Indirect Electrooxidation of Alcohols by a Double Mediatory System with Two Redox Couples of [R₂N⁺=O]/R₂NO･ and [Br･ or Br⁺]/Br^- in an Organic- Aqueous Two-Phase Solution

Inokuchi, T.; Matsumoto, S.; Torii, S. J. Org. Chem. 1991, 56, 2416.

An indirect electrooxidation method for alcohol to aldehyde or ketone conversion has been developed. This method, applicable to chemoselective oxidation, employs two redox couples, consisting of 2,2,6,6-tetramethyl-piperidine-1-oxyl derivatives 6 and active bromine species. The former is required for the chemical process and recycled, whereas the latter is to be involved in the electrochemical process. Three chemical events play an important role in this system: (1) the formation of [Br･ or Br⁺] from bromide ion by discharge on the anode in an aqueous solution, (2) the reaction of N-oxyl compounds 6 with active bromide species to generate N-oxoammonium ion 7, and (3) the oxidation of alcohols with 7 in an organic phase. Optimum conditions were established as follows: an aqueous 25% NaBr solution buffered at pH 8.6 in a binary system, the use of 1-10 mol % of 4-(benzoyl-oxy)piperidine derivatives 6, and adjustment of an electric current at 10-100 mA/cm². The successful applications of the present method to the oxidation of a variety of primary and secondary alcohols including 1,n-diols, giving the corresponding carbonyl compounds, have delineated its synthetic utility. The chemoselective oxidation of a primary hydroxy group in the presence of secondary one has been achieved with a high selectivity by the present procedure.