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 [OsH 2 (η 5 -C 5 H 5 )(η 2 -H 2 )(P i Pr 3 )]BF 4 ( 1 ) reacts in acetone with 1-phenyl-1-propyne and 2-butyne to give the γ-(η 3 -allyl)-α-alkenylphosphine derivatives [OsH(η 5 -C 5 H 5 ){κ 4 -( P, C, C, C )-CH 2 C[CH 2 C( CH 2 )P i Pr 2 ]CHR}]BF 4 (R = Ph ( 2 ), CH 3 ( 3 )), by means of one-pot tandem processes of four reactions. The stable intermediates have been isolated and characterized. In acetone, complex 1 dissociates H 2 and coordinates the solvent to afford [OsH 2 (η 5 -C 5 H 5 )(κ 1 -OCMe 2 )(P i Pr 3 )]BF 4 ( 4 ), which reacts with a molecule of 1-phenyl-1-propyne or 2-butyne to form [OsH(η 5 -C 5 H 5 )(η 3 -CH 2 CHCHR)(P i Pr 3 )]BF 4 (R = Ph ( 5 ), CH 3 ( 6 )), containing the CHR group cisoid disposed to the phosphine and the R substituent anti to C meso . 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 C meso . The reactions of 5 and 6 with a second molecule of the respective alkyne lead to the corresponding Z -olefin and [Os(η 5 -C 5 H 5 ){η 2 -( Z )-CH(CH 3 ) CHR}{κ 3 -( P, C, C )-[CH 2 C(CH 3 )]P i Pr 2 }]BF 4 (R = Ph ( 9 ), CH 3 ( 10 )). The isopropenyl group of the phosphine of 9 and 10 undergoes coupling with a third alkyne molecule to give 2 and 3 .