The history of the Cope rearrangement is ripe with controversy, mostly stemming from a debate about the mechanism. Ultimately it was determined that the mechanism changed based on substitution therefore the mechanism was classified “chameleonic.” We showed that the mechanism of the Pd(II)-promoted Cope rearrangement is also chameleonic (now converting between concerted and stepwise zwitterionic pathways). Further, we developed the idea of transition state complexation, in which the transition state for a reaction is complexed to a metal without changing the mechanistic pathway, only lowering the barrier.
Highest occupied molecular orbital (HOMO) for the transition state for the cope rearrangement (above).
HOMO-4 of the transition state for the Pd(II)-catalyzed Cope rearrangement (above). The barrier is lowered by approximately five kcal/mol, but the mechanism remains concerted.
Siebert, M.R. and Tantillo, D.J. Journal of the American Chemical Society, 2007, 129, 8686-8687. “Transition State Complexation in Palladium-Promoted [3,3]-Sigmatropic Shifts.” 10.1021/ja072159i.