Aluminum-hydride-catalyzed hydroboration of carbon dioxide

• Main Authors: Chia, Cher-Chiek, Teo, Yeow-Chuan, Cham, Ning, Ho, Samuel Ying-Fu, Ng, Zhe-Hua, Toh, Hui-Min, Mézailles, Nicolas, So, Cheuk-Wai
• Other Authors: School of Physical and Mathematical Sciences
• Format: Article
• Language: English
• Published: 2021
• Subjects: Science::Chemistry; Mixtures; Hydrocarbons
• Online Access: https://hdl.handle.net/10356/151886
• Institution: Nanyang Technological University

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.