Well-Defined Bis(NHC)Mn(I) Complex Catalyzed Tandem Transformation of α,β-Unsaturated Ketones to α-Methylated Ketones Using Methanol

Tandem transformation of α,β-unsaturated ketones to α-methylated ketones by utilizing methanol as both the hydrogen and C1 sources is reported in the presence of a phosphine-free bis-N-heterocyclic carbene-Mn(I) (bis-NHC-Mn(I)) catalyst. The dehydrogenative coupling between methanol and α,β-unsaturated ketones produces the corresponding α-methylated ketones along with 1 equiv of formaldehyde and water. Among different bis-NHC-Mn(I) complexes, the ethyl wingtip-substituted complex with a triflate counteranion showed the highest catalytic activity. This catalytic system was highly effective for the reductive methylation of a wide variety of substrates including aromatic, heteroaromatic, and aliphatic straight chains containing α,β-unsaturated ketones. Notably, this methodology led to the synthesis of pharmaceutically important drug molecules such as eperisone and lanperisone. Several control experiments, kinetic studies, Hammett studies, and density functional theory (DFT) calculations were carried out to understand this tandem catalytic process.