Substituent-dictated partitioning of intermediates on the sulfide singlet oxygen reaction surface. A new mechanism for oxidative C--S bond cleavage in alpha-hydroperoxy sulfides.

The reactions of singlet oxygen with 17 sulfides bearing either anion or radical stabilizing substituents are reported. The abilities of substituents to modify product compositions in both the oxidative cleavage and sulfide oxidation pathways are analyzed in terms of partitioning of the hydroperoxy sulfonium ylide intermediate. Evidence is presented that suggests that the hydroperoxy sulfonium ylide exists in both diradical and zwitterionic forms. In addition, both inter- and intramolecular pathways for decomposition of alpha-hydroperoxy sulfides are suggested to rationalize the substituent-dependent formation of oxidative C--S bond cleavage products.     

MOLECULAR REARRANGEMENTS. VI. THERMOLYSIS OF α-(N-ARYL)BENZYL ARYL SULFIDES

Abstract

In view of the significance of studying the effect of heat on organic compounds especially those containing nitrogen and/or sulfur, six substituted α-(N-aryl) benzyl aryl sulfides (4–9) were prepared and subjected to thermolysis at 200°C.

Surveying of the reaction products showed the formation of amines, benzylidene amines, sulfides, disulfides, dibenzyl, stilbene, benzthiazole, benzimidazole, tetraphenyl thiophene as principle products in variable ratios according to the type of the starting sulfide.

A mechanism involving the homolytic cleavage of the C[sbnd]S and the C[sbnd]N bond is suggested to rationalize the thermolysis products.     

Oxygen transfer reactions. 4. Reaction of high valent oxoruthenium compounds with sulfides

The oxidation of methoxy substituted benzyl phenyl sulfides can be used to distinguish between oxidants that react by single electron transfer (followed by oxygen rebound) and those which react by direct oxygen atom transfer in a two-electron process. Transfer of a single electron results in the formation of an intermediate radical cation, which can undergo C-S bond cleavage and deprotonation reactions leading to the formation of methoxy substituted benzyl derivatives, methoxy substituted benzaldehydes, and diphenyl disulfide. The oxidation of 4-methoxybenzyl phenyl sulfide and 3,4,5-trimethoxybenzyl phenyl sulfide by oxidants known to participate in single electron transfers (Ce(4+), Mn(3+), and Cr(6+)) results in the formation of the corresponding benzaldehydes, benzyl alcohols, benzyl acetates, and benzyl nitrates in variable yields. However, the only products obtained from the oxidation of the same compounds with RuO(4), RuO(4-), and RuO(4)(2-) are sulfoxides and sulfones. Therefore, it is concluded that the oxidation of sulfides by oxoruthenium compounds likely proceeds by a concerted mechanism.     

Photofragmentation of Dibenzyl Ketone in the Presence of Tetramethylpiperidine Derivatives. Studies on light stabilizer mechanisms [1]

The interaction of selected tetramethylpiperidine derivatives with radicals arising from the Norrish-type I cleavage of dibenzyl ketone under oxygen was studied. Product analyses and kinetic studies showed that the investigated sterically hindered piperidine derivatives have a pronounced effect on both the nature and distribution of the products of photolysis of dibenzyl ketone in the presence of oxygen. Observations indicated that the phenylperacetoxyl radical is formed as an intermediate during irradiation and that it interacts with the additives used. Possible mechanisms of the reactions studied are discussed. The observation that oxidation of an isolated double bond by the radicals formed in dibenzyl ketone photolysis under oxygen is strongly inhibited in the presence of the studied sterically hindered amines is discussed in the light of the results presented. The findings are considered in relation to the problem of polymer stabilization.     

The Cp2ZrCl2-catalyzed cycloalumination of functionally substituted olefins with triethylaluminum

Reactions of functionally substituted olefins (allylamines, sulfides and ethers, homoallylic alcohols and amines, as well as vinyl ethers) with Et₃Al in the presence of Cp₂ZrCl₂ as a catalyst were studied. Cycloalumination of allylamines occurs with high regioselectivity to furnish after subsequent deuterolysis 4-deutero-2-(deuteromethyl)butyl-substituted amines. Cycloalumination of alkyl allyl sulfide is accompanied by a side process of the C-S bond cleavage. In the case of allyl and vinyl ethers, no cycloalumination products are formed under the reaction conditions. However, the reactions with homoallylic alcohol and amine after deuterolysis gave the corresponding dideutero-containing compounds in good yields.     

Bonded exciplexes. A new concept in photochemical reactions

Charge-transfer quenching of the singlet excited states of cyanoaromatic electron acceptors by pyridine is characterized by a driving force dependence that resembles those of conventional electron-transfer reactions, except that a plot of the log of the quenching rate constants versus the free energy of electron transfer is displaced toward the endothermic region by 0.5-0.8 eV. Specifically, the reactions with pyridine display rapid quenching when conventional electron transfer is highly endothermic. As an example, the rate constant for quenching of the excited dicyanoanthracene is 3.5 x 10(9) M(-1)s(-1), even though formation of a conventional radical ion pair, A*-D*+, is endothermic by approximately 0.6 eV. No long-lived radical ions or exciplex intermediates can be detected on the picosecond to microsecond time scale. Instead, the reactions are proposed to proceed via formation of a previously undescribed, short-lived charge-transfer intermediate we call a "bonded exciplex", A- -D+. The bonded exciplex can be formally thought of as resulting from bond formation between the unpaired electrons of the radical ions A*- and D*+. The covalent bonding interaction significantly lowers the energy of the charge-transfer state. As a result of this interaction, the energy decreases with decreasing separation distance, and near van der Waals contact, the A- -D+ bonded state mixes with the repulsive excited state of the acceptor, allowing efficient reaction to form A- -D+ even when formation of a radical ion pair A*-D*+ is thermodynamically forbidden. Evidence for the bonded exciplex intermediate comes from studies of steric and Coulombic effects on the quenching rate constants and from extensive DFT computations that clearly show a curve crossing between the ground state and the low-energy bonded exciplex state.     

Photochemistry of carboxylic acids containing the phenyl and thioether groups: Steady-state and laser flash photolysis studies

The mechanisms for the direct photolysis of phenylthioacetic acid (PTAA) and S-benzylthioglycolic acid (SBTGA) in acetonitrile were investigated using steady-state and laser flash photolysis. For both compounds, a variety of stable photoproducts were found under steady-state, 254 nm irradiation of acetonitrile solutions. The products from the direct photolysis of PTAA included carbon dioxide (photodecarboxylation), acetic acid, diphenyl disulfide, diphenyl sulfide, thiophenol, thioanisole, di(phenylthio) methane, and S-phenyl benzenethiosulfate. The products from the direct photolysis of SBTGA included carbon dioxide, toluene, dibenzyl, dibenzyl sulfide, dibenzyl disulfide, thioglycolic acid, benzyl mercaptan, benzyl alcohol, and benzaldehyde. These stable photoproducts were identified and characterized using HPLC, GC, GC–MS, and UV–vis methods. Quantum yields were determined for the formation of the various stable products following steady-state irradiations in the absence and in the presence of oxygen. In laser flash photolysis (266 nm Nd:YAG laser) experiments, a variety of transients (e.g., phenylthiyl radical, benzyl radical, etc.) was found. For both substrates (PTAA and SBTGA), photoinduced CS bond cleavage was the main primary process. It was also found that for both acids, photoinduced CC bond cleavage occurred, but as a minor process relative to CS bond cleavage. Detailed mechanisms of the primary and secondary processes are proposed and discussed. The validity of these proposed mechanisms was supported by an analysis of the quantum yields of stable products and their transient precursors. Supplementary observations on reactions between the radicals (e.g., C₆H₅–S, C₆H₅–CH₂) and oxygen are also consistent with the proposed mechanisms.     

Mechanism of the oxidation of aromatic sulfides catalysed by a water soluble iron porphyrin

The oxygen atom transfer-electron transfer (ET) mechanistic dichotomy has been investigated in the oxidation of a number of aryl sulfides by H2O2 in acidic (pH 3) aqueous medium catalysed by the water soluble iron(III) porphyrin 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p",p"'-tetrasulfonic acid iron(III) chloride (FeTPPSCl). Under these reaction conditions, the iron-oxo complex porphyrin radical cation, P+. Fe(IV)=O, should be the active oxidant. When the oxidation of a series of para-X substituted phenyl alkyl sulfides (X = OCH3, CH3, H, Br, CN) was studied the corresponding sulfoxides were the only observed product and the reaction yields as well as the reactivity were little influenced by the nature of X as well as by the bulkiness of the alkyl group. Labelling experiments using H(2)18O or H(2)18O2 clearly indicated that the oxygen atom in the sulfoxides comes exclusively from the oxidant. Moreover, no fragmentation products were observed in the oxidation of a benzyl phenyl sulfide whose radical cation is expected to undergo cleavage of the beta C-H and C-S bonds. These results would seem to suggest a direct oxygen atom transfer from the iron-oxo complex to the sulfide. However, competitive experiments between thioanisole (E degree = 1.49 V vs. NHE in H2O) and N,N-dimethylaniline (E degree = 0.97 V vs. NHE in H2O) resulted in exclusive N-demethylation, whereas the oxidation of N-methylphenothiazine (10, E degree = 0.95 V vs. NHE in CH3CN) and N,N-dimethyl-4-methylthioaniline (11, E degree = 0.65 V vs. NHE in H2O) produced the corresponding sulfoxide with complete oxygen incorporation from the oxidant. Since an ET mechanism must certainly hold in the reactions of 10 and 11, the oxygen incorporation experiments indicate that the intermediate radical cation, once formed, has to react with PFe(IV)=O (the reduced form of the iron-oxo complex which is formed by the ET step) in a fast oxygen rebound. Thus, an ET step followed by a fast oxygen rebound is also suggested for the other sulfides investigated in this work.     

Quenching of n,pi*-excited states in the gas phase: variations in absolute reactivity and selectivity

The quenching of the n,pi*-excited azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene by 19 heteroatom-containing electron and hydrogen donors, that is, amines, sulfides, ethers, and alcohols, was investigated in the gas phase. Deuterium isotope effects were measured for 9 selectively deuterated derivatives. The data support the involvement of an excited charge-transfer complex, that is, an exciplex, for tertiary amines and sulfides, and a competitive direct hydrogen transfer from the C-H bonds of ethers or from the N-H or O-H bonds of secondary and primary amines or alcohols. The recently observed "inverted" solvent effect for the fluorescence quenching of azoalkanes by amines and sulfides in solution is supported by the observed rate constants in the gas phase, which are substantially larger than those in solution. A more pronounced inverted solvent effect for the weaker electron-donating sulfides and a presumably faster exciplex deactivation result in a switch-over in absolute reactivity relative to tertiary amines in the gas phase. Most importantly, the kinetic data demonstrate that the reactivity of the strongly dipolar O-H and N-H bonds in photoinduced hydrogen abstraction reactions shows a larger decrease upon solvation than that of the less polar C-H bonds. The azoalkane data are compared with previous studies on quenching of n,pi*-triplet-excited ketones in the gas phase.     

CHEMISTRY OF SINGLET OXYGEN—XXVIII*. STERIC AND ELECTRONIC EFFECTS ON THE REACTIVITY OF SULFIDES WITH SINGLET OXYGEN

Abstract—The rates of reaction of singlet oxygen with a number of alkyl and aryl sulfides have been determined in CH₃OH. The rate for diethylsulfide is 1.71 ± 0.06 × 10⁷M^-1 s^-1. The addition of methyl groups α to the sulfur causes an approximate tenfold decrease in rate for each symmetrical pair. A similar effect is caused by replacement of alkyl by phenyl groups. The rates of substituted thioanisoles correlate well with σ(ρ= -1.6). but poorly with E_P/2 (half-peak oxidation potentials). A mechanism involving nucleophilic reaction of the sulfide with oxygen, rather than charge transfer is suggested.     

Photoproduct selectivity in reactions involving singlet and triplet excited states within bile salt micelles

Generally, photochemical reactions tend to give more than one product. For such reactions to be useful one should be able to control them to yield a single product. Of the many approaches used in this context, the use of reaction media with features different from those of isotropic solutions has been very effective. We provide results of our studies on four reactions within bile salt micelles (cholic acid and deoxycholic acid). These four reactions involve homolytic cleavage of a C-C or C-O bond to yield either a singlet or triplet radical pair. The bile salt micelles control the rotational and translational mobilities of the radical pair, resulting in photoproduct selectivity. The dynamic nature of the bile salt micelles results in differential effects on the singlet and triplet radical pairs.     

Reaction of singlet oxygen with some benzylic sulfides

Product distribution, total quenching rate (k_T), and rate of chemical reaction (k_r) with singlet oxygen have been determined for some alkyl, benzyl, α-methylbenzyl, and cumyl sulfides. Their contributions depend on the steric hindering around the sulfur atom. In protic solvents, the sulfoxide is the main product via a hydrogen-bonded persulfoxide. In apolar solvents, intramolecular α-H abstraction leads to oxidative C-S bond cleavage, with varying efficiency. The behavior of sulfides is compared to that of alkenes and amines.     

SELECTIVE PHOTOOXIDATION OF THIOLS SENSITIZED BY AQUEOUS SUSPENSIONS OF CADMIUM SULFIDE

-Finely powdered cadmium sulfide in aqueous, air-saturated, phosphate buffered suspension sensitizes the photooxidation of cysteine to cystine with good efficiency; several additional thiols and inorganic sulfides are also photooxidized. The other amino acids (histidine, methionine, tryptophan, tyrosine) known to be rapidly photooxidized with typical organic photosensitizers are photooxidized only very slowly. The quantum yield of oxygen uptake during cysteine photooxidation is pH dependent with a maximum (0.021) at pH 9.5; the yield is not increased in D₂O and is not decreased appreciably by sodium azide, suggesting that singlet oxygen is not involved in the photooxidation process. The slow rate of photooxidation of histidine, which is known to react efficiently with singlet oxygen, also suggests that little if any singlet oxygen is produced by illuminated cadmium sulfide. Superoxide dismutase inhibits the yield of cysteine photooxidation to a maximum of approximately 50%, suggesting the partial involvement of superoxide in the reaction mechanism. The quantum yields of the photooxidation of cysteine, ethylenediaminetetraacetate and inorganic sulfides decrease as the wavelength of the exciting light is increased. Yeast alcohol dehydrogenase, a sulfhydryl enzyme, is inactivated by photodynamic treatment with cadmium sulfide; lysozyme, which has no free sulfhydryl groups, is not.     

Dicyanoanthracene sensitized photo-oxygenation of olefins : Electron transfer and singlet oxygen mechanisms

Cyanoaromatic sensitizers, in particular 9,10-dicyanoanthracene (DCA), sensitize the photo-oxygenation of olefins by two distinct mechanisms. In the case of aryl substituted olefins (OL), which react extremely slowly (if at all) with singlet oxygen, the reaction proceeds by way of electron transfer to produce discrete radical ions (DCA^-and OL⁺). In the presence of oxygen, this ionic process results, ultimately, in the cleavage of the olefin to carbonyl compounds along with production of some epoxide and other minor by-products. Aromatic ethers can interfere with this process by reducing the radical cation by electron transfer, resulting in net quenching of the reaction. With simple alkenes the DCA-sensitized reaction takes a different course, producing hydroperoxide products with distributions which are very similar to those obtained with the singlet oxygen ene reaction. Careful study has shown that this reaction does, indeed, proceed by way of singlet oxygen, which is produced by at least two mechanisms : (1) enhanced intersystem crossing, in which ¹DCA is quenched by interaction with the olefin, leading to a low yield of ³DCA, which subsequently reacts with oxygen to produce singlet oxygen; and (2) direct reaction of ¹DCA with oxygen. At limiting high oxygen concentration, this process produces 2 mol of singlet oxygen for each mol of ¹DCA quenched; the mechanism involves energy transfer to produce ³DCA and 1 mol of singlet oxygen ; the ³DCA reacts again with oxygen to produce a second mol of singlet oxygen. The complex kinetic behaviour of simple olefins in the presence of DCA can be satisfactorily rationalized by these mechanisms.     

ON THE MECHANISM OF QUENCHING OF SINGLET OXYGEN BY AMINES-III. EVIDENCE FOR A CHARGE-TRANSFER-LIKE COMPLEX

Abstract— A series of amines were found to quench singlet oxygen in the order tertiary > secondary > primary, with a reasonable correlation between the log of their rate constant of quenching and their ionization potential. In addition, a Hammett rho plot gave a rho value of - 1.39 for the quenching of singlet oxygen by a series of substituted N, N-dimethylanilines, in good agreement with the results obtained by a different method. It was found that some of the amines (anilines) quenched the triplet state of the dye-sensitizer (Rose Bengal) used for the production of singlet oxygen. Corrections in the results were made in the calculations of rates of quenching of singlet oxygen to allow for the triplet-state quenching. No extensive quenching of the singlet state of the dye was observed at the concentrations of the amines necessary for singlet-oxygen quenching. In one case (N, N, N', N'-tetramethylphenylenediamine) there was no observable chemical reaction between singlet oxygen and the amine. It was concluded that singlet oxygen undergoes physical quenching by the amines via partial charge-transfer intermediates.     

Reactivity and mechanistic insight into visible-light-induced aerobic cross-dehydrogenative coupling reaction by organophotocatalysts

With visible-light irradiation, a mild, simple, and efficient metal-free photocatalytic system for the facile construction of sp(3)-sp(3) C-C bonds between tertiary amines and activated C-H bonds has been achieved. Spectroscopic study and product analysis demonstrate for the first time that photoinduced electron transfer from N-aryl tetrahydroisoquinolines to eosin Y bis(tetrabutylammonium salt) (TBA-eosin Y) takes place to generate TBA-eosin Y radical anion, which can subsequently react with nucleophiles and molecular oxygen. More strikingly, electron spin resonance (ESR) measurements provide direct evidence for the formation of superoxide radical anions (O(2)(-.)) rather than singlet oxygen ((1)O(2)) during visible-light irradiation. This active species is therefore believed to be responsible for the large rate of acceleration of the aerobic photocatalytic reactions.     

PHOTOSENSITIZED OXYGENATION OF CARBONYL STABILIZED SULFUR AND PYRIDINIUM YLIDS, AND RELATED DIAZO COMPOUNDS. CARBON-SULFUR AND NITROGEN BOND CLEAVAGES

Abstract— The dye sensitized photooxygenation of sulfur and pyridinium ylids proceeds by way of singlet oxygen under the reaction conditions studied. The most remarkable results were that dimethyl sulfoxide and pyridine were obtained in the photooxygenations of oxosulfonium and pyridinium methy-lids, respectively. as the major cleavage products. These results suggested that the 1,2-dioxetane like intermediates were not significant ones in these reactions. The oxygenation reactions in the presence of diphenyl sulfide which was known as the most unreactive one to singlet oxygen afforded diphenyl sulfoxide efficiently. A new type of intermediate instead of 1,2-dioxetane. that can monooxygenate the substrate may be formed. Photosensitized oxygenations of corresponding diazo compounds were also studied.     

Efficient trapping of the intermediates in the photooxygenation of sulfides by aryl selenides and selenoxides

Aryl selenides and selenoxides trap efficiently the intermediates in the reaction of singlet oxygen with sulfides. In the co-photooxygenation of 1 equiv of an aryl selenoxide with 1.3 equiv of dimethyl sulfide, the aryl selenone is formed quantitatively. Aryl selenides require 4-5 equiv of sulfide for their complete co-oxidation to selenones.     

S′H type reactions of substituted allylic compounds

S′_H reactions of allyl sulfides and halides with phenyl radicals are reported. Thermal decomposition of phenylazotriphenylmethane with allyl sulfides and bromide has been shown to give allylbenzene. This apparent substitution reaction involves attack of a phenyl radical on the terminal unsaturated carbon atom of the allyl sulfide; the reaction in α,α-dimethylallyl ethyl sulfide produced 2-methyl-4-phenylbutene-2. To estimate the relative reactivities of allylic substrates towards phenyl radicals, competitive reactions of phenyl radicals with allylic compounds and carbon tetrachloride were investigated. The data indicate that the radical formed by addition of a phenyl radical to the allylic sulfide looses thiyl radicals almost quantitatively.