Oxidation of N,N,N,2,2,6,6-Heptamethyl-piperidine-4-ammonium Chloride to Water-Soluble N-Oxyl Radicals: A Comparative Study

A vast variety of oxidation procedures is known to obtain 2,2,6,6-tetramethylpiperidinyl-4-oxyl (TEMPO) moieties from its piperidinyl precursor. However, up to now no comprehensive study has been performed concerning the performance and feasibility for lab-scale synthesis. With the focus on the ease of synthesis, occurring impurities, the reaction yield and the degree of oxidation (DoO), this study represents a thorough investigation of different oxidation processes yielding N-oxyl radicals in a one-step procedure. The resulting materials were investigated by redox titration and electron spin resonance spectroscopy to determine the DoO, which is the determining parameter for the yield of this oxidation reaction. The most promising procedures have been transferred to a polymeric material to evaluate the feasibility of the catalysts for more complex substrates.