Radical reactions TEMPO-mediated oxidation

The mechanistic details of these laccase/mediator catalyzed aerobic oxidations are still a matter of conjecture (51-54). However, experiments with a probe alcohol point towards one-electron oxidation of the mediator by the oxidized (cupric) form of the laccase followed by reaction of the oxidized mediator with the substrate, either via electron transfer (ET), e.g., with ABTS, or via hydrogen atom transfer (HAT), e.g., with N-hydroxy compounds which form N-oxy radicals (55). TEMPO and its derivatives form a unique case one-electron oxidation of TEMPO affords the oxoammonium cation which oxidizes the alcohol via a heterolytic pathway (Fig. 6), giving the carbonyl product and the hydroxylamine. The Tl copper center in fungal laccases has a redox potential of ca. 0.8 V vs. NHE. Consequently, fungal laccases can easily oxidize TEMPO to the corresponding oxoammonium cation, since the oxidation potential of the latter, which was first measured by Golubev and co-workers (55,57), is 0.75 V. This was confirmed by EPR measurements, which showed that laccase is reduced in the presence of TEMPO One equivalent of laccase could oxidize at least three equivalents of TEMPO within a few minutes under anaerobic conditions (58). 

There are fewer reports on the preparation of block copolymers via the combination of anionic polymerization with nitroxide-mediated syntheses [132,133]. As shown in Scheme 3.30, the reaction product of sodium with 4-hydroxyl-TEMPO initiated the anionic polymerization of ethylene oxide at 60 °C in THF solution. After treatment with methanol, TEMPO-terminated PEO was obtained, and then used in the nitroxide-mediated radical polymerization of St at 120 °C resulting in block copolymers of type CLB-17 [133]. Another method is the transformation of anionic polymerization into nitroxide-mediated radical polymerization. A poly(butadienyl)lithium solution in... 

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