Origin of Regioselectivity in the Reactions of Nitronate and Enolate Ambident Anions

摘要

The reactions of nitronates of ring-substitutednphenylnitromethanes and enolates of ring-substituted 1-phenyl-2-npropanones with MeOBs gave exclusively the O-methylated and Cmethylatednproducts, respectively. DFT calculations suggested thatntwo factors, namely, intrinsic barriers and metal-cation coordination,ncontrol the C/O selectivity. The kinetic preference for Omethylationnin the reactions of nitronates arises from the intrinsicnbarriers, which are ca. 10 kcal/mol lower for O-methylation thannfor C-methylation. The situation is the same for the gas-phase reaction of an enolate, in which the O-methylation is morenfavorable than the C-methylation. The experimentally observed C-selectivity of enolate reactions in solution is due to the metalcationncoordination, which hinders O-methylation for enolates. The effects of the enolate reactivity and the solvent on the C/Onselectivity are also rationalized to arise from the two factors.