Pieces for a metal-alkoxide mediated mechanism in the hydrogenation and dehydrogenative coupling of carbonyl compounds
There is great interest at present to use transition-metal complexes to catalyze the dehydrogenative coupling of alcohols into esters or carboxamides as well as the hydrogenation of carbonic- and carboxylic-acid derivatives. These reactions promise a revolution in green synthesis and can be relevant to contemporary applications such as biomass production of H2, energy storage and CO2 fixation. Octahedral metal-hydride complexes, such as those developed by the Milstein group, are among the best known catalysts used for these purposes, and their activity is generally attributed to metal-ligand cooperation. We argue there are ample experimental observations from the chemical literature that point to an altogether different mechanism involving the intermediacy of octahedral metal-alkoxides. We support this proposition with DFT calculations.