Nitroxyl Radical-Mediated Oxidation of Alcohols in Continuous Microreactors

Primary and secondary alcohols are oxidized to corresponding aldehydes and ketones, respectively, with nitroxyl radicals. The stable radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is used as a mediator for selective oxidation of anisyl alcohol to anisaldehyde. This reaction is operated in different continuous microreactors either in single-phase or in multiphase applications like double emulsions. The latter are used for a simple separation of the hydrophilic coproduct 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPO-H) and the lipophilic product anisaldehyde. In addition, cyclic voltammetry is applied to determine conditions and parameters for electrochemical recycling of TEMPO-H. TEMPO-H is reactivated in a continuous electrolysis cell by anodic oxidation for reuse as mediator.