Oxidation of thioethers to sulfoxides by iodine. II. Catalytic role of some carboxylic acid anions (original) (raw)
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Oxidation of Thioethers by Iodine to Sulfoxides. Catalytic Role of Certain Inorganic Nucleophiles1
Journal of the American Chemical Society, 1966
The rate of oxidation of several thioethers by iodine in aqueous solution has been found to be a complex function of the concentrations of sulfide, iodine, nucleophilic species, and iodide. In the absence of buffer species the rate law agrees with the formulationd[RSR']/dt = k[RSR'][(Iz)t~~l]/~+][I-]2. In the presence of species such as HP02-the rate has been found, however, to be substantially increased (IO4 times in 0.1 MHP042') and follows the relationship-d[RSR']/dt = k[RSR'][02)t,~3[HP042-]/[Hf][I-]2 over a limited range. A mechanism is proposed on the basis that the catalyzed reaction probably involves in this instance an oxidative phosphorylative step.
Oxidation of Thioethers by Iodine to Sulfoxides. Catalytic Role of Certain Inorganic Nucleophiles1
Journal of the American Chemical Society, 1966
The rate of oxidation of several thioethers by iodine in aqueous solution has been found to be a complex function of the concentrations of sulfide, iodine, nucleophilic species, and iodide. In the absence of buffer species the rate law agrees with the formulationd[RSR']/dt = k[RSR'][(Iz)t~~l]/~+][I-]2. In the presence of species such as HP02-the rate has been found, however, to be substantially increased (IO4 times in 0.1 MHP042') and follows the relationship-d[RSR']/dt = k[RSR'][02)t,~3[HP042-]/[Hf][I-]2 over a limited range. A mechanism is proposed on the basis that the catalyzed reaction probably involves in this instance an oxidative phosphorylative step.
The Journal of Physical Chemistry A, 2004
The mechanism of the • OH-induced oxidation of S-ethylthioacetate (SETAc) and S-ethylthioacetone (SETA) was investigated in aqueous solution using pulse radiolysis and steady-state γ radiolysis combined with chromatographic and ESR techniques. For each compound, • OH radicals were added, as an initial step, to the sulfur moiety, forming hydroxysulfuranyl radicals. Their subsequent decomposition strongly depended on the availability of Ror -positioned acetyl groups, pH, and the thioether concentration. For SETAc, which contains the R-positioned acetyl group, hydroxysulfuranyl radicals SETAc(> • S-OH) subsequently decay into secondary products, which do not include intermolecularly three-electron-bonded dimeric radical cations, even at high concentrations of SETAc. At low pH, these observations are rationalized in terms of the highly unstable nature of sulfur monomeric radical cations SETAc(>S +• ) because of their rapid conversion via deprotonation to the R-(alkylthio)alkyl radicals H 3 C-• CH-S-C(dO)-CH 3 (λ max ) 420 nm). However, at low proton concentrations, the R-positioned acetyl group destabilizes SETAc(> • S-OH) radicals within a five-membered structure that leads to the formation of alkyl-substituted radicals, H 3 C-CH 2 -S-C(dO)-• -CH 2 . A somewhat different picture is observed for SETA, which contains a -positioned acetyl group. The main pathway involves the formation of hydroxysulfuranyl radicals SETA(> • S-OH) and R-(alkylthio)alkyl radicals H 3 C-CH 2 -S-• CH-C(dO)-CH 3 (λ max ) 380 nm). The latter radicals are highly stabilized through the combined effect of both substituents in terms of the captodative effect. At low pH, SETA(> • S-OH) radicals undergo efficient conversion to intermolecularly three-electron-bonded dimeric radical cations SETA-(>S∴S<) + (λ max ) 500 nm), especially for high SETA concentrations. In contrast, at low proton concentrations, SETA(> • S-OH) radicals decompose via the elimination of water, formed through intramolecular hydrogen transfer within a six-membered structure that leads to the formation of alkyl-substituted radicals, H 3 C-CH 2 -S-CH 2 -C(dO)-• CH 2 . The latter radicals undergo a 1,3-hydrogen shift and intramolecular hydrogen abstraction within the six-membered structure, leading to the R-(alkylthio)alkyl radicals H 3 C-CH 2 -S-• CH-C(dO)-CH 3 and H 3 C-• CH-S-CH 2 -C(dO)-CH 3 , respectively. To support our conclusions, quantum mechanical calculations were performed using density functional theory (DFT-B3LYP) and second-order Møller-Plesset perturbation theory (MP2) to calculate the bond-formation energies of some key transients and the location and strength of their associated optical absorptions. * Corresponding we have extended our studies to the reactions of the • OH radicals with two model thioether compounds, S-ethylthioacetate and S-ethylthioacetone, containing acetyl groups in the R and positions with respect to the sulfur atom, respectively. Furthermore, to support our conclusions, we performed quantum mechanical ab initio calculations by using density functional theory (DFT-B3LYP) and second-order Møller-Plesset perturbation theory (MP2) to calculate the bond formation energies of some of the key transients and the location and strength of their associated optical absorptions.
International Journal of Chemical Kinetics, 2000
The substituted thiourea, 4-methyl-3-thiosemicarbazide, was oxidized by iodate in acidic medium. In high acid concentrations and in stoichiometric excess of iodate, the reaction displays an induction period followed by the formation of aqueous iodine. In stoichiometric excess of methylthiosemicarbazide and high acid concentration, the reaction shows a transient formation of aqueous iodine. The stoichiometry of the reaction is: 4IO 3 Ϫ ϩ 3CH 3 NHC("S)NHNH 2 ϩ 3H 2 O : 4I Ϫ ϩ 3SO 4 2Ϫ ϩ 3CH 3 NHC("O)NHNH 2 ϩ 6H ϩ (A). Iodine formation is due to the Dushman reaction that produces iodine from iodide formed from the reduction of iodate: IO 3 Ϫ ϩ 5I Ϫ ϩ 6H ϩ : 3I 2 (aq) ϩ 3H 2 O (B). Transient iodine formation is due to the efficient acid catalysis of the Dushman reaction. The iodine produced in process B is consumed by the methylthiosemicarbazide substrate. The direct reaction of iodine and methylthiosemicarbazide was also studied. It has a stoichiometry of 4I 2 (aq) ϩ CH 3 NHC (" S)NHNH 2 ϩ 5H 2 O : 8I Ϫ ϩ SO 4 2Ϫ ϩ CH 3 NHC("O)NHNH 2 ϩ 10H ϩ (C). The reaction exhibits autoinhibition by iodide and acid. Inhibition by I Ϫ is due to the formation of the triiodide species, I 3 Ϫ , and inhibition by acid is due to the protonation of the sulfur center that deactivates it to further electrophilic attack. In excess iodate conditions, the stoichiometry of the reaction is 8IO 3 Ϫ ϩ 5CH 3 NHC("S)NHNH 2 ϩ H 2 O : 4I 2 ϩ 5SO 4 2Ϫ ϩ 5CH 3 NHC("O)NHNH 2 ϩ 2H ϩ (D) that is a linear combination of processes A and B.
Journal of Photochemistry and Photobiology A: Chemistry, 2016
The TiO 2 photosensitized oxidation of benzyl methyl sulfides (X-C 6 H 4 CH 2 SCH 3) and benzyl thiols (X-C 6 H 4 CH 2 SH) has been investigated in Ar-saturated CH 3 CN. Steady-state irradiation produced benzaldehydes or dibenzylsulfides as oxidation products with sulfides and thiols, respectively. The results obtained through kinetic competitive experiments, aimed to evaluate the ring substituent effect on the reactivity, suggested the involvement of radical cation intermediates, formed by the favorable electron transfer from the substrate to the TiO 2 photogenerated hole, which reasonably deprotonate to give the final products. This process was poorly affected by the adsorption of the substrate at the TiO 2 surface, as demonstrated by similar results, both in terms of products and reactivity, collected for the homogeneous photooxidation of the same substrates sensitized by N-methoxyphenanthridinium hexafluorophosphate (MeOP + PF 6 À). This behavior is likely due to the low hydrogen-bond acceptor ability of divalent sulfur systems. Density functional theory calculations pointed out that the most stable conformations of X-C 6 H 4 CH 2 SH + are characterized by having the CÀ ÀS bond almost collinear with the p system of the aromatic ring and by a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom. 2016 Elsevier B.V. All rights reserved.
Asymmetric oxidation of thioethers
Tetrahedron Letters, 1989
p-hydroqsulfoxides. of fairly high optical purity (up to 80%) may be prepared by direct asymmetric oxidation [EutO~H. Ti(OPr')d, (+)-DET] of acetylated or silylated S-methyl p-hydronysuljides and subsequent deprotection. The upgrading of the optical purity from 78% up to ~98% by simple crystallization has been obtained. Access to optically pure a-hydroxysulfoxides by direct asymmetric oxidation of the parent a-hydroxysulfides would be a significant synthetic achievement in view of their vast utilization in asymmetric transformations2. Unfortunately, the titanium-catalyzed procedure developed in our laboratory for the asymmetric oxidation of thioethers3, while rather effective for some classes of substrates e.g. methylaryl sulfides3 and 2-substituted-dithiolanes4~5 (e.e. 70-98%), gives poor results in the oxidation of a-hydroxysulfides6. Unsatisfactory results have also been obtained when the Sharpless reagent7 or the Kagan's procedure* were employed. We have recently reported6 that the enantioselection in the oxidation of a series of I+hydroxysulfides (Ph-CH(OH)-CHa-S-R; R=t-Butyl, o-Tolyl, Naphtyl; e.e. 20-47%) is lower than that attainable in the oxidation of structurally similar unfunctionalyzed sulfides. Therefore our data indicate that, at variance with the asymmetric epoxidation of allylic alcohols by the Sharpless reagent7, in p-hydroxysulfide oxidations the hydroxy group plays a negative role as far as the enantioselection is concerned. At the same time we have critically examined all the data on sulfide asymmetric oxidations9 reaching the conclusion that the simple steric differentiation of the two groups attached to the sulfur atom plays a dominant role in determining high enantioselections9.
Thiocarbonyl versus carbonyl compounds: A comparison of intrinsic reactivities
Journal of The American Chemical Society, 1993
The first systematic comparison of structural effects on the intrinsic reactivities of carbonyl and thiocarbonyl compounds has been carried out. To this end, the gas-phase basicities (GB) of a wide variety of thiocarbonyl compounds XCSY (as well as of some carbonyl derivatives) were determined by means of Fourier transform ion cyclotron resonance spectrometry (FTICR) and SCF and MP2 ab initio calculations at different levels of accuracy were performed on 27 different neutral compounds and their protonated forms. The same set, enlarged by the inclusion of very large systems such as di-tert-butyl-and bis-( 1-adamanty1)thioketones was also investigated at the AM 1 semiempirical level in order to get a more complete view of structural effects. The agreement between the calculated and the experimental changes in thermodynamic state functions is good in all instances. Correlation analysis of the experimental data shows that (i) substituent effects on the gas-phase basicity of thiocarbonyl compounds are linearly related to those of their carbonyl homologs with a slope of 0.80 and (ii) these effects can be quantitatively analyzed in terms of polarizability, field, and resonance effects (Taft-Topsom model). Comparison of the GBs of thiocarbonyl and carbonyl compounds with solution basicities and nucleophilicities sheds light on differential structural and solvation effects. Substituent effects on both neutral and protonated species were explored by means of appropriate isodesmic reactions. These results confirm that all thiocarbonyl compounds investigated are sulfur bases in the gas phase. The features revealed by correlation analysis can be rationalized in terms of the interactions between the MOs of the substituent and the parent compound. (4) See, for instance: (a) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley: New York, 1986. (b) Bouchoux. G.; Flament, J. P.; HoDDilliard, Y.; Tortaiada, J.; Flammang, R.; Maquestiau, A. J. Am. Chem. SoE.i989,11,5560. (cjAlcaml, M.; M6,O.; YBBez, M.; Anvia, F.; Taft, R. W. McMahon, T. B.; Surjasasmita, I.; Roth, L. M.; Gord, J. R.; Freiser, B. S. Int. J. Mass Spectrom. Ion Processes 1991, 109, 15. (f) Alcami, M.; M6.0.; Ydfiez, M.; Abboud, J.-L. M. Phys. Org. Chem. 1991.4, 177. (9) Tortajada, J.; Total, A,; Morizur, J. P.; Alcami, M.; Mb, 0.; Yifiez, M. The pioneering work on the gas-phase ion chemistry of thiocarbonyl compounds was mostly focused on the reactivity of thioacylium cations: (a) Caserio, M. J.; Kim, J. K. J. Am. Chem. Soc. 1983, 105, 6896. (b) Caserio, M. J.; Kim, J. K. In Nucleophilicity; Harris, J. M., McManus, S. P., Eds.; Advances in Chemistry Series 2 15; American Chemical Society: Washington, DC, 1987; Chapter 5. (c) Paradisi, C.; Kenttamaa, H.; Lee, Q. T.; Caserio, M. J. Org. Mass Spectrom. 1988, 23, 521 and references therein.