Hiscotropic Rearrangements: Hybrids of Electrocyclic and Sigmatropic Reactions

An unusual reaction manifold for cyclopropylcarbinyl cations, uncovered using B3LYP and MP2 calculations, is described. This reaction is a hybrid of a [1,2] sigmatropic hydrogen shift and a two-electron electrocyclic ring opening. These two processes occur through a single transition structure (i.e., they are concerted), although they are not synchronous. We call these reactions "hiscotropic" rearrangements. The potential energy surfaces for these reactions are often complicated, in some cases involving flat plateaus and bifurcations. Torquoselectivity occurs in some cases as a result of favorable orbital interactions between the breaking C-C and C-H bonds.