On the Mechanism of Cyclization of 5-Hexenylchromate Intermediates in the Reactions of Fischer Carbene Complexes with a Lithium Enolate and Allylmagnesium Bromide

The mechanisms for the evolution of pentacarbonyl-5-hexenylchromate complexes, unsubstituted and methyl substituted at C2. formed from a pentacarbonyl(alkoxy)carbene complex of chromium, the corresponding ketone lithium enolate, and allylmagnesium bromide, were theoretically Investigated by using DFT (Density Functional Theory) at the B3PW91/6-31G* level (LANL2DZ for Cr and Br) taking into account the effect of THF solvent through the PCM model (Polarizable Continuum Model). Methyl substitution at C2 provokes a shortening of about 5 degrees in the C1-C2C3 angle that favors the formation of the pentacyclic product. Also, the presence of this methyl substituent at C2 sterically disfavors the formation of the hexacyclic product, Thus, our results yield the hexacyclic system as the most favored product for the evolution of the unsubstituted alkylpentacarbonylchromate complex, and the pentacyclic product in the case of the substituted system, in good agreement with the experimental findings. The stereochemistry of the products experimentally observed is determined at the transition state for the migration of the Cr(CO)5 fragment from C1 to C6 and the conformational rearrangement of the C1-C6 skeleton. Amine molecules, present in the reaction medium, can play a catalytic role by assisting the 1,2-H migration in the last step for the formation of hexacyclic products.