Mechanism of Sulfone Formation in the Reaction of Sulfides and Singlet Oxygen:  Intermediacy of S-Hydroperoxysulfonium Ylide

Abstract

H-D exchange was observed in the methyl group during the formation of sulfones in the reaction of dimethyl sulfide-d 6 or thioanisole-R,R,R-d 3 with singlet oxygen in aprotic solvents. No exchange was observed in the sulfoxides obtained. The proton in the sulfones was shown to come from adventitious water, since the oxidation of C 6 H 5 SCH 3 in the presence of D 2 O lead to the formation of sulfones with monodeuteriation. The 16 O 2 -18 O 2 tracer study demonstrated no oxygen scrambling in the sulfones. All the results indicate that the sulfones are formed intramolecularly via an intermediate possessing one activated proton exchangeable with trace water, a suggested structure for which is S-hydroperoxysulfonium ylides (RS + (OOH)CH 2 -). Kinetic isotope effects (k H /k D ) 2-4) observed for methyl protons in the sulfone formation suggest that the rate-determining step is the intramolecular proton abstraction in the persulfoxides (RS + (OO -)CH 3 ) generating S-hydroperoxysulfonium ylides. The conversion of the ylide intermediates to sulfones is shown to be facile on the basis of PM3 theoretical calculations.

Sequential Photooxidation of a Pt(II) (Diimine)cysteamine Complex: Intermolecular Oxygen Atom Transfer versus Sulfinate Formation

“…However, no H/D exchange was observed at the methylene site adjacent to the sulfur atom during the photooxidation of 1 in D 2 O, in contrast with the photooxidation of organic sulfides which does proceed via a hydroperoxysulfonium ylide and concomitant H/D exchange at the methylene group adjacent to the sulfur atom. 14 Thus, alternatively, the intermediate responsible for intermolecular oxygen atom transfer may possibly be a thiadioxirane-type moiety. Both of these possible secondary intermediates are depicted in Scheme 2 below.…”

Photooxygenations of Sulfur Compounds

“…Several groups [53][54][55][56][57][58] have suggested this ylide as an intermediate and it has been located as a viable species using a variety of different computational models [49,[59][60][61]. Dimethyl S-hydroperoxysufonium ylide exists in several conformations but the most stable is the rotomer in which the peroxy hydrogen is located 3.15 Å above the CH 2 group [46].…”

“…However, a detailed theoretical study of the thiadioxirane was unable to locate a path to sulfone [73]. Instead a mechanism involving the hydroperoxy sulfonium ylide, SY, is currently thought to be responsible for sulfone formation in the early part of photooxygenation reactions [61] (Figure 32.11). This mechanism is supported by observation of hydrogen-deuterium exchange during sulfone formation and a kinetic isotope effect, k H /k D , of 2-4 in a variety of different solvents [61].…”

“…Instead a mechanism involving the hydroperoxy sulfonium ylide, SY, is currently thought to be responsible for sulfone formation in the early part of photooxygenation reactions [61] (Figure 32.11). This mechanism is supported by observation of hydrogen-deuterium exchange during sulfone formation and a kinetic isotope effect, k H /k D , of 2-4 in a variety of different solvents [61]. Theoretical calculations (RHF-PM3) also suggest that this pathway is energetically accessible.…”

Photooxygenation of Non-Aromatic Heterocycles

“…The widely accepted mechanism is that the reagents initially form a weakly bound persulfoxide 72 which collapses to thiadioxirane 73 (a) and this, in turn, reacts with sulphide substrate 71 to give two sulfoxides 74 (b) (Scheme 20) [45]. The reaction is instead very complex as proven by ab initio calculations [43,46], isotopic effects [47,48], alcohol [49,50] or protic medium effects [51]. Depending on sulphide structure, In protic solvents stabilization occurs by hydrogen bonding (c) [45,[49][50][51] (or in MeOH by formation of a sulfurane intermediate 75 (d) [45]) and attack to a second molecule of sulphide is promoted leading to two sulfoxides.…”