Milstein's seminal 2005 and 2006 reports of pincer-ruthenium catalysts for ester hydrogenation and the reverse reaction, acceptorless dehydrogenative coupling of alcohols, sparked a revolution in the design of metal-ligand-cooperative catalysts for the hydrogenation and dehydrogenation of polar bonds. The originally proposed mechanism, supported by several computational studies, involved reversible deprotonation of a CH2 linker on the ligand and reversible dearomatization of a pyridine ring. This presentation describes recent work by the Chianese group demonstrating that Milstein's catalyst is inactive in its original form, and activates by releasing ethane, resulting in the conversion of a diethylamino side group into an NHEt group. Through computational studies, kinetics, and isolation of key intermediates, we have demonstrated that the N-H functional group is required for catalysis, and is intimately involved in the catalytic mechanism. By trapping the activated form of the catalyst as a ruthenium(0)-pincer complex that can enter the catalytic cycle at room temperature, we have developed the most efficient catalyst for base-free ester hydrogenation known to date.

How does Milstein's catalyst really work?

ACS Spring Meeting 2021

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