Sulfuranyl Radicals Research Articles

Regioselectivity in the Reductive Bond Cleavage of Diarylalkylsulfonium Salts: Variation with Driving Force and Structure of Sulfuranyl Radical Intermediates

This investigation was stimulated by reports that one-electron reductions of monoaryldialkylsulfonium salts never give aryl bond cleavage whereas reductions of diarylmonoalkylsulfonium salts preferentially give aryl bond cleavage. We studied the product ratios from the reductive cleavage of di-4-tolylethylsulfonium and di-4-tolyl-2-phenylethylsulfonium salts by a variety of one-electron reducing agents ranging in potential from -0.77 to +2.5 eV (vs SCE) and including thermal reductants, indirect electrolyses mediated by a series of cyanoaromatics, and excited singlet states. We report that the cleavage products vary from regiospecific alkyl cleavage to predominant aryl cleavage as a function of the potential of the reducing agent. We conclude that differences between the reductive cleavages of mono- and diarylsulfonium salts are direct consequences of the structures of the sulfuranyl radical intermediates and the bond dissociation energies of the alkyl and aryl bonds. Competitions between the rates of cleavage and oxidation of the intermediate sulfuranyl radicals and between concerted and stepwise mechanisms are discussed to explain the variations in bond cleavage products as a function of the driving forces for the reductions. Density functional theory investigations of the nature of the antibonding S-alkyl and S-aryl orbitals of the starting sulfonium salts provide additional insight.

The Influence of Solid‐State Molecular Organization on the Reaction Paths of Thiyl Radicals

Electron paramagnetic resonance (EPR) spectroscopy has been employed to investigate the effect of solid-state molecular organization on the reaction of thiyl radicals with thiols. In an irradiated C18H37SH/thiourea clathrate, the conversion of thiyl to perthiyl radicals is substantial, due to the head-to-head arrangement of the reactants within the channels and the suppression of other possible competing reactions due to hindrance by the clathrate walls. The perthiyl radical was identified using EPR analysis of its molecular dynamics within the clathrate channels. Irradiated polyethylene film containing 30% C18H37SH afforded a negligible conversion of thiyl to perthiyl radicals because of the random distribution of reactants. These results suggest that in the absence of favorable structure-control effects, the reaction between RS* and RSH is unimportant with respect to other competing reactions. Perthiyl radicals are also the major product in the vacuum solid-state radiolysis of lysozyme. A proposal of the mechanism involved in all cases is based on the equilibrium RS* + RSH <==> RSS*(H)R, followed by the irreversible conversion of the sulfuranyl radical to the perthiyl radical. As the equilibrium is strongly shifted to the left, the intermediate sulfuranyl radicals were not detected, but the lack of other competing reactions for the thiyl radicals caused the formation of perthiyl radicals to become the major path in the clathrate and in solid lysozyme radiolysis.

Computational characterization of sulfur-cxygen-bonded sulfuranyl radicals derived from alkyl- and (carboxyalkyl)thiopropionic acids: evidence for sigma-type radicals.

Sulfide radical cations are known to stabilize via sulfur-oxygen bond formation with carboxylic acid functions. However, structural information on the resulting sulfuranyl radicals has so far only been provided by ESR spectroscopy for more stable, aromatic-substituted species, while the rather short-lived aliphatic sulfuranyl radicals have largely been characterized by time-resolved UV spectroscopy only. Therefore, we have obtained theoretical structural information on S-O-bonded sulfuranyl radicals from three model compounds, tert-butyl 2-(methylthio)peroxybenzoate, 3-methylthiopropionic acid, and 3,3'-thiodipropionic acid, using density functional theory, semiempirical, and molecular mechanics methods. All S-O-bonded species exist predominantly as three-electron-bonded sigma-radicals with an estimated heterolytic bond dissociation energy of the S therefore O bond on the order of 23-27 kcal mol(-1). Characteristic optical absorption bands, vibrational frequencies, and hyperfine coupling tensors are evaluated to facilitate the identification of such radicals by time-resolved UV, IR, and ESR spectroscopy.

Conformational Flexibility Controls Proton Transfer between the Methionine Hydroxy Sulfuranyl Radical and the N-Terminal Amino Group in Thr−(X)n−Met Peptides

For Thr−(X)n−Met peptides, we have correlated the efficiency of proton transfer and an intramolecular radical reaction between a methionyl hydroxy sulfuranyl radical and the N-terminus with the flexibility of the spacer sequence −(X)n−, X = Gly and Pro and n = 0−4. Hydroxy sulfuranyl radicals accept a proton from the N-terminus to yield a cyclic (S∴Ν)+ three-electron-bonded intermediate which ultimately expels acetaldehyde. Acetaldehyde formation was used to quantify the efficiency of proton transfer and cyclization, whereas peptide flexibility was calculated by molecular modeling methods. For X = Gly, proton transfer and cyclization proceeds in a concerted manner, and peptide flexibility mainly controls the proton transfer step. For X = Pro, no low-energy conformation was located, which allows concerted proton transfer and cyclization, indicating that proton transfer occurs over longer distances, likely via bridging water molecules. These sequence-dependent differences are reflected in different β-values...

Oxidation of (Carboxyalkyl)thiopropionic Acid Derivatives by Hydroxyl Radicals. Mechanisms and Kinetics of Competitive Inter- and Intramolecular Formation of σ- and σ*-type Sulfuranyl Radicals

The substituent effects on kinetics and yields of specific intermediates and products for the one-electron oxidation by hydroxyl radicals of various (carboxylalkyl)thiopropionic acid derivatives, 3-(methylthio)propionic acid (3-MTPA), 3,3‘-thiodipropionic acid (3,3‘-TDPA), 3-(carboxymethylthio)propionic acid (3-CMTPA), and 2-(carboxymethylthio)succinic acid (2-CMTPA) have been investigated employing pulse radiolysis on the nanosecond to microsecond time scale, and γ-radiolysis. For each derivative, the initial step was a formation of a hydroxysulfuranyl radical proceeding with absolute rate constants of kOH+3-MTPA = 9.1 × 109 M-1 s-1 and kOH+3,3‘-TDPA = 5.8 × 109 M-1 s-1. The subsequent formation of one-electron-oxidized intermediates such as dimeric sulfur−sulfur (S∴S)-three-electron-bonded and monomeric sulfur−carboxylate oxygen (S−O)-bonded sulfide radical cations strongly depended on pH, thioether concentration, and the availability of α- or β-positioned carboxylate functions. A spectral resolution procedure permitted the quantification of all transients present in solution at any time after the pulse. Whereas both (S∴S)- and (S−O)-bonded intermediates were formed for 3-MTPA at neutral solution, electrostatic repulsion nearly prohibited the formation of dimeric (S∴S)-bonded intermediates between an overall negatively charged sulfide radical cation of 3,3‘-TDPA and a second nonoxidized, overall twice negatively charged, molecule of 3,3‘-TDPA. Neither sulfide radical cation complex (S∴S)+ was observed for 3-CMTPA and 2-CMPTA rationalized by a fast decarboxylation of the α-positioned carboxylate group, yielding α-(alkylthio)alkyl radicals which were the only products optically observed on the pulse radiolysis time scale. For 3,3‘-TDPA, the conversion of the initially formed hydroxysulfuranyl radicals into the (S−O)-bonded intermediates occurred unimolecularly with k ≅ 108 s-1 whereas the formation of the (S∴S)-bonded intermediates proceeded bimolecularly with k = (1.9−2.0) × 108 M-1 s-1. These processes did not occur competitively, as the intercepts of plots of pseudo-first-order rate constants for the formation of the S∴S bonded intermediates as a function of thioether concentration were too small (2.7 × 107 s-1) to contain the unimolecular rate constant for the formation of the (S−O)-bonded intermediate (k = 108 s-1). Therefore, a mechanism was proposed according to which initially formed hydroxysulfuranyl radicals rapidly converted into the σ*-type (S−O)-bonded intermediate. Subsequently, these either converted into (S∴S)-bonded radical cation complexes via a displacement of the carboxylate oxygen by a second nonoxidized sulfide function, or reversibly ring-opened to yield the monomeric sulfur-centered radical cation. The latter either associated with a nonoxidized sulfide or irreversibly cyclized to a σ-type (S−O)-bonded sulfuranyl radical.

One-Electron Oxidation and Reduction of Sulfites and Sulfinic Acids in Oxygenated Media: The Formation of Sulfonyl and Sulfuranyl Peroxyl Radicals

Bisulfite anions and sulfinic acids are shown by ESR spectroscopy to undergo reduction with radiolytically produced free electrons and oxidation by hole centers (Cl2•-) in a low-temperature aqueous LiCl glass. Electron-reduced intermediates display g anisotropy within the 2.010−2.002 range and are considered as neutral or singly deprotonated forms of sulfuranyl radicals (HO)3S•, (RO)2SOH•, and RS(OH)2•. The oxidized species produced by hole trapping are sulfite (SO3•-) and sulfonyl (RSO2•) radicals. Dialkyl sulfites scavenge free electrons only and are not oxidized by Cl2•-. Both oxidized and reduced intermediates add oxygen on annealing forming peroxyl radicals which are identified by their characteristic oxygen-17 hyperfine couplings. A large 100 G oxygen-17 coupling with the terminal oxygen atom found for oxidized sulfite−oxygen adduct -O3SOO• is in accord with its known high reactivity. Sulfuranyl peroxyls formed by the reductive pathway show lower 17O couplings (e.g., 90 G with terminal oxygens) and are expected to be far less reactive than alkyl peroxyls. Ab initio MO calculations predict stable structures for these sulfuranyl peroxyls, suggesting a trigonal-bipyramid coordination of the central sulfur atom. However, the sulfuranyl intermediates formed through one-electron reduction of dimethyl and methylphenyl sulfoxides are found to be unstable even in low-temperature glass, fragmenting into a methyl radical and sulfenic acid or into a hydroxyl anion and sulfide radical cation, respectively.

Formation and characterization of 1-substituted 1,2-dithiolanyl radicals by laser flash photolysis

Photolyses of 1-alkylthio-3-propanethiols, 1,3-propanedtthiol and 1-phenylthio-3-propanethiol in acetonitrtle, 2-propanol or aqueous mixtures of these solvents lead to five membered cyclic sulfuranyl radicals of the type \(\left[ {R - \overline {S - S - CH_2 - CH_2 - C} H_2 } \right]\) which exhibit characteristic uv/vis absorption spectra. These 1-substituted 1,2-dithiolanyl radicals are formed by intramolecular cyclization of the primarily formed thiyl radicals generaeed by S-H- bond cleavage in the excited starting material. The wavelength of the absorption maximum observed for these transients depends on the natuee of the substituntt R on the sulfur atom (R=H, alkyl, aryl).

Organic sulfuranyl radicals

Sulfuranyl radicals are intermediates in a variety of addition/elimination reactions of sulfides, disulfanes, sulfoxylates and S-neterocycles as well as in the reduction of triorgano- and heteroorgano sulfonium salts. Their lifetimes — depending on their substutution pattern — range from < 1 μs to being thermally stabee in solution at room temperature with no reduction in concentration over severll months.

Qualitative valence bond analysis of (10-S-4) sulfuranes and (9-S-3) sulfuranyl radicals

Valence-bond (VB) correlation diagrams are used for the analysis of sulfur hypervalent compounds. It is shown that the stability of (10-S-4) sulfuranes is closely related to the electronegativity of the apical substituents. Moreover, it is predicted that an antisymmetrical bond stretch of apical bonds is easier with less-electronegative substituents. The electronic structure of (9-S-3) sulfuranyl radicals is analyzed by means of ab initio calculations. VB correlation diagrams are then used to discuss the factors which favor the σ or π nature of these radicals. Qualitative arguments are developed for predicting the possible metastability of these radicals.

An ab initio SCF + CI study of the SH 3 and SF 3 radicals

The symmetric sulfuranyl radicals SH 3 and SF 3 are studied by means of ab initio SCF + CI calculations. All geometries are optimized at the UHF level using analytical gradients. SH 3 is found to be a transition state corresponding to a hydrogen exchange reaction, whereas SF 3 is stable with respect to decomposition to SF 2 + F.

ChemInform Abstract: Structures and Optical Absorption Spectra of Some Sulfuranyl Radicals in Solution.

AbstractMit dem Radikal (II) aus Di‐tert.‐butylperoxid (I) erhält man durch Reaktion mit dem Dioxathian (III) ein Radikal (IV).

Structures and optical absorption spectra of some sulfuranyl radicals in solution

Structures and optical absorption spectra of some sulfuranyl radicals in solution

Electron spin resonance spectra and molecular orbital theory of sulfuranyl radicals. Trans influence in free radicals

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectron spin resonance spectra and molecular orbital theory of sulfuranyl radicals. Trans influence in free radicalsA. R. Gregory, S. E. Karavelas, J. R. Morton, and K. F. PrestonCite this: J. Am. Chem. Soc. 1975, 97, 8, 2206–2208Publication Date (Print):April 1, 1975Publication History Published online1 May 2002Published inissue 1 April 1975https://doi.org/10.1021/ja00841a035RIGHTS & PERMISSIONSArticle Views61Altmetric-Citations14LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (333 KB) Get e-Alerts Get e-Alerts