. MINES. Can. J. Chem. 67, 689 (1989). Alkenes, conjugated with a phenyl group, can be converted to nonconjugated tautomers by sensitized (electron transfer) irradiation. For example, irradiation of an acetonitrile solution of the conjugated alkene 1-phenylpropene, the electron accepting photosensitizer 1,4-dicyanobenzene, the cosensitizer biphenyl, and the base 2,4,6-trimethylpyridine gave the nonconjugated tautomer 3-phenylpropene in good yield. Similarly, 2-methyl-1-phenylpropene gave 2-methyl-3-phenylpropene, and l-phenyl-1-butene gave E-andZ-1-phenyl-2-butene. The reaction also works well withcyclic alkenes. For example, 1-phenylcyclohexene gave 3-phenylcyclohexene, and 1-(phenylmethylene)cyclohexane gave 1-(phenylmethyl)cyclohexene. The proposed mechanism involves the initial formation of the alkene radical cation and the sensitizer radical anion, induced by irradiation of the sensitizer and mediated by the cosensitizer. Deprotonation of the radical cation assisted by the base gives the ambident radical, which is then reduced to the anion by the sensitizer radical anion. Protonation of the ambident anion at the benzylic position completes the sequence. Reprotonation at the original position is an energy wasting step. Tautomerization is driven toward the isomer with the higher oxidation potential, which is, in the cases studied, the less thermodynamically stable isomer. The regioselectivity of the deprotonation step is dependent upon the conformation of the allylic carbon-hydrogen bond. The tautomerization of 2-methyl-1-phenylbutene gave both 2-phenylmethyl-1-butene and 2-methyl-1-phenyl-2-butene (E and Z isomers), while 2,3-dimethyl-1-phenylbutene gave only 3-methyl-2-phenylmethyl-1-butene. In the latter case, steric interaction of the methyls on the isopropyl group prevents effective overlap of the tertiary carbon-hydrogen bond with the singly occupied molecular orbital, thus inhibiting deprotonation from this site.Key words: photosensitized, electron transfer, alkene, tautomerization, radical cation. , on peut transformer des alcknes conjuguCs avec un groupement phCnyle en tautomkres qui ne le sont pas. Par exemple, l'irradiation du phCnyl-1 propkne, un alckne conjuguC, en solution dans I'acCtonitrile, en prCsence de dicyano-1,4 benzkne agissant comme photo-sensibilisateur acceptant des electrons, de biphCnyle agissant comme cosensibilisateur et de trimCthy1-2,4,6 pyridine agissant comme base, fournit de phCnyl-3 propkne, le tautomkre qui n'est pas conjuguCe, avec un bon rendement. De la m&me manikre, le methyl-2 phCnyl-1 propkne fournit du mCthyl-2 phCnyl-3 propkne alors que le phCnyl-1 butkne-1 conduit i un melange des phCnyl-1 butknes-2-(E) et -(a. La rCaction donne aussi de bons rtsultats avec les alcknes cycliques. Par exemple, le phinyl-1 cyclohexkne fournit du phCnyl-3 cyclohexkne alors que le (phCny1mCthylkne)-1 cyclohexane conduit au (phCnylmCthy1)-1 cyclohexene. Le micanisme proposC implique la formation initiale du cation radical de l'alckne et l'anion radical du sensibilisateur qui est induit...

This paper is dedicated to Professor Douglas E . Ryan DONALD R. ARNOLD and SHELLEY A. MINES. Can. J. Chem. 65, 2312 (1987). The photosensitized (electron transfer) irradiation of several conjugated I , I-diphenyl alkenes, in acetonitrile with 1,4-dicyanobenzene or I-cyanonapthalene as electron accepting sensitizer and 2,6-lutidine as base, leads essentially quantitatively to tautomerization to the less stable unconjugated isomer(s). The proposed mechanism for this reaction involves formation of the alkene radical cation and sensitizer radical anion followed by deprotonation of the radical cation, reduction of the resulting radical to the ambident anion by back electron transfer from the radical anion, and reprotonation. There are several steps in this mechanism that could control the ratio of isomers. Evidence is provided that, at least in some cases, it is the relative rate of deprotonation from the isomeric radical cations that is the determining factor. This rate is influenced by the conformation of the radical cation; the carbon-hydrogen bond involved in the deprotonation step must overlap with the singly occupied molecular orbital. DONALD R. ARNOLD et SHELLEY A. MINES. Can. J. Chem. 65, 2312Chem. 65, (1987. L'irradiation photosensibilisCe (transfert d'tlectron) de plusieurs diphknyl-I , 1 alcbnes conjuguts, effcctute dans l'acttonitrile avec du dicyano-l,4 benzbne ou du cyano-l naphtalene comme sensibilisateur acceptant les tlectrons et de la lutidine-2,6 comme base, conduit essentiellement et avec un rendement quantitatif une tautomCrisation vers le (ou les) isomkre(s) le(s) moins stable(s). Le mtcanisme proposC pour cette reaction implique (a) la formation du cation radical de l'alctne et de I'anion radical du sensibilisateur, ( b ) une dkprotonation du cation radical, (c) une rtduction du radical qui en rtsulte, par un retransfert d'electron de la part du radical anion, conduisant a l'anion ambidentate et (d) une reprotonation. Plusieurs Ctapes de ce mCcanisme pourraient contr6ler le rapport des isomtres. On prksente des donntes qui suggerent, au moins dans quelques cas, que c'est la vitesse relative de dCprotonation des cations radicaux isomeres qui est le facteur determinant. Cette vitesse est influencCe par la conformation du cation radical; il doit y avoir un recouvrement de la liaison carbone-hydrogene impliquee dans 1'Ctape de dkprotonation avec l'orbitale moltculaire qui est occupke d'une faqon ~Clibataire.[Traduit par la revue]The tautomerization of alkenes, by either acid or base catalysis, is an old and relatively well-understood process (1). The composition of the equilibrium mixture reflects the thermodynamic stability of the isomers. Conjugation of a double bond with an aryl group has a profound effect on stability; for example, at equilibrium P-methyl styrene dominates over allylbenzene by three to four orders of magnitude (2). Until now there has been no direct way to counteract the thermodynamic driving force of this equilibrium, that is, to convert a conjugated aryl alkene into a less ...

Alkenes such as (I), (III), or (V), conjugated with a phenyl group, can be converted to nonconjugated tautomers by sensitized (electron transfer) irradiation.