Aluminum-Hydride-Catalyzed Hydroboration of Carbon Dioxide

This
study describes the first use of a bis­(phosphoranyl)­methanido
aluminum hydride, [ClC­(PPh2NMes)2AlH2] (2, Mes = Me3C6H2), for the catalytic hydroboration of CO2. Complex 2 was synthesized by the reaction of a lithium carbenoid [Li­(Cl)­C­(PPh2NMes)2] with 2 equiv of AlH3·NEtMe2 in toluene at −78 °C. 2 (10 mol
%) was able to catalyze the reduction of CO2 with HBpin
in C6D6 at 110 °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·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(PPh2NMes)2Al­(H)­{OC­(O)­H}] (4), which was
subsequently reacted with BH3·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.