Recyclable Catalyst Reservoir: Oxidation of Alcohols Mediated by Noncovalently Supported Bis(imidazolium)-Tagged 2,2,6,6-Tetramethylpiperidine 1-Oxyl

Bis(imidazolium)-tagged 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) catalysts were adsorbed on different supports such as silica gel, silica gel modified with highly cross-linked polymeric imidazolium networks, and highly cross-linked polymeric imidazolium networks entrapping magnetic particles. These systems provided a convenient tool for the oxidation of both primary and secondary alcohols working as recyclable reservoirs for the bis(imidazolium)-tagged TEMPO catalysts. By using EPR spectroscopy it was demonstrated that the catalyst was released as the corresponding oxoammonium salt in the solution during the recycling step, thus promoting the oxidative process in a homogeneous fashion. After solvent removal, the catalyst was readsorbed on the support allowing an easy recovery and recycle of the catalytic material up to 13 consecutive cycles with no loss in activity. The bis(imidazolium)-tagged TEMPO catalyst could be used in only 1mol% both for the oxidation of benzylic and aliphatic alcohols. The catalytic material was highly recyclable if used on silica or imidazolium-modified silica gel in 10mol% loading. Loading could be scaled down to 1mol% and the catalyst proved to be recyclable up to 8cycles only with imidazolium-modified silica gel. Such a catalyst-sponge-like system permits to combine the benefits of homogeneous and heterogeneous catalysis.