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

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Catalysis, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://dx.doi.org/10.1021/cs300619qA ruthenium(II) catalyst containing an NHC−amine (NHC = Nheterocyclic carbene) ligand (C−NH2) catalyzes the H2-hydrogenation of various esters and lactones at 50 °C and 25 bar of H2 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 (tBuCOOCH3) is hydrogenated much more rapidly than is tert-butyl acetate (CH3COOtBu). This is explained by the energetics of the rate-determining step of the proposed Ru−H/N−H bifunctional mechanism