4. CARBOXYLIC ACID REDUCTION TO ALCOHOL
Several methods have been reported for the reduction of carboxylic acids to alcohols. Borane47 is a very common reagent for this purpose, either as BH3·THF (1 M in THF)67 or BH3·SMe2 (usually employed neat on scale, although solutions in various solvents and molarities are commercially available).68 BH3·THF is more reactive than BH3·amine complexes, and it
may work for substrates where the latter fails.67c An aqueous,acidic workup (e.g., HCl, citric acid) usually follows reduction with BH3·THF. For reductions with BH3·SMe2, THF and CH2Cl2 are the solvents of choice, and either aqueous, methanolic KF68a or aqueous NaOH quenches have been employed after
reaction completion. Acids can be chemoselectively reduced to alcohols with borane in the presence of esters