Electro-organic reactions. Part 25. The role of chromium(III) in the modification of cathodic pinacolisation

Abstract

The presence of chromium(III) chloride, both hydrated and anhydrous, profoundly alters the course of cathodic reduction in dimethylfomamide solution of benzophenone, benzaldehyde, β-ionone, β-ionylidene acetaldehyde, and retinal. Pinacolisation is enhanced at the expense of formation of the corresponding alcohol. Furthermore the relevant reduction potential is lowered. The mechanism of the reaction has been investigated using voltammetric and coulometric experiments combined with a detailed analysis of preparative-scale reductions under a variety of conditions. The stereochemical course of the reactions has also been considered. In total the results provide compelling evidence in favour of the key reducible intermediate being a carbonyl compound–Cr^III complex which is formed via the corresponding Cr^II species with homogeneous, inner sphere, re-oxidation playing a crucial role. This is contrary to earlier suggestions invoking the participation of electrogenerated Cr^II as the reductant.