Reduction and C(sp²)−H Bond Activation of Ketones Promoted by a Cyclopentadienyl-Osmium- Dihydride-Dihydrogen Complex

The dihydride-dihydrogen complex [OsH2(η5-C5H5)(η2-H2)(PiPr3)]BF4 (2) has been prepared by addition of HBF4·OEt2 to OsH3(η5-C5H5)(PiPr3) (1), and its reactions with benzylideneacetone, methyl vinyl ketone, acetophenone, and benzylideneacetophenone have been studied. The reaction with benzylideneacetone leads initially to [OsH2(η5-C5H5){κ1-OC(CH3)CHCHPh}-(PiPr3)]BF4 (3), which in dichloromethane is converted to the hydroxyallyl derivative [OsH(η5-C5H5){η3-CH2C(OH)CHCH2Ph}(PiPr3)]BF4 (4). Complex 4 releases 4-phenylbutan-2-one, and the resulting metallic fragment activates a Cβ(sp2)−H bond of a new molecule of benzylideneacetone to give (5), which affords (6) by deprotonation with NaOCH3. The reaction of 2 with methyl vinyl ketone gives ethyl methyl ketone and (9). The latter can also be obtained from Os(η5-C5H5)Cl{η2-CH2CHC(O)CH3}(PiPr3) (7) via the intermediate [Os(η5-C5H5){CH2CHC(O)CH3}(PiPr3)]BF4 (8). Treatment of 9 with NaOCH3 leads to an equilibrium mixture of (10) and the hydride-vinylidene OsH(η5-C5H5){CCHC(O)CH3}(PiPr3) (11). The reaction of 2 with acetophenone gives 1-phenylethanol and the orthometalated derivative (13), which is deprotonated with NaOCH3 to give (14), while the reaction of 2 with benzylideneacetophenone leads to (15), which yields (16) by deprotonation. Complexes 3, 10, and 13 have been characterized by X-ray diffraction analysis.