Hydrogen atom transfer mechanism

The kinematic theory can now be used to determine the appropriate values of Js, lQtotal, and It (Equations 15, 16, and 17, respectively) for various assumed values of k, the rate constant of Reaction O—a quantity about which nothing is known—and for the various mechanisms. Obviously from the chemistry of the system, Reactions U, V, and W cannot occur by a hydrogen atom transfer mechanism hence, only two cases need be considered—Reactions S, T, U, V, and W, occurring by... 

Suggestions that the sulphate catalysed paths may involve a mechanism with a sulphate-bridged activated complex, as opposed to a hydrogen-atom transfer mechanism, have been made. ... 

The last three entries in Table 13 reflect a marked preference for a-methylene versus methyl proton transfer for electron-withdrawing a substituents. These amines are also unusual in that they react with t in nonpolar solvents and do not display exciplex fluorescence. While this unusual behavior was initially attributed to a free radical hydrogen atom transfer mechanism leading to the formation of exceptionally stable "merostabilized" a-aminoallyl radicals (115), our current view is that the high kinetic acidity of the a-C-H bond of these amines when complexed with t is responsible for their behavior. 

Recent studies by Baciocchi et al. on kinetic deuterium isotope effect profiles and substituent effects in the oxidizing N-demethylation of iV,JV-dimethylanilines catalyzed by tetrakis (pentafluorophenyl)porphyrin also supports the electron-transfer mechanism and exclude the hydrogen-atom transfer mechanism in such processes [218]. 

P-450 amine dealkylation mechanisms. Because kn/ko values for A -demethylation of / -substituted A, A -dimethylanilines by cytochrome P-450 are nearly identical with those for the hydrogen abstraction reactions of t-BuO , it was proposed that P-450 reacted by a direct hydrogen-atom-abstraction mechanism (B) rather than a sequential electron-proton-electron-transfer mechanism (A) [78, 80]. Identical relationships between A h/ d values for reactions of t-BuO and P-450 has been expanded for other substrates including />-xylene, toluene, benzyl alcohol, and tertiary trialkylamine [78]. It has thus been suggested that all these P-450 reactions proceed by a common hydrogen-atom transfer mechanism [78]. 

A similar hydrogen atom transfer mechanism for the oxidation of some transition-metal complexes by C-centred radicals was proposed in Ref. 382. 

The trityl radical is able to abstract a hydrogen atom from OsH2(CO)4, yielding the 17-electron OsH(CO)4, which rapidly dimerizes to Os2H2(CO)g [33]. Although this intermediate was not directly observed, kinetic evidence is in agreement with the proposed hydrogen atom transfer mechanism [34,35]. 

These residues do not show CIDNP, whereas several other residues do (Tyr 76, Tyr 115, His 119) (20). This strongly suggests that in the cases of tyrosine and histidine free phenolic OH and imidazole NH protons are required for the photoreaction with flavin in accordance with the hydrogen atom transfer mechanism. 

As will be discussed later, two or more mechanisms have been proposed for the "net hydride transfer reactions in mimetic systems. One is, of course, the one-step "hydride transfer mechanism and the other is a multistep mechanism involving the initial "electron" transfer process. The latter mechanism is further subdivided into two categories the two-step electron-hydrogen atom transfer mechanism and the three-step electron-proton-electron transfer mechanism as shown in Scheme 8. 

Photolysis of bicyclo[2.2.2]octan-2-one (A) gives B in good yield. When A labeled as shown is used, the aldehyde group carries deuterium to the extent of 51.7%. Write a mechanism to account for the overall transformation. Calculate the isotope effeet for the step in which hydrogen-atom transfer occurs. What mechanistic conclusion do you... 

FIGURE 24.21 A mechanism for the methylmalonyl-CoA mntase reaction. In the first step, Co is rednced to Co dne to homolytic cleavage of the Co —C bond in cobalamin. Hydrogen atom transfer from methylmalonyl-CoA yields a methylmalonyl-CoA radical that can undergo rearrangement to form a snccinyl-CoA radical. Transfer of an H atom regenerates the coenzyme and yields snccinyl-CoA. 

Vitamin E actually consists of a family of compounds, the most active of which is a-tocopherol. The mechanism of the vitamin s action is not completely certain, but it seems likely that it might undergo hydrogen atom transfer reactions with free radicals to give a stable radical (see also Chapter 17, Problem 7). 

Other postulated mechanisms for spontaneous initiation include electron transfer followed by proton transfer to give two monoradicals, hydrogen atom transfer between a charge-transfer complex and solvent,110 and formation of a di radical from a charge-transfer complex, JJ[Pg.111]

Cases of addition-abstraction" polymerization have also been reported where propagation occurs by a mechanism involving sequential addition and intramolecular 1,5-hydrogen atom transfer steps (Section 4.4.3.4). 

The vinylsilanes (e.g. 40, 41) do not readily homopolymerize. Forsyth et al.Mj explored the mechanism of grafting these monomers using dodecane as a model for PE. Their work suggests that multiple monomer units are attached through a sequence of addition and intramolecular hydrogen atom transfer steps by a mechanism analogous to that shown in Scheme 7.33 on page 394. 

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

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. 

An understanding of kinetic acidity is necessary in order to distinguish such mechanisms from other ways in which hydrogen may become attached to a substrate, e.g., hydrogen atom transfer, reaction 8, and hydride transfer, reaction 9. 

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