Sulfur-Centered Three-Electron Bonded Radical Species
TL;DR: In this article, it was shown that if one of the antibonding σ* electrons is removed, the electronic σ/σ* configuration would be a slight repulsion of the two atoms.
Abstract: We all know that it is not possible to stabilize a He2 molecule. Any interaction of the filled He is orbitals would lead to the establishment of bonding a and antibonding σ* energy levels, both doubly occupied. As σ*, for quantum mechanical reasons, is raised a little bit more than σ is lowered relative to the original atomic energy levels the net result of this electronic σ/σ* configuration would be a slight repulsion of the two atoms. The situation becomes significantly different though if one of the antibonding σ* electrons is removed.
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TL;DR: 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 O2-*, NMQ*, and DCA-*.
Abstract: 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-• a...
141 citations
TL;DR: Hydroxyl radicals were generated radiolytically in N(2)O-saturated aqueous solutions of thiourea and tetramethylthiou Andrea and it has been shown that, in both cases, OH(-) and a positively charged species are produced.
Abstract: Hydroxyl radicals were generated radiolytically in N(2)O-saturated aqueous solutions of thiourea and tetramethylthiourea. The rate constant of the reaction of OH radicals with thiourea (tetramethylthiourea) has been determined using 2-propanol as well as NaN(3) as competitors to be 1.2 x 10(10) dm(3) mol(-1) s(-1) (8.0 x 10(9) dm(3) mol(-1) s(-1)). A transient appears after a short induction period and shows a well-defined absorption spectrum with lambda(max) = 400 nm (epsilon = 7400 dm(3) mol(-1) cm(-1)); that of tetramethylthiourea has lambda(max) = 450 nm (epsilon = 6560 dm(3) mol(-1) cm(-1)). Using conductometric detection, it has been shown that, in both cases, OH(-) and a positively charged species are produced. These results indicate that a radical cation is formed. These intermediates with lambda(max) = 400 nm (450 nm) are not the primary radical cations, since the intensity of the absorbance depends on the substrate concentration. The absorbance build-up follows a complex kinetics best described by the reversible formation of a dimeric radical cation by addition of a primary radical cation to a molecule of thiourea. The equilibrium constant for this addition has been determined by competition kinetics to be 5.5 x 10(5) dm(3) mol(-1) for thiourea (7.6 x 10(4) dm(3) mol(-1) for tetramethylthiourea). In the bimolecular decay of the dimeric radical cation (thiourea, 2k = 9.0 x 10(8) dm(3) mol(-1) s(-1); tetramethylthiourea, 1.3 x 10(9) dm(3) mol(-1) s(-1)), formamidine (tetramethylformamidine) disulfide is formed. In basic solutions of thiourea, the absorbance at 400 nm of the dimeric radical cation decays rapidly, giving rise (5.9 x 10(7) dm(3) mol(-1) s(-1)) to a new intermediate with a broad maximum at 510 nm (epsilon = 750 dm(3) mol(-1) cm(-1)). This reaction is not observed in tetramethylthiourea. The absorption at 510 nm is attributed to the formation of a dimeric radical anion, via neutralization of the dimeric radical cation and subsequent deprotonation of the neutral dimeric radical. The primary radical cation of thiourea is deprotonated by OH- (2.8 x 10(9) dm(3) mol(-1) s(-1)) to give a neutral thiyl radical. The latter reacts rapidly with thiourea, yielding a dimeric radical, which is identical to the species from the reaction of OH(-) with the dimeric radical cation. The dimeric radical cations of thiourea and tetramethylthiourea are strong oxidants and readily oxidize the superoxide radical (4.5 x 10(9) dm(3) mol(-1) s(-1) for thiourea and 3.8 x 10(9) dm(3) mol(-1) s(-1) for tetramethylthiourea), phenolate ion (3 x 10(8) dm(3) mol(-1) s(-1) for tetramethylthiourea), and even azide ion (4 x 10(6) dm(3) mol(-1) s(-1) for thiourea and similar to 10(6) dm(3) mol(-1) s(-1) for tetramethylthiourea). With Oar the dimeric radical cation of thiourea reacts relatively slowly (1.2 x 107 dm(3) mol(-1) s(-1)) and reversibly (2 x 10(3) s(-1)).
107 citations
TL;DR: In this paper, a review of the synthetic aspects and mechanism/schemes of the reactions discussed in the manuscript are based on stoichiometry and products of the reaction reactions and the reaction conditions and types of oxidants steer the formation of different products.
84 citations
TL;DR: The thermodynamic driving force for the observed enhanced free-radical repair reactivity of RSSH compared to RSH is attributed to the resonance stabilization energy of 8.8 kJ mol-1 within the RSS.
Abstract: 2-(3-Aminopropyl-amino) ethaneperthiol (RSSH, the perthiol analogue of the thiol radioprotector, WR-1065) reacts with the α-hydroxy alkyl radical (CH3)2C.OH by donating a hydrogen atom as indicated by the characterization of perthiyl radicals (RSS.; λmax = 374 nm, ≈374 = 1680 ≈ 20 dm3 mol−1 cm−1) by pulse radiolysis. The perthiyl radical abstracts a hydrogen from the alcohol to establish a reversible hydrogen-transfer equilibrium. This equilibrium lies predominantly on the side of radical repair since the rate constants for the forward and reverse reactions at pH 4 are: k(RSSH + (CH3)2C−OH) = (2.4 ± 0.1) ± 109dm3 mol−1 s_1 and k(RSS− + (CH3)2CHOH) = (3.8 ± 0.3) × 103 dm3 mol−1 s−1 respectively. The pKa, (RSSH ← RSS− + H+) = 6.2 ± 0.1 was determined from the pH dependence of the rate of perthiol repair. Identical experiments have been performed with WR-1065 allowing a direct comparison of free-radical repair reactivity to be made with the parthiol analogue. At pH ≈ 7.4 the reactivities of the thiol and per...
83 citations
TL;DR: Sulfur radical cations are novel reaction intermediates that have attracted considerable attention recently as mentioned in this paper, in part due to renewed interest in the chemistry of sulfur cations in general, as well as four other factors.
Abstract: Sulfur radical cations are novel reaction intermediates that have attracted considerable attention recently. This in part is due to the renewed interest in the chemistry of radical cations in general, as well as four other factors. The first is the basis for understanding the structure and reactions of radical cations — can their inherent nature be best depicted by analogy with radicals, with cations, or is a new algorithm required? As is exemplified in this review, sulfur radical cations show novel behavior. Second, some reactions of sulfur radical cations are attracting interest for their application in organic synthesis. Third, biological electron-transfer may be mediated by sulfur and, indeed, sulfur radical cations may be intermediates in biological redox processes. Finally, materials based on tetrathiafulvalene, polythiophene, and related sulfur compounds have high electrical conductivity which may be understood in terms of p-delocalized sulfur radical cations or the related dications. Consequently, a comprehensive and critical review of this field appeared timely.
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