A charge-remote allylic cleavage reaction: Mechanistic possibilities

The collision-induced allylic cleavage reactions of deuterium-labeled [M - H + 2Li)⁺ and [M - H]^- ions of monounsaturated fatty acids were investigated. Three concerted mechanistic possibilities were considered for this process: a l,4-elimination of a vinylic H, a retro-ene reaction, and a l,4-conjugate elimination. A fourth mechanistic possibility, a two-step radical version of the retro-ene and l,4-conjugate elimination reactions, was also considered. The radical reactions are in accord with the isotopic labeling results and offer certain mechanistic consistencies for cleavage of both C-C allyl bonds; they are expected, however, to have large activation energies. The lower-energy concerted alternatives, the retro-ene reaction for cleavage of the proximal and the l,4-conjugate elimination for cleavage of the distal C-C allyl bond, are also consistent with experimental results. The alternative of two different concerted mechanisms for cleavage of the two allyl bonds, however, is at odds with the charge-remote concept.