Solvent-Assisted Ruthenium Complex Catalyzes Hydrogenation and the Reductive Amination of Carbon Dioxide

Catalytic hydrogenations represent fundamental processes in the synthesis of fine chemicals and chemical intermediates, allowing for atom-efficient and clean functional group transformations. Herein, we have developed a highly efficient, robust catalytic system consisting of a ruthenium hydride complex [RuH(CO)(dppp)(en)]Cl and tetrabutylammonium acetate (TBAOAc), where the former was employed as a catalyst for selective hydrogenation of methyl levulinate (ML) to gamma-valerolactone (GVL) with molecular hydrogen under mild reaction conditions without any base additives, while the latter was used as a low-temperature molten salt solvent assisting the hydrogenation reaction. After the reaction, solid salt TBAOAc can settle down spontaneously due to the immiscibility with weak polar extraction solvent, leading to the clean separation of products from reaction mixture. Furthermore, the catalytic system was highly leaching-resistant and maintains its catalytic activity in the consecutive recycles. Notably, TBAOAc not only acted as reaction media but also played an important role in improving the catalytic performance via the coordination of OAc- with Ru sites. In contrast to poor activity of ML hydrogenation catalyzed by Ru complex in conventional organic solvents, the superior catalytic activity was achieved by using TBAOAc as media. NMR, HR-ESI-MS analysis, and deuterium-labeling studies indicated that the ligand exchange could happen between TBAOAc and the ruthenium hydride complex via the formation of Ru-OAc species, which can promote H2 activation, dissociation and sequential hydrogenation. Finally, this catalytic system has been extended smoothly for N-formylation of a large variety of amines with carbon dioxide and hydrogen and showed high catalytic activity, stability, and selectivity toward formamides. This study identifies the reason for TBAOAc-assisted ruthenium complex catalyzing hydrogenation and provides new insights to optimize the sustainability of a procedure for the conversion of biomass-derived platform molecules and CO2.