Anti-Markovnikov 1,3-CH Addition of Allenes to Allenes: A Straightforward Method To Prepare Osmium–Dienylcarbene Complexes

One of the acetone molecules of the solvento complex [OsTp(κ 1 -OCMe 2 ) 2 (P i Pr 3 )]BF 4 ( 1; Tp = hydridotris(1-pyrazolyl)borate) is displaced by cyclohexylallene to give [OsTp(η 2 -CH 2 ═C═CHCy)(κ 1 -OCMe 2 )(P i Pr 3 )]BF 4 ( 2 ). Treatment of 2 with a second molecule of cyclohexylallene leads to an 80:20 mixture of the conjugated ( Z, Z )-osmahexatriene [OsTp{═CH-( Z )-C(CH 2 Cy)═CH[η 2 -( Z )-CH═CHCy]}(P i Pr 3 )]BF 4 ( 3 ) and the ( Z, Z )-dienylcarbene [OsTp{═CH-( Z )-C[CH 2 -η 2 -( Z )-CH═CHCy]═CHCy}(P i Pr 3 )]BF 4 ( 4 ), as a result of an anti-Markovnikov 1,3-C–H addition of the coordinated allene of 2 to the CH 2 ═C double bond of the second allene molecule. In fluorobenzene at 85 °C, 3 evolves into the hydride-( E, E )-dienylcarbyne [OsHTp{≡C-( E )-C(CH 2 Cy)═CH[( E )-CH═CHCy]}(P i Pr 3 )]BF 4 ( 5 ), whereas 4 isomerizes into the ( E, Z )-dienylcarbene [OsTp{═CH-( E )-C[CH 2 -η 2 -( Z )-CH═CHCy]═CHCy}(P i Pr 3 )]BF 4 ( 6 ). Complex 2 also reacts with ethyl carboxylateallene. In dichloromethane at room temperature, the reaction gives [OsTp{═CH-( Z )-C[CH 2 -η 2 -( Z )-CH═CHCO 2 Et]═CHCy}(P i Pr 3 )]BF 4 ( 7 ), which isomerizes into [OsTp{═CH-( E )-C[CH 2 -η 2 -( Z )-CH═CHCO 2 Et]═CHCy}(P i Pr 3 )]BF 4 ( 8 ) in fluorobenzene at 85 °C. Treatment of 7 with NaOMe yields OsTp{( E )-CH═C[CH═ C(CH 2 ) 4 CH 2 ]CH 2 -η 2 -( Z )-CH═CHCO 2 Et)(P i Pr 3 ) ( 9 ). The addition of HBF 4 ·OEt 2 to 9 gives 8 .