Dynamic Effects Responsible for High Selectivity in a [3,3] Sigmatropic Rearrangement Featuring a Bispericyclic Transition State.

Bispericyclic transition states appear when two independent pericyclic transition states merge into one. They are a particular case of the more general ambimodal concept applied to a transition state that connects reactants with two or more products involving reaction path bifurcations through valley-ridge inflections. In the present computational work, the first example of a bifurcating sigmatropic reaction featuring a bispericyclic transition state is reported for a cyclohexane featuring opposing methylene and a vinylidene fragments. Perhalogenation of a C-C bond in this substrate imparts a strong desymmetrization to the bifurcating potential energy surface. These systems undergo the [3,3] sigmatropic rearrangement with high selectivity, with a preferred product that depends on which halogen decorates the C-C bond. We have found that dynamic effects have a paramount role in the selectivity observed for these reactions.