Acyl oxyl radical

Many O-centered radicals undergo facile P-fragmentation. For example, acyl-oxyl radicals which are intermediates in the electrolytic oxidation of acids (Kolbe electrolysis), rapidly decompose into alkyl radicals and C02 [reaction (1)]. The rate of these reactions is in the order of 109 s 1 and increases with increasing branching of the alkyl substituent, i.e., decreasing C-C02 bond energy (Table 7.1). 

Taken together, the various reactions and interconversions of these manganese porphyrin complexes have allowed the examination of each step in the activation of molecular oxygen by the mechanism suggested for P-450. Detailed mechanistic studies of the 0-0 bond cleavage event in 29 by kinetics, substituent effects, and product analysis showed that the reaction proceeds via heterolysis to produce 27 when acid is present, whereas homolysis is predominant in the absence of acid but in the presence of hydroxide ion (95). Under basic conditions, homo-lytic cleavage of the 0-0 bond of 29 forms Mn (=0)TMP (28) and an acyl-oxyl radical. Thus, when an alkyl peroxy acid is employed, decarboxylation competes with electron transfer, as shown in Scheme IX, to afford a mixture of 27 and 28. Yuan and Bruice have proposed a similar heterolysis mechanism based on the kinetic analysis for the reaction of mCPBA with catalytic amounts ofMn TPP(/(W). 

The formed acyl radicals are reactive towards efficient radical trapping reagents such as 2,2,6,6-tetramethylpiperidine-l-oxyl radical (TEMPO), diphenyl diselenide and diphenyl disulphide, and A-f-butyl-a-phenylnitrone giving the respective adducts. ... 

A new process for the homolytic acylation of protonated heteroaromatic bases has been developed by Minisci et al. An A-oxyl radical generated from iV-hydroxyphthalimide by oxygen and Co(ll) abstracts a hydrogen atom from an aldehyde. The resulting nucleophilic acyl radical adds to the heterocycle which is then rearomatized via a chain process. Under these conditions, quinoline and benzaldehyde afford three products (Equation 108) <2003JHC325>. A similar reaction with 4-cyanopyridine gives 2-benzoyl-4-cyanopyridine in 96% yield. 

In the case of the pinene-derived acyl nitroxide 31 [50], as well as the phthalimide-iV-oxyl radical 32 [56], the mechanism of the oxidation involves hydrogen abstraction by the reactive acyl nitroxide [58] (or phthalimidyl nitroxide), followed by trapping of the resulting carbon radical with either a second equivalent of nitroxide or by molecular oxygen (Scheme 19). Table 2 summarizes the results of oxidative desymmetrization using optically active nitroxides. 

The radical C-H transformation of ethers is generally initiated by a-hydrogen abstraction with highly reactive radicals generated from such initiators as peroxides [3a, g], photo-activated carbonyl compounds [3b—d], metallic reagents [3i, j], and redox systems [3f, h[. Various combinations of ethers, radical initiators, and radical acceptors (e.g. carbon-carbon multiple bonds) may be used as the reaction components [6], Several notable means of direct C-C bond formation via the radical a-C-H transformation of ethers involve the use of triflon derivatives [7], the phthalimide-N-oxyl (PINO) radical [8], 2-chloroethylsulfonyl oxime ethers [9], and N-acyl aldohydrazones [10],... 

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