Sulfuranyl radical

A limited number of radiation chemical results pertains to sulfuranyl radicals. The simplest of these three-coordinate sulfur-centered radicals, namely, R3S are very short-lived (tj/2 1 M-s) and so far escaped direct detection. Their transitory formation can, however, be deduced from a fast, close to diffusion controlled, reduction of the corresponding sulfonium salts by hydrated electrons and scavenging of the radicals cleaved as an overall result of reaction sequence 35. 

In case of cyclic sulfonium salts (two of the three substituents backbone-linked) the results indicate that sulfur-carbon cleavage leading to ring opening seems to prevail over expulsion of the open chain alkyl group.76 

Better candidates for stabilization of a sulfuranyl type intermediate appear to be those which offer the possibility for resonance and electron delocalization. Reduction of 1-methyl-1,2-dithiolanium, for example, results in a transient with tj/2 = 10 is and 396 nm which can be been assigned to a structure 1 but in the light of all the flndings concerning three-electron bonded species vide infra) is probably even be better represented by structure 2.77 

The reaction of thiyl radicals with disulfides results in even much longer-lived ( 100 fis) intermediates which are embedded in an extended equilibrium system (eq. 37) constituting, in its essence, a thiyl displacement process.7  

Within the adduct the unpaired electron is suggested to be located in an antibonding orbital extending over the entire sulfur bridge, thereby facilitating breakage of either of the two S-S bonds. Direct identification of the transient adduct is possible through its distinct optical absorption nm, e = 

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It has been demonstrated that the MCR enzyme is active only if the metal center of coenzyme F430 is in the Ni1 form.1857 The natural substrate Me-CoM or simple methyl thioethers, however, do not react with Ni1 F430, which has lead to the proposal of a catalytic mechanism in which the addition of a thiyl radical to the S atom of the thioether giving a sulfuranyl radical intermediate is... 

The nature of the cleavage and the possible stability of sulfuranyl radicals formed after electron transfer could be advantageously revealed by ESR and the associated technique of spin trapping as well [247,248]. Spin traps such as f-butylphenylnitrone and nitrosodurene could contribute to demonstrate the structure of the leaving radical formed by cathodic decomposition of the sulfonium substrate. 

Anderson RF, Vojnovic B, Patel KB, Michael BD (1986) Reaction of alcohol radicals with cyclic disulfides. An optical and conductometric pulse radiolysis study. Radiat Phys Chem 28 219-222 Anklam E, Margaretha P (1989) Organic sulfuranyl radicals. Res Chem Intermed 11 127-155 Armstrong D (1990) Applications of pulse radiolysis for the study of short-lived sulphur species. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 121-134... 

Chatgilialoglu C, Castelhano AL, Griller D (1985) Structures and optical absorption spectra of some sulfuranyl radicals in solution. J Org Chem 50 2516-2518 D Alessandro N, Bianchi G, Fang X, Jin F, Schuchmann H-P, von Sonntag C (2000) Reaction of superoxide with phenoxyl-type radicals. J Chem Soc Perkin Trans 2 1862-1867 Das S, von Sonntag C (1986) Oxidation of trimethylamine by OH radicals in aqueous solution, as studied by pulse radiolysis, ESR and product analysis. The reactions of the alkylamine radical cation, the aminoalkyl radical and the protonated aminoalkyl radical. Z Naturforsch 41b 505-513 Das S, Schuchmann MN, Schuchmann H-P, von Sonntag C (1987) The production of the superoxide radical anion by the OH radical-induced oxidation of trimethylamine in oxygenated aqueous solution. The kinetics of the hydrolysis of (hydroxymethyl)dimethylamine. Chem Ber 120 319-323... 

Pogocki D, Schoneich C (2002) Computational characterization of sulfur-oxygen-bonded sulfuranyl radicals derived from alkyl- and (carboxyalkyl)thiopropionic acids evidence for a -type radicals. J Org Chem 67 1526-1535... 

In this regard the electrochemical reduction behavior of arylmethylalkylsulfo-nium salts is of interest. One-electron reduction produces C—S bond cleavage by a concerted process. We believe that the sulfuranyl radical may be an intermediate in the one-electron reduction of sulfonium salts only when the cation-radical and radical species are extremely unstable. It has been concluded that as the electron enters the C—S a orbital bond cleavage occurs concomitantly [89],... 

Scheme 13. The catalytic reaction was proposed to go through a sulfuranyl radical by addition of a thiyl radical to the sulfur atom of the thioether MeSPh. Interestingly, these studies are consistent with the proposed mechanism of the enzyme the thiyl radical of cofactor HS-CoB resulting from one-electron reduction of F430 by S-CoB couples with Me-CoM to afford the sulfuranyl radical, CoB-S-S (Me)-CoM. The sulfuranyl radical transfers a methyl radical to Ni -p43o to generate Me-Ni -F43o and the heterodisulfide CoM-S-S-HTP...

Pogocki D., Ghezzo- Schbneich E., Schbneich C., Conformational flexibility controls proton transfer between methionine hydroxy sulfuranyl radical and the N-terminal amino group in Thr-(X) -Met peptides, J. Phys. Chem. B, 2001,105,1250-1259. 

On the basis of structural variations in the hydridophosphorane it was previously suggested that the final step in this sequence was consistent with reversible formation of a sulfuranyl radical intermediate (43) followed by rate-determining sulfur-sulfur cleavage or a concerted process with a T.S. having a relatively high degree of phosphorus-sulfur bond formation . 

The dependence of the reduction potential on leaving group structure is greater than can be accounted for by a two-step reductive cleavage decomposition of the sulfuranyl radical on the picosecond time scale would be expected to result in at most a 360 mV shift in the reduction potential [52,62b]. A shift of almost 1.3-1.4 V is observed in systems where the change in substituents is insulated from the aryl-sulfur group by a saturated carbon... 

This route corresponds with Eberson s proposal (44) for the SET reaction of ArH + with R that is, the reaction of ArH with R- would follow. We think that this is not likely to have occurred in our reactions. Saeva and Morgan (46) showed that phenyl and (l-naphthyl)dialkylsulfuranyl radicals are highly unstable. When formed by anodic reduction of sulfonium ions, (ArRR )S +, the sulfuranyl radicals appear to lose an alkyl radical at the time of electron transfer (46). This observation suggests that sulfuranyl radicals of the type ThR- may also have a short life. This condition does not rule them out as intermediates in the alkylation reaction (equations 27 and 28) however, it does not encourage belief in that route. 

Scheme 10-5 shows the simplest version for a possible mechanism of enzyme activation. However, this mechanism does not reveal much about the control of the S-C bond cleavage within the intermediate sulfuranyl radical. Spontaneous cleavage of this radical within the active site is not unlikely if one takes the great instabihty of such radicals into account. Furthermore it has to be emphasized that no direct evidence for intermediate free radicals, such as the sulfuranyl radical or the 5 -deoxyadenosyl radical, is available yet. Whether these radicals are true intermediates or not need to be demonstrated, by using rapid freeze quench EPR methods for example. 

Interest in the electronic structure of 9-S-3 sulfuranyl radicals (267) and (268) and a bisulfuranyl species, the dimer of 9-5 -3 sulfuranyl radical (267), has led workers to obtain the ESR and electron spin echo envelope modulation (ESEEM) data <86JA3206,86JA32ii>. 

The conversion of (551) to compounds (552)-(554) was accomplished in the straightforward manner shown in Scheme 48. The preparation of T-shaped 9-S -3 7t-sulfuranyl radicals permitted Electron Spin Resonance and Electron Spin Echo studies <86JA3206,86JA3211). 

Also reported in the early 1970s was the photochemistry of some alkyl a-substituted p-ketosulfoxides 290 [148]. Stereomutation at suliiir was observed in all cases where it could be detected due to diastereomeric interconversions. In addition to the products that are parallel to the previous example, ketone 293 and thioester 294 were observed. A mechanism in which Type I cleavage competed with p-cleavage was proposed. It included a cyclic sulfuranyl radical 297 in order to accomplish the oxygen migration. An alternative h3rpothesis has formation of a sulfenic ester 301, secondary photolysis, and typical chemistry of alkoxy radicals to get to the same intermediates. Sulfenic esters were not detected, but analysis was by GC or after column chromatography, neither of which would have been survived by such compounds. 

A sulfuranyl-alkyl biradical has been proposed [13] as intermediate in this ring contraction (Scheme 8). Nevertheless, the question whether trialkyl-sulfuranyl radicals are to be considered stable intermediates [37] or just transition states [38] remains open. 

A sulfuranyl radical of particular interest in the sulfide oxidation is R2S (0H) formed, for example, as an adduct in the reaction of R2S with OH radicals. 

In the absence of suitable redox partners sulfide and disulfide radicals cations decay mainly by disproportionation or deprotonation. Considering their positive charge they are also prone for nucleophilic attack. Examples for the latter are the reaction of R2S with OH , leading to sulfuranyl radicals R2S (0H), or with halide ions, yielding sulfur-halide coupled radicals. Both these product radical species will be dealt with in more detail in separate sections. 

Being cationic species, both conjugates are susceptible to reactions with nucleophiles such as halide ions (to form S.. Hal radicals) or hydroxide ions (to yield >S -OH sulfuranyl radicals). (For more details on these product species see corresponding later sections). 

Oxidation of methionine residues has been widely studied. Reaction of OH is fast (k= 10 mol I s l) and proceed via formation of an OH-adduct > S-OH, the hydroxy sulfuranyl radical (42). This radical eliminates water, yielding the monomeric radical cation >S + which then stabilizes through formation of >S-X radical species, X= N, O or S (56). In methionine amino-acid, the free radical undergoes ring closure with the amine function to give cyclic >SN radical (57) which then gets decarboxylated. Final compound seems to be methionine sulfoxide. When methionine is in a peptide, the fate of the hydroxy sulfuranyl radical is strongly dependent on the position of methionine in the peptide. 

Pfaltz, Jaun, and co-workers developed a related system that utilized photochemical homolysis of Ni -thiolate bonds to generate sulfuranyl radical intermediates, via an intramolecular cycliza-tion involving a Ni-bound aliphatic thioether. Scheme 10. " Methane yields ranged from 2.5% to 48%. These authors repeated the Tada and Masuzawa photochemical experiments in order to quantitate methane formation. While they succeeded in reproducing methane and disulfide formation, the methane yields in the presence of Ni complex (0.3-1.4%) were consistently much lower than in the absence of Ni. 

Finally, it has been shown that sulfuranyl radicals, for example, 52 (cf. Scheme 6), exhibit absorption spectra, which can be detected with ultraviolet-visible (UV-Vis) spectroscopy absorption maxima were found to be sensitive to the a- versus 7t-electronic configurations of these radicals and thereby to be a sensitive guide in the assignment of the electronic stmcture of sulfuranyl radicals <1985JOC2516>. 

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