Fluorous TEMPO: An efficient mediator for the aerobic oxidation of alcohols to carbonyl compounds

The aerobic oxidation of alcohols to aldehydes and ketones mediated by a fluorous derivative of the stable nitroxyl radical 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) has been investigated. The applicability of literature protocols originally developed for TEMPO-mediated reactions was assessed. Best results were obtained using molecular oxygen and a catalytic system combining the fluorous nitroxyl radical with Co(NO3)(2)/Mn(NO3)(2) (Minisci's conditions). Primary and benzylic alcohols were smoothly oxidized to the corresponding aldehydes under both homogeneous and fluorous biphasic conditions, whereas the oxidation of secondary aliphatic alcohols to ketones proved more challenging, in line with the behavior of similar catalytic systems based on polymer-supported TEMPO derivatives. The fluorous radical and related byproducts were easily removed from the reaction mixture by simple phase separation or fluorous liquid-liquid extraction, thus greatly simplifying the purification of the products. Recycling of the fluorous radical was also attempted with good results in the case of oxidation reactions requiring mild conditions.