Phenyl free radicals

The liquid products contain benzene (21%), phenol (19%), diphenyl (8%), benzonitrile (5%). dibenzofurane (8.5%), benzoquinoline (2.5%) among other less defined substances. Among earlier papers see also that of Fields and Meyer-son (4J (see p. 113). Hand,. Merrit and Di Pietro [5] which referred to phenyl free radical arrd rritrobenzene formed by pyrolysis. 

Nitrosobenzene was also used as a quantitative spin trap for phenyl free radicals, to study their involvement in arylation reactions with organobismuth reagents. Again, ESR observations indicated the formation of phenyl free radicals. However, quantitative trapping measurements showed that only a small amount of free radical derived adducts (2-3%) was formed. The yield of C-arylated products was not affected by the presence of this free radical trap. ... 

The outcome of the reaction of pyridine with diaryliodonium salts was dependent upon the nature of the counterion. With a non nucleophilic counterion as the tetrafluoroborate, the iV-phenylpyridinium salt (31) was obtained in high yield. 23 with bromide, no reaction occurred and with chloride, ring arylation was observed as a result of the formation of phenyl free radicals. 25... 

Such a possibility was quickly supported by qualitative ESR studies, which revealed the presence of phenyl free radicals in the C-phenylation of phenols and enols. For example, when the reaction of tetraphenylacetone with tetraphenylbismuthonium tosylate and BTMG as a base was performed in the presence of a spin-trap, phenyl-rerf-butylnitrone, (PBN), the phenyl free radical adduct (101) was detected by ESR. The same free radical adduct (101) was seen in the absence of tetraphenylacetone.28 (Scheme 6.17)... 

Therefore, the observation of ESR signals due to phenyl free-radicals resulted from a minor competing decomposition pathway of the bismuth reagent, and the arylation reaction itself does not proceed by a free-radical pathway. 

Diphenyl is a commonly reported product of the reaction of benzene in electrical discharges and will not be considered further in this paper. Most authorities suggest that it arises via combination of phenyl free radicals. 

These initiators thermolytically decompose at the bond shown, i.e., dibenzoyl peroxide, BPO, to benzoyloxy free radicals, and, in, certain solvents, also further to phenyl free radicals ... 

Primarily formed benzoyloxy free radicals, and not the secondary phenyl free radicals produced in the absence of monomer molecules, also induce free radical polymerization. But, in the case of pinacol-started polymerizations, primary free radical fragments are not found as end groups. The starting free radicals in this case are presumably monomer-free radicals produced by a transfer reaction ... 

Fig, H8. (a) Partial rale factors of free radical phenylation, relative to benzene (397). (b) Free valence calculated by HMO method (117). (c) Radical localization energy (in units) calculated by HMO method (117). 

Fig. 1-19. Partial rate factors of free radical phenylation relative to benzene (398).

CycJohexyl free radicals, generated by photolysis of t-butyl peroxide in excess cyclohexane, also possess nucleophilic character (410). Their attack on thiazole in neutral medium leads to an increase of the 2-isomer and a decrease of 5-isomer relative to the phenylation reaction, in agreement with the positive charge of the 2-position and the negative charge of the 5-position (6). 

The free-radical reactivity of thiazoles has been well studied with various radicals such as methyl, phenyl, substituted phenyl, cyclohexyl, and aromatic-heterocyclic, in nonpolar solvent or strong acids (180-182). 

In Volume 2, in the chapter on Free-Radical Substitutions of Heteroaromatic Compounds by R. O. C. Norman and G. K. Radda, p. 166, Table VI, 6-R-Acridine should read 9-R-Acridine p. 167, lines 17 and 18, 6-phenylacridine should read 9-phenyl-acridine... 

The absolute rate constants for the reaction of a variety of electrophilic free radicals with 4-(dimethylamino)-l,5-dimethyl-2-phenyl-l,2-dihydro-3//-pyrazol-3-one (aminopyrine) and l,5-dimefliyl-2-phenyl-l,2-dihydro-3//-pyrazol-3-one... 

There are very few homolytic reactions on triazolopyridines. A suggestion that the ring opening reactions of compound 1 involved free radical intermediates is not substantiated (98T9785). The involvement of radical intermediates in additions to ylides is discussed in Section IV.I. The reaction of radicals with compound 5 and its 1-substituted derivatives gives 4-substituted compounds such as 234 (96ZOK1085). A more detailed study of the reaction of the 1-methyl and 1-phenyl derivatives with r-butanol and ammonium persulfate produced 4-methyl substitution with a silver nitrate catalyst, and the side chain alcohol 235 without the catalyst (96ZOK1412). 

Free radical polymerization of n / yJro-(2-hydroxy-3-[(3-vinylphenyl)-methyl]-l-phenyl-4-oxo-4//-pyrido[l,2-n]pyrimidinium)hydroxide in DMF at 60 °C for 2 days in the presence of 2,2 -azoizobutyronitrile led to a mesoionic polymer (00MI26). 

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. 

Pyridine has been phenylated with the following free-radical sources benzenediazonium chloride with aluminum trichloride the Gomberg reaction " phenylhydrazine and metal oxides A -nitroso-acetanilide dibenzoyl peroxide phenylazotriphenylmethane di-phenyliodonium hydroxide and electrolysis of benzoic acid. ° Although 2-phenylpyridine usually accounts for over 50% of the total phenylated product, each of the three phenyl derivatives can be obtained from the reaction by fractional recrystallization of the... 

It is difficult to treat the effect of a heteroatom on the localization energies of aromatic systems, but Brown has derived molecular orbital parameters from which he has shown that the rates of attack of the phenyl radical at the three positions of pyridine relatively to benzene agree within 10% with the experimental results. He and his co-workers have shown that the formation of 1-bromoisoquinoline on free-radical bromination of isoquinoline is in agreement with predictions from localization energies for physically reasonable values of the Coulomb parameters, but the observed orientation of the phcnylation of quinoline cannot be correlated with localization ener-... 

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 ... 

If 2,2 -dimethyoxy-2-phenyl acetophenone was used as the photolnitlator, the exited molecule decomposes into a free radical as follows ... 

The situation is not as clearly solved in a positive or negative sense for arenediazo phenyl ethers. Here three alternatives have to be considered, namely an intramolecular rearrangement of the arenediazo phenyl ether (Scheme 12-11, A), and two types of intermolecular rearrangement, either by heterolytic dissociation into a diazonium ion and a phenoxide ion (B) or by homolytic dissociation into a radical pair or two free radicals (C). 

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

Se-phenyl areneselenosulfonates (24) undergo facile free-radical addition to alkenes to produce / -phenylseleno sulfones (25) in excellent yield86,87 (see Scheme 7). The addition occurs regiospecifically and affords anti-Markovnikov products contrary to the analogous boron trifluoride catalyzed reaction which produces exclusively Markovnikov and highly stereospecific products86 (equation 37). Reaction 36 has been shown to have the radical... 

Free-radical addition of Se-phenyl areneselenosulfonates to acetylenes is also a facile process, occurring regiospecifically and stereoselectively to afford the E-isomer of a -(phenylseleno) vinyl sulfone (26) in high yield88. 

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