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(PiPr3)2 (1) and OsH2Cl2(PiPr3)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)­(PiPr3)2 (3).
The protonation of 3 with triflic acid (HOTf) produces
the release of H2 and the formation of the unsaturated
osmium­(IV) dihydride [OsH2(acac)­(PiPr3)2]­OTf (4), which is also prepared by starting
from 2 via the intermediate OsH2Cl­(acac)­(PiPr3)2 (5). Treatment of
an acetylacetone solution of 5 with KOH affords Os­(acac)2(PiPr3)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 H2. 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 H2 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).