Free radicals hydrogen abstraction reactions

A study of the photodeconjugation reaction of the ester (136) in the presence of a variety of optically active amines such as (IR,2S)-l-phenyl-2-isopropylamino propanol has shown that the product can be obtained with an enantiomeric excess of about 70 X. Irradiation of the alkynylketones (137) in alcohols (methanol, ethanol or propanol) affords the furan derivatives (138) by a free radical hydrogen abstraction path. Excitation of the cyclohexenone derivatives (139) yields the cyclized products (140) the structures of which were verified by X-ray crystallography. The formation of the amide products is reminiscent of a Norrish Type II process. 

The reactivity and specific behavior of free radicals produced during initiator s thermal decomposition strongly depend on the type of the radicals formed, which is determined by the nature of peroxide (28,37). Table 10.2 lists primary and secondary radicals formed during the decomposition of an initiator, while Table 10.3 gives data on the activity of certain types of free radicals in abstraction reactions of hydrogen atoms from carbon (33). Primary radicals are formed directly at breakdown of an initiator molecule secondary radicals result from transformations of primary radicals by a monomolecular mechanism. 

Pertubation interactions between FMOs have been utilized to predetermine the regioselectivity of allylic and benzylic hydrogen-abstraction reactions. Thus, it is shown that, in unsaturated compounds containing two non-conjugated and non-equivalent r-systems, one system contributes mainly to the HOMO and the other to the HOMO-1. The important FMO interaction in these free-radical hydrogen-abstraction processes is SOMO-HOMO. Analysis of four representative examples corroborates empirical evidence and previously published studies/ ... 

The peioxy free radicals can abstract hydrogens from other activated methylene groups between double bonds to form additional hydroperoxides and generate additional free radicals like (1). Thus a chain reaction is estabhshed resulting in autoxidation. The free radicals participate in these reactions, and also react with each other resulting in cross-linking by combination. 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

Fundamental studies of coal liquefaction have shown that the structure of solvent molecules can determine the nature of liquid yields that result at any particular set of reaction conditions. One approach to understanding coal liquefaction chemistry is to use well-defined solvents or to study reactions of solvents with pure compounds which may represent bond-types that are likely present in coal [1,2]. It is postulated that one of the major routes in coal liquefaction is initiation by thermal activation to form free radicals which abstract hydrogen from any readily available source. The solvent may, therefore, function as a direct source of hydrogen (donor), indirect source of hydrogen (hydrogen-transfer agent), or may directly react with the coal (adduction). The actual role of solvent thus becomes a significant parameter. 

Comparison of the behavior of free (mono) radicals with that of electronically excited sensitizers has led to the assumption that the excited sensitizer, Sensrad, is best described as a phototropic-isomeric diradical. Thus, photodimerization and photodehydrogenation, exhibited by certain sensitizers in the absence of oxygen, reflect radical-combination and hydrogen-abstraction reactions. Furthermore, fluorescein (a photosensitizer) becomes paramagnetic when excited... 

REACTION WITH FREE RADICALS HYDROGEN ATOM ABSTRACTION AND ONE- OR THREE-ELECTRON BONDING... 

Photoinitiation of polymerization can be obtained through a variety of photochemical reactions which produce reactive free radicals. These radicals then lead to the formation of the polymer chains through the addition of further monomer units to the end of a chain in a sequence of radical addition reactions (Figure 6.10). A photoinitiator of polymerization is therefore a molecule which produces free radicals under the action of light. Benzo-phenone and other aromatic ketones can be used as photoinitiators, since a pair of free radicals is formed in the hydrogen abstraction reaction. Some quinones behave similarly, for example anthraquinone in the presence of hydrogen donor substrates such as tetrahydrofuran. 

The reaction of silylene centers with dihydrogen has been studied in some detail346. It appears to proceed by a free-radical chain mechanism, initiated by radical sites on the silica surface which abstract hydrogen from H2. The H atoms react with the silylene center to give a free radical, and a reaction chain can then ensue (equations 103-105). The hydrogen addition to silylene centers is reversible the hydrogen is completely removed at temperatures near 1000 K. 

Reid DL, Armstrong DA, Rauk A, Nese C, Schuchmann MN, Westhoff U, von Sonntag C (2003) H-atom abstraction by C-centered radicals from cyclic and acyclic dipeptides. A theoretical and experimental study of reaction rates. Phys Chem Chem Phys 5 3278-3288 Roberts BP (1996) Understanding the rates of hydrogen abstraction reactions empirical, semi-em-pirical and ab initio approaches. J Chem Soc Perkin Trans 2 2719-2725 Russell GA (1973) Reactivity, selectivity, and polar effects in hydrogen atom transfer reactions. In Kochi JK (ed) Free radicals. Wiley, New York, pp 275-331 Russo-Caia C, Steenken S (2002) Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution. Phys Chem Chem Phys 4 1478-1485... 

The ratio of 2,2-dimethyl butane to 2-methyl pentane produced by these reactions will be k5a/k5h. In general, chlorine atom is less selective in hydrogen abstraction reactions than are hydrocarbon free radicals and hence fc2a/ 2b > > k5a/k5h. Consequently, one would expect that the first increment of HCl would decrease the ratio of 2,2-dimethyl butane to 2-methyl pentane in the C6 alkylation product. 

As an odd-electron molecule, many of the reactions of nitrogen dioxide are typical of free radicals. This applies particularly to its association with atoms such as, H, F, and Cl or other radicals such as, the OH, NO, alkyl, and alkoxy radicals its addition reactions with unsaturated systems such as, olefins, acetylenes, and aromatic compounds, and its hydrogen abstraction reactions. The ability of the NO2 free radical to abstract hydrogen from an organic molecule ... 

The PVK copolymer blend shows a lower rate and less sensitization of degradation in the homqpolymer. The PVK copolymer is known to have a low yield of type I reaction 0 and this supports the idea that tiie sensitization of photooxidation is due to free radical formation rather than more elaborate mechanisms such as via formation of singlet oxygen. It also appears that the hydrogen abstraction reaction of excited phenone groups (1 which is the basis of the use of benzo-phenone as a prodegradant in polystyrene, is not as important in this system. 

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