Substituent effects on the TiO2 photosensitized oxidation reaction of benzyl thioethers and thiols in deaerated acetonitrile (original) (raw)
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Journal of Photochemistry and Photobiology A-chemistry, 2004
The quantum yields (Φ) of the colloidal TiO 2 -sensitized photooxidation of 4-(1) and 3-methoxybenzyl alcohol (2) together with 4-methoxybenzyltrimethyl-(3) and 4-methoxybenzyltriisopropylsilane (4) were determined in CH 3 CN, in the presence of HClO 4 for 3 and 4. The true quantum yields (Φ 0 ) of 1, 2 and 3, obtained from a Langmuir-Hinshelwood isotherm treatment of Φ at different substrate concentrations, are linearly correlated with I −1/2 A , where I A is the light intensity. According to the previously suggested mechanisms for alcohols and silanes, a kinetic scheme that justifies this correlation is suggested. It is shown that the ratio of the slopes (from the Φ 0 versus I −1/2 A plots) for 1 and 2 is equal to the Φ 1 0 /Φ 2 0 ratio at any I A ; this ratio depends on the rate constants in the kinetic scheme, in this case principally on the electron transfer constant, k et . On the contrary, the Φ 3 0 /Φ 4 0 ratio depends on k p , the cation radical desilylation rate constant, confirming a steric hindrance to nucleophylic assistance in the C-Si fragmentation by the bulky isopropyl group in the SiR 3 moiety. Differently from Φ 0 , the adsorption constants on the semiconductor under irradiation (K D , obtained from the above isotherm treatment) are independent of I A . Moreover, as K 1 D = K 2 D and K 3 D = K 4 D , the structural modifications within the two alcohols and within the two silanes should be far enough away from the adsorption site. For all the substrates, K (the dark adsorption constant) is five times greater than K D , showing that this change does not depend on the substrate structure but is the result of different experimental conditions.
Unsensitized photooxidation of sulfur compounds with molecular oxygen in solution
Tetrahedron, 1997
The short wavelength irradiation of aliphatic disulfides, sulfides and of n-butanethiol in alcohols or aqueous acetonitrile in the presence of oxygen was investigated : the corresponding sulfonic acids are produced in good yields for short alkyl chain compounds, together with smaller amounts of sulfuric and carboxylic acids. In acetonitrile, the influence of added water on the reaction course is evidenced : increased reaction rate and acid yields, control of sulfuric acid formation. Intermediates such as sulfinic acid and thiosulfonate were detected and their rates of formation were monitored. The reaction appears to involve thiyl radicals giving rise to sulfonyl radicals in the presence of oxygen. A first tentative hypothesis concerning the mechanism is advanced. © 1997, Elsevier Science Ltd. All rights reserved.
Journal of the American Chemical Society, 2003
Photooxygenations of PhSMe and Bu2S sensitized by N-methylquinolinium (NMQ + ) and 9,10-dicyanoanthracene (DCA) in O2-saturated MeCN have been investigated by laser and steady-state photolysis. Laser photolysis experiments showed that excited NMQ + promotes the efficient formation of sulfide radical cations with both substrates either in the presence or in absence of a cosensitizer (toluene). In contrast, excited DCA promotes the formation of radical ions with PhSMe, but not with Bu2S. To observe radical ions with the latter substrate, the presence of a cosensitizer (biphenyl) was necessary. With Bu2S, only the dimeric form of the radical cation, (Bu2S)2 +• , was observed, while the absorptions of both PhSMe +• and (PhSMe)2 +• were present in the PhSMe time-resolved spectra. The decay of the radical cations followed second-order kinetics, which in the presence of O2, was attributed to the reaction of the radical cation (presumably in the monomeric form) with O2 -• generated in the reaction between NMQ • or DCA -• and O2. The fluorescence quenching of both NMQ + and DCA was also investigated, and it was found that the fluorescence of the two sensitizers is efficiently quenched by both sulfides (rates controlled by diffusion) as well by O2 (kq ) 5.9 × 10 9 M -1 s -1 with NMQ + and 6.8 × 10 9 M -1 s -1 with DCA). It was also found that quenching of 1 NMQ* by O2 led to the production of 1 O2 in significant yield (φ∆ ) 0.86 in O2-saturated solutions) as already observed for 1 DCA*. The steadystate photolysis experiments showed that the NMQ + -and DCA-sensitized photooxygenation of PhSMe afford exclusively the corresponding sulfoxide. A different situation holds for Bu2S: with NMQ + , the formation of Bu2SO was accompanied by that of small amounts of Bu2S2; with DCA, the formation of Bu2SO2 was also observed. It was conclusively shown that with both sensitizers, the photooxygenations of PhSMe occur by an electron transfer (ET) mechanism, as no sulfoxidation was observed in the presence of benzoquinone (BQ), which is a trap for O 2 -• , NMQ • , and DCA -• . BQ also suppressed the NMQ + -sensitized photooxygenation of Bu2S, but not that sensitized by DCA, indicating that the former is an ET process, whereas the second proceeds via singlet oxygen. In agreement with the latter conclusion, it was also found that the relative rate of the DCA-induced photooxygenation of Bu 2S decreases by increasing the initial concentration of the substrate and is slowed by DABCO (an efficient singlet oxygen quencher). To shed light on the actual role of a persulfoxide intermediate also in ET photooxygenations, experiments in the presence of Ph 2SO (a trap for the persulfoxide) were carried out. Cooxidation of Ph2SO to form Ph2SO2 was, however, observed only in the DCA-induced photooxygenation of Bu2S, in line with the singlet oxygen mechanism suggested for this reaction. No detectable amounts of Ph2SO2 were formed in the ET photooxygenations of PhSMe with both DCA and NMQ + and of Bu2S with NMQ + . This finding, coupled with the observation that 1 O2 and ET photooxygenations lead to different product distributions, makes it unlikely that, as currently believed, the two processes involve the same intermediate, i.e., a nucleophilic persulfoxide. Furthermore, the cooxidation of Ph 2SO observed in the DCA-induced photooxygenation of Bu2S was drastically reduced when the reaction was performed in the presence of 0.5 M biphenyl as a cosensitizer, that is, under conditions where an (indirect) ET mechanism should operate. This observation confirms that a persulfoxide is formed in singlet oxygen but not in ET photosulfoxidations. The latter conclusion was further supported by the observation that also the intermediate formed in the reaction of thianthrene radical cation with KO 2, a reaction which mimics step d (Scheme 2) in the ET mechanism of photooxygenation, is an electrophilic species, being able to oxidize Ph 2S but not Ph2SO. It is thus proposed that the intermediate involved in ET sulfoxidations is a thiadioxirane, whose properties (it is an electrophilic species) seem more in line with the observed chemistry. Theoretical calculations concerning the reaction of a sulfide radical cation with O 2 -• provide a rationale for this proposal.
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.
Oxidation of nauseous sulfur compounds by photocatalysis or photosensitization
Catalysis Today, 2007
Reduced sulfur compounds such as methanethiol (MSH), dimethylsulfide (DMS) and dimethydisulfide (DMDS) are nauseous by-products produced by a great number of industrial processes. Oxidation of these reduced sulfur compounds in polluted atmospheres and hence the decrease of their harmful and malodorous effects is thus a matter of concern in numerous industrial and water treatment plants. Photocatalytic treatment of gaseous flow polluted by these sulfur compounds has been actively investigated for the last few years.
Photo-oxidation and Thermal Oxidations of Triptycene Thiols by Aryl Chalcogen Oxides
ACS Omega, 2020
Oxidation of thiols yield sulfenic acids, which are very unstable intermediates. As sulfenic acids are reactive, they form disulfides in the presence of thiols. However, sulfenic acids also oxidize to sulfinic acids (−SO 2 H) and sulfonic acids (−SO 3 H) at higher concentrations of oxidants. Hydrogen peroxide is a commonly used oxidant for the oxidation of thiols to yield sulfenic acids. However, hydrogen peroxide also oxidizes other reactive functional groups present in a molecule. In this work, the reaction intermediates arising from the oxidation of sterically hindered thiols by aryl chalcogen oxides, dibenzothiophene S-oxide (DBTO), dibenzoselenophene Se-oxide (DBSeO), and dibenzotellurophene Te-oxide (DBTeO), were investigated. Photodeoxygenation of DBTO produces triplet atomic oxygen [O(3 P)], which has previously shown to preferentially react with thiols over other functional groups. Similarly, aryl selenoxides have also shown that they can thermally react selectively with thiols at room temperature to yield disulfides. Conversely, aryl telluroxides have been reported to oxidize thiols to disulfides thermally with no selectivity toward thiols. The results from this study demonstrate that sulfenic acids are an intermediate in the oxidation of thiols by DBTeO and by photodeoxygenation of DBTO. The results also showed that the oxidation of thiols by DBSeO yields sulfonic acids. Triptycene-9thiol and 9-fluorotriptycene-10-thiol were for the thiols used in this oxidation reaction. This work expands the list of oxidants that can be used to oxidize thiols to obtain sulfenic acids.
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.
Mechanism of oxidation of α,β-unsaturated thiones by singlet oxygen
Tetrahedron, 1985
Photo-oxidation of &unsaturated thioncs yields the corresponding ketones as the only product. Studies carried out on three systems, namely thioketones, a&unsaturated thiones and thioketenes, have revealed that there exists a similarity in their mechanism of oxidation. It has been suggested that the thiocarbonyl chromophore is the site of attack by singlet oxygen in a&unsaturated thionea and that the adjacent CC double bond is inert under these conditions. Absence of sulphine during the oxidation of IX,/?unsaturated thiones is attributed to the electronic factors operating on the zwitterionic/diradical intermediate. While a&unsaturated ketones are poorly reactive, a&unsaturated thiones are highly reactive toward singlet oxygen.
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.
Physical Chemistry Chemical Physics, 2010
The TiO 2 photosensitized oxidation in water of a series of X-ring substituted benzyl alcohols gives the corresponding benzaldehyde. Kinetic evidence (from competitive experiments) suggests a single electron transfer (SET) mechanism with a changeover of the electron abstraction site from the aromatic moiety (X = 4-OCH 3 , 4-CH 3 , H and 3-Cl) to the hydroxylic group (X = 3-CF 3 and 4-CF 3 ), probably due to the preferential adsorption of the above OH group on the TiO 2 surface. The same photo-oxidation of a series of 1-(X-phenyl)-1,2-ethanediols and of 2-(X-phenyl)-1,2-propanediols gives the corresponding benzaldehyde and acetophenone, respectively, accompanied by formaldehyde, whereas a series of symmetrically X-ring-substituted 1,2-diphenyl-1,2-ethanediols yields the corresponding benzaldehyde (substrate/product molar ratio = 0.5). The relative rate values suggest a SET mechanism in all of the series, with electron abstraction from one of the two OH groups of all the considered diols, probably due to the much higher adsorption of the above groups (due to the chelation effect) on the semiconductor. Further confirmation of this mechanistic behaviour has been obtained from laser flash photolysis experiments.