Stoichiometric, catalytic, and enantioface-selective hydrogenation of C==N bonds by an ionic mechanism.

The hydrogenation of carbon-carbon double bonds is a classic application of homogeneous catalysis, and numerous complexes are known to catalyze it. Furthermore, the use of chiral ligands to produce enantioface selectivity in such catalytic hydrogenations is now well developed.1 The catalytic cycle for such reactions involves an olefin dihydride complex (with the addition of H2, step A, and the coordination of the olefin, step B, occurring in either order), giving the hydrogenated product by insertion and reductive elimination (Scheme 1). However, there have been reports that transition metals can add the elements of hydrogen across double bonds stoichiometrically by a different mechanism. In separate papers in 1985 the Darensbourg2 and Gibson3 groups reported the reduction of aldehydes (ketones were much less reactive) by group 6 carbonyl hydrides and CH3CO2H. In 1987 Vos and co-workers reported the hydrogenation of acetone, by a hydride ligand and H+ in aqueous solution.4 In 1989 Bullock and Rappoli reported the hydrogenation of tetra-, tri-, and 1,1-disubstituted olefins by HMo(CO)3(C5H5) and CF3SO3H. In 1992 we and the Bullock group reported the reduction of aldehydes and ketones under similar conditions, and isolated an isopropyl alcohol complex formed thereby (eq 1);6 similar results have recently been reported with H2Re(NO)(CO)(PR3)2/CF3CO2H by Bakhmutov and co-workers.7 Ionic hydrogenation has been extended to CdN double bonds by Ito and co-workers.8 These reactions resemble the “ionic hydrogenations” long known with silanes as hydride donors.9