The acyloxy radical

As was indicated previously, primary alkyl diacyl peroxides undergo decomposition to give two acyloxy radicals, which subsequently undergo rapid decarboxylation. The rate coefficient for decarboxylation of the acetoxy radical is calculated to be 1.6 X 10 sec at 60 °C in n-octane . The corresponding activation parameters were = 6.6 kcal.mole and A = 3.5 x 10 sec An earlier report estimated the activation energy to be about 5 kcal.mole . It was pointed out by 

Martin et that attempts to trap the acetoxy radical with iodine, galvinoxyl or diphenylhydrazyl were either unsuccessful or ambiguous. However, isotopic labeling experiments show that cage recombination does occur and the mechanism of acetyl peroxide decomposition may be formulated as 

In isooctane at 80 °C about 38 % cage recombination is observed. The rates of isotopic scrambling are given in Table 89. Furthermore, it is seen that the rate of scrambling for acetyl peroxide is dependent on the viscosity of the solvent as would be expected for cage recombination. The inversion mechanism was shown to be 

RATES OF SCRAMBLING OF CARBONYL LABEL IN DIACYL PEROXIDES AT 80 

Secondary isotope studies are consistent with the suggested mechanism for acetyl peroxide decomposition. It was concluded from the data given in Table 90 that little or no methyl radical character was developed in the rate-determining step . This view has been contested with data from a study of and 0-iso-tope effects in the decomposition of acetyl peroxide. The carbon isotope effect in 

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Radicals react at the sulfur, and decomposition generating an acyloxy radical ensues. The acyloxy radical undergoes decarboxylation. Usually, the radieal then gives produet and another radical which can continue a chain reaction. The process can be illustrated by the reactions with tri-w-butylstannane and bromotrichloromethane. 

The rates of thermal decomposition of diacyl peroxides (36) are dependent on the substituents R. The rates of decomposition increase in the series where R is aryl-primary alkyKsecondary alkyKtertiary alkyl. This order has been variously proposed to reflect the stability of the radical (R ) formed on (i-scission of the acyloxy radical, the nucleophilicity of R, or the steric bulk of R. For peroxides with non-concerted decomposition mechanisms, it seems unlikely that the stability of R should by itself be an important factor. 

The current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

The homolytic decomposition of diacyl peroxides proceeds via splitting of the weakest O—O bond. The acyloxy radicals formed are very unstable and a cascade of cage reactions follows this decomposition [4,42-46] ... 

This step is unlikely. There is no relief of steric strain as the acetoxy group is still present and the C-H bond strength is higher than the 0=C0-C bond strength by about seven kcal/mole (26). In addition the acyloxy radical is unstable and decomposes exothermically (26). If this happens during dismutation, loss of acetoxy is favored even more, up to thirty kcal/mole. 

The acyloxy radicals produced in reaction (6-40) of diacyl peroxides can initiate polymerization or undergo side reactions as described in Section 6.5.5. Other peroxides behave similarly. 

Acetic acid gives the methyl radical as well as CHg. COgH, and malonic acid gives only 0112.00211. These decarboxylation products possibly arise from abstraction of carboxylic hydrogen followed by, or concerted with, decarboxylation of the acyloxy radical, e.g. 

The benzyl radical might be formed by abstraction of hydrogen from the carboxyl group and decarboxylation of the acyloxy radical (PhCHs.COaH- PhOHz.COa- PhCHs. + COa), but the effect of pH on the observed spectra, considered in the light of the results already discussed for the behaviour of glycol with the titanous-peroxide system, reveals a more likely mechanism namely, that addition of hydroxyl to an aromatic carbon atom is followed, in sufficiently acidic media, by the elimination of hydroxide ion from the ring concerted with the loss of carbon dioxide and a proton. A convenient representation, in the case of an initial reaction at the para position, is as follows ... 

It is also interesting that the yield of monomethyl succinate (R —H), which presumably arises from the abstraction of a hydrogen atom by the acyloxy radical, is higher in the presence of aromatic than aliphatic substrates. This may mean that the acyloxy radical is mainly responsible for the abstraction of a hydrogen atom from the initial adduct (26) formed between the alkyl radical and the aromatic substrate (cf. Section II,B). 

The photochemical cleavage of naphthylmethyl alkanoates in methanol is reported to proceed by homolytic cleavage to naphthylmethyl radical and acyloxy radical,the latter decarboxylates in competition with electron transfer to give naphthylmethyl cation and carboxylate anion. Using known rates of electron transfer as a clock the rate constants for decarboxylation of the acyloxy radicals has been estimated.The light induced homolysis of 1-chloromethyl-naphthalene has also been studied using chemically induced dynamic electron polarisation (CIDEP) spectroscopy to detect the naphthyl-... 

Benzylic esters have been studied in considerable detail often as a continuation of the pioneering work by Zimmerman and co-workers (Scheme 2) in 1963 [44]. There are several reasons for this. First, the synthesis of compounds with the structural variables required to probe specific mechanistic questions is often straightforward. Second, products are usually formed from both ion pairs and radical pairs and, therefore, the structural variables that control this partitioning can be systematically studied. Third, the radical pair (ARCH2-O CO)— R) incorporates a built-in radical clock, the decarboxylation of the acyloxy radical, which serves as a useful probe for the reactivity of the radical pair. If the carbon of the acyloxy radical is sp hybridized, this decarboxylation rate is on the 1- to 1000-ps time scale, depending on R, so that decarboxylation will often occur within the solvent cage before diffusional escape. The topic of benzylic ester photochemistry has been recently reviewed twice by Pincock [5,98] and therefore only a brief summary will be given here. 

N-acetylated derivatives leads to the formation of Fe(II) porphyrins together with the acyloxy radical which undergoes decarboxylation to give ammonio-alkyl or amidoalkyl radicals. Large differences in the observed rates of Fe(II) porphyrin formation can be accounted for in terms of two factors, the binding affinity of the carboxyl to form a photoactive complex, and competitive reactions of acyloxy radicals following photolysis. 

Decarboxylation of the acyloxy radical then competes with electron transfer (k f) for formation of ion pairs. The rates of electron transfer for both substituted 1-naphthylmethyl 2 and benzyl substrates 6 follow Marcus theory in both the normal and inverted region when correlated with the oxidation potential of the arylmethyl radical. The meta-methoxy compounds give high yields of ion-derived products because the oxidation potentials of their arylmethyl radicals place them near the maximum on the Marcus plot therefore, kg is competitive with fcco2- This work has been reviewed in the previous volume of this Handbook and in other places." ... 

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