Oxygen bond scission

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 ... 

This ladical-geneiating reaction has been used in synthetic apphcations, eg, aioyloxylation of olefins and aromatics, oxidation of alcohols to aldehydes, etc (52,187). Only alkyl radicals, R-, are produced from aliphatic diacyl peroxides, since decarboxylation occurs during or very shortiy after oxygen—oxygen bond scission in the transition state (187,188,199). For example, diacetyl peroxide is well known as a source of methyl radicals (206). 

Equation 4 indicates a rapid, irreversible formation of hydroxypentacyano-cobaltate(III) and is analogous to the cleavage of hydroperoxides by lithium aluminum hydride (17) involving oxygen-oxygen bond scission. Equation 5... 

Recently, tin(II) butoxide was used in the polymerization of l-LA [88]. The initiation is fast and quantitative and no transesterification or back-biting reactions are observed. The reaction proceeds with acyl-oxygen bond scission with retention of the configuration, and can be used both in bulk and solution (THF, 20-80 C) polymerization. It is possible to control the molecular weight in the range of 103 to 106 with a MWD of 1.15-1.85. The polymerization is very fast, kp=5 x 10 1 mol-1 L s-1, with only the rare earth alkoxides being faster. 

W. A. Lee, T. C. Bruice, Homolytic and heterolytic oxygen-oxygen bond scissions accompanying oxygen transfer to iron(III) porphyrins by percarboxylic adds and hydroperoxides. A mechanistic criterion for peroxidase and cytochrome P-450, ]. Am. Chem. Soc. 107 (1985) 513. 

Carbon-Oxygen Bond Scissions with Boron Trichloride, T. G. Bonner and E. J. Bourne, Methods Carbohydr. Chem., 2, 206-207 (1963). 

The cationic polymerisation of lactones takes place in the presence of the usual cationic catalysts (Lewis acids and Bronstedt superacids) and the active propagating species are oxonium cations, formed by the attack of the exocyclic oxygen atoms of lactone and the ring opening of the lactone cycle takes place by alkyl - oxygen bond scission [31] ... 

In the case of ethers, the C-O and the C-C (adjacent to the oxygen) bond scission is favored over the C-H bond scission. The products formed in the case of the radiolysis of diethyl ether are shown in Table 11.7. 

Chan, H.W.-S., Prescott, F.A.A., and Swoboda, P.A.T. (1976) Thermal Decomposition of Individual Positional Isomers of Methyl Linoleate Hydroperoxide Evidence of Carbon-Oxygen Bond Scission, J. Am. Oil Chem. Soc. 53, 572-576. 

Oxygen Bond Scission and Catalysis 5.3.1. MECHANISM OF 0-0 BOND SCISSION... 

Initiation of [3-lactone polymerization with ionic alkoxides proceeds in such a way that it initially results in acyl-oxygen and alkyl-oxygen bond scission in c. 1 1 proportion (Scheme 2). However, independent of the chemistry at earlier stages, the eventual active species are carboxylate... 

The alkyl-oxygen bond scission is known only for P lactones, whereas for higher laaones and LAs exclusively the acyl-oxygen bond scission proceeds and the active species are alkoxide anions. 

On the other hand, in the purely anionic polymerization of five- or higher-membered cyclic esters, the carbonyl carbon of the monomer is attacked with subsequent acyl-oxygen bond scission and reformation of the alkoxide anion. In the coordination polymerization, this is also the carbonyl carbon that is now first coordinated with alkoxide species and then the acyl-oxygen bond is broken with reforming of the covalent alkoxide chain end. In the already formed macromo-lecular chains, the same ester bonds are present as those being the site of the nucleophilic attack in the monomer molecules. These processes are illustrated in Scheme 12, where the active centers are shown as. ..-OMt, for both anionic and covalent centers. 

The nature of the active centers formed at various stages of the polymerization has also been elucidated in several model experiments. The authors concluded that the propagation would proceed on alkoxide and carboxylate active centers, respectively formed by the acyl-oxygen and alkyl-oxygen bond scissions of the monomer. The number of carboxylate active sites was also found to increase with the DP, such that they became prevalent on completion of the reaction (Scheme 9.14) [48]. 

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