Controlled Evolution of the Cope Rearrangement: Transition from Concerted to Interrupted and Aborted Pericyclic...

We show how synergy between properly placed acceptor and donor groups allows rational design of interrupted and aborted pericyclic reactions, using the Cope rearrangement as a model process. When placed at C2 and C5 carbons of 1,5-hexadienes, a frustrated Lewis pair made of two complementary groups assures that the C-C bond formation is assisted by the flow of electron density from a donor at C5 into an acceptor at C2 through the formation of the C1-C6 bond. If the electron excess at the accepting group is strongly stabilized by the electronic nature of substituents, the pericyclic transition state can become an energy minimum, leading to a switch from a concerted sigmatropic shift to its aborted or interrupted versions. Depending on the electronic nature of acceptors at C5 and the donor groups at C2, a range of possibilities from concerted to aborted pathways is accessible, including the first example of an aborted Cope rearrangement in the absence of a metal catalyst. Furthermore, the zwitter-ionic strategy stabilizes the usually unfavorable boat TS that can potentially evolve into rarely accessible bicyclohexane products.