Simple Ligand Modifications with Pendent OH Groups Dramatically Impact the Activity and Selectivity of Ruthenium Catalysts for Transfer Hydrogenation: The Importance of Alkali Metals

Remarkable differences in selectivity and activity for ruthenium-catalyzed transfer hydrogenation are described that are imparted by pendent OH groups. Kinetic experiments, as well as the study of control complexes devoid of OH groups, reveal that the pendent OH groups serve to orient the ketone substrate through ion pairing with an alkali metal under basic conditions. The deprotonation of the OH groups was found to modulate the electronics at the metal center, providing a more electron rich ruthenium center. The effects of the ion pairing between alkali metals and the pendent alkoxide groups were highlighted by demonstrating chemoselective transfer hydrogenation of ketones in the presence of olefins. The results illustrate that a simple ligand modification (installation of OH groups) imparts dramatic changes to catalysis. Pendent OH groups turn on catalysis through electronic perturbations at the metal site under basic conditions and can also change the mechanism of catalysis, the latter of which can be used to promote chemoselective reductions.