A Direct Reduction of Aliphatic Aldehyde, Acyl Chloride, Ester, and Carboxylic Functions into a Methyl Group

The aliphatic carboxylic group was efficiently reduced to the methyl group by HSiEt₃ in the presence of catalytic amounts of B(C₆F₅)₃. To the best of our knowledge, this is the first example of a direct exhaustive reduction of aliphatic carboxylic function. Aliphatic aldehydes, acyl chlorides, anhydrides, and esters also underwent complete reduction under similar reaction conditions. Aromatic carboxylic acids, as well as other carbonyl functional equivalents, underwent smooth partial reduction to the corresponding TES-protected benzylic alcohols. It was shown that, unlike the reduction of aliphatic substrates, the exhaustive reduction of aromatic substrates was not straightforward:  a concurrent Friedel−Crafts-like alkylation process competed with the reduction yielding trace to notable amounts of dimeric products, thus decreasing the overall selectivity of the reduction process.