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.