Selective reduction of less reactive carbonyl groups in the presence of diborane and sodium bisulfite on silica gel

Abstract

The less reactive carbonyl group of a mixture of reducible groups of carbonyl compounds was preferentially reduced with diborane on silica gel by first forming an adduct of the more reactive carbonyl group with sodium bisulfite. For example, 4-acetylbenzaldehyde could be converted to 4-(1-hydroxylethyl)benzaldehyde in 93% selectivity; in a mixture of 4-phenyl-acetophenone and 4-phenylbenzaldehyde, biphenylethanol was preferentially formed in 95% yield with 16% yield of 4-phenylbenzyl alcohol. Silica gel and sodium bisulfite were essential for this selective reaction. Aliphatic and aromatic aldehydes and unhindered cyclohexanones could be selectively protected by this method. Keywords: sodium bisulfite, silica gel, carbonyl compound, selective reduction, diborane.