Osmium Hydride Acetylacetonate Complexes and Their Application in Acceptorless Dehydrogenative Coupling of Alcohols and Amines and for the Dehydrogenation of Cyclic Amines

The preparation of new osmium hydride complexes, starting from OsH6((PPr3)-Pr-i)(2) (1) and OsH2Cl2((PPr3)-Pr-i)(2) (2), and their catalytic activity in acceptorless dehydrogenative coupling of alcohols and amines and in dehydrogenation of cyclic amines are reported. Complex 1 reacts with acetylacetone (Hacac) to give the classical trihydride OsH3(acac)((PPr3)-Pr-i)(2) (3). The protonation of 3 with triflic acid (HOTf) produces the release of H-2 and the formation of the unsaturated osmium(IV) dihydride [OsH2(acac)((PPr3)-Pr-i)(2)]OTf (4), which is also prepared by starting from 2 via the intermediate OsH2Cl(acac)((PPr3)-Pr-i)(2) (5). Treatment of an acetylacetone solution of 5 with KOH affords Os(acac)(2)((PPr3)-Pr-i)(2) (6). In the presence of 5 mol % of KOH, complexes 3-6 promote the coupling of benzyl alcohol and aniline to give N-benzylideneaniline and H-2. Under the same conditions, complex 3 catalyzes a wide range of analogous couplings to afford a variety of imines, including aliphatic imines, with yields between 90 and 40% after 1-48 h. Complex 3 also catalyzes the dehydrogenation of cyclic amines. According to the amount of H-2 released by each 1 g of employed substrate, the amines have been classified into three classes: poor (1,2,3,4-tetrahydroquinaldine, 2-methylindoline, and 2,6-dimethylpiperidine), moderate (1,2,3,4-tetrahydroquinoline and 6-methyl-1,2,3,4-tetrahydroquinoline), and good hydrogen donors (1,2,3,4-tetrahydroisoquinoline).