Borohydride Compounds Catalyze the Selective 4e Reduction of CO with 9-BBN.

The selective double hydroboration of CO into bis(boryl)acetal (BBA) is a challenging yet appealing reduction process since BBA can be used as a versatile C and C 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 CO 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 LiHBBN catalyzed the hydroboration of CO with 9-BBN as a reductant into the corresponding BBA in 77% yield with TON and TOF of 385 and 196 h, respectively. The simplicity of borohydride contrasts with the more elaborate catalytic systems used so far for the 4e reduction of CO.