Radical reactions hydrogen atom transfer

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. 

Keywords Bond dissociation energies Catalysis Chain reactions Hydrogen atom transfer Metal-hydrogen bonds Radicals... 

It seems obvious to assume a priori that the facility of the transfer reaction depends on the strength of the broken and newly formed bond between radical and hydrogen atom transferred. The dependence between the activation energy E and dissociation energy D of the broken bond R —H ... 

Finally, some termination step occurs, two of which are shown in the scheme. The most common is coupling, in which two radicals combine, leading to one larger macromolecule. Polystyrene radicals typically undergo termination by coupling. Another reaction that is common with some monomers (e.g., methyl methacrylate) is called disproportionation in which on the reaction of two radicals, a hydrogen atom transfers from one species to the other. 

Radical transfer reactions involve abstraction of an atom or group B by a radical A from a molecule B-C (reaction 6.24). B is nearly always an atom transfer of a group, which would correspond to substitution at a polyvalent atom, though important in nucleophilic and electrophilic reactions, is very uncommon in radical reactions. The atom transferred is almost always a hydrogen or a halogen atom. 

Iminyl radicals are particularly useful for the construction of nitrogen heterocycles, since the cyclization products are functionally disposed for further elaboration into a variety of useful systems. Zard and coworkers have pioneered the development of methodology for the generation and cyclization of iminyl radicals (Table 8) [5, 71]. Iminyl radicals cyclize one order of magnitude more rapidly than the related neutral aminyl radicals but react less rapidly than the aminyl radicals with hydrogen atom transfer trapping agents, and one would predict that iminyl radicals formed in chain reaction sequences could prove to be as versatile as carbon radicals [78]. 

Transfer/Abstraction Reaction (hydrogen-atom transfer reaction between methyl radical and ethane) ... 

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. 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

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). 

It is important to emphasize that the hydroxy dithioketal cyclization can be conducted under mild reaction conditions and can be successfully applied to a variety of substrates.15 However, the utility of this method for the synthesis of didehydrooxocane-contain-ing natural products requires the diastereoselective, reductive removal of the ethylthio group. Gratifyingly, treatment of 13 with triphenyltin hydride and a catalytic amount of the radical initiator, azobisisobutyronitrile (AIBN), accomplishes a homolytic cleavage of the C-S bond and furnishes didehydrooxocane 14 in diastereo-merically pure form (95 % yield), after hydrogen atom transfer. 

To avoid these stability problems, it is necessary to minimize the proportion of chains that terminate by radical-radical reaction. One way of achieving this is to conduct the polymerization in the presence of an appropriate chain transfer agent. For example, if polymerization is performed in the presence of a H-donor chain transfer agent, conditions can be chosen such that most chains terminate by hydrogen-atom transfer. Bagby et al.iA examined the thermal stability of PMMA formed with dodecanethiol. These polymer chains will then possess, more... 

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... 

These reactions result in iodine atom transfer and introduce a potential functional group into the product. The trialkylborane method of radical generation can also be used in conjunction with either tri-n-butyl stannane or fnT-(trimethylsilyl)silane, in which case the product is formed by hydrogen atom transfer. 

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... 

Free Radical Substitution and Hydrogen Atom Transfer Reactions... 

Alkoxyl radicals are very active and rapidly enter into bimolecular reaction (see Chapter 2). Moreover, alkoxyl radicals with sufficiently long alkyl substituents react with intramolecular hydrogen atom transfer, for example [37] ... 

The intramolecular hydrogen atom transfer occurs with lower activation energies in comparison with the intermolecular transfer (see the values of Ee for both types of reactions in Table 6.11). The values of the activation energies of intramolecular radical H-atom abstraction calculated by the IPM method are given in Table 6.15. 

FREE RADICAL SUBSTITUTION AND HYDROGEN ATOM TRANSFER REACTIONS... 

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