n f r a r e d spectrum of the crude product was taken. F o l l o w i n g t h i s , the crude m a t e r i a l was c r y s t a l l i z e d from CHCl^-hexane to g i v e s p a r k l i n g c o l o r l e s s p l a t e s s u i t a b l e f o r s i n g l e c r y s t a l X-ray s t r u c t u r e d e t e r m i n a t i o n . Mp** 265.0° w i t h decomposition. I r (KBr) , 5.82 and 5.90 (C=0) y. NMR (CDC1 3) 6, 5.83 ( s , 4H, v i n y l s ) , 3.47 ( s , 4H, cyclobutane r i n g methines), 3.22 (dd, J=3 and 2 Hz, 4H, C4, C9, C14 and C19 methines), 2.50 (m, 4H, C5, C8, C15 and C18 methines), 1.10 (d, J=7 Hz, 12H, m e t h y l s ) . Mass spectrum m/e parent 380 A n a l . C a l c d . f o r C 2 4 H 2 8 ° 4 : C ' 7 5 > 7 6 ; H» 7 ' 4 2 ' Found : C, 75.48; H, 7.42. P r e p a r a t i o n Of 6,7-Dimethyl-4a8,5,8,8aB-tetrahydro-l,4-naphthoquinone 9 6, 2. R e c r y s t a l l i z e d p-benzoquinone (3.14 g, 0.03 mole) was suspended i n 4.65 g of 2,3-dimethyl-l,3-butadiene and heated at 65° - 174 - under r e f l u x f o r 2 hours. Removal of excess diene i n vacuo gave 2.92 g (93%) of crude 2_ which was d i s s o l v e d i n ether and f i l t e r e d . Removal of ether f o l l o w e d by c r y s t a l l i z a t i o n once from ethanol and twice from petroleum ether gave pa l e y e l l o w c r y s t a l s of 2. Mp 115-116°. ( l i t . 9 6 115-117°). I r (KBr), 5.93 (C=0), 6.24 (conj. C=C) y. NMR (CDC1 3) 6, 6.58 ( s , 2H, C2 and C3 v i n y l s ) , 3.10 (m, 2H, C4a and C8a methines), 2.20 (m, 4H, C5 and C8 methylenes), 1.63 ( s , 6H, methyls). - I r r a d i a t i o n o f Adduct 2_ i n KBr A 0.5% KBr p e l l e t of 2 was i r r a d i a t e d (X >_ 340 nm) and the r e a c t i o n monitored by i r . A f t e r 5 hours of i r r a d i a t i o n , the 6.24y ab s o r p t i o n of 2_ was completely gone. In a d d i t i o n , the 5.93y C=0 s t r e t c h of 2_ had been r e p l a c e d by a 5.85y C=0 a b s o r p t i o n . I r r a d i a t i o n of 2_ i n the S o l i d State C r y s t a l s of compound 2_ (93.0 mg) were i r r a d i a t e d i n a covered Pyrex p e t r i d i s h f o r 0.4 hour. The r e s u l t i n g s o l i d was washed three times w i t h chloroform and the washings concentrated and analyzed by TLC ( s i l i c a g e l ; c h l o r o f o r m ) . With the e x c e p t i o n of a base spot, the TLC showed only r e s i d u a l 2_. The dry weight of recovered 2_ was 55.8 mg. The s o l i d product 2A was d r i e d i n vacuo. The crude y i e l d was 36.1 mg (97%). The extent of r e a c t i o n (% conversion) - 175 - c a l c u l a t e d from unrecovered 2_ was 40%. Y i e l d s of 2A from other runs at v a r y i n g conversions are given i n the t e x t . The i n f r a r e d spectrum of the crude product was recorded. C r y s t a l l i z a t i o n from chloroform a f f o r d e d t i n y , s p a r k l i n g , c o l o r l e s s p l a t e s of 2A. Mp** 264.7° w i t h decomposition. I r (KBr), 5.85 (C=0) y. . NMR (CDC1 3) 6, 3.64 ( s , 4H, cyclobutane r i n g methines), 3.18 (m, 4H, C4, C9, C14 and C19 methines), 2.22 (m, 8H, methylenes), 1.66 ( s , 12H, methyls). Mass spectrum m/e parent 380. A n a l . Calcd. f o r C 2 4 H 2 8 ° 4 : C, 75.76; H, 7.42. Found : C, 75.61; H, 7.29. Notes of 2A C r y s t a l Used f o r X-ray S t r u c t u r e Determination The c r y s t a l s obtained by c r y s t a l l i z i n g from chloroform were too s m a l l f o r use i n the X-ray s t r u c t u r e determination. Larger s i n g l e c r y s t a l s f o r t h i s purpose were obtained by c r y s t a l l i z i n g crude 2A from a c e t o n i t r i l e . The i r of t h i s batch of 2A c r y s t a l s was i d e n t i c a l i n every respect to the i r of crude 2A as w e l l as to the 2A t h a t had been c r y s t a l l i z e d from c h l o r o f o r m . P r e p a r a t i o n of 4aB,5,8,8ag-Tetrahydro-l,4-naphthoquinone 5 5, 2. A s o l u t i o n pf 1,4-benzoquinone which had been c r y s t a l l i z e d twice from acetone-petroleum ether was made by d i s s o l v i n g 13.0 g - 176 - (0.12 mole) i n 200 ml of benzene i n a hydrogenation b o t t l e . The s o l u t i o n was cooled to 0°, and 18 ml of 1,3-butadiene which had been condensed to l i q u i d u s i n g d r y - i c e acetone was added and the b o t t l e f i r m l y stoppered and replaced i n i t s s h i e l d . The stopper was secured by screws i n a rocker of a hydrogenation apparatus and the s o l u t i o n m e c h a n i c a l l y rocked f o r 21 days. The r e s u l t i n g y e l l o w s o l u t i o n was s t r i p p e d of s o l v e n t and excess diene l e a v i n g an o i l which s o l i d i f i e d on c o o l i n g i n i c e . I t was c r y s t a l l i z e d once from petroleum ether and twice from ether-petroleum ether. The y i e l d a f t e r the three c r y s t a l l i z a t i o n s was 61%. Mp 53.5-54° ( l i t . 5 5 52-54°). I r (KBr), 5.97 (C=0), 6.24 ( c o n j . C=C) y. NMR (CCl^) 6, 6.57 ( s , 2H, C2 and C3 v i n y l s ) , 5.63 (m, 2H, C6 and C7 v i n y l s ) , 3.15 (m, 2H, 4a,8a methines), 2.28 (m, 4H, C5 and C8 methylenes). I r r a d i a t i o n of 3 i n KBr A 0.7% KBr p e l l e t of 3 was i r r a d i a t e d (A >_ 340 nm) f o r 3 hours. I r of the i r r a d i a t e d p e l l e t showed complete disappearance of the conjugated C=C s t r e t c h a t 6.24y. The C=0 a b s o r p t i o n had a l s o s h i f t e d from 5.97 to 5.88 y. I r r a d i a t i o n of _3 i n the S o l i d State C r y s t a l s of compound _3 (135.5 mg) were i r r a d i a t e d at 0.01 t o r r between 4.3° and 0.0° f o r 1 hour. The s o l i d was washed o f f the - 177 - reactor's cavity with acetone and chloroform. Removal of solvents and drying in vacuo le f t 124.0 mg of so l id (H92% recovery). It was redissolved in chloroform and suction f i l t e r e d . The f i l t r a t e was concentrated and checked by TLC ( s i l i c a ge l ; chloroform). Apart from a base spot, the only other spot was that of start ing material , _3. The chloroform soluble portion was 8.2 mg. The chloroform insoluble so l id was 117.6 mg (92% y i e l d ) . Infrared spectra of the crude sol id and also of the recovered starting material were recorded. The l a t ter was confirmed to be residual _3. The product was crys ta l l ized from refluxing chloroform. The crystals thus obtained were colorless , sparkling flakes. The i r of the crysta l l ized material was identical to that of the crude product, 3A. Yields of 3A from other runs.are given in the text. Compound 3A had Mp** 280° ± 2 ° with decomposition. Ir (KBr), 5.85 (C=0) y. NMR (CDC13) 6, 5.70 (m, 4, v iny l s ) , 3.64 (s, 4H, cyclobutane r ing methines), 3.20 (m, 4H, C4, C9, C4 and C19 bridgehead methines), 2.30 (m, 8H, methylenes). Mass spectrum m/e parent 324. Anal. Calcd. for c 2 o H 2 0 ° 4 : C ' 7 4 * 0 6 ; H » 6 - 2 2 - Found : C, 73.88; H, 6.15. Notes on 33. Crystal Used for X-ray Structure Determination The crystals of _3A obtained from crys t a l l i z ing from chloroform V - 178 - were too small and thin for single crystal X-ray structure determination. Larger crystals for this purpose were obtained by cr y s t a l l i z i n g from acetonitrile. The infrared spectrum of this mate- r i a l (3B) i s discussed in the text. Its NMR was identical to that of 3A crystallized from chloroform. Irradiation of Adduct 3. in Benzene A solution of 406.0 mg of compound _3 in 200 ml benzene was degassed and photolysed for 24.2 hours. The resulting milky solution was f i l t e r e d and the solid component dried and weighed. Its weight was 165.0 mg. Ir of the crude material showed a broad OH at 2.90u and a C=0 absorption at 5.87u. The absorptions were generally very broad especially in the fingerprint region. NMR ((CD 3) 2CO) 6, 6.80 (s), 5.07 (br,m), 2.51 (br,s, broadens on adding D 20), 1.53 (m). Mass spectrum: highest observed m/e = 495. The f i l t r a t e was concentrated and separated by GLC (5' x V of 20% DEGS on 60/80 Chromosorb W at 150° and 150 ml/min). Two peaks with retention times of 4 and 6 minutes, respectively were obtained. They were subsequently identified as 3_C and 3D by comparing their ir's with those of authentic samples. Of the two, 3D, retention time = 6 minutes, was the major. - 179 - P r e p a r a t i o n of 6,7-Dipheny.l-4aB,5,8,8aB^etrahydro-l,4-naphthoquinone :* /, 4. This was a f o u r - s t e p s y n t h e s i s comprising 97 a (a) The p r e p a r a t i o n of b e n z i l dihydrazone from b e n z i l - , 97 a (b) the c o n v e r s i o n of the b e n z i l dihydrazone to d i p h e n y l a c e t y l e n e , (c) the c o n v e r s i o n of the d i p h e n y l a c e t y l e n e to 2 , 3 - d i p h e n y l - l , 3 - b u t a d i e n e 9 ^ , and (d) the D i e l s - A l d e r r e a c t i o n between the diene from p a r t (c) and 97 c p-benzoquinone to g i v e k_ . (a) A s o l u t i o n of 105.1 g (0.5 mole) of b e n z i l and 76.0 g of 85% hydrazine hydrate i n 325 ml of n-propyl a l c o h o l was r e f l u x e d f o r 61.5 hours. The r e s u l t i n g red s o l u t i o n was f i r s t cooled t o room temperature and then cooled i n an i c e - b a t h f o r one hour. The b e n z i l hydrazone was f i l t e r e d , washed w i t h a t o t a l of 200 ml of ethanol i n p o r t i o n s and then d r i e d i n vacuo f o r one hour. This gave 100.4 g of b e n z i l hydrazone. (b) The b e n z i l hydrazone was t r a n s f e r r e d to a o n e - l i t r e t h r e e - necked f l a s k f i t t e d w i t h a r e f l u x condenser and a mechanical s t i r r e r and 480 ml of benzene added. Y e l l o w HgO (240 g) was added i n s m a l l amounts to the suspended mixture over a p e r i o d of 1.5 hours w i t h s t i r r i n g and g e n t l e h e a t i n g over a steam bath. A f t e r the a d d i t i o n , the mixture was f u r t h e r s t i r r e d f o r 1.5 hours during which time a vigorous r e a c t i o n accompanied by f r o t h i n g occurred. I t was kept under c o n t r o l by c o o l i n g i n an i c e bath. The mixture was l e f t a t room temperature o v e r n i g h t and then f i l t e r e d . The r e s i d u e was washed w i t h 100 ml of benzene and the washings added to the f i l t r a t e . The l a t t e r - 180 - was d r i e d over anhydrous Na2S0^, f i l t e r e d and s t r i p p e d of s o l v e n t . The r e s i d u e was d i s t i l l e d at 0.02 t o r r and 125°. This gave 62.1 g of p a l e y e l l o w s o l i d d i p h e n y l a c e t y l e n e . C r y s t a l l i z a t i o n from 95% ethanol gave 53.7 g of d i p h e n y l - a c e t y l e n e , m.p. 56-58° ( l i t . 9 7 a 60-61°). (c) A 500 ml three-necked round-bottomed f l a s k was f i t t e d w i t h a s e a l e d mechanical s t i r r e r , a r e f l u x condenser and a thermometer. A T - j o i n t was attached to the top of the condenser. The remaining two arms of the T - j o i n t were connected to a source of pure n i t r o g e n and a bubbler r e s p e c t i v e l y . The f l a s k was f l u s h e d w i t h n i t r o g e n and 5 g of dry sodium hydr i d e and 120 ml of anhydrous dimethyl s u l f o x i d e introduced i n t o the f l a s k . The mixture was heated to 75° under N 2 w i t h s t i r r i n g f o r 40 minutes. The f l a s k was then cooled to 30° u s i n g a water-bath. The thermometer was replaced by a dropping f u n n e l c o n t a i n i n g 17.8 g (0.1 mole) of d i p h e n y l a c e t y l e n e i n 80 ml of anhydrous dimethyl s u l f o x i d e . T h i s l a t t e r s o l u t i o n was added dropwise to the grey contents of the f l a s k . F o l l o w i n g the a d d i t i o n , the dropping funnel was r e p l a c e d by the thermometer and the mixture heated to and maintained at 70° f o r 2.5 hours. I t was subsequently cooled to room temperature and the reddish-brown mixture s l o w l y poured over i c e w i t h s t i r r i n g and l e f t a t room temperature u n t i l a l l the i c e had melted. The crude mixture was e x t r a c t e d f i v e times w i t h 200 ml p o r t i o n s of ether and the combined e t h e r e a l e x t r a c t s washed w i t h 300 ml of water and then d r i e d over anhydrous sodium s u l f a t e . - 181' - Removal of the ether in vacuo le f t 16.4 g of a reddish-brown o i l which was divided into two portions. Each portion was chromatographed on 200 g columns of neutral alumina using as eluents benzene-hexane, (1:7) and then (1:3). The pale yellow fractions were combined and stripped of solvents in vacuo to give 12.0 g of impure 2,3-diphenyl- 1,3-butadiene as an o i l . It was used i n the procedure described below without further pur i f icat ion. (d) A l l of the o i l from part (c) and 3.0 g (0.03 mole) of p- benzoquinone were dissolved in 80 ml of benzene and refluxed for 18 hours. The mixture was cooled to room temperature and stripped of solvent. Hexane was added in small portions u n t i l precipitat ion had ceased. The precipitate was f i l tered and washed twice with cold hexane. It was dried i n a i r to give 3.9 g of compound 4 as a greenish-yellow so l id (45% y ie ld based on p-benzoquinone). It was crys ta l l i zed twice from acetone-hexane to give pale yellow needles. Mp 1 6 3 - 1 6 4 . 5 ° ( l i t . 9 7 ° 1 6 3 ° ) . Ir (KBr), 5.92 and 5.95 (C=0), 6.24 (conj. C=C) y. NMR (CDC13) 6, 7.10 (s, 10H, aromatic), 6.68 (s, 2H, C2 and C3 v i n y l s ) , 3.47 (m, 2H, C4a and C8a methines), 2.77 (m, 4H, C5 + C8 methylenes). Irradiation of 4 i n KBr The i r rad ia t ion , X >_ 340 nm, of a 0.4% KBr pel let of 4_ for twenty minutes led to i t s complete depletion as shown by the disappearance of a moderately intense peak (^46% of the absorbance of the C=0 peak) at 11.52y. - 182 - In the carbonyl region, the,5.92 and 5.95 y absorptions due to 4̂  had been replaced by a s i n g l e C=0 absorption at 5.97y. In a d d i t i o n , a broad but d i s t i n c t OH absorption at 2.91y was noted. The i r recorded a f t e r f u r t h e r i r r a d i a t i o n i n d i c a t e d a new product formation a f t e r a t o t a l i r r a d i a t i o n time of 0.5 hour. This new product had a 5.72y C=0. It increased i n i n t e n s i t y at the expense of the 5.97y C=0 absorption of the primary product t i l l i t eventually became the most intense of the two carbonyl peaks a f t e r 6 hours. I r r a d i a t i o n of A i n the S o l i d State C r y s t a l s of compound 4̂  (50.0 mg) were i r r a d i a t e d at 0.05 t o r r between -10.4° and -9.8° f o r 15 minutes. 94% of m a t e r i a l was recovered. Preparative TLC ( s i l i c a g e l ; 15% e t h y l acetate-, benzene) of the mixture gave 41.3 mg of r e s i d u a l 4̂  and 5.8 mg of the product 4A. The extent of r e a c t i o n , based on recovered s t a r t i n g m a t e r i a l , was 17% and the y i e l d of product 67%. In another run, ,136.7 mg of h_ was i r r a d i a t e d at 0.02 t o r r between 18.5° and 22.0° for 2 hours. Preparative TLC as before gave 46.4 mg of r e s i d u a l 4̂  7.1 mg of 4B_ and 41.2 mg of 4A. The y i e l d s of 4B and 4A were, thus, 8% and 47% r e s p e c t i v e l y . L a s t l y , i r r a d i a t i o n of 83.0 mg of 4_ at 0.01 t o r r between 17.3° and 18.5° f o r 11 hours followed by preparative TLC gave 6.2 mg of r e s i d u a l 4̂, 11.7 mg of 4B and 25.7 mg of 4A. The y i e l d s of these - 183 - two products were, t h e r e f o r e , 15% and 33% r e s p e c t i v e l y . ] The products 4A and 4B were c r y s t a l l i z e d from acetone- hexane; 4A was w h i t e , f e a t h e r y needles. Mp 189.5°-190.0°. I r (KBr), 2.92 (OH), 5.97 (C=0) y. NMR (CDC1 3) 6, 7.17 (m, 10H, p h e n y l s ) , 6.44 (d, J=10 Hz, IH, C2 v i n y l ) , 6.30 (d, J=3 Hz, IH, C9 v i n y l ) , 6.01 (dd, J=10 and 2 Hz, IH, C3 v i n y l ) , 3.45 (d, J=3 Hz, IH, CIO methine), 2.73 ( s , IH, disappears on adding I^O, OH), 2.65 (m, 2H, C6 methylenes), 2.13 (dd, J=14 and 9 Hz, IH, C5 methine). Mass spectrum m/e parent 314. Anal. Calcd. f o r C__H 1 o0_: C, 84.05; H, 5.77. Found : C, 84.06; H, 5.87. Product 4B c r y s t a l l i z e d as c o l o r l e s s needles. Mp 170.5°-171.5° I r (KBr), 5.73 (C=0) 6.25 ( c o n j . C=C) y. NMR (CDC1 3) 6, 7.23-6.87 (m, 10H, a r o m a t i c ) , 3.23 (m, 2H, C7 and CIO methines), 3.07 (m, 1H-, C5 methine), 2.83 (m, 3H, C l methine and 69 C4 methylenes), c a l c d . 2.49 (dd, J=20 and 5 Hz, IH, C8 exo), c a l c d . 6 9 2.30 (dd, J=20 and 1.5 Hz, C8 endo). Mass spectrum m/e parent 314. Anal. Calcd. f o r C-.H._0o: C, 84.05; H, 5.77. 2.2. l o 2. Found : C, 83.95; H, 5.71. - 1 8 4 - Base-Catalyzed Deuterium Exchange of Ene-Dlone 4B S i x drops of a 2N s o l u t i o n , of KOH i n T)^0 were added t o the NMR sample of 4B. The tube was p e r i o d i c a l l y shaken. A spectrum recorded a f t e r 12 hours showed no changes. However, a f t e r 4.5 days, the 2.496 resonances a t t r i b u t e d to the C8 exo proton had disappeared, the doublet of doublet resonances due to.t h e C8-endo proton-had c o l l a p s e d to a broad s i n g l e t at 2.316 and the s m a l l s p l i t t i n g of the m u l t i p l e t at 3.236 was no longer present. The remainder of the spectrum remained unchanged. P h o t o l y s i s of 4. i n Benzene A s o l u t i o n of 214.9 mg of compound 4_ i n 100 ml of benzene was degassed and i r r a d i a t e d , f o r 3.0 hours. The so l v e n t was removed i n vacuo and the r e s u l t i n g mixture separated by p r e p a r a t i v e TLC ( s i l i c a g e l ; 15% e t h y l acetate-benzene). This gave 31.0 mg of 4B and 85.2 mg of 4A.(Combined y i e l d s = 54%. R a t i o of 4A:4JL =3:1.) In another r u n , 100.0 mg of 4_ i n 100 ml of benzene was degassed and photblysed f o r 3.1 hours. Treatment of t h i s mixture as above gave 8.2 mg of r e s i d u a l 4_, 24.8 mg of 4B_ and.34.1 mg of 4A. (Combined y i e l d of products = 64%. R a t i o of 4A:4B = 3:2.) F i n a l l y , p h o t o l y s i s of a s o l u t i o n of 159.7 mg of 4_ i n 100 ml of benzene f o r 3.9 hours f o l l o w e d by p r e p a r a t i v e TLC gave 57.1 mg of B̂_ and 26.3 mg of 4A. (Combined y i e l d s = 52%. R a t i o of 4A:4B = 1:2.) - 185 - The m.p.'s and s p e c t r a of products 4A and 4B were i d e n t i c a l to those r e p o r t e d f o r the s o l i d s t a t e p h o t o l y s i s products. A n a l y t i c a l Run:- A s o l u t i o n of 23.0 mg of adduct 4, i n 10.ml of benzene was degassed and i r r a d i a t e d . A l i q u o t s were withdrawn at 0.1 hour i n t e r v a l s , and used f o r analyses by TLC ( s i l i c a g e l ; 15% e t h y l acetate-benzene) and i r . A f t e r 0.1 hour of i r r a d i a t i o n , a peak at 8.42u c h a r a c t e r i s t i c of enone-alcohol product, 4A, had begun to develop i n the i r . The presence of 4A was f u r t h e r confirmed by TLC. A l s o i n t h i s i r , a broadening was observed i n the 5.70y r e g i o n i n d i c a t i n g the presence of the ene-dione, 4B. I n the i r of the sample withdrawn a f t e r 0.2 hour of i r r a d i a t i o n , the 2.97u OH s t r e t c h a t t r i b u t a b l e to 4A became apparent. In subsequent s p e c t r a of samples withdrawn a f t e r 0.3, 0.4 and 0.5 hour of i r r a d i a - t i o n r e s p e c t i v e l y , peaks due to 4A and 4B s t e a d i l y i n c r e a s e d . In none of the f i v e samplings was the s t a r t i n g m a t e r i a l , depleted. This was shown by the p e r s i s t e n c e of the moderately i n t e n s e peak at 11.52u due to 4/ ( I t s i n t e n s i t y - 46% the absorbance of the C=0 peak.) This was confirmed by TLC. I r r a d i a t i o n of Photoproduct AA i n the S o l i d S t a t e C r y s t a l s of the enone-alcohol photoproduct, 4A (17.0 mg) were photolysed at 0.05 t o r r between 17.5° and 18.2° f o r 12 hours. I r of the r e s i d u e showed a 5.73y C=0 c h a r a c t e r i s t i c of the ene-dione 4B. _ 186 _ P h o t o l y s i s of Photoproduct 4A 'in Benzene A s o l u t i o n of 24.4 mg of photoproduct 4A i n 20 ml of benzene was degassed and photolysed. The r e a c t i o n was monitored by TLC ( s i l i c a g e l ; 15% e t h y l acetate-benzene). A f t e r 0.6 hour of i r r a d i a t i o n , two spots were detected i n a d d i t i o n to t h a t of 4A. The r e a c t i o n was stopped a f t e r 2.1 hours of t o t a l i r r a d i a t i o n . I r of the crude m i x t u r e showed broad C=0 s t r e t c h e s at 5.65, 5.75 and 5.90 u. The mixture was separated by p r e p a r a t i v e TLC. The uppermost spot had 5.68 and 5.80 u C=0 s t r e t c h e s i n the i r and was assigned s t r u c t u r e 4C. The middle spot had a 5.73u C=0 c h a r a c t e r i s t i c of 4B, and the lower spot had a 2.92u OH and a 5.90u C=0 and was consequently i d e n t i f i e d as r e s i d u a l 4A. I r r a d i a t i o n of Photoproduct 4B i n KBr The i r r a d i a t i o n of a 0.4% KBr p e l l e t of 4JS was f o l l o w e d by i r at h o u r l y i n t e r v a l s up to 11.1 hours of t o t a l i r r a d i a t i o n . There was no change i n the i r . I r r a d i a t i o n of Photoproduct AB_ i n Benzene A s o l u t i o n of 10.0 mg of ene-dione 4B_ i n 10 ml of benzene was degassed and i r r a d i a t e d f o r 2 hours. The i r of the i r r a d i a t e d sample was i d e n t i c a l to that of 4B. - 187 " Preparation of 2,3,6,7-Tetramethyl-4aB,5,8,8aB-tetrahydro-l,4- 98 naphthoquinone , 5_ (a) 2,3-Dimethyl-l,4-benzoquinone. 98a The procedure is that described by Pieser for the preparation of quinones. A two-liter flask having a long neck was set up for steam d i s t i l l a t i o n . The condenser from this flask was connected by means of an adapter to a two-liter, round-bottomed, short-necked flask which served as the receiver. This latter was submerged in a bucket of ice slurry and was equipped with a second condenser clamped i n a v e r t i c a l position. These measures were taken to prevent loss of the volatile quinone. Water was started running through both condensers and the reaction flask disconnected. Dry, well powdered 3-amino-o-xylene sulfate (12.63 g) was introduced into the reaction flask. Concentrated ^SO^ (45 ml) was diluted with 200 ml of 1^0, cooled, and added to the contents of the flask. Powdered Mn02 (20.44 g) was added next and the flask quickly swirled to mix the contents and then immediately connected to the rest of the apparatus. The contents were steam d i s t i l l e d . Occasionally, the condenser between the reaction flask and the receiver became clogged with the quinone and i t became necessary to temporarily stop the water running through this condenser to allow the quinone to d i s t i l l into the receiver. After 1 hour, the d i s t i l l a t i o n was ended. A small amount of the quinone lodged i n the condenser was washed into the receiver with ether. - 188 - An additional 100 ml of ether was added to the quinone and the resulting ethereal solution dried over anhydrous Na^SO^ for a few hours. The ether was removed in vacuo and the o-xyloquinone so produced used i n the next step without further purification. 98b (b) 2,3,6,7-Tetramethyl-4ag,5,8,8af3-tetrahydro-l,4-naphthoquinone - , 5. A l l of the quinone prepared above was dissolved i n 10 ml of ethanol in a 50 ml round-bottomed flask. Three ml of 2,3- dimethyl-l,3-butadiene was added and the mixture refluxed for 8.5 hours. It was subsequently stripped of solvent in vacuo and the resulting solid crystallized from petroleum ether using Norit to decolorize. A further recrystallization gave crystals of _5. Mp 104.0-104.5° ( l i t . 9 8 b 105-106.5°). Ir (KBr), 5.95 and 5.98 (C=0), 6.18 (conj. C=C) y. NMR (CCl^) 6, 3.07 (m, 2H, C4a and C8a methines), 2.13 (m, 4H, C5 and C8 methylenes), 1.97 (s, 6H, C2 and C3 methyls), 1.63 (s, 6H, C6 and C7 methyls). Irradiation of 5_ i n KBr A 0.7% KBr pellet of 5_ was irradiated discontinuously for a total of 12 hours. Ir recorded at hourly intervals showed no new peaks. - 189 - Irradiation of 5_ i n the Solid State Crystals of compound 5_ (35.8 mg) were irradiated at 0.03 torr between 8.0° and 9.3° for a total of 30.3 hours. The recovery was 86%. TLC ( s i l i c a gel; 15% ethyl acetate-benzene) showed a single spot with an Rf identical to that of 5_. The i r and NMR of the irradiated sample were also identical to those of 5_. 99 Preparation of 2>3-Dimethyl-l,4>4aS>9aB-tetrahydro-9>10-anthraquinone , 6, A solution of 8.0 g (0.05 mole) of 1,4-naphthoquinone and 8.0 g (0.1 mole) of 2,3-dimethyl-l,3-butadiene in 100 ml of ethyl alcohol was refluxed for 5 hours. On cooling to room temperature, the mixture s o l i d i f i e d . It was l e f t in the refrigerator overnight. The solid was broken up with a spatula and sucked dry. It was washed three times with 15 ml portions of cold ethanol and dried in vacuo for 3 hours. The yi e l d was 10.9 g (91%). It was crystallized from acetone with Norit to remove colored impurities. This gave 7.7 g of sparkling colorless crystals. During a subsequent recrystallization from acetone, two distinct crystals were formed, one (the major) was colorless rods of the desired adduct, j6; the other was bright yellow plates which darkened on exposure to air for prolonged periods. The i r and NMR of this latter compound suggested i t might be 2,3-dimethyl-l,4-dihydro-9,10-anthraquinone formed by p a r t i a l a i r oxidation of j6. Adduct 6_ had mp . 148.0- 148.5° ( l i t . 150.0°). - 190 - Ir (KBr), 5.92 (C=0), 6.26 (conj. C=C) y. NMR (CDC13) 6, 8.23-7.56 (m, AH, phenyl), 3.38 (m, 2H, C4a and C9a methines), 2.31 (m, 4H, Cl and C4 methylenes), 1.68 (s, 6H, methyls). Irradiation of & in KBr A 0.4% KBr pellet of 6̂  was irradiated (X >_ 340 nm) and the reaction monitored by i r . The i r taken after 26.4 hours of irradiation showed an OH peak at 2.90u. The C=0 absorption of §_ at 5.92u was unchanged but the intensity of the conjugated C=C absorption at 6.26u was reduced relative to the C=0 absorption. Irradiation of 6_ i n the Solid State Crystals of compound b_ (132.4 mg) were irradiated in vacuo (0.02 torr) between 8.3° and 10.0° for a total of 30.8 hours. A pale yellow solid (131.9 mg = 100% recovery) was obtained. Preparative TLC ( s i l i c a gel; 15% ethyl acetate-benzene) "of.the reaction mixture gave 58.3 mg of residual 6̂  and 44.6 mg of product 6A (60% yield). Small amounts (^5% each) of the quirioi and the dihydro analog of 6_ were also isolated. The product, 6A, was washed with hexane, dried and sublimed at 103° and 0.03 torr. Subsequent crystallization from acetone-benzene gave colorless crystals of 6A. 2Aa Mp ^24.5-125.5°, ( l i t . 126;0-126.5°). Ir (KBr), 2.90 (OH), 5.93 (C=0), 6.23 (conj. C=C) y. . NMR (CDC1,) 6, 8.00-7.25 (m, 4H, aromatic), 5.74 (m, IH, C9 v i n y l ) , - 191 - 3.22 (d, J=3 Hz, IH, CIO methine), 2.79 (s, IH, disappears on adding D20, OH), 2.54 (dd, J=8 and 3 Hz, IH, C5 methine), 1.83 (d, J=1.5 Hz, 3H, C8 methyl), 1.58 (dd, J=13 and 3 Hz, IH, C6 endo), 1.34 (dd, J=13 and 3 Hz, IH, C6 exo), 1.01 (s, 3H, C7 methyl). Mass spectrum m/e parent 240. Irradiation of jj i n the Solid State. Calculation of Extent of Reaction Crystals of compound 6_ (161.0 mg) were.:irradiated.at 0.03 torr between 8.1° and 9.5° for 31.5 hours. The amount of material recovered from the reactor was 149.4 mg (=93% recovery). A portion of this mixture (101.0 mg) was analyzed by NMR using 0.4 ml of a stock solution of nitromethane containing 228.3 mg of nitromethane per 10 ml of CDCl^ solution as the internal standard. The NMR integration of the C9 v i n y l of the product 6A was used for the analysis. The calculated extent of reaction was 28.5%. (For method of calcu- lation see under General i n the Experimental Section.) Photolysis of 6. in Benzene A solution containing 116.6 mg of 6̂  in 100 ml of benzene was degassed and irradiated for 30.1 hours. Removal of solvent i n vacuo, l e f t a thick yellow o i l . Preparative TLC of this material gave 39.3 mg of residual 6̂  and 49.7 mg (64% yield), of the alcohol, 6A. The latter was washed with hexane, dried and sublimed at 0.04 torr and 104°. Subsequent cry s t a l l i z a t i o n from acetone-hexane gave colorless 192 - p l a t e s of 6A. ?4a Mp . 125.0°-126.0° ( l i t . a 126.0-126.5°). I r and NMR were i d e n t i c a l to those reported f o r 6A i s o l a t e d i n the s o l i d s t a t e p h o t o l y s i s of 6_. P r e p a r a t i o n of 2,3-Dicyano-l,4-benzoquinone 1 0° Commercial 2,3-dicyanohydroquinone (8.1 g, 0.05 mole) was suspended i n 50 ml of C C l ^ I n a 250 ml f l a s k . N i t r o g e n d i o x i d e (as n i t r o g e n t e t r o x i d e ) was condensed i n a r e c e i v i n g tube cooled by l i q u i d n i t r o g e n . The condensed l i q u i d was added i n s m a l l p o r t i o n s to the suspension of the q u i n o l w i t h s t i r r i n g . The a d d i t i o n was continued u n t i l brown fumes of n i t r o g e n oxides p e r s i s t e d f o r s e v e r a l minutes. S t i r r i n g was continued f o r a f u r t h e r 1 hour a f t e r which excess oxides of n i t r o g e n were removed by a stream of n i t r o g e n gas. The r e s u l t i n g mixture was f i l t e r e d - t o g i v e a y e l l o w s o l i d which was c r y s t a l l i z e d from CHCl^-benzene to a f f o r d 6.9 g (87% y i e l d ) of golden c r y s t a l s of 2,3-dicyano-l,4-benzoquinone. Mp 181-182° ( l i t . 1 0 0 178-180°). P r e p a r a t i o n of 4ag,8ag-Dicyano-6,7-dimethy 1-43.6,5,8,8ag-tetrahydro- 1,4-naphthoquinone 1 0 1, J_ C r y s t a l s of 2,3-dicyano-l,4-benzoquinone (4.0 g, 0.025 mole) and 100 ml of a s o l v e n t mixture of acetone-benzene (1:1 V/V) were introduced i n t o a 250 ml three-necked f l a s k equipped w i t h a - 193 - condenser. The mixture was warmed and m a g n e t i c a l l y s t i r r e d u n t i l the quinone d i s s o l v e d . The s o l u t i o n was cooled t o room temperature and 3.7 g (0.05 mole) of 2,3-dimethyl-l,3-butadiene added. The. mixture was s t i r r e d a t room temperature f o r 22 hours and then s t r i p p e d of s o l v e n t s and excess diene. This l e f t a t h i c k o i l which p r e c i p i t a t e d upon a d d i t i o n of l i g h t petroleum (30°-60°). The f l a s k was cooled i n i c e f o r 30 minutes and the r e s u l t i n g golden y e l l o w s o l i d was f i l t e r e d ; washed w i t h petroleum-ether (30°-60°) and d r i e d . This gave 5.9 g (98%) of compound _7. I t was c r y s t a l l i z e d twice from acetone-petroleum ether to g i v e l a r g e , s p a r k l i n g , y e l l o w . -plates of 7_. 101 Mp 156.5°-157.5° ( l i t . 157°-158°). I r ( K B r ) , 4.44 (C=N), 5.79 and 5.86 (C=0), 6.24 ( c o n j . C=C) u. NMR (CDC1 3) 6, 6.88 ( s , 2H, C2 and C3 v i n y l ) , 2.71 ( s , 4H, C5 and C8 methylenes), 1.73 ( s , 6H, m e t h y l s ) . P h o t o l y s i s of J_ i n KBr A 0.4% KBr p e l l e t of 1_ was i r r a d i a t e d . A f t e r 0.3 hour of i r r a d i a t i o n , a new broad but i n t e n s e peak was observed at 2.91u (OH). In the c a r b o n y l r e g i o n , the 5.79u peak had decreased r e l a t i v e to the 5.86u peak. A f t e r 0.5 hour of i r r a d i a t i o n , a new a b s o r p t i o n began to develop at 5.62u. This peak increased at the expense of the 2.91 and 5.86 u a b s o r p t i o n s u n t i l i t became the more in t e n s e of the two c a r b o n y l peaks a f t e r a t o t a l of 18 hours of i r r a d i a t i o n . - 194 - I r r a d i a t i o n of Compound X i n the S o l i d State C r y s t a l s of compound 7_ (230.4 mg) were i r r a d i a t e d at 0.01 t o r r between 19.3° and 19.7° f o r 1.5 hours. The r e a c t i o n mixture which was r e t r i e v e d from the r e a c t o r weighed 195.2 mg (85% r e c o v e r y ) . To 22.0 mg of the r e a c t i o n m i x t u r e , 0.2 ml of a stock s o l u t i o n of benzophenone ( i n t e r n a l standard) i n chloroform was added. GLC a n a l y s i s of t h i s m ixture was c a r r i e d out on a 5' x V 10% OV-210 column at 175° and a f l o w r a t e of 160 ml/min. The d e t e c t o r response f a c t o r f o r 7_ under i d e n t i c a l c o n d i t i o n s was determined to be 1.34. The c a l c u l a t e d % c o n v e r s i o n of 1_ from the GLC a n a l y s i s of the r e a c t i o n mixture was 34%. (For method of c a l c u - l a t i o n , see under General i n the Experimental S e c t i o n ) . The r e s t of the r e a c t i o n mixture (172.5 mg) was separated by p r e p a r a t i v e TLC ( s i l i c a g e l ; 10% acetone-chloroform). This gave 109.2 mg of r e s i d u a l 1_ and 57.3 mg of 7A (91% y i e l d ) . The y i e l d s from three other runs were 98%, 94% and 87% at conversions of 19%, 30% and 26% r e s p e c t i v e l y . The product from a l l the runs was combined and c r y s t a l l i z e d from acetone-hexane to g i v e c o l o r l e s s c r y s t a l s of 7A. 2 4 h Mp 195-196° (reported 188-190°). I r (KBr), 2.93 (OH), 4.41 and 4.45 (C=N), 5.87 (C=0) , 6.15 ( c o n j . C=C) y. NMR (CDCl 3-acetone d &) 6, 7.00 (d, J=10 Hz, IH, C2 v i n y l ) , 6.22 (d, J=10 Hz, IH, C3 v i n y l ) , 5.79 (d, J=1.5 Hz, IH, C9 v i n y l ) , 2.80 ( s , IH, - 195' - OH, disappears on adding D 2 0 ) , 2.16 (d, J=13.5 Hz, IH, one of the C6 methylenes), 1.92 (d, J=13.5 Hz, IH, the other C6 methylene), 1.91 (d, J=1.5 Hz, 3H, C8 m e t h y l ) , 1.21 ( s , 3H, C7 m e t h y l ) . Mass spectrum m/e parent 240. A n a l . C a l c d . f o r C 1 4 H 1 2 N 2 ° 2 : C ' e 9'"> H» 5 - 0 4 ; N» H-66. Found : C, 69.99; H, 5.00; N, 11.54. I r r a d i a t i o n of Product 7A i n KBr The i r r a d i a t i o n of a 0-4% KBr p e l l e t of 7A f o r 2.5 hours l e d to new ca r b o n y l a b s o r p t i o n s at 5.61 and 5.76 u. A f t e r 8.4 hours, the 5.61p C=0 was the most i n t e n s e of the three carbonyl peaks. I r r a d i a t i o n of J_ i n Benzene A s o l u t i o n of 242.0 mg of compound 7_ i n 150 ml of benzene was degassed and photolysed f o r 0.5 hour. P r e p a r a t i v e TLC ( s i l i c a g e l ; 10% acetone-chloroform) gave 121.4 mg of r e s i d u a l 1_, 89.8 mg of product 7A (74% y i e l d ) and 8.3 mg of compound 7B (7% y i e l d ) . The mp and s p e c t r a of 7A were i d e n t i c a l t o those of.7A i s o l a t e d from the s o l i d s t a t e p h o t o l y s i s of ]_. Compound 7_B had m.p. 156-158°. I r ( K B r ) , 4.44 and 4.46 (C=N); 5.90 (C=0) u. NMR (CDC1 3) 6, 7.44 (d, J=10 H z , l H , C2 v i n y l ) , 6.41 (d, J=10 Hz, IH, C3 v i n y l ) , 2.56 ( s , 2H, C6 or" C9 methylenes), 2.45 (d, J=15 Hz, IH, one of theC9 orC6 methylenes), 1.72 (d, J=15 Hz, the other C9 or C6 methylene), 1.5.6 ( s , 3H, C8 m e t h y l ) , 1.38 ( s , 3H, C7 methyl). - 196 - Mass spectrum m/e parent 240. A possible structure for J7B_ i s discussed in the text. Preparation of 4ag-8ag-Dicyano-5a,8a-dimethyl-4ag,5,8 ,8ag-tetrahydro- 1,4-naphthoquinone^^*, jj A solution of 1.5 g (0.009 mole) of 2,3-dicyano-l,4- benzoquinone was dissolved in 60 ml of benzene-acetone (1:1) and 1.5 g of trans,trans-2,4-hexadiene added. The mixture was magnetically stirred at room temperature for 21 hours. The resulting yellow solution was stripped of solvents and residual diene. The solid was crystallized from acetone-hexane to give 1.4 g (65% yield) of sparkling, pale yellow crystals. 2 Ah Mp 153°-154° ( l i t . 155°-156°). Ir . (KBr), 4.45 (CsN), 5.78 and 5.88 (C=0), 6.24 (conj. C=C) y. NMR (CDC13) 6, 6.94 (s, 2H, C2 and C3 vinyls), 5.62 (s, 2H, C6 and C7 v i n y l s ) , 3.09 (q, J=8 Hz, 2H, C5 and C8 methines), 1.29 (d, J=8 Hz, 6H, C5 and C6 methyls). Irradiation of B_ in KBr A 0.4% KBr pellet of J3_ was irradiated. The i r taken after 1.3 and 4.3 hours, respectively showed no new peaks. However, after 24 hours of total irradiation, the 5.87y C=0 had grown i n intensity relative to the 5.78y C=0. - 197- - A. Irradiation of Adduct SL i n the Solid State Crystals of adduct 8^ (172.0 mg) were photolysed discontinuously at 0.02 torr between 7.2° and 8.3° for a total of 35 hours. Recovery of material from the reaction stage was 166.1 mg (97%). Preparative TLC ( s i l i c a gel, 30% ethyl acetate- benzene) gave 130.3 mg of residual JJ, and 27.0 mg (75% yield) of a thick o i l which s o l i d i f i e d on standing. The yields from two other runs were 74% and 65% respectively. The product from a l l runs was combined and crystallized twice from ether-petroleum ether with a few drops of acetone. The resulting colorless crystals, 8A, had 24b mp 138-139° ( l i t . 137.5°-139°). Ir (KBr), 4.45 (C=N), 5.90 (C=0), 6.28 (conj. C=C) y. NMR (CDC13) 6, 7.54 (d, J=10 Hz, IH, C2 v i n y l ) , 6.27 (d, J=10 Hz, IH, C3 v i n y l ) , 4.77 (d, J=3.5 Hz, IH, C8 methine), 3.18 (d, J=3.5 Hz, IH, C7 methine), 2.88 (q, J=7 Hz, IH, C6 or C9 methine), 2.64 (q, J=8 Hz, IH, C6 or C9 methine), 1.39 (d, J=7 Hz, 3H, C6 or C9 methyl) 0.94 (d, J=8 Hz, 3H, C6 or C9 methyl). Mass spectrum m/e parent 240. \ B. Irradiation of 8. i n the Solid State and Determination of the Extent of Reaction Crystals of J3 (147.5 mg) were irradiated discontinuously as in A above at a vacuum of 0.03 torr and temperatures between 7.5° and 9.0° for a total of 31 hours. (The reactor was positioned - 198 - at the same distance from the lamp as in A). The recovery of material from the reactor was 94%. A 0.3 ml aliquot of a stock solution of nitromethane containing 228.3 mg of CH^M^ per 10 ml of deuterochloroform solution was added to 32.5 mg of the reaction mixture. This mixture was analysed quantitatively by NMR using the integrations of the C2 and C3 Hs of 8̂  at 6.946 relative to those of the methyl resonance of nitromethane, the internal standard. Using the method outlined under General i n the Experimental Section, the residual E[ i n the 32.5 mg of mixture was found to be 26.0 mg. The extent of reaction was thus 20%. To check the accuracy of the method, 33.2 mg of authentic (3 and 0.3 ml of the internal standard stock solution were analysed as above. The amount of j8 as calculated from the NMR analysis was 33.1 mg. The accuracy of this method was thus >99%. Irradiation of Adduct jj i n Benzene i - A solution containing 131.7 mg of compound 8̂  in 100 ml of benzene was degassed and photolysed, discontinuously for 3 hours. The reaction was followed by TLC ( s i l i c a gel; 30% ethyl acetate- benzene). ' The irradiated solution was concentrated and subjected to column chromatography ( s i l i c a gel; 30% ethyl acetate-benzene). Combination of the f i r s t six fractions followed by removal of solvent gave 20.0 mg of residual 8̂. Fractions 7-14 gave 67.8 mg (60% yield) - 199 - of a pale yellow o i l , 8A, which was c r y s t a l l i z e d twice from ether- petroleum ether with a few drops of acetone. 24h Mp 141.5°-142.5° ( l i t . 137.5°-139°). I t s i r and NMR were i d e n t i c a l to those of s o l i d s tate product 8A. Preparation of 2,3,4a&6,7,8ag-Hexamethyl-4a,5,8,8a-tetrahydro-l,4- . „. . 102 Q naphthoquinone , 9_ A mixture of 2.0 g (0.024 mole) of 2,3-dimethyl-l,3- butadiene, 1.61 g (0.098 mole) of duroquinone and a few c r y s t a l s of hydroquinone were heated i n a sealed Pyrex tube at 197° f o r 23 hours. The contents of the tube were washed out with chloroform. Subsequent removal of the solvent and excess diene l e f t a s o l i d which was c r y s t a l l i z e d t h r i c e from petroleum ether (68°). This gave 1.36 g (56% y i e l d ) of large, pale-yellow c r y s t a l s of 9/. 10? Mp 113-114° ( l i t . 115-117°). I r (KBr), 5.97 (C=0), 6.15 (conj. C=C) y. NMR (CC1 4) 6, 2.70-1.50 (m, 4H, C5 and C8 methylenes), 1.86 (s, 6H, C6 and C7 methyls), 1.60 (s, 6H, C2-and C3 methyls), 1.10 (s, 6H, C9 and C10 methyls). I r r a d i a t i o n of 9_ i n KBr A 0.4% KBr p e l l e t of 9^was i r r a d i a t e d and the r e a c t i o n followed by i r . A f t e r 3 hours of i r r a d i a t i o n , the spectrum showed a new broad peak at 2.90y i n d i c a t i v e of OH and two new carbonyl absorptions at 5.64 and 5.83 y r e s p e c t i v e l y . _ 2 0 0 _ Irradiation of 9. l n the Solid State In a series of reactions, 50-80 mg of 9^were irradiated at 0.03-0.05 torr between - 2 6 ° to - 2 3 ° for periods ranging from 1.5 to 24 hours. The recovery of material from the reactor, i n a l l cases was >78%. A l l the reaction mixtures were combined and separated by column chromatography ( s i l i c a ge l ; 8% ethyl acetate- benzene) . The order of elution was 9_, 915, and j)A. Compound 9_ was crysta l l ized from petroleum ether and shown by m.p. i r and NMR to be residual starting material. Product 9B was purif ied by short path vacuum d i s t i l l a t i o n at 0.01 torr and 65-70° using a Kugelrohr. Mp* 2 4 . 5 - 2 6 . 0 ° . Ir (KBr), 5.67, 5.84 (C=0) y. NMR (CC14) 6, 2.44 (q, J=7.5 Hz, IH, C7 methine), 2.02 (m, 3H, C l methine and C4 methylenes), 1.68 (m, 6H, v i n y l methyls), 1.21 (s, 3H, methyl), 1.03 (s, 3H, methyl), 1.03 (d, J=7.5 Hz, 3H, C7 methyl), 0.95 (s, 3H, methyl). Product 9A crysta l l ized from petroleum ether as colorless crystals . Mp 1 0 1 . 0 - 1 0 2 . 0 ° ( l i t . 2 4 a 1 0 1 . 0 - 1 0 2 . 0 ° ) . Ir (KBr), 2.87 (OH), 6.03 (C=0) y. NMR (CCl^) 5, 5.38 (m, IH, C9 v i n y l ) , 2.24 (s, IH, disappears on adding D 2 0, OH), 1.85 (d, J=2 Hz, 3H, C3 methyl), 1.76 (d, J=13 Hz, IH, one of the C6 methylenes), 1.08 (s, 3H, methyl), 0.97 (d, J=13 Hz, IH, the other C6 methylene), 0.85 (s, 3H, methyl), 0.80 (s, 3H, methyl). - 201 ~ The above spectra reported for 9A and JJB were identical to authentic samples prepared by photolysis of adduct 9̂  i n benzene. Authentic Samples of Products i!A and 33. Samples of 9A and £B for comparative purposes and for the determination of the detector response factors (below) were kindly supplied by Mr. J.P. Louwerens to whom the author i s grateful. Irradiation of 9. i n the Solid State. Quantitative GLC Analyses Analyses were carried out on a 5' x V column of 20% DEGS on 60/80 Chromosorb W operated at 150° and 150 ml/min. Retention times were 16.4, 19.0 and 22.0 minutes for 9B_, and 9A respectively. The detector response factors for 9B_, 9_> and 9A were determined to' be 1.5, 1.2 and 1.3 respectively, using a solution, containing weighed amounts of the three compounds and an aliquot.of a stock solution of biphenyl as the internal standard. (For method of calculations, see under General i n the Experimental Section). Crystals of 9_ (49.3 mg) were irradiated between -34.0° and -33.5° for 8 hours. A 0.5 ml aliquot of the stock solution of biphenyl (173.2 mg per 10 ml of benzene solution) was added to the reaction mixture and the mixture analysed by GLC. Results of this and other runs are tabulated in the text under Results and Discussion. - 202 - P h o t o l y s i s of 9_ i n D i e t h y l Ether Below the E u t e c t i c Temperature These r e a c t i o n s were c a r r i e d out i n the apparatus f o r low temperature s o l u t i o n r e a c t i o n s d e s c r i b e d under General i n the Experimental S e c t i o n . A s o l u t i o n c o n t a i n i n g 86.8 mg of £ i n 40 ml of anhydrous d i e t h y l ether was degassed and p h o t o l y s e d , between -31.5° and -29.0° f o r 6.5 hours. The r e a c t i o n m ixture was analyzed by GLC using b i p h e n y l as i n t e r n a l standard. For the r e s u l t s of t h i s and other runs, see t e x t . P r e p a r a t i o n of 2,3,4ag,5a,8a,8ag-Hexamethyl-4ag,5,8,8ag-tetrahydro- 24b N 1,4-naphthoquinone , JLO, A mixture of 1.6 g (9.7 m i l l i m o l e ) of duroquinone, 2.0 g of trans,trans-2,4-hexadiene and a few c r y s t a l s of hydroquinone were heated i n a sealed Pyrex tube at 143° f o r 46 hours. A f t e r c o o l i n g , the s o l i d was washed out w i t h acetone. The mixture was s t r i p p e d of s o l v e n t and approximately 100 ml of petroleum ether added. The mixture was warmed w i t h s w i r l i n g and then f i l t e r e d . The o f f - w h i t e s o l i d p o r t i o n (0.611 g) was sublimed at 0.03 t o r r and 150° and subsequently i d e n t i f i e d by m.p., i r and NMR to be durohydro- quinohe. The f i l t r a t e was concentrated and cooled f i r s t to room temperature and then i n i c e . This gave 250.3 mg of r e s i d u a l duro- quinone. The mother l i q u o r was chromatographed on 30 g of s i l i c a g e l (<0.08 mm) u s i n g benzene as e l u e n t . F r a c t i o n s were checked by GLC - 203 - (7 1 x V column of 20% DEGS on 60/80 Chromosorb W at 170° and 200 ml/ min.)' The fractions containing duroquinone and 10 were concentrated and separated by GLC. This gave 311.3 mg (15% yield) of 10 as a pale yellow o i l . It was crystallized twice from hexane to give ?4h material melting at 52-54° (reported 47-50°). Ir (KBr), 5.91 and 6.00 (C=0), 6.13 (conj. C=0) y. NMR (CDC13) 6, 5.44 (s, 2H, C6 and C7 v i n y l s ) , 2.02 (q, J-8 Hz, C5 and C8 methines), 1.91 (s, C2 and C3 methyls), - 1.27 (s, 6H, C4a and C8a methyls), 1.01 (d, J=8 Hz, 6H, C5 and C8 methyls). Irradiation of 10 i n KBr A 0.4% KBr pellet of 10 was irradiated. The i r of the pellet was recorded at hourly intervals for the f i r s t 2 hours and then at longer intervals up to 17 hours total time of irradiation. There were no changes in the spectrum for the f i r s t 2 hours. However, after 6 hours of total irradiation, broadening in the 5.6-5.78 y region became apparent. The i r recorded after 17 hours of irradiation showed two new carbonyl stretches at 5.70 and 5.76 y. Irradiation of 10 i n the Solid State A solution containing 56.6 mg of 10_ in a cet one-hexane was slowly evaporated on the reaction stage and l e f t overnight. It was observed that no crystallization had occurred. The apparatus was assembled as for a normal run except that the sample was neither - 204; - cooled nor irradiated. It was l e f t under vacuum thus for an hour. The reaction stage was subsequently cooled, down to -9.5° and the sample kept under vacuum and at this temperature for 6 hours. It • was subsequently l e f t under vacuum overnight. This procedure allowed 10 to c r y s t a l l i z e out on the reaction stage. The sample was irradiated at 0.03 torr between -9.9° and -9.1° for a total of 14 hours. The material which was recovered from the reactor weighed 53.4 mg (95% recovery). The irradiated sample was checked by TLC ( s i l i c a gel; 15% ethyl acetate-benzene), GLC (7' x V column of 20% DEGS on 60/80 Chromosorb W, at 170° and 150 ml/min.), i r and NMR. A l l analyses showed the presence of only 10. In a similar procedure 43.2 mg of 10_ evaporated from petroleum ether was irradiated at 0.04 torr and -1.4° to -0.5° for 13 hours. Analyses as above again showed no reaction had occurred. Preparation of 2,3,4ag,56,86,8ag-Hexamethyl-4a,5,8,8a-tetrahydro- l,4-naphthoquinone 2 4k, jy^ A mixture of 3.20 g (19.5 millimoles) of duroquinone, 4 g of trans,trans-2,4-hexadiene and a few crystals of hydroquinone was heated at 185° i n a sealed Pyrex tube for 22.4 hours. the resulting dark-brown mixture was washed out with chloroform. An insoluble solid (1.6 g) was f i l t e r e d and purified by sublimation (150°, 0.03 torr). It had mp 228-229° and was identified by infrared,NMR and - 205 - 84 elemental analysis to be durohydroquinone, mp 233°. The mother liquor was concentrated and subjected to column chromatography on 64 g of neutral alumina act i v i t y grade 1 from M. Woelm. It was eluted f i r s t with benzene and approximately 110 ml collected and shown by GLC ^(5' x V stainless steel, 20% DEGS on 60/80 Chromosorb W; 130°, 180 ml/min) to contain none of the desired Diels-Alder adduct. The eluting solvent was changed to 15% ethyl acetate-benzene and subsequent fractions checked by GLC. The fractions containing adduct 11 were combined and stripped of solvents. The resulting o i l s o l i d i f i e d on standing to give 160 mg (3% yield) of compound 11. It was crystallized twice from petroleum-ether to give pale-yellow ?4h rods of 11 melting at 104-105° ( l i t . 103-104°). Ir (KBr) 5.99 (C=0), 6.12 (conj. C=C) y. NMR (CDC13) 6, 5.42 (s, 2, vinyls ) , 2.85 (q, 3=7 Hz, 2, C5 and C8 methines), 1.97 (s, 6, C2 and C3 methyls), 1.13 (s, 6, C4a and C8a methyls), 0.93 (d, J=7 Hz, 6, C5 and C8 methyls). Irradiation of Compound 11 in KBr A 0.4% KBr pellet of compound LL was irradiated (X >̂  340 nm) and the reaction monitored by infrared. After 0.5 hour, the reaction was complete as judged by the disappearance of a moderately intense peak of 11 at 7.98y. A f a i r l y sharp and intense peak had developed at 2.88y. In addition, there were new carbonyl absorptions at 5.67, 5.85 and 6.05 y, respectively. Further irradiation up to 1.5 hours of - 206 - total i rradiat ion time produced no further changes in the spectrum. A. Irradiatibn of Compound 11 in the Solid State Crystals of compound 11 (56.9 mg) were irradiated (X >_ 355 nm) at 0.005 torr and between -32.4 and - 3 1 . 7 ° for 5.6 hours. The reaction mixture was washed off the reactor 's cavity with chloroform, stripped of solvent and dried in vacuo. The recovered material weighed 49.2 mg (86% recovery). A small amount of this mixture (5 mg) was used i n GLC analysis (5' x V stainless steel column of 20% DEGS on A/W Chromosorb W; 60/80 mesh) at 1 5 0 ° and 100 ml/min. It showed two peaks with retention times of approximately 16 and 22 minutes, respectively. The rest of the reaction mixture (44.2 mg) was subjected to preparative TLC ( s i l i c a gel ; 8% ethyl acetate-benzene). This gave 11.6 mg of a white sol id (lower band) and 25.9 mg of an o i l . The infrared and NMR spectra of these two samples showed that the sol id was enone-alcohol 11A and the o i l ene-dione 11B. The start ing material 11 which has the same Rf (TLC) and the same GLC retention time as product 11B was found to be absent from the NMR sample of 11B. The conversion of starting material to photoproducts i s thus complete. The combined isolated y ie ld of the two products i s 85% and the 11A:11B ra t io i s 1:2. In another run, 70.4 mg of crystals of adduct 11 were irradiated at 0.005 torr and between - 3 1 . 6 ° and - 2 7 . 3 ° for 5 hours. The recovered material from the reactor weighed 60.8 mg (86% recovery). - 207 - A portion of this material (29-0 mg) was set aside for GLC analysis. The rest of the reaction mixture was separated by preparative TLC ( s i l i c a gel ; 8% ethyl acetate-benzene) and shown by NMR to contain only photoproducts 11A and 11B. An aliquot (0.2 ml) of a stock solution of internal stan- dard (173.2 mg of biphenyl in 10 ml of benzene) was added to the 29.0 mg of reaction mixture and the result ing solution analyzed by GLC (5' x V column of 20% DEGS at 150° and 150 ml/min) . The peak areas for biphenyl,11B and 11A were calculated for each of 4 injections. The re la t ive areas (area of sample peak/area of biphenyl peak) were calculated and the average re lat ive area for each of the two peaks found. The detector response factors for 11B and 11A under identical GLC conditions were 1.9 and 1.7, respectively. Using these values, the average relat ive areas and the weight of internal standard in the GLC mixture, the weight of each of the two products was calculated as outlined under General in the Experimental Section. This analysis showed, that the 29.0 mg sample of reaction mixture contained 8.4 mg of enone-alcohol 11A and 16.7 mg of ene-dione 11B. The combined GLC y ie ld of the two products i s thus 86% and the 11A:11B rat io i s 1:2. B. Low Conversions of Adduct JLL Since the starting material 11 and one of i t s photoproducts, namely 11B could not be separated by GLC or TLC under the conditions - 208 - t r i ed , the analyses of reaction mixtures containing 11, 11A and 11B were carried out as f o l l o w s : the reaction mixture was f i r s t separated . by TLC into two fractions, one containing solely 11A and the other containing product 11B and residual start ing material 11. The LI + 11B mixture was then analyzed by quantitative NMR using the integrated peak area of the C2 and C3 methyl resonance of jLl at 61.97 and the peak area of the resonances of added biphenyl as internal standard. This analysis allowed.for the calculation of the weight of residual 11 i n the reaction mixture. The yie ld of photoproduct 11B i s then eas i ly found by subtracting the calculated weight of residual 11 from the 11̂  + 11B mixture. Below i s one of the low conversion runs which was analyzed by this method. Crystals of compound 11 (53.6 mg) were irradiated at 0.005 torr between - 3 2 . 1 ° and - 3 1 . 5 ° for 2 hours. The weight of recovered reaction mixture from the reactor was 49.1 mg (92% recovery). Preparative TLC ( s i l i c a gel ; 8% ethyl acetate-benzene) gave 6.4 mg of enone-alcohol 11A and an upper band material of 29.5 mg. Crystal l ized biphenyl (8.4 mg) was added to the upper band material to serve as the internal standard. The mixture was dissolved i n chlorof orm-d and analyzed quantitatively by NMR. The ten protons of biphenyl integrated for 39.5 and the six protons of 11 integrated for 26. . Using the formula given under General i n the Experimental Section, the residual starting material 11 i n the sample was calculated to be 15.8 mg. The conversion of 11 to products i s thus , 56% and the weight of product 11B i s 13.7 mg. The combined y ie ld - 209 - of products 11A and JIB by this analysis i s 81% and the l l A t l l B rat io i s again 1:2. The results of similar runs are given in the text. The enone-alcohol product from a l l the runs was combined and crysta l l ized from petroleum-ether to give colorless rods of 11A melting at 1 5 8 - 1 5 9 ° ( l i t . 2 4 b 1 5 6 . 5 - 1 5 7 ° C ) . Ir (KBr) 2.94 (OH), 6.04 (C=0) and 6.15 (conj. C=C) y. NMR (CDC10) 6, 5.92 (m, 1, v i n y l ) , 2.63 (dd, J , =3 Hz, „=3 Hz, o o, / / ,o 1, C7 methine), 2.57 (s, 1, OH, disappears on adding D 2 0). 2.23 (m, 1, C6 methine), 1.91 (d, J<2 Hz, 3, C3 methyl), 1.82 (d, J<2 Hz, 3, C2 methyl), 1.78 (d, J<_2 Hz, 3, C9 methyl), 0.87 (s, 3, C5 methyl), ' 0.77 (s, CIO methyl), 0.75 (d, J=7 Hz, C6 methyl). Mass spectrum m/e parent 246. The above spectra were ident ica l to those of 11A isolated from the 24b photolysis of adduct 11 in benzene A l l the upper band materials from preparative TLC were combined and photolysed for 0.3 hours to photolyse residual 11. The resulting mixture was separated by preparative TLC (8% ethyl acetate- benzene) . The o i l (upper band material) was assigned the structure 11B based on the following spectra data: Ir (Film), 5.64 and 5.83 (C=0) y. NMR (CDC13) 6, 6.04 (dd, J 2 3 = 1 0 Hz, J 3 4=5.5 Hz, 1, C3 v i n y l ) , 5.59 (dd, J =10 Hz, J ,=1.5 Hz, 1, C2 v i n y l ) , 2.82 (q, J=7.5 Hz, 1, C7 methine), 2.26 (m, 1, C4 methine), 1.15 (s, C8 methyl), 1.15 (d, J=7.5 Hz, C7 methyl), 1.13 (s, Cl methyl), 1.11 (d, J=7.5 Hz, C4 methyl), 1.09 (s, methyl), 1.04 (s, methyl). - 210 - Mass spectrum m/e parent 246. 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Fourier Transform 100 MHz PMR Spectrum of 6,7,16,17- Tetramethylpentacyclo [ 10.8.0.0 2» 1 1.0 4 >9.01** > 1 9 ] eicosa- 6,16-dien-3,10,13,20-tetrone, 2A. 223 Figure 37. A 60 MHz PMR Spectrum of 4af3,5,8,8a3-Tetrahydro-l,4- naphthoquinone, 3_. 2A. residual C tJCt, 1L. • * ^ . C D C L i 3 impurity * * — noise spike see Figure 55/ 2 THS 0 J •H-H-f- Figure 38. Fourier Transform 100 MHz PMR Spectrum of Pentacyclo- [10.8.0.0 2» 1 1.0 4;» 9.0 1 4» 1 9]eicosa-6,16-diene-3,10,13,20- tetrone, 3A. - 224 - Figure 39- A 60 MHz PMR Spectrum of 6,7-Diphenyl-4aB,5,8,8ag- tetrahydro-1,4-naphthoquinone, 4̂ Figure 40. A 100 MHz PMR Spectrum of l-Hydroxy-7,8-diphenyltricyclo- [5.3.0.0 5» 10]deca-2,8-dien-4-one, 4A. Figure 41. A 60 MHz PMR Spectrum of 2,3,6,7-Tetramethyl-4aB,5,8,8a6- tetrahydro-1,4-naphthoquinone, 5_. Figure 43. A 100 MHz PMR Spectrum of l-Hydroxy-2,3-benzo-7,8- dimethyltricyclo[5.3.0.05>10]deca_8-ene-4^-one, 6A. - 227 - Figure 44. A 60 MHz PMR Spectrum of 4a3,8a3-Dicyano-6,7-dimethyl- 4a8,5,8, 8ag-tetrahydro-l,4-naphthoquinone, 7_. Figure 45. A 100 MHz PMR Spectrum of l-Hydroxy-5,10-dicyano-7,8- dimethyltricyclo[5.3.0.0 5 ' 1 0 ]deca-2,8-dien-4-one, 7A. Figure 47. A 100 MHz PMR. Spectrum of 5,10-Dicyano-6,9-dimethyl-ll- oxatetracyclo[6.2.1. 0^»7.0^»10]undec-2-ene-4-one, 8A. - 229 - Figure 48. A 60 MHz PMR Spectrum of 2,3,4a3,6,7,8aB-Hexamethyl- 4ag,5,8,8aB-tetrahydro-l,4-naphthoquinone, 9. Figure 49/ A 100 MHz PMR Spectrum of l-Hydroxy-2,3,5,7,8,10- hexamethyltricyclo[5.3.0.05»10]deca-2,8-dien-4-one, 9A. - 230 - Figure 50. A 100 MHz PMR Spectrum of 2,3,5,7,8,10-Hexamethyl- tricyclo[6.2.0.0 5 >10]deca-2-en-6,9-dione, 9B. - 231 - Figure 51. A 60 MHz PMR Spectrum of 2,3,4ag,5a,8a,8ag-Hexamethyl- 4af3,5,8, 8ag- tetrahydro-1,4-naphthoquinone, 10. - 232 - -Figure 52. A 60 MHz PMR Spectrum of 2,3,4a3,5g,8r3 ,8aB-Hexamethyl- 4ag,5,8,8a8-tetrahydro-l,4-naphthoquinone, 11. - 233 - Figure 54. A 100 MHz PMR Spectrum of 1,4,5,7,8,10-Hexamethyltri- cyclo[6.2.0.05»-L0]deca-2-en-6,9-dione, 11B; (a) 1000 Hz sweep width; (b) 250 Hz sweep width of the 6.5-5.25 6 region with amplitude magnification of xlO; (c) 250 Hz sweep width of the 3.05 - 0.8 6 region. \ - 234 - A SAMPLE CF CDCL residual CHCl 0 * = impurity * * =noise spikes 6 5 TMS Figure 55. Fourier Transform 100 MHz PMR Spectrum of CDC1 from Merck Sharp & Dohme Canada Limited, Kirkland, Quebec. - 235 - UV Absorption Spectra of Substrates 1-11 A l l the tetrahydro-1,4-naphthoquinones which have been studied in this investigation show two principal absorptions in the UV. The absorption due to the H,TI* transition reminiscent of a,$-unsaturated ketones occurs in the range 225 - 280' nm and i s the more intense of the two bands. The absorption at longer wavelength, >_340 nm, i s due mostly to a forbidden n , T i * transition and i t s extinction coefficient for a l l substrates was _<150. The intensity of this latter absorption 30 may be enhanced through mixing with the allowed T r , T r * transition Absorptions for the individual compounds are given in Table XVI. Table XVI* UV Absorption Spectra of Substrates JL-11 Compound Band 1 Band II 1 (benzene) A ,nm(e) max 371 (68) 2 (MeOH) 226 (8.7xl0 3) 370 (70) 3 (benzene) 280 (7.5xl0 3) ^ 370 (63) h_ (MeOH) 280 (7.5x103) 342 (72) _5 (MeOH) 225 (9.4xl0 3) 330 (115) 6_ (MeOH) 250 296 ( l . l x l O 4 ) (1.9xl0 3) 340 (150) 7_ (MeOH) 225 (8.9xl0 3) 340 (93) 8 (MeOH) 240 (6.2xl0 3) 352 (64) 9_ (MeOH) 251 (l . l x l O 4 ) 280-400 (e340=146) 10 (benzene) 350 (83) 11 (MeOH) 251 (8.7xl0 3) 340 (70) * (Compiled from Reference 24). - 236 - As mentioned i n the text, the irradiations of these substrates in the sol i d state were carried out at the same wavelengths as 24 reported for the irradiations i n solution , i.e., X >̂  340 nm. This has allowed for a more valid comparison of reactivity differences in the two phases than would have been possible otherwise. With 22 regard to earlier investigations of substrate _3 using sunlight and 23 Pyrex-filtered UV li g h t , respectively, the results presented here would seem to indicate a wavelength dependence for these reactions. 22 23 Thus, i t i s very l i k e l y that tar formation as earlier observed ' is promoted by excitation of a l l the chromophores while a more selective excitation (mostly n , T r * ) leads cleanly to dimer formation when intermolecular separation and geometry favor i t or to intramolecular processes when the former process i s blocked.

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