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On the basis of ab initio calculations we propose a revised mechanism for the reaction of organic sulfides with singlet oxygen, which is more consistent with experimental evidence than previous schemes. In aprotic solvents the reagents initially form a weakly bound peroxysulfoxide, with a small barrier due to entropy. The peroxysulfoxide may decay back to ground state (triplet) oxygen, be trapped by sulfoxides, or rearrange to a S-hydroperoxysulfonium ylide with a barrier of ∼6 kcal/mol. The latter is ∼6 kcal/mol more stable than the peroxysulfoxide, and can be trapped by sulfides or rearrange to a sulfone. In some cases, like five-membered rings or benzylic sulfides, the S-hydroperoxysulfonium ylide may undergo a 1,2-OOH shift to an α-hydroperoxysulfide, which eventually leads to cleavage products. In protic solvents the peroxysulfoxide is rapidly converted to a sulfurane by solvent.

Persulfoxide (R(2)S(+)-OO(-) <--> R(2)S(.)-OO(.)) is the first formed intermediate in the reactions between singlet oxygen and organic sulfides. It is a weakly bound species that nevertheless has a sufficient lifetime to undergo a myriad of inter- and intramolecular reactions. Its behavior suggests that it can be considered as a resonance hybrid of zwitterionic and diradical canonical structures. It primarily acts as a nucleophile/base at oxygen but has a tendency to interconvert to secondary intermediates that often behave as electrophilic oxidizing agents. Judicious selection of reaction conditions and substituents can allow the use of the persulfoxide as a synthetically useful intermediate.

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