Hydrogen atom, free-radical transfer

This conforms to the common rale of free radical reactions in radical substitution reactions, i.e. when reactions between free radicals and molecules lead to atom (most often, hydrogen atom) or radical transfer from a molecule to a free radical, and the reaction progresses in the direction of lesser active radical formation. 

Cobalt porphyrin complexes are involved in the chain transfer catalysis of the free-radical polymerization of acrylates. Chain transfer catalysis occurs by abstraction of a hydrogen atom from a grow ing polymer radical, in this case by Co(Por) to form Co(Por)H. The hydrogen atom is then transferred to a new monomer, which then initiates a new propagating polymer chain. The reaction steps are shown in Eqs. 12 (where R is the polymer chain. X is CN), (13), and (14)." ... 

One step or two-step transfer Another major question about dehydrogenases is whether the hydrogen atom that is transferred moves as a hydride ion, as is generally accepted, or as a hydrogen atom with separate transfer of an electron and with an intermediate NAD or NADPH free radical. In one study para-substituted benzaldehydes were reduced with NADH and NAD2H using yeast alcohol dehydrogenase as a catalyst.30 This permitted the application of the Hammett equation (Box 6-C) to the rate data. For a series of benzaldehydes for which o+ varied widely, a value... 

Two distinct peripheral processes can be identified, once again mirroring the outer and inner sphere mechanisms just mentioned. The first involves simple orbited overlap between the reactant s rr-system and that of the porphyrin near the mrao-position. No o-bonds are formed or broken, only electron transfer (i.e. an outer sphere process) takes place, and as such is likely to be reversible. The second process involves transfer of an atom along with an electron (i.e. an inner sphere process). The species can be hydrogen or a free radical. Transfer is likely to be irreversible and involves formation and breaking of a-bonds at the meso-position. Both of these peripheral processes require the metal to be in conjugation with the porphyrin TT-system for oxidation of the metal to take place. This happens... 

Two types of sensitizers are recognized for photosensitized oxidation. These sensitizers can proceed by two pathways. A Type I sensitizer serves as a photochemically activated free radical initiator. The sensitizer in the triplet state reacts with the lipid substrate by hydrogen atom or electron transfer to form radicals, which can react with oxygen (1). The hydroperoxides produced... 

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. 

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). 

If a vinyl monomer is polymerized in the presence of cellulose by a free radical process, a hydrogen atom may be abstracted from the cellulose by a growing chain radical (chain transfer) or by a radical formed by the polymerization catalyst (initiator). This leaves an unshared electron on the cellulose chain that is capable of initiating grafting. As cellulose is a very poor transfer agent [10], very little copolymer results from the abstraction of hydrogen atoms by a growing chain radical. The... 

Hydrogen-Atom Transfer. Many oxidation and reduction reactions are free-radical substitutions and involve the transfer of a hydrogen atom. For example, one of the two main propagation steps of 14-1 involves abstraction of... 

Establishment of a free radical mechanism via H-atom transfer for hydrogenation using HMn(CO)5 (see Section II,D), and possibly also HCo(CO)4 (see Section II,C), suggests that more serious consideration for such mechanisms should be given for other hydridocarbonyl catalyst systems, and indeed for other homogeneous catalysts systems in general. The pentacyanocobaltate(II) catalyst can certainly operate by such a mechanism (see Section II,D). 

Free Radical Substitution and Hydrogen Atom Transfer Reactions... 

FREE RADICAL SUBSTITUTION AND HYDROGEN ATOM TRANSFER REACTIONS... 

Termination can take place either by the combination of two free radical chains to form one molecule or by transfer of a hydrogen atom in a disproportionation reaction. 

Sulfenic acids undergo hydrogen atom transfer to free radicals extremely readily (Koelewijn and Berger, 1972), rate constants of at least 107 M-1 s-1 being observed for (18a) when R is a peroxy radical. Block and O Connor (1974a) believe the marked antioxidant activity of most thiolsulfinates is... 

---