Phenylthiyl

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

Mortensen, A. 2000. Mechanism and kinetics of scavenging of the phenylthiyl radical by carotenoids. A laser flash photolysis study. Asian Chem. Lett. 4 135-143. 

The fonnation of these substances contradicts common ideas on nucleophilic substitution. The presence of radical traps (oxygen or tetrabromobenzoquinone) decelerates the formation of both unexpected compounds and product of thioarylation. Consequently, the first stage of the reaction depicted in Scheme 4.5 produces phenylthiyl radical and anion-radical of the substrate. Both electron-transfer products undergo further conversions The phenylthiyl radical gives diphenyldi-sulfide, and the anion-radical of the substrate produces 9-fluorenyl radical. The latter reacts in two directions—dimerizing, it forms bifluorenyl reacting with the nucleophile, it gives the anion-radical of the substitution product. The chain continues because the electron from the anion-radical is transferred to the unreacted molecule of the substrate. The latter loses bromine and then reacts with the nucleophile, and so on (Scheme 4.6). 

Zoltewicz and Oestreich (1973) employed sodium methylate to accelerate the reaction between 4-bromo-iso-quinoline and sodium thiophenolate. In this case, the CHjO ion acts as a competing electron donor with respect to the PhS ion. On electron transfer to the substrate, thiophenolate converts into the phenylthiyl radical and then to diphenyldisulhde. Diphenyldisulfide is inactive in further transformations. The methylate ions generate the anion-radicals of the substrate, thus preserving the greater part of the thiophenolate for use in substitution. The observed rate of thioarylation and the yield of 4-phenylthio-iso-quinoline increase in the presence of sodium methylate. Azobenzene inhibits the action of sodium methylate. Scheme 5.7 summarizes what has been mentioned. 

The authors hold the opinion that the thiophenolyc moiety adds to the olefin bond and an electron adds to the nitro group. Hence, the anion-radical [R R C(SPh)CH(R )N02] controls the reaction. The final product is formed as a result of the cleavage of the latter anion-radical with the expnlsion of the nitrite ion and the phenylthiyl radical. The radical normally transforms into diphe-nyldisulfide. The yields of the denitrated olefines are high and reach 80-95%. 

Scheme 6.2 represents Sj j l substitution that takes place when sodium thiophenolate attacks e,4-tert-butyl-c,2-methyl-fl,4-nitro-e,4-(4-nitrophenyl) cyclohexane. Light irradiation stimulates the reaction. It is carried out under nitrogen in HMPA. The ion-radical type of the process has been established by means of inhibitors. It was found that the stereochemical outcome of the reaction depends on the concentration of the PhSNa nucleophile. At a low concentration of PhSNa, the reaction leads to a mixture of phenylthiyl derivatives the content of a,SPh-substituted product is higher than that of c,SPh product by 20%. At a high concentration of PhSNa, the reaction produces practically a single stereoisomer bearing the a-PhS group. 

The stereospecific substitution produces a sterically less strained product when a more bulky p-nitrophenyl substituent occupies the equatorial position and the less bulky phenylthiyl substituent occupies the axial position. Steric limitations of the nucleophile attack caused by strains in the final product are thus removed. And therefore, ion-radical addition ofp-nitrobenzyl radical to the lithium salts of 5-nitro-l,3-dioxanes proceeds stereospecifically (Zorin et al. 1983 Scheme 6.3). 

The generation of active radicals as a result of bond breakage makes cation-radicals useful as syn-thons. For example, arylsulfenamide cation-radicals may be used as sources of sulfenyl radicals. The reaction of 4 -nitrobenzenesulfenanilide with Lewis acids, such as BF3 and AICI3, leads to the formation of sulfonamide cation-radical. The latter appears to be an active sulfenyl transfer species. In the presence of anisol, ethenes, or ethynes, it gives phenylthiyl derivatives (Benati et al. 1990, Gross and Montevecchi 1993). 

Electrochemical studies provide irreversible potential data and thus will not be considered here. However, accurate determinations of several E° values in non-aqueous solvents, pertaining to para-substituted phenylthiyl radicals, have been reported. The oxidation of RS anions, generated in MeCN by deprotonation of the corresponding thiophenols, has been studied by using gold micro or ultramicroelectrodes. 

Where it is possible to distinguish the products, thiol additions show stereospecificity. The products of addition of hydrogen sulfide, thio-phenol, and thiolacetic acid to 1-chlorocyclohexene are to be 75%, 94%, and 66% cis-l, 2-disubstituted cyclohexane, respectively.88 The addition of thiolacetic acid is less stereopecific than the other thiols. The stereospecificity apparently depends upon the ratio of addendum to 1-chlorocyclohexene, Phenylthiyl radical addition to 1-methylcyclo-... 

Photolysis of aryl-desyl sulfides (LV, It = C H6) yielded didesyl (LVII).240 Desylthiobe n zoate (LV, R = SCOC6H6) gave didesyl and dibenzoyl disulfide. It is probable that in the former case the phenylthiyl radical (C0H6S ) abstracts hydrogen from the solvent to give thio-phenol and that in the latter reaction dibenzoyl disulfide is formed from the dimerization of benzoylthiyl radical (C H6COS-). The formation of LVII may be attributed to the dimerization of the desyl radical (LVI). O OH... 

Radical attack on the central bond in [1.1.1 [propellane 1 occurs 2-3 times faster than attack on styrene96 and yields bridgehead bicyclo[l.l.l]pent-l-yl radicals6. Laser flash photolysis techniques were used to measure the rate constants for the reactions of la with five different radicals (Table 12)96"99. The addition of the phenylthiyl radical to la is... 

A potential method for the preparation of novel amino acids via the highly selective addition of radicals to the glyoxylic oxime derivative of Oppolzer s camphor sultam (88) has been reported.181 Both Lewis acid and non-Lewis acid-mediated reaction conditions for the addition of alkyl radicals generated from alkyl iodides and Et3B/Bu3SnH were examined. A new chiral auxiliary based upon (R,R)-2,5-diphenylpyiTolidine has been used in the addition of phenylthiyl radicals to unsaturated methacrylamides. The selectivity was found to be better than that reported for the structurally related 2,5-dimethylpyrrolidine derivative.182... 

At a low concentration of PhSNa, the reaction leads to a mixture of phenylthiyl derivatives the content of a,SPh-substituted product is higher by 20% than that of e,SPh product. At a high concentration of PhSNa, the reaction produces practically the single stereoisomer bearing the a-PhS group. 

Armstrong D, Sun Q, Schuler RH (1996) Reduction potentials and kinetics of electron transfer reactions of phenylthiyl radicals comparisons with phenoxyl radicals. J Phys Chem 100 9892-9899 Asmus K-D (1979) Stabilization of oxidized sulfur centers in organic sulfides. Radical cations and odd-electron sulfur-sulfur bonds. Acc Chem Res 12 436-442 Asmus K-D (1990a) Sulfur-centered free radicals. Methods Enzymol 186 168-180 Asmus K-D (1990b) Sulfur-centered three-electron bonded radical species. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 155-172... 

A highly diastereoselective radical cascade involving addition of a phenylthiyl radical to a terminal alkyne followed by a [1,5]-hydrogen transfer and a 5-exo-cyclization (g) has been reported (Scheme 112).165... 

---