Oxidation with Immobilized Oxoammonium Salts

6-tetramethylpiperidinoxyl (TEMPO) radical was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor. TEMPO is a highly persistent radical, resistant to air and moisture, which is stabilized primarily by the steric hindrance of the NO-bond. Paramagnetic TEMPO radicals can be used as powerful spin probes for investigating the structure and dynamics of biopolymers such as proteins, DNA, and synthetic polymers by ESR spectroscopy [7]. A versatile redox chemistry has been reported for TEMPO radicals. The radical species can be transformed by two-electron reduction into the respective hydroxyl-amine or by two-electron oxidation into the oxoammonium salt [8]. One-electron oxidations involving oxoammonium salts have also been postulated [9]. The TEMPO radical is usually employed under phase-transfer conditions with, e.g., sodium hypochlorite as activating oxidant in the aqueous phase. In oxidations of primary alcohols carboxylic acids are often formed by over-oxidation, in addition to the de- 

Thus isolation of oxoammonium salts on insoluble, cross-linked polymer supports was investigated together with integration of the active resins in polymer-assisted solution phase synthesis [11], These isolated oxoammonium salts could be employed in a water-free system with the intention of generating a highly reactive oxidizing agent that avoids over-oxidation to the acid, because of the absence of water. 

The versatility of the novel reagent was investigated with a diverse selection of alcohols at room temperature with 3 equiv. of the reagent for 1 h. Results from the oxidations can be summarized as follows. Clean and rapid quantitative conversion to the respective aldehyde or ketone product was observed for all benzylic, allylic, 

In summary, the polymer-bound oxoammonium reagent was highly efficient in polymer-supported oxidation of different alcohols and was capable of cleanly converting complex compound collections. No overoxidation to carboxylic acids was observed. It is obvious that the reagent will be of great value in polymer-supported transformations in solution, in automated parallel synthesis operations, and in flow-through reactors in up-scaled production processes. 

Immobilized TEMPO radical has also been used for one-pot oxidation of alcohols to carboxylic acids [14]. For this purpose TEMPO-resin 1 was combined with two 

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