Benzoyloxyl

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). 

In reactivity, the same series of mutagens exhibited positive Hammett correlations for AaiI acid catalysed solvolysis (p — + 0.32) and SN2 reactivity with A-methyl-aniline (p — + 1.7), hydroxide (p — +0.55) and L-cysteine ethyl ester (p — + 1.1), all reactions in which the benzoyloxyl group leaves with electrons and which are therefore accelerated by electron-withdrawing groups. The negative Hammet p-value for... 

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. 

Copper-catalyzed allylic oxidation allows the functionalization of unactivated alkenes into chiral allylic carboxylates.1347 The use of oxazoline-containing ligands give good enantioselectivities, but the reaction is extremely slow.1348-1351 Chiral bipyridine complexes, in turn, are much more active and give products in good yields and enantioselectivities up to 70% when applied in benzoyloxylation of cycloalkenes with rm-butyl perbenzoate.1352,1353... 

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.

Benzoyloxylation of p-lactams.1 Reaction of p-lactams with this reagent in the presence of copper(II) octanoate effects benzoyloxylation at C4, a to the nitrogen when the nitrogen is substituted by a phenyl or /-butyl group. However, reaction at an exocyclic carbon a to the nitrogen is a competing reaction when possible. 

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. 

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