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 pote ntial surface for the Cope rearrangement of 1,2,6-heptatriene (1) to 3-meth ylene-1,5-hexadiene (3). Although an allylic diradical intermediate (2) has been located, the transition states connecting it to 1 and 3 show little a llylic delocalization. The first of these transition states is higher in en ergy than the second; and, when the geometries of intermediate points are c onstrained so as to prevent allylic delocalization, a pathway from the form er to the latter has been found along which the energy decreases monotonica lly. The existence of a second pathway from 1 to 3, which bypasses 2, is co nsistent with the experimental results of Roth and co-workers, who found th at roughly half of this rearrangement proceeds without formation of a trapp able intermediate. In the absence of appreciable allylic delocalization in the rate-determining transition state, the relative strengths of the pi bon ds broken and the sigma bond formed in this transition state explain why De lta H-double dagger for Cope rearrangement is lower for 1 than for 1,5-hexa diene and more highly unsaturated derivatives.