Electron/hydrogen atom transfer reactions

To explore the spontaneity of electron/hydrogen atom transfer reactions AG° can be calculated by the following expression ... 

Quantification of antioxidant action usually relies on the reducing ability of antioxidants, measured either by electron transfer, reaction [16.15], or by hydrogen atom transfer reactions, reaction [16.16] ... 

In vitro tests, used in evaluation of antioxidant properties make use of the ability of antioxidants to quench free radicals. Based on this mechanism, the methods are divided into two groups SET - single electron transfer, and HAT - hydrogen atom transfer. Reactions with antioxidants in assays with the DPPH radical, ABTS and the Folin-Ciocalteu reagent both operate according to the SET and HAT mechanism. Due to the kinetics of the reaction, they are included in the... 

The 3(do po) excited state of the dS-d complexes has been shown to be involved in the photochemical hydrogen atom-transfer reaction. The atom-transfer reactivity of this state is attributed to the presence of a hole in the do orbital, analogous to the 3nir state of organic ketones. Interaction of the oxidizing hole wiUi the electron pair of the C-H bond is the presumed pathway. 

The other possible mechanism is the removal of a proton from the transition state concerted with the electron transfer. Overall, the equations can be combined and expressed as a hydrogen atom transfer reaction (equation 41) °. ... 

Electron-transfer reactions and hydrogen-atom-transfer reactions are physically possible with NADH and its analogs. Much evidence indicates such a mechanistic versatility in redox reactions for this class of compounds. 

Hydrogen atom transfer reactions involving 17-electron hydrides have been considered in a number of cases [54, 58, 86, 99, 112] as alternatives to proton and electron transfers, on the basis of the known atom transfer processes of hydride compounds of tin, germanium, and silicon. In addition, as discussed in section 6.4.3, there is some evidence for M-H bond weakening upon oxidation, suggesting that the homolytic rupture of this bond may take place under favourable circumstances. 

A property of superoxide to act as an oxidizing agent is much more ambiguous. Thermodynamically, an electron transfer to bare superoxide is almost impossible because the product of this reaction, peroxide dianion (02 ), is highly imstable. Therefore, the reduction of superoxide is either a proton-coupled process (Eq. (3)) or metal-assisted reaction (Eq. (4)), where the latter requires coordination of 02 and subsequent inner-sphere electron transfer. It is also possible to think in terms of hydrogen atom transfer reactions as a special sort of proton-coupled electron-transfer processes (Eq. (5)). 

Scheme 10 Interrelation and energetics of electron transfer, proton transfer, and hydrogen atom transfer reactions of CpFePeMe ) complexes in five different oxidation states. Adapted from Trujillo, H. A. Casado, C. M. Ruiz, J. Astruc, D. J. Am. Chem. Soc. 1999, 121, 5674-5686, with permission from American Chemical Society.

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. 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

In triethylamine instead of benzene the reaction products are completely different, and are indicative of a homolytic process involving an initial electron transfer from triethylamine followed by a hydrogen atom transfer. Scheme 10-68 gives the major products, namely 1,3,5-tri-tert-butylbenzene (10.36, 20%), the oxime 10.39 (18%), formed from the nitroso compound 10.38, and the acetanilide 10.37 (40%). ESR and CIDNP data are consistent with Scheme 10-68. In their paper the authors discuss further products which were found in smaller yields. 

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. 

In systems of this type, the electrochemical reactions can be realized or greatly accelerated when small amounts of the components of another redox system are added to the solution. These components function as the auxiliary oxidizing or reducing intermediates of the primary reactants (i.e., as electron or hydrogen-atom transfer agents). When consumed they are regenerated at the electrode. 

As with carbocation-initiated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant provide potential reaction sites. Cyclization may be followed by a second intramolecular step or by an intermolecular addition or alkylation. Intermediate radicals can be constructed so that hydrogen atom transfer can occur as part of the overall process. For example, 2-bromohexenes having radical stabilizing substituents at C(6) can undergo cyclization after a hydrogen atom transfer step.348... 

Roberts and co-workers have employed a number of chiral carbohydrate-derived thiols as polarity reversal catalysts in the radical hydrosilylation of electron-rich prochiral alkenes [68-70]. In these thiols, the SH group is attached to the anomeric carbon atom. Scheme 21 demonstrates the non-catalyzed reaction and in step b, the hydrogen atom transfer from the silane... 

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