Oxidation in Flow Using an Ionic Immobilized TEMPO Catalyst on an Ion Exchange Resin

An ionic heterogenized catalyst system for Anelli oxidation has been developed using potassium 4-sulfonato-oxy-2,2,6,6-tetramethylpiperidine-1-yloxyl (TEMPO-4-sulfate) and anion exchange beads as support material. The catalytic oxidation of benzyl alcohol by bis(acetoxy)iodobenzene (BAIB) in acetonitrile with the modified beads gives a 94% yield of benzaldehyde within 60 min (batch operation). The beads give about the same conversion of benzyl alcohol in six consecutive cycles when reused after simple washing, albeit with a somewhat longer half-life time. The TEMPO entity could be removed from the beads using a sodium chloride/sodium hydroxy mixture. Reloading the beads with TEMPO-4-sulfate restored about 80% of their initial catalytic action. This exemplifies that the catalytic activity in a fixed bed can be regained without the need for cleaning and repacking the reactor. Preliminary experiments in a fixed bed show that a constant benzyl alcohol conversion of 84% over 10 residence times (as plug flow) can be achieved by the in-flow execution of the oxidation reaction.