One-step transformation of carboxylic acids into aldehydes induced by the electrochemical oxidation of Ph3P

Article abstract of DOI:10.1248/cpb.42.1041

Constant-current electrolysis (CCE) of Ph₃P and a carboxylic acid (RCO₂H, 1) in CH₂Cl₂ 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⁺H·ClO₄/^- 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 = cyclo-C₆H₁₁- and (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 CH₂Cl₂ solutions of Ph₃P, Ph₃P⁺H·ClO₄/^-, 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 corresponding acyl triphenylphosphonium ion (4); reduction of 4 to the α-hydroxyalkyl triphenylphosphonium ion (7) at 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.

Full text of DOI:10.1248/cpb.42.1041

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