Carbonyl reduction by the Cannizzaro reaction

The Cannizzaro reaction, that is, the base-catalysed disproportionation of a carbonyl compound to an alcohol and a carboxylic acid, has gained some importance as an economically viable alternative to the reduction with borohydrides. However, the reaction is restricted to carbonyl compounds without any a-hydrogen, which do not undergo competing aldol reactions. Thus, mainly aromatic aldehydes are used for this kind of transformation. The protocols developed for microwave applications typically involve solvent-free conditions using alumina as the solid support. Under these conditions, a significant acceleration of the reaction was achieved. 

The so-called crossed Cannizzaro reaction is synthetically more useful than the Cannizzaro reaction itself, as it can be applied for the preparation of alcohols in high yields, without loss of 50% of the product in the formation of the corresponding carboxylic acid. Typically, paraformaldehyde is used as a sacrificial reducing agent, together with the carbonyl compound which is to be transformed into the alcohol. The reaction thus serves as an alternative method to the use of complex hydrides for the reduction of aromatic aldehydes. 

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