Carbon-oxygen bonds radical additions

The reaction of organic radical cations have been the focus of much interest, and their synthetic reactions - including addition to alkenes and nucleophilic capture by alcohols resulting in carbon-carbon and carbon-oxygen bond formation, respec-... 

Electron transfer from Me2C=C(OMe)OSiMe3 to Q is made possible by the strong interaction between Q" and Mg + (or 2Mg +) to produce the radical ion pair. Since the spin of the ketene silyl acetal radical cation is mainly localized on the terminal carbon atom [229], the carbon -oxygen bond is formed before the cleavage of the Si-0 bond to yield the adduct (Scheme 15). This contrasts with the 1,2-addition of nonsubstituted ketene silyl acetal [H2C=C(OEt)OSiEt3] via nucleophilic attack to the positively charged carbonyl carbon of the quinone rather than via an alternative electron transfer pathway [228]. 

From these dai a Gray selected a set of best values for the heats of formation of alkoxy radicals, which are shown in Table 6. These heats of formation, when combined with the heats of formation of the alkyl radicals may be used to calculate the carbon-oxygen bond dissociation in the alkoxy radicals. Values are shown in Table 6. In addition, these heats of formation of alkoxy radicals may be combined with similar data for the alcohols and ethers to obtain bond dissociation energies i)(R—OH), i>(RO—H) and D(R—OR ). These derived energies are given in Table 7. 

It is believed that this reaction involves a rapid, reversible addition of the stannyl radical to the thiocarbonyl group followed by a slower fragmentation of the radical, with cleavage of the carbon oxygen bond. " ... 

We shall see in Chapter 2 that the hybrid radical (A ) formed by reaction (8) and the resulting peroxyl radicals by reaction (9) can assume different stereochemical conformations, which determine the geometric configurations (cis and trans) of the isomeric hydroperoxides from various unsaturated lipids oxidized under different conditions. The oxygen addition reactions (8) and (9) can be reversible for many radicals. The reverse reaction (-9) proceeds by /J-scission or j8-fragmentation of the carbon-oxygen bond (-C-I-00 ) of the peroxyl radicals and may also determine the stereochemical configuration of the isomeric hydroperoxides,... 

This comparison suggests that of these two similar reactions, only alkene additions are likely to be a part of an efficient radical chain sequence. Radical additions to carbon-carbon double bonds can be further enhanced by radical stabilizing groups. Addition to a carbonyl group, in contrast, is endothermic. In fact, the reverse fragmentation reaction is commonly observed (see Section 10.3.6) A comparison can also be made between abstraction of hydrogen from carbon as opposed to oxygen. 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

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