Ester Hydrogenation Catalyzed by a Ruthenium(II) Complex Bearing an N-Heterocyclic Carbene Tethered with an "NH2" Group and a DFT Study of the Proposed Bifunctional Mechanism

A ruthenium(II) catalyst containing an NHC-amine (NHC = N-heterocyclic carbene) ligand (C-NH2) catalyzes the H-2-hydrogenation of various esters and lactones at 50 degrees C and 25 bar of H-2 pressure, mild reaction conditions compared with other reported catalysts. A maximum turnover frequency of 1510 h(-1) for the hydrogenation of phthalide with a conversion of 96% is achieved in 4 h. DFT calculations suggest a concerted, asynchronous bifunctional mechanism for homogeneous ester hydrogenation; a proton transfer step from the N-H group of a ruthenium hydride-amine complex to the carbonyl group has the largest energy barrier in the catalytic cycle. A surprising observation is that methyl pivalate ((BuCOOCH3)-Bu-t) is hydrogenated much more rapidly than is tert-butyl acetate ((CH3COOBu)-Bu-t). This is explained by the energetics of the rate-determining step of the proposed Ru-H/N-H bifunctional mechanism.