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UBC Theses and Dissertations

Stability relationships for cis-trans olefin pairs Page, Brian Denis 1970

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STABILITY RELATIONSHIPS FOR CIS-TRANS OLEFIN PAIRS by BRIAN DENIS PAGE B.Sc. (Hons.)* University of British Columbia, 1963 A THESIS SUBMITTED IN PARTIAL FULFILMENT 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 October, 1970 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r equ i r emen t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y pu rposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l owed w i thou t my w r i t t e n p e r m i s s i o n . Depa rtment The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada - i i -TO JAN ABSTRACT An i n v e s t i g a t i o n was undertaken to examine some of the factors which may a f f e c t the thermodynamic p o s i t i o n of equilibrium f or a number of c i s - t r a n s o l e f i n p a i r s . The positions of equilibrium were measured for selected aldehydes, ketones, n i t r i l e s and propenyl ethers. For these unsaturated compounds i t has been found that t h e i r p o s i t i o n of equilibrium may be a t t r i b u t e d to four d i f f e r e n t f a c t o r s . These factors are: i ) a s t e r i c f a c t o r involving van der Waals repulsion between c i s groups; i i ) a polar repulsive term involving e l e c t r o s t a t i c repulsion between polar groups; i i i ) an e f f e c t termed d i f f e r e n t i a l resonance s t a b i l i z a t i o n , which r e s u l t s from a conjugative s t a b i l i z a -t i o n by resonance which i s greater i n the trans isomer than i n the c i s isomer, where s t e r i c d i s t o r t i o n from coplanarity of the carbonyl group and the double bond i s present; and iv) an a t t r a c t i v e i n t e r a c t i o n a r i s i n g between c i s substituents. The previously unreported Z isomers of crotonaldehyde and tiglaldehyde have been prepared as a mixture with the E_ isomers by photoisomerization of the E isomers. The presence of the Z isomers was r e a d i l y detected by n.m.r. spectroscopy. Several g-halo a,3-unsaturated aldehydes have been prepared. The d i f f e r e n t i a l resonance s t a b i l i z a t i o n , which res u l t e d i n >98% of the E isomer i n the crotonaldehyde and tiglaldehyde equilibrium, was found to be p a r t i a l l y compensated by an additional halogen-formyl polar repulsive term. The replacement of the 3-methyl group i n - i v -8-chlorocrotonaldehyde by a t e r t - b u t y l group increased the d i f f e r e n t i a l resonance s t a b i l i z a t i o n e f f e c t such that the presence of the E_ isomer i n the equilibrium was not detected. The e f f e c t of t h i s t e r t - b u t y l group i s p a r t i a l l y compensated by.the-replacement of the 3-halogen'by methoxyl. The equilibrium positions f o r the series of aldehydes were compared to those of the corresponding ketones and esters. The p r e f e r e n t i a l s t a b i l i z a t i o n of a more polar isomer of a given isomer p a i r by a polar solvent has been investigated f o r several n i t r i l e s and h a l o o l e f i n s . The magnitude of the solvent s t a b i l i z a t i o n e f f e c t i n the above compounds was found to depend on the difference i n p o l a r i t y of the two solvents and the difference i n p o l a r i t y of the isomeric p a i r . With the above information, i t was possible to extend the work of Gardner and McGreer on the a d d i t i v i t y of free energy terms (m u l t i p l i c a -t i o n of equilibrium r a t i o s ) to predict o l e f i n i c e q u i l i b r i a . In most cases c o r r e l a t i o n of the equilibrium r a t i o to a non-polar solvent, gave reasonable agreement between the predicted and experimental positions of equilibrium. - V -TABLE OF CONTENTS Page TITLE PAGE i ABSTRACT i i i TABLE OF CONTENTS v LIST OF TABLES ,. v i i LIST OF FIGURES i x ACKNOWLEDGMENTS x INTRODUCTION 1 Determination of Configuration 2 Thermodynamics of cis-trans Equilibrations 6 Factors Affecting Equilibrium Position 7 Methods of Establishing and Measuring Equilibria 11 Examples of Known Equilibrium Positions 13 Free Energy Additivity for Olefin Pairs 19 OBJECT OF THE PRESENT WORK 24 RESULTS AND DISCUSSION 26 PREPARATION OF a,3-UNSATURATED OLEFINS 26 The Synthesis of Aldehydes 26 The Synthesis of Methyl Ketones 44 The Synthesis of Nitri l e s 50 The Synthesis of Propenyl Ethers 60 EQUILIBRATION OF PREPARED OLEFINS 61 SOLVENT STUDIES 74 ERRORS ' 80 - vi -Page DISCUSSION 81 Photoisomerization of Crotonaldehyde and Tiglaldehyde ... 81 Factors Affecting Equilibrium Positions 84 Nuclear Magnetic Resonance Spectra 98 Solvent Effects on Equilibrium. Positions 103 Free Energy Additivity of Olefin Pairs 105 EXPERIMENTAL 124 GENERAL STATEMENT 124 ALDEHYDES 128 METHYL KETONES 145 NITRILES 153 PROPENYL ETHERS 164 SOLVENT EFFECTS ON THE EQUILIBRIUM 167 BIBLIOGRAPHY 174 - v i i -LIST OF TABLES Table Page 1 Percent composition of E_ isomer at equilibrium from reference (11) 17 2 Equilibrium positions for various olefins .. 21 3 Aldehydes prepared for equilibrium determination 27 4 Chemical shifts and coupling constants for the a,3-Unsaturated aldehydes 29 5 Photoisomerization of crotonaldehyde, (E-) (5) and tiglaldehyde, (E-)(6) 32 6 Isomeric aldehydes prepared by the method of Arnold 39 7 Methyl ketones prepared for equilibrium determination 45 8 Chemical shifts and coupling constants for the a,g-unsaturated methyl ketones 46 9 Nit r i l e s prepared for equilibrium determination 51 10 Chemical shifts and coupling constants for the a,g-unsaturated n i t r i l e s 52 11 Chemical shifts and coupling constants for propenyl ethers .. 62 12 Equilibrium determination of some a,g-unsaturated aldehydes 63 13 Equilibrium determination of some a,g-unsaturated methyl ketones ..... 69 14 Equilibrium determination of some a,g-unsaturated n i t r i l e s .. 71 15 Thermal equilibration of 1-thioethoxypropene and 1-methoxypropene 73 - v i i i -Table Page 16 Solvent stabilization factors for crotononitrile, (Z-) and ( E - H 1 7 J 76 17 Solvent stabilization factors for 3-bromoacrylonitrile, (Z-) and CE-)(5£) • ••• 77 18 Solvent stabilization factors for a-bromocrotononitrile, (Z-) and (E-) (55) 77 19 Solvent stabilization factors for 1,2-dibromoethylene, (Z-) and CE-H70) 78 20 Solvent stabilization factors for 1,2-dichloroethylene, (Z-) and (E-)(71) 79 21 Chemical shifts and coupling constants for haloacrylic acids and their corresponding acid chlorides, amides and n i t r i l e s 100 22 Chemical shift differences between Z_ and E_ 3-protons of 3-haloacrylic acid derivatives 102 23 Solvent effects on the equilibrium of some n i t r i l e s and dihaloolefins 106 24 Corrections of equilibrium positions to non-polar solvent . 110 25 Solvents used for equilibration studies 168 26 Compounds used for equilibration studies 170 27 Vapour phase chromatographic data for the isomer pairs used in solvent studies. (10 f t x 0.25 in, 20% FFAP) 173 28 Group moments 180 29 Calculated and literature dipole moments 181 - i x -LIST OF FIGURES Figure Page 1 van der Waals curve 9 2 Reaction diagram for At;B i n solvents 1 and 2 10 3 An example of free energy a d d i t i v i t y i n olefins 20 4 oc-Vinyl protons of crotonaldehyde, (Z-) and (E-) (5) ....... 34 5 g-Vinyl protons of crotonaldehyde, (Z-) and (E-)(5) ....... 35 6 Amine catalyzed rearrangement of $-dialkylamino-3-tert-butyl acroleins . 42 7 Preparative sequence for g-bromoacrylonitrile, Z-54 54 8 E q u i l i b r a t i o n of C i t r a l , (Z-) and (E~) (40) 67 9 S t e r i c interactions i n Z- and E-39 V. 92 10 S t e r i c interactions i n Z-50 93 11 Equilibrium positions f o r 8-tert-butyl aldehydes, ketones, n i t r i l e s and esters 95 12 Equilibrium values of selected aldehydes, ketones and esters • 96 13 Examples of free energy a d d i t i v i t y i n o l e f i n pairs being used to predict d l e f i n i c e q u i l i b r i a ^115 14 Bulb-to-bulb d i s t i l l a t i o n apparatus ...127 - X -ACKNOWLEDGMENTS I wish to express my sincere gratitude to Dr. D i E . McGreer for his constant interest and encouragement throughout the course of this research project. Grateful acknowledgment is also made to the University of British Columbia for the award of a Graduate Fellowship, 1967-1968; and to Standard Oil of British Columbia for a Graduate Fellowship, 1968-1969. INTRODUCTION The possibility of stereoisomerism, or geometrical isomerism, involving differing arrangements of groups about ethylenic double bonds was f i r s t recognized by van't Hoff in 1875 (I). This concept arose from his theory of tetrahedral carbon valencies. By placing two carbon tetrahedra edge to edge, van't Hoff proposed a structure for ethylene involving a coplanar arrangement of a l l six atoms. This model, although basically incorrect, at least indicated the existence of two stable planar arrays which Baeyer (2) designated as cis-trans isomers. The prefix cis described the situation involving two equal groups on the same side of the double bond and the prefix trans indicated that these groups were on opposite sides of the double bond. The configurational descriptors cis and trans, however, have led to a great deal of confusion in the literature (3,4). Recently, a new set of ' ' configurational descriptors, Z_ and E, have been suggested (3) to define the ethylenic stereochemical environment. These descriptors, based on the sequence rule of Cahn> Ingold and Prelog (5); unambiguously define the stereochemistry about a l l double bonds. Briefly, the descriptor _Z (zusammen) refers to a cis (on the same side of the double bond) arrangement of the two groups with the highest priority, and the descriptor E_ (entgegen) refers to a trans (on opposite sides of the double bond) arrangement of these groups. to designate 1,2-interactions, either on the same side of the double bond (cis) or on opposite sides of the double bond (trans). In referring to a general class of compounds, the terms cis and trans may also be used; only i f the assignment is totally unambiguous or defined by a drawn structure. The descriptors Z_ and E w i l l be used exclusively for stereochemical description. Determination of Configuration The determination of the stereochemistry of unsaturated systems may be based on either chemical or physical methods. The early work involving stereochemical assignments was, by necessity, mainly chemical in nature. These chemical methods (6) usually involved reactions between cis substituents, conversions of unknown stereoisomers to compounds of known stereochemistry, and assignments based on kinetic measurements. At the present time, however, modern instrumentation and new spectroscopic techniques have largely supplanted the . In this thesis, therefore, the terms cis and trans w i l l be reserved CIS trans - 3 -e a r l i e r chemical methods. Of the spectroscopic methods available f o r stereochemical determination about double bonds, the methods of major importance to the present-day organic chemist are i n f r a r e d ( i . r . ) spectroscopy and e s p e c i a l l y nuclear magnetic resonance (n.m.r.) spectroscopy. Other methods, such as dipole moment measurements (7) and u l t r a v i o l e t spectroscopy (8) are also used. • The use of i . r . spectroscopy i n the configurational assignment of o l e f i n s i s generally r e s t r i c t e d to the 1,2-disubstituted o l e f i n p a i r s (9). With the t r i -and t e t r a s u b s t i t u t e d o l e f i n s assignments are un r e l i a b l e as there are no absorptions which may be r e a d i l y correlated to structure. Differences i n the i . r . spectra of 1,2-disubstituted isomer p a i r s are mainly observed i n the C=C stretching v i b r a t i o n . Generally speaking, the c i s isomer, shows strong absorption whereas the trans isomer shows l i t t l e or no absorption due to C=C st r e t c h i n g . However, as the i n t e n s i t y of absorption i s roughly proportional to the change i n dipole moment f o r the v i b r a t i o n considered, compounds such as crotonic acid, (Z-) (JL) , show strong absorptions. H H / / \ CH 3 COOH Z-1 _ 4 -When both c i s and trans C=C stretching vibrations are present i n the i . r . spectrum, then generally the ci s isomer absorbs at lower frequencies than the trans isomer. Conjugation of a carbonyl with the double bond generally lowers the C=C stretching frequency by about 30 cm \ The C=C-H out-of-plane bending vibrations are most helpful i n discerning the stereochemistry about a 1,2-disubstituted double bond. The position of the band i n the cis-compounds i s not very clear and cannot be employed e f f e c t i v e l y for structural determinations. The C=C-H out-of-plane bending vibrations i n the trans o l e f i n , however, are most useful i n assigning the stereochemistry as th i s band almost invariably absorbs at 990 to 895 cm \ Conjugative or substituent effects are not pronounced. During the past decade or so the u t i l i z a t i o n of nuclear magnetic resonance (n.m.r.) spectroscopy to assign the configuration of a,8-unsaturated olefins has become increasingly widespread. Information concerning the stereochemistry of not only 1,2-disubstituted, but also t r i - and tetrasubstituted olefins has been made available by n.m.r. spectroscopy (i0) . There are two basic methods by which stereochemistry can be deduced by n.m.r. spectroscopy; namely by shielding or deshielding effects and coupling constants. Probably the most useful information i s derived from diamagnetic anisotropic deshielding effects. Thus a cis B-vinyl proton or a proton on the f i r s t carbon of a 3-alkyl group attached cis to any diamagnetic anisotropic group, such as a carbonyl, w i l l be deshielded with respect to the trans arrangement. A te r t - b u t y l group w i l l also be deshielded i n this arrangement (11). The groups, whose'^d'eshieldThg^ e*f£ects"'"on a "3-vinyl group or proton are considerably greater in the cis than in the trans-configuration are n i t r i l e , acetylene, ketone, carboxyl, ester, amide, acyl halide and aldehyde. Other groups may also shield or deshield 3-vinyl protons or groups. An extensive compilation of these shielding or deshielding effects on.a vinyl proton by any group which is cis, trans or geminal to the proton has been published by Pascual and co-workers (12). They report that chemical shifts may be estimated by empirical additive shielding parameters. The use of such an additive method is obvious; for estimating chemical shifts, and especially in cases where distinction between stereoisomers is being made. The authors point out, however, that there are certain structural features, associated with large discrepancies between calculated and experimental values. Of importance to the stereochemical assignments of a,^-unsaturated olefins are difficulties"• encountered in planar carbonyl compounds (12c). In this respect, criticism of Pascual's attempted correlations has been levied by Rapoport et a l . (13). Information concerning the stereochemistry about double bonds of 1,2-disubstituted olefins is readily accessible from the proton-proton coupling constants of the vinyl hydrogens. Typical values for the trans and cis coupling are 13-18 and 7-12 Hz, respectively.; It has been shown that there is an inverse relationship between cis and trans coupling constants and the electronegativity of the double bond substituents (14,15), That is lower cis and trans coupling constants correlate to higher substituent electronegativity. In a l l cases i t was observed that the trans coupling constant was clearly - 6 -greater than the cis coupling constant for a given compound. Recently, much attention has been given to stereochemical assignments based on the chemical shift of formyl protons in.2-methyl-2-enals ( 1 3 , 1 6 ) . It has been found that the aldehyde proton cis to a vinyl hydrogen, appears at about 0 . 7 5 to 0-.70T and the formyl,proton in the trans orientation appears at about 0 . 0 T „ T t i i s phenomenon is particularly suited to the study of the configuration of natural" products. Thermodynamics-of cis-trans Equilibrations In order to understand some of'the factors affecting an isomeric equilibrium, i t i s perhaps instructive;to consider some of the thermodynamic properties, of the equilibrium state,: This brief introduction is by no means considered to be a rigorous thermodynamic treatment of. the subject. For the isomeric equilibrium A ^ B the thermodynamic equilibrium constant K is given by where [B] / [A] is the equilibrium concentration ratio of A and B. (Ideally, K should.be expressed as the equilibrium partial pressure ratio or as the activity ratio). It'can be shown (17) that AG°, the standard free energy change, is given by the following expression AG° = -RTlnK - 7 -Thus, a determination of the equilibrium position is a measure of the relative values of the standard free energies of isomer A and isomer B. It is important to note that the free energy- term must be evaluated at constant temperature and pressure. With these conditions met, as the' ratio of the isomers approaches the equilibrium value, the total free energy of the system approaches i t s minimum value. The value of AG, by definition,.depends upon two factors,-AH, the enthalpy difference between the two isomers; and AS, the entropy difference between the: two isomers. AG =. AH - TAS The free energy term, is temperature dependent and changes in the equilibrium position can be expected i f AS ? 0. Such entropy:differences w i l l presumably be relatively unimportant for olefins with spherically symmetrical groups, such as halogens, or methyls, but w i l l be considerably more important for unsymmetrical groups (18). Factors Affecting Equilibrium Position There are many factors which may affect the equilibrium position in cis-trans olefin pairs. Of these factors, bonding interactions between cis-substituents, when present, usually dominate most equilibria. These bonding interactions include anhydride formation (an extreme case) and the formation of hydrogen bonds. An example of the latter i s found in ethyl 3raminoerotonate, (Z-) and (E-)(2), in which the _ isomer dominates the equilibrium (19). 5> / / — \ H H-N \ r° C2 H5 H-iN \ H H 0 H Z-2 E-2 These bonding interactions, because of their,dominance of the equilibria, do not allow investigation of other effects which may influence, to a much lesser degree, the equilibrium. factor to be considered in dealing with cis-trans s t a b i l i t i e s . This emphasis has probably arisen for a variety of reasons. The main reason may be that the s t a b i l i t i e s of hydrocarbon olefins, stilbenes, diethyl maleate and fumarate, and the cinnamate esters (20) were among the f i r s t cis-trans olefin pairs to be studied. In a l l these cases steric effects dominate. More recently, Viehe (21) has observed that a number of cis -1,2-disubstituted olefins are more stable than their trans isomers. This c i s - s t a b i l i t y may be thought to occur by van der Waals or electrostatic attraction between the groups involved. long distances (r) there exists an attractive force between two atoms which is proportional to (1/r^). This force may be termed an induced dipole-induced dipole interaction, a London force or a dispersion force. The phenomenon involves the polarizabilities and the ionization potentials of the two atoms involved. A short range repulsive term, called the van der Waals repulsive interaction, comprises the second part of the van der Waals function. This force is Traditionally, steric factors have been emphasized as the major The van der Waals function is in theory composed of two parts. At 12 proportional to ( l / r ) and becomes important quite abruptly. It arises from the repulsion between nuclei and electrons of the atoms concerned. Since the repulsive force becomes important quite abruptly and the attractive force increases in importance more gradually; the combination of' the two forces leads to the familiar van der Waals curve. Figure 1 - van der Waals curve Thus, i f the distance between the cis-substituents is just outside the combined van der Waals r a d i i , then an attractive force would result. For ethylenes, the distance between the interacting cis-groups or atoms would be a function of the size of the groups or atoms, the bond angles with the ethylenic carbons and the bond lengths from the double-bond carbon to the groups or atoms in question. Dipolar interactions, involving either electrostatic repulsions or attractions may be a factor involved in isomer sta b i l i t y . The possibility of dipolar interactions arises when polar groups or electronegative atoms such as halogens, n i t r i l e s , carboxyl, carbonyl, etc. are on one carbon and similar groups - 10 -are present on the other. In addition to dipole-dipole interactions, there is also a possibility of dipole-induced dipole interactions in which one group is polar and the other group is polarizable. Another effect which has not been emphasized in cis-trans olefinic equilibria, but has been considered i n other equilibrium situations (22) is that of solvent stabilization of a more polar isomer. Intuitively, the more polar isomer should be stabilized in a more polar solvent; in fact, this polar solvent may be the neat solution of isomers i t s e l f in which the equilibrium is established. One example of this effect is described by Wood and Dickinson (23) who noted that the cis-trans equilibrium constant varied slightly between three non-polar solvents and the neat solution. McMulIen and S t i r l i n g (24) also described solvent effects on the cis-trans equilibrium of various enamines. The differences in equilibrium in polar and non-polar solvents were explained as a balance between intra- and intermolecular hydrogen bonding. The thermodynamic basis of solvent stabilization has been described by Leffler and Grunwald (25). Figure 2 shows the standard partial molar free energies (G^) for a system undergoing isomerization in two solvents where the subscripts A and B refer to the isomers and the subscripts 1 and 2 refer to the different solvents. „—. Figure 2 A , B - Reaction diagram for A^ =B in solvents 1 and 2. 6 is termed the solvent stabilization operator. Figure 2 interprets the m preferential solution of one isomer by a particular solvent (2) with respect to a reference solvent (1). Methods of Establishing and Measuring Equilibria The interconversion or stereomutation of isomers to an equilibrium position may be achieved in a variety of ways. The most straightforward, although not necessarily the best, method of interconversion is by heating. The equilibrium established by heating, however, does not necessarily correspond to the equilibrium established at low temperatures, as the free energy difference between isomers i s , in most cases, temperature dependant. The usual method*of effecting equilibration is by catalysis. Among the many catalytic agents used are the free radical catalysts, such as the oxides of nitrogen or halogens in the presence of light; acids, such as halogen acids, sulfuric acid, p-toluenesulfonic acid and boron trifluoride, and hydrogenation-dehydrogenation catalysts such as platinum and selenium. Many other catalysts may be employed (26). The mechanism of radical catalysis probably involves addition to the double bond to form an adduct radical followed by rotation and departure of the radical. Acid catalysts are particularly useful in isomerization of a,B-unsaturated carbonyl compounds such as diethyl maleate. It is believed that the isomerization involves protonation of the carbonyl groups followed by the formation of a 3-carbcnium ion through which isomerization occurs. (27) - 12 -H C-OC H \ / 2 5 H C - OC H II 0 H L 3 0 H H 9i C - ° C 2 H 5 -H+ H . H C HO — C' 2 5 5 OH C-OC H S 2 5 H C-OC H 6 2 5 OH C-OC H 2 5 'H H C-OC H ^ 2 5 H C-OC H 6 2 5 OH H C-OC H r \ ^ 2 5 C H O-C 2 5 a H Ideally, an equilibrium should be approached from both sides, however, i n some cases e q u i l i b r a t i o n can be achieved from one side only. In t h i s s i t u a t i o n i t i s necessary to ensure that the equilibrium state has been reached, either by graphically p l o t t i n g the approach to the equilibrium value asymptotically or by correlation with s i m i l a r compounds, whose equ i l i b r a t i o n i s achieved by the same method employed i n the same period. The estimation of the equilibrium p o s i t i o n , once achieved, can be carried out by a variety of methods. The most accurate method i s probably v.p.c.j i f tha isomers can be readily separated without isomerization or decomposition. Nuclear magnetic resonance spectroscopy i s also f a i r l y accurate and the equilibrium position i s ea s i l y measured. Some other methods that have been used are i . r . spectroscopy, dipole moment measurements, and refra c t i v e index measurements. - 13 -Examples of Known Equilibrium Positions The preparation of a great number of cis-trans olefin pairs have been reported in the literature.- Unfortunately, in the great majority of these reports no attempt has been made to evaluate the composition of the isomeric mixture. In fact, very few early papers deal at a l l with cis-trans equilibria and fewer s t i l l present arguments attempting to explain the equilibrium position. In the following discussion an attempt w i l l be made to divide the text into classes of compounds. The aldehydes and ketones w i l l be discussed f i r s t followed by esters and then n i t r i l e s . Finally, a group of miscellaneous equilibria w i l l be presented in tabular form. The Z and E isomers of g-chloroacrolein, (3) and methyl g-chlorovinyl ketone, (4.) were prepared by Ivanov et a l . (28). Although no equilibrium values were given, they indicated that the Z_ isomer in both compounds readily isomerized to the E_ isomer. -H H H CHO H H H COCH \ / \ / \ / \ / 3 / \ / — \ / — v / — \ Cl CHO Cl H Cl COCH Cl H 3 Z-3 E-3 Z-4 E-4 Both isomers of 3 and 4 decomposed readily; however, Shapet'ko et a l . noted that the chloroaldehyde, Z-3, completely isomerized to E-3 in 2 h at 22° (29). The preferred s t a b i l i t y of the E_ over the Z_ form in both 3_ and 4_ was interpreted by Ivanov as a destabilizing dipole-dipole interaction between the cis chloro atoms and carbonyl groups; however, they also pointed out that -Im-possible steric interactions could not be excluded. Landini and Montanari (30) later prepared other 8-chlorovinyl ketones by the addition of hydrogen chloride in ether to ethynyl ketones at -40°. Again the E_ isomers were shown to be more stable. The unstable _ isomers were found to isomerize to the trans with a slight excess of acid or with a higher reaction temperature. A study of crotonaldehyde, (Z-) and (E-)(5) and 2-methyl-2-butenal, (Z-) and (E-)(6) (that i s , angelaldehyde and tiglaldehyde, respectively) has been carried out by McGreer and the author (31). These results w i l l be discussed later. CH CHO CH, H CH, CHO CH, CH, 3\ / 3N J 3N / \ / 3 / \ / \ / \ / \ H H H CHO H CH3 H CHO Z-5 E-5 Z-6 E-6 Concurrent with this work, Rapoport and co-workers (13) reported the synthesis of the isomeric 2-methyl-2-pentenoic acids and their corresponding esters, alcohols and aldehydes. They found that the 2-methyl-2-pentenal, (Z-)(7) was unstable and readily isomerized to the E isomer. C^ H,. CHO C_H_ CH, 2 5N / 2 5 \ / 3 / \ / \ H CH3 H CHO - 15 -They also prepared 7_ by the method of Doebner (32), which involves the self condensation of propionaldehyde, and found the product to be entirely E-7. In his report, Rapoport also refers to many naturally occurring a,B-unsaturated aldehydes in which the possibility of acid catalyzed cis to trans isomerization may have occurred. The corresponding ethyl homologs of crotonaldehyde; 2-pentenal, (Z-) and (E-)(8), have been prepared by Thomas and Warburton (33). C H CHO CH. H 25' J 2 \ / / \ / \ H H H CHO Z-8 E-8 They report that the _Z isomer can be isomerized to the E_ isomer by an acid resin or by perchloric acid. In both cases the isomerization was not complete and with the perchloric acid decomposition occurred. With hydrogen chloride in ether isomerization did not occur and instead the saturated chloroaldehyde was produced. Under acidic conditions both Z- and E_-8 gave the E_ 2,4-dinitrophenylhydrazone. Thomas (16a,16b) has also reported on the properties of some 2-methyl-2-enals. In his latter report (16b) he suggests that the isomerization of the _Z isomers of various 2-methyl-2-enals was not as rapid and complete as that suggested by Rapoport. - 16 -For methylcrotonate, (Z-) and ( E - ) ( 9 ) , the equilibrium was found to be approximately 82% in favour of the E_ isomer between 200 and 500° (34). CH CO CH CH H \ / z 5 \ / / \ / \ H H H C02CH3 Z-9 E-9 3-Substituted ethyl crotonates have been found to be more stable i n the _E configuration (35-37) despite the varying steric requirements of the 3-substituent. Cl H PhS H C H 0 H \ / \ / 2 5 N / / \ / \ / \ CH- CO C H CH CO C H CH, CO C H 3 2 2 5 3 2 2 5 3 2 2 5 (CJIJ N H MesS H 2 5 2 \ / \ / / \ / \ CH3 C0 2C 2H 5 CH3 C0 2C 2H 5 Recently. Gardner and McGreer (11) have prepared a series of a,3-unsaturated n i t r i l e s and esters substituted at the 3-position with tert-butyl or isopropyl groups and at the same carbon with methoxyl, thioethoxyl, dimethylamino, diethylamino, chlorine and hydrogen. The equilibrium positions for the various compounds are listed i n Table 1. -17 -TABLE 1 Percent composition of E_ isomer at equilibrium from reference (11) CH 0 H (CH )Ji H Cl H 3 \ / 3 / N f \ / / \ / \ / \ t-Bu CN t-Bu CN t-Bu CN E-10,60% E-11,22.5% E-12,0.5% CH 0 H CHO CH Cl H 3 \ / 3 \ / 3 \ / / \ / \ / \ t-Bu CO C H H CO CH t-Bu CO CH - 2 2 5 2 3 - 2 3 E-13,23% E-23,98.8% E-14,1.5* CH,0 H Cl H Cl H 3\ / \ J \==s / \ / \ / \ i-Pr CO C H CH CO R i-Pr CO CH - 2 2 5 3 2 - 2 3 E-15,97% E-24,87.5% E-16,8.5,13% CH„ H H 3N / / \ CN E-17,42% C H H 2 5 X / / \ H COoR 2 rE-18,87.5% i-Pr H H \ / / \ CO R 2 E-19,90% CH, H J / \ H CO^CH, 2 3 i-Pr H \ / / \ H CN t-Bu H \ / / \ H CN E-20,82% •E-21,33% E-22,98.5% - 18 -The explanation offered to reconcile the equilibrium positions reported i n Table 1 i s based on a balance of three factors. The f i r s t factor to be considered i s steric in nature. Compounds 17, 18, 19, 20, 21, and 22_ demonstrate the effect of only alkyl groups on the 3-carbon. In a l l cases, except when the 8-substituent i s tert-butyl, there i s a significant percentage of the Z_ isomer. Another effect which must be considered is the sensitivity of the carbonyl compounds to a cis-substituent (28,30,31). This effect i s undoubtably due to the requirement that for maximum conjugation the carbonyl group and the double bond must be coplanar, and in this arrangement i t i s very sensitive to cis substituents. The contribution of the zwitterion form, (25), to the st a b i l i t y of carbonyl containing olefins would be expected ... S+ \> ^  V C=C} or C—"L\ / / R R 25_ 25_ to show the following order (38): C0CH3 - CHO > COCl >;C02R > C0NR2 > C0~ Thus, the aldehydes and ketones would show a greater sensitivity to steric inhibition to conjugation. When group X in 25 is electron donating (+M effect), the importance of the zwitterion form i n esters i s increased and the E_ form w i l l be more highly favoured. This effect is readily observed i n compound; 2.3. It can also be seen in compound 10_ that the methoxyl group, when i n conjugation, can be effectively larger than a tert-butyl group. - 19 -Gardner and McGreer also feel a repulsive term exists between methoxyl and n i t r i l e or ester groups. This helps explain the equilibrium position for 13_. A repulsive interaction also is found in 24 as the chlorine, which i s st e r i c a l l y expected to be smaller than methyl, prefers to be trans to the ester. This repulsive term is compensated by the steric effect of the isopropyl group in 16 and completely overcome by that of the tert-butyl group in 14. Gardner and McGreer, of this laboratory, have recently reported the equilibrium positions of a number of B-substituted methacrylonitriles (39). The equilibrium positions of these compounds are shown below. Cl Br CH, CHO CH_ CH, CH, \ / 3 \ / 3 3 \ / 3 3 \ / 3 / \ / \ / \ / \ H CN H CN H CN H CN E-26,60% E-27,60% E-28,78% E-29,20% In this series of compounds, in contrast to Gardner and McGreer's other publication (11), in which steric and conjugative effects were of considerable importance; an important stabilization due to van der Waals or electronic attraction between cis substituents i s suggested. Thus, only in 2_ is the steric factor possibly determining the equilibrium. Free Energy Additivity for Olefin Pairs Gardner and McGreer indicate, that where steric effects are not dominant, one might expect that the additive contribution of two 1,2-disubstituted olefins, with a common substituent, may give the equilibrium - 20 -position of the trisubstituted olefin. The following examples were offered. K H H a H CN \ / . \ / / \ V \ Cl CN Cl H H H ^ H CH \ / , \ / 3 / \ / \ Cl CH- Cl H K H CN c H CH \ / ' \ / 6 / \ ^ / \ Cl CH Cl CN 3 K _ S ?_i? — - — c " K ~ 75.5 31 ~ 58 a Figure 3 - An example of free energy additivity in olefins. In concluding the introduction, i t would be of interest to l i s t some of the equilibrium positions measured of some cis-trans olefin pairs that have not been mentioned earlier. Table 2 displays these values. - 21 -TABLE 2 Equilibrium positions for various olefins Structure % as Temp. State or Reference written solvent 63.4 185 vapour 40 Ci Cl 70.8 150 neat 23 \ / 70.9 150 benzene 23 / \ 68.0 150 cyclohexane 23 H H 68.1 150 decalin 23 CH_ Br 3N / 68 40 neat 41 j \ 80.5 24-26 vapour 42 H H Br Br 50.0 144-178 vapour 43 \ / 56. 50 CC1 44 / \ 4 H H 63 25 neat 45 CH_ Cl 3^ / 75 30-35 neat 46 / \ H. H CH, CN •\ / 57±2 20-35 neat 46 / \ H H - 22 -TABLE 2 (continued) Structure written Temp. State or solvent Reference CH J / \ H OCH. H 49.3 25 neat 47 CH. H CH. r~~\ H 24 37 33 27 26 25 110 127 95 25 vapour 11 CH Cl-CH Cl 2 2 48 49 50 51 52 Cl CN \ / / \ H H 69 30-35 neat 46 CH. H Br J CH. 83±2 r . t . neat 53 CH, H Cl / \ CH. = 80 r . t . pentane 54 CH 2 Br \ / 65 r . t . neat 55 Br H - 23 -An interesting view on cis-trans equilibria has been taken by Viehe (21) who reports the following rule for s t a b i l i t y considerations of cis-trans isomers: "the forces of attraction predominate between substituents, provided H-H interaction (as in pure hydrocarbons) or extreme overcrowding are absent". - 24 -OBJECT OF THE PRESENT WORK In the preceeding examples i t was shown that many factors may influence the equilibrium position of cis-trans olefin pairs. In the ester series, an interplay of steric, polar and conjugative forces determined the equilibrium. In the n i t r i l e s , an additional attractive interaction was also present. It was decided, therefore, to undertake a study of some representative a,3-unsaturated aldehydes to determine how the above factors may influence the position of equilibrium. The starting point for such a study must be the simple 8-substituted acroleins. The B-substituent may be varied to investigate the effects arising from steric, polar or conjugative interaction. To the simple B-substituted aldehydes an a-substituent may be added and the effect observed. With two different 8-substituents a comparison of their relative influence on the equilibrium position can be made. Again the effect of an a-substituent on the equilibrium position of a B ,B-disubstituted aldehyde may be studied. Finally, a B-tert-butyl group may be incorporated into the aldehyde to observe i t s effect in conjuction with strongly conjugating B-substituents. - 25 -As the solvent stabilization effect on the equilibrium position of very few compounds has been studied, i t w i l l be of interest to see i f any trends are apparent. These solvent effects on the equilibrium become most important i f one attempts to extend the work of Gardner and McGreer (39) on the concept of free energy additivity of olefin pairs. This principle of additivity w i l l be investigated in greater detail using equilibrium positions which have been correlated into a non-polar environment at room temperature. The equilibrium positions w i l l be drawn from this work and the available literature. - 26 -RESULTS AND DISCUSSION PREPARATION OF a,8-UNSATURATED OLEFINS The Synthesis of Aldehydes The isomeric pairs of aldehydes prepared for equilibrium determinations are listed i n Table 3. A compilation of the n.m.r. data i s presented i n Table 4. (i) Photoisomerization of crotonaldehyde and tiglaldehyde The irradiation of crotonaldehyde, (E-) (5) as a neat degassed liquid at 40° gave crotonaldehyde, (Zj) (5) as a mixture with the starting material. The photoisomerization was carried out in a Rayonet Photochemical Reactor equipped with lamps emitting radiation at 3000A. The _Z:E_ ratio was found to attain a maximum value of 29:71 after 85 h. The _Z:E_ ratio was readily measured by integration of the formyl proton resonances in the n.m.r. spectrum. Some typical results of the photoisomerizations are listed in Table 5. It i s interesting to note that the rate at which the photostationary state was attained depended on the dimensions of the sample tube and the amount of aldehyde irradiated. - 27 -TABLE 3 Aldehydes Prepared for Equilibrium Determination B A \ / / A C CHO Aldehyde A B C Reference Z- s H H CH^ This work E-<5 H CH H 11 - — , 3 Z--6 CH H CH " - - 3 3 E- _ CH CH H " 3 "j E-30 Br CH^ H 56 E-31 H OCH3 H 57 E-32 H OC H H 57 — 2 5 Z-33 CH CH Cl 58,59,60. - — 3 3 E-33 CH Cl CH " - -— 3 3 Z-34 H CH^ Cl 58,60 E-34 H Cl CH3 " ^-35 H C(CH 3)3 Cl " E-35 H C 1 CGC1I ) . " 3 3 Z_-36_ CH CH Br 61 3 3 E-36 CH Br CH " 3 3 Z-37 H CH Br " 3 E-37 H Br CH » 3 - 28 -TABLE 3 (continued) Aldehyde A Reference Z-38 E-38 Z-39 E_-39 £-40 E-40 Br Br H H H H Br C(CH ) 3 3 OCH CH -C H 6 11 CH Br OCH c c o n 3 3 -C H 6 11 This work CH„ 62 TABLE 4 Chemical S h i f t s and Coupling Constants f o r the a .3-Unsaturated Aldehydes B A \ / / \ C CHO Aldehyde A B C Chemical S h i f t (T) Ca) Coupling Constant (b) (Hz) A B C CHO J A,B J A,C J B,C J A, CHO (c) Z- 5_ H H CH 3 4.19m 3.36m 7.90m -0.04d 11.4 1.8 7.6 8.0 E- 5_ H CH 3 H 4.01m 8.07m 3.15m 0.60d 1.5 15.8 6.7 7.6 7 ^ C c ) ( d ) L- 0 C H 3 H C H 3 -8.27m =3.20m 7.92d -0.38s 1 ? =7 E_- 6_ CH 3 CH 3 H 8.27m 8.00d 3.20m 0.45s 1.0 1.3 7.0 Z_-3£ Br C H3 H 7.85d 2.72q 0.78s 6.9 E-31 H OCH 3 H 4.46q 6,20s 2,59d 0,69d 12,6 &-3_2 H OC H r 2 5 H 4.50q 5.99q 2.68d 0.73d 12.6 JCH 2,CH 3 = 7 A TABLE 4 (continued) (a) (b) Aldehyde A B C Chemical S h i f t (T) Coupling Constant (Hz) A B C CHO J J J J A,B A,C B,C A,CHO Z-33_ CH 3 CH 3 Cl 8.21q 7.63q -0.17s 1.1 E-33_ CH 3 Cl CH 3 8.13q 7.38q 0.00s 1.5 . Z-3£ H CH 3 C l 3.95m 7.63q 0.07d 1.7 7.0 E_-34 H Cl CH 3 3.80m 7.37q 0.22d 1.7 6.7 Z-35 H C(CH 3) 3 Cl 3.94d 8.72s 0.05d 7.3 E-35 H Cl C C C H 3 ) 3 3.85d 8 - 5 3 s -0.05d 7.7 Z_-36_ CH 3 CH 3 Br 8.20q 7.42q 0.03s 1.1 E-36 CH 3 Br CH 3 8.12q 7.13q 0.03s 1.6 _Z-37 H CH 3 Br 3.77m 7.43q 0.23d 1.3 6.5 E-37 H Br CH 3.57m 7.21q 0.27d 1.3 6.5 TABLE 4 (continued) Aldehyde A B C Chemical Shift ( T ) ^ Coupling Constant ( H z ) ^ A,B A,C B,C A,CHO Z-38 E-38 Br Br Br CH CH. Br 7.23s 7.09s 0.35s 0.24s _Zj-39_ E-39 H H C(CH )„ ^ 3J3 OCH OCH 4.68d 8.85s 5.90s O.Old 4.85d 6.32s 8.65s -0.12d 7.3 7.7 (a) s, Singlet; d, doublet; t, tripl e t ; q, quartet; m, multiplet or two doublets. A l l spectra were recorded as 10-20% solutions in CC1 , with internal TMS, except 5_ and 6_ which were recorded neat and 38_ which was run in CDC13. In general, x =±0.02 ppm. (b) J = ±0.1 Hz. (c) The n.m.r. spectra at high dilution may be found in reference (63). (d) Values for Z isomer partially obscured by overlapping E_ isomer. - 32 -TABLE 5 Photoisomerization of Crotonaldehyde, (E-) (5) and Tig1aldehyde, (E-)(6) Aldehyde Volume of Sample Tube Time _:E_ Sample (ml) , Size O.D. (mm) (h) E-5_ 0.20 4 20 24:76 39 27:73 85 29:71 E-5_ 5.0 12 66 16:84 216 29:71 E-5_ 4.2 8 190 27:73 E-6_ 0.20 4 19 18:82 85 28:72 E-6_ 5.0 12 66 15:85 216 28:72 E-6 5.0 310 36:64 - 33 -The presence of the previously unreported Z. isomer of croton-aldehyde was readily detected by n.m.r. spectroscopy. In the spectrum of the Z isomer, the methyl and formyl proton resonances were shifted downfield (0.17 ppm and 0.64 ppm, respectively), whereas the a- and 3-hydrogen resonances were shifted upfield (0.18 ppm and 0.21 ppm, respectively) relative to the E isomer. The n.m.r. spectra of the olefinic hydrogen regions of the _ and E_ isomers are reproduced in Figure 4 and Figure 5. The coupling constants between these protons were found to be 11.4 and 15.8 Hz for the _ and E_ isomers, respectively. A complete tabulation of the n.m.r. data is given in Table 4. The separation of the isomers by physical methods was unsuccessful. The Z_ isomer was determined to have a slightly lower boiling point than the E_ isomer, as a sample enriched i n the _ isomer was obtained by a spinning-band d i s t i l l a t i o n . A l l v.p.c. separations failed. The possibility of isomerization occurring during the attempted v.p.c. separation was ruled out as the mixture of isomers was recovered unchanged or only partially isomerized from the v.p.c. The irradiation of tiglaldehyde, (E-)(6), was carried out in an identical manner to that of crotonaldehyde. The maximum Z_:E_ ratio obtained was 36:64 after 310 h. The photoisomerization results are displayed in Table 5. The presence of the previously unreported Z_ isomer was detected by n.m.r. spectroscopy. The B-methyl resonance and the formyl proton resonance were both shifted downfield (0.08 ppm and 0.83 ppm, respectively) in the _ isomer relative to the E_ isomer. The position of the a-methyl resonance and the B-proton resonance were virtua l l y unchanged. The _ and E_ formyl proton resonances occurred at -0.38 and 0.45x, respectively. Figure"5 - B^Vinyi / protons of crotonaldehyde," (Z-) and (E-) (5). - 36 -The separation of the two isomers by physical methods proved unsuccessful. D i s t i l l a t i o n failed to yield any enriched samples. A l l v.p.c. separations attempted failed. Isomerization during the v.p.c. work was ruled out by the same method used for crotonaldehyde. ( i i ) a-Bromocrotonaldehyde, (Zj) (30) The 2,3-dibromobutanal was prepared by adding bromine to E-5 in pentane solution. The addition of bromine was rapid and the reaction could be carried out as a ti t r a t i o n ; the bromine colouration disappearing rapidly as i t was added. The addition of potassium carbonate to the acetic acid solution of the dibromide generated potassium acetate, which dehydrobrominated the dibromide. CH, H CH, Br 3 / Br 0 3 , \ / 2 KOAc \ / / \ - CH -CHBr-CHBr-CHO — — *" / \ H CHO V^-tane 3 HOAc H ^ E-5 Z-30 This method has been used effectively (64) i n removing the acidic a-hydrogen and the 3-halogen. The method is reasonably mild and gives a good yield. The acetylene, resulting from removal of two moles of hydrogen bromide, or the 3-bromocrotonaldehyde, resulting from removal of a-bromine, were not detected in the n.m.r. spectrum. The n.m.r. data is reported i n Table 4. The stereochemical assignment w i l l be discussed later. - 37 ( i i i ) g-Alkoxyacroleins The 3-methoxyacrolein, (E-)(31), was prepared by the method of Kalinina and co-workers (57). It was found, however, that a significant fraction (35%) of the product was 3-ethoxyacrolein, (E-)(32). The reaction scheme i s as follows: (C 2H 50) 2CH-CH 2-CH(C 2H 50) 2 H CHO-CH—C 42 ,0-C-OCH3 CHO-CH -CHO l l CH0-CH=CH0H KOH H C1-C02CH3 •* K+ 0-CH=CH-CHO H+ 41 CH30 H \ / / \ H CHO C H O H 2 5 \ / / \ H CHO E-31 E-32 Mixed solvents containing ethanol were used to wash the crystalline products 4_ and 42_. Thus, ethanol adhering to the 41_ and 42_ may have resulted in the production of the ethoxyaldehyde by exchange with methoxyl i n 42_, or by inclusion during the acid catalyzed decomposition. For the acid catalyzed decomposition of 42, Kalinina (57) proposes an eight-membered transition state. - 38 -H H )c=0 V CH„ H N H X C ' XCH, 0 0 H X0--C o E-31 Shramova (65), however, prepared an analogous compound (43) CH -C0-CH=CH-0C0 CH 3 2 3 43 CH -CO-CH='CH-OCH 3 3 44 which was decomposed to give 44. (iv) B-Haloaldehydes and derivatives The formylation of ketones with dimethylformamide-phosphorus oxychloride or dimethylformamide-phosphorus tribromide as described by Arnold and co-workers (58,61) was used to prepare the 3-chloroaldehydes and the 8-bromoaldehydes, respectively. CO-CH, P0C1„ - DMF PBr 3 - DMF, R2 R l I2 r ,CH Br-C=C-CH=N 3  VCH_ Br H20 I I Br-C=C-CHO R R I I ,CH_ C1-C=C-CH=N XCH_ H20 R7 R, ,2 ,1 C1-C=C-CH0 Cl - 39 -The previously unreported Z:E ratio of the isomers obtained from these reactions are liste d in Table 6. TABLE 6 Isomeric Aldehydes Prepared by the Method of Arnold (58,61) X-R2C=CR-CHO Aldehyde X R i R2 33 Cl CH 3 CH 3 30:70 34 Cl H 50:50 35 Cl H C(CH ) 3 3 100:0 36 Br CH 3 34:66 37_ Br H 50:50 (a) estimated by n.m.r. / - 40 -The yields in the preparation of both the chloro- and bromoaldehydes were generally less than those reported by Arnold and co-workers (58,61). It was found that the B-haloaldehydes. prepared were unstable at room temperature. The bromoaldehydes were found to be most unstable and the aldehyde 3_5_, the most stable. Although Benson and Pohland (66) reported that the decomposition of the unstable chloxoaldehydes could be inhibited by dissolution cf the aldehyde i n toluene containing a small amount of trimethy1amine, i t was found that the haloaldehydes could be conveniently stored in sealed ampoules kept at -78° in a Dry Ice-acetone slush bath. The part i a l separation of the isomeric haloaldehydes for equilibrium determinations was carried out by v.p.c. Generally speaking, the best separations were achieved with Carbowax 4000 Monostearate, FFAP or Ethofat. In a l l cases the stereochemical assignments for the _ and E_ isomers of the 3-haloaldehydes were based on the diamagnetic anisotropic deshielding effect of the aldehyde group on the S-substituent. Thus, the methyl or tert-butyl group cis to the aldehyde function appears at lower f i e l d than the same group in the trans orientation. In the 3-chloroaldehydes this deshielding factor i s 0.25 to 0.19 ppm, and for the 3-bromoaldehydes this deshielding factor i s 0.29 to 0.22 ppm. In the dibromoaldehyde, 38_, the above difference is 0.14 ppm. Further confirmation of the stereochemical assignment is obtained from the chemical s h i f t of the a-proton or a-methyl group. The 3-chlorine deshields these groups by 0.15 to 0.08 ppm, whereas the 8-bromine deshields the same groups by 0.20 and 0.08 ppm respectively. The above effects may be - 41 -observed i n Table 4. The E_ isomer of the chloraldehyde 35_ was prepared as a mixture with Z-55 by photoisomerization of the latter in ether solution with a 450 W Hanovia lamp. The Z_:E_ ratio was found to be 60:40. The nucleophilic displacement of chlorine in the chloroaldehyde Z-35 by methoxide ion yielded the 8-methoxy aldehydes, (Z-) (39) and (E-)(39), with a Z_:E_ ratio of 38:62. The stereochemical assignment was based on n.m.r. chemical shifts. The aldehyde group cis to the tert-butyl deshields that group 0.20 ppm with respect to the tert-butyl group i n the trans position. A similar situation i s observed with the methoxyl group for which the above described difference i s 0.42 to 0.46 ppm. Although the n.m.r. spectrum and the v.p.c. results on several columns indicated that the mixture of isomers did not contain any extraneous material, a correct microanalysis could not be obtained. The separation of the Z_ and E, isomers was somewhat d i f f i c u l t to achieve despite the fact that apparent separation i s readily attainable on many columns. The existence of a saddle region between the two peaks indicated that isomerization was occurring on the column. A new column (FFAP) operating at relatively low temperatures was used to achieve the desired separation. The nucleophilic displacement of chlorine by dialkylamine i n the chloroaldehyde, Z-35 did not result in the normal product. The 8-dimethylaminovinyl tert-butyl ketone,, (E-)(45) and the 8-diethylaminovinyl tert-butyl ketone, (E-)(46) were obtained as the result of a base catalyzed - 42 -rearrangement. This rearrangement has been reported by Arnold and co-workers (58). t-Bu H \ / / \ Cl CHO Z-35 R2NH t-Bu /C=CH-CH=iNR R2N 2 t-Bu V-CH-CH-NR^ R N 2 2 OH. OH t-Bu C=CHCHO R N 2 R NH 2 t_-Bu / V=CH-CH « n ' \ NR- NR, t-Bu HO—C-CH=CH-NR„ I 2 NR„ -R„NH 2 t-Bu-CO-CH CH-NR„ E-45 (R = CH3) E-46 (R = C 2H 5) Figure 6 - Amine catalyzed rearrangement of 8-dialkylamino-g-tert-butyl acroleins. The stereochemistry of the rearranged products, E-45 and E-46, was readily established by their trans coupling constants of 12.6 Hz. The preparation of the a,g-dibromocrotonaldehydes, (Z-)(38) and (E-)(38) was carried out in an analogous manner to that of the - 43 -a-bromocrotonaldehydes (Z-)(30). Br CH 2 3,C=CH-CHO ^ CHT-CBr -CHBr-CHO Br" pentane 3 2 37 HOAc KOAc CH3-CBr=CBr-CH0 3_8 The 3-bromoaldehyde 37, was brominated and the racemic tribromides were dehydrobrominated by potassium acetate in acetic acid solution. The a,3-dibromocrotonaldehyde was particularly prone to decomposition during d i s t i l l a t i o n . This decomposition may be the result of unreacted tribromide debrominating or dehydrobrominating upon heating; the resulting bromine or hydrogen bromide catalyzing the decomposition. In addition, Arnold and Holy (61) report that 3-bromoacrylaldehydes are particularly unstable to even mild overheating. In one attempted d i s t i l l a t i o n , the d i s t i l l e d product rapidly decomposed to a black viscous liquid before i t could be removed from the d i s t i l l a t i o n apparatus. The n.m.r. spectrum of the d i s t i l l e d product indicated a Z_:E_ ratio of 19:81. The stereochemical assignment i s based on the deshielding effect (0.13 ppm) of the aldehyde on the 3-methyl group i n the Z_ isomer. The n.m.r. data i s reported in Table 4. It was not,possible to separate the isomeric aldehydes by v.p.c. with the columns tried. Decomposition i n the injection chamber was the major deterrent. - 44 -(v) C i t r a l A commercially available sample of c i t r a l was separated into i t s two isomers by fractional d i s t i l l a t i o n . The characteristics of the higher and lower boiling fractions were consistent with the n.m.r. assignments of Verieuto and Day (62). The Synthesis of Methyl Ketones The ketones prepared for equilibrium determination are listed in Table 7. The unstable isomer for equilibrium determination was obtained i n some cases as a mixture with the stable isomer, E-47, E-49 by photoisomerization of the latter. The ketone 48_ was prepared in the unstable form, whereas the methoxy ketones 5£ were obtained as a mixture and then separated by v.p.c. A compilation of the n.m,r. parameters are presented in Table 8. (i) 3-Methyl-3-penten-2-i6ne, (Z-) (47) The 3-methyl-3-penten-2-one, (Z-)(47), was prepared by irradiation of the E_ isomer, followed by fractional d i s t i l l a t i o n . The Z_ isomer from the d i s t i l l a t i o n was further purified by v.p.c. The above work was performed by Miss P. Balmos of this laboratory. ( i i ) 4-methoxy-3-penten-2-one, (Z-) and (E-)(48) The method of Eistert (68) was used to prepare the 4-methoxy-3-penten-2-one, (Z-)(48). The product was thermally labile and isomerized to the E_ isomer at room temperature. The Z_ isomer was found to be higher boiling, and by a simple d i s t i l l a t i o n , a sample containing 96% of the Z_ isomer was obtained. - 45 -TABLE 7 Methyl Ketones Prepared for Equilibrium Determination B A / — \ C COCH Ketone A B C Reference Z-47 CH 3 H CH 3 This work 1E-47 CH 3 CH 3 H ti Z_-48 H CH 3 OCH 3 68 E-48 H O C H 3 C H 3 II Z-49 H CtCH 3) 3 Cl This work E-49 H Cl C(CH ) 3 3 II Z-50_ H C(CH ) 3 3 OCH 3 it E-50 H OCH . 3 C(CH ) 3 3 II Z_-5_l H C(CH ) 3 3 NfCH ) 3 2 II TABLE 8 Chemical Shifts and Coupling Constants for the a,B-Unsaturated Methyl Ketones B A \ / / \ C COCH, Ketone A Chemical Shift (t) B C CO(CH3) Coupling Constants (Hz) Cb) Z-47 E-47 Cc) Cc) CH CH. H CH CH, H 8.15m 4.23m 8.19d 7.84s 8.27m 8.18d 3.36q 7.82s J = 7.0 B,C J = 7.0 B,C Z-48 E-48 Cd) (d) H CH OCH 4.94s 7.92s 6.12s 7.76s H OCH CH, 4.37s 6.31s 7.88s 7.74s Z_-49 E-49 H H CCCH 3) 3 Cl Cl CCCHJ 3 3 3.84s 8.74s 7.67s 3.72s 8.74s 7.82s Z_-5£ E-50 H C C C H 3 ) 3 0 C H 3 4.72s 8.93s 6.23s 7.95s H 0CH3 C ( C H 3 ) 3 4 ' 8 6 s 6 « 4 4 s 8 - 8 3 s 7.95s TABLE 8 (continued) Ketone A B C C a ) Chemical Shift ( T ) ^ A B C C0(CH3) Coupling Constants^ (Hz) Z-51 H C(CH ) 3 3 N(CH ) 3 2 4.88s 8.94s 7.05s 7.59s (a) S, Singlet; d, doublet; m, multiplet. A l l spectra were recorded as 10-20% solutions in CC14 with internal TMS, except 48_ which was recorded neat, T = ±0.02 ppm. (b) J = ±0.1 Hz. (c) See reference (67). (d) Assignments for C and COCH in Z-48 and B and COCH in E-48 may be reversed. - 48 -The stereochemistry of the two isomers was assigned by boiling point (68) and by n.m.r. spectroscopy. The diamagnetic anisotropy of the acetyl group deshields the methoxyl or methyl group when cis by 0.19 or 0.12 ppm, respectively, compared to the trans orientation. The methoxyl group also deshields the a-hydrogen when cis by 0.57 ppm compared to the trans orientation. ( i i i ) 6-Chloro ketones and derivatives The previously unreported 4-chloro-5,5-dimethyl-3-hexen-2-ones, (Z-) and (E-)(49), were prepared from the aldehyde, Z-55, by the following sequence: t-Bu H \ / Cl CHO CH3MgI ether t-Bu H " \ / / \ Cl CH-CH, OH -Z-35 Z-52 t-Bu COCH - v _ y 3 / — \ Cl H hv MnO, pentane t-Bu H " \ / / \ Cl COCH E-49 Z-49 - 49 The aldehyde Z-55 was converted to the alcohol by methyl magnesium iodide. Activated manganese dioxide, prepared by the method of Attenburrow et a l . (69), was used to oxidize a pentane solution of Z-52 to the ketone, Z-49. The reaction was followed by v.p.c. The E_ isomer of the chloroketone, 49_, was prepared as a mixture with Z-49 by photoisomerization of the latter i n ether solution with a 450 W Hanovia lamp. The Z_:E_ ratio produced was determined to be 75:25. The stereochemical assignment of the isomer pair Z- and E-49 was determined by n.m.r. Although the tert-butyl resonance position does not change in the _Z and E_ isomer, the deshielding effect of the chlorine when cis to the a-proton allows configurational assignment. This effect i s also noted in the corresponding aldehyde (see Table 4). This stereochemical assignment i s substantiated by the fact that the preparation of the ketone by the above method should be stereospecific (13,28,33). The nucleophilic displacement of chlorine in the chloroketone, 49, by methoxide ion resulted in an isomeric mixture of the 6-methoxy ketones, Z- and E-50. The product Z_:E_ ratio was 75:25. t-Bu H CHz0H t-Bu H t-Bu COCH " \ / 5 > ~ \ / + " \ ( / \ NaOCH / \ ' \ Cl COCK 3 CH 0 COCH, CH,0 H 3 3 5 5 Z-49 Z-50 E-50 The separation of the 1_ and E_ isomers was carried out by v.p.c. The stereochemical assignments were based on the n.m.r. spectra. The tert-butyl group and the methoxyl group were deshielded when cis to the carbonyl by - 50 -0.10 and 0.21 ppm, respectively, compared to their chemical shifts i n the trans orientation. It was found that the reaction of the chloroketone, Z-49, with dimethylamine or diethylamine paralleled the reactions of the chloroaldehyde Z-34 (see Figure 6). In this instance, however, some enamino methyl ketone, Z-51, was produced. 0 t-Bu H (CHJ NH t _ B u H t-Bu-C CH - \ / —3 2 . - \=J + - w 3 / \ / \ / \ Cl C0CH3 ^ 3 ^ 2 N C ° C H 3 H N C a i 3 } 2 1-49 Z_-51_ E-53_ The two products Z-51 and E-53 were separated by v.p.c. and identified by n.m.r. spectroscopy. The acetyl group of Z-51 appeared at lower f i e l d than the vinyl methyl group i n E-53. The stereochemical assignment of Z-51 is not definite as i t was not possible to prepare E-51 for n.m.r. spectral comparison. This matter, however, w i l l be discussed later in the thesis. The stereochemical assignment for E-53 is based purely on analogy with the aldehyde situation (i.e. compound E-45). With diethylamine only the rearranged product was obtained. Synthesis of Nit r i l e s The n i t r i l e s prepared for equilibrium study are listed in Table 9. They were prepared by a variety of methods which w i l l be described below. The n.m.r. parameters are presented i n Table 10. - 51 -TABLE 9 Nitriles Prepared for Equilibrium Determination B A \ / / \ C CN Ni t r i l e A B C Reference £-17 H H CH^ E-17 H CH H 3 Z-54 H H Br This work E_-54 H Br H £-55_ Br CH3 H E_T55_ Br H CH3 Z-56_ H H OCH^  E-56 H OCH H 3 Z-r57 H H SC H - — 2 5 E-57 H SC H H 2 5 Z-58 CH CH Cl . 3 3 E-58 CH Cl CH TABLE 10 Chemical Shifts and Coupling Constants for the a,B-Unsaturated N i t r i l e s B A \ / / \ C CN r a ) hi Ni t r i l e A B C .Chemical Shift ( T ) ^ Coupling Constant 1 J A B C CHz) Z-17^ H H CH, 4.84m 3.72m 8.05m J A D = 11.0 J „ = 6.8 J A „ = 1.4 — 3 A,D D,L A , L H CH H 4.78m 8.20m 3.47m J = 1.5 J = 6.7 J. = 16.0 3 A,B B',C A»C Z-54 H H Br 3.68d 2.72d J A _ = 8.4 A,B E-54 H Br H 3.88d 2.56d J. = 14.9 A,C Z-55 Br CH H 8.05d 3.02q J =7.1 3 • B,C E-55 Br H CH, 3.22q 7.94d J „ = 7.3 3 .- n B,C Z-56 H H OCH, 5.61d 3.18d 6.03s J = 6.3 — 3 A, B E-56 H OCH H 5.35d 6.28s 2.71d J = 12.9 3 A,C N) TABLE 10 (continued) N i t r i l e A B C Chemical S h i f t fx) ^  Coupling Constant^'' A B C (Hz) Z-57 CH H SC H 4.75d 2.89d 7.04a J = 10.7 J , =7.5 - — 3 2 5 8 . 6 1 t A,B CH 2,CH 3 E-57 CH, SC H H' 4.85d 7.19q 2.71d J = 15.6- J , = 7.5 - — 3 2 5 g.63? A,C C H ^ C ^ Z-58 CH, CH, C l 8.03q 7.75q J = 1.1 ' 3 3 . A,B E_-58 CH 3 Cl CH 3 8.00q 7.56q J A C = 1 , 6 (a) s, Si n g l e t ; d. doublet; t , t r i p l e t ; q, quartet or two doublets; m, m u l t i p l e t . A l l spectra were recorded as 10-20% solutions i n CC1 with i n t e r n a l TMS, except 5_4, which was recorded i n CDC1 . x = ±0.02 ppm. (b) J = ±0.1 Hz (c) Ref. (70). - 54 -In contrast to the aldehydes prepared (see Table 3), which were d i f f i c u l t i f not impossible to separate with the v.p.c. columns tried, the n i t r i l e s prepared (see Table 9) were readily separated on most columns. (i) Crotononitrile, (Z-) and (E-)(17) The crotononitriles, Z-17 and E-17, were separated by fractional d i s t i l l a t i o n . ( i i ) 8-Bromoacrylonitrile, (Z-) and (E-)(54) The previously unreported B-bromoacrylonitrile, Z-54, was prepared by the following method: • H-C==C-COOH Br V H j CN / \ H 48% HBr P2°5 Br H Br \ / / \ H COOH E-59 \ / / \ K H CONH, NH3, 0° ether Br COOH \ / / \ H H _Z-59 S0C1, Br COCl \ / / \ H H Z-54 Z-61 Z-60 Figure 7 - Preparative Sequence for 3-bromoacrylonitrile, Z-54 - 55 -This procedure follows the same sequence described by Gryszkiewicz-Trochimowski and co-workers for preparing the E_ isomer (71). Gryszkiewicz-Trochimowski, et a l . , however, did not report any yields. The addition of hydrogen bromide to propiolic acid was carried out by the method of Alder et a l . (72) . It i s interesting to note that their reported yield was only 22%. This low yield i s probably due to the loss of the soluble Z_ isomer in the aqueous reaction mixture. An overall yield of 88% was obtained i n this work by extraction of the reaction mixture with chloroform. The £:E_ product ratio was found to be 82:18. The melting points for the 1_ and E_ isomers corresponded to those reported by Alder (72). Starting with the Z_ acid, the previously unreported acid chloride, Z-60 ; amide, Z-61 ; and n i t r i l e £-54 ; were prepared. The acid chloride was prepared by the action of thionyl chloride on the solid acid at room temperature. The n.m.r. spectrum of the acid chloride showed that no isomerization had occurred during i t s preparation. Ammonia, bubbled through an ethereal solution of the acid chloride produced the amide, Z-61, which was characterized. This compound was readily isomerized with bromine and sunlight to the E_ isomer, which was identified by i t s melting point and n.m.r. spectrum. Finally, dehydration with phosphorus pentoxide of the neat amide produced the liquid n i t r i l e , Z-54. This Z_ isomer could readily be isomerized to a mixture of the Z_ and E_ isomers in a ratio of 70:30 by bromine catalysis (Rayonet, 3100A, 2 h) of the neat isomer. In the preparative sequence outlined in Figure 7, i t is of interest to note that the scheme by which Gryszkiewicz-Trochimowski et a l . prepared E-54 must have started with E-59. Thus, the series of compounds prepared from this isomer a l l have the E_ configuration. In this work, the compounds - 56 -prepared have the Z_ configuration. Isomerization of the _Z compounds to the corresponding E_ isomers throughout the series provides a check of configuration and also provides an interesting series of n.m.r. correlations. This series of bromo compounds w i l l later (see Table 21, p. 100 ), be compared to a similar series of Z and E chloro compounds given by Kurtz (73). The a-bromocrotononitrile (Z-) and (E-)(55) was prepared by an analogous method to that of the a-bromocrotonaldehyde and the ct,8-dibromo-crotonaldehyde. Crotononitrile was brominated and resulting dibromide CH 3 N C = C H C N H 17 Br pentane CH. V CH -CHBr-CHBr-CN KOAc HOAc Br / J \ H CN CH. V CN J / \ H Br E-55 dehydrobrominated with potassium acetate in acetic acid. The product was found to contain the _Z and E_ isomers in a ratio of 69:31. The separation of the isomers was readily carried out by v.p.c. The stereochemical assignment was based on the deshielding effect of the n i t r i l e on the cis methyl or cis proton in the n.m.r. spectra. ( i i i ) q-Brompacrylonitrile The method of bromination of an a,B-unsaturated n i t r i l e followed by dehydrobromination of the resulting dibromide to give an a-bromo n i t r i l e was employed with aerylonitrile. 57 Br. CH2—CHCN ether CH^Br-CHBr-CN KOAc HOAc CHf=CBrCN 62 Thus, the a-bromoacrylonitrile 6_2 was prepared. (iv) g-Methoxyacrylonitrile, (Z-) and (E-)(56) A reaction s i m i l a r to that of Gregory (74) was used to prepare the B-methoxyacrylonitriles, (Z-) and (E-)(56). I t was found, however, that the excess of methoxide used i n the reaction led to the formation of 8,B-dimethoxy-p r o p r o p i o n i t r i l e , (63) . Br CH=C 62 CN CH3OH NaOCHr H (CH 0) CH-CH -CN 63 H \ / / \ CH 0 CN 3 H 3P0 4, 185° CH 0 3 H 7 / \ H CN V Z-56 E-56 - 58 -Methanol could be eliminated from the acetal by an acid catalyzed pyrolysis using phosphoric acid. This gave the desired methoxynitrile, 5_6_, which could readily be separated into i t s isomers by v.p.c. The £:E_ ratio of the n i t r i l e s in the product was 38:62. The coupling constants of the Z_ and E_ isomers were used to assign stereochemistry. (v) 8-Thioethoxyacrylonitrile, (Z-) and (E-)(57) A similar reaction to that above was used to prepare the B-thioethoxy-acrylonitrile,(Z-) and (E-)(57). Br N^ C2 H5^3 H H C H S CH = C ** \ / 2 5 V 2 C N ether, C ^ S H j \ / \ C H S CN H CN •62 2 5 Z-57 E-57 The product, readily separated by v.p.c. was found to contain the 1_ and E_ isomers in a ratio of 43:57. The cis and trans coupling constants of the two isomers readily indicated the stereochemistry. (vi) 8-Chlbrotiglonitrile, (Z-) and (E-)(§8) The oxime of 8-chlorotiglaldehyde, 6_4, was prepared using the acetate method. The n.m.r. and i . r . spectra of the oxime were consistent with the spectra reported by Benson and Pohland (66). The oxime could readily be dehydrated employing a phosphoric acid ester as described by Makaiyama and Hata (75). - 59 -H CH, VC=NOH CH, CH_ 3. / 3. / 3 / 2 P 0 , 5 C H OH V / Z.-58 / \ 2 5 2 5 , / \ Cl CH, , Cl CN 3 benzene 64 CH, CN 3 V J E-58 Cl CH, The Z_ and E_ n i t r i l e s were readily separated by v.p.c. The difference in deshielding effects of the n i t r i l e on the cis methyl group compared to the trans arrangement resulted i n the cis methyl appearing 0.19 ppm downfield. It is probable that the solid oxime w i l l undergo elimination of hydrogen chloride to give the 4,5-dimethylisoxazole, (65), although this problem was not encountered i n the above preparation. The isoxazole was formed, however, from the crude, unreacted oxime recovered from the above preparation after 12 days. It was tentatively identified from i t s n.m.r. spectrum by comparison to the spectra of 5-tert-butylisoxazole (66) and 3,5-dimethylisoxazole (67). CH. CH. N OH CH, CH. H r .N Cl Z-64 65 The 3-proton resonance in 65_ appears at 2.15T, whereas in 66_ i t appears at 2.02T (76). The 4- and 5-methyl resonances in 65 appear at 8.08T;.and 7.73T, respectively. In the reported spectrum of 67, the unassigned 3- and 5-methyl resonances appear at 7.77 and 7.63T (77). - 60 -Synthesis of Propenyl Ethers The method of Boonstra et a l . (78) was used to prepare the isomeric 1-ethylthiopropenes, (Z-) and (E-)(68). The propionaldehyde diethylthioacetal was prepared and then pyrolyzed to give the mixture of ethylthiopropenes 68. The Z^ E_ product ratio was determined to be 61:39 by n.m.r. spectroscopy. C H CHO 2 5 H C H SH 2 5 CH„ C H CH(SC H ) 2 5 2 5 2 K H \ / / \ S C2 H5 Z-68 H 3P0 4, 240s H SC H \ / 2 5 CH K E-68 The cis and trans proton-proton coupling constants provided an unambiguous stereochemical assignment. A related reaction sequence was used to prepare the 1-methoxypropenes, (Z-) and (E-)(69). This method was developed by Howard (79). The propion-aldehyde dimethyl acetal was prepared and then pyrolyzed at 320°. The catalyst used for the elimination reaction (phosphoric acid) was coated on glass beads which were packed in the pyrolysis chamber. H CH C(0CH ) CH + 3 3 2 3 CH CH CHO 3 2 CH^ OH CH3CH2-CH(0CH3)2 CH, H H \ / / \ H PO , 320' 3 4 CH, H V J OCH, / \ H OCH, Z-69 E-69 - 61 -The yield for the pyrolysis was only 30%; the pyrolyzate, however, may have contained unreacted acetal. The separation of the isomers was readily achieved by the v.p.c. The stereochemical assignments were based on the cis and trans proton-proton coupling constants. The n.m.r. data for the thioether Z- and E-68, and the ethers Z- and E-69 are presented i n Table 11. EQUILIBRATION OF PREPARED OLEFINS The equilibrium positions determined for the various a,g-unsaturated aldehydes are shown i n Table 12. In most cases where appreciable amounts of both isomers were available at equilibrium, the equilibrium position was approached from both sides. In cases where the equilibrium considerably favours one isomer, the equilibrium was only approached from the side of the less stable isomer. The equilibrium position was also approached from only one side in cases where the separation of isomers was not attained. It was assumed that equilibrium was attained when the ratio of isomers did not change over a period of time; and the equilibration of analogous compounds had occurred in the same time under identical conditions. Most equilibrations were carried out by acid or bromine catalysis as described on p.125 ; some equilibrations were achieved thermally. Finally, i n some cases no indication of the other isomer was found and attempts to produce this isomer failed. Thus the equilibrium position was assumed to be almost completely in favour of the isomer present. TABLE 11 Chemical Shifts and Coupling Constants for Propenyl Ethers CH-C = CH1-R „/ 1 H2 (a) Compound R Chemical Shifts (x) Coupling Constants C b ) CH H H R (Hz) 3 1 2 ^ J Z-68 SC H 8.32m 4.15m 4.46m 7.37q J = 6.2 J = 7.2 J = 9.4 „ „ „ „{ „ 3 2 2 5 g i 7 1 t CH 3 >H 2 CH ,CH H ^ E-68 SC H 8.24m 4.12m 4.51m 7.44q J = 5.6 J = 7.2 J = 15.0 2 5 8 7 5 t CH ,H CH ,CH H ,H Z.-6J9 OCH3 8.49m 4.23m 5.71m 6,47s J C H H = 6.5 J C H =1.6 J H H = 6 - l 3*2 3*1 1*2 E-69 OCH, 8.46m 3.79m 5.43m 6.58s J = 6.6 J = 1.5 J = 12.8 3 CH 3 >H 2 CH3,Hi H ^ (a) s, Singlet; t , t r i p l e t ; q, quartet; m, multiplet. A l l spectra were recorded as 10-20% solutions i n CC1 with in t e r n a l TMS. x = ±0.02 ppm. 1 ( b ) J = ±0.1 Hz. TABLE 12 f a ) Equilibrium Determination of some a.3-Unsaturated Aldehydes^ Aldehyde Temp, Method of Time I n i t i a l Cone. Final Cone. °C Equilibration^) (h) %Z_ %E_ %Z %E_ _5 r . t / 0 ' ' A 24 29 71 2 98 2 98 2 98 _6 r . t . A 24 28 72 1 99 1 99 1 99 30 r . t . A 24 <1 >99 <1 >99 Cd) 35 B(3000A)V 4.5 <1 >99 <1 >99 51 r . t . A 24 <1 >99 <1 >99 Cd) 35 B(3000A) 6 <1 >99 <1 >99 32 r . t . A 24 <1 >99 <1 >99 35 B(3000A) ^  6TABLE 12 (continued) Aldehyde Temp. °C Method of Equilibration (b) Time G O I n i t i a l Cone. %Z %E Final Cone. %Z %E 33 35 B(3500A) 1.5 88 10 37 12 90 63 31 69 32 68 31 69 34 r.t. A 24 96 24 50 50 75 50 50 25 68 67 72 32 33 28 ON 35 r.t. 72 40 60 <1 >99 36 35 B(3000A) 1.5 81 32 40 19 68 60 40 37 40 60 63 60 37 r.t. A B(3500A) 3 1.5 67 66 33 34 70 69 30 31 38 r.t. 24 19 81 21 79 TABLE 12 (continued) Aldehyde Temp. Method of Time I n i t i a l Cone. Final Cone. °C E q u i l i b r a t i o n ^ (h) %Z %E %Z %E 39 r . t . A 96 67 33 48 52 24 67 33 47 53 9 7 ^ 7 93 47 53 40 140 Thermal 15 91 9 43 57 19 6 94 36 64 (a) Analysis by n.m.r. A l l equilibria determined on 10-20% solution in CC1 except 5_, 6_ and 40; which were determined on the neat liquid. ^ (b) Refers to Method A or Method B on p. 125, (c) r.t., Room temperature. (d) No bromine catalyst added. (e) Acid catalyst added after 96 h, at which time Z = 41%. - 66 -The probable errors involved in n.m.r. integration are discussed on p. 81. Where the presence of one isomer of an isomer pair was not detectable in the n.m.r. spectrum, the percentage of this isomer is designated <1% i n Tables 12-15. In the text of the discussion, however, this value w i l l also be referred to as -0%, or as =100% for the stable isomer. In cases where bromine catalyzed photoisomerization was used to determine the thermodynamic position of equilibrium, i t was shown that only very slow isomerization occurred when the bromine was not present. The availability of both isomers of c i t r a l by fractional d i s t i l l a t i o n prompted a study of the thermal isomerization. The isomerizations were carried out neat at 140° and followed by n.m.r. spectral integral of the aldehyde resonance. The equilibrium position in the naturally occurring isomer mixture i s shown to be readily approached in Figure 8. Unfortunately, spurious peaks appearing in the aldehyde region of the n.m.r. spectra after 19 h prevented following the equilibration to completion. The acid catalyzed equilibration of the c i t r a l isomers was unsuccessful. After 14 days the E_ isomer increased from 6% to 34% and the Z_ isomer increased from 27% to 35%. Accurate integration of the formyl proton resonances after p a r t i a l isomerization was not possible as extraneous peaks hindered the integration. -68 -The ketones prepared for equilibrium determination and their equilibrium positions are listed i n Table 13. The equilibrium positions either have been approached from both sides, or from the unstable isomer when the equilibrium is considerably biased towards one isomer. The method of analysis i s indicated. The n i t r i l e s prepared for equilibrium determination and their equilibrium positions are lis t e d in Table 14. In a l l cases the equilibrium position has been approached from both sides. For the n i t r i l e s JL7, 54, and 55, equilibrium determination have been made in various solvents. For completeness, the equilibrium values determined as 5% solutions in pentane are entered in this table. The photochemically induced bromine catalyzed isomerization was shown to give the thermodynamic position of equilibrium. Only very slow isomerization was observed when the catalyst was not present. The equilibration of the two propenyl ethers, 6_8 and 69_ are shown in Table 15. The analysis was carried out by v.p.c. For the 1-methoxypropene, the sample par t i a l l y decomposed and i t was necessary to dissolve the sample in ether for analysis. The ether peak tailed, necessitating the use of the cut and weigh method (discussed on p.'80)' for analysis. TABLE 13, Equilibrium Determination of some a,8-Unsaturated Methyl Ketones Ketone Temp. Method of Time I n i t i a l Cone. Fina l Cone. Method of °C Equilibration 00 • %Z %E_ • % i %E Analysis 47 214 ' Thermal 24 12 88 <i >99 v.p.c. 35 B(3000A) 0.75 12 88 <l >99 n.m.r. 48 r . t . Thermal 336 96 4 <i >99 n.m.r. 49 200 Thermal 0.75 83 17 <i >99 v.p.c. r . t . A 24 75 25 <i >99 n.m.r. 50 r . t . A 144 95 5. • 77 23 t-Bu* 83 17 ° C H3* 85 15 v.p.c. r . t . A 144 32 78 77 23 t-Bu* 85 15 * OCH 3 85 15 v.p.c. TABLE 13 (continued) Ketone Temp. Method of Time I n i t i a l Cone. Fin a l Cone. Method of °C Equilibration (h) % 1 % i % 1 %E Analysis 51 208 Thermal 7.5 >,99 <1 >99 <1 n.m.r. 35 (SIOOA)^ 260 >99 <1 >99 <1 n.m.r. * Analysis by n.m.r. integration peaks integrated for e q u i l i b r i a determination, (a) No catalyst used. TABLE 14 Equilibrium Determination of some a,B-Unsaturated Nitriles N i t r i l e Temp. Method of Time In i t i a l Cone. Final Cone. Method of °C Equilibration (h) %Z_ %E %1_ . • %E ' Analysis 57 225 . Thermal 7.5 >80 <20 44 56 v.p.c. (neat) <10 >90 44 56 11.0 >80 <20 44 56 <10 >90 44 56 57 - 297 Thermal 0.33 >85 <15 44 56 v.p.c. (neat) <30 >70 43 57 r. t . B(Sunlight) 13 100. 0 25 75 n.m.r. 23 78 v.p.c. Q . 100 26 74 n.m.r. r.t . B(3500A) 1.5 <5 >95 11 89 n.m.r, >99 <1 10 90 ,1 7 ™ 35 B(3000A) 61 39 v.p.c. TABLE 14 (continued) N i t r i l e Temp. Method of Time In i t i a l Cone. Final Cone. Method of °C Equilibration (h) %Z_ %E %Z %E Analysis 54 C b ) . ,35 B(3000A) 51 49 v.p.c. 55C b ) ' 35 . B(3000A) 67 33 v.p.c. (a) 20% solution in CC1 . 4 (b) 5% solution in pentane. TABLE 15 Thermal Equilibration of 1-Thioethoxypropene and 1-Methoxypropene Compound Temp. Time I n i t i a l Cone. Fin a l Cone. Analysis (v.p.c.) °C (h) %Z %E %Z %E 68 207 7.25 100 0 52.5 47.5 integration 19 81 53 47 20.75 100 0 53 47 19 81 53 47 69 300 256 >95 <5 55 45 integration <5 >95 53 47 cut-weigh 335 44.5 83 17 55 45 integration 20 80 52 48 cut-weigh - 74 -SOLVENT STUDIES In order to investigate the effect of solvent on the position of equilibrium five compounds were isomerized by bromine catalysis in a variety of solvents. It was found that n i t r i l e s were the most useful compounds through which solvent effects could be investigated, the 1,2-dichioro and 1,2-dibromo-ethylenes were also suitable. There are many factors which must be considered in choosing a pair of isomers for such an investigation. The compound must not decompose during the isomerization of v.p.c. analysis and i t must not isomefize while in the v.p.c. Both isomers of the compound should be readily available, and they must be separable by v.p.c. with no overlapping, so that integration is accurate. The retention times of the isomers chosen should be different from the majority of the solvents chosen. Finally, the equilibrium value of the isomers should f a l l i n the 20 to 80% range. This i s because the equilibrium position, which i s a measure of the standard free energy difference between the two isomers, w i l l be more sensitive to solvent effects in systems where the two isomers are in more or less equal proportions. This may be illustrated by the following example. For an isomer pair with an equilibrium ratio of 55:45 (K, = 1.22), the standard free energy difference between the two isomers i s given by AG = -RT InK. Thus at room temperature (25°) AG = -0.119 kcal./mole. If by solvent stabilization, the ratio becomes 65:35 (a change of 10% for one isomer) then AG = -0.367 kcal./mole; or a stabilization energy of 0.248 kcal./mole. If the original equilibrium ratio was 90:10 (K = 9.00) or AG = -1.30, kcal./mole, then the:same solvent - 75 -stabilization energy would give AG = -1.55, kcal./mole, corresponding to an equilibrium ratio of 93:7 (a change of 3% for one isomer). Thus, i t is readily seen that solvent stabilizations w i l l be more readily detected i n systems where the equilibrium ratio i s closer to 50:50 than those with more biased equilibrium ratios. In the experimental section evidence indicates that extraneous peaks did not arise from solvent blanks. It was also shown that under the conditions of isomerization, the isomerization was due only to catalysis, hot photoisomerization. The detector response was tested for a weighed sample mixture and found to be the same for both isomers. Thus, i t may be concluded that the method of equilibration and analysis gives an accurate and reliable measure of the equilibrium position in the given solvent. The following tables give the equilibrium percentages and free energy differences for different isomer pairs i n a variety of solvents. The errors w i l l be discussed after these tables. - 76 -TABLE 16 Solvent Stabilization Factors for Crotononitrile, (Z-) and (E-)(17) Solvent No. of % E_ Range Determ. in % E neat 7 43.7±0.3 0.8 0.156±.007 0.019 pentane 3 38.8±0.7 1.1 0.2801.002 0.003 methanol 3 43.110.3 0.5 0.170±.005 0.009 acetonitrile 4 44.3±0.5 1.0 0.1421.004 0.009 ethyl acetate 4 43.1±0.5 1.1 0.170±.009 0.018 benzene 6 43.5±0.4 1.0 0.1611.010 0.024 chloroform 4 41.8±0.6 1.2 0.2021.015 0.031 ether 4 40.7±0.4 0.8 0.2321.006 0.013 carbon tetrachloride 3 38.6±0.2 0.3 0.2841.005 0.008 benzonitrile 4 46.4±0.4 6.9 0.0911.011 0.022 -AG^a^ Range in AG (a) K = %Z / % E / A 6 a ..RTlnK. - 77 -TABLE 17 Solvent Stabilization Factors for 8-Bromoacrylonitrile, (Z-) and (E-)(54) Solvent No. of % _E Range -AG^ Range Determ. in % E_ in AG pentane 4 49.3+0.1 0.1 0.016±0.002 .004 acetonitrile 4 31.8±0.5 1.0 0.468+0.014 .029 benzonitrile 5 34.811.2 2.7 0.385±0.032 .074 carbon tetrachloride 6 44.8±1.1 2.8 0.127±0.028 .070 (a) K = %Z /. .%E , AG = -RTlnK. TABLE 18 Solvent Stabilization Factors for a-Bromocrotononitrile,(Z~) and (E-) (55) Solvent No. of Determ. % _E Range in % E_ -AG ( a ) Range in AG pentane 3 33.2±0.6 1.1 0.429±0.018 0.031 benzene 4 31.3±0.8 1.7 0.481±0.025 0.052 benzonitrile 2 31.5±0.7 0.8 0.477±0.021 0.024 acetonitrile 6 31.5+1.0 2.6 0.477±0.029 0.073 (a) K = %Z / %E, AG = -RTlnK. - 78 -TABLE 19 Solvent Stabilization Factors for 1,2-Dibromoethylene, (Z-) and (E-)(70) (a) Solvent No. of % E_ Range -AG Range Determ. in % E in AG neat 7 39.4±0.3 0.9 0.26510.009 0.024 pentane 6 47.610.7 1.8 0.05810.018 0.045 ether 4 40.010.5 1.0 0.24910.014 0.039 benzene 4 37.5±0.4 0.8 0.31310.011 0.023 benzonitrile 8 34.4±1.4 4.0 0.39610.038 0.107 o-nitroanisole 3 32.910.2 0.4 0.43810.007 0.011 acetone 5 32.010.7 1.6 0.46310.019 0.044 ethyl acetate ! 4 32.810.8 1.6 0.44010.022 0.045 methanol 4 34.110.4 0.9 0.40310.013 0.026 acetonitrile 5 27.610.4 0.9 0.60210.003 0.006 toluene 4 40.410.9 1.8 0.23610.018 0.036 chloroform 5 39.410.9 2.0 - 0.26510.022 0.051 (a) K = %Z / %E t A G = ^RTlnK. - 79 -TABLE 20 Solvent Stabilization Factors for 1,2-Dichloroethylene, (Z-) and (E-) (71) Solvent No. of Determ. % E ' Range in % E_ <•••" (a) -AG^ Range in AG neat 3 19.0+0.2 0.4 0.88910.008 0.014 pentane 4 26.5±0.4 0.9 0.62710.019 0.040 ether 5- 20.2±0.7 1.6 0.84310.025 0.059 benzonitrile 5 16.1±1.2 2.8 1.01 ±0.030 0.071 acetonitrile 4 13.210.5 1.1 1.16 ±0.029 0.060 tolune 4 20.410.2 0.3 0.83510.053 0.011 nitromethane 4 15.311.0 2.0 1.05 10.041 0.090 (a) K = %Z / %E, AG = -RTlnK. - 80 -ERRORS Disc integration was used to measure the area under the gas chromatographic peaks; McNair and Bonelli (80) report the relative standard deviation of this method for a given sample to be 1.3%. A surprising result of their study indicates that the cut and weight method also has reasonable precision (1.7%). They mention the main disadvantages of the cut and weigh method l i e in the use of thin chromatographic paper and the destruction of the chromatogram by cutting. These problems have been circumvented by using fast chart speed, giving broader peaks with greater area (greater paper weight), and by taking photocopies of the chromatograms. The photocopy paper is heavier than the chromatographic paper and of uniform density. This cut and weigh technique was generally applied in this work in cases where the baseline was curving, such as for a peak on the t a i l of a large solvent peak. The precision of the measurements i n the section dealing with solvent effects on the position of equilibrium is expressed as the average value and one standard deviation. The standard deviation is calculated as an "instant" standard deviation (80). This means that the range is multiplied by a factor determined by the number of measurements made. The range i s the difference between the largest and smallest measurement. The r e l i a b i l i t y of this method in giving a value close to the calculated value decreases as the number of measurements made increases. Although v.p.c. is the preferred method of analysis, problems such as overlapping or inseparable isomers, and isomers unstable or isomerizing under the v.p.c. conditions, have made necessary the use of n.m.r. spectral integration. - 81 -The errors involved in n.m.r. integration are greater than those involved i n v.p.c. work. The integral measurements on a spectrum depends on various factors. The concentration of the sample, the size of the peaks integrated, extraneous peaks, the operation of the n.m.r. instrument, and the interpretation of the integrals, can a l l affect the r e l i a b i l i t y of measurements. The equilibrium values obtained by n.m.r. spectroscopy were generally based on triplicate integrals of the peaks in question. The integrals for these peaks were recorded at the optimum integral height. In general, i t was found that the ratio of the two peaks of an equilibrated isomer pair in different samples could be integrated within 2%. With pure samples and higher concentrations i t was possible to reduce this range, thus, for a given pair of isomers which were d i f f i c u l t to separate, small amounts of the separated isomers were equilibrated and their n.m.r. integrals measured. In addition, a larger sample of the unseparated isomers was equilibrated in the same manner. As this sample was in greater concentration in the n.m.r. solvent, a more reliably integrated n.m.r. spectrum was obtained. DISCUSSION Photoisomerization of Crotonaldehyde § tiglaldehyde Prior to 1969, the Z_ isomer of crotonaldehyde, (5) had not been reported in the literature (31).. This fact is remarkable, considering the extensive amount of work that has been reported on the chemistry of crotonaldehyde (81). In this thesis we report the preparation of the Z_ isomer of crotonaldehyde as a mixture with E-5 by the photoisomerization of E-5_. The presence of the Z isomer - 82 -was readily detected by n.m.r. spectroscopy. The i r r a d i a t i o n of E-5 has been investigated by a number of workers (82-86) over the past t h i r t y years. In some reports (82,86), an e f f o r t was made to detect the presence of Z-5_; i n other reports, the presence of t h i s compound i s inferred as an intermediate i n a photochemical process (84). In any event, no v a l i d report revealing knowledge of Z-5_ has been presented. One early attempt to detect Z-5_ i n commercially available sample was reported by Blacet et a l . (82), who irradiated an acidic solution of E_-5_. By analogy to other compounds (87), this treatment was expected to convert a l l the Z_ isomer to the more stable E_ isomer. The absorption spectra of the p u r i f i e d i r r a d i a t e d sample was found to be i d e n t i c a l to that of the p u r i f i e d commercially available sample and Blacet and co-workers concluded that the commercial crotonaldehyde was e s s e n t i a l l y the pure E_ isomer. More recently, during the vapour phase photolysis of E-5_, Allen and P i t t s (86) investigated the p o s s i b i l i t y of isomerization to the Z_ isomer. During the i r r a d i a t i o n of the aldehyde vapour, the sample c e l l , equipped with suitable windows, was placed i n the path of the sample beam of an infrared spectrophotometer. The Z_ aldehyde was not detected. The presence of 3-butenal, (72) was not detected i n this work; nor was the presence of ethylketene, (73) or enol-crotonaldehyde (74). H rj= c=0 ^C^CHOH H H 72 73 74 - 83 -These results are not surprising as Sifniades reports that the equilibrium of 5 and 72 contains only 0.35 and 1.0% of the unconjugated isomer at 150 and 210"', respectively (88). Furthermore, P i t t s and co-wofkers (85,86) report that 73 and 74 are short-lived" unstable compounds. In the formation of 72 McDowell and Sifniades report two possible t r a n s i t i o n states, A and B (84). hv Z-6 CH -H i 2 i CH CH - CHO A B Yang and Jorgerisen (89) l a t e r showed that a t r a n s i t i o n state analogous to A was involved i n a,B to 8,Y-isomerizations i n ketones. In deuterated solvents, deuterium was incorporated i n the 0,Y-isomer, as expected for an enol intermediate. In l i g h t of Yang and Jorgensen's study and the fact that 7_2 i s reported to be a primary photochemical product i n the photolysis of croton-aldehyde (84), suggests that the formation of the deconjugated isomer. 72, from Z-5 rather than from E-5, may be an important step i n the photochemical processes of 5_. One may speculate as to why Z-5 i s observed by i r r a d i a t i o n with 3000A * l i g h t i n th i s work and riot i n other photolyses. The * f f i a x for the n,ir absorption f o r E-5_ occurs at about 3010A (90); thus i r r a d i a t i o n at 3000A may produce Z-5 by " o p t i c a l pumping1* (91,92) as 95% of the i r r a d i a t i o n produced by the Rayonet tubes f a l l s between 2800 and 3200A with a maximum output at 3000A (93) - 84 -In a recent communication from Professor R. J. Crawford, he reports the preparation of Z-5_ as a mixture with E-5_ by pyrolysis of the monoepoxide of butadiene (75). 290°, 2 h Z-5 and E-5 0-(35%) (65%) Factors Affecting Equilibrium Positions The use of conformational free energy differences for a group on cyclohexane, a measure of the group's space f i l l i n g a b i l i t y , to determine the possible steric interactions in an ethylenic system must be executed with caution. In the ethylenic system, bond angles, and bond lengths d i f f e r from those in cyclohexane. In the olefin, cis substituents are coplanar whereas in cyclohexane the substituents are staggered with respect to the adjacent methylene. With the use of molecular models (Framework Molecular Models, Prentice-Hall, Englewood C l i f f s , N.J.) i t can be shown that approximately the same relative steric environment surrounds a group bn cyclohexane as on ethylene. With the ethylenic system, a methyl group was used as a standard cis group in comparing steric interactions with other groups. With the ethylenic system i t was necessary to keep carbonyl groups and the ethylenic system coplanar in order to correlate the steric environment of this group to the cyclohexane system in which the group may freelyrotate. In this work, the relative bulkiness of substituents in the cyclohexane system (measured by a comparison of free energy differences) w i l l be correlated to the relative bulkiness in an ethylenic environment, keeping in mind the fact that their use is only qualitative in nature. - 85 -Before beginning a discussion of the factors affecting the equilibrium p o s i t i o n of various olefins i t would be useful to label" each possible interaction. Effect I: This effect involves a purely s t e r i c i n t e r a c t i o n , involving only van der Waals repulsion. Effect I I : This effect involves an e l e c t r o s t a t i c repulsion or a polar  repulsive term which may arise between two polar groups. Effect I I I : This effect involves a conjugative s t a b i l i z a t i o n by resonance which i s greater i n the trans isomer than i n the cis isomer, where s t e r i c d i s t o r t i o n from coplanarity of the carbonyl group and double bond i s present. This d i f f e r e n t i a l resonance s t a b i l i z a t i o n , (DRS), must be considered as a single effect involving two inseparable interactions, a s t e r i c factor (effect I) and a resonance s t a b i l i z a t i o n . Effect IV: t h i s attractive force may resu l t from a van der Waals attraction or an e l e c t r o s t a t i c (polar) attraction between cis oriented groups. This attractive interaction i s found between halogens, between methyl and n i t r i l e , between methyl and halogen, and between halogen and n i t r i l e (21). The Z:iE equilibrium r a t i o for crotonaldehyde, established by acid c a t a l y s i s was found to be 2:98. This equilibrium position may seem anomalous, CH, H CH H Cl H 3N / \ / \ / / \ / \ / \ H CHO H CH, H CHO E-5,98% E-76,76% E-3_, = 100% - 86 -as the Z:E_ equilibrium r a t i o f o r 2-butene, (76) i s reported to be approximately 24:76 (48-52). ; I f one considers solely the s t e r i c interactions of the methyl and formyl groups (effect,1), one may expect the %1 isomer i n 5_ to be not greater, and perhaps less than the % E isomer i n 7_6_: as the conformational free energy difference, -AG, for the formyl group on cyclohexane i s 1.35 kcal./mole (94) compared to 1.7 kcal./mole for the methyl group (95). Therefore, there must be some additional consideration, other than a pure s t e r i c interaction, to explain the equilibrium position for crotonaldehyde. Shapet'ko et a l . (29) reported that the cis form of 3_ completely converted to the trans form at 22° within 2 h. Ivanov, et a l . (28) f e l t that the s t a b i l i t y of E-3 compared to _Z-3 was due to a repulsive term (effect II) exi s t i n g between the polar chlorine atom and formyl group; however, they did not t o t a l l y discount a s t e r i c i nteraction (effect I or effect I I I ) . With crotonaldehyde a polar repulsive interaction between the formyl and methyl groups i s u n l i k e l y and a purely s t e r i c interaction i s not adequate to account f o r the equilibrium; therefore the greater s t a b i l i t y of the E- isomer must be explained by d i f f e r e n t i a l resonance s t a b i l i z a t i o n (effect I I I ) . For crotonaldehyde the equilibrium position may be r a t i o n a l i z e d as follows: a purely s t e r i c interaction (effect I ) , which would be expected to produce an equilibrium with less than 75% E_ (as i n 76); and the d i f f e r e n t i a l resonance s t a b i l i z a t i o n term (effect I I I ) , which together with effect I, produces an equilibrium f o r 5^  with 98% of the E_ isomer. The dominance of the crotonaldehyde equilibrium by DRS suggests that t h i s effect may be present i n other carbonyl compounds, notably ketones and esters. I t has been reported that for methyl crotonate, (9), 82% of the E isomer i s - 87 -present at equilibrium (34). As the conformational free energy terms for methyl and carbomethoxyl are 1.7 and 1.1 kcal./mole, respectively (95), the equilibrium value of 82% E_ in 9_ suggests that DRS is much less important for carbomethoxyl than for aldehyde. CH, H CH CH \ / \ / / \ / \ H CO CH H COCH, 2 3 3 E-9,82% E-47, >99.5% For the methyl ketone 47, the E isomer i s at least 99.5% at equilibrium, despite a methyl-methyl steric interaction. This suggests that DRS is an important stabilizing factor for the trans isomers of a,8-unsaturated ketones. Consequently, for crotonaldehyde and the corresponding methyl ester and ketone we may state that the conjugative stabilization a b i l i t y is of the order: C0CH3 = CHO > C02CH3 This order i s expected to correspond with the -M effect described by Ingold (38). The above order is also supported by the relative amounts of the deconjugated B.,y-isomer present at equilibrium. For crotonaldehyde, 5_ at 210°, 1% of the B,Y-isomer i s present (88); for methyl crotonate, 9_ at 300°, 5% of the B , Y - i s o m e r is found (96) . The other factor that may now be varied is the 8-substituent. In 6, 9 and 47 only methyl groups were on the B-position. In the following aldehydes and ketones the B-methoxyi group would be expected to increase the DRS by a +M effect (38). - 88 -CHO H CHO H CH_0 CH 3 \ / 3 \ / 3 \ / 3 / \ / \ / \ H CHO CH3 COCH3 H C0 2 E-3i, = 1 0 0 % * 1 0 0 % Uri2!.' >99«8% The results show that this enhancement of DRS dominates the equilibrium of 23, 31 and 48; although a certain amount of polar repulsion probably exists between the methoxyl and formyl groups. This polar repulsion (effect n ) combined with the d i f f e r e n t i a l resonance s t a b i l i z a t i o n may be seen i n the methacrylate esters. CH, CH, Br CH, CH 0 CH \ / 3 \ / 3 3 \ / 3 / \ / \ / \ H C02R H C02R H C02R E-78,83% E-79, >99% E-23_, >99.8% In t h i s series a correlation of +M effects with percent E_ isomer at equilibrium i s found despite differences i n the s t e r i c requirements of the B-substituent. The next step i n the discussion involves the addition of an a-substituent to the above compounds. I t has already been shown that i n the ester series an a-methyl group has l i t t l e e f f e c t ; compare 9_ to 78. In the aldehyde series the equilibrium for tiglaldehyde, (6) was found to be 99% i n favour of E_ form. CH, H CH_ CH- CH Br \ / \ / 3 \ / / \ / \ / \ H CHO H CHO H CHO E_-5,98% E-6,99% _Z-?0_, =100% - 89 -Considering the experimental errors involved, the equilibrium values, f o r 5_ and 6_ must be assumed to be very s i m i l a r . The possible 1,1-steric interaction can be considered unimportant i f one considers the work of Chen and Le Fevre (97) who measured the s-cis : s-trans ratios for crotonaldehyde and tiglaldehyde and found them to be 1:23 and 1:18, respectively. The addition of an a-bromine to 5_ was also found to have a negligible effect on the equilibrium. This i s shown i n compound 30. In continuing t h i s study of equilibrium positions the obyious approach i s to consider the effect of an additional 8-substituent. I t would be of interest to compare the halogens, which influenced the equilibrium of the 8-chloroacrolein, (3) by polar and to a lesser degree, s t e r i c and d i f f e r e n t i a l resonance s t a b i l i z a t i o n e f f e c t s ; to the methyl group, whose influence on the equilibrium of crotonaldehyde (5) involves only DRS. Furthermore, i n compounds with two 8-subst.ituents d i f f e r e n t i a l resonance s t a b i l i z a t i o n w i l l be evident i n both isomers. In the aldehyde series the following e q u i l i b r i a for the 6-chloro and B-bromocrotonaldehydes were measured. CH, H CH H \ / 3N / / \ / \ Cl CHO Br CHO Z-34,68-72% Z-37_, 70% These results indicate that the dominating factor i n the equilibrium i s DRS, as the methyl-formyl repulsive term i n the J .isomer results i n conjugative d e s t a b i l i z a t i o n of that isomer r e l a t i v e to t h e i s o m e r . The halogen-formyl repulsion, a combination of s t e r i c and polar repulsive effects, i s not as important. - 90 -St e r i c interactions give r i s e to the same conjugative s t a b i l i z a t i o n -d e s t a b i l i z a t i o n effects i n the halocrotonaldehydes as i n 6_. No data for the analogous haloketones i s available but for the esters we f i n d that the DRS for the methyl group, which was^minor i n the methyl crotonate, 9^  i s readily overcome by the repulsive polar term (effect II) between halogen and ester i n 24 (11). I t should also be noted that the conjugative a b i l i t y of the chlorine, which i s greater than that of methyl, must also influence the equilibrium. Cl H \ J / \ CH CO CH 3 2 3 E-24,87.5%' Continuing to increase the complexity of our compounds one can now consider the effect of an additional a-substituent to the 6-halccrotonate series. The following aldehydes 33, 36 and 38 were prepared and equilibrated. C H 3 _ V C H 3 - ^ B r / \ / \ / \ Cl CHO Br CHO Br CHO Z-33,31% Z-36,39?o E-3jB,78% In the aldehydes, 33 and 36, apparently the o r i g i n a l d i f f e r e n t i a l resonance s t a b i l i z a t i o n that existed i n crotonaldehyde, E_-5_, i s now overcome by the following combined effects: the polar and/or s t e r i c repulsion between the formyl group and halogen atoms (effects I and I I ) , the attractive interaction between - 91 -the halogen and methyl (effect iy) 3 ^ the s t e r i c repulsion between the two methyls (effect I) . In comparison of aldehydes 33_ and 36, with 3_4 and 37, indicates the only added factors are the halogen-methyl at t r a c t i o n (effect IV) and the methyl-methyl s t e r i c repulsion (effect I ) , which i s enough to account f o r the difference. The 1,1-interaction between the methyl and formyl groups i s not considered important (see p. 89). Again there are no ketones of the same structure available for equilibrium determinations; however, D. P. Kaushal of this laboratory has recently prepared the methyl 8-bromotiglate, (80) and determined the equilibrium position. In t h i s compound the additional cy-methyl group has increased the amount of E_ isomer compared to 24. Br CH, \ / 3 / \ CH3 C0 2CH 3 E-80,96% This assumes that both the chlorine i n 24 and the bromine i n 80_ have s i m i l a r interactions; at least the effect of the a-methyl group i s shown to increase the amount of E isomer. The increase i n the amount E_ i n 80_ by halogen-methyl att r a c t i o n and methyl-methyl repulsion compared to 24_ i s the same factor operating for the aldehydes 33_ and 36_; With 38, the new interactions i n comparison to 37_ are a bromine-bromine and a bromine-methyl interaction. The former i s expected to have l i t t l e effect on the equilibrium (21) and the l a t t e r i s expected to be a t t r a c t i v e , s t a b i l i z i n g - 92 -the E isomer (effect IV). F i n a l l y the aldehydes 35_ and 3_9 were prepared and equilibrated. t-Bu H t-Bu H ~ \ / ~ \ / / \ / \ Cl CHO CH 0 CHO 3 Z-35, =100% Z-39,47% In aldehyde 35, which i s s i m i l a r to the 8-halocrotonaldehydes 34 and 37, the s t e r i c effect of the t e r t - b u t y l group controls the equilibrium. With aldehyde 39/ however, the methoxyl group, which has a very strong +M effect ( r e c a l l compound 48) has an influence equal to that of the t e r t - b u t y l on the equilibrium. While the effect of the t e r t - b u t y l group i s completely s t e r i c , the effect of the methoxyl i n the equilibrium position of 39_ i s complex. The methoxyl group must be coplanar with the conjugated system for most effective overlap i n the conjugated system, and i n th i s orientation i t i s forced into a cis interaction with the formyl group. Figure 9.- S t e r i c interactions i n Z- and E-39 - 93 -It i s important to note that the s t e r i c repulsion between the S-substituents i s least when the methoxyl i s directed towards the formyl group." I t i s also l i k e l y that there i s a polar repulsive term existing between the methoxyl and formyl groups, s i m i l a r to that between halogen and ester (effect I I ) . In the methyl ketone series compounds s i m i l a r to 35_ and 39_ have been prepared and equilibrated. t-Bu H t-Bu H t-Bu H r = \ / — \ / — \ Cl COCH • CHO C0CH_ (CH ) N COCH 3 3 ^ 3 2 J Z-49, =100% £-50,85% Z.-5_l_, =100% As expected the chloroketone 49_ exists e n t i r e l y i n the Z_ configuration. With the ketone 50, however, we note that the s t e r i c interactions between the B-substituents are ess e n t i a l l y the same as i n 39. (Compare Figures 9 and 10). The cis interaction i n 50_ i s much more severe and there may be some doubt concerning the coplanarity of the methoxyl and double bond, a requirement for maximum overlap. CH3 CH3 C^ = C^  .0 ,C—CH 3 CH„ :o planar non-planar Figure 10 - St e r i c interactions i n Z-50 - 94 -In Figure 10 the acetyl group i s drawn i n the expected s-cis conformation (98*99). The enamino ketone 51 was prepared f o r study. Attempts to isomerize this ketone to produce the other isomer f a i l e d . This ketone has a 3-dimethyl-amino group, whose +M character i s greater than that of the methoxyl group (38). However, the s t e r i c bulk of the dimethylamino group, i f coplanar with the double bond, must be considered to be as great of greater than that of a t e r t - b u t y l group. The resulting s t e r i c s i t u a t i o n i s improbable and the dimethyiamino group must exist i n a non-planar arrangement with i t s +M character "much reduced (11). The configuration of the isomer may also be deduced by comparison to the equilibrium positions of the compounds prepared by Gardner and McGreer (11) and the aldehydes mentioned e a r l i e r . The information given i n Figure 11 gives the correlation of the equilibrium positions f o r various 8-tert-butyl substituted compounds prepared i n t h i s work and'by Gardner and McGreer.,(11) . Thus, the p o s i t i o n of equilibrium-, fo r 51 l i e s between the e q u i l i b r i a for the ketones 49 and 50. In concluding the discussion of the equilibrium positions of the various aldehydes, ketones and esters studied i t would be useful to present t h e i r equilibrium i n tabular form. Figure 12 presents these e q u i l i b r i a . Column I of Figure 12 i l l u s t r a t e ? the effect of a single g-substituent on the equilibrium of aldehydes, ketones and esters. Whether the effect i s due to s t e r i c interactions (effect I ) , polar repulsion (effect II) or d i f f e r e n t i a l resonance s t a b i l i z a t i o n (DRS, effect I I I ) ; very l i t t l e of the _Z isomer i s present at equilibrium. - 95 -t-Bu COCH, J 3 / \ Cl H fa") £-49,0%^ 3 t-Bu COCH " \ / 3 / \ c a y 2 N H E-51,0% 0 0 t-Bu COCH, " \ / 3 / \ H CH30 E-50,15% Ca) t-Bu CHO \ / / \ Cl H E-35,6% Ca) t-Bu CHO " \ / / \ CH 0 H 3 E-39,53%^ t-Bu CC) CK / 2 3 / \ Cl E-14,1.5% H Cb) t-Bu V r CH 0 3 CO C H J 2 2 5 H E-15,23% Cb) •Bu V r C l J CN t-Bu CN " \ / / \ H E-12,0.5%^ CCH )^ N J i. E-11,22.5% H (b) t-Bu \ / CN CH 0 H 3 E-10,60% t b ) Figure 11 - Equilibrium positions for g-tert-butyl aldehydes, ketones, n i t r i l e s and esters. (a) - this work. (b) - reference (11). CH H J / \ H CHO E-5_,98% (a) II CH. CH, J 3 / \ H CHO E-6,99% (a) III CH. H J / \ Cl CHO Z-34,70% Ca) IV CH. Br J Br / \ CHO E-3_8,78% Ca) V CH. CH, J 3 Br CHO Z-36_,39% Ca) Cl H \ / / \ H CHO E-3,*100% Cb) CH. Br J / \ H CHO Z-30_,=400% Ca) CH H J / \ Br CHO Z-37,70% Ca) CH CH y 3 / \ Cl CHO Z-33_,31% Ca) CH,0 H 3 V _ / / \ H CHO E-31_,=100% Cc) CH. CH, y / \ H COCH, E-47,=O.00% (a) CH 0 H 3\ / / \ CH, COCH, 3 3 E-48,=100% Ca) Cl H / — \ H COCH. E-4_,*100% Cb) ^3 / H ^3 P1* CH H CH CH / V / 3 3 w V \ / 3 / \ / \ / — A > = \ H C02CH3 H C02CH3 Cl CO CH Br CO CH 1-9,82% Cd) E-78,83% Ce) Z-24,12.5% Cf) 3 Z-80,4%Cg)3 Figure 12 - Equilibrium values of selected aldehydes, ketones and esters. (a) - this work, (b) - ref. C28,29),. Cc) - ref. (30). Cd) - ref. f34) fe) - ref CM Cf) - ref. CH). Cg) - D. P. Kaushal, this laboratory. 1 J 1 J ' C 9 6 ) - 97 -Column III indicates that a second B-substituent can have considerable effect; either predominantly polar (effect II) as in 2£, 3_4 and 37, or predominantly conjugative (effect III) enhanced by the +M character of the B-substituent, as in 48. This last example completely reverses the equilibrium situation which would be expected in column I. Columns II, IV and V demonstrate the effect of an additional a-methyl or halogen. In these cases the effect depends upon the interactions between the group added and the B-substituents. Attractive forces between halogen and methyl (effect IV) and repulsive forces between methyl and methyl (effect I) must be weighed to predict the total effect. This w i l l be shown in the section dealing with free energy additivity. In considering the above equilibrium situations, i t may be instructive to consider how the n i t r i l e compounds compare to the aldehydes, ketones and esters. The n i t r i l e group is a r i g i d symmetrical polar group with l i t t l e steric bulk (conformational free energy term (95) is 0.2 kcal./mole), while the aldehydes, methyl-ketones and methyl esters are freely-rotating polar groups with preferred orientations and a much larger bulk. The conformational free energy terms for aldehyde (92) and methyl esters (95) are 1.35 and 1.1 kcal./mole, respectively. Therefore, we may expect the steric factor, which dominated the equilibrium of the carbonyl containing olefins, to be less important when studying the n i t r i l e s . Thus, for the B-methyl, B-methoxyl, B-bromo, and B-chloroacrylonitriles the steric factor is of less importance and attractive forces in some cases become the major influence on the equilibrium position (21,46). - 98 -CH, / CN CH30 / \ H H Z-17,61% \ / / \ H CN Br H H Z-56,25% CN Cl \ / / \ H H Z-54,51% CN J H Z-81,69% It i s especially interesting to note 56_ in which the methoxyl can adopt a planar conformation in either isomer, thus 25% of Z-56 is present at equilibrium compared to =0% for the g-methoxy ketones and esters. Gardner and McGreer have recently measured the equilibrium position of a variety of a,g-unsaturated n i t r i l e s (11,39). It was found that the above attractive forces between halogen or methyl and the n i t r i l e group could be overcome by the steric requirements of a bulky g-substituent, such as isopropyl or tert-butyl. Nuclear Magnetic Resonance Spectra The nuclear magnetic resonance spectra of the aldehydes seems worthy of comment. Using Pascual's data (12), one may predict that the resonance positions of the g-vinyl proton cis to the formyl group should be at higher f i e l d compared to the trans g-vinyl proton. The results of this work, in accord with that of Rapoport (13), indicate the opposite trend; the cis g-vinyl proton in crotonaldehyde (5) appeared at lower f i e l d (0.21 ppm) than when trans. The stereochemistry of crotonaldehyde (5) was unambigously assigned by the cis and trans coupling constants. - 99 -With the tiglaldehyde, (Z-) and (E-)(6), the formyl proton resonances appeared at -0.38 and +0.45T, respectively. This i s consistent with earlier studies (13,16) which allow configurational assignment of 2-methyl-2-enals on the basis of formyl chemical shifts. When a formyl proton i s cis to the 3-substituent, i t occurs at O.OT; when trans, i t appears at 0.75 to 0.80T. The spectra of 6^were recorded on neat samples and thus may be affected by the solvent, however, their relative positions may be used to assign the stereochemistry. The similarities i n the n.m.r. spectra of tiglaldehyde, (Z-) and (E-)(6) and the 2-methyl-2-pentenal, (Z-) and (E-)(7), prepared by Rapoport (13) further defend the stereochemical assignments. In both 6^  and 7, the g-vinyl protons and the a-methyl groups show no difference in the chemical shift for the Z and E_ isomers. The g-methylene protons of 7 and the methyl protons of 6^  are both deshielded i n the cis environment of the aldehyde by 0.17 and 0.08 ppm, respectively compared to the trans positions. With the g-haloaldehydes the deshielding effect of the g-halogen on the cis a-methyl or a-proton was consistent with the assigned stereo-chemistry. Finally, the stereochemical assignments for a l l aldehydes could be readily determined by the g-methyl or g-tert-butyl resonance, which in a l l cases f e l l 0.08 to 0.30 ppm to lower f i e l d when cis oriented, with respect to the aldehyde, than when trans. The g-bromoacrylonitriles, (Z-) and (E-)(54) were prepared for equilibration study by the scheme shown in Figure 7. This follows a - 100 -TABLE 21 Chemical s h i f t s ^ and coupling constants^ for haloacrylic acids and their corresponding acid chlorides, amides and n i t r i l e s . Structure JCHz) Ha Chemical Shift ( T ) HB Other Solvent X C00H \ / J \ H H H H \ / / \ COOH Br 8.5 3.37 2.88 Cl* 8.0 3.80 3.16 Br 13.8 3.47 2.23 Cl* 13.2 3.78 2.52 •1.94 CDC1 CC1 •2.13 CC1 =CC1 2 2 CC1 \ / / \ H COC1 H Br 8.3 3.08 2.85 Cl* 8.0 3.38 3.05 CC1 4 neat ( c ) X CONH „ , , o j „ r , , . ^ / 2 Br 8.1 3.24 3.18 2.5-3.3 Acetone / \ Cl* 8.0 3.66 3.31 " X H \ / Br 13.5 3.28 2.56 2.7-3.7 / \ :ONH 2 H C Q N K Cl 13.0 3.52 2.71 - 101 -TABLE 21 (continued) Chemical Shift (T) Structure X J(H Z) Ha Hg Other Solvent \ / CN H H X H \ / / \ H CN Br Cl Br Cl* 8.4 7.8 14.9 14.0 3.68 4.05 3.88 4.11 2.72 2.89 2.56 2.75 CC1 CC1 CC1 CC1 (a) x = ±0.02 ppm. (b) J = ±0.1 Hz. (c) External TMS. * Reference (73). - 102 -similar scheme used by Kurtz et a l . for the g-chloroacrylonitriles (73). Thus a comparison of the n.m.r. spectra at each stage in the preparative sequence i s valuable in stereochemical assignments. Table 21 demonstrates the similarities in the n.m.r. spectra of the chloro and bromo compounds. In the bromo series of compounds the coupling constants between the vinyl protons were generally higher; 0.1 to 0.6 Hz for the cis coupling and 0.5 to 0.9 Hz for the trans, compared to the corresponding chloro compounds. The a-proton resonances occur from 0.30 to 0.43 ppm to lower f i e l d and those of the 3-proton appear 0.23 to 0.31 ppm to lower f i e l d than those of the corresponding hydrogens in the chloro compounds. The diamagnetic anisotropy of the acid, amide and n i t r i l e in the chloro series was noted by Kurtz et a l . (73) who reported the differences in chemical shifts for the cis and trans isomers. The corresponding data for the bromo olefins is shown with that of Kurtz in Table 22. TABLE 22 Chemical Shift Differences between _Z and E_ 3-protons of 3-haloacrylic Acid Derivatives Compound ( E ) T - (Z)T Cl (a) Br (b) acid 0.64 0.65 amide 0.60 0.62 n i t r i l e 0.14 0.16 (a) Reference (73). (b) This work. - 103 -Table 22 demonstrates that the diamagnetic anisotropic deshielding of the B-proton cis to the functional group, compared to the trans arrangement, is the same in both the chloro and bromo series; thus supporting a l l the stereochemical assignments made. Solvent Effects on Equilibrium Positions In order to undertake a quantitative or semiquantitative study of the equilibrium positions in cis and trans olefin pairs i t was necessary to investigate the magnitude of the solvent stabilization effect on the equilibrium position. As pointed out i n the introduction, l i t t l e i f any work of a quantitative nature has been done in this area. Solvent effects have been tabulated for a wide variety of keto-enol equilibria, reaction rates and spectral shifts. In most of these reports, attempts have been made to correlate the solvent effects to a physical property of the solvent. These attempted correlations have been largely unsuccessful on a quantitative plane, due to the complex behaviour of solvent-solute interactions. Therefore, empirical parameters for solvent "polarity" have been developed. The recent interest in this problem is evidenced by an extensive review article (98). This review contains most of the relevant references dealing with this problem. In this thesis, although quantitative solvent effects are measured for five pairs of compounds; only a semiquantitative or qualitative picture is desired in order to estimate possible solvent effects where less quantitative data is available. - 104 -In the literature, despite the fact that equilibrium positions have been measured, no attempt has been made to standardize the conditions of equilibration as to temperature or solvent. In order to correlate the literature data on equilibrium positions, an estimate of the solvent effects on the equilibria must be made. The standard conditions chosen for this work are room temperature in dilute non-polar solvent. Tables 16 to 20 demonstrate the effect a solvent may have on the equilibrium position. From these tables the following trends should be noted: 1. The magnitude of the solvent effect on the equilibrium depends upon the "polarity" of the solvent. 2. The magnitude of the solvent effect also depends on the difference in "polarity" between the two isomers being equilibrated. 3. The direction of solvent effect depends upon the polarity of the individual isomers; the more polar isomer being stabilized in a more polar solvent. In the above statements the term "polarity" is used to designate a l l the intermolecular interactions of which the molecule i s capable; excluding protonation, complex formation and other interactions which produce changes in the substrate. The most readily available solvent or solute parameters; by measurement, calculation or i n the literature; which roughly corresponds to "polarity", are the dipole moment and dielectric constant. Therefore, solvents - 105 -of high dipole moment (polar solvents) w i l l be expected to stabilize the more polar isomer of an isomer pair compared to a non-polar solvent. Also for a given solute, the solvent effect w i l l be greatest for isomers of greater differing dipole moments (polarity). Table 23 shows these relationships. These relationships become particularly important i f one attempts to extend the work of Gardner and McGreer of this laboratory on the concept of free energy additivity in olefin pairs (39). Free Energy Additivity of Olefin Pairs The additivity of free energy terms of simple olefins to predict the equilibrium position in more complicated olefins has recently been proposed by Gardner and McGreer (39). The following example^wi11 demonstrate this idea: X Y X X Y \ / \ = \ / \ \ z z (a) (b) (c) X X Z X z \ \ / = z \ / — \ — — — \ Y Y (a') (b') Cc') For olefin (a), the equilibrium value K is given by the ratio (a) / (a'); the equilibrium value is a measure of the sum total of a l l steric, polar and TABLE 23 Solvent effects on the equilibrium of some n i t r i l e s and dihaloolefins Free Energy difference between isomers (-AG) * CH 3 VC=CHCN H Br XC=CHCN H CH 3^C=CBrCN H Br SC=CHBr H Cl H = CHC1 Neat l i q u i d 0.156 =0.45 =0.47 0.265 0.889 (a) Calc. Dipole moment difference Ca) Exper. Dipole moment difference 0.54 0.45 2.22 0.37 2.72 1.31 2.76 1.91 Solvent D i e l e c t r i c Constant Pentane 1.90 0.280 , 0.016 0.429 0.058 0,627 Carbon tetrachloride .2.23 0.284 0.127 Benzene 2.27 0.161 0.481 0.313 Ether 4.22 0.232 0.249 0.843 Benzonitrile 34.6 0.091' 0.385 0.477 0.396 1.01 Ac e t o n i t r i l e 37.5 0.142 0.486 0.477 0.602 1.16 (a) See Appendix. Values i n Debye units. (b) Reference ( 1 0 0 ) . * Data from Tables 16-20, K = %Z_ / %E, AG = -RTlnK. - 107 -any other interactions between the groups X and Y. In olefin (b), K measures b the total interactions between X and Z. In olefin Cc), i f we neglect interactions between Y and Z, then the interactions between X and Y, and X and Z (which may be measured by K and K ) w i l l determine K . As a b c equilibrium values and are free energy terms their additivity w i l l be expressed as a product of the corresponding equilibrium constants. K, • K, = K a b c As mentioned above, this approach to calculating equilibrium positions does not take into account any 1,1-interactions, nor does i t allow for the possibility of a change in the electron density or the electronic distributions in the double bond. This principle of additivity of free energy terms involving 1,2-disubstituted olefins to give the free energy term for a trisubstituted olefin may be expanded in the following form: X Y \ / \ r w J / — \ w z \ / / w V \ z / — \ w = \ / / \ w \ / V / w z y - 108 -The additivity of free energy terms has been successfully applied to conformational free energies in the cyclohexanes (18). In order to further test the applicability of free energy additivity we can use the equilibrium positions measured throughout this work in conjunction with values found in the literature. Table 24 gives some of these values. In light of the discussions involving solvent stabilization effects on the equilibrium position of olefins, i t is necessary to at least indicate in which direction the equilibrium would be expected to change in moving into a non-polar environment. The f i r s t factor to be considered in attempting to relate an equilibrium measurement made in neat solution (presumably polar) to a non-polar solution is the direction in which the equilibrium w i l l be shifted. The 1,2-dihaloolefins illustrate one aspect of this correlation. The neat solution at equilibrium contains both cis and trans isomers, the neat polar solution stabilizing the cis (more polar) isomer. In a non-polar solvent (pentane), the cis isomer w i l l be destabilized relative to the situation in the neat (polar) solution; and the equilibrium w i l l shift to include more of the less polar (trans) isomer; i.e., -AG approaches zero. This is shown to be the case in Table 23. The other aspect of this situation may be illustrated by crotononitrile. In this compound the more polar isomer is the trans isomer, as the bond moments of the methyl and n i t r i l e group in the trans isomer,are parallel and in_the same direction. Changing to a non-polar solution the equilibrium shifts to include more of the less polar (cis) isomer; i.e., -AG becomes.more negative. This trend is shown in Table 23. In estimating the magnitude of the change in equilibrium position in - 109 -moving from a p o l a r to a non-polar environment, the trends noted on p. 104 w i l l a l l o w a crude estimate to be made. A large e f f e c t i n the e q u i l i b r i u m p o s i t i o n i s found when two p o l a r groups are v i c i n a l on the double bond. This s i t u a t i o n leads to a reasonably p o l a r s o l v e n t and a large d i f f e r e n c e i n p o l a r i t y between the two isomers. A smaller e f f e c t on the e q u i l i b r i u m p o s i t i o n i s found when one p o l a r group and a r e l a t i v e l y non-polar group are v i c i n a l on the double bond. The r e s u l t i n g compound i s p o l a r but the d i f f e r e n c e i n p o l a r i t y between the isomers i s smaller than w i t h two v i c i n a l p o l a r groups. A large e f f e c t w i l l be found i n the 1,2-dichloro- and 1,2-dibromo-ethylenes and 8 - b r o m o a c r y l o n i t r i l e ; a smaller e f f e c t i n c r o t o n o n i t r i l e . A f a c t o r which cannot be estimated i n v o l v e s e q u i l i b r i a measured at high temperatures and perhaps high pressures. U n c e r t a i n t i e s i n v o l v i n g pressure and entropy e f f e c t s and u n c e r t a i n t i e s due to solvent e f f e c t s (neat or i n vapour phase) do not allow a v a l i d b a s i s f o r e s t i m a t i n g c o r r e c t i o n s to room temperature and a non-polar medium. In Table 24 an estimate has been made of the expected d i r e c t i o n of e q u i l i b r i u m s h i f t i n moving from the s t a t e i n which the e q u i l i b r i u m was measured (neat s o l u t i o n ) , to a non-polar s o l u t i o n . P r e d i c t i o n s f o r compounds with f r e e l y -r o t a t i n g groups are d i f f i c u l t to make, due to the dependence of d i p o l e moment on conformation, but perhaps the d i r e c t i o n of change may be determined. A solvent s t a b i l i z a t i o n energy of 0.2 kcal./mole, which w i l l probably r e s u l t w i t h v i c i n a l p o l a r groups, such as two halogens, corresponds to percentage changes of the f o l l o w i n g order: 15%-20%, 20%-26%, 27%-34%, 32%-39% and 39%-47%. - 110 -TABLE 24 Corrections of Equilibrium Positions to Non-polar solvent Compound and Correction to Experimental Temp. State non-polar Reference Equilibrium environment* . Position °C 1 \ / 225 neat ? This work / \ (vapour) H CN E-57,56% C H S H 2 5 \ / 207 neat / \ (vapour) ? This work H CH 3 E-68,47% CH 0 H 3 \ / 300 neat ? This work / \ (vapour) H CH 3 E-69,45-48% E-69,51% 25° neat small 47 Cl H \ / r . t . neat decrease / \ (small) 41 H E-82,25% - I l l -TABLE 24 (continued) Compound and Correction to Experimental Temp. State non-polar Reference Equilibrium environment* Position °C Br H \ / r . t . neat decrease •/ \ (small) 41 H CH 3 E-83,32% CH3 Br \ / r . t . neat increase / \ (moderate) 55 Br H E-84,65% CH CH X / 3 r . t . neat ? 53 / \ H Br E-85,17% Cl CN \ / r . t . neat decrease / \ (large) 46 H H E-81,69% - 112 -TABLE 24 (continued) Compound and Experimental Equilibrium Position Temp. °C State Correction to non-polar environment* Reference CH CH7 / \ H H E-76_,24% " 33% " 33% " 27% " 26% 25° (calc.) vapcur iio " 127 " 95 " 25 CH Cl-CH Cl 2 2 48 49 50 51 52 Cl CH \ / * r . t . neat increase 39 / \ (large) H CN E-26,60% Br CH \ / r . t . neat increase 39 / \ (large) H CN E-27,60% - 113 -E-28,78% E-86,68% E-29,20% TABLE 24 (continued) Compound and Experimental Equilibrium Position Temp. State Correction to non-polar environment* Reference CH3.O H CH. \ / / \ CN r . t . neat 39 C H S 2 5 \_ r H CH„ J CN 210 neat (vapour) 39 CH_ CH, \ / / \ H CN 25 CH OH 3 solution decrease (small) 39 Direction and magnitude of change of the indicated equilibrium position on moving from given state to non-polar solvent. - 114 -Such an effect w i l l be termed a large effect. Using the equilibrium positions from the literature, which are tabulated and commented upon in Table 24; and the equilibria measured in non-polar solvents i n this work which are displayed in Tables 12 to 20, the concept of free energy additivity may now be more extensively examined. Figure 13 explores the p o s s i b i l i t i e s of free energy additivity. The correlation between the experimental and calculated values is quite good considering the possible errors involved in the experimental measurements. With two bulky substituents on the same carbon atom i t is possible that the additivity concept w i l l give poor results, especially i f one of the substituents acts i n a conjugative manner and has a preferred orientation. The free energy additivity displayed in Figure 13 deserves comment. In general, agreement is reasonable between the experimentally measured equilibrium values and the equilibrium values calculated by additivity, without any consideration of the solvent effect. The term solvent effect used here w i l l apply to any correction needed in the equilibrium value on changing from a polar solvent (usually the neat solution) to a non-polar solvent. The term component equilibrium w i l l apply to the measured equilibrium value of a compound used to predict the equilibrium of the more highly substituted olefin. The term predicted equilibrium w i l l apply to the equilibrium calculated by free energy additivity (equilibrium ratio multiplication) for the more highly substituted olefin. The term experimental equilibrium w i l l apply to the experimentally measured equilibrium of the more highly substituted olefin. - 115 -Figure 15 Examples of free energy a d d i t i v i t y i n o l e f i n pairs being used to predict o l e f i n i c e q u i l i b r i a . ALDEHYDES l a . l b . CH. H Cl J / \ 70 30 CHO NP CH, H J CH. J J \ CH. H H i i e NP 74 — CH. CH. V J / \ / \ Cl CHO Cl H 22* 30 — 20 6 NP 80 — H V CH J 3 Cl / — \ H 25 6 P 75 -22 P 78 37* 63 CH. J CH. / \ Cl CHO NP 69 — 2a. 2b. CH. H y / — \ Br CHO ^ W 30 — C H 3 \_ r Br H J 70 t — NP 30 — CH„ CH. y / \ H 26 74 H NP CH. y CH. ! \ CHO Br H 17_ 83 L. H V r Br CH J 3 A H 32 68 2_8" 72 32 P 68 CH. CH. y / \ Br CHO 39* IT-NP - 116 -Figure 13 (continued) 3a. 3b. CH. H J Br CHO CH. 70 30 N?_ H J Br / \ CHO 70 30 "NL CH. Br J / \ H H 68" 32 P CH, r Br J Br H ^ P 35 L. H V r Br J Br H 48 c 52 NL H 18 81 p 19 CH. r Br J Br CHO 79* NP 21 — NITRILES 4b. CH, CH„ CH, H H CH H H 4a. \., / 3 . \ / . \ / 3 . \ / 14 P / \ " T \ / \ / \ 86 H H H CN Cl H Cl CN t 39 256 69 e 74 —- 6 T E. 75 P 3T £ CH. F H J CH. H H CN \ / V ci CN / \ CH y 3 Cl H 16_P 84 CH CH y 3 / — \ Cl CN l l 1 89 NP - 117 -Figure 13 (continued) CH. J / \ CH. H CN 80 -CH. CH y 3 K H 26 74 — CH CH y 3 Cl H NP 80 — CH. Cl \ / / \ CN CH, 60 -H H J A CN 39* 6T ML CH y / — \ H CN <9* H CH. Cl V / — \ CN 60 L. H H \ / / \ Cl CN 31 -14? 86 13* 87 26 74 CH CH, ^ / 3 / \ Cl CN 89 CH, Br CH, H \ / . N 1 / \ / \ H H H CN ,32 -39 t 61 NP i 8 42 CH. H Br y CN I T NP Cl CH. Cl H \ / H A \ / / \ H H CN 58 P 42 Cl CH. \ / / \ H CN 118 -Figure 15 (continued) 7. Br V r H CH J 3 H e 68 P 32 ~ Br 49l 51 H \ / 67 P 33 CN NP Br H CH„ \ / / \ CN e 60 p 40 -8. CH30 H CH„ \ / / \ CH30 H 53 47 H H \ f / \ CN 75' 25 if 23 CHjO H CH, \ / / \ CN 22 ' -9. C H S 2 5 H CH„ \ / / \ H C 2 H 5 S H H \ / I \ CN 59x 41 C H S 2 5 H CH. \ / / \ CN 53 47 56l 44 68 32 P ^ 3 ^ 3 ,oP ^ / C H3 io. A / \ — / 2 2 \ / / \ * / \ 78 / \ H H H CN H CN 26 e 39*- 20 6 74 NP 6 i NP 80 ?_ RALOOLEFINS 11. CH„ J / \ H Br CH, H 6 8 % 32 — H H J / \ 86* 14 CH, 74 e -26 — H Br J "A CH, 83 6 17 £ - 119 -Figure 13 (continued) 12. CH J / \ H Br II H Br Br \ / / \ H 66? 34 CH. J / \ 3r Br H 68 e P 32 ~ 48* 52 NP 65 c p 35 -13. CH J / \ H CH, CH H H H / \ Cl i i P ) 89 ' CH J CH. / \ H Cl 26 e NP 74 — 25 6 p 20* NP 80 — 14. CH. H y "\ Cl H Cl y / — \ Cl H H V 52_P 48 CH y Cl / \ Cl H 75 e: 25 P 27' 7T 'NP 39 61 P t. Experimental isomeric equilibrium ratio, this work. e. Experimental isomeric equilibrium ratio, from table 2, p. 21; and p. 19. p. Predicted equilibrium ratio using free energy additivity. * The ratios given i n the table represent the % r a t i o for the drawn structure over that of i t s isomer. NP Non-polar environment. P_ Polar environment (neat solution) . - 120 -When more than one set of component equilibria were available to obtain predicted equilibria, the agreement between these values is quite good. There are, however, two notable exceptions. Agreement is poor between Examples la and lb, and 4a-d and 4e. An examination of the component equilibria show that the common term is the equilibrium ratio of 2-chloro-2-butene. Reinsertion of the predicted equilibrium from Example 13 (the bracketed value) into Examples lb 22 15 and 4e, gives ratios of yg- and g^ -, respectively, which are comparable to the other predicted equilibria. This indicates that the experimental value of 2-chloro-2-butene may be in error. In discussing the solvent effect, mention w i l l only be made where the effect is expected to be large or in the wrong direction with respect to the product equilibrium. It should also be noted that the measured experimental equilibrium w i l l also be subject to solvent effects. The difference between the predicted equilibrium values and the experimental values in Examples 1 and 2 cannot be reconciled with the expected solvent effect on the component equilibrium values. For the 1,2-dibromoaldehyde in Example 3 the agreement is good. In Example 3b the expected solvent effect for the dibromopropene would increase the difference between the predicted and experimental values. In the n i t r i l e series a l l predicted values for Example 4 are high, except 4e which was discussed earlier. Solvent effects (on the component equilibrium of the B^chloronitriles) would be expected to reduce the value of the predicted equilibrium, by about 5%, which would improve the correlation between predicted and experimental ratios. - 121 -In Example 5 solvent effects on the bromopropene would be expected to increase the predicted equilibrium slightly and give better agreement with the experimental equilibrium. In Example 6 the solvent effect on the chloropropene would be expected to increase the component equilibrium slightly whereas both the 3-chloroacrylonitrile and the B-chloromethacrylonitrile would be expected to increase considerably. The net result of the solvent effects i s to maintain good agreement between experimental and predicted equilibrium positions. The solvent effect on the bromopropene in Example 7 would be expected to increase the equilibrium value slightly, whereas the solvent effect on the 8-bromomethacrylonitrile would bring the experimental equilibrium i n line with the predicted value. With Examples 8 and 9 corrective terms are d i f f i c u l t to apply, as the 1-methoxypropene equilibrium in Example 8, and the thioether and 3-thioethoxymethacrylonitrile equilibrium in Example 9 were determined at high temperature. In Examples 10 and 11 l i t t l e solvent effect i s anticipated. With Example 12 the solvent effect on the bromopropene would increase the predicted value, however the solvent effect on the dibromopropene would also increase the experimental value, possibly to a greater extent. In Example 14 a situation similar to Example 12 exists; the solvent effect on the dichloropropene would be expected to be large, - 122 -increasing the predicted value whereas the solvent would increase the equilibrium of the chloropropene slightly. In most of the examples considered above,the solvent effect would lead to a better correlation of predicted and experimental values. The value of being able to predict the equilibrium position of complex olefins by addition of less complex terms is obvious. Another use of this concept is in the configurational assignment of olefins. This may be illustrated for compound 30_. CH Br CH H CH, Br \ / — 3 N / \ / / \ / \ / ~\ H CHO H CHO H H Z-30, ? E-5_, 98 Z-83, 68 2 32 The attractive interaction between the a-bromine and 8-methyl group, noted in 83, further stabilizes the trans arrangement favoured in 5. The expected equilibrium percentage for the Z_ isomer of 30_ is calculated to be 99%, which substantiates the earlier assignment. The approximate equilibrium position of unstable compounds may be predicted in the following manner. CH, H H H CH H 3 \ / \ - \ / / \ / ~ \ — — / \ H CHO Br CHO Br CHO E-5, 98 h ? Z-37, 70 2 30 - 123 -The equilibrium position of the unstable bromoaldehyde may be expected to contain about 5% of the Z_ isomer. - 124 -EXPERIMENTAL GENERAL STATEMENT A l l boiling points (b.p.) and melting points (m.p.) are uncorrected. Most boiling points were determined by the microcapillary method. Infrared (i.r.) spectra were recorded on either a Perkin Elmer Model 137 or Model 457 spectrophotometer. The i . r . spectra were recorded neat on liquid films or on Nujol mulls between sodium chloride plates. The 60 MHz nuclear magnetic resonance (n.m.r.) spectra were recorded on a Varian Associates Model A-60 or Model T-60 spectrometer. Some n.m.r. spectra were taken with a Jeolco Model C-60 spectrometer. The spectra were recorded by Miss C. Burfitt and Miss P. Watson. The 100 MHz n.m.r. spectra were recorded using a Varian Associates Model HA-100 spectrometer by Mr. R. Burton. The n.m.r. spectra were run as 20% (v/v) solutions in either carbon tetrachloride, deuteriochloroform (Merck, Sharp and Dohme) or tetrachloroethylene, unless otherwise stated. Tetramethylsilane was used as an internal reference. Spectra not tabulated in the Experimental section w i l l be found in Tables 4,8,10,11 and 21. - 125 -Vapour phase chromatography (v.p.cO was carried out using an Aerograph Model A-90-P or Model A-90-P3 equipped with a thermal conductivity detector. The v.p.c. peak areas were measured by integration, unless other-wise stated, using a Honeywell Model Electronik 15 recorder equipped with a Disc chart integrator Model 201-B. A l l v.p.c. work was carried out on 10 f t x 0.25 i n stainless s t e e l columns with 20% l i q u i d phase on 60/80 Chromosorb W (Varian-Aerograph), unless otherwise noted. The QF-i, Carbowax 20M l i q u i d phase referred to consisted of 15% QF-1 and 5% Carbowax 20M. Helium was used as a c a r r i e r gas f o r a l l v.p.c. work. The helium i n l e t pressure was 50 p . s . i . U l t r a v i o l e t (u.v.) i r r a d i a t i o n was carried out i n a Rayonet Photochemical Reactor (The Southern New England U l t r a v i o l e t Co., Middletown, Ccnn.), equipped with 16 interchangeable RPR lamps. Lamps were available which emitted radiation with maxima at 2537A, 3000A, 3100A and 3500A. A 450 W Hanovia lamp (2537A) was also used. Equilibrium determinations were carried out either thermally or by ca t a l y s i s . The two c a t a l y t i c methods used are referred to as Method A or Method B. Method A entailed acid c a t a l y s i s , using a solution of carbon tetrachloride saturated with dry hydrogen chloride. A few drops of t h i s s o l u t i were added to the n.m.r. tube and the isomerization could be followed by xi.m.r integration. Method B consisted of photochemically induced bromine isomerization. To a sample i n a pyrex tube, either neat or i n solution, was added 1-2 mole % bromine as a 10% solution (v/v) i n carbon tetrachloride. In one instance dry - 126 -ethanol was used as the bromine solvent. The sample tube was then sealed and placed i n direct sunlight or irradiated (Rayonet, 3100A or 3500A). The bulb-to-bulb d i s t i l l a t i o n apparatus used to d i s t i l small samples is shown in Figure 14. The sample to be d i s t i l l e d i s placed i n bulb A, a vacuum is applied and the bulb is immersed in a heated o i l bath to level D. The sample d i s t i l s to bulb B. Bulb B is lowered into the o i l bath to level E and the sample d i s t i l s to bulb C. The sample may be removed through the B-10 opening. The elemental microanalyses were performed by Mr. P. Borda. - 1?.7 -Figure 14 - Bulb-to-bulb d i s t i l l a t i o n apparatus. - 128 -ALDEHYDES Crotonaldehyde,- (Z-) (5) . (a) Preparation Crotonaldehyde, (Z-)(5J , was prepared as a mixture with crotonaldehyde, (E-)f5), by irradiation (Rayonet, 3000A) of a degassed, neat solution of E-5_. The analysis of the isomeric mixture was achieved using n.m.r. spectral integration of the aldehyde proton signal in the neat samples. (b) Attempted separation The separation of the two isomers, Z- and E_-5_, by d i s t i l l a t i o n or v.p.c. using a variety of columns was not successful. D i s t i l l a t i o n attempts were carried out on a Nester and Faust 600 mm x 8 mm Teflon spinning-band s t i l l under reduced pressure and at atmospheric pressure. Samples were obtained that were enriched in the lower boiling, Z_ isomer. However, i t was not practical to obtain larger samples i n this manner. The mixture of isomers, in several instances, appeared to undergo thermal or acid catalyzed isomerization during the d i s t i l l a t i o n . Separation of the isomers by v.p.c. was not achieved." In order to evaluate the abi l i t y of a particular column to effect the desired separation, the following procedure was adopted. Samples of small volume, usually 1 u l , were injected into the v.p.c. and the chromatogram assessed. When only one peak was observed, a larger sample, usually 50-100 y l , was injected and collected - 129 -from the column. The collected sample was then analyzed by n.m.r. In some cases p a r t i a l isomerization was found to occur, but oniy to a minor extent (30-40%). Thus i f any separation actually was achieved, the resolution of the isomers was such that a useful separation did not occur. The columns used i n the attempted separation included; DECS, Apiezon J , DIDP, Ucon Polar 2000, DNP, g l y c e r o l - s i l v e r n i t r a t e , polypropylene g l y c o l - s i l v e r n i t r a t e , Poropak Q, SE-30 and QF-1, Carbowax 20M. (c) Equilibrium determination E q u i l i b r a t i o n was readily achieved using Method A. The acid catalyst was added either to the neat mixture of isomers or to a solution of the isomers. In f a c t , i t was found that residual acid i n the carbon tetrachloride used as a n.m.r. solvent caused isomerization. The position of equilibrium was evaluated using n.m.r. spectroscopy. The i n t e g r a l of the aldehyde proton signals were measured taking care to avoid spinning side bands. The Z_:E_ r a t i o for crotonaldehyde was found to be 2:98. The equilibrium value obtained from a commercially available sample of crotonaldehyde was found to be the same. Tiglaldehyde, (Z-) (6) (a) Preparation Tiglaldehyde, (Z-)(6), was prepared as a mixture with tiglaldehyde, (E-) (6), by i r r a d i a t i o n (Rayonet, 3000A) of a degassed neat solution of E-6_. The analysis of the isomeric mixture was achieved using n.m.r. spectral integration of the aldehyde proton signal i n the neat samples. - 130 -(b) Attempted separation The separation of the two isomers, Z- and E_-6_, by d i s t i l l a t i o n or by v.p.c. was unsuccessful. D i s t i l l a t i o n attempts were carried out i n the same manner as that for the crotonaldehyde isomers Z- and E-5. In t h i s case, however, even enrichment of the low b o i l i n g isomer, presumably, Zj-6_> was not achieved. Attempted separation by v.p.c. was carried out i n the same manner as that for the crotonaldehyde isomers. (c) Equilibrium determination E q u i l i b r a t i o n was readily achieved using Method A. Results p a r a l l e l i n g the crotonaldehyde determinations were observed. The Z_:E_ r a t i o for tiglaldehyde was found to be 1:99. The equilibrium value obtained from a commercially available, sample of tiglaldehyde was found to be the same. ,2,3-Dibromobutanal, (87) A solution of bromine (34.0 g, .0.214 mole) i n pentane (75 ml) was slowly added to a w e l l - s t i r r e d solution of crotonaldehyde (15.0 g, 0.214 mole) i n pentane (75 ml)at 0°. After the addition was complete (25 min), the pentane was removed by rotary evaporation leaving 47.4 g of the crude dibromide. The n.m.r. spectrum indicated the presence of about 7% pentane, thus the overall y i e l d of the bromine addition product was 90%. The n.m.r. spectrum (CCi>4) : • 8.03-8. 16T (multiplet, 3H, CH <-) » 5.45-5.77T (multiplet, 2H, -CHBr-CHBr-), 0.57, 0.77T (multiplet, IH, CHO). - 131 -a-Bromocrotonaldehyde,. (Z-)(30) The above prepared 2,3-dibromobutanal, (87), (44 g, 0.19 mole) was taken up i n acetic acid (102 ml). Potassium carbonate (13.9 g, 0.102 mole) was added gradually at room temperature with s t i r r i n g over 0.5 h (64). A white precipitate formed. The resulting mixture was stirred for 2 h and then added to water (260 ml). The oily lower layer was removed and the aqueous solution extracted with ether (3 x 100 ml). The combined organic layer and the ether extracts were washed with 3M potassium hydroxide (3 x 120 ml), dried over magnesium sulphate and concentrated by rotary evaporation. The crude product d i s t i l l e d at 64-66° / 15 mm ( l i t . (56) 63° / 15 mm) to yield 22.8 g (80%) of the t i t l e bromoaldehyde. Attempted isomerization of a-bromocrotonaldehyde (Z~)(30) Method A was used. After 1 day the n.m.r. spectrum did not show any peaks which could be attributed to the E_ isomer. A sample of the bromocroton-aldehyde, Z-30, was purified by v.p.c. (20 f t DC-550, 150°, 45 ml per min), degassed, sealed and irradiated (Rayonet, 3000A) for 4.5 h. The n.m.r. spectrum, taken immediately after irradiation, indicated that only the Z_ isomer was present. Attempted photoisomerization (Hanovia, 2537A) in ether resulted in a viscous dark liquid. Potassium salt of malondialdehyde This compound was prepared by the method of Kalinina (57), 1,1,3,3-Tetraethoxypropane (22 g, 0.1 mole) was hydrolyzed by acid and the reaction mixture was .;then made basic with potassium hydroxide. The water was - 132 -removed under reduced pressure and the salt remaining was washed with a mixture of acetone and anhydrous ethanol (10:1). A yield of 6.0 g (54%) of an orange salt was obtained. B-Methoxyacrolein, (E-)(31) and B-ethoxyacrolein, (E-)(32) These compounds were prepared by the method of Kalinina (57), An aqueous solution of the potassium salt of malondialdehyde (6.0 g in 25 ml, 0.055 mole), obtained by the hydrolysis of 1,1,3,3-tetraethoxypropane, was added drop by drop over 10 min to a solution of methylchloroformate (5.1 g, 0.055 mole) in 100 ml of methylene chloride with s t i r r i n g . The reaction mixture was stirred an additional 0.5 h and the organic layer was removed. The solvent was evaporated under vacuum and the orange crystalline residue was washed with a solution of petroleum ether (b.p. 30-60°) and anhydrous ethyl alcohol (10:1). The crude product was taken up in 30 ml of methylene chloride and a few crystals of p-toluenesulfonic acid hydrate were added. A vigorous evolution of carbon dioxide ensued. After the bubbling stopped, 0.5 g of sodium bicarbonate was added and the solution was dried over magnesium sulfate. F i l t r a t i o n , evaporation of the solvent and d i s t i l l a t i o n of the residue yielded 2.5 g of an almost colourless product: b.p. 43-45° / 2.5 mm ( l i t . (57) 35.5-37° / 1.5 mm). Analysis of the d i s t i l l a t e by v.p.c. (QF-1, Carbowax 20M, 145°, 120 ml per min) showed two major peaks, A (65%, retention time 11.5 min) and B (35%, retention time 15.3 min) comprising 75% of the total d i s t i l l a t e . Separation of these peaks by v.p.c. and subsequent n.m.r. spectroscopy showed that peak A was 3-methoxyacrolein, E-31, and peak B was B-ethoxyacrolein, E-32, The n.m.r. spectrum of the ethoxyacrolein, E-32, - 133 -was consistent with the spectrum tabulated in the literature (101). The overall yield of alkoxyacroleins was determined to be 37%. Attempted isomerization of 3-methoxyacrolein, (E_-)(31) and 3-ethoxyacrolein, (E-)(32) Samples of the alkoxyacroleins, E-31 and E-32, were purified by v.p.c. (QF-1, Carbowax 20M, 145°, 60 ml per min). Irradiation (Rayonet, 3000A) of the neat degassed samples for periods of up to 6 h, followed by n.m.r. analysis did not indicate the presence of any Z_ isomer. Further irradiation discoloured the samples. Method A was also unsuccessful. 3-Chlorotiglaldehyde, (Z-) and (E-)(33) (a) Preparation The isomeric chloroaldehydes, Z- and E-35, were prepared by slight modification of the procedures of Arnold and Zemlicka (58), Brederek et a l . (59), and Bodendorf and Mayer (60). Dimethylformamide (109 g, 1.5 mole) was placed in a 250 ml three-necked round-bottom flask equipped with a paddle s t i r r e r , condenser and thermometer. The flask was cooled in an ice bath and phosphorus oxychloride (192 g, 1.25 mole) was added over 15 min. The mixture was slowly warmed to 25° over 40 min. Methyl ethyl ketone (36 g, 0.5 mole) was added drop by drop over 20 min with s t i r r i n g . During the addition, the reaction temperature was kept below 55° by periodic immersion of the reaction flask in an ice bath. After the addition was complete, the contents of the flask were heated at 40° for 35 min and then poured onto 1500 g of ice with s t i r r i n g . Solid sodium bicarbonate was added to the solution, but complete neutralization - 134 -could not be attained u n t i l the reaction mixture had warmed to room temperature. The reaction mixture was diluted with two l i t e r s of water and extracted with ether (5 x 200 ml). The combined ether extracts were dried over magnesium sulphate, concentrated by rotary evaporation and d i s t i l l e d under reduced pressure. The f a i n t l y yellow product d i s t i l l e d at 54-56° / 24.5 mm ( l i t . (60) 53-55° / 23 mm) y i e l d i n g 41.3 g (72%) of the isomeric chloroaldehydes, Z_ and E-33. The _Z:E_ product r a t i o , measured by integration of the n.m.r^ spectrum, was found to be 30:70. The i . r . spectrum of the Z and E_ mixture (neat) C=0 1678 cm"1, C=C 1622 cm 1. (b) P a r t i a l separation The p a r t i a l separation of the two isomers, Z- and E-33, was carried out using v.p.c. (Carbowax 4000 Monostearate, 125°, 60 ml per min). The * following fractions were collected and analyzed by n.m.r. spectral integration of the 8-methyl resonances. Fraction A (retention time 19.9 min) contained 88% Z-33 while f r a c t i o n B (retention time 23.4 min) contained 90% E-33. (c) Equilibrium determination Method B was used. The sample was photoisomerized for 1.5 h (Rayonet, 3500A). 8-Chlorocrotonaldehyde, (Z-) and (E-)(34) (a) Preparation The isomeric chlorocrotonaldehydes, Z- and E-34, were prepared i n an analogous manner to that of the B-chlorotiglaldehydes, 33_. To the formylation - 135 -reagent prepared from dimethylformamide (55 g, 0.75 mole) and phosphorus oxychloride (96 g, 0.63 mole) was added acetone (14.5 g, 0.25 mole). The reaction mixture was heated at 60° for 3 h and 3.0 g (12%) of the crude light yellow product was isolated. The Z_:E ratio i n the crude product was calculated to be 50:50, based on integration of the methyl proton signals in the n.m.r. spectrum. Purification of the crude product by v.p.c. (QF-1, Carbowax 20M, 145°, 60 ml per min) and subsequent n.m.r. spectroscopy showed the crude product to contain 87% of the isomeric aldehydes, Z- and E-34, and 13% of an unidentified product. (b) Partial separation In order to determine the position of equilibrium of the isomeric chloroaldehydes, samples containing non-equilibrium compositions must be obtained for both sides of the equilibrium position. The crude reaction mixture, purified by v.p.c. (QF-1, Carbowax 20M, 145°, 60 ml per min), supplied a sample with a Z_:E_ ratio of 50:50. This sample was adequate for equilibrium determination. The only v.p.c. column tried that could effect any noticable separation of the _Z and E_ isomers was Poropak Q at 210°. A sample containing about 25% of the E isomer (lower retention time) was obtained using this column. Chromatographic columns giving no separation were polypropylene glycol-silver nitrate, DEGS, DIDP, SE-30 and Ucon Polar 2000 HB. Cc) Equilibrium determination Method A was used. An excess of acid catalyst effected rapid isomerization. In some cases the sample decomposed during isomerization. - 136 -3-Chloro-4,4-dimethyl^2^pentenal, (Z-)(35) The chloroaldehyde, Z-35, was prepared in an analogous manner to that of the 8-chlorotiglaldehydes, 33_. To the formylation reagent prepared from dimethylformamide (109 g, 1.50 mole) and phosphorus oxychloride (192 g, 1.25 mole) was added pinacolone (54 g, 0.50 mole). The reaction mixture was heated for 3.5 h at 60° and the crude product was isolated. The colourless product d i s t i l l e d at 71-76° / 20 mm ( l i t . (60) 63° / 14 mm). The yield was 46.3 g (61%). 3-Chloro-4,4-dimethyl-2-pentenal, (E-)(35) (a) Preparation The chloroaldehyde, E-35, was prepared as a mixture with the chloroaldehyde, Z-35, by irradiation through s i l i c a of the Z_ isomer i n ether solution with a 450 W Hanovia lamp. Removal of the solvent followed by n.m.r. analysis indicated the presence of 40% of the E_ isomer. (b) Equilibrium determination Equilibration was effected by method A i n an n.m.r. tube. The n.m.r. spectrum taken three days after the addition of acid failed to show the presence of any detectable amount of the E_ isomer. - 137 -B-Bromotiglaldehyde, (Z-) and (E-)(56) (a) Preparation The isomeric bromotiglaldehydes, Z- and E-36, were prepared by the method of Arnold and Holy (61) with s l i g h t modification. A solution of dimethylformamide (21.9 g, 0.30 mole) i n 80 ml of chloroform was placed i n a 250 ml three-necked round-bottom flask equipped with a paddle s t i r r e r , condenser and thermometer. The flask was cooled i n an ice-bath and freshly d i s t i l l e d phosphorus tribromide (67.7 g, 0.25 mole) was added over 15 min with s t i r r i n g . After 15 min, the l i g h t yellow c r y s t a l l i n e adduct appeared. The solution was l e f t for 3 h at 0°. A solution of methyl ethyl ketone (7.2 g, 0.1 mole) i n 40 ml of chloroform was then added and the reaction mixture was heated at 60° for 7 h with s t i r r i n g . After the reaction period was over the dark red-brown mixture was transferred to a 500 ml flask and the chloroform removed on a rotary evaporator. The syrupy mixture was then poured onto 200 g of i c e . The aqueous solution was immediately neutralized with s o l i d sodium bicarbonate, 100 ml of ether was added, and the solution was allowed to come to room temperature. The ether layer was separated and the aqueous solution extracted with ether (4 x 100 ml). The combined ether extracts were washed with water, dried over magnesium sulphate, concentrated by rotary evaporation and d i s t i l l e d at 51-57° / 5.5 mm ( l i t . (61) 30° (bath)/ 0.25 mm) yi e l d i n g 1.9 g (12%) of the bromoaldehydes, Z- and E-36. The _Z:E_ product r a t i o , based on n.m.r. spectroscopy was found to be 34:66. The i . r . spectrum of the _Z and E_ mixture (neat) C=0 1677 cm 1, C=C 1613 cm \ ( l i t . (61) - 138 -(CC1 ) C=0 1687 cm"1, C=C 1618 cm"1). (b) P a r t i a l separation The p a r t i a l separation of the two isomers Z- and E-36 was carried out using v.p.c. (Carbowax 4000 Monostearate, 135°, 90 ml per min). The following fractions were collected and analyzed by n.m.r. spectral integration. Fraction A (retention time 20.7 min) contained 81% Z-36 while f r a c t i o n B (retention time 23.4 min) contained 68% E-36. The v.p.c. l i q u i d phases used i n the attempted separation were FFAP, Ethofat, Zonyl E-7, Carbowax 4000 Monostearate, QF-1, Carbowax 20M, DC-550, Ionox, SF-96, SE-30, DNP, and DIDP. The only l i q u i d phases giving any separation were the FFAP, Ethofat and Carbowax 4000 Monostearate. (c) Equilibrium determination Method B was used. The samples were photoisomerized f o r 1.5 h (Rayonet, 3000A). 8-Bromocrotonaldehyde, (Z-) and (E-)(57) (a) Preparation The isomeric bromocrotonaldehydes, Z_- and E-57, were prepared i n an analogous manner to that of the 8-bromotiglaldehydes, Zj- and E_-56. To the formylation reagent prepared from dimethylformamide (44 g, 0.60 mole) - 139 -and phosphorus tribromide (135 g, 0.50 mole) was added acetone (11.6 g, 0.20 mole) i n chloroform. The reaction mixture was l e f t at room temperature for 12 h and the crude product isolated and d i s t i l l e d . The clear pale yellow product d i s t i l l e d at 56-58.5° / 15 mm ( l i t . (61) 53° (bath) / 11 mm) yiel d i n g 6.68 g (23%) of the isomeric aldehydes, Z- and E-37. The _Z:E_ product r a t i o based on n.m.r. spectral integration was found to be 50:50. The i . r . spectrum -1 -1 of the Z_ and E_ mixture (neat) C=0 1685 cm ^ C=C 1620 cm ( l i t . (61) (CC1 ) -1 -1 OO 1692 cm , C=C 1629 cm ). (b) P a r t i a l separation I t was not possible to separate the _Z and E_ isomers by v.p.c. using the l i q u i d phases available. (c) Equilibrium determination As d i f f i c u l t y was encountered i n the attempted separation of the isomers, the equilibrium p o s i t i o n was approached from one side only. Both Method A and Method B (Rayonet, 3500A) were used. 2,3,3-Tribromobutanal To a mixture of the bromocrotonaldehydes, Z_ and E-37, (1.25 g, 0.0084 mole) i n 10 ml of ether was added bromine (1.35 g, 0.0085 mole). The ether was removed on a rotary evaporator. The n.m.r. spectrum of the residue indicated the addition to be complete. The n.m.r. (CDC1 ) 7.35T (singlet, 3H, - 140 -a y , 5.39T (doublet, J = 4.3 Hz, IH, -CHBr-), 0 .65x (doublet, J = 4.3 Hz, IH, CHO). 3-Dibromocrotonaldehyde, (Z-) and (E-)(38) (a) Preparation To the crude tribromoaldehyde prepared above was added 10 ml acetic acid. Potassium acetate (0.85 g, 0.0087 mole) was added with s t i r r i n g (64). The solution immediately turned milky. The reaction mixture was s t i r r e d for 12 h and then dilu t e d with water (50 ml). The aqueous solution was extracted with ether (3 x 25 ml). The combined ether extracts were washed with water (2 x 25 ml), neutralized with saturated sodium bicarbonate solution, and dried over magnesium sulphate. The ether was removed by rotary evaporation and the residue d i s t i l l e d i n a bulb-to-bulb d i s t i l l i n g apparatus at 70-80° (bath temperature) at a pressure of 0.3 mm. The y i e l d was 0 .65 g (34%). The n.m.r. spectrum of the d i s t i l l e d product indicated the presence of the aldehydes, Z- and E-38, i n a r a t i o of 19:81. For the mixture of isomers: i . r . (neat) C=0 1695 cm"1, C=C 1587 cm"1. The aldehyde rapidly darkens at room temperature i n the presence of a i r . The 2,4-dinitrophenylhydrazone was prepared and r e c r y s t a l i i z e d three times; twice from chloroform and once from acetic acid: m.p. 193° (dec.). A correct microanalysis, however, was not obtained. Anal. Calcd. f o r C H Br N 0 : C, 29.44; H, 1.98; N, 13.73. 10 8 2 4 4 Found: C, 30.18, 30.30; H, 2.10, 2.00; N, 14.33, 14.38. - 141 -Cb) Equilibrium determination The isomeric aldehydes, Z- and E-38, were not separable by v.p.c. In most attempted separations the aldehydes turned to tar i n the injection chamber of the v.p.c. Method A was used to approach the equilibrium from one side only. 4,4,-DimethyI-3-methoxy-2-pentenal, CZ-) and CE~)(39) Ca) Preparation To the chloroaldehyde, Z-34, C3.6 g, 0.025 mole) i n a 25 ml round bottom flask was added sodium methoxide (1.35 g, 0.025 mole) i n 7.5 ml of methanol. The reaction mixture was stirred for 20 h, neutralized with acetic acid, diluted with water and extracted with ether (4 x 30 ml). The combined ether extracts were dried over magnesium sulphate, concentrated under reduced pressure and d i s t i l l e d in a bulb-to-bulb d i s t i l l i n g apparatus at 75-80° (bath temperature) under 1.5 mm pressure. The yield was 2.1 g C60%). Subsequent n.m.r. and i . r . spectroscopy showed the d i s t i l l a t e to contain the isomeric methoxyaldehydes, Z- and E-39. The £:E_ product ratio was estimated to be 38:62 by integration of the n.m.r. spectrum. For the mixture of isomers: b.p, 199° / 760 mm. A satisfactory elemental microanalysis could not be obtained for samples purified by v.p.c. Anal. Calcd. for c 8 ^ ^ 2 ' ^' ^ 7 , 5 7 » H, 9.92. Found: C, 63.98; H, 9.19. - 142 -Cb) Separation The separation of the two isomers, Z- and E-(39), was carried out by v.p.c. (FFAP, 132", 120 ml per min). The following fractions were collected and analyzed by n.m.r, spectral integration. Fraction A contained 84% _Z isomer and fraction B contained 93% E_ isomer. For Z-39: i . r . (neat) C=0 1655 cm"1, C=C 1600 cm - 1. For E-59: i . r . (neat) C=0 1655 cm"1, C=C 1580 -1 cm . (c) Equilibrium determination Method A was used. An excess of acid catalyst led to the appearance of extraneous peaks in the n.m.r. spectrum which hindered the accuracy of the integration. Attempted preparation of 4,4-dimethyl-3-dimethylamino-2-pentenal, (88) To the chloroaldehyde, Z-35, (2.5 g, 0.017 mole) in a round-bottom flask was added anhydrous dimethylamine (1.4 g, 0.030 mole) in 5 ml of ether. A Dry Ice condenser was attached and the reaction mixture was refluxed for 5 h. A yellow, ether-insoluble o i l formed. The ether was evaporated under reduced pressure to leave the oily liquid. This residue was stirred with 3 portions of ether over 24 h. The combined ether extracts were evaporated. The n.m.r. spectrum of the crude product showed only 4,4-dimethyl-l-dimethylamino-l-penten-3-one, Z-45, the rearranged product, identified by i t s n.m.r. spectrum (CCl^) 8.93x (singlet, 9H, C(CH )_), 7.06x (singlet, 6H, N-CH ), 4.90x (doublet, J = 12.5 Hz, IH, H-C=C-N), 2.61x (doublet, J = 12.5 Hz, IH, C0-C=C-H). - 143 -Attempted preparation of 4,4-dimethyl-5-diethylamino-2-pentenal. (89) To the chloroaldehyde, Z-35, (0.4 g, 0.0027 mole) in a round-bottom flask was added 1 ml of anhydrous diethylamine. An exothermic reaction ensued. When the flask cooled, the reaction mixture was refluxed 1 h and l e f t at room temperature a further 24 h, during which time the amine hydrochloride crystals appeared. Ether was added and the salt removed by f i l t r a t i o n . The ether and excess diethylamine were removed under reduced pressure and the oily residue was d i s t i l l e d i n a bulb-to-bulb d i s t i l l a t i o n apparatus with a bath temperature of 100° / 0.5 mm to yield 0,3 g (60%) of the product. The product was further purified by v.p.c. (QF-1, Carbowax 20M, 210°, 90 ml per min). Subsequent n.m.r. and i . r . spectroscopy showed the product to be 4,4-dimethyl-l-diethylamino-l-penten-3-one, (Z-) (46) : i . r . -1 -1 -1 -1 C=0 1650 cm , C=C 1570 1570 cm ( l i t . (102) C=Q 1652 cm , C=C 1566 cm ); n.m.r. (CC14) 8.94T (singlet, 9H, C(CH 3) 3, 8.81T (t r i p l e t , J = 7.1 Hz, 6H -Q^-CH^), 6.77x (quartet, J = 7.1 Hz, 4H, N-qy , 4.88T (doublet, J = 12.6 Hz, IH, H-C=C-N), 2.67T (doublet, J = 12.6 Hz, IH, C0-C=C-H). Citr a l a, (E-)(40) and C i t r a l b, (Z-)(40) (a) Separation A commercially available sample of c i t r a l (Brickman and Co.) was fractionally d i s t i l l e d on a Nester and Faust 600 mm x 8 mm stainless steel spinning-band s t i l l under reduced pressure. The d i s t i l l a t i o n was monitored by v.p.c. (5 f t x 0.25 i n , 20% SE-30, 158°, 110 ml per min) and d i s t i l l e d - 144 -fractions with the following compositions were collected. A low b o i l i n g f r a c t i o n , b.p. 63° / 0.9 mm, was found to contain the c i t r a l isomers, Z-and E-40, i n a r a t i o of 91:9. A high b o i l i n g f r a c t i o n , b.p. 63° / 0.7 mm, was found to contain the c i t r a l isomers with a _Z:E_ r a t i o of 6:94. The r a t i o of isomers was determined by integration of the aldehyde proton signals i n the n.m.r. spectrum. I d e n t i f i c a t i o n of the two isomers by b.p. and n.m.r. spectral characteristics was consistent with the data recorded i n the l i t e r a t u r e (62) (b) Thermal isomerization Neat samples of the above separated isomers were degassed and sealed under vacuum i n n.m.r. tubes. The samples i n the n.m.r. tubes were thermally isomerized by heating i n an o i l bath at 140 ±1.5°. The n.m.r. tubes were removed from the o i l bath at fix e d time intervals and analyzed by integration of the aldehyde proton signals i n the n.m.r. spectrum. In this manner the thermal isomerization of c i t r a l could be followed. Unfortunately, the equilibrium could not be followed to completion as decomposition after 19 h resulted i n inaccurate integrations. (c) Acid catalyzed isomerization Method A was used. - 145 -METHYL KETONES 3-Methyl-3-penten-2-one, (Z-)(47) (a) Preparation A sample of the unsaturated ketone, Z-47, was prepared by Miss P. Balmos. 3-Methyl-3-penten-2-one, E-47, was irradiated (Rayonet, 3000A) i n ether solution to produce a mixture of the Z_ and E_ isomers. Further p u r i f i c a t i o n was accomplished by f r a c t i o n a l d i s t i l l a t i o n on a Nester and Faust 600 mm x 8 mm stainless s t e e l spinning-band s t i l l , followed by preparative v.p.c. The given sample was analyzed by v.p.c. (20 f t x 0.25 i n , 20% DEGS, 168°, 60 ml per min) and found to be a mixture of the unsaturated ketones with a Z_:E_ r a t i o of 91.5 : 8.5 (retention times 8.0 min (Z) and 6.0 min (E)). Further p u r i f i c a t i o n by v.p.c. using the above column yielded the ketone, Z-47, containing only 0.7% of the E_ isomer. The n.m.r. spectra of the _Z and E_ isomers were, i d e n t i c a l , with those recorded:.in t'Ke l i t e r a t u r e (67). (b) Equilibrium determination Ten microliter, samples of-4T (Z:E_ = 12:88) were i n d i v i d u a l l y sealed at atmospheric pressure and heated at 214°. Samples were removed at fixed time intervals and analyzed by v.p.c. (DC-550, 175°, 20 ml per - 146 -min). The retention times f o r the Z_ and E_ isomers were 2.6 min and 3.2 min, respectively. Method B was also used; the samples being i r r a d i a t e d (Rayonet, 3100A) for 0.75 h. N-Nitros6-N-methyl urea The N-nitroso-N-methyl urea was prepared by the method of Arndt (103). Diazomethane The diazomethane i n an ether-methanol solution was prepared by the method of Chiu (104). 4-Methoxy-3-penten-2-one (Z-)(48) (a) Preparation The t i t l e compound was prepared by the method of E i s t e r t et a l . (68). Freshly d i s t i l l e d 2,4-pentanedione (3.2 g, 0.032 mole, Eastman) i n 25 ml of anhydrous ether was added to an ether-methanol solution of diazomethane (prepared from 20 g of N-nitroso-N-methyl urea). The reaction mixture was l e f t 16 h. The solvent was removed by rotary evaporation and the residue was immediately d i s t i l l e d at 71-75° / 8 mm ( l i t . (68) 85° / 10 mm) to y i e l d 2.0 g (55%) of the methoxy ketone, Z-48, which was stored at -5°. The n.m.r. spectrum of the d i s t i l l e d product, recorded neat after 48 h at -5° and 18 h at room temperature, indicated the presence of 79% of the Z_ isomer. After - 147 -an additional 24 h at room temperature, the amount of Z_ isomer had dropped to 60%. Attempted v.p.c. analysis, even with short columns and low temperature conditions proved impossible due to isomerization of the unstable Z isomer after injection. This isomerization! was evident as the baseline of the v.p.c. tracing did not return.to zero between the two peaks. (JO Separation The sample prepared above was d i s t i l l e d under reduced pressure through a short column containing stainless steel turnings. The last third of the d i s t i l l e d portion, (b.p. 77-79° / 9 mm), expected to contain the Z_ isomer, was collected. An integration of the n.m.r. spectrum showed the _2:E_ ratio to be 96:4. (c) Equilibrium determination As described above, Z-48.was found to be thermally labile at room temperature. A sample kept for two weeks at room temperature was analyzed by n.m.r. and found to contain less than 1% of the Z_ isomer. The equilibrium position could not be measured by v.p.c, as the Z_ isomer was labile under the conditions of analysis. 4-Chloro-5,5-dimethyl-3-hexene-2-ol, (Z-)(52) To a 250 ml three-necked round-bottom flask, equipped with a mechanical s t i r r e r , condenser and dropping tunnel was added magnesium turnings (2.88 g, 0.12 mole). The apparatus was swept with dry nitrogen while being - 148 -flamed. When the apparatus was cool, the magnesium was covered with 40 ml of anhydrous ether and a solution of methyl iodide (17 g, 0.12 mole) i n 20 ml of ether was added. The mixture was refluxed f o r 15 min. The chloro-aldehyde, Z-35, i n 80 ml of ether was added over 10 min with cooling. The reaction mixture was allowed to stand for 12 h during which time a c r y s t a l l i n e p r e c i p i t a t e appeared. An aqueous solution of ammonium chloride (65 ml, 35% w/v) was added and the ether layer separated. The aqueous solution was extracted with ether (3 x 100 ml). The combined ether extracts were dried over magnesium sulphate, concentrated under reduced pressure and the residue was d i s t i l l e d . The colourless product d i s t i l l e d at 80° / 12 mm. The y i e l d on -1 was 10.6 g (54%): b.p. 197° / 760 mm; n^ 1.4617; i . r . (neat) C=C 1660 cm , -C(CH_)- 1360 cm"1; n.m.r. (CC1 =CC1 ) 8.84x (s i n g l e t , 9H, -C(CH ) ), 8.78T —*o «J 2 ^ ~~*3 3 (doublet, J = 6.7 Hz, 3H, -CH(OH) - qy , 6.07T (broad s i n g l e t , IH, -OH), 5.32T (5 line m u l t i p l e t , J = 6.7 Hz, IH, H-C-OH), 4.42t (doublet, J =7.3 Hz, IH, C=C-H). Anal. Calcd. for C H C10: C, 59.07; H, 9.28. Found: C, 58.96; 8 15 H, 9.13. 4?Chloro-5,5-dimethyl-3-hexen-2-one, (Z-)(49) The chloroalcohol, (Z-)(52), (10.55 g, 0,065 mole) i n 80 ml of dry pentane was added to activated (69) manganese dioxide (60 g, 0.69 mole) and s t i r r e d f o r three days (105). Analysis by v.p.c. (QF-1, Carbowax 29M, 155°, 90 ml per min) showed the reaction to be 89% complete. F i l t r a t i o n , rotary evaporation of the pentane solution and d i s t i l l a t i o n of the residue yielded - 149 -7.55 g (72%) of a colourless liquid: b.p. 60° / 2.4 nun, 202-203° / 760 mm; n 2° 1.4702; i . r . (neat) C=0 1675 cm"1, C=C 1600 cm"1, C(CH ) 1365 cm"1. D 3 3 Anal. Calcd. for C H CIO: C, 59.81; H, 8.15. Found C, 59.88; 8 13 H, 7.77. 4-Chloro-5,5-dimethyl-3-hexen-2-one, (E-)(49) (a) Preparation The chloroketone, (E-,49), was prepared as a mixture with the chloroketone, Z-49, by irradiation ( s i l i c a tube) of 0.25 g of Z-49 in 5 ml of ether with a 450 W Hanovia lamp for 1.8 h. Removal of the solvent followed by n.m.r. analysis indicated the presence of 25% of the E_ isomer. (b) Equilibrium determination Equilibration was effected by method A in an n.m.r. tube. The n.m.r. spectra failed to show the presence of any E isomer. After thermal isomerization for 45 min at 200° in a sealed tube, a mixture of isomers with an i n i t i a l Z:E ratio of 83:17, failed to show the presence of any E_ isomer when analyzed by v.p.c. (5 f t Apiezon .J, 160°, 32 ml per min; Z_ and E_ retention times were 11.0 and 8.3 min respectively). 5,5-Dimethyl-4-methoxy-3-hexen-2-one, (Z-) and (E-)(50). (a) Preparation The isomeric methoxy ketones, Z- and E-50, were prepared in an - 150 -analogous manner to that of the methoxyaldehyd.es, Z^and E-59. Sodium methoxide (0.010 g, 0.019 mole) i n 5.8 ml of methanol was added to the chloroketone, Z-49, (2.7 g, 0.017 mole), and the reaction mixture was s t i r r e d for 4 days. Isolation of the crude product and bulb-to-bulb d i s t i l l a t i o n with a bath temperature of 60° under 0.1 mm pressure afforded 1.48 g (56%) of the methoxyketones, Z- and E-50. The Z_:E_ product r a t i o was estimated to be 75:25 by integration of the n.m.r. spectrum. For the 20 mixture of isomers: b.p. 188-189° / 760 mm; n^ 1.4604. Anal. Calcd. f o r C H 0 : C, 69.19; H, 10.33. Found: C, 69.42; 9 16 H, 10.55. (b) Separation The separation of the two isomers, _Z- and E-50, was carried out using v.p.c. (SE-30, 135°, 200 ml per min). The following fractions were collected and analyzed by n.m.r. spectral integration. Fraction A (retention time 12.7 min) contained 95% Z-50 and f r a c t i o n B (retention time 15.7 min) contained about 65% E-50.' For Z-50: i . r . (neat) C=0 1670 cm 1, C=C 1580 cm"1. For E-50: i . r . (neat) C-0 1670 cm"1, C=C 1565 cm"1. (c) Equilibrium determination No acid catalyst was used. The samples appeared to isomerize with residual acid i n the n.m.r. solvent (CC1 ) or isomerize thermally. Integration 4 of the t e r t - b u t y l resonances i n the n.m.r. spectra was unreliable as a broadening of the base of the peaks occurred. The equilibrium position was measured by v.p.c. and by integration of the methoxyl resonances i n the n.m.T. spectrum. - 151 -5,5-Dimet>iyl-4-dimethylamino-3-hexen-2-one, (Z-) (51) To the chloroketone, Z-49, (0.3 g, 0.0019 mole) in a round-bottom flask was added 1 ml of anhydrous dimethylamine. The flask was stoppered, the greased stopper being held securely i n place with wire and rubber bands. After 12 h crystals had appeared in the flask and after 24 h the flask was opened. The crystals were washed with ether and the ether extract concentrated by rotary evaporation. The n.m.r. spectrum of the crude product showed i t to be a mixture of the desired product, Z-51, and the rearranged product, 5,5-dimethyl-2-dimethylamino-2-hexen-4-one, E-53, in a ratio of 60:40. Purification by bulb-to-bulb d i s t i l l a t i o n with a bath temperature of 100° at 0.3 mm gave 1.5 g (47%) of the mixed products. The products were separated by v.p.c. (Apiezon J , 2009, 85 ml per min). Their retention times were 6.2 min (E-53) and 13.2 min (Z-51). For the rearranged product, E-53: n.m.r. (CCl^) 8.74T (singlet, 9H, C(CHJ,J, 8.07T (singlet, 3H, OC-CHJ, 7.12x (singlet, 6H, N-CHJ, 4.77T — J .5 —3 —3 (singlet, IK, H-OC). 20 For the desired product, Z-51: n^ 1.5260; i . r . (neat) C=0 1635 cm"1, OC 1540 cm"1. Anal. Calcd. for C 1 0 H 1 9 N O » Z.~£l.: c» 7 0 - 9 6 J H, 11.31. Found: C, 70.86; H, 11.45. Attempted isomerization of 5,5-dimethyl-4-dimethylamino-5-hexen-2-,One, (Z-) (51) None of the methods tried were successful in producing the E_ isomer. Photoisomerization under the following conditions was unsuccessful: Rayonet, - 152 -2537A, s i l i c a f i l t e r , ether solution, 2.25 h; Rayonet, 3100A, pyrex f i l t e r , neat, 11 days; Rayonet, 3100A, pyrex f i l t e r , ether solution, 15.5 h. . Thermal isomerization of neat solutions, either degassed or at atmospheric pressure, for periods up to 7 h at 208° was unsuccessful. Bromine catalysis (sunlight) produced only a brown viscous l i q u i d . Attempted preparation of 5,5~dimethyl-4-diethylamino-3-hexen-2-one, (Z-) (90) To the chloroketone, Z-49,- (0.3 g, 0.0019 mole) i n a round-bottom flask was added 1 ml of diethylamine. The reaction mixture was refluxed for 1 h, during which time a precipitate appeared. The solution was l e f t overnight. Ether was added and the hydrochloride s a l t removed by f i l t r a t i o n . The ether and excess diethylamine were removed under reduced pressure. The n.m.r. spectrum indicated the presence of unreacted chloroketone, Z-49. Thus, 1 ml of diethylamine was added to the mixture of st a r t i n g material and product and the mixture was l e f t 42 h at room temperature. The product (1.5 g, 44%) was isola t e d as before and d i s t i l l e d i n a bulb-to-bulb d i s t i l l a t i o n apparatus with a bath temperature of 110° at 0.5 mm. Subsequent i . r . and n.m.r. spectroscopy on the p u r i f i e d sample (v.p.c, Apiezon J , 198°, 85 ml per min) showed the product to be 2-diethylamino-5,5-dimethyl-2-hexene-4-one, (Z-) (91): i . r . (neat) C=0 1640 cm"1, C=C 1550 cm"1; n.m.r. ( C C l ^ 8.99t ( t r i p l e t , J = 7.1 Hz, 6H, -OU-CH,). 8.78T f s i n e l e t , IH, CCCH ) ). 8.05t (si n g l e t . 3H. C=C-CH,), 6.87x 2 —3 - - - - —3- 3- ' ' —3 (quartet, J = 7.1 Hz, 4H, N-CH2-), 4.58T ( s i n g l e t , IH, H-C=C). Anal. Calcd. f o r C H NO: C, 73.04; H, 11.75. Found: C, 72.82; 12 23 H, 11.93. - 153 -NITRILES Crotononi t r i l e , (Z-) and (E-)(17) (a) Separation The separation of the isomeric c r o t o n o n i t r i l e s , Z- and E-17, was carried out by Mr. M. Vinje, of t h i s laboratory by f r a c t i o n a l d i s t i l l a t i o n on a Nester and Faust 600 mm x 8 mm stainless s t e e l spinning-band s t i l l . Fractions were collected with b.p. 107°, Z-17, and 121-122°, E-17. (b) Equilibrium determination The determination of the equilibrium position i s described i n the section dealing with solvent e f f e c t s . 8-Bromoacrylic acid, (Z-) and (E-)(59) The bromoacrylic acids, Z- and E-59, were prepared by modification of the procedures of Gryszkiewicz-Trochimowski et a l . (71) and K. Alder et a l . (72). Pr o p i o l i c acid (15.6 g, C.22 mole) was added drop by drop over 0.5 h tc 91 ml of 48% hydrobromic acid at 0°. The reaction mixture was allowed to come to room temperature over 3 h and was l e f t to s t i r a further 30 h. During t h i s period white crystals appeared. Vacuum f i l t r a t i o n of these crystals yielded 2.1 g (6%) of the crude bromo acid, E-59: m.p. 115.5-116.5° ( l i t . (72) 121°); i . r . (Nujol) C=0 1710 cm'1, C=C 1599 cm"1. - 154 -The n.m.r. spectrum indicated the presence of 12% of the E isomer. 8-Bromoacryloyl chloride, (Z-)(60) To the crude 8-bromoacrylic acid, Z-59, (10.1 g, 0.067 mole) was added freshly d i s t i l l e d t h i o n y l chloride (8.4 g, 0.075 mole) at room temperature with s t i r r i n g . After 18 h the excess thionyl chloride was removed under reduced pressure, a d i s t i l l a t i o n head and condenser were attached and 7.25 g of a pale yellow l i q u i d d i s t i l l e d at 63° / 18 mm. To the residue remaining i n the f l a s k , 3 ml of thionyl chloride was added and the mixture refluxed f o r 0.5 h. After removal of the thionyl chloride a further 1.8 g of product were d i s t i l l e d . A combined y i e l d of 9.05 g (80%) of the acid chloride, Z-60, was obtained: i . r . -1 -1 (neat) C=0 1757 cm , C=C 1581 cm . 6-Bromoacrylamide, (Z-)(61) The procedure outlined by Gryszkiewicz-Trochimowski et a l . (71) was followed. To the acid chloride, Z-60, (9.05 g, 0.053 mole) was added 50 ml of anhydrous ether and the f l a s k was cooled i n an ice bath. Anhydrous ammonia was bubbled into the ether solution and a white flocculent precipitate immediately appeared. The ammonia addition was continued f o r 0.5 h with magnetic s t i r r i n g . The ether and excess ammonia were removed under reduced pressure and the r e s u l t i n g white s o l i d was ether extracted f o r 66 h using a Soxhlet extractor. Colourless needle-like crystals formed i n the ether solution - 155 -during extraction. These crystals were removed, (3.15g), m.p. 90-91° and the ether evaporated to y i e l d a further 4.9 g of f l u f f y white c r y s t a l s : m.p. 81-84°. A sample r e c r y s t a l l i z e d from methanol-ether had a m.p. 90-90.5°. The combined y i e l d was 8.0 g (98%). The i . r . (Nujol) N-H 3350, -1 -1 -1 3175 cm , C=0 1667 cm , C=C, 1599 cm . Anal. Calcd. f o r C H BrN : C,24.02; H, 2.69; N, 9.34. Found: C, 3 4 24.12; H, 2.90; N, 9.55. B-Bromoacrylamide, (E-)(61) To a solution of 3-bromoacrylamide, Z-61, (0.2 g, 0.0013 mole) i n 1 ml of methanol contained i n an 8 mm O.D. pyrex tube was added 3 drops of bromine i n carbon tetrachloride (1:9). The tube was sealed and i r r a d i a t e d (sunlight) f o r 5 h. The l i q u i d was removed by rotary evaporation leaving l i g h t brown cr y s t a l s . These crystals were r e c r y s t a l l i z e d from methanol-ether to y i e l d white crystals of the bromo amide, E-61: m.p. 153-155° ( l i t . (71) 155-156°); i . r . (Nujol) N-H 3355, 3170 cm"1, C=0 approximately 1665 cm"1, C=C 1695 cm"1. B-Bromoacrylonitrile, (Z-)(54) The n i t r i l e Z-54 was prepared by the method of Gryszkiewicz-Trochimowski et a l . (70). The bromc amide, Z-61, (3.0 g, 0.020 mole) was added i n one operation to a 50 ml round-bottom f l a s k containing phosphorus pentoxide (3.8 g, 0.026 mole). The fla s k was heated i n an o i l bath at 75-90° - 156 -at 40 mm pressure with s t i r r i n g and 2.25 g (85%) of a colourless l i q u i d d i s t i l l e d : b.p. 171° / 760 mm; n*° 1.5006; i . r . (neat) C=H 2225 cm"1, C=C 1590 cm 1. The l i q u i d was sealed i n an ampoule and stored at -78°. Anal. Calcd. f o r C H BrN: C, 27.31; H, 1.53. Found C, 26.96; 3 2 ' H, 1.75. B-Bromoacrylonitrile, (E-)(54) Three drops of bromine i n carbon tetrachloride solution (1:9) were added to 250 y l of the bromoacrylonitrile, E-54. The sample was sealed i n 6 mm O.D. pyrex tubing and ir r a d i a t e d (Rayonet, 3100A) for 2 h. The resulting mixture of isomers was analyzed and separated by v.p.c. (FFAP, 153°, 87 ml per min). Two peaks, A (30%) and B (70%), had retention times of 6.1 min and 11.8 min, respectively. These peaks were shown to be the E_ and Z. isomers, respectively, by n.m.r. spectroscopy. For the bromoacrylonitrile, E-54: m.p. 58-59° ( l i t . (71) 56-58°). Equilibrium determination f o r 8-Bromoacrylonitrile, (Z-) and (E-)(54) The determination of the equilibrium position f o r the bromonitriles, Z- and E-54, i s described i n the section dealing with solvent e f f e c t s . - 157 -a-Bromocrotononitrile, (Z-) and (E-) (55) (a) Preparation To a solution of cr o t o n o n i t r i l e , Z- and E-17, (6.7 g, 0.10 mole) i n 50 ml of ether was added bromine (16.0 g, 0.10 mole). The solution was l e f t at room temperature for 1 h with s t i r r i n g . The ether was removed by rotary evaporation and the residue taken up i n 50 ml of acetic acid. Potassium acetate (9.9 g, 0.10 mole) was added and the solution was s t i r r e d for 9 h. The reaction mixture was diluted with 300 ml of water and extracted with chloroform (4 x 25 ml). The combined chloroform extracts were rinsed with 25 ml of saturated sodium bicarbonate solution and dried over magnesium sulfate. The solvent was removed under reduced pressure to y i e l d 10.5 g (70%) o£ the crude a-bromonitriles, Z- and E-55. P u r i f i c a t i o n was carried out by d i s t i l l a t i o n at 63-68° / 20 mm ( l i t . (106) 65-74 / 40 mm). Separation and analysis of the isomers by v.p.c. (FFAP; 153°, 90 ml per min) and subsequent n.m.r. spectroscopy showed the d i s t i l l a t e to contain the Z_ and E_ isomers i n a r a t i o of 69:31. (b) Separation The separation of Z- and E-55 was carried out by v.p.c. (FFAP, 153°, 90 ml per min). The following fractions were collected and analyzed by re i n j e c t i o n into the v.p.c. Fraction A (retention time 5.6 min) contained about 80% E-55 and f r a c t i o n B (retention time 7.2 min) contained about 80% -1 -1 - 1 Z-55. For Er'55: i . r . C=N 2220 cm i For Z-55: i . r . C=N 2225 cm , C=C 1623 cm . - 158 -(c) Equilibrium determination The determination of the equilibrium position i s described i n the section dealing with solvent effects. 2,5-Dibromopropionitrile, (92) Bromine (320 g, 2.0 mole) was added drop by drop over 0.75 h to a well s t i r r e d solution of a c r y l o n i t r i l e (110 g, 2.0 mole) i n 200 ml of ether. The free r a d i c a l addition was catalyzed by the addition of a few crystals of iodine to the gently refluxing solution; the bromine then being added at such a rate that the ether was maintained i n c o n s t a n t r e f l u x . The ether was removed under reduced pressure to give a nearly quantitative y i e l d of the yellow crude dibromide. q-Bromoacrylonitrile, (62) To a mixture of p a r t i a l l y dissolved potassium acetate (181 g, 1.87 mole) i n 400 ml of acetic acid (64) was added the crude dibromide. The res u l t i n g milky suspension was s t i r r e d at room temperature for 12 h. Two l i t e r s of water was then added and the organic (lower) layer removed. The aqueous (upper) layer was extracted with ether (3 x 300 ml). The combined organic and ether layers were washed with water (5 x 100 ml). The ether layer was then transferred to a large beaker and 200 ml of water were added. Sol i d sodium bicarbonate was added with s t i r r i n g u n t i l the aqueous layer was neutral (pH paper). The ether layer was separated and dried over magnesium sulphate. The ether was removed under reduced pressure and the - 159 -residue d i s t i l l e d to yield 138 g (57%) of the light yellow a-bromoacrylo-n i t r i l e : b.p. 62° / 95 mm ( l i t . (107) 116° / 760 mm). The n.m.r. spectrum was consistent with that recorded in the literature (108). 3,3-Dimethoxypropionitrile, (65) Sodium methoxide (2 mole) in 700 ml of methanol was added drop by drop over 3 h to a solution of crude 2,3-dibromopropionitrile, (92), (215 g, 1 mole) in 120 ml of methanol at 0° with s t i r r i n g . The reaction mixture was allowed to warm to room temperature and was left overnight. The mixture was then bri e f l y heated to reflux. Two l i t e r s of water were added, the solution was saturated with sodium chloride and the resulting aqueous solution was extracted with chloroform (6 x 200 ml). The combined chloroform extracts were washed with water (2 x 50 ml). The aqueous washing were then back-extracted with chloroform (50 ml). The combined chloroform extracts were dried over ' magnesium sulphate, concentrated by rotary evaporation and d i s t i l l e d under reduced pressure to yield 81.7 g (71%) of the n i t r i l e (63): b.p. 93-97° / 20 mm ( l i t . (109) 57° / 1 mm, 94-98° / 25 mm). g-Methoxyacrylonitrile, (Z-) and (E-)(56) (a) Preparation The acetal, (63), (1.2 g, 0.0096 mole) with one drop of 85% phosphoric acid was placed i n a bulb-to-bulb d i s t i l l i n g apparatus. The o i l bath was rapidly raised to 185° and held at this temperature for 5 min as the methanol d i s t i l l e d . The temperature of the o i l bath was allowed to drop to 110° at which time the - 160 -pressure was reduced to 18 mm and a mixture of the acetal, (63), and the t i t l e compounds, Z- and E-56, d i s t i l l e d . Analysis of the d i s t i l l a t e was carried out by n.m.r. spectral integration, which showed 75% conversion to the o l e f i n ; an overall y i e l d of 52%. The Z_:E_ r a t i o of the methoxy n i t r i l e s was found to be 38:62. '(b) Separation The separation of the two isomers, Z- and E-56, was carried out using v.p.c. (FFAP, 125°, 90 ml per min). The E_ isomer and the st a r t i n g material, the B,8-dimethoxypropionitrile, could not be separated. The retention times were 5.9 min (E_ isomer and st a r t i n g material) and 7.8 min (Z isomer). (c) Equilibrium determination The _Z and E_ isomers were dissolved i n carbon tetrachloride (20% v/v) for n.m.r. analysis. After the n.m.r. spectra had been taken, isomerization was carried out by Method B. The samples were ir r a d i a t e d (sunlight) f o r 1 day (13 h i r r a d i a t i o n ) and the n.m.r. integration of the methoxyl proton signals gave the Z to E r a t i o s . The sample s t a r t i n g from the Z_ isomer was also analyzed by v.p.c. (FFAP, 175°, 87 ml per min). B-Thioethoxyacrylonitrile, (Z-) and (E-)(57) (a) Preparation Triethylamine (5.6 g, 0.055 mole) was added drop by drop over 20 min to a magnetically s t i r r e d solution of a-bromoacrylonitrile, (62), i i i 20 ml of ether at -78° (Dry Ice - acetone slush bath). To the resu l t i n g l i g h t brown solution, ethanethiol (3.1 g, 0.05 mole) was added over 20 min. The solution - 161 -was allowed to warm to room temperature and was le f t overnight. The resulting dark brown precipitate was removed by vacuum f i l t r a t i o n . The f i l t r a t e was concentrated under reduced pressure leaving a dark brown liquid-solid mixture. The liquid phase was removed and d i s t i l l e d under reduced pressure to yield 2.3 g (41%) of a yellow liquid : b.p. 70° / 0.3 mm ( l i t . (74) 111-115° / 18 mm). Separation and analysis of the isomers by v.p.c. (FFAP, 183°, 33 ml per min), and subsequent identification by n.m.r. spectroscopy showed the d i s t i l l a t e to contain the Z_ and E_ isomers i n a ratio of 43:57. (b) Separation The separation of Z- and E-57 was carried out using v.p.c. (FFAP, 183°, 33 ml per min). The following fractions were collected and analyzed by n.m.r. spectral integration and by v.p.c. integration by reinjection. Fraction A (retention time 13.2 min) contained 80% E-57 and Fraction B (retention time 15.2 min) contained 90% Z-57. For E-57: i . r . C=N 2215 cm"1, C=C, 1565 cm"1. For Z-57: i . r . C=N 2210 cm"1, C=C 1560 cm"1. (c) Equilibrium determination Three or 5 ul samples of >70% E-57 and >80% Z-57 were individually sealed at atmospheric pressure in 4 mm O.D. pyrex tubing. Samples were placed in a furnace at 225° or 297° and then analyzed by v.p.c. (FFAP, 183°, 33 ml per min) at fixed time intervals. Due to some charring i t was convenient to dissolve the samples in 5ul of ether for v.p.c. analysis. - 162 -3-Chlorotiglaldoxime, (64) A solution of the chloroaldehydes, Z- and Ej-33, (4.3 g, 0.0365 mole) in 15 ml of ethanol was added to a solution of hydroxylamine (3.5 g, 0.050 mole) and sodium acetate trihydrate (10.0 g, 0.074 mole) in 20 ml of water. The resulting white crystals"were removed by vacuum f i l t r a t i o n . To the f i l t r a t e was added an equal volume of"water and a second crop of crystals was recovered. The yield of oxime was 4.05 g (83%). A sample recrystallized once from aqueous methanol had m.p. 100-106° ( l i t . (66) 118-119°); i . r . (Nujol) OH 3260 cm"1 (broad) C=C-C=N 1630 cm"1; n.m.r. (CCl^ 8.02T (quartet, 3H, C1-C=C-CH3) 7.73x (quartet, 3H, CH_3C1C=C) , 1.85T (singlet, IH, HC=N) 1.50T (broadened singlet, IH, OH). The n.m.r. spectrum also contained smaller peaks which were assigned to the other isomer: 8.12T (quartet, 3H, ClC^-CRj) and 1.61T (singlet, IH, HC=N) 3-Chlorotiglonitrile, (Z-) and (E-)(58) (a) Preparation The method of Mukaiyama and Hata (75) was used to dehydrate the oxime (64). A slurry of the oxime, (64), (3.5 g, 0.026 mole) in 10 ml dry benzene was added to a syrup of ethanol (2.0 g) and phosphorus pentoxide (4.5g). The reaction mixture was refluxed and stirred for 35 min and then cooled. The benzene layer was decanted. Water (25 ml) was added and the resulting aqueous solution was extracted with ether (3 x 10 ml). The combined benzene and ether extracts were dried over magnesium sulphate, concentrated on a rotary evaporator and d i s t i l l e d under reduced pressure. Three fractions were collected: Fraction A (0.1 g), b.p. 65-68° / 50 mm; Fraction B (0.4 g), b.p. 59° / 20 mm; Fraction C - 163 -CO.4 g), b.p. 65° / 5 mm. The combined yield was 0.9 g (33%. Separation and analysis by v.p.c. (DC-550, 175°, 60 ml per min) and subsequent n.m.r. spectroscopy showed Fractions A, B and C to contain the Z_ and E_ isomers in ratios of 1:99, 5:95 and 65:35 respectively. From the residue remaining after the above d i s t i l l a t i o n was recovered 0.5 g of the unreacted oxime C64). Upon standing at room temperature for 12 days most of the solidJoxime had spontaneously dehydrochlorinated to give the 4,5-dimethylisoxazole (64), identified by i t s n.m.r. spectrum CCC14) 8.08T (singlet, 3H, CH_3-C-0); 7.73T (singlet, 3H, CH^-CsC-O); 2.15T (singlet, IH, -CH=N-) . (b) Separation The separation of Z- and E-58 was carried out using v.p.c. (DC-550, 175", 60 ml per min). The following fractions were collected and analyzed by n.m.r. spectral integration. Fraction A (retention time 5.0 min) contained >95% E-58 and Fraction B (retention time 10.2 min) contained >99% £-58. For the Z_ isomer: b.p. 190-191° / 760 mm; i . r . (neat) C=N 2220 cm"1, C=C 1634 cm \ For the E_ isomer: b.p. 139-140° / 760 mm; i . r . (neat) C=N 2220 cm"1, C=C 1631 cm"1. Anal. Calcd. for C H.C1N (Z isomer): C, 51.96; H, 5.23. 5 6 — Found: C, 51.78; H, 5.44. The n.m.r. spectrum of the E_ isomer indicated the presence of about 10% 4,5-dimethylisoxazole and was not suitable for microanalysis. (c) Equilibrium determination Method B was used. The isomerizations and analyses were carried out on 20% (v/v) solutions i n carbon tetrachloride in n.m.r. tubes. The samples - 164 -were photoisomerized for 1.5 h (Rayonet, 350QA). PROPENYL ETHERS 1-Ethylthiopropene, (Z-) and (E-)(68) (a) Preparation The isomeric ethylthiopropenes, Z- and E-68, were prepared by the method of Boonstra et a l . (78). Propionaldehyde diethylthioacetal (43 g) was prepared in 80% yield by heating ethanethiol (42 g, 0.67 mole) with propionaldehyde (19.5 g, 0.33 mole) in the presence of a trace of a trace of p-toluenesulfonic acid (78). The crude propionaldehyde diethylthioacetal (10.0 g, 0.060 mole) and 3 drops of phosphoric acid were placed in a 25 ml round-bottom flask equipped with a 40 cm Vigreux column and descending condenser. The mixture was heated. At an o i l bath temperature of 170° decomposition began and the ethanethiol-ethylthiopropene mixture began to d i s t i l . The d i s t i l l a t e was collected in 50 ml of a stirred 20% sodium hydroxide solution. As the " decomposition progressed, the bath temperature was raised to 238° and the temperature of the d i s t i l l a t e reached 95°. At this point, the d i s t i l l a t i o n was momentarily stopped and 3 drops of phosphoric acid were added at the top of the column. The d i s t i l l a t i o n was continued u n t i l only a black residue remained. The sodium hydroxide solution was extracted with ether (3 x 25 ml). The combined ether extracts were washed once with 25 ml of water and dried over magnesium sulfate. The ether was removed under reduced pressure and the - 165 -resulting yellow liquid was d i s t i l l e d at atmospheric pressure. Two g (32%) of a colourless liquid b.p'. 121-123° / 760 mm ( l i t . (78) 121-130°) were obtained. Separation and analysis of the isomers by v.p.c. (QF-1, Carbowax 20M, 93°, 22 ml per min) and subsequent n.m.r. spectroscopy showed the d i s t i l l a t e to contain the Z and E_ isomers i n a ratio of 61:39. (b) Separation The separation of Z- and E-68, was carried out by v.p.c. (QF-1, Carbowax 20M, 93°, 22 ml per min). The following fractions were collected and analyzed by n.m.r. spectral integration and by v.p.c. upon reinjection. Fraction A (retention time 8.5 min) contained 100% Z-68 and Fraction B (retention time 9.6 min) contained 81% E-68. (c) Equilibrium determination Five ul samples of >85% Z-68 and >95% E-68 were individually sealed in 4 mm O.D. pyrex tubing at atmospheric pressure. Samples were heated at 207° and analyzed by v.p.c. (QF-1, Carbowax 20M, 93°, 22 ml per min) after 7.25 h and 20.75 h. 1-Methoxypropene, (Z-) and (E-)(69) (a) Preparation The isomeric methoxypropenes, Z- and E-69, were prepared by the method of Howard et a l . (79). - 166 -Propionaldehyde dimethylacetal (82.5 g) was prepared i n 61% yield by the reaction of propionaldehyde (89 g, 1.53 mole), 2,2-dimethoxy-propane (160 g, 1.53 mole), and methanol (5.9 g, 0.18 mole) in the presence of a trace of p-toluenesulfonic acid (79). The pyrolysis of the propionaldehyde dimethyl acetal was carried out in a 300 mm (heated portion) pyrex tube (24 mm I.D.) placed vertically in an electric furnace at 320°. The column was packed with 4 mm glass beads which had been washed with 1% phosphoric acid and dried 24 h at 110°. Dry nitrogen was fed through the column at a rate of 10 ml per min. Propionaldehyde dimethyl acetal (35.4 g, 0.34 mole) was slowly dropped through the column over 30 min. The pyrolyzate was collected in a stirred receiver containing 0.5 g of sodium methoxide in 25 ml of methanol. Toluene (60 ml) was added, and the resulting solution washed with 2N sodium hydroxide (3 x 25 ml). The organic layer was separated, dried over potassium carbonate and d i s t i l l e d . The d i s t i l l a t e boiling up to 60° (7.0 g, 30%) was collected. Separation and analysis of the isomers by v.p.c. (polypropylene glycol-silver nitrate, 35°, 21 ml per min) and subsequent n.m.r. spectroscopy showed the d i s t i l l a t e to contain the Z_ and E_ isomers in a ratio of 69:31. (b) Separation The separation of Z- and E-69 was carried out by v.p.c. (polypropylene glycol-silver nitrate, 35 p, 21 ml per min). Fractions containing >95% and 83% Z-69 and >95% and 80% E-69 were collected. Analysis was achieved by reinjection into the v.p.c. The retention time for the Z_ isomer was 10.2 min; for the E isomer, 11.8 min. - 167 -(c) Equilibrium determination Two or 3 y l samples of Z- and E-69 were individually sealed at atmospheric pressure i n 4 mm O.D. pyrex tubing. Samples were heated at 300° and at 335°. Analysis was accomplished by v.p.c. (polypropylene glycol-s i l v e r nitrate, 35°, 21 ml per min). Due to charring and the v o l a t i l i t y of the products i t was necessary to dissolve some of the samples in ether for v.p.c. analysis. The unsuccessful methods of equilibration tried were p-toluenesulfonic acid catalysis, sodium methoxide catalysis and bromine (2%) in sunlight (Method B). 1,2-Dibromoethylene, (Z-)(70) and 1,2-dibromoethylene, (E-)(70) (a) Separation The isomeric dibromoethylenes were separated by v.p.c. (FFAP, 135°, 60 ml per min). (b) Equilibrium determination The determination of the equilibrium position i s described in the section dealing with solvent effects. SOLVENT EFFECTS ON THE EQUILIBRIUM (a) Solvents used Table 25 l i s t s the solvents employed in the solvent study and their sources. - 168 -TABLE 25 Solvents used for Equ i l i b r a t i o n Studies Solvent Grade and Source P u r i f i c a t i o n acetone spectro none benzene " " a c e t o n i t r i l e " " ethyl acetate " " carbon tetrachloride " " chloroform 11 " toluene reagent " ether anhydrous reagent " nitromethane C.P. " benzonitrile Eastman (pract.) dried § d i s t i l l e d o-nitroanisole " " pentane pract. " methanol 100% " - 169 -A l l solvents were checked for purity by injecting 50 y l samples into the v.p.c. At low attenuation, no false peaks were present which would interfere with the analysis of the isomeric mixture. Cb) Solutes used Table 26 l i s t s the solutes selected for solvent studies. Cc) Method of equilibration and analysis For solvent studies, sample tubes were prepared from 4 mm O.D. pyrex tubing. In order to f a c i l i t a t e the sealing of the sample tube, the neck of the tube was constricted before the sample was placed in the tube; when sealed, the sample tube was 60 to 70 mm in length. In most cases, 0.5 y l of bromine-carbon tetrachloride solution (1:9) was placed i n the sample tube with 5.0 y l of the olefin and 95 yl of the chosen solvent. In the case of Z_ and E-71, the bromine was added in dry ethanol solution. Thus, 2 mole % bromine was present in a 5% solution of the olefin. The constriction in the sample tube was heated and the sample was sealed at atmospheric pressure. In the case of 3-bromoacrylonitrile, (E-)(54), a solid; a weighed sample was dissolved in a measured volume of ether and aliquots containing 5 mg were placed i n the sample tubes. The ether was removed in a stream of dry nitrogen. The equilibration of the isomers was achieved by irradiation (Rayonet, 3100A) of the sample tubes for various time periods. - 170 -TABLE 26 Compounds used fo r Eq u i l i b r a t i o n Studies Compound Configuration Source Crotononitrile Z E See p. 153 6-bromoacrylonitrile Z E See p. 155 a-bromocr.ot ononit r i l e Z E See p. 157 1,2-dibromoethylene Z E Eastman, See p. 167 1,2-dichloroethylene Z E Eastman (pract) - 171 -The analysis of the equilibrium mixture was accomplished by injecting 15-20 ul of the equilibrated solution into the v.p.c. and measuring the peak areas by integration. Table 27 tabulates the v.p.c. data. In some cases where the solvent tailed into the peaks of the isomers being integrated, a solvent blank was run to establish a baseline for the isomer peaks. The v.p.c. tracing was then photocopied and the peak areas estimated by the cut and weigh method. Cd) Solvent blanks Samples were, also prepared in the manner described above except that the bromine catalyst was omitted. The rates of isomerization under these conditions were very slow compared to the catalyzed isomerization. (e) Detector response In order to detect any variation in detector sensitivity between a pair of isomers, a sample was prepared by mixing carefully weighed quantities of the isomerically pure Z_ and E_ 1,2-dibromoethylenes. The purity of these samples was verified by v.p.c. (FFAP, 135, 60 ml per min). This mixture of isomers was injected into the v.p.c. Integration disclosed that the detector sensitivity was vir t u a l l y the same for both isomers; in fact, the difference between the percentage of the E_ isomer in the weighed sample and the integrated peaks averaged 1 part i n 340. (f) Concentration effects An investigation of the effect of concentration of a given solute on the equilibrium position was carried out be preparing 10, 5, 1 and 0.5% - 172 -solutions of 1,2-dibromoethylene, Z-70, and 1,2-dibromoethylene, E-70, i n pentane. Equilibration by bromine catalysis (Rayonet, 3100A) indicated that there was no significant differences in the equilibrium position due to solute concentration. - 173 -TABLE 27 Vapour Phase Chromatographic Data f o r the Isomer Pairs Used i n Solvent Studies. CIO f t x 0.25 i n , 20% FFAP). Compound Temp. Flow Rate Retention °C (ml per min) Time (min) _Z-17 4.4 (5.8) 135 60 (33) E-17 6.0 (7.9) Z-54_ 8.3 153 150 E-54 4.2 Z-55 7.2 153 90 E-55 5.6 Z-70 7.5 150 60 E-70 5.0 £-71 6.3 90 90 E-71 3.6 - 174 -BIBLIOGRAPHY 1. J . H. van't Hoff. B u l l . Soc. Chinu France, 23, 295 (1875). cited i n 1(a). E.L." E l i e l . Stereochemistry of Carbon Compounds. McGraw-Hill, New York, 1962. p.3. Other general references. 1(b). L. Crombie. Quart. 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Tables of Experimental Dipole Moments. W. H. Freeman and Co., San Francisco, 1963. 111. R. A. Beaudet. J . Chem. Phys. 58, 2548 (1965). 112. C. P. Smyth. D i e l e c t r i c Behaviour and Structure. McGraw-Hill, New York, 1955. - 180 -APPENDIX Calculation of Dipole Moments The dipole moments of the c i s and trans olefins l i s t e d i n Table 23 were calculated using the group moments and methods found i n reference (7). Bond angles of 120° are assumed. The group moments used i n the calculations 2 are for the group bonded to an sp carbon (benzene). These values are l i s t e d i n Table 28. TABLE 28 Group Moments Bond or Group U (D) (a) =C-H 0 =C-CH +0.37 3 =C-C1 -1.59 =C-Br -1.57 =C-CN -4.05 (a) The positive sign i s taken for groups which are positive poles with respect to the double bond. D = Debye u n i t s . - 181 -TABLE 29 Calculated and Literature Dipole Moments Compound Dipole Moment (D) fa) Experimental Calculated Z-17 4.08 ^  3.88 E-17 4 « 5 3 4 « 4 2 Z-5_4 2.48 E-54 4.70 Z-5_5 3.88 E-55 3.51 7,-70 1.35 2.72 E-70 0 0 Z-7l_ 1.91 2.76 E-71 0 0 (a) Reference (110). (b) Reference (111) - 182 -Using these values, a projection of the vectors of the group dipole moments on an arbitrarily chosen coordinate system, followed by a summation of the projected vectors allows the dipole moment (u) to be calculated from the following formula: In the formula u is the calculated dipole moment of the olefin and m^ and my are the projections of the group moments on the rc. and y reference axes, respectively. The calculated dipoles are listed in Table 29. The large discrepancies existing between the calculated and experimental dipole moments for the Z 1,2-dihaloethylenes are reported to be due to a mutual induction of the two halogens CH2). A sample calculation follows: N N Z-55 E-55 - 183 -=C-Br m = +1.57 Cos60 mv = -1.57 Sin60 =C-CN » = -4.05 m = 0 T. J =C-CH3 = -0.37 = 0 = -3.63 T^w^  = -1.36 U = C3.63)Z + (1.36) 2 = 3.88 D =C-Er = +1.57 Cos60 = -1.57 Sin60 =C-CN = -4.05 «ly = 0 =C-CH /n-c = +0.37 Cos60 jn v = -0.37 Sin60 5 > = -3.08 ^ J l ) y = -1.68 u = |^C3.08)2 + (1*68) 2] 1/2 = 3.51 D 

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