Photoinduced One-Electron Oxidation of Benzyl Methyl Sulfides in Acetonitrile: Time-Resolved Spectroscopic Evidence for a Thionium Ion Intermediate

Abstract

The photo-oxidation of 4-methoxybenzyl methyl sulfide (1a), benzyl methyl sulfide (1b), and 4-cyanobenzyl methyl sulfide (1c) has been investigated in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP(+)PF6(-)) under nitrogen in CH3CN. The steady-state photolysis experiments showed for the investigated sulfides exclusively the formation of the corresponding benzaldehyde as the oxidation product, reasonably due to a deprotonation of the sulfide radical cations. Photo-oxidation of 1a-1c occurs through an electron transfer process. Indeed, laser flash photolysis measurements showed an efficient formation of sulfide radical cations, detected in their dimeric form [(4-X-C6H4CH2SCH3)2(+•)] at ≈520 nm. At longer delay times, the absorption of the dimer radical cation was replaced by an absorption band assigned to the (α-thio)benzyl cation (thionium ion, λmax = 420-400 nm), formed by oxidation of the benzyl radical and not by that of the (α-thiomethyl)benzyl radical, as expected if a Cα-H bond cleavage is operative. This finding highlights a particular stability of this kind of cation never reported before, even though its involvement in one-electron oxidation mechanisms of various sulfides has already been invoked. Density functional theory calculations allowed identification of a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom of the radical cations.

Efficient Metal‐Free Aerobic Photooxidation of Sulfides to Sulfoxides Mediated by a Vitamin B2 Derivative and Visible Light

“…Our method may also be used for other substrates that hardly undergo oxidation, such as tert ‐butyl phenyl sulfide 2i (entry 9) and diphenyl sulfide 2l (entry 12), which is relatively resistant to ET photooxidation and is unreactive towards singlet oxygen 33. Oxidation of benzylic substrates 2k and 2m took place without side oxidation to aldehyde (entries 11 and 13), which is often observed in photooxidations 14a,20a,d,34. We also did not observe side oxidation of 2j (entry 10) and substrates of high practical importance, e.g., sulfide 2o , smoothly afford the sulfoxidation product modafinil ( 3o , Provigil; see entry 15), a psychostimulant used for the treatment of narcolepsy, extensive daytime sleepiness and hypersomnia 35 .…”

Photochemistry of a 9‐Dithianyl‐Pyronin Derivative: A Cornucopia of Reaction Intermediates Lead to Common Photoproducts

“…Therefore, we suggest that the formation of singlet oxygen is negligible and does not interfere with the reaction course. Cyclic thioethers readily undergo single‐electron‐transfer oxidation due to their low oxidation potential, which was utilized in the photochemical ring‐opening of 1,3‐dithiane to give carbonyl compounds in the presence of oxygen via dithianyl radical cation intermediates . Such a radical cation undergoes either single‐electron transfer to form dithioacetal dication to trigger the C−S bond cleavage and subsequent hydrolysis to give ketone, or it can be attacked by air oxygen or superoxide radical anion to form persulfides .…”

Acceptor Influence on Thiolate Sensing by Hemicyanine Dyes

“…For the minor enamine product, Ha′ (5.33) is directly attached to the sp 3 -C 4 ′ that resonates at 66.1 ppm, which is considerably shifted downfield compared to the sp 3 -C 4 (46.8) of the C 4 –SMe adduct (Figure A). As chemical shift references, it is diagnostic that the sp 3 -methylene carbon of benzyl alcohol resonates ∼65 ppm, while the methylene carbon of benzyl methyl sulfides resonates upfield at ∼40 ppm . The H a ′ proton displayed through-bond correlations with C 2 ′ (157.3), C 5 ′, C 6 ′, and C 3 ′ (85.6) but failed to exhibit a correlation with the SMe group.…”

“…As chemical shift references, it is diagnostic that the sp 3 -methylene carbon of benzyl alcohol resonates ∼65 ppm, 19 while the methylene carbon of benzyl methyl sulfides resonates upfield at ∼40 ppm. 20 The H a ′ proton displayed through-bond correlations with C 2 ′ (157. 1.…”