Ab Initio and DFT Calculations on the Cope Rearrangement of 1,2,6-Heptatriene

(8/8)CASSCF, CASPT2, and B3LYP calculations have been performed on the potential surface for the Cope rearrangement of 1,2,6-heptatriene (1) to 3-methylene-1,5-hexadiene (3). Although an allylic diradical intermediate (2) has been located, the transition states connecting it to 1 and 3 show little allylic delocalization. The first of these transition states is higher in energy than the second; and, when the geometries of intermediate points are constrained so as to prevent allylic delocalization, a pathway from the former to the latter has been found along which the energy decreases monotonically. The existence of a second pathway from 1 to 3, which bypasses 2, is consistent with the experimental results of Roth and co-workers, who found that roughly half of this rearrangement proceeds without formation of a trappable intermediate. In the absence of appreciable allylic delocalization in the rate-determining transition state, the relative strengths of the π bonds broken and the σ bond formed in this transition state explain why ΔH for Cope rearrangement is lower for 1 than for 1,5-hexadiene and more highly unsaturated derivatives.