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Aliphatic and aromatic carboxylic acids are reduced rapidly and quantitatively to the corresponding alcohols by borane in tetrahydrofuran, either at 0°or 25°. Even sterically hindered acids, such as 1-adamantanecarboxylic acid, dicarboxylic acids, such as adipic acid, phenolic acids, and amino acids undergo facile and quantitative reduction with borane. Aliphatic carboxylic acids are reduced at faster rates than aromatic carboxylic acids. Unlike more conventional, very powerful reducing agents, such as lithium aluminum hydride, the mildness of the reagent, borane, permits the presence of other functional groups less susceptible to the reducing action of the reagent, groups such as ester, nitro, halogen, nitrile, keto, etc. The remarkable utility of this reagent for the selective reduction of carboxylic acids was confirmed by the selective conversion of adipic acid monoethyl ester to ethyl 6-hydroxyhexanoate and p-cyanobenzoic acid to p-cyanobenzyl alcohol in yields of 88 and 82%, respectively. This reaction provides a highly convenient synthetic procedure for the selective reduction of the carboxylic acid group where this is required in synthetic operations.

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