Reduction of cyclic group 6 Fischer carbene complexes: a case of exquisite regioselectivity

The reduction of alpha,beta-unsaturated cyclic group 6 metal-carbene complexes strongly depends on the electronic profile of the groups attached both to the carbene and the beta-carbon and occurs with exquisite regioselectivity. Thus, for complexes 8 the reduction does not take place at the carbene carbon but exclusively at the gamma-carbonyl group. The resulting alkoxide 20 evolves to a tricyclic epoxide structure 21, which precludes additional hydride transfers. Complex 9 experiences the exclusive 1,4-reduction because of the imino character of the beta-carbon (due to the participation of the aromatic resonance form 22). In contrast, monocyclic carbene complexes 10 behave as their acyclic congeners and experience 1,2-hydride addition followed by the 1,3-migration of the metal center. In this case, the participation of eta(3)-Cr(CO)(5) species 31 allows us to understand the labeling pattern found in the final products.