Ketene Formation or Phenyl‐Group Migration as the Favorable Intramolecular Rearrangement in Phenyliodonium Ylides of Hydroxyquinones

Phenyliodonium ylides of hydroxyquinones easily undergo intramolecular rearrangements associated with phenyl-group migration or ketene formation under ambient conditions. The mechanism of the two rival potential reaction paths was explored by electronic structure calculation techniques at the B3LYP/6-311+G(d,p)∪SDD(I) level. It was found that both rearrangements follow a single-step concerted mechanism occurring via a four-membered [1.1.0] bicyclic ring transition state for the ketene-formation process and a five-membered transition state for the phenyl-group migration process. The ketene-formation pathway is predicted to be kinetically and thermodynamically more favorable than the phenyl-group migration pathway, and this is in excellent agreement with the experimental findings that these ylides predominantly form ketenes. An alternative pathway to ketene formation that involves intramolecular rearrangement through a singlet-carbene transition state resulting from dissociation of iodobenzene was also examined. This process was found to be kinetically unfavorable, asit demanded a relatively high activation barrier of 40.6 kcal mol–1. The geometric and energy reaction profile of both intramolecular rearrangements were thoroughly scrutinized, whereas the electronic effects accompanying the intramolecular rearrangements were analyzed by means of electronic structure calculation methods.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)