Ruthenium-Catalyzed Chemoselective Olefin Transfer Hydrogenation of alpha, beta-Unsaturated Carbonyl Systems By Using EtOH as Hydrogen Source

The use of EtOH as hydrogen donor is remarkably under explored in transfer hydrogenation reactions, even though EtOH represents an appealing hydrogen source. With the cationic ruthenium complex [Ru(PYA)(cymene)]⁺, 1, containing a N,N-bidentate amino-functionalized pyridinium amidate (PYA) ligand as catalyst precursor, we here demonstrate the first example of chemoselective room temperature transfer hydrogenation of the C═C bond in α,β-unsaturated ketones using EtOH as benign hydrogen source to yield a variety of functionalized ketones. The reaction proceeds under mild conditions with K₂CO₃ as base and conventiently at room temperature. A broad substrate scope, including various functionalized α,β-unsaturated carbonyl groups, demonstrates the general applicability of this method. Preliminary mechanistic studies suggest the formation of an alkoxide complex [1]–OEt as an initially formed species, and the requirement for EtOH to induce hydride transfer, suggesting a protic activation of the catalyst.