Hydrogen atom transfer from benzene

In a photochemical experiment, irradiation of benzene leads to Sj, which connects to the ground-state surface via the conical intersection shown. Benzene, the much more stable species, is expected to be recovered preferentially, but the prebenzvalene structure which hansfomis to benzvalene is also fomied. Another possible route from the prebenzvalene, along a different coordinate, will lead to fulvene [90, p.357] after a hydrogen-atom transfer from... 

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. 

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. 

Srivastava et al. showed that the parent nitrenium ion is capable of adding to simple aromatics such as benzene, toluene and anisole (91) to give aniline derivatives (92a-c, Fig. 13.47). Unfortunately, these reactions give a distribution of regio-isomers as well as products derived from competing hydride and/or hydrogen atom transfer. 

The case of benzoin alkyl ethers illustrated in Figure 8.15 is a remarkable example of the effect of complexation with cyclodextrins. Such molecules normally undergo homolytic dissociation in solution (the Norrish type 1 process described in section 4.4) and there is practically no intramolecular hydrogen atom abstraction (Figure 8.15). When the benzoin alkyl ethers are complexed with a cyclodextrin to form a 1 1 association, it can be shown that one of the phenyl rings fits inside the cyclodextrin cavity in aqueous solution. When the solid complex is irradiated only the photoproducts resulting from hydrogen atom transfer are detected the opposite behaviour from irradiation of the crystal of benzoin alkyl ether as well as of solutions in benzene. 

The quantum-yields for reaction of compounds 120a, 120b, and 120 c in benzene solution were 0.054, 0.50, and 0.57 (0.62 in t-butyl alcohol) respectively 100). Rates of hydrogen atom transfer were obtained from Stem-Vollmer treatment of quenching... 

Hydrogen atoms add to benzene rather than abstracting H2 therefore does not come from hydrogen atoms. Burns " has shown that there is a dependence of G c,h . andGcjHi on the linear energy transfer of the system studied, with the C -values increasing for increased let. This supports the occurrence of reactions (9) to (11) in the track of the radiation. 

Some papers report on the influence of aromatic compounds on the polymerization of vinyl compounds other than vinyl acetate. Mayo et al.48 found that bromo-benzene acts as a chain transfer agent in the polymerization of styrene, although no fragments of bromobenzene are incorporated into the polymer. They concluded that a complex is formed between the solvent molecule and either the propagating poly-styryl radical or hydrogen atom derived from the latter. 

Useful synthetic methodologies are based on the cyclization or rearrangement of the nitrogen-centered radicals generated in the reaction of the appropriate amides with (diacetoxyiodo)benzene in the presence of iodine [652-655]. Specific examples are illustrated by the synthesis of bicyclic spirolactams 622 from amides 621 [653] and preparation of the oxa-azabicyclic systems (e.g., 624) by the intramolecular hydrogen atom transfer reaction promoted by carbamoyl and phosphoramidyl radicals generated from the appropriately substituted carbohydrates 623 (Scheme 3.244) [654],... 

The enthalpy of activation for the turnover-limiting hydrogen-atom transfer step depends upon the affinity of tire anthracene substrate for free radicals. The 9,10-dihydroanthracene is formed as the product because the radical is most stable in these positions, and the mixture of stereoisomers is formed because the process does not involve migratory insertion. This mechanism also explains why simple benzene derivatives are imreactive. The 1,4-hexadienyl radical lies too far uphill of benzene and the cobalt hydride to be a product from a reaction of a catalytic cycle that occurs under mild conditions. 

Photodecomposition of some ortfeo-substituted aryldiazonium ions leads to unusual reaction products due to hydrogen atom transfer between the ortho-substituent and the radical center derived by loss of nitrogen from the diazonium ion. Thus, 2-diethylaminobenzenediazonium salts yield N-ethylaniline via the intermediate 3. Deuterium is not incorporated into the benzene ring from either DjO or CD3OD.2 Decomposition of the diazonium salt 4 leads to a benzocyclobutene as the principal product. The influence of the nitro substituent is important, because the un-nitrated diazonium salt is decomposed to give a phenol in the normal manner. ... 

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