Hydrogen atom transfer, and

In contrast to phenols (Section 5,3.4), phenothiazine (39) is reported to be an excellent scavenger of both carbon-centered and oxygen-centered radicals by hydrogen atom transfer and is also used to stabilize monomers in storage.198... 

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

The mechanism shown in Scheme 4.9 has been proposed for the hydrogen atom transfer from phenols (ArOH) to radicals (Y ) in non-aqueous solvents, a kinetic effect ofthe solvent (S) being expected when ArOH is a hydrogen bond donor and the solvent a hydrogen bond acceptor. Steps with mechanistic rate constants k, k-1 and k>, involve proton transfer (the latter two near to the diffusion-controlled limit), and kj involves electron transfer. The step with rate constant fco involves a direct hydrogen atom transfer, and the other path around the cycle involves a stepwise alternative. 

This approach has been taken for the reaction of chlorinated ethenes with Zn° [125,165] and Fe° [88,166], resulting in separate rate constants for all the reactions shown in Fig. 3. Care must be taken in using these parameters in predictive modeling, however, as it is not yet known how sensitive the relative values of these rate constants are to pH, thickness and composition of the oxide film, etc. The same caution applies where the approach represented by Eq. (25) is used to describe parallel mechanisms of transformation. For example, it has recently been reported that several experimental factors influence the relative contributions of dissociative electron transfer, hydrogen atom transfer, and reductive elimination to the dechlorination of carbon tetrachloride and TCE by Fe° [177],... 

I, 4-dehydrobenzene intermediates. Trapping by inter- and intramolecular hydrogen atom transfer and observation of high-temperature CIDNP. 

Klingler RJ, Rathke JW. High pressure NMR investigation of hydrogen atom transfer and related dynamic processes in 0x0 catalysis. J Am Chem Soc 1994 116 4772-4785. 

SOS I 17 The Relation between Hydrogen Atom Transfer and Proton-coupled Electron Transfer... 

Several types of thermally induced monomolecular reactions of silenes have been reported (Chart 8) (a) geometrical isomerization, (b) 1,2-shift, (c) other sigmatropic shifts, (d) electrocyclic ring closure and opening, (e) ene and retro-ene processes, (f) hydrogen atom transfer and (g) nucleophilic attack. 

As noted above in the introduction, in principle, carotenoids can react with free radicals in a number of ways, namely electron transfer, hydrogen atom transfer and addition ... 

The reactions described above and published in 1971 are the first compelling experimental evidence showing that the observed intermolecular hydride shift consists, first, in an SET, followed immediately by a hydrogen-atom transfer, and that this observation has been made possible on account of steric hindrance in the latter process. This work has been overlooked in all recent SET and hydride-shift articles and reviews. 

A review of the phototochemistry of A-oxides has appeared and this includes a section on the photo-induced deoxygenation of heterocyclic A-oxides. A second review deals with the mechanistic aspects of the photochemistry of bicyclic azoalkanes and considers their photo-reduction by hydrogen donors, a process for which hydrogen-atom transfer and CT mechanisms have been suggested. 

Decomposition without hydrogen-atom transfer and with formation of monomeric products ... 

Another example is the photolysis of mercaptoles. Compound 139 yielded the rearranged product 140 cis-trans mixture, ratio 8 1). Its formation is best explained by a homolytic scission of the C—S bond, followed by hydrogen atom transfer and addition of the thiol to the double bond . 

---