Phenyl radicals, reactions

Phenyl radical reactions with 02, O, or H02 seem to be the most likely candidates for the first steps in the aromatic ring-breaking sequence [54, 61]. A surprising metathesis reaction that is driven by the resonance stability of the phenoxy product has been suggested from flow-reactor studies [54] as a key step in the oxidation of the phenyl radical ... 

It should be stressed that in case of the ethynyl-acetylene reaction, a molecular hydrogen loss channel synthesizing the 1,3-butadienyl radical is open as well. Since the reactions of cyano and ethynyl radicals have no entrance barrier, are exoergic, and aU transition states involved are lower than the energy of the separated reactants, these reaction classes are extremely important to form nitriles and complex unsaturated hydrocarbons in low-temperature environments. On the other hand, the corresponding phenyl radical reactions are—due to the presence of an entrance barrier—closed in those environments. However, the elevated temperature in combustion systems helps to overcome these barriers, thus making phenyl radical reactions important pathways to form aromatic molecules in combustion flames. 

The data are consistent with polymer formation by way of phenyl radical reaction with benzene or the fulvene produced to give cyclic radical intermediates which then polymerize to yield a polycyclopentene chain with pendant phenyl and/or benzyl groups. The average non-olefinic to olefinic proton ratio of 2.9 indicates that the many possible... 

Unfortunately, for reactions in Table 23 where values of Fr + c jh and D(R—H) are available, none of the data for the phenyl radical reactions are consistent with the above relation. Assuming the bond dissociation energies are reliable, which seems likely, it follows that one or both of the activation enei es are in error. On the whole, activation energies for radical reactions with benzene are easier to study than for phenyl radicals with hydrocarbons, although complications with radical addition to the benzene ring can arise [280]. 

There is little or no thermochemical property data available for unsaturated alkylperoxy and peroxide species. Peroxides are often impure and/or instable, and therefore difficult to isolate and characterize by experimental methods. There is no experimental data on vinyl, phenyl, ethynyl, or allyl peroxides that we are aware of. Experimental studies on the reaction of vinyl radical and allyl radical with O2 to form the corresponding peroxy radical have been reported by Gutman et al. [11, 12]. The phenyl-peroxy radical was reported by Lin s group as a major product in the phenyl radical reaction with O2 at ambient temperatures [17]. 

Yu T, Lin MC. 1995. Kinetics of phenyl radical reactions with selected cycloalkanes and carbon-tetrachloride . J. Phys. Chem. 99(21) 8599-8603. 

The distinction is that a polyatomic molecule has many vibrational degrees of freedom. Temporary stabilization occurs if energy can be transferred from the C-C vibration to some of the other vibrational modes of the biphenyl molecule. Such a process occurs readily so that three-body collisions are not immediately required for the recombination of phenyl radicals. Reactions such as... 

Benzoyl peroxide has been the most common source of phenyl radicals. But in reaction with thiazoles the benzoyloxy radical abstracts a hydrogen atom from the thiazole nucleus or from a methyl group in the case of methylthiazoles, giving by-products such as dithiazolyls or 2.2 -dithiazolylethane (183). The results obtained with benzoyl peroxide are summarized in Tables III-23, III-24. and III-25. 

The thiazolyl radicals are, in comparison to the phenyl radical, electrophilic as shown by isomer ratios obtained in reaction with different aromatic and heteroaromatic compounds. Sources of thiazolyl radicals are few the corresponding peroxide and 2-thiazolylhydrazine (202, 209, 210) (see Table III-34) are convenient reagents, and it is the reaction of an alky] nitrite (jsoamyl) on the corresponding (2-, 4-, or 5-) amine that is most commonly used to produce thiazolyl radicals (203-206). The yields of substituted thiazole are around 40%. These results are summarized in Tables III-35 and IIT36. 

Part B of Table 12.2 gives some addition reaction rates. Comparison of entries 19 and 20 shows that the phenyl radical is much more reactive toward addition than the benzy 1 radical. Comparison of entries 22 and 23 shows that methyl radicals are less reactive than phenyl radicals in additions to an aromatic ring. Note that additions to aromatic rings are much slower than additions to alkenes. 

Substituted pyrazolin-5-ones have only three and -substituted pyrazolin-3-ones only two tautomers, since now the corresponding 19c and 19d structures are isomers. The calculations involved l-methylpyrazolin-5-one (PM3/6-3H-G, anions and cations), l-phenyl-3-methyl-2-pyrazolin-5-one (DFT, radical reactions) [97JPC(A)3769], and l-(2, 4 -dinitrophenyl)-3-methyl-2-pyrazolin-5-one [B3LYP/6-31G and the crystal structure (Section V,D,2)] (98NJC1421). 

The results are consistent with the rate-determining step being addition of the aryl radical to the aromatic ring, Eq. (9). Support for this mechanism is derived from the results of three other studies (a) When A -nitrosoacetanilide is decomposed in pyridine, the benzene formed by abstraction of hydrogen from pyridine by phenyl radical accounts for only 1 part in 120 of the reaction leading to phenyl-pyridines. (b) 9,9, 10,lCK-Tetrahydro-10,10 -diphenyl-9,9 -bianthryl is formed in the reaction between phenyl radicals and anthracene, probably by the addition mechanism in Eq. (11). Adducts are also formed in the reactions of benzyl radicals with anthracene- and acridine. ... 

The reactivity of pyridine relative to that of benzene has been measured using the competitive technique developed by Ingold and his schoool for corresponding studies of electrophilic aromatic substitution. The validity of the method applied to free-radical reactions has been discussed. Three sources of the phenyl radical have been used the results obtained are set out in Table II. 

In many cases, however, the ortho isomer is the predominant product, and it is the meta para ratio which is close to the statistical value, in reactions both on benzenoid compounds and on pyri-dine. " There has been no satisfactory explanation of this feature of the reaction. One theory, which lacks verification, is that the radical first forms a complex with the aromatic compound at the position of greatest electron density that this is invariably cither the substituent or the position ortho to the substituent, depending on whether the substituent is electron-attracting or -releasing and that when the preliminary complex collapses to the tr-complex, the new bond is most likely to be formed at the ortho position.For heterocyclic compounds such as pyridine it is possible that the phenyl radical complexes with the nitrogen atom and that a simple electronic reorganization forms the tj-complex at the 2-position. 

There is an early report that thiophene reacts at the 3-position in phenylation with benzenediazonium chloride and aluminum trichloride, but in the Gomberg reaction thiophene has been found to substitute mainly at the 2-position both with p-tolyl and with p-chloro-phenyl radicals.Bcnzothiazole is phenylated at the 2-position in low yield by dibenzoyl peroxide a small quantity of the 4-isomcr is also obtained. ... 

Qualitatively, the results shown in Tables IV and V indicate that the methyl radical, just as the phenyl radical, substitutes pyridine preferentially in the 2- and 4-positions. The absence of the 3-isomer in these reactions is probably a result of the method of analysis... 

The very reactive phenyl radical reacts with the aromatic substrate 2, present in the reaction mixture. Subsequent loss of a hydrogen radical, which then combines with 7 to give 4, yields a biaryl coupling product e.g. the unsymmetrical biphenyl derivative 3 ... 

The initiating radicals are assumed to be SCN, ONO or N3 free radicals. Tris oxalate-ferrate-amine anion salt complexes have been studied as photoinitiators (A = 436 nm) of acrylamide polymer [48]. In this initiating system it is proposed that the CO2 radical anion found in the primary photolytic process reacts with iodonium salt (usually diphenyl iodonium chloride salt) by an electron transfer mechanism to give photoactive initiating phenyl radicals by the following reaction machanism ... 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

For reactions with S, specificity is found to decrease in the series cyanoisopropyl mcthyl Fbutoxy>phcnyl>bcnzoyloxy. Cyanoisopropyl (Scheme 3.3),7 f-bntoxy and methyl radicals give exclusively tail addition. Phenyl radicals afford tail addition and ca l% aromatic substitution. Benzoyloxy radicals give tail addition, head addition, and aromatic substitution (Scheme 3.4). ... 

Only a few diacvl peroxides see widespread use as initiators of polymerization. The reactions of the diaroyl peroxides (36, R=aryl) will be discussed in terms of the chemistry of BPO (Scheme 3.25). The rate of p-scission of thermally generated benzoyloxy radicals is slow relative to cage escape, consequently, both benzoyloxy and phenyl radicals are important as initiating species. In solution, the only significant cage process is reformation of BPO (ca 4% at 80 °C in isooctane) II"l only minute amounts of phenyl benzoate or biphenyl are formed within the cage. Therefore, in the presence of a reactive substrate (e.g. monomer), tire production of radicals can be almost quantitative (see 3.3.2.1.3). 

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M"1 s"1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or... 

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