Cleavage reactions homolytic

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

This suggests that at the transition state of this homolytic cleavage reaction 40% of the ground state strain is still present. Under the reasonable assumption that the radicals, which are the cleavage products, are more or less strain-free this means,... 

Termination reactions convert radicals to closed-shell compounds. Radical-radical coupling reactions are the reverse of homolytic cleavage reactions and are common, but radicals with (3-hydrogen atoms also react in disproportionation reactions as shown for 13. The selectivity of radical-radical terminations is low because the... 

Type I initiators are compounds that upon irradiation undergo a homolytic cleavage reaction ( -or p-cleavage) to generate two radicals.18... 

The cleavage reaction of Equation 23-2 reveals other useful generalizations. Whatever its source, a parent molecular ion, M+, has one unpaired electron and is properly described as an odd-electron ion (a radical cation). When a parent molecular ion fragments, it does so homolytically, as shown in Equation 23-2, and produces a radical and an ion in which the electrons are paired—an even-electron ion. The m/e value of an even-electron ion is an even number for any elemental composition of C, H, O in combination with an odd number of nitrogens. These generalizations are summarized in Table 23-2 and can be useful in the interpretation of mass spectra, as illustrated by Exercises 23-4 and 23-5. 

The C—Ge bond is less stable toward heterolytic and homolytic cleavage reactions than the C—Si bond, but it is more stable than the C—Sn and C—Pb bonds. This is consistent with the bond energies of these bonds (see Chapter 2). 

Among the C—Pb bond cleavage reactions, thermo- and photo-induced homolytic cleavage is of special theoretical and practical interest. 

Pincock and DeCosta [96] have recently described photoinduced bond cleavage reactions in a series of naphthylmethyl esters to produce both ionic and radical products. The authors attribute their results to excited state homolysis to form a caged radical pair and ET between the radicals to form an ion pair. The rates of ET have been rationalized by the Marcus theory which shows a Marcus inverted region where ET becomes slow when AG T is highly exothermic. In this case ET appears to follow homolytic bond cleavage rather than prior to bond breaking. 

The known coenzyme Bi2-dependent enzymes all perform chemical transformations in enzymatic radical reactions that are difficult to achieve by typical organic reactions. Homolytic cleavage of the Co bond of the protein-bound coenzyme B12 (3) to a 5 -deoxy-5 -adenosyl radical (9) and cob(n)alamin (5) is the entry to reversible H-abstraction reactions involving the 5 -position of the radical (9). Indeed, homolysis of the Co bond is the thermally most easily achieved transformation of coenzyme B12 (3) in neutral aqueous solution (with a homolytic (Co-C)-BDE of about 30 kcal mol ). However, to be relevant for the observed rates of catalysis by the coenzyme B12-dependent enzymes, the homolysis of the Co-C bond of the protein-bound coenzyme (3) needs to be accelerated by a factor of about 10 , in the presence of a substrate. Coenzyme B12 might then be considered, first of aU, to be a structurally sophisticated, reversible source for an alkyl radical, whose Co bond is labihzed in the protein-bound state (Figure 8), and the first major task of the... 

Heterolytic versus Homolytic Bond-Cleavage Reactions... 

Scheme 5. Heterolytic and homolytic C-S bond-cleavage reactions.

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