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

The dihydride-dihydrogen complex [OsH 2 (η 5 -C 5 H 5 )(η 2 -H 2 )(P i Pr 3 )]BF 4 ( 2 ) has been prepared by addition of HBF 4 ·OEt 2 to OsH 3 (η 5 -C 5 H 5 )(P i Pr 3 ) ( 1 ), and its reactions with benzylideneacetone, methyl vinyl ketone, acetophenone, and benzylideneacetophenone have been studied. The reaction with benzylideneacetone leads initially to [OsH 2 (η 5 -C 5 H 5 ){κ 1 -OC(CH 3 )CH CHPh}-(P i Pr 3 )]BF 4 ( 3 ), which in dichloromethane is converted to the hydroxyallyl derivative [OsH(η 5 -C 5 H 5 ){η 3 -CH 2 C(OH)CHCH 2 Ph}(P i Pr 3 )]BF 4 ( 4 ). Complex 4 releases 4-phenylbutan-2-one, and the resulting metallic fragment activates a C β (sp 2 )−H bond of a new molecule of benzylideneacetone to give ( 5 ), which affords ( 6 ) by deprotonation with NaOCH 3 . The reaction of 2 with methyl vinyl ketone gives ethyl methyl ketone and ( 9 ). The latter can also be obtained from Os(η 5 -C 5 H 5 )Cl{η 2 -CH 2 CHC(O)CH 3 }(P i Pr 3 ) ( 7 ) via the intermediate [Os(η 5 -C 5 H 5 ){CH 2 CHC(O)CH 3 }(P i Pr 3 )]BF 4 ( 8 ). Treatment of 9 with NaOCH 3 leads to an equilibrium mixture of ( 10 ) and the hydride-vinylidene OsH(η 5 -C 5 H 5 ){ C CHC(O)CH 3 }(P i Pr 3 ) ( 11 ). The reaction of 2 with acetophenone gives 1-phenylethanol and the orthometalated derivative ( 13 ), which is deprotonated with NaOCH 3 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.