Amplifying the Reactivity of Anionic Mn(I)-H Catalysts via the Cation Effect: Mechanistic Investigation and Application to the Hydrogenation of α-Trisubstituted Carboxylic Esters.

Despite significant advancements in ester hydrogenation using homogeneous metal catalysts over the past two decades, the catalytic hydrogenation of sterically hindered α-trisubstituted carboxylic esters remains a formidable challenge. Herein, we present a class of imidazole-based NNP-pincer manganese (Mn) catalysts capable of efficiently hydrogenating α-trisubstituted carboxylic esters to their corresponding β-trisubstituted primary alcohols, which are widely applied in the pharmaceutical and fine-chemical industry. Under catalytic conditions, the imidazole moiety is deprotonated by tBuOK to generate a highly reactive anionic Mn-H species with a potassium countercation. Mechanistic studies reveal that the potassium cation in this anionic Mn-H intermediate interacts synergistically with the substrate's carbonyl group and the alkoxide group during hydrogen activation. This cation effect significantly lowers the energy barriers for both hydride transfer and heterolytic H2 cleavage, thereby enhancing catalytic activity. Leveraging this potent anionic Mn-H catalyst, we achieved the hydrogenation of a diverse array of α-trisubstituted carboxylates (80 examples) under mild conditions.