Base-Free Catalytic Transfer Hydrogenation of Alkyl Formates and Organic Esters at Mild Temperature

The pincer-ligated ruthenium complex (iPrPNHP)Ru(CO)H₂ (iPrPNHP = (iPr₂PC₂H₄)₂NH) is an active catalyst for the transfer hydrogenation of alkyl formates (HCO₂R) and organic esters (RCO₂R′) to the corresponding alcohols under base-free reaction conditions at mild temperatures. Specifically, a range of alkyl formate esters were reduced to MeOH and the corresponding alcohols in high yields using (iPrPNHP)Ru(CO)H₂ as the catalyst and isopropanol (iPrOH) as the hydrogen donor at 30 °C. The first step in the process is the metal-catalyzed transesterification of the alkyl formate with iPrOH to generate isopropyl formate, which is then reduced. The use of iPrOH as the hydrogen donor is crucial. (iPrPNHP)Ru(CO)H₂ can also catalyze the transfer hydrogenation of a broad range of organic esters, including cyclic, acyclic, heteroatom-substituted, and long-chain bio-derived esters, to the corresponding alcohols in good yields using ethanol (EtOH) as the hydrogen donor at 55 °C. Computational studies were used to elucidate the proposed pathway for alkyl formate reduction and the underlying reasons why iPrOH is the most effective hydrogen donor for alkyl formate reduction, while EtOH is optimal for organic ester reduction. Overall, this work describes a highly active catalyst for alkyl formate and organic ester transfer hydrogenation and provides mechanistic insight into the factors responsible for the strong catalytic performance. These findings will be valuable for designing catalysts for both transfer hydrogenation and related reactions.

Kumar, A.; Ertem, M. Z.; Hazari, N.; Miller, A. J. M.; Travis, B. D. Base-Free Catalytic Transfer Hydrogenation of Alkyl Formates and Organic Esters at Mild Temperature. ACS Catalysis 2026, 16 (13), 12474–12485. https://doi.org/10.1021/acscatal.6c02242