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Electron transfer from the hydroxide ion to a carbon-carbon triple bond Halliwell, Janet Elizabeth 1970

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ELECTRON TRANSFER FROM THE HYDROXIDE ION TO A CARBON - CARBON TRIPLE BOND Janet E. Halliwell . B.Sc, Queen's University, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Chemistry We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January, 1970 In presenting th i s thes i s in pa r t i a l f u l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make i t f r e e l y ava i l ab le for reference and study. I fu r ther agree tha permission for extensive copying of th i s thes i s for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of th i s thes i s f o r f i nanc ia l gain sha l l not be allowed without my wr i t ten permission. Department of CH&n ,1 T «a 1 The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada Date •^/£3<n6c<2^ ABSTRACT Supervisor: Professor R. Stewart A one-electron transfer from the hydroxide ion to the electron acceptor p-nitrotolan was studied i n aqueous dimethylsulfoxide by means of vis i b l e spectroscopy and electron spin resonance techniques. The p o s s i b i l i t y of i n i t i a l nucleophilic attack followed by electron transfer from the carbanion so formed was eliminated. The analogous reaction of methoxide with p-nitrotolan was brie f l y examined. i i TABLE OF CONTENTS page INTRODUCTION 1 A . The F o r m a t i o n of R a d i c a l - A n i o n s i n O r g a n i c Systems 1 B . H y d r o x i d e Ion as an E l e c t r o n Donor 4 C . E l e c t r o n T r a n s f e r t o a Carbon-Car ton T r i p l e Bond 5 D . The D i m e r i z a t i o n of t h e T o l a n R a d i c a l - A n i o n 6 OBJECT OF THE PRESENT RESEARCH 7 EXPERIMENTAL 9 A» P u r i f i c a t i o n o f S o l v e n t s and P r e p a r a t i o n o f S o l u t i o n s 9 B , P r e p a r a t i o n o f Compounds 11 C , S p e c t r a l Measurements 12 D, E l e c t r o n S p i n Resonance Measurements 13 RESULTS 14 A. E l e c t r o n S p i n Resonance S t u d i e s 14 B . S p e c t r o s c o p i c S t u d i e s - 20 C . NMR Study 32 D . F u r t h e r R e a c t i o n s o f p - N i t r o t o l a n R a d i c a l - A n i o n 35 i i i DISCUSSION / 39 A. Electron Transfer as the Elementary Act of the Reaction 39 B. Further Reactions of the p-Nitrotolan Radical-Anion 41 SUGGESTIONS FOR FURTHER WORK 46 BIBLIOGRAPHY 48 APPENDIX 51 iv LIST OP FIGURES page 1* E.S.R. Spectrum (High Modulation) of p-Nltrotolan Radical-Anion Produced by Hydroxide Ion Reaction 15 2. p-Nitrotolan Radical-Anion: E.S.R. Spectrum 16 3« E.S.R. Spectrum of p-Nitrotolan Radical-Anion Produced Electrochemically 18 4* Calibration of Spectral Data for p-Nitrodeoxybenzoin Carbanion 22 5« U.V. - Visible Spectra of p-Nitrotolan and p-Nitrotolan Radical-Anion 24 6» Relative Rates of Radical-Anion Formation as a Function of Solvent 26 7. Rate of p-Nitrotolan Radical-Anion Formation 28 8. N.M.R. Spectrum of p-Nitrotolan in DMSO 33 9» N.M.R. Spectrum of Methoxide Addition Product of p-Nitrotolan 34 10. Rate of Formation of p-Nitrodeoxybenzoin Carbanion in the Presence of Oxygen 36 11. U.V. Absorption of Product 38 V ACKNOYiL EDGEMENT I would like to thank Professor R. Stewart for his guidance throughout this work. My thanks are also extended to Mr D. Kennedy for his assistance with the E.S.R. work. Grateful acknowledgement is made to the National Research Council for its financial assistance. 1 . INTRODUCTION A. The Formation of Radical-Anions i n Organic Systems It i s only in the last 10 years that the existance of aromatic radical-anions as important reaction intermediates has been realized. Classifying reactions into "electrophilic" and "nucleophilic" attacks on a substrate disguise the possibility of heterolytic electron transfer as an i n i t i a l step. As electron resonance spectroscopy became widely used i n organic chemistry, large numbers of radical-anions were reported and their role i n mechanism was more c r i t i c a l l y examined. In I 9 6 3 . G. Ai Russell and E. J. Geels* published a summary 2 of the standard methods of generating aromatic radical-anions , i n addition to a number of newer methods^ 1) Reduction by an al k a l i metal 2) Electrochemical reduction 3) Zinc or glucose reduction i n basic media 4) Photolytic generation 5) Electron transfer between anions and unsaturated systems 6) Homolytic dissociation of 1,4 dianions 7) Electron transfer between dianions and molecular oxygen 8) Formation of radical-anions i n basic solution from hydroxide addition products of unsaturated systems and electron acceptors 3-5 G. A. Russell and co-workers have carried out a great deal of work 2 on radical-anion formation in aromatic nitro compounds i n a t-butyl-alcohol or dimethylsulfoxide (DMSO) solution of potassium t-butoxide. For example, p-nitrotoluene yields a high concentration of radical-anion i n either solvent system. The mechanisms postulated involve i n i t i a l ionization of the benzylic protons, i n both solvent systems, but a complex sequence of disproportionation reactions had to be invoked to rationalize the kinetics and products: t-butyl alcohol: C H -CH 3 DMSO: C H / \ N O , • + ( C H ^ C O C H paramagnetic + other products C H , N O . C H . N O . It was then concluded that the main difference in this disproportionation reaction of p-nitrotoluene i n t-butyl'alcohol and DMSO was merely a change i n the rate determining step. In the f i r s t case, i t was ionization of the benzylic proton, and in the second, the formation of the charge transfer complex or an electron transfer step. The authors then generalized that this "spontaneous disproportionation 1* reaction was to be expected whenever hydrogen atoms are o( to an easily reducible group. Other work has been done by Russell et al on one-electron transfer to nitrobenzenes from strong bases^. Here the rate of radical-anion formation i s solvent dependent. On the increase in concentration of a dipolar aprotic solvent i n a mixture, the rate of production of the radical-anion i s increased. These studies indicate the extreme probability of encountering radical-anion formation when 4 working with aromatic nitro compounds in highly basic media, and the 7 different manners i n which electron transfer can occur . Most radical-anions reported are very highly coloured, but vi s i b l e absorption techniques have not been frequently employed in Q studies of these species. Chambers and Adams have reported a large solvent dependence of radical-anion absorptions i n the visible region on studies of the nitrobenzene anion radical i n a dimethylformamide: water solvent mixture. This effect was attributed to hydrogen bonding or solvation, and may be used as an indication that the absorption i s due to a radical-anion species. B. The Hydroxide Ion as an Electron Donor In the literature, there have been reports of a wide variety of anions acting as donors of an unpaired electron, as was indicated by Russell's suismary^. Alkoxide ions have, perhaps, received the most attention, whereas l i t t l e work has been done on the hydroxide ion as 9 donor. Some work has been done i n this f i e l d on the well known electron acceptors: tetracyanoethylene, chloranil, p-benzoquinone, and duroquinone i n an aqueous ethanol solution of potassium hydroxide. Here a direct one electron transfer from hydroxide was postulated: OH" + A : >Ai + OH* , 2 OH * H2°2 5 and a qualitative correlation was observed between the yield of the radical anion and the electron a f f i n i t y of the acceptor. Oxygen was observed to react rapidly with the radicals produced. C. Electron Transfer to a Carbon-Carbon Triple Bond Recently, an investigation of electron transfer to a number of acetylenic compounds i n DMF was reported by Dvorko and S h i l o v ^ ; SCN~ and I~ both acted as donors to the substrates Me02C.C=C.CO^Me , H.C=C.C02Me , and H.C=C.C020Ac , and a one electron transfer was postulated to explain the observed E.S.R. signals. However, a competing nucleophilic attack was also proposed to rationalize the "inhibitory effects" of a proton donor on radical formation. By both mechanisms a polymeric product could be formed: - C = C - • x" As previously indicated,a proton donor ( acetic acid ) decreased the concentration of radical observed, as did oxygen at temperatures above 40°C. 6 D. The Dimerizatiori of the Tolan Radical-Anion The dimerization of the tolan radical-anion produced by a l k a l i 11 12 metal anion reduction has been quite extensively studied ' , which i s interesting i n the light of the suggestion that the acetylenic compounds studied 1^ produced polymers of low molecular weight. Dadley and Evans looked at the reaction of an al k a l i metal-aromatic hydrocarbon with tolan i n tetrahydrofuran ( THF ) . The following reaction sequence was postulated: J2f C=C 0 + M+ArH~ f t > ( 0 C=C 0 )~M+ + ArH ...1 2 (jer c=:c 0 rii+ > j6 c—c 0-c 0='c 0 ...2 ft ti ( S6 C=C 0 ) iM + + ArH~M+ > 0 C= C 0 + ArH ...3 The relative rates of reactions (2) and (3) are dependent on the gegen-ion employed, i. e . K+, Na +, L i + , that i s , there i s a competition between the dimerization reaction and the second electron transfer. These reactions were followed by vis i b l e spectroscopy and by E.S.R. . An estimate of the electron a f f i n i t y of tolan placed i t between 2.70 eV and 2.96 eV . The influence of the a l k a l i metal cation here i s so strong that i t i s d i f f i c u l t to draw analogies with a s t r i c t l y organic system. I OBJECT OF PRESENT RESEARCH The i n i t i a l purpose of t h i s research was to examine the p o s s i b i l i t i e s of forc i n g the hydration of an acetylenic t r i p l e bond by nucleophilic attack on the normally electron-rich centre. For t h i s purpose, the acetylene bond must be s u f f i c i e n t l y activated by electron withdrawing substituents, and contain no acidic protons. Thus, the substituted diphenylacetylene ( tolan ) system was chosen f o r study i n highly basic DMSO : H20 : OH" medium. This study was considered to be an extension of the work on nucleophilic attacks on carbon-carbon double bonds, which has received considerable attention over the past ten y e a r s ^ ' 1 ^ ' ^ . Here the primary nucleophilic attack was shown to be always on the p o s i t i v e l y polarized carbon atom of the double b6nd giving an unstable carbanion of the same general type. The further reaction of t h i s carbanion was governed by a number of factors: 1) the nature of the nucleophile, 2) the substituent immediately adjacent to the double bond, and 3) the nature of the solvent system. R R R / + YA A \ R 9 EXPERIMENTAL Ao Purification of Solvents and Preparation of Solutions 1. Purification of Dimethylsulfoxide (DMSO) Baker analyzed grade DMSO was degassed with nitrogen and stirred over calcium hydride for 24 - 36 hours. It was then d i s t i l l e d under reduced pressure at 35 - 40°C. The f i r s t 15$ of the d i s t i l l a t e was discarded and subsequent fractions taken off under nitrogen by means of a Perkin triangle. Each flask was sealed with a ground glass stopper and waxed polyethylene film, and once opened was kept over molecular sieves. The DMSO obtained by this treatment contained no more than 0.01% water by t i t r a t i o n with Karl Fischer reagent. 2. Preparation of DMSO : TCater Stock Solutions The water used for DMSO : water stock solutions was d i s t i l l e d and degassed by boiling and cooling with nitrogen bubbling through i t . The stock DMSO : water solutions were prepared i n liq u i d reagent bottles and made up approximately by volume. The weights of DMSO and water added were recorded and then Karl Fischer titrations carried out on each solution to determine the exact mole % DMSO concentration. These solutions were sealed with a rubber serum stopper with a flap that bent over the l i p and neck of the bottle. This 10 formed an airtight seal and i t was possible to penetrate the serum cap many times without losing its seal. 3. Hydroxide Ion Source Tetramethylammonium hydroxide was employed as the hydroxide ion source using a constant concentration of approximately 0.01 mole/1. It was obtained as a 10/6 aqueous solution ( Eastman Organic Chemicals). Before use i t was transferred under nitrogen to a number of small vials sealed with rubber serum caps. In this manner, exposure of the base to air was minimized. The base was transferred to the reaction mixture by means of a calibrated micro syringe. 4. Preparation of Stock Solutions of Reactants Knowing the concentration of substrate necessary to follow the reaction in the U.V. - visible, the required concentration of the stock solution was calculated for a ten fold dilution. The approximate amount of reactant was accurately weighed out in a 10 ml volumetric flask. To this was added DMS0 to make the volume up to 10 ml, and the solution stored in a desiccator. 8 The only reported instance of such an attack on acetylenic substances was i n 1952 by Haszeldine^, who studied the attack on hexafluorobut-2-yne by methoxide, ethoxide, and a secondary amine i n ethanol solution y i e l d i n g v i n j ' l i c ethers: e.g. CF , c=C.CF + OMe -> C F _ . C = C . C F , 5 I • 3 OMe OMe C F i - C - C H . C P , <~ 3 | 3 OMe I f OMe" more stringent conditions CF-, o C — CHo CF-3 | 3 OMe CF,.C(OMe)_CH_CF, 3 2 2 3 The reaction occurred readily at less than 0 C, but i t must be noted that CPj has such tremendous electron withdrawing properties that the electronic environment of the t r i p l e bond may be substantially altered from that of the tolan system,, In working with p-nitrotolan, i t became obvious that some form of r a d i c a l reaction was occurring, although there was also "evidence" of hydration of the substrate. Thus the system was examined by v i s i b l spectroscopy and E. S.R. to determine the mechanising s) operative. 11 ^° P r e P a r a t i o n of Compounds 1. Diphenylacetylene (tolan) Tolan was prepared from trans stilbene by bromination and subsequent double dehydrobrominatlon of the dibromide i n triethylene T0 glycol,and KOH . The l i g h t brown cr y s t a l s melted 60 - 6 l°C ( Buchi mp ), o 19 cf l i t e r a t u r e value of 6 0 - 61 C , and the overall y i e l d of p u r i f i e d tolan was 6 5 $ a f t e r three r e c r y s t a l l i z a t i o n s from 9 5 $ ethanol. 2o (p-Nitrophenyl)-phenylacetylene ( p-nitrotolan) p-Nitrotolan was sjnithesized from phenylacetylene and p-nitro-20 21 iodobenzene by the method of Castro et a l ' . Phenylacetylene was converted to the cuprous phenylacetylide by reaction with a Cu"*" salt produced by the reaction of hydroxylamine hydrochloride with cupric sulphate. The product was extensively washed with water s ethanol, and ether, and then dried i n a vacuum oven at 65°C f o r 12 hours 0 The purity of the cuprous phenylacetylide was essential f o r a reasonable y i e l d of the disubstituted acetylene. The reaction of the acetylide was carried out i n pyridine under nitrogen at ^0°C f o r 9 hours. After extraction, the product was r e c r y s t a l l i z e d twice from pet ether ( 30 - 6 0 ) and melted 1 1 9 . 7 - 1 2 0 . 1 ° C ( Buchi ) ; ( l i t e r a t u r e value: 119 - 1 2 0°C 2 0). Microanal. C a l c : Cs 7 5 . 3 4 » Hs 4 * 0 4 , N: 6 . 28 Expt. ; Cs 7 5 » 5 5 , Hs 4 . 0 2 , Ms 6.41 12 3. p-Nitrodeoxybenzoin p-Nitrodeoxybenzoin was prepared from p-nitrophenylacetic acid by a Friedel Crafts aoylation i n the following manner. p-Nitro-phenylacetic acid ( 5 ) was suspended i n benzene and anhydrous sodium hydride ( 0.7 gm ) added i n benzene, after washing i t free of the storage o i l . The mixture was stirred at room temperature for three hours. To the sodium salt of the p-nitrophenylacetic acid was added 22 5 drops of pyridine, followed by oxalyl chloride ( 4.7 ml ) • After one hour of s t i r r i n g at room temperature, the mixture was refluxed for one hour and f i l t e r e d . This f i l t r a t e was concentrated to 5 0 ml, to which was added anhydrous AlCl^ ( 3.8 gm ) . After s t i r r i n g for one hour, the dark reaction mixture was poured onto 25 gm of ice and 10 ml of concentrated HC1 . The cream coloured precipitate was f i l t e r e d , washed, and recrystallized from absolute ethanol to yield cream o o 23 coloured crystals melting 144.0 - 145.2 C;( literature value: 1 4 5 C ). C. Spectral hfeasurements A l l U.V. - visible spectral studies were carried out using a Bausch and Lomb recording spectrophotometer, Model No. 5 0 2 , with 1 cm. quartz c e l l s . This instrument was equipped with a constant temperature c e l l holder through which water was pumped from a thermostated bath. 13 Measurements were carried out i n a i r , or under nitrogen at 25 C unless otherwise noted. The general proceedure for measurements under nitrogen was as follows. The solutions were prepared i n the quartz c e l l s ( which contain approximately 3«2 ml.) , f i t t e d with neoprene stoppers which are easily punctured by syringe needles. The c e l l was flushed with dry nitrogen for a short time, and then 2.98 ml of the DMSO : water stock solution added by means of a syringe equipped with a Chaney adaptor. Twenty five ^ i l of the reactant stock solution was then added, and nitrogen bubbled through the solution for three minutes. The base solution was added after the c e l l had been allowed to equilibrate i n the c e l l block of the spectrophotometer for at least 10 minutes. The reference c e l l s were handled i n the same manner as described above, except for the addition of the 25 ^A1 of the substrate stock solution. These solutions were always prepared simultaneously with the sample to be studied. -Wavelength scans, or the change of absorption with time at one particular wavelength, were followed with the addition of hydroxide ion as zero time. • Electron Spin Resonance Measurements E.S.R. measurements were carried out on a Varian E - 3 spectrometer at room temperature. Care was taken to use degassed solutions and to not expose the reaction mixture to a i r . 14 RESULTS Ao Electron Spin Resonance Studies 1. Examination of p-Nitrotolan : Hydroxide Ion Reaction i n DMSO -2 A thoroughly degassed DMSO solution, approximately 1.9 x 10 molar i n p-nitrotolan, was treated with an excess of tetramethyl-ammonium hydroxide. A very large E.S.R. signal was observed immediately from the deep green solution, and after a number of dilutions with degassed DMSO, a spectrum of high resolution was obtained. It consisted of seven major peaks when examined at high modulation ( FIGURE 1 , p . 15 ) i which were s p l i t into about 65 doublets or 120 distinct peaks i n the high resolution spectrum ( FIGURE 2 , p. 16 ). This spectrum did exhibit the unusual characteristic of being broader and less resolved at the high f i e l d end. The results are consistent with the formation of a p-nitrotolan radical-anion, produced by one-electron transfer to the neutral p-nitrotolan molecule from some donor anion or neutral species. The radical-anion may be pictures as: - 0 - 0 FIGURE E. S.R. Spectrum ( High Modulation) of Anion Produced b^j H^droxicle. Ton p-N ifvo+olan Radical" Reacfion 16 p -N i t r o t o l an R a d i c a l - A n i o n FIGURE 2 17 Because of the interaction with the nitrogen nucleus, the free electron resonance i s s p l i t into three components of equal intensity and with s p l i t t i n g a^ T « A further interaction with the two equivalent H^ 's w i l l further s p l i t each of these components into three of intensity 1 : 2 s 1, and.with s p l i t t i n g a^ . But i f a^ — , a spectrum of seven components would he expected, as was actually obtained. Further analysis of the s p l i t t i n g due to more distant protons was consistent with those peaks observed. 2. Electrolytic Production of the p-Nitrotolan Radical-Anion It was necessary to prove that the E.S.R. spectrum observed was actually due to the p-nitrotolan radical-anion, and not due to the radical of an addition product, which was a possibility analogous to some of the work of G. A. Russell et a l . Hence, a radical was produced electrochemically from a solution of p-nitrotolan and TPAE ( tetra-N-propylammonium perchlorate ), used as charge carrier, in DMSO. A low current of about 1»5 volts was passed through the solution for a short time, and a greenish colour developed. A strong E.S.Ro signal was recorded, which again, consisted of seven major peaks. This i s illustrated i n FIGURE 3, p. 18. It was not possible to resolve this spectrum as completely as that of the previous case due to the high viscosity of the pure DMSO medium* In the f i r s t experiments, the 00 FIGURE E-5.R. S p e c t r u m o f p - N if rololaw RoLcUeotl- An ion P r o d u c e d EI e c f ro c We w i"c ex JI y 19 hydroxide ion was added as an aqueous solution, and the resulting viscosity decrease allowed a much higher degree of resolution. However, this spectrum was seen to be superimposible on that produced by the reaction mixture and observed at high modulation. 3. Computer Analysis of Splitting Values To obtain good approximations of the s p l i t t i n g constants from the spectrum a method of " best f i t " was employed. The values of the splittings were estimated from the spectra, and were used i n a computer programme which plotted the appropriate derivative E.S.R. signal. By comparison with the original spectrum, and by successive approximations, values for the s p l i t t i n g constants were obtained as follows: = 7.60 gauss = 3.83 gauss ; a^ = 1.3 gauss ; a^ =0.5 gauss ; = 0.25 gauss • 20 B. Spectroscopic Studies 1. Unsubstituted Diphenylacetylene _5 A tolan solution of concentration 4*55 x 10 molar was prepared i n anhydrous DMSO. To this , at 25°C was added OH" ( 0.01 molar) and the U.V. - visible spectrum observed as a function of time. No change was noted over 72 hours. This was repeated at 35°C, and again no change i n spectrum was observed* 2. p-Nitrodeoxybenzoin I f a nucleophilic attack on p-nitrotolan were to occur, an unstable carbanion would be expected to form, which would then undergo rapid tautomerism to a more stable species according to the following scheme: HI 21 Thus, to absolutely identify intermediate I I I , i t was prepared by the reaction of hydroxide ion on p-nitrodeoxybenzoin: The oLhydrogens of p-nitrodeoxybenzoin are weakly acidic, and a proton may be removed by KOH i n ethanol. Thus i t was assumed that 0H~ i n DMSO caused complete ionization, giving one a means of calculating the extinction coefficient. On the addition of base ( 0 . 0 1 molar ) to a p-nitrodeoxybenzoin - 5 solution of 1 . 8 x 1 0 mole/litre an intense blue colour develops, corresponding to a wide absorption peak at 5 8 4 mu. An extinction coefficient of 5 3 » 3 0 0 was calculated for this species at 5 8 4 mu i n 9 4 . 7 4 mole % DMSO. A distinct solvent dependence of ^ and O.D. was r max max observed for this carbanion ( III ) and hence these were calibrated against solvent molarity as indicated i n TABLE 1 , and FIGURE 4 , p. 2 2 . 22 t QD. •1 + 70 80 100 •MOLE % D M S O — * 585 580 575 70 3 o 90 MOLE 7o DMSO-FIGURS 4 Calibration of Spectral Data for p-Nitrodeoxybenzoin Carbanion 23 TABLE 1 Mole % DMSO O.D. max 96.74 O.96I 584 92.20 0.959 583 87.52 0.944 581 77.03 - 0.907 578 68.05 0.875 576 3. p-Nitrotolan and Hydroxide Ion Prom a solution of p-nitrotolan ( 2.99 x 10* ^  mole/litre ) i n DMSO, an extinction coefficient of 20,890 was calculated for the one major absorption peak, with no fine structure, at 332 mu. ( see FIGURE 5, p. 24 ). The reaction of p-nitrotolan with hydroxide ion was f i r s t examined i n the presence of oxygen, and a slow buildup of a blue species absorbing at 583 nip was noted. The rate of production of this species increased with temperature, and with an increase of the DMSO content of the solvent. The rate of production of this species, identified as the p-nitrodeoxybenzoin carbanion, was reproducable, but after some time, the carbanion disappeared. 24 400 300 /{my FIGURE 5 U.V. - Visible Spectra of p-Nitrotolan and p-Nitrotolan Radical-Anion -5 1. p-Nitrotolan ( 3 x 10 mole/litre ) 2. p-Nitrotolan Radical-Anion produced i n 96.74 mole % DMSO 3. p-Nitrotolan Radical-Anion produced i n 94.04 mole % DMSO 4* p-Nitrotolan Radical-Anion produced i n 92.20 mole % DMSO 25 The same reaction was then carried out i n pure DMSO under nitrogen employing degassed solutions. On following the 500 - 600 mu region of the vi s i b l e spectrum, a broad absorption band was observed to form i n the 590 - 428 mu area with a maximum at 427 mn. This had some fine structure, and the absorption maximum seemed extremely sensitive to solvent composition, which i s ill u s t r a t e d i n FIGURE 5» p. 24 * A small absorption peak was observed to form i n the 583 mu region. These changes were accompanied by a rapid decrease i n the p-nitrotolan peak ( 332 mu ) and the creation of an absorption peak at slightly lower % . On passing oxygen through the reaction mixture, the 390 - 428 mu peak disappeared immediately, and l i t t l e change was i n i t i a l l y observed i n the 582 mu region. The wide absorption over 390 - 428 mu was thus attributed to a radical-anion. The reaction of hydroxide ion with p-nitrotolan was then carried out i n solvents of varying basicity ( as measured by the H function ), but always with a large excess of OH". The rate of increase of the radical-anion concentration was followed by monitoring the solution at the appropriate ^ m a x ( see FIGURE 5» P» 24 ) and typical results are plotted i n FIGURE 6, p. 26. Here values of O.D. vs time are plotted for DMSO : H20 solvent systems: 1) 96.74 mole % DMSO at 427 mu ; 2) 94.04 mole % DMSO at 422 mu ; 3) 92.20 mole % DMSO at 410 mu . The absolute value of the extinction coefficient of the radical-anion i s not known, and hence, only a relative picture can be 26 S 10 —TIME (min) — * FIGURE 6 Relative Rates of Radical-Anion Formation as a Function of Solvent Composition: 1) i n 96.74 mole % DMSO at 427 mu. 2) i n 94.04 mole % DMSO at 422 mu. 3) i n 92.20 mole % DMSO at 410 mu. 27 obtained. The runs were carrefully carried out i n t r i p l i c a t e under nitrogen, but results were not completely reproducible, although runs for each solvent system f e l l i n the same range. This was attributed to the action of oxygen as an electron scavenger. Due to the low -5 concentration of substrate, 3 x 10 mole/litre, a very small amount of oxygen could have a relatively large effect on radicals i n solution. Thus a l l solutions would have to be prepared and handled i n an oxygen free atmosphere to obtain meaningful results. The effect of a change in the i n i t i a l concentration of the substrate, p-nitrotolan, was examined by using concentrations of 3 x 10* ^  and 6 x 10 molar. Plots of O.D. at 427 mp vs time for these reactions are shown in FIGURE 7» P* 28. The i n i t i a l slopes of the curves give a relative measure of the rate of radical-anion production, and i t was found that: _i •. S 1 ° P e ^ n i t . d c l / dt .665 - . 0 - _ [p-"i*rotol*tt] j _________ - _______ — _____ = X»yj — —————————————— Slope 2 i n i t ^ d c ^ d t .342 [p-nitrotolanj 2 Thus, assuming that the radical-anion i s formed i n the rate determining step of the reaction, or i s a direct monitor of the rate determining step, this result indicates that the reaction of p-nitrotolan and 0H~ i n excess 0H~ i s pseudo f i r s t order in the p-nitrotolan. 29 An analysis of the results up to this point, taken i n conjunction with the literature on radical-anion formation, allows one to postulate two plausible mechanisms to account for the formation of the p-nitrotolan radical-anion. Mechanism 1: A nucleophilic attack followed by tautomerism and electron transfer from the carbanion so formed. 50 Mechanism 2: Single electron transfer from the hydroxide ion directly to p-nitrotolan. Both mechanisms presented are consistent with the fact that the p-nitro-tolan radical-anion i s produced during the reaction and with the qualitative kinetic results. To distinguish between the two mechanisms, a test reaction was prepared as follows. A 0.36 x 10"^ molar solution of p-nitrodeoxy-benzoin was prepared in degassed DMSOi to which was added a very small amount of dilute OH* solution. About one third of the p-nitrodeoxy-benzoin was ionized to the carbanion as was seen from the O.D. of 0.7 at 584 np. Thus i t was assumed that there was no excess base i n the system, merely a source of carbanion as large or larger than could be present under reaction conditions. To this was added p-nitrotolan -5 ( to a concentration of 3 x 10 mole/litre ) and the optical density at 427 njji was monitored. No change was observed for about 30 minutes. Then, a large excess of base was added, whereupon the immediate formation of the radical-anion was observed. Thus mechanism 1 may be ruled out. 31 It has been shown that this reaction sequence i s pseudo f i r s t order i n p-nitrotolan, and i n an analogous manner, an attempt was made to quantitatively examine the influence of hydroxide concentration. By employing a dilute base solution, i t was found that the minimum amount of hydroxide required to observe any radical-anion formation was approximately a three fold excess over p-nitrotolan. Attempts to were unsuccessful due to an i n a b i l i t y to keep the system free of oxygen. This w i l l also vastly affect the minimum required hydroxide concentration to observe radical-anion production that was previously suggested. 4. Methoxide Ion as an Electron Donor To further substantiate the idea of a rapid one-electron transfer from a small strongly nucleophilic reagent, the reaction of methoxide ion with p-nitrotolan was br i e f l y studied. -5 To a solution of p-nitrotolan ( 3 x 10 molar ) i n anhydrous degassed DMSO was added a very small amount of solid sodium methoxide ( Fisher chemical ) . A fine stream of nitrogen gas was then passed through the mixture to further degass and agitate i t . An immediate green colour developed and the characteristic U.V. - v i s i b l e spectrum examine the rate of radical anion production as a function of of the radical-anion was observed, with ' max 32 C NMR Study To a f a i r l y concentrated solution of p-nitrotolan ( 1.0 molar ) i n anhydrous DMSO was added a large excess of sodium methoxide ( s o l i d ) i n the presence of oxygen. An immediate deep red-green colour was observed, and the change i n the NMR spectrum noted. The NM spectrum of the pure p-nitrotolan i n DMSO c l e a r l y showed two sets of doublets downfield from the main aromatic resonance ( at about 2*5 t ) with the same coupling of ~" 8 cps. These doublets, due to the n i t ro-aromatic ri n g appear at X, = 1.8 and T= 2.3 based on © a reference of f^gQ = 7«'5 • This spectrum i s i l l u s t r a t e d i n FIGURE 8 , p. 33 » On the addition of methoxide, two effects were immediately evident: 1) the resonance i n t e n s i t y decreased markedly? and 2) the remaining signal shifted u p f i e l d . This i s i l l u s t r a t e d i n FIGURE 9 » P» 34 . Here a clear doublet was observed at T= 3.1 with J^8 cps as i n the parent spectrum. The main aromatic resonance i s centred on t*= 2.7> a very s l i g h t u p f i e l d s h i f t . Thus the n i t r o -aromatic r i n g has been shifted u p f i e l d r e l a t i v e to the unsubstituted r i n g , i n that substance observed, and some other non resonating species seems to have been formed. Thi3 l a t t e r may be attributed to the r a d i c a l - -anion, as the green colour disappeared f a i r l y r a p i d l y , and a s l i g h t broadening of the spectrum was observed ( although i t was s t i l l reasonably 33 2.0 3.0 FIGURE 8 NMR Spectrum of p-Nitrotolan i n DMSO 34 FIGURE 9 NMR Spectrum of Methoxide Addition Product of p-Nitrotolan 35 well resolved ) . The NMR signal of the •product' increased with time, and concurrent with th i s , better resolution was obtained. The NMR spectrum, and probably the red colour observed may be attributed to a carbanion addition product of the form: where there i s free rotation around the bond indicated, as observed by the (HaJgCHyOg system i n the spectrum. The negative charge next to the aromatic ring has shifted the H 0 and Hb resonances upfield. )« Further Reaction of the p-Nitrotolan Radical-Anion It has been mentioned previously, that i n the reaction of p-nitrotolan with hydroxide i n the presence of a i r , there i s a reprod-uceable build-up of p-nitrodeoxybenzoin carbanion. Typical plots of the carbanion concentration as a function of time i n these systems are shown i n FIGURE 10, p. 36 . It was also observed that a product 36 C mole/l X id 2.S TIME (min) FIGURE 10 Rate of Formation of p-Nitrodeoxybenzoin Carbanion in the Presence of Oxygen: 1) in 96.74 mole % DMSO 2) in 94.04 mole % DMSO. 37 absorbing i n the 270 - 310 mu range was formed slowly with time. The absorption was a broad band with maxima at 309 and 290 mu, and i s ill u s t r a t e d i n FIGURE 11, p. 38 • Owing to the intense p-nitrotolan absorption at 290 - 340 mu, no rate studies were carried out on the formation of this product. However, i t was noted to form more rapidly i n the better degassed systems, and i t s formation was independant of the nature of the electron donor. 38 39 DISCUSSION A. Electron Transfer aa the Elementary Act of the Reaction The necessary requirements for an electron transfer as the elementary act of any reaction are that an electron donor and an electron acceptor he present i n the reaction medium. The capacity to receive one electron, and thus form a radical-anion, i s exhibited by a number of classes of organic compounds; i.e. 1) aromatic compounds, 2) heterocyclic compounds, 3) compounds containing conjugated systems, and 4) those with strong electron withdrawing groups. p-Nitrotolan i s a conjugated system with electron withdrawing groups, and hence i s a potential electron acceptor. The actual ease of reduction of this system could, however, only be determined by studies with various electron donors and i n relation to other acceptors. The hydroxide ion has previously been shown to be involved i n electron transfer reactions as a donor. In the DMSO : water medium employed, the 0H~ has a high enough activity that an electron transfer to even a relatively weak acceptor i s feasible. Any nucleophilic attack on the acetylenic bond has been ruled out by the U.V. - v i s i b l e studies reported, and thus the electron transfer from hydroxide must be viewed as the elementary act, where the radical-anion may be simply pictured as either of the following species: 40 From an a n a l y s i s of the s p l i t t i n g constants, i t can be seen that approximately 0.3 of the t o t a l spin density i s l o c a l i z e d on the nitrogen atom, and at most 0.23 of the spin density on the /3 carbon ( APPENDIX 1 ). T h i s r a d i c a l - a n i o n i s inherently a r e l a t i v e l y stable organic r a d i c a l , as coupling reactions between n i t r o groups ( to form N-N compounds ) are u n f e a s i b l e , as are r i n g coupling reactions. P i c t u r i n g the r a d i c a l e l e c t r o n as being on the nitrogen, i t i s / p l a c e d i n a favour-able p o s i t i o n f o r d e r e a l i z a t i o n i n t o the benzene r i n g . The basic seven l i n e E.S.R. spectrum, which can be resolved i n t o more than 100 l i n e s , i s also consistent with t h i s formulation. The formation of the p - n i t r o t o l a n r a d i c a l - a n i o n i s only p o s s i b l e because of the low r e a c t i v i t y of the substrate towards a nucleophile, and the f a c t that i t contains no a c i d i c protons. Thus the number of systems i n which t h i s e l e c t r o n t r a n s f e r from OH* may be expected to be seen i s severely l i m i t e d . The i n f l u e n c e of solvent on r a d i c a l formation has not been extensively studied, although some work has been done on the e f f e c t of the medium on the behaviour of free r a d i c a l intermediates ( f o r a 41 review, see ref. 24 ). The solvent effects that have been noted for radical formation by decomposition of azo compounds are extremely small. They are attributed to differences i n solvation of the ground state, the transition state being poorly solvated by both polar and non-polar solvents. But here the hydroxide ion i s known to be poorly solvated and any transition state complex would be expected to be sligh t l y better solvated, and hence reverse this situation. The addition of protic solvents almost always causes a rapid disappearance of the radical-anion, so the f i e l d of study i s restricted to f a i r l y aprotic solvents. For example, the hydrolysis of the radical-anion of an aromatic hydrocarbon yields the i n i t i a l hydrocarbon and the corresponding 25 dihydro derivative . B. Further Reaction of the p-Nitrotolan Radical-Anion The action of oxygen on the p-nitrotolan radical-anion appears to be rather anomolous. When exposed to a large excess of oxygen, the p-nitrotolan i s immediately regenerated; however, i n the presence of a small amount of oxygen, the deoxybenzoin carbanion i s formed. Carbon radicals are known to react very readily with molecular oxygen to form a peroxy radical: 42 R • -N 0 2 —> R-0-0' The resulting radical i s unreactive relative to a carbon radical, and what actually happens to i t depends on the particular system involved,, It i s reasonable to assume that this type of reaction i s a step i n the formation of the deoxybenzoin carbanion, followed by hydride abstraction and the decomposition of the hydroperoxy radical so formed to the keto radicals 43 The decomposition of hydroperoxides i s a relatively complex chain 27 28 process ' and the scheme suggested may well be a vast oversimp-l i f i c a t i o n , as temperature, solvent, and the presence of other molecules i n solution have considerable influence on the decomposition pathways The reaction of the p-nitrotolan radical-anion i n the presence of excess oxygen may be simply pictured as an oxidation by electron transfer to the oxygen to produce neutral p-nitrotolan, which can again be reduced by hydroxide. The overall scheme can be pictured i n terms of the relative oxidation levels of the various speciess 4 Red°n p-Nitrotolan Carbanion The possibility of the formation of the apparent copolymer of the peroxy radical ( -0-O-RH- ) should not be overlooked, as this system could be considered analogous to the styrene polymerization where the following mechanism has been observeds 44 -?» Rad-O-0 • Rad-O-0• * CH_=CH0 o Rad«0-0-CH2-CH 0 However9 no evidence, other than that mentioned i n the next paragraph, was seen to substantiate t h i s product formation, and the regeneration of most of the p-nitrotolan from the radical-anion by oxygen indicated that no~*rapid polymerisation was occuring. The slow b u i l d up of the product showing the broad absorption band i n the 290 - 3 4 0 np region has s t i l l to be explained, and t h i s can be done i n two ways<> The f i r s t i s the formation of the copolymer of the peroxy r a d i c a l that was previously discussed, and the second i s a slow polymerisation of the p-nitrotolan radical-anion i t s e l f by the following schemes where? 45 R'-C=C-R R -CH=C-R ' • + . R ' -CH=C-R | termination j R'-CH=C-R R'-C = c - R further polymerization This mechanism of product formation i s analogous to that suggested by Dvorko et a l ~ ^ i n the formation of polymeric products from acetylene radicals generated by electron transfer from negative ions. However, neither scheme has been definitely substantiated at present. 46 SUGGESTIONS FOR FURTHER WORK In this thesis, i t has been shown that the hydroxide ion can act as an electron donor towards p-nitrotolan i n a highly basic DMSO : water medium. However, the effect of basicity on the formation of the p-nitrotolan radical-anion has not been thoroughly studied, and could lead to a further confirmation of the mechanistic picture involved. The correlation of kinetic behavior with acidity ( basicity ) has been 29 30 31 32 examined by a number of authors for homolytic reactions * * ' but there has not been complete agreement on the va l i d i t y of this treat-ment. For example, Kresge^ asserted in 1965 that kinetic acidity i s a property of the substrate only, and hence provides insight into the structure of the transition state and not the mechanism. But this point of view has been widely disputed. -For a reaction of the type: R • OH" v R. + OH* R....0-H T.S. k (OH") (R) rate: v = . ' w f * where f * i s the activity coefficient of the transition state. By using a number of assumptions, particularly with respect to the use of the H s c a l e ^ a correlation of log k Q b s and H + log a g Q can be predicted. 47 For this work i t i s necessary to prepare and handle a l l solutions i n an inert atmosphere to obtain accurate data. As reported, i n the present research considerable trouble was experienced with oxygen contamination. The further reaction of the p-nitrotolan radical-anion again poses some experimental problems, particularly i n the presence of oxygen. However, electrolytic generation of this radical-anion i s a promising method of approach, as relatively concentrated solutions can be handled. The fact that the corresponding electron transfer reaction with unsubstituted tolan did not proceed under the same conditions il l u s t r a t e s the necessity of the polarization of the acetylenic bond by a ring substituent. In the lig h t of this, i t would be interesting to examine a series of substituted tolans to see i f rates.and substituent constants may be correlated by the Hammett equation. Careful definition of the reaction taking place must be made due to the p o s s i b i l i t y of competing nucleophilic attack, especially i n such compounds as p,p*-dinitrotolan. Here the effect of substituents i s to remove electron density from the acetylenic bond rather than a polarization of charge. Interesting extensions to the studies suggested would be a variation of both the nucleophile employed as electron donor and the 35 dipolar aprotic solvent. Hexamethylphosphoramide has been reported to have the lowest dielectric constant and the highest basicity of a l l dipolar aprotic solvents, and hence would be of particular interest i n this system. 48 BIBLIOGRAPHY 1. G. A. Russell and E. J . Geels, Tet. Letters,1333 (1963). 2. P. B. Ayscough and F. P. Sargent, Proc. Chem. Soc, (London) , 94 (1963). 3« G. A. Russell and E. G. Janzen, J . Am. Chem. Soc, 84 , 4153, 4154, 4155 (1962). 4. G. A. Russell and R. C. Williamson, J . Am. Chem. Soc, 86 , 2357» (1964). 5. G. A. Russell and E.G. Janzen, J . Am. Chem. Soc, 8j? , 300 (1967). 6. G. A. Russell, E. G. Janzen and E. T. Strom, J . Am. Chem. Soc, 86 , 1807 (1964). 7. E. Buncel, A. R. Norris and K. E. Russell, Quart. Rev., 22 , 125 (1968). 8. J . Q. Chambers and R. N. Adams, Kolec Phys., £ , 413 (1965). 9. G. V. Fomin, L. A. Blyumenfel'd and V. I. Sukhorukov, Dokl. Akad. Nauk., SSSR , 15J , 1199 (1964). 10. G. F. Dvorko and E. A. Shilov, Teor. Eksper. Khim., j> , 606 (1967). 11. D. Dadley and A. G. Evans, J . Chem. Soc (B) , 418 (1967). 12. D. Dadley and A. G. Evans, J . Chem. Soc (B) , 107 (1968). 13. S. Patai and Z. Rappoport, J . Chem. Soc, 377, 383, 392 (1962). 14. S. Patai and Z. Rappoport, "The Chemistry of the Alkenes" Interscience, London (1964). 15. C A. Fyfe, Can. J . Chem., 42 , 2331 (1969). 16. R. N. Haszeldine, J . Chem. Soc, 3490 (1952). 49 17. K. A. Bilevich and 0. Yu Okhlobystin, Russ. Chem. Rev., 32 , 954 (1968). 18. "Organic Synthesis", Co l l . Vol. 5 , 350, J . Wiley and Sons, N.Y., 1955. 19. Buttenberg, Ann., 2J_9 , 327 (1894). 20. C. E. Castro, E. J . Gaughan and D. C. Owsley, J . Org. Chem., 31 * 4071 (1966). 21. R. D. Stevens and C. E. Castro, J . Org. Chem., 2B , 3515 (1963). 22. H. 0. House, S. G. Boots and V. K. Jones, J . Org. Chem., 30 , 2522 (1965). 23. Petrenko-Kritschenko, B., 25_ , 2242 . 24. "Advances i n Free Radical Chemistry",^ , 77 , Logos Press, 1965* 25. D. E. Paul, D. Lipkin and S. J . Weissman, J . Am. Chem. Soc, 78 , 116 (1956). 26. "Radical Ions" Ed. E. T. Kaiser and L. Kevan, Interscience, 1968. 27. C. Walling, "Free Radicals i n Solution", J . Wiley and Sons Inc., 1957. 28. E. S. Gould, "Mechanism and Structure i n Organic Chemistry", Holt, Rinehart and T/inston, 1959. 29. M. Anbar, M. Bobtelsky, D. Samuel, B. Silver and G. Yagil, J . Am. Chem. Soc, 8_5_ , 2380 (1963). 30. R. Stewart, J . P. O'Donnell, D. J . Cram and B. Rickborn, Tetrahedron, 18 , 917 (1962). 31. G. Yagil and M. Anbar, J . Am. Chem. Soc, 84 , 1797 (1962). 32. W. D. Kollmeyer and D. J . Cram, J . Am. Chem. Soc, 9_0 , 1784 (1968). 50 33* A. J . Kresge, Chem. Comm., 46 (1965). 34. D. Dolman, PhD Thesis, University of Br i t i s h Columbia, 1966. 35. H. Normant, Angew. Chem. Int., 6 , IO46 (1967). 51 APPENDIX The Calculation of Spin Density from Hyperfine Splitting Constants The use of hyperfine splittings i n E.S.R. spectra of radical-anions to determine the distribution of the unpaired electron over the 26 molecule has been extensively reviewed . Thus, this appendix w i l l serve merely as a description of the calculation used, and not the f u l l derivation. It has been shown thats 4 - ^ where p i s the spin density, 'a* i s the hyperfine s p l i t t i n g , and 'Q* i s a numerical factor ( gauss/spin density ). Por the benzene negative •H ion was found to be -22.5 G-» and hence this value was used for both proton s p l i t t i n g and as an approximation to the nitrogen s p l i t t i n g . The distribution of the electron density over the molecule was reqired only to indicate i f any of the electron density localized on the acet-ylenic carbons, so an accurate value was not essential. The splittings and spin densities calculated with this equation are tabulated below: •a» (splitting) p (spin density) N 7.72 0.305 H i 3.83 0.340 H2 1.00 0.090 H 3 0.51 0.046 H4 0.12 0.005 


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