New paradigms for the peroxy acid epoxidation of CC double bonds: the role of the peroxy acid s-trans conformer and of the 1,2-H transfer in the epoxidation of cyclic allylic alcohols.

RB3LYP calculations, on reaction of performic acid with cyclic allylic alcohols, demonstrate that the less stable s-trans conformer of peroxy acids can be involved in epoxidations of C=C bonds. Transition structures (TSs) arising from s-trans performic acid retain some of the well-established characteristics of the TSs of the s-cis isomer such as the perpendicular orientation of the O-H peroxy acid bond relative to the C=C bond and a one-step oxirane ring formation. These TSs are very asynchronous but collapse directly (without formation of any intermediate) to the final epoxide-peroxy acid complex via a 1,2-H shift. Thus, our findings challenge the traditional mechanism of peroxy acid epoxidation of C=C bonds by demonstrating that the involvement of the s-trans isomer opens an alternative one-step reaction channel characterized by a 1,2-H transfer. This novel reaction pathway can even overcome, in the case of the reaction of cyclic allylic alcohols in moderately polar solvents (e.g., in dichloromethane), the classical Bartlett's mechanism that is based on the s-cis peroxy acid form and that features a 1,4-H shift. However, the latter mechanism remains strongly favored for the epoxidation of normal alkenes.