Radicals benzoyloxyl radical

These substitution reactions were discovered by Nozaki and Bartlett [57,58] in their study of benzoyl peroxide decomposition in different solvents. When benzoyl peroxide is decomposed, the formed benzoyloxyl radical attacks the solvent (RH), and the formed alkyl radical (R ) induces the chain decomposition of the peroxide (see Chapter 3). 

As noted above, the rate of decarboxylation of the acetoxyl radical (k = 1.3 X 109 s-1) is too high for spin trapping to be feasible. The rate of decarboxylation of the benzoyloxyl radical is 103 times slower,... 

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

Figure 1. Nonexponential decay of methyl benzoyloxyl radical pairs in a single crystal of acetyl benzoyl peroxide after long photolysis at 77 K. The initial rapid decay has an effective rate constant of 1.1 min" while the later decay has an effective rate constant of 0.06 min -. Shorter photolysis gave clean exponential decay indicating a more uniform radical-pair structure (see Refs. 16b and 66).

The last alkaloid, compound L, named knightolamine (12), is closely related to (11) except for the lack of a carbonyl group in an acetyl ester and the presence of a benzoyloxy-group (1740 cm ) the parent ion (M+ 367 C HzsNOJ is cleaved to an ion of m/z 246 and a benzoyloxyl radical. The fragment of m/z 94 usually appears in the mass spectrum of those tropanes8 that have a hydroxy-group in the pyrrolidine moiety. 

With triphenylmethyl radicals, pyrrole behaves like a 1,3-diene giving the adduct 201. /V-Mcthylpyrrolc undergoes radical benzoyloxylation with dibenzoyl peroxide to give the 2-benzoyloxypyrrole 202 and 2,5-dibenzoyloxypyrrole 203. Furan, however, is converted in good yield into a mixture of as and trails addition products analogous in structure to 201. 

This equation is too complicated to be confronted with experimental results. It can be greatly simplified when reaction (46) is neglected. The assumption about the small importance of macroradical termination by phenyl radicals appears acceptable. First of all, the concentration of phenyl radicals is much smaller than that of benzoyloxyl radicals, kt pr should not substantially differ from kt pr, and finally the reactive phenyl radical should be immediately consumed by initiation. Equation (51) then assumes the form... 

The reaction of benzoate anion with S04 " has been established to proceed by electron transfer from the aromatic ring leading to the corresponding radical zwit-terion [223], followed by decarboxylation of an intermediate benzoyloxyl radical to yield the phenyl radic (Scheme 67). Both benzoyloxyl and phenyl radicals have been identified by ESR spectroscopy through their adducts with CH2N02 [224, 225], and in particular, in the case of polycarboxylated benzenes, the (carboxylated) phenyl radicals have been directly seen by ESR [224]. 

Odm Processes - The benzophenone sensitised reaction of the N,0-diacyl N-phenylhydroxylamine (189) in hexadecyltrimethylanunonium chloride micelles yields benzoyloxy-migrated products (190) and (191) derived from amidyl-benzoyloxyl radical pairs located at the micellar surface, whereas amidyl-phenyl radical pairs that penetrate more deeply into the micelle are responsible for phenyl-migrated products (192) and (193). Irradiation of ethanolic solutions of N-aryl-N-nitrosohydroxylamine ammonium salts produces azoxy compounds. 

Before discussing Breslow s results, which involve remarkable specificity, it seems worthwhile to discuss a monocyclization reaction described by Julia (Scheme 142). Treatment of the diene 380 with benzoyl peroxide, cuprous chloride, and cupric benzoate in acetonitrile at 80 C affords the (Cy5) product 383 in 35% yield and less than 2% of the (Cy6) product. Thus a significant proportion of the benzoyloxyl radicals add to 380 at the more substituted double bond giving radical 381, which then cyclizes to the (Cy 5) radical 382. 

To methyl and phenyl radicals To benzoyloxyl radical To t-butoxyl radical... 

It appears possible to interpret the observations with the carbon radicals, which are electron-donating, in terms of the domination of the resonance effect over the polar effect. On the other hand, those with the electron-accepting benzoyloxyl radical may be attributed only to the polar effect, as has been pointed out by Bevington et al,... 

The order of reactivity with the t-butoxyl radical differs from that with either carbon or benzoyloxyl radical. Probably, the resonance effect plays some role in the monomer reaction with this radical. 

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