Homolytic cleavage catalysis

Holmium, tris(2,2,6,6-tetramethyl-3,5-heptanedione)-photosubstitution, 1,408 Holmium complexes 1,3-diketones, 2,387 phenanthroline, 3,1069 Homoazaporphyrins synthesis, 2, 817 Homolytic cleavage catalysis... 

Pathway 1 may not be relevant to Tyr catalysis homolytic cleavage of the peroxide O—O bond is presumably not the case in Tyr because the two copper ions are held by the protein chains so as to reverse the O—O bond cleavage. But pathway 2 can occur in Tyr as well. A proposal for the mechanism of Tyr is illustrated in Fig. 22. In Tyr, only one phenol can be inserted into the substrate-binding pocket in the protein,... 

EPR experiments have shown that the redox ability of WZ catalysts is sufficient to initiate a homolytic cleavage of C-H bonds in alkanes. Exposure of a WZ catalyst to n-pentane at 523 K led to the formation of W5+ species and organic radicals on the surface.27 The formation of organic radicals also occurred when WZ catalysts interacted with other hydrocarbons, including benzene.31 We therefore infer that one-electron transfer, although it is not regarded as a step in the catalytic cycle, can initiate catalysis by a process that leads to the formation of the carbenium ion chain carriers,27 as also occurs in acidic solutions.32 We emphasize that a strong redox reactivity is necessary but not sufficient for the catalytic activity of WZ. 

Classical catalysis of the addition by a redox system utilizes iron or copper salts. The usual catalyst is copper(I) chloride which is slowly oxidized by moist air to yield a green mixture of copper(I) and copper(II) salts. The example of the addition of 1,2-dibromo-l-chloro-l,2,2-trifluoroethane to oct-l-eiie to give 5 shows selective carbon-halogen homolytic cleavage." " ... 

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... 

Proper kinetic and thermodynamic meshing of the reactants is necessary for hydride transfer from a donor to an acceptor. There are at least three obviously different mechanisms by which hydride equivalents can be transferred. These are (i) concerted transfer of the proton and two electrons (equation 5) (ii) homolytic cleavage of the carbon-hydrogen bond followed by subsequent transfer of an electron (equation 6) and (iii) initial loss of a proton followed by transfer of two electrons, either together or stepwise (equation 7). Fusion of pathways is imaginable, (tependent on the structures of the participating molecules and possibilities for catalysis. 

When RX is easily reduced, as in the case of allyl iodides and benzyl bromides, the competing further reduction of the intermediate radical is suppressed and radical reactions such as dimerization, addition to double bonds and aromatic compounds or reaction with anions can be favored. The radical pathway can be also promoted by catalysis with reduced forms of vitamin Bn, cobaloximes or nickel complexes. These react with the alkyl halide by oxidative addition and release the alkyl radical by homolytic cleavage. 

The above homolytic cleavage of the 0-0 bond has an extensive chemistry, involving also catalysis by metal ions or complexes. The spontaneous autoxidation of liquid hydrocarbons (without added initiator) is due to the slow formation of radicals via the thermodynamically unfavorable reaction ... 

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