Acyloxy radicals aliphatic

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... 

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). 

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). 

Whether or not a fragmentation according to reactions (15) and (16) takes place depends on the reactivity of the primary formed oxygen-centered radicals toward the monomer. In the case of BPO, there is a fragmentation with phenyl radical formation [reaction (15)] only in the absence of the monomer. In the presence of the monomer, the benzoyl oxy radicals react with monomer before decarboxylation. Aliphatic acyloxy radicals, on the other hand, undergo fragmentation already in the solvent cage whereby recombination products are produced that are not susceptible to further radical formation. As a result, the radical yield Ur for these initiators is smaller than 1 ... 

Cobalt complexes derived from Schiff bases 388 catalyzed the hydroxyacylation of electron-deficient alkenes (Fig. 90) [431, 432]. Thus, methyl acrylate 387 reacted with aliphatic aldehydes 386 in the presence of 5 mol% of the in situ generated catalyst, molecular oxygen, and acetic anhydride to 2-acyloxy-4-oxoesters 389 in 56-77% yield. When acetic anhydride was omitted, the yields of products were lower and mixtures of the free hydroxy compounds and acylated compounds resulting from Tishchenko reactions were obtained. Electron-rich alkenes did not undergo the transformation, since the addition of the acyl radical is much slower. The acylcobalt species inserts oxygen instead and acts as an epoxidation catalyst. 

Thallium(m) acetate effects a one-step synthesis of aliphatic a-acyloxy-carboxylic acids (Scheme 28) the transition state (92) is proposed. a-Aminoesters are readily oxidized to the corresponding a-diazoesters by isoamyl nitrite in the presence of acetic acid. A review has been published on the radical addition of carboxylic acids and derivatives to unsaturated linkages. ... 

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