Aluminum-Hydride-Catalyzed Hydroboration of Carbon Dioxide

This study describes the first use of a bis(phosphoranyl) methanido aluminum hydride, [ClC(PPh(2)NMes)(2)AlH2] (2, Mes = Me3C6H2), for the catalytic hydroboration of CO2. Complex 2 was synthesized by the reaction of a lithium carbenoid [Li(Cl)C(PPh(2)NMes)(2)] with 2 equiv of AlH3 center dot NEtMe2 in toluene at -78 degrees C. 2 (10 mol %) was able to catalyze the reduction of CO2 with HBpin in C6D6 at 110 degrees C for 2 days to afford a mixture of methoxyborane [MeOBpin] (3a; yield: 78%, TOF: 0.16 h(-1)) and bis(boryl)oxide [pinBOBpin] (3b). When more potent [BH3 center dot SMe2] was used instead of HBpin, the catalytic reaction was extremely pure, resulting in the formation of trimethyl borate [B(OMe)(3)] (3e) [catalytic loading: 1 mol % (10 mol %); reaction time: 60 min (5 min); yield: 97.6% (>99%); TOF: 292.8 h(-1) (356.4 h(-1))] and B2O3 (3f). Mechanistic studies show that the Al-H bond in complex 2 activated CO2 to form [ClC(PPh(2)NMes)(2)Al(H){OC(O)H}] (4), which was subsequently reacted with BH3 center dot SMe2 to form 3e and 3f, along with the regeneration of complex 2. Complex 2 also shows good catalytic activity toward the hydroboration of carbonyl, nitrile, and alkyne derivatives.