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

Abstract

One of the acetone molecules of the solvento complex [OsTp(κ1-OCMe2)2(PiPr3)]BF4 (1; Tp = hydridotris(1-pyrazolyl)borate) is displaced by cyclohexylallene to give [OsTp(η2-CH2═C═CHCy)(κ1-OCMe2)(PiPr3)]BF4 (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(CH2Cy)═CH[η2-(Z)-CH═CHCy]}(PiPr3)]BF4 (3) and the (Z,Z)-dienylcarbene [OsTp{═CH-(Z)-C[CH2-η2-(Z)-CH═CHCy]═CHCy}(PiPr3)]BF4 (4), as a result of an anti-Markovnikov 1,3-C–H addition of the coordinated allene of 2 to the CH2═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(CH2Cy)═CH[(E)-CH═CHCy]}(PiPr3)]BF4 (5), whereas 4 isomerizes into the (E,Z)-dienylcarbene [OsTp{═CH-(E)-C[CH2-η2-(Z)-CH═CHCy]═CHCy}(PiPr3)]BF4 (6). Complex 2 also reacts with ethyl carboxylateallene. In dichloromethane at room temperature, the reaction gives [OsTp{═CH-(Z)-C[CH2-η2-(Z)-CH═CHCO2Et]═CHCy}(PiPr3)]BF4 (7), which isomerizes into [OsTp{═CH-(E)-C[CH2-η2-(Z)-CH═CHCO2Et]═CHCy}(PiPr3)]BF4 (8) in fluorobenzene at 85 °C. Treatment of 7 with NaOMe yields OsTp{(E)-CH═C[CH═C(CH2)4CH2]CH2-η2-(Z)-CH═CHCO2Et)(PiPr3) (9). The addition of HBF4·OEt2 to 9 gives 8.