Radical 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%. 

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... 

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... 

The related phenylthiyl radical complexes are also known.31 The Ru complex of L3 (Scheme 4) is oxidized by four electrons in an unusual reaction to give the dimeric macrocyclic complex.32... 

The dependence on substituent of the reduction potential and other properties of p-substituted phenoxyl radicals has been compared with the properties of the analogous phenylthiyl radicals. From this comparison it is evident that the electronic interaction... 

As shown in Scheme 2, radical 13 can be trapped by an external trap such as diphenyl diselenide to give y -lactam 14 in competition with the 5-endo cyclization leading to y-lactam 16. -Lactams can also be prepared by introducing a fragmentation step such as yS-fragmentation of sulfide [7]. Thus compound 24 is converted to yS-lactam 26 in 50% yield after rapid elimination of phenylthiyl radical within 25 (Scheme 3). Under the same conditions, 27 afforded 28 in 65% yield. 

Studies on prostaglandin biosynthesis in the early 1970s have shown that molecular oxygen is incorporated into polyunsaturated lipids. It was shown that autoxidation of polyunsaturated species leads to peroxyl radical intermediates that can undergo yff-scission, H-atom abstraction, and allylic rearrangement or/and cyclization. Beckwith looked into the oxygenation of dienes initiated by phenylthiyl radicals... 

The synthesis of natural products has been accomplished by this strategy examples are shown in Eqs. (18), (19). The key step used by Keck and Wager in the total synthesis of en/-lycoricidine [43a] relies on the addition of phenylthiyl radical to the... 

The observation that thiyl radicals are able to isomerize alkenes was first made by Walling et al. many years ago [55] and, now the addition-elimination sequence of the phenylthiyl radicals is an established methodology in fine chemical synthesis [56]. It has been utilized as key step in the synthesis of biologically active compounds, such as (-)-gloeosporone [57a], (-l-)-hitachimycin [57b] and other naturally occurring macrolides [5b], as well as for preparing non-natural isomers of phospholipids [58]. 

Vinylidenecyclopentanes 45 have been synthesized by a sequential radical reaction. A careful evaluation of the suitable starting materials led to the choice of thiohydroxamic esters like 43 for starting the radical chain shown in Scheme 15. Photolytical decomposition of 43 in the presence of 5-fold molar excess of a radicophilic olefin generated the 5-hexynyl radical 44 which cyclized onto the triple bond. jS-Elimination of a phenylthiyl radical afforded the vinylidenecyclopentane 45 in 60-70% yield. Other insights in the mechanism of these radical chain reactions are given in the paper. 

The Radiation Boundary Model (RBM1-- The optical density of the Scheme 2 system at 435nm, produced by 35-40 psec (gaussian fwhm) laser pulses (0.5mJ per pulse at 354.7 nm), is due to the sum of the free and cage pair phenylthiyl radical concentrations. The expression for the time dependence of this sum [SR (t)], according to the RBM, is equation (9). This equation would be directly applicable if the temporal widths of the exciting and probing... 

The Battelle group (JAF) has recently measured the diffusion coefficient of benzyl phenyl sulfide in nonane and compared it with the value calculated by the Spernol-Wirtz (SW) method. The agreement between experiment an SW is within 3.8 % in this case. The calculated (SW) termination rate constants for phenylthiyl radicals in heptane are 30% higher than observed . The direction of this discrepancy could be explained by a 70% efficiency of the collisional cage in heptane. 

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