Electrochemical reduction of carbonyl compounds in aqueous media in the presence of β-cyclodextrin. Reduction of acetophenone and p-methoxyacetophenone in alkaline solution

Abstract

The electrochemical reduction of acetophenone, 1, and 4-methoxy-acetophenone, 2, has been investigated in aqueous alkaline solutions, on mercury electrode, in the absence and in the presence of β-cyclodextrin (β-CD). Macroelectrolyses afforded mixtures of d,l and meso pinacols (head-to-head dimers) as preferential products and, in the case of 1, also head-to-tail dimers. These dimers showed some optical activity (enantiomeric excess of ca 24% for each diastereoisomer) when obtained by reduction in the presence of β-CD, whereas d,l pinacols did not. The same results were obtained by reduction of pre-formed 1:1 complexes with β-CD. A mechanism involving dimerization of radical anions is proposed for the formation of both type of dimers, and the asymmetric induction observed in the head-to-tail dimerization products is attributed to chiral effects exerted by β-CD on the guest molecules in this particular process.