Photosensitized Oxidation of Sulfides: Discriminating between the Singlet‐Oxygen Mechanism and Electron Transfer Involving Superoxide Anion or Molecular Oxygen

Abstract

The oxidation of diethyl and diphenyl sulfide photosensitized by dicyanoanthracene (DCA), N-methylquinolinium tetrafluoroborate (NMQ(+)), and triphenylpyrylium tetrafluoroborate (TPP(+)) has been explored by steady-state and laser flash photolysis studies in acetonitrile, methanol, and 1,2-dichloroethane. In the Et(2)S/DCA system sulfide-enhanced intersystem crossing leads to generation of (1)O(2), which eventually gives the sulfoxide via a persulfoxide; this mechanism plays no role with Ph(2)S, though enhanced formation of (3)DCA has been demonstrated. In all other cases an electron-transfer (ET) mechanism is involved. Electron-transfer sulfoxidation occurs with efficiency essentially independent of the sulfide structure, is subject to quenching by benzoquinone, and does not lead to Ph(2)SO cooxidation. Formation of the radical cations R(2)S(*+) has been assessed by flash photolysis (medium-dependent yield, dichloroethane>>CH(3)CN>CH(3)OH) and confirmed by quenching with 1,4-dimethoxybenzene. Electron-transfer oxidations occur both when the superoxide anion is generated by the reduced sensitizer (DCA(*-), NMQ(*)) and when this is not the case (TPP(*)). Although it is possible that different mechanisms operate with different ET sensitizers, a plausible unitary mechanism can be proposed. This considers that reaction between R(2)S(*+) and O(2)(*-) mainly involves back electron transfer, whereas sulfoxidation results primarily from the reaction of the sulfide radical cation with molecular oxygen. Calculations indeed show that the initially formed fleeting complex RS(2)(+)...O-O(*) adds to a sulfide molecule and gives strongly stabilized R(2)S-O(*)-(+)O-SR(2) via an accessible transition state. This intermediate gives the sulfoxide, probably via a radical cation chain path. This mechanism explains the larger scope of ET sulfoxidation with respect to the singlet-oxygen process.

Electron self-exchange activation parameters of diethyl sulfide and tetrahydrothiophene

“…Compared to typical carbonyl sensitizers such as benzophenone or xanthone, its reduction potential (−0.29 V in acetonitrile versus SCE [31]) is more favorable by more than 2 V, yet its triplet energy (222 kJ/mol [32]) is only lower by less than 90 kJ/mol. Its first excited singlet state has a lifetime of 2.9 ns [32], so reacts only marginally with the millimolar sulfide concentrations used in our experiments, but its triplet state (intrinsic intersystem-crossing efficiency, 0.48 [32], with evidence for a noticeable increase in the presence of DES by an intermolecular heavy-atom effect [33]) is quenched [34] quantitatively (lifetime, 10 μs [32]; diffusion-controlled electron transfer from DES [35]) under our experimental conditions. Protonation of the pyranyl radical TPP • is unknown.…”

Photooxygenations of Sulfur Compounds

“…Instead, they located a loose ion-dipole complex between oxygen and dimethyl sulfide radical cation bound by 3.2 kcal/mol, with a sulfur-oxygen distance only slightly shorter than the sum of the van der Waals radii. In 2006, Albini and coworkers pointed out that unproductive electron transfers between superoxide and sulfur radical cations are substantially more exothermic than reactions to form thiadioxiranes [112]. Consequently, they rejected the Baciocchi thiadioxirane mechanism [104] and instead suggested the chain reaction depicted in Figure 32.20.…”

Selective photocatalytic reactions with organic photocatalysts

“…110 It should be noted that bromination of thiophenes is particularly important in the preparation of oligothiophenes and polythiophenes, [111][112][113][114][115][116][117][118][119][120][121][122][123] which have many applications as conductive, semiconductive, nonlinear optical and liquid crystalline materials. 124 Visible light irradiation of [Acr + -Mes]ClO 4 -(λ max = 430 nm, 0.20 mM) in an oxygen-saturated MeCN solution containing 1,2,4-trimethoxybenzene (TMB, 4.0 mM), 50 % aqueous HBr ([HBr] = 20 mM, [H 2 O] = 100 mM) with a xenon lamp attached with color glass filter (λ < 320 nm) for 20 min resulted in formation of a brominated product, 2,4,5trimethoxybromobenzene.…”