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.