Hydrogen isotope exchange in the aldehyde group during the reduction of benzaldehyde by tritiated sodium borohydride (tetrahydridoborate)

Abstract

The reduction of benzaldehyde by tritiated sodium tetrahydridoborate in dimethyl sulphoxide and dimethyl sulphoxide–water mixtures as solvents is accompanied by the incorporation of tritium into the aldehyde group of unchanged benzaldehyde. The occurrence of the exchange implies that the hydride-transfer step of the reduction is reversible. As it is improbable that there is reversibility after the formation of a stable boron–oxygen bond, it is concluded that the hydride-transfer step precedes the step in which the boron–oxygen bond is formed. However, attempts to detect the implied intermediate borane in dimethyl sulphoxide solution by trapping with amines had proved negative. It follows that borane is destroyed by reaction with the other product of the first reaction step (the benzyl oxide ion) more effectively than it can react with an amine. Since the reaction of borane with a tertiary alkylamine is likely to be very rapid, the reactions between borane and benzyl oxide ion (leading either back to the starting materials with hydrogen exchange or, predominantly, to an alkoxyhydridoborate species) are considered to occur competitively as cage reactions before the primary reaction products have diffused apart.

The exchange reaction exhibits kinetic complications which are attributed to a more rapid hydrogen exchange between benzaldehyde and the reaction product sodium tetrakisbenzyloxyborate (which is shown to take place) and to the instability of this product in the media employed. It is suggested that the high basicity of the reaction solutions is due to the presence of some sodium benzyl oxide in equilibrium with sodium tetrakisbenzyloxyborate, leading to the formation of dimsyl ions.