Radicals acyloxy

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

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. 

Earlier sections have already provided several examples of radical fragmentation reactions, although this terminology was not explicitly used. The facile decarboxylation of acyloxy radicals is an example. 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo decarboxylation. Such electron-transfer reactions can be effected by strong one-electron oxidants, such as Mn(HI), Ag(II), Ce(IV), and Pb(IV) These metal ions are also capable of oxidizing the radical intermediate, so the products are those expected from carbocations. The oxidative decarboxylation by Pb(IV) in the presence of halide salts leads to alkyl halides. For example, oxidation of pentanoic acid with lead tetraacetate in the presence of lithium chloride gives 1-chlorobutane in 71% yield ... 

A classic reaction involving electron transfer and decarboxylation of acyloxy radicals is the Kolbe electrolysis, in which an electron is abstracted from a carboxylate ion at the anode of an electrolysis system. This reaction gives products derived from coupling of the decarboxylated radicals. 

The low bond-strength of the O—0 bond renders peroxides susceptible to homolytic fission to give oxy radicals on heating. Diacyl peroxides give rise to acyloxy radicals which then decompose to aryl radicals and carbon dioxide, Eq. (5). For example, dibenzoyl... 

The hyponitrites (16), esters of hyponitrous acid (HO N=N OH), are low temperature sources of alkoxy or acyloxy radicals. A detailed study of the effect of substituents on k4 for the hyponitrite esters has been reported by Quinga and Mendenhall,114... 

Diacyl or diaroy] peroxides (36, R- alkyl or aryl respectively) are given specific coverage in reviews by Fujimori,141 Bouillion et c//.,14 and Hiatt.14j They are sources of acyloxy radicals which in turn are sources of aryl or alkyl radicals. Commercially available peroxides of this type include dibenzoyl peroxide (BPO), didodecanoyl or dilauroyl peroxide (LPO), didecanoyl peroxide (42) and succinic acid peroxide (43). 

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. 

In general, aliphatic diacyl peroxide initiators should be considered as sources of alkyl, rather than of aeyloxy radicals. With few exceptions, aliphatic acyloxy radicals have a transient existence at best. For certain diacyl peroxides (36) where R is a secondary or tertiary alkyl group there is controversy as to whether loss of carbon dioxide occurs in concert with 0-0 bond cleavage. Thus, ester end groups observed in polymers prepared with aliphatic diaeyl peroxides are unlikely to arise directly from initiation, but rather from transfer to initiator (see 3.3,2.1.4),... 

The high rate of decarboxylation of aliphatic acyloxy radicals is also the prime reason behind low initiator efficiencies (see 3.3.2.1.3). Decarboxylation occurs within the solvent cage and recombination gives alkane or ester byproducts. Cage return for LPO is 18-35% at 80 °C in -octane as compared to only 4% for BPO under similar conditions.144... 

The chemistry of peroxyesters (38) also commonly called pcrcsters has been reviewed by Sawaki,199 Bouillion el al.m and Singer.194 The peroxyesters are sources of alkoxy and acyloxy radicals (Scheme 3.32). Most commonly encountered peroxyesters are derivatives of /-alkyl hydroperoxides (< .g. /-butyl peroxybenzoate, BPB). 

The initiators (31) and (65) are low temperature sources of alkyl and hydroxy or acyloxy radicals respectively (Scheme E4/. " The ra-hydroperoxy... 

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

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] ... 

The mechanism of the ester formation would seem to be either an S i decomposition of the peroxide to give the ester directly, or a front side reaction between two acyloxy radicals. 

The question of rearrangement in acyloxy radicals is still unsettled. In the case of the decomposition of -methoxy- -nitrobenzoyl peroxide rearrangement is observed, but the other circumstances indicate a polar mechanism for the reaction.118... 

Since ditriptoyl peroxide is electrically symmetrical, and since benzene is not outstanding in its ability to solvate polar transition states, it seems probable that the inversion reaction in this case is due to the rearrangement of an acyloxy radical rather than cation. It may be that failure to isolate comparable products from other peroxides under free radical conditions is due to competition from very fast substitution... 

Few examples have been reported (5, 8, 9, 10, 12, 24) of cage recombination of simple alkoxy or acyloxy radicals to form O—O bonds in isolable molecules. This paper explores further the implications of the observed (17, 22, 23) scrambling of label seen in acetyl peroxide carbonyl-18O recovered after partial decomposition. 

Figure 7.4. Interactions which determine the relative reactivities of carboxyalkyl (left) and acyloxy radicals.

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