Formation from hydrogen abstraction

These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398). 

Gassman and Cryberg solvolyzed 125 in a number of solvents containing methanol.170 The predominant products were 126, 127, and 128. The first two, 126 and 127, are solvolysis products derived from rearrangement to electron-deficient nitrogen. But 128 almost surely results from hydrogen abstraction from the solvent. The proposed mechanisms for their formation are shown in Scheme 11. 

As in the carbon series, this photolytic rearrangement involved a nitrene intermediate. Indeed, in the case of thio phospho-razides, beside the formation of dimers 3 and polymers, the product was characterized and arises from hydrogen abstraction from the solvent by the nitrene intermediate. 

Hydrogen chain transfer reaction, which may occur as intermolecular or intramolecular processes, leads to the formation of oleflnic species and polymeric fragments. Moreover, secondary radicals can also be formed from hydrogen abstraction through an intermolecular transfer reaction between a primary radical and a polymeric fragment. 

Photoreduction of benzophenone by isopropanol is accompanied by the formation of a coloured compound which Filipescu and Minn (59) proposed was an adduct (see below) of the radicak resulting from hydrogen abstraction. These authors further proposed that this adduct was a key intermediate in the photoreduction, semipinacol radicals being generated by its reactions with ground state benzophenone. 

Two papers have been presented on the photochemistry of 5-methylphena-zinium salts in aqueous solution. Fluorescence, optical flash photolysis, and electron paramagnetic resonance (e.p.r.) techniques have been used to elucidate various aspects of product formation and quantum yield. Two products have been identified, namely the 5-methyl-10-hydrophenazinium cation radical (MPH ) and the pyocyanine (l-hydroxy-5-methyl-phenozinium) cation (PyH ) in a stoicheiometric ratio of 2 1. The quantum yield of formation of (MPH ) was found to be 0.29 0.03 at pH 7.0 and 1.1 0.1 at pH 3.0. The triplet state of MP (Ti) has also been detected by triplet-triplet absorption and is found to have a lifetime of 0.5 ns. Flash photolysis and e.p.r. have also been used to study a geminate triplet radical pair obtained from hydrogen abstraction by excited triplet acetone from propan-2-ol. The authors demonstrate that the geminate pairs contribute most of the polarization in photochemically-induced dynamic electron polarization (CIDEP) as compared with free random-phase pairs. 

Figure 10 shows the O NMR spectra of poly(isoprene) gamma irradiated in the presence of O-enriched oxygen. The authors have identified classes of peaks through comparison with model compounds it is conceivable that in future a database of O chemical shifts will aid researchers in this area. The authors were able to show that irradiation of poly(isoprene) in oxygen results in the formation largely of alcohols and ethers from hydrogen abstraction and termination reactions, respectively, of alkoxy radicals. These radicals are products of homolysis of hydroperoxy radicals. 

The first chemists working in the area of the carbonylation of nitro compounds immediately noted many similarities between the products obtained from these reactions and the ones obtained from the reactions of organic azides (R-N3), which were known to be sources of free nitrenes when activated either photochemically or thermally (for some examples see [255, 256]). Similarities were especially found in the formation of coupled products such as azoarenes and in the formation of products derived from hydrogen-abstraction from the solvent or the nitro compound itself [255], a reaction typical for nitrenes in the triplet state [257]. It was also noted that high pressures were required to drive the reactions of both azides and nitro compounds with CO towards the synthesis of isocyanates rather than towards hydrogen-abstraction-derived products and this... 

Benzophenone is a common photosensitiser. Free radical formation by hydrogen abstraction from solvent leads to use in UV-curing applications such as inks, imaging, and clear coatings in the printing industry. By way of contrast, hydroxy benzophenones with the OH group adjacent to the carbonyl are useful photostable UV absorbers because reversible proton transfer is an efficient deactivation route leading to very short lived excited-states. 

Product mixtures from radical-chain addition of hydrogen chloride to alkenes are much more complicated than is the case for addition of hydrogen bromide. The problem is that the rate of abstraction of hydrogen from hydrogen chloride is not fast relative to the rate of addition of the alkyl radical to the alkene. This results in the formation of low-... 

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

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