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(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.