Carbonyl Ylides Photogenerated from Isomeric Stilbene Oxides. Temperature Dependence of Decay Kinetics

Article abstract of DOI:10.1021/j150667a036

Upon 266-nm laser excitation in fluid solutions, trans- and cis-stilbene oxides form isomeric carbonyl ylides (λ_max's = 470 and 490 nm) that are spectrally and kinetically distinct from one another.Red shifted in absorption and considerably shorter-lived, the ylide photogenerated from the cis oxirane is assigned the cis-exo, exo structure on the basis of symmetry rules for electrocyclic ring opening; its counterpart from the trans oxirane is given the trans-exo, endo structure.The short lifetimes of the ylides arise in part from moderately small activation enthalpies (6-7 kcal mol^-1) and in part from small and negative activation entropies (-5 to -7 eu).Laser flash photolysis in acetonitrile gives rise to a second, longer-lived transient species (λ_max = 335 nm) assignable as the nitrile ylide derived from phenylcarbene and an acetonitrile molecule.Bimolecular rate constants in the range 10⁶ - 10⁹ M^-1s^-1 are presented for the reactions of carbonyl and nitrile ylides with several dipolarophiles.