REDUCTION OF CARBONYL COMPOUNDS, CARBOXYLIC ACIDS AND THEIR DERIVATIVES

The chapter describes the process of reduction of carbonyl compounds, carboxylic acids, and their derivatives. The first step in the reduction of carbonyl compounds is the addition of an electron to the LUMO of the carbonyl group to form a radical-anion in which the unpaired electron is delocalized over the π-system. Conjugated aliphatic enones, aromatic aldehydes, and aromatic ketones show reversible polarographic waves at more accessible potentials in aprotic solvents due to one-electron addition with the formation of the radical-anion. The preparative scale reduction of oximes at a mercury or lead cathode in acid solution has been used in the conversion of the carbonyl function to amine. Originally, 30–50% of sulphuric acid was used as a solvent, but ethanol with dilute hydrochloric acid is usually satisfactory. Aliphatic and aromatic oximes give amines in 64–86% yields. Aromatic ketoximes are also reducible in an alkaline solution, and acetophenone oxime is converted to 1-phenylethylamine in a tri-potassium orthophosphate solution. Phenylglyoxal monoxime in acid solution is however reduced at both the carbonyl and the oxime centers by sodium amalgam to yield 2-ammo-l-phenylethanol.