A voltammetric study of the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation of benzyl alcohol in tert-butanol/water

摘要

The mechanism of the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation of benzyl alcohol in alkaline tert-butanol/water mixtures has been investigated in order to understand the preferred conditions for the mediated oxidation of alcohols in an electrochemical microflow reactor. It is shown that TEMPO and the corresponding hydroxylamine both undergo facile oxidation to the oxoammonium ion at the potential and pH used for the mediated oxidation although the hydroxylamine/TEMPO couple is electrochemically irreversible. The initial reaction between the oxoammonium ion and benzyl alcohol is a rapid reaction but the overall rate of the catalytic cycle, and hence the rate of conversion of alcohol to aldehyde, is determined by a deprotonation step of an intermediate formed in this reaction. The mediated oxidation is never mass transport controlled with respect to the alcohol but the rate of the reaction increases with pH. The oxoammonium ion also reacts with hydroxide and hence the selection of pH for electrosynthesis is a compromise between the increased rate of the catalytic cycle and the loss of mediator due to reaction of the oxoammonium ion with hydroxide as the pH is increased. At a pH of ～11.5, the rate of the conversion to benzaldehyde is high enough for the reaction to be synthetically useful and much of the TEMPO can be recovered after electrolysis.