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 C_β(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.