Radical, Benzenethiol

The experiments with 2-(3-butenyloxy)benzenediazonium ions (10.55, Z = 0, n = 2, R=H) and benzenethiolate showed a significant shift of the product ratio in favor of the uncyclized product 10.57. They also indicated that the covalent adduct Ar — N2 — SC6H5 is formed as an intermediate, which then undergoes homolytic dissociation to produce the aryl radical (Scheme 10-83). Following the bimolecular addition of the aryl radical to a thiolate ion (Scheme 10-84), the chain propagation reaction (Scheme 10-85) yielding the arylphenylsulfide is in competition with an alternative route leading to the uncyclized product 10.57. 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

Carboxylic acids, a-bromination of 55, 31 CARBOXYLIC ACID CHLORIDES, ketones from, 55, 122 CARBYLAMINE REACTION, 55, 96 Ceric ammonium nitrate [Ammonium hexa mtrocerate(IV)[, 55, 43 Chlorine, 55, 33, 35, 63 CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Cinnamomtnle, a-phenyl- [2-Propeneni-tnle 2,3-diphenyl-], 55, 92 Copper(l) iodide, 55, 105, 123, 124 Copper thiophenoxide [Benzenethiol, copper(I) salt], 55, 123 CYCLIZATION, free radical, 55, 57 CYCLOBUTADIENE, 55, 43 Cyclobutadieneiron tricarbonyl [Iron, tn-carbonyl(r)4-l,3-cyclo-butadiene)-], 55,43... 

The photochemical reduction of Barton ester 40 is depicted in Scheme 12. A series of hydrogen atom donors were screened. A stoichiometric amount of benzenethiol at - 78 °C provided the product in 86% ee (entry 3). This implies that, in the presence of an efficient hydrogen atom donor, radical trapping is competitive with the ring/radical inversion, generating an enantiomeri-... 

Bromonaphthalene does not react with benzenethiol (thiophenol) salts. However, if electric current is passed through a solution containing 1-bromonaphthalene, the tetrabutylammonium salt of thiophenol, and DMSO, then l-(phenylthio)naphthalene is produced in 60% yield. When the reaction is conducted in acetonitrile, it leads to naphthalene above all (Pinson and Saveant 1978, Saveant 1980, Amatore et al. 1982). In the electrochemically provoked reaction, it is sufficient to set up the potential difference corresponding to the initial current of the reduction wave to transform 1-bromonaphtahalene into 1-naphthyl radical. The difference in the consumption of electricity is rather remarkable In the absence of thiophenolate, bromonaphthalene is reduced, accepting two electrons per molecule in the presence of thiophenolate, 1-bromonaphthalene is reduced accepting two electrons for every ten molecules. The reaction with the thiophenolate ion is catalyzed by electric current and takes a reaction path shown in Scheme 5.2. 

For substituted tricyclo[4.1.0.02,7]heptanes, similar addition of benzenethiol in diethyl ether gave an isomeric mixture of bicyclo[3.1.1]heptanes.35 As shown in the mechanistic scheme, the 1,3-disubstituted patterns of the bicyclo[3.1, l]heptanes are governed by the regiospecific attack of the thiol radical on the sterieally less hindered bridgehead carbon. The results of these radical additions arc summarized for bicyclo[n.l.l]alkanes (Table 8)35 and bicyclo[1.1.0]butanes (Table 9). 

TableS. Bicyclo[n.l.l]alkanes by Radical Addition Using Benzenethiol...

Bicyclobutanes as a structural subunit of complex hydrocarbon frameworks also undergo radical addition reactions with benzenethiol and bromotrichloromethane. In compounds such as 23, where the azo functionality can also be involved in a radical reaction, a half-cage compound 24 was produced in 64% yield.38 The generation of other half-cage compounds from 23 and its analogs are shown in Table 10.38... 

Table 10. Half-Cage Compounds by Radical Addition Using Benzenethiol or Bromotrichloromethane...

The Srn 1 reaction has been applied to heterocycles. Among five-membered ring compounds, halothiophenes have been the most studied they have been shown to be susceptible to both electron-stimulated and photostimulated reactions, and have been converted to the corresponding acetonitriles [150], acetones [151], and phenyl-sulfides [ 152] in low to medium yields. In the study with the benzenethiolate anion it has been shown that the yield is low because of fragmentation of the adduct radical anion it can be increased by adding an electron acceptor, e.g. benzonitrile which prevents decomposition. Further applications include the thermally activated SrnI reaction between 3-iodobenzothiophene and enolates [153] and the photo-stimulated reaction of 3-halo-2-aminobenzothiophenes [154]. 

Sam and Sutherlandin a continuation of their studies on the cyclization of germacratriene (222) have shown that both radical- and cation-induced cycliza-tions follow similar directional and stereoselective pathways. Thus irradiation of germacratriene in the presence of carbon tetrachloride or benzenethiol leads to (236 = CCI3, = Cl) and (236 R = SPh, R = H) respectively. 

The stannacyclopentanes are again particularly reactive 138). Alk-oxyl radicals will now react at the tin center in the fully alkylated compounds, with opening of the ring, and benzoyloxyl and alkylthiyl radicals will also induce ring cleavage. For example, 1,1-dibutylstan-nacyclopentane reacts homolytically with benzenethiol to give tribu-tyl(phenylthio)tin. 

Bromoisoquinoline may be made to react with benzenethiolate ion to give 4-phenylthioisoquinoline in methanol at 147 °C by a radical chain route. The interesting feature is that the radical ion process requires methoxide ion for initiation. 

In the absence of methoxide ion substitution takes place at a slower rate by an S jAr route. Formerly, the presence of methoxide ion in substitution reactions involving thiolate ions in methanol was avoided studiously in order to prevent a competing substitution by the methoxide ion. Although methoxide ion is required for the radical chain process to occur, only traces of substitution product containing the methoxy group are present, demonstrating that benzenethiolate ion is superior to methoxide ion in trapping the intermediate 4-isoquinolyl radical,116 Scheme 12. 

The radical addition of benzenethiol to norbomadiene afforded four 1 1 adducts, including 3-phenylsulfanyltricyclo[2.2.1.0 ]heptane, in a mixture that also depended on the concentration of the starting material and on reaction temperature, At 150°C, 3-phenylsulfanyl-tricyclo[2.2.1.0 ]heptane was the major compound (73%) in a mixture obtained in 63% yield. 

Nucleophilic substitutions have also been carried out by irradiating mixtures a of a bromocyclo-propane, benzenethiol, and ammonia. When 7-bromobicyclo[4.1.0]heptane and 1-bromo-l-chloro-2,2,3,3-tetramethylcyclopropane were photolyzed under these conditions replacement of the bromine with the phenylsulfanyl group takes place, but the yields of the corresponding cyclopropyl sulfides 9 and 10 were low. Mechanistic studies support a radical chain mechanism for the reactions. ... 

Cyclopropanes without electron-withdrawing substituents usually resist the attack of nucleophiles. Exceptions are strained systems and a-halo-substituted derivatives which undergo homoallylic substitution reactions.[l.l.l]Propellane structures, though usually formed only as intermediates, underwent addition of benzenethiol with cleavage of the central cyclopropyl bond. This addition possibly follows a radical chain mechanism. 

Under free-radical conditions carbon tetrachloride and benzenethiol were added across the vinylcyclopropane system in methyl 2-methylene-6,6-dimethylbicyclo[3.1.0]hexane-l-carbox-ylate. ... 

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