Reduction and C(sp2)-H bond activation of ketones promoted by a cyclopentadienyl-osmium-dihydride-dihydrogen complex

The dihydride-dihydrogen complex [OsH2(eta(5)-C5H5)(eta(2)-H-2)((PPr3)-Pr-i)]BF4 (2) has been prepared by addition of HBF4 center dot OET2 to OsH3(eta 5-C5H5)((PPr3)-Pr-i) (1), and its reactions with benzylideneacetone, methyl vinyl ketone, acetophenone, and benzylideneacetophenone have been studied. The reaction with benzylideneacetone leads initially to [OsH2(eta 5-C5H5){k(1)-OC(CH3)CH=CHPh}-((PPr3)-Pr-i)]BF4 (3), which in dichloromethane is converted to the hydroxyallyl derivative [OsH-(eta 5-C5H5){eta(3)-CH2C(OH)CHCH2Ph}((PPr3)-Pr-i)]BF4 (4). Complex 4 releases 4-phenylbutan-2-one, and the resulting metallic fragment activates a C-beta(sp(2))-H bond of a new molecule of benzylideneacetone to give [OsH(eta 5-C5H5){C(Ph)CHC(O)CH3}((PPr3)-Pr-i)]BF4 (5), which affords Os(eta 5-C5H5){C(Ph)CHC(O)CH3}(PiPr(3)) (6) by deprotonation with NaOCH3. The reaction of 2 with methyl vinyl ketone gives ethyl methyl ketone and [OsH(eta(5)-C5H5){CHCHC(O)-CH3}((PPr3)-Pr-i)]BF4 (9). The latter can also be obtained from Os(eta(5)-C5H5)Cl{eta(2)-CH2=CHCH(O)-CH3}((PPr3)-Pr-i) (7) via the intermediate [Os(eta(5)-C5H5){CH2CHC(O)CH3}((PPr3)-Pr-i)]BF4 (8). Treatment of 9 with NaOCH3 leads to an equilibrium mixture of Os(eta(5)-C5H5){CHCHC(O)CH3}-((PPr3)-Pr-i) (10) and the hydride-vinylidene OsH(eta(5)-C5H5){=C=CHC(O)CH3}((PPr3)-Pr-i) (11). The reaction of 2 with acetophenone gives 1-phenylethanol and the orthometalated derivative [OsH(eta(5)-C5H5{C6H4C(O)CH3}((PPr3)-Pr-i)]BF4 (13), which is deprotonated with NaOCH3 to give OsH(eta(5)-C5H5){C6H4C(O)CH3}((PPr3)-Pr-i) (14), whle the reaction of 2 with benzylidene-actophenone leads to [OsH(eta(5)-C5H5){C(Ph)CHC(O)Ph}((PPr3)-Pr-i)]BF4 (15), which yields Os(eta(5)-C5H5){C(Ph)CHC(O)Ph}((PPr3)-Pr-i) (16) by deprotonation. Complexes 3, 10, and 13 have been characterized by X-ray diffraction analysis.