Hydrogen abstraction aminoxyl radicals

For a review concerning mechanistic discussion on the Cu-TEMPO-catalyzed aerobic oxidation, see C. GaUi, P. GentiU, O. Lanzalunga, Angew. Chem. Int. Ed. 2008, 47, 4790-4796. Hydrogen abstraction and electron transfer with aminoxyl radicals synthetic and mechanistic issues. 

The spin trapping of NO as aminoxyl radicals (ARs) Rj-NO -R was observed by electron spin resonance (ESR) spectroscopy in various radical reactions. For example, such ARs are formed in the course of the photodecomposition of 2,2 -azobisisobutyronitrile (AIBN) in aliphatic alcohols (methanol, ethanol, 2-propanol) [15]. In this case, Rj is the radical NhC-CICHj) from AIBN, and R is derived from alcohol molecules as a result of a hydrogen atom abstraction by Rj. 

Diphenylphosphinoyl radicals 16 were generated by hydrogen-abstraction from diphenylphosphine oxide 15 by rm-butoxyl radicals. Thus, heating a mixture of di-r rr-butyl peroxalate (5 x IQ- M), the aminoxyl 1 (1.2 x 10-2 M) and diphenylphosphine oxide (1 x 10-2 M) in acrylonitrile at 60 C for 70 mins, gave a mixture of the addition product 17 and the phosphinate ester 18 in the ratio of 1 0.28 (Scheme 5). Clearly, the addition of diphenylphosphinoyl radicals to acrylonitrile is competitive with aminoxyl trapping, and as expected, increasing the aminoxyl concentration (to 5 x 10 2 M) resulted in an increase in the ratioof 17 18(to 1.0 0.78). 

Solvents essentially influence the decay rate. Rate constants in polar solvents are less because of AR blocking due to formation of hydrogen bonds with solvents. In conditions where hydrogen-atom abstraction from surrounding molecules is difficult, aminoxyls are stable up to 200-220 °C [18]. ARs can accept one radical with the formation of diamagnetic compounds ... 

Along with reactions without the participation of the radical centre, ARs react as typical radicals. At elevated temperatures, they abstract hydrogen atoms, chlorine, bromine and other elements. There are examples of sufficiently reactive ARs in H-atom abstraction at standard temperatures. The benzotriazole-N-oxyl (BTNO) generated by the oxidation of 1-hydroxybenzotriazole (HBT) with a Ce" salt in acetonitrile spontaneously decays with a first-order rate constant of 6.3 x 10" s" at room temperature [31]. The decay of this aminoxyl is strongly accelerated in the presence of H-donor substrates such as alkylarenes, benzyl and allyl alcohols ... 

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