Benzoyloxyl radical

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. 

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. 

The reaction of Scheme 4.10 yields only products of ortho and para substitutions the meta isomer is lacking. If it were a standard radical substitution, the meta-isomer would obviously be formed in a certain amount (i.e., in the same amount as that for ortho-substituted product). Introduction of electron-acceptor substituents enhances stability of the substrate to oxidation and prevents electron transfer to benzoyloxy radical. As a result, phenylation takes place instead of benzoyloxylation, and the phenyl radical enters into any free position. 

A radical ipso substitution at the 3-position of 2,3-disubstituted indoles has also been reported in their reaction with benzoyl-r-butyl nitroxide leading to (227) or, with the 2-substituted indole, the dimer (228) (cf. Section 3.05.1.4) (81CC694). In contrast with the benzoyloxylation reactions the nitroxide radical initially abstracts the hydrogen atom at the 1-position to form the indolyl radical. This mechanism is supported by the failure of the corresponding 1 -methylindole to undergo an analogous oxidation. 

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).

Figure 7. Photolytic and thermal decomposition pathways in crystalline ABP. Initial photolysis at 300-400 nm gives methyl-benzoyloxyl (MB) radical pairs, which can either collapse to give methyl benzoate, or decarboxylate thermally or photochemi-cally to give methyl-phenyl radical pairs.

Introduction of the electron-acceptor substituents enhances the stability of the substrate to oxidation and prevents electron transfer to the benzoyloxy radical. As a result, phenylation takes place instead of benzoyloxylation, and the phenyl radical enters into any free position. 

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. 

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]. 

The stannacyclopentanes are again particularly reactive 138). Alk-oxyl radicals will now react at the tin center in the fully alkylated compounds, with opening of the ring, and benzoyloxyl and alkylthiyl radicals will also induce ring cleavage. For example, 1,1-dibutylstan-nacyclopentane reacts homolytically with benzenethiol to give tribu-tyl(phenylthio)tin. 

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