Homolysis accelerated

Chromophores (Ch) are transformed after absorption of the actinic solar radiation in excited singlet ( Ch ) and triplet (3Ch ) states (Rabek, 1996) (Eq. 3-11). Excited chromophores sensitize the formation of macroalkyls from the matrix polymer (Eq. 3-12a) and singlet oxygen from the ground state oxygen (Eq. 3-12b) and accelerate homolysis of POOH via an exciplex (Eq. 3-12c), Reaction scheme 3-3. 

An accelerated homolysis is defined as one that proceeds at an accelerated rate over that expected for a simple unimolecular homolysis. If a compound A-B undergoes unimolecular homolysis, eq 5, the rate constant can be predicted from the Arrhenius equation, eq 6, where BDE(A-B) is the bond dissociation energy of the A-B bond (18). However, if AB undergoes an assisted homolysis... 

The failure to observe any radical products 7 or acetone) in the decomposition of 6 allows us to rule out a mec Hanism for the anchimerically accelerated homolysis of 3 which begins with a biphilic insertion of sulfenyl sulfur into the 0-0 bond of 3 to yield 6 directly with 6 serving as the precursor to radical products. "Analogous bipliilic insertion reactions have been used to produce sulfuranes from the reaction between sulfoxylates and dioxetanes (18). 

Table 15 shows that peroxyester stabiUty decreases for the alkyl groups in the following order tert — butyl > tert — amyl > tert — octyl > tert — cumyl > 3 — hydroxy — 1,1 dimethylbutyl. The order of activity of the R group in peroxyesters is also observed in other alkyl peroxides. Peroxyesters derived from benzoic acids and non-abranched carboxyUc acids are more stable than those derived from mono-a-branched acids which are more stable than those derived from di-a-branched acids (19,21,168). The size of the a-branch also is important, since steric acceleration of homolysis occurs with increasing branch size (236). Suitably substituted peroxyesters show rate enhancements because of anchimeric assistance (168,213,237). 

Methylmalonyl-CoA mutase (MCM) catalyzes a radical-based transformation of methylmalonyl-CoA (MCA) to succinyl-CoA. The cofactor adenosylcobalamin (AdoCbl) serves as a radical reservoir that generates the S -deoxyadenosine radical (dAdo ) via homolysis of the Co—C5 bond [67], The mechanisms by which the enzyme stabilizes the homolysis products and achieve an observed 1012-fold rate acceleration are yet not fully understood. Co—C bond homolysis is directly kineti-cally coupled to the proceeding hydrogen atom transfer step and the products of the bond homolysis step have therefore not been experimentally characterized. 

The role of the base in this case is not clear, but in catalytic Mn porphyrins the addition of imidazole accelerates the peroxide O—O bond homolysis [105],... 

Usanov and Yamamoto recently found that catalytic amounts of Co(TPP) 367 led to a dramatic rate acceleration of Nozaki-Kishi-Hiyama reactions catalyzed by chromium complex 368 (Fig. 101) [460]. The authors attributed the rate enhancement to initial reduction of 367 to a Co(I) complex. The latter is able to undergo an Sn2 substitution at propargyl bromide 366 giving an allenylCo(ffl) species. It was proposed that its homolysis leads to allenyl radical 366A, which couples to Cr(II) complex 368. The resulting allenyl Cr(III) complex adds in an SN2 process... 

During weathering, phenolic antioxidants are photooxidized into hydroperoxycy-clohexadienones, such as 59 (Pospisil, 1993 Pospisil, 1980). The presence of peroxidic moieties in 57 and 59 renders them thermolabile at temperatures exceeding 100 °C and photolysable under solar UV radiation. Both processes account for homolysis of the peroxidic moieties. As a result, the oxidative degradation of the polymeric matrix is accelerated by formed free-radical fragments (tests were performed with atactic polypropylene and acrylonitrile-butadiene-styrene terpolymer (ABS) (PospiSil, 1981 PospiSil, 1980). Low-molecular-weight products of homolysis, such as 60 to 63 are formed in low amounts. 

Vlasie M, Banerjee R (2003) Tyrosine 89 Accelerates Co-Carbon Bond Homolysis in Methylmalonyl-CoA Mutase. J. Am. Chem. Soc. 125 5431-5435... 

As noted above, dioxygen reacts with organic molecules, e.g. hydrocarbons, via a free radical pathway. The corresponding hydroperoxide is formed in a free radical chain process (Fig. 4.3). The reaction is autocatalytic, i.e. the alkyl hydroperoxide accelerates the reaction by undergoing homolysis to chain initiating radicals, and such processes are referred to as autoxidations [1]. 

The role of coenzyme B12 as cofactor of enzymatic rearrangement reactions requires the (reversible) formation of the 5 -adenosyl radical by homolysis of the organometal-lic bond of (3) (estimated bond-dissociation energy ca. 30kcalmol to be accelerated there by a factor of... 

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... 

Secondary initiation (and sometimes spectacular acceleration of rate) may well occur through formation and subsequent homolysis of a molecular hydroperoxide such as CH3CO3H, particularly as the temperature is increased above 400 K. Marked autocatalysis and lengthy induction periods may result. Near quantitative yields of CH3CO3H have been observed [15] consistent with the following type of sequence ... 

The substantial acceleration of radical ion formation observed in the homolysis of peroxide bonds in orf/zo-substituted sulfenyl perbenzoates, e.g. (41), is in agreement with the postulated formation of bridged radicals (42) (Figure 7). 

As discussed in the introduction, chlorine substituents may be expected to influence the autoxidation of organic substrates as a result of their electron-directing properties, but these effects may be complex. Thus, for example, Kulicki [161] has reported that halogen and nitrate substituents, particularly in the ortho position, have an effect on the autoxidation of cumene by inhibiting the primary oxidation, but they also accelerate the homolysis of any hydroperoxide that is formed the net result is an overall acceleration of oxidation. Similar effects were noted by Kovalev and... 

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