One-Pot Dehydrogenative Addition of Isopropyl to Alkynes Promoted by Osmium: Formation of γ-(η3-Allyl)-α-Alkenylphosphine Derivatives Starting from a Dihydride−Dihydrogen−Triisopropylphosphine Complex

The dihydride−dihydrogen complex [OsH2(η5-C5H5)(η2-H2)(PiPr3)]BF4 (1) reacts in acetone with 1-phenyl-1-propyne and 2-butyne to give the γ-(η3-allyl)-α-alkenylphosphine derivatives [OsH(η5-C5H5){κ4-(P,C,C,C)-CH2C[CH2C(CH2)PiPr2]CHR}]BF4 (R = Ph (2), CH3 (3)), by means of one-pot tandem processes of four reactions. The stable intermediates have been isolated and characterized. In acetone, complex 1 dissociates H2 and coordinates the solvent to afford [OsH2(η5-C5H5)(κ1-OCMe2)(PiPr3)]BF4 (4), which reacts with a molecule of 1-phenyl-1-propyne or 2-butyne to form [OsH(η5-C5H5)(η3-CH2CHCHR)(PiPr3)]BF4 (R = Ph (5), CH3 (6)), containing the CHR group cisoid disposed to the phosphine and the R substituent anti to Cmeso. In dichloromethane, complexes 5 and 6 evolve to the thermodynamic isomers 7 and 8, containing the CHR group cisoid disposed to the hydride and the R substituent syn to Cmeso. The reactions of 5 and 6 with a second molecule of the respective alkyne lead to the corresponding Z-olefin and [Os(η5-C5H5){η2-(Z)-CH(CH3)CHR}{κ3-(P,C,C)-[CH2C(CH3)]PiPr2}]BF4 (R = Ph (9), CH3 (10)). The isopropenyl group of the phosphine of 9 and 10 undergoes coupling with a third alkyne molecule to give 2 and 3.