Diacyl peroxides, thermolysis

Phenols with bulky ortho- and para-substituents, eg, phenoHc antioxidants, do not undergo this reaction however, they scavenge radicals generated by thermolysis of diacyl peroxides and other peroxides. Diacyl peroxides react with potassium superoxide, KO2, forming singlet oxygen (207). 

Diacyl peroxides have continuous weak absorptions in the UV to ca 280 nm (e ca 50 M cm 1 at 234 nm),147 Although the overall chemistry in thermolysis and photolysis may appear similar, substantially higher yields of phenyl radical products are obtained when BPO is decomposed photochemically. It has been suggested that, during the photodecomposition of BPO, (3-scission may occur in... 

The free radical substitution reactions, other than phenylation, of pyridine and its derivatives have received but scant attention. Alkylation of pyridine itself has been studied briefly, the alkyl radicals being generated either by the thermolysis of diacyl peroxides or of lead tetraacetate in acetic acid, or by electrolysis of the carboxylic acid precursor (for summary, see Norman and Radda369). Most of the available results are summarized in Table XIV. These figures on isomer ratios are not very reliable since the analyses were carried out by... 

Feldhues, M. and Schafer, H.J. (1985) Selective mixed coupling of carboxylic adds (I). Electrolysis, thermolysis and photolysis of unsymmetrical diacyl peroxides with acyclic and cydic alkyl groups. Tetrahedron, 41, 4195M212. 

Free radicals may be generated by oxidation, reduction, or by homolytic cleavage of one or more covalent bonds, such as C—C bonds e.g. dimers of triarylmethyl radicals), N—N bonds e.g. tetrasubstituted hydrazines), O—O bonds e.g. hydroperoxides, dialkyl and diacyl peroxides, peroxycarboxylic esters), C—N bonds e.g. dialkyl azo compounds), and N—O bonds (as in the thermolysis of nitrogen pentoxide O2N— O—NO2). Two typical examples, which have been investigated in different solvents, are given in Eqs. (5-56) and (5-57) cf. also reaction (5-39a) in Section 5.3.2. 

In the case of the thermolysis of uns5mrmetrical diacyl peroxides, R—CO—O2 —CO—Ar, with negatively substituted phenyl groups e.g. Ar = 3-chlorophenyl), there is a moderate increase in reaction rate with increasing solvent polarity. They are generally considered to involve ion-pair intermediates e.g. R 02C—Ar), formed via dipolar activated complexes. A typical example is that of endo- and evo-(2-norbomyl)formyl 3-chlorobenzoyl peroxide /ri(CH3CN)/A i (cyclohexane) = 320 for the exu-reactant [563]. 

Estimates of the probability of escape of radical pairs in conventional solvents have been made by product analysis of the decomposition of diacyl peroxides. For example, Braun et al. [22] estimated that 60 to 80% of the methyl radicals produced in the thermolysis of acetyl peroxide escape geminate cage recombination. However, Guillet and Gilmer [25] showed that for longer chain and Cjj radicals the probability was much lower, ranging from 5% at 760C to 16% at 262<>C (Table VI). 

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