Stereochemistry of Carbenic 1,2-Vinyl Shifts

Various 1-phenylbut-3-enylidenes, (Ph)CCR2CHCHR‘, were generated thermally and photolytically from tosylhydrazone (diazo) precursors. 1,2-Vinyl shifts, leading to 1,3-dienes, R‘CHCHC(Ph)CR2, were found to predominate over γ-C−H insertion (R = Me) and to compete with 1,2-H shifts (R = H). Intramolecular addition to the double bond was detected in the case of R = R‘ = Me. The resulting bicyclobutane is thermally stable and does not mediate the vinyl shift. Stereospecific migration of 1-propenyl groups (R‘ = Me), with retention of configuration, was observed on thermolysis and direct photolysis of appropriate substrates. These data exclude the intervention of a triplet diradical and point to vinyl migration in the singlet manifold. Benzophenone-sensitized generation of the carbenes led to partial stereomutation but did not provide conclusive evidence for a triplet rearrangement (isomerization of the diene products could not be avoided under these conditions).