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. The oxidation of anisyl alcohol to anisaldehyde with 2,2,6,6‐tetramethyl‐1‐oxopiperidinium bromide as a model reaction for nitroxonium salt‐mediated oxidation of alcohols is transferred into a continuous‐flow system. In double emulsions, a selective interface reaction runs with full conversion while alcohol/aldehyde and mediator remain separated. The latter is recycled by continuous electrolysis.