Homo lytic Mechanisms

These concepts are important for an understanding of the roles played by metal ions and their complexes in the catalysis of oxidation reactions via homo-lytic mechanisms. Thus, metal complexes may function as catalysts by interfering with any of the various initiation, propagation, and termination steps outlined earlier. 

It has recently been suggested that a free radical mechanism i.e., homo-lytic cleavage of the oxygen-oxygen bond rather than the heterolytic cleavage pictured) may be involved in the reaction of some substituted benzophenones and peroxyacetic acid. 

The configurational stability of sulfinamides was investigated by Cram and Booms (257). They found that sulfinamides undergo racemization at room temperature but with a very long induction time. This indicates that the racemization process is a consequence of a homo-lytic scission of the sulfur-nitrogen bond involving the formation of an achiral sulfinyl radical. The free radical mechanism of the racemization of sulfinamides is supported by the fact that they initiate the polymerization of styrene and methyl methacrylate. 

Let us examine the different elementary steps of the mechanism. Initiators [In in Eq. (2)] can be chemical species of different origins. Very often they are peroxy (ROO ) and oxy (RO ) radicals produced by thermal homo-lytic decomposition of organic peroxides,... 

Taken together, the various reactions and interconversions of these manganese porphyrin complexes have allowed the examination of each step in the activation of molecular oxygen by the mechanism suggested for P-450. Detailed mechanistic studies of the 0-0 bond cleavage event in 29 by kinetics, substituent effects, and product analysis showed that the reaction proceeds via heterolysis to produce 27 when acid is present, whereas homolysis is predominant in the absence of acid but in the presence of hydroxide ion (95). Under basic conditions, homo-lytic cleavage of the 0-0 bond of 29 forms Mn (=0)TMP (28) and an acyl-oxyl radical. Thus, when an alkyl peroxy acid is employed, decarboxylation competes with electron transfer, as shown in Scheme IX, to afford a mixture of 27 and 28. Yuan and Bruice have proposed a similar heterolysis mechanism based on the kinetic analysis for the reaction of mCPBA with catalytic amounts ofMn TPP(/(W). 

In the case of copper complexes, the chemistry is less well understood. High valent copper-oxo species are not likely to be formed, consequently copper-hydroperoxo or copper-hydroxo species are usually proposed as active species in DNA oxidation. These species are thus more susceptible to homo-lytic cleavage of the peroxide or the metal-hydroxo bond and consequently, to 1-electron oxidation mechanism. However, the labeling of the product of deoxyribose oxidation at Cl by Cu(l,10-phenanthroline)2 clearly demonstrated that these complexes can mediate a 2-electron oxidation mechanism of DNA damage since the oxygen atom incorporated in DNA originates from H2O. 

Isomerization and Exchange.— The rate of isomerization of cyclopropylmethyl-cobalamin (10)- (11) is virtually independent of the polarity of the solvent and is unaffected by oxygen. This excludes a mechanism involving polar intermediates and the rearrangement probably proceeds either via pathways involving homo-lytic or heterolytic Co—C bond fission or alternatively by a mechanism in which the Co—C bond remains intact. 

The loss of the X halide is in competition with the corresponding X radical that supplies the [M-X] ion. This last mechanism is presented in Figure 9.60 in the case of a tetrachlorodioxin. Unlike the elimination of halide ions that results from homo-lytic cleavage of the C-X bond, the formation of [M-X]- results from a heterolytic cleavage. For polyhalogenated aromatic compounds, the relative proportions of M-, [M-X]-, and X- in the mass spectrum depend on the conditions of the spectrometer source (temperature, pressure) and also and especially on the nature of the thermal-ization gas used."... 

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