One-Step Transformation of Carboxylic Acids into Aldehydes Induced by the Electrochemical Oxidation of Ph3P

Abstract

Constant-current electrolysis (CCE) of Ph3P and a carboxylic acid (RC02H, 1) in CH2C12 using a one-compartment cell equipped with two graphite plates as an anode and a cathode under a nitrogen atmosphere gave the corresponding aldehyde (2) when Ph3P+HC104 was used as a supporting electrolyte. The partial reduction was affected by the choice of electrolysis conditions, especially temperature and current density, depending on the structure of 1. Thus, the CCE of 1 with R = Ph-, o-Cl-Ph-, p-MeO-Ph-, Ph(CH2)2-, CH3(CH2)8-, Ph2CH-proceeded smoothly at — 30 °C in fair yields. In the case of 1 with R =kland (CH3)2CH-, the electrolysis had to be carried out under reflux, and with R = (CH3)3C- the aldehyde was obtained only in a poor yield even under reflux. Based on the results of cyclic voltammetry of the anolyte obtained by the CCE of CH2C12 solutions of Ph3P, Ph3P+H-CIC4 and 1 in a divided cell, the sequence of the partial reduction is proposed to be as follows: formation of acyloxy triphenylphosphonium ion (3) at the anode; the reaction of 3 with Ph3P to afford the corres-ponding acyl triphenylphosphonium ion (4); reduction of 4 to the a-hydroxyalkyl triphenylphosphonium ion (7) al the cathode; decomposition of 7 to 2 and Ph3P during the aqueous work-up after the CCE. The voltammetric study also suggested that the ease of the transformation of 3 to 4 is reflected in the required CCE conditions and may determine the yield of 2. © 1994, The Pharmaceutical Society of Japan. All rights reserved.