Unsensitized photooxidation of sulfur compounds with molecular oxygen in solution (original) (raw)
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Photosensitized oxidation of phenyl and tert-butyl sulfides
Photochemical & Photobiological Sciences, 2004
The photosensitized oxygenation of diphenyl (1), di-tert-butyl (2) and phenyl tert-butyl sulfide (3) was studied. Bimolecular rate constants of singlet oxygen quenching are low (1 to 5 × 10 4 M Ϫ1 s Ϫ1) since the sulfides are poor nucleophiles due to sterical hindrance (2, 3) or the HOMO on the sulfur atom being a less accessible p z orbital (1). The quenching is mainly physical, but chemical reaction leading to sulfoxides also takes place in methanol and, to a lower degree, in acetonitrile. Catalysis by carboxylic acids considerably enhances the rate of sulfoxidation. Inefficiency in the chemical reaction is again due to the poor nucleophilicity of the sulfides, which limits oxygen transfer by electrophilic intermediates such as the protonated persulfoxide.
Direct Irradiaton of Aryl Sulfides: Homolytic Fragmentation and Sensitized S-Oxidation
The Journal of Organic Chemistry, 2017
The direct irradiation of diphenyl sulfide and p-substituted thioanisoles in the presence of oxygen was investigated by means of both steady state and laser flash photolysis experiments. Two competitive pathways took place from the triplet excited state of thioanisoles, C-S bond cleavage, finally leading to aryl sulfinic acid and sensitized oxidation leading to S-oxidation. Co-oxidation of dodecyl methyl sulfide occurred efficiently implying that an S-persulfoxide intermediate is involved during the sensitized oxidation. On the other hand, triplet state of diphenyl sulfide also showed competitive C-S bond cleavage giving phenyl sulfinic acid and ionization to diphenyl sulfide radical cation that in turn led to diphenyl sulfoxide. The rate constants of the above reactions were determined by time-resolved experiments.
Explorative and Mechanistic Studies of the Photooxygenation of Sulfides
2003
The results of recent work on the dye-sensitized photooxygenation of sulfides is discussed. In the case of dialkyl sulfides, the weakly bonded adduct initially formed with singlet oxygen (the persulfoxide) decays unproductively unless protonation by an acid (an alcohol or a carboxylic acid) facilitates its conversion to the sulfoxide. The effect is proportional to the strength of the acid ( eg., less than 0.1% chloroacetic acid in benzene is sufficient for maximal efficiency) and corresponds to general acid catalysis, suggesting that protonation of the persulfoxide occurs. On the other hand, with sulfides possessing an activated hydrogen in α position (eg., benzyl and allyl sulfides), hydrogen transfer becomes an efficient process in aprotic media and yields a S-hydroperoxysulfoniumm ylide, possibly arising from a conformation of the persulfoxide that is different from the one protonated in the presence of acids. Calculations on some substituted sulfides support this hypothesis. Thi...
Photochemical carbon–sulfur bond cleavage in some alkyl and benzyl sulfides
Inorganica Chimica Acta, 2007
Irradiation (254 nm) of five alkyl and benzyl ethyl sulfides causes efficient (U r 0.27-0.90) homolytic cleavage of the C-S bond. Of the resulting fragments, thiyl radicals mainly couple, while alkyl radicals abstract hydrogen, disproportionate or couple when stabilized (benzyl). Selective trapping of either of the two types of radicals occurs in the presence of nucleophilic (methyl vinyl ether and 1-hexene) and, respectively, electrophilic (acrylonitrile) alkenes. When an easily oxidized radical is formed, e.g. cumyl, secondary electron transfer leads to the corresponding cation.
The Journal of Organic Chemistry, 1995
The photochemistry of aryl benzyl sulfoxides is described. The initial event is homolytic cleavage to form a singlet sulfinylhenzyl radical pair. This radical pair partitions between reversion to starting material with at least partial racemization and closure to form a sulfenic ester. With acetone sensitization, the primary radical pair also undergoes quite significant escape, leading to formation of diphenylethane and aryl arenethiosulfonates. Secondary photolysis of the sulfenic ester leads exclusively to S-0 homolysis, yielding the radical pair from which isolated products are derived. Quantum yields and other mechanistic observations are discussed.
Effect of Protic Cosolvents on the Photooxygenation of Diethyl Sulfide
The Journal of Organic Chemistry, 2000
The sluggish (k r < 10k q) photooxygenation of diethyl sulfide in both benzene and other aprotic solvents such as acetone and acetonitrile is made efficient by addition of small amounts of alcohols and, with a much more conspicuous increase, of phenols and carboxylic acids (,0.1% additive is sufficient in this case). A kinetic analysis shows that the effect is accounted for by interaction of the protic additives with the first formed intermediate, the persulfoxide, in competition with cleavage to the components. The thus obtained rate constants k H linearly correlate with the acid strength of the additives, and the effect is rationalized as a general acid catalysis. Hydrogen bonding of the persulfoxide under this condition accounts in an economic way for the observed data, including co-oxygenation of Ph 2 SO in mixed solvents. The photooxygenation of sulfides continues to be a topic of mechanistic debate. 1 A key issue that must be addressed is why alkyl sulfides are good quenchers of singlet oxygen (1.7 × 10 7 M-1 s-1 for diethyl sulfide, 2 independently from the nature of the solvent), 3 while the rate of chemical reaction (k r , eq 1) is at least an order of
Journal of the American Chemical Society, 1997
A detailed ab initio study of the structures and energetics of the persulfoxides and thiadioxiranes derived from sulfenic acid derivatives (RSX) is reported. The persulfoxides adopt structures in which the O-O bond bisects the RSX angle while the thiadioxiranes prefer a distorted trigonal bypyramidal geometry. The thiadioxiranes are more stable in every case than their persulfoxide isomer. The exothermicities of the interconversions of the persulfoxide to the thiadioxirane increase in the substituent (X) order CH 3 < NH 2 < Cl < OCH 3 < SCH 3 < F from a low of 3 kcal/mol to a high of 31 kcal/mol. The activation barriers, on the other hand, decrease in the substituent order Cl > CH 3 ≈ NH 2 > OCH 3 ≈ SCH 3 > F from 27 to 10 kcal/mol. Only those persulfoxides which do not have a hydrogen on a heteroatom X exist in well-defined minima on the potential energy surface. Attempted minimization with tight convergence criteria of persulfoxides with heteroatom X-H bonds resulted in collapse via ene-like reactions to give hydroperoxy sulfonium ylides. In the case where X-H is N-H, the resulting hydroperoxysulfonium ylide (iminopersulfinic acid) adopts a hydrogen-bonded structure reminiscent of peracids. Experimental evidence for the formation of these new peroxides was obtained by photooxidations of N-methyl-, N-n-butyl-, and N-tertbutylbenzenesulfenamides.
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.