A formal [1,3] sigmatropic reaction involving free radical intermediates: a mechanistic investigation

Abstract

. 63, 745 (1985).The quantitative isornerization of 2,2-bis(ethylthi0)-3,3-dimethylpent-4-en to 2.2-bis(ethy1thio)-5-methylhex-4-enal was studied over the temperature range 130-170°C. An investigation of the generality and specific mechanism of this formal [I ,3] sigmatropic shift was conducted with six related compounds. The rearrangements were found to obey first-order kinetics, and on the basis of significant positive entropies of activation (52-106 J deg-I mol-I), crossover and trapping experiments, and the lack of a solvent effect (decane vs. DMF), an intermolecular, free-radical chain pathway has been proposed for the isomerization. During the rearrangement of several of the compounds esr signals were observed that were consistent with the presence of the proposed free-radical intermediates. These esr signals have been computer simulated. GORDON S. BATES et S. RAMASWAMY. Can. J . Chem. 63, 745 (1985).OpCrant a des tempCratures allant de 130 170°C, on a CtudiC I'isomCrization quantitative du bis(Cthylthi0)-2,2 dimethyl-3,3 pentkne-4 al en bis(Cthy1thio)-2,2 mCthyl-5 hexknc-4 al. Utilisant six composCs apparent&, on a dCterminC la gCnCralitC et le mCcanisme spCcifique de cette transposition qui est, d'un point de vue formel, du type sigrnatropique [I ,3]. On a trouvC que la cinCtique de ces transpositions est du premier ordre et, sur la base d'entropies d'activation qui sont assez positives (52-106 J deg-' mol-I), d'expkriences d'echanges et de piegeages ainsi que sur Ic fait qu'il n'y a pratiquement pas d'effet de solvant (dCcane vs. DMF), on suggkre que cettc isomCrisation sc produit par le biais d'un proccssus radicalaire intermolCculaire. Au cours des transpositions de plusieurs cornposCs, on a pu observer des signaux de rpe qui sont en accord avec la presence des interrnkdiaires radicalaires proposCs. Utilisant un ordinateur, on a simule ces signaux de rpc.[Traduit par le journal]Although the chemical literature contains many examples of [1,3] alkyl shifts in cyclic systems, their occurrence in acyclic analogues is relatively rare (1). Cookson and Kemp (2) have reported a benzyl migration in the thermal conversion of 3,3-dicyano-2-methyl-4-phenylpent-1 -ene to 1,l-dicyano-2-methyl-4-phenylpent-1-ene which occurs with retention of configuration. Baldwin et al. have evidence for the presence of a radical pathway in the [1,3] sigmatropic rearrangements of a number of systems (3). More recently, Ollis and coworkers (4) have reported rearrangements of various dimethylaminobutenes that again can formally be regarded as [1,3] alkyl shifts. These authors speculate that, although a stereoselective transformation involving a concerted pathway is a likely possibility in the case of several of the compounds, a process involving a weakly interacting radical pair is not unlikely.We recently observed2 a high yielding3 thermal rearrangement of 2,2-bis(ethylthi0)-3,3-dimethylpent-4-enal 1 into compound 2. This rearrangement, which is an example of a [1,3] carbon-carbon shift in an acyclic system, should be favoured si...