Sigmatropic Rearrangements of 1,1-Diarylindenes. Migratory Aptitudes of Aryl Migration in the Ground and Electronically Excited States

Article abstract of DOI:10.1021/jo00318a018

The photochemical and thermal rearrangements of 1,1-diarylindenes to give 2,3-diarylindenes have been investigated.Migratory aptitudes of p-X-phenyl vs. phenyl were determined for X =Br,CN, and OCH3 in the photochemical and thermal migrations.The identities of the products of these rearrangements were established by unambiguous synthesis, and the synthetic work is described.Product ratios were generally determined by NMR techniques, but VPC and isotope dilution were also used in the case of 1-(p-cyanophenyl)-1-phenylindene reactions.The excited-state reactions (direct and triplet sensitized) are highly selective, migration of the substituted phenyl group being favored for all three substituents.The thermal reactions, in contrast, are quite unselective, phenyl migrating almost as readily as the substituted phenyl group in all cases.Quantum yields for the rearrangement in the case of 1,1-diphenylindene and 1-(p-cyanophenyl)-1-phenylindene were 0.80 and 0.4, respectively (direct irradiation), and 0.43 and 0.53 (sensitized reactions).The results of the thermal reactions and results from the literature are discussed in terms of bond-dissociation energies and transition-state-delocalization energies calculated by using the Hueckel theory.Neither approach led to a satisfactory interpretation.The excited-state migrations are consistent with charge-transfer stabilization of the transition state, which can be estimated from oxidation and reduction potentials by using Weller's equation.