Hydrogen atom transfer from solvent

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. 

The use of benzene as a solvent eliminates the chlorinated organic products in Table II. This point and the products in the table are consistent with radical abstraction (hydrogen atom transfer) from the substrate, cyclohexene, by the high valent (and likely oxometal) intermediate form of the TMSP complexes, followed by chlorine abstraction from the solvent by intermediate organic radicals. Separation and analysis of the two phases after the reaction reveals that the polyoxometalate is intact. 

Both of the above reaction pathways are suppressed if 443-D and perdeuteriated solvents are used simultaneously in cyclization reaction 265. Mainly oligomer and less than 5% of alkane 450 were then produced. This has been explained by different reactivities of radical 445 formed directly from 444 or formed by abstraction of hydrogen atom from 443 through complex 453. The more energetic radicals formed by hydrogen atom transfer from 443, add intermolecularly to an alkene and initiate preferentially oligomerization512, while... 

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. 

Scheme 4.9 Proposed mechanism for hydrogen atom transfer from phenols to radicals in non-aqueous solvents [19],...

Ingold and coworkers have also expressed a kinetic equation (equation 43) to determine the equilibrium constant ( T ) for hydrogen bonding between the phenolic antioxidant and dilute HBA solvent. A, if rate constants in neat HBA and non-HBA solvent, nA, CCI4 in this example, are known. For this study they monitored hydrogen-atom transfer from the antioxidant phenol to cumyloxyl radicals . ... 

A photoreductive scheme was proposed in which the triplet state of the ketone (3B) first abstracts a hydrogen atom from the solvent or from a hydrogen donor (YH2) to give a ketyl radical (BH). Hydrogen atom transfer from the ketyl radical to the azo linkage of the dye (D) leads to the formation of a hydrazl radical (DH), which is finally reduced to the colourless hydrazobenzene (DH2) as follow ... 

The product ratio can be explained by a hydrogen atom transfer from one M (cod) species to another M cod) species. An ET disproportionation reaction between two M cod) species to give M cod) and M (cod) followed by a proton transfer from an allylic position of M cod) to M (cod) was ruled out (182, 183). Electrochemical data suggest that the solvent is easier to oxidize... 

The two ketyl units that are part of the ion quadruplet undergo (1) interionic dimerization to give pinacolates, (2) intraionic disproportionation via hydrogen atom transfer from one unit to the other to give alcoholate and enolate, and (3) intermolecular hydrogen atom abstraction from the solvent molecule (THF) in the solvent shell to provide the alcoholate. Thus ketyl radicals appear to undergo the three basic reactions that are characteristic for free radicalsld. 

Oxidations of arylalkanes by "Bu4NMn04 in toluene solvent led to the conclusion that rate-limiting hydrogen atom transfer from the substrate to a permanganate oxo group is consistent with all of the experimental evidence [69a] ... 

Reactions involving a hydrogen atom transfer from a solvent or a coordinated group. 

Hydrogen atom transfer from BH solvent to tbe 4-nitropbenolate and formation of 4-AP, and Desorption of products from the surface of the catalyst. 

Only a-hydrogen atoms are exchanged because the reaction occurs through an enolate ion intermediate. All possible enolizable hydrogen atoms are eventually replaced. The hydrogen atoms are lost in the solvent, which contains many more deuterium atoms than the number of hydrogen atoms transferred from the carbonyl compound. 

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]

Oxidation of carboxylate ions in homogeneous solution using some one-electron transfer agents gives in varying proportions the Kolbe dimer and the product from hydrogen atom abstraction from the solvent by the intermediate alkyl radical. Persulphate ion [109], hexachloco-osmate(v) [110] and the radical-cation from tris(4-bromophenyl)amine [111] all have been used to promote this reaction. 

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