Cobalt- and Silver-Promoted Methylenecyclopropane Rearrangements.

The rate of the methylenecyclopropane rearrangement is enhanced by an alkyne-Co(CO) complex bonded to the para position of a benzene ring. This is explained by a stabilizing effect on the transition state leading to the biradical intermediate. Computational studies indicate that the benzylic-type biradical intermediate is stabilized by a delocalization mechanism, where spin is delocalized onto the two cobalt atoms. Silver cation also enhances the rate of the methylenecyclopropane rearrangement. Computational studies suggest that silver cation can also stabilize a benzylic radical by spin delocalization involving silver. In the case of the silver-promoted reactions, the rate enhancements in a series of aryl-substituted methylenecyclopropanes correlate with σ values. The question remains open as to whether the silver-catalyzed methylenecyclopropane rearrangement proceeds via an argento-stabilized biradical or whether the reaction involves an argento-substituted allylic cation.