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(κ 1-OCMe 2) 2(P iPr 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 iPr 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 2Cy)=CH[η 2-(Z)-CH=CHCy]}(P iPr 3)]BF 4 (3) and the (Z,Z)-dienylcarbene [OsTp{=CH-(Z)-C[CH 2-η 2-(Z)-CH=CHCy]=CHCy}(P iPr 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 2Cy)=CH[(E)-CH=CHCy]}(P iPr 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 iPr 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 2Et]= CHCy}(P iPr 3)]BF 4 (7), which isomerizes into [OsTp{=CH-(E)-C[CH 2-η 2-(Z)-CH=CHCO 2Et]= CHCy}(P iPr 3)]BF 4 (8) in fluorobenzene at 85 °C. Treatment of 7 with NaOMe yields OsTp{(E)-CH=C[CH=C(CH 2) 4CH 2]CH 2-η 2-(Z)-CH=CHCO 2Et)(P iPr 3) (9). The addition of HBF 4•OEt 2 to 9 gives 8. © 2012 American Chemical Society.