Well-Defined Molecular Alkali Metal Catalysts for Mild and Selective Reduction of Carboxylic Acids by Hydrosilylation.

The selective transformation of carboxylic acids into alcohols remains a challenge, although many catalytic methods have been developed recently. Hydrosilylative reduction appeared to be an attractive strategy in the presence of a metallic catalyst. In those processes, transition metals were usually employed for the catalyst design, and the reactions often suffered from poor selectivity, limited functional group tolerance, and the requirement of elevated temperature. Here, we report inexpensive and easy-to-make alkali metal catalysts that effectively promote the hydrosilylation of carboxylic acids to synthesize primary alcohols. The optimized catalyst could be produced on a gram scale in minutes from commercial potassium triethylborohydride and 18-crown-6. A broad range of aliphatic and aromatic carboxylic acids were employed, and even challenging nitro and cyano groups could survive the hydrosilylation, although some limitations of the substrate scope were disclosed. Preliminary mechanistic studies proved a distinct mechanism of substrate activation from those reported in the literature, through the isolation of a new adduct from the reaction of substrate and the potassium catalyst.