Reductive catalysis that uses hydrogen to assist with the activation of strong carbon-oxygen bonds is of industrial importance, as found itself in the catalytic cycles for the reduction of CO2, hydrogenation of carbonyl functional groups, and hydrogenolysis of strong C-O bonds in lignin derived intermediates. In these reactions, their catalytic cycles share a common mechanistic sequence, starting from the activation of hydrogen, which forms reactive proton and hydride intermediates, before their sequential attack to the carbon-oxygen bond, leading to the reduction or bond scission. Introduced in this lecture is the general mechanistic traits, in their similarities and differences on the hydrogen activation catalysis at vapor-transition metal and liquid-transition metal interfaces among these reductive reactions. Depending on the chemical properties of the solvent or ligand, the hydrogen intermediates on transition metal surfaces may acquire different electronic charges and therefore acquire the catalytic function with characteristics of hydrogen adatoms, protons, or even hydrides in the series of hydrogen attack reactions. Similarly, solvent and ligand could also mediate the charge and stability of the carbon containing precursors and the transition states. Together, these two factors lead to changes in the mechanism and catalytic routes. In particular, the formation of interfacial protons, enabled by the protic solvents at the liquid-transition interface, promotes the reduction of OC=O, RC(H)=O, and RC(O)R’, as well as the scission of Ar-OCH3 bonds. Its formation opens up a new path of proton-electron transfer in carbonyl reduction and of concerted intra-molecular proton transfer and Ar-OCH3 scission in the deoxygenation of phenolic species. The ability to tune the charges of the reactive hydrogen atoms and alter their catalytic roles through inner and outer sphere engineering presents new catalytic strategies in reducing strong carbon-oxygen bonds.
Ya-Huei (Cathy) Chin
Professor at University of Toronto