Mechanism of N,N,N-Amide Ruthenium(II) Hydride Mediated Acceptorless Alcohol Dehydrogenation: Inner-Sphere β-H Elimination versus Outer-Sphere Bifunctional Metal–Ligand Cooperativity

Abstract

The reversible transformations between ketones and alcohols via sequential hydrogenation–dehydrogenation reactions are efficiently achieved using a single precatalyst HRu(bMepi)(PPh3)2 (bMepi = 1,3-bis(6′-methyl-2′-pyridylimino)isoindolate). The catalytic mechanism of HRu(bMepi)(PPh3)2 mediated acceptorless alcohol dehydrogenation (AAD) has been investigated by a series of kinetic and isotopic labeling studies, isolation of intermediates, and evaluation of Ru(b4Rpi)(PPh3)2Cl (R = H, Me, Cl, OMe, OH) complexes. Two limiting dehydrogenation scenarios are interrogated: inner-sphere β-H elimination and outer-sphere bifunctional double hydrogen transfer. Isotopic labeling experiments demonstrated that the proton and hydride transfer in a stepwise manner. Catalyst modifications suggest that the imine group on the bMepi pincer scaffold is not necessary for catalytic alcohol dehydrogenation. Evaluation of the kinetic experiments and catalyst modifications suggests a pathway whereby HRu(bMepi)(PPh3)2 operates via ...