ONE-POT TRANSFORMATION OF CARBOXYLIC-ACIDS INTO ALDEHYDES BY A DOUBLE ELECTROLYSIS IN THE PRESENCE OF TRIBUTYLPHOSPHINE

Constant-current electrolysis (CCE) of a mixture of Bu3P, CH3SO3H, Bu4NBr, and a carboxylic acid (1) in CH2Cl2 using a one-compartment cell at room temperature gave the corresponding aldehyde (2) without any over-reduction products. The partial reduction was affected by the choice of cathode materials, depending on the structure of 1. Thus, by the CCE with two graphite plates as an anode and a cathode, aliphatic carboxylic acids were reduced smoothly into 2 in fair to good yields, regardless of the branching situation at their alpha-carbons. In the case of aromatic acids, a tin cathode was effective for the successful preparation of 2. Based on the results of cyclic voltammetry and P-31-NMR spectroscopy, acyl tributylphosphonium ions generated through anodic reaction are reduced to the alpha-hydroxylakyl tributylphosphonium ions at the cathode, which remain intact during the electroylsis and are decomposed to 2 and Bu3P only by the work-up under weakly basic conditions.