Borohydride Compounds Catalyze the Selective 4e^– Reduction of CO₂ with 9-BBN

The selective double hydroboration of CO₂ into bis(boryl)acetal (BBA) is a challenging yet appealing reduction process since BBA can be used as a versatile C₁ and C_n sources for the synthesis of value-added products. In the present study, we demonstrate that simple borohydride compounds are efficient and selective catalysts for the synthesis of BBA when using 9-borabicyclo(3.3.1)nonane (9-BBN) as a reductant. The experimental and theoretical investigations show that while the borohydride species catalyzes the first reduction step of CO₂ into formoxyborane (2e^- reduction intermediate), the observed 4e^- reduction selectivity is mostly due to the ability of 9-BBN to reduce the formoxyborane into BBA without a catalyst. Notably, 0.2 mol % of LiH₂BBN catalyzed the hydroboration of CO₂ with 9-BBN as a reductant into the corresponding BBA in 77% yield with TON and TOF of 385 and 196 h^-1, respectively. The simplicity of borohydride contrasts with the more elaborate catalytic systems used so far for the 4e^- reduction of CO₂.