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The Pummerer reaction involves the formation of an α‐functionalized sulfide from a sulfoxide bearing at least one α‐hydrogen atom. The reaction can also be described as an internal redox process where the SX group is reduced and the α carbon is oxidized. The first report by Pummerer on the reaction which now bears his name appeared in 1909 and described the formation of thiophenol and glyoxylic acid on heating phenylsulfinylacetic acid with mineral acids. The products Pummerer observed resulted from hydrolysis of the initially formed α‐substituted sulfides, which are the typical products of the reaction. The term “Pummerer reaction” was later extended to the reaction of sulfoxides with acid anhydrides. Selenium and nitrogen analogs undergo similar reactions. The former is known as the seleno–Pummerer reaction, and the latter is usually referred to as the Polonovski reaction. The sila–Pummerer reaction, which is also discussed in this chapter, is the rearrangement of sulfoxides bearing a silyl group on the α carbon. From a mechanistic point of view there are many other reactions, sometimes given specific names, such as the Sommelet–Hauser, Stevens, and Vilsmeier rearrangements, that appear to resemble the Pummerer reaction. Reactions in which the sulfoxide group acts as an oxidant in an intermolecular redox process have characteristics similar to the typical Pummerer reaction. The α‐halogenation of sulfides, in which the sulfide sulfur may first be oxidized to a halosulfonium salt that rearranges to the final product, is formally similar to the Pummerer reaction. For the sake of clarity and to be as exhaustive as possible, we have limited the scope of this chapter to the restrictive definition.

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Triphosgene was decomposed quantitatively to phosgene by chloride ion. The reaction course was monitored by IR spectroscopy (React-IR), showing that diphosgene was an intermediate. The methanolysis of triphosgene in deuterated chloroform, monitored by proton NMR spectroscopy, gave methyl chloroformate and methyl 1,1, 1-trichloromethyl carbonate in about a 1:1 ratio, as primary products. The reaction carried out in the presence of large excess of methanol (0.3 M, 30 equiv) was a pseudo-first-order process with a k(obs) of 1.0 x 10(-)(4) s(-)(1). Under the same conditions, values of k(obs) of 0.9 x 10(-)(3) s(-)(1) and 1.7 x 10(-)(2) s(-)(1) for the methanolysis of diphosgene and phosgene, respectively, were determined. The experimental data suggest that, under these conditions, the maximum concentration of phosgene during the methanolysis of triphosgene and diphosgene was lower than 1 x 10(-)(5) M. Methyl 1,1,1-trichloromethyl carbonate was synthesized and characterized also by the APCI-MS technique.

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The X-ray structures of c-2,t-3-di-tert-butyl-r-1-methylthiiranium 8 BF(4)(-), t-2,t-3-di-tert-butyl-r-1-methylthiiranium ion 10 BF(4)(-), and 2,3-di-tert-butyl-1-methylthiirenium 11 BF(4)(-) have been determined. The DeltaG()(298) values for the rearrangements from the cis and the trans tert-butyl groups of 8 SbCl(6)(-) to thietanium ion (two intramolecular S(N)2 displacements) and for the rearrangement of 11 SbCl(6)(-) to thietium ion (an intramolecular S(N)2-Vin displacement) are linearly correlated with the strengths of the C-S breaking bonds, suggesting that the two mechanisms are, in the absence of steric hindrance, uniquely governed by the nucleofugality of the sulfonium leaving group.

The thiiranium hexachloroantimonates 1a, 3, and 5a − c and the thiirenium hexachloroantimonates 6a − c and 7a with exocyclic S-R substituent (R = Me, Et, i-Pr) react at sulfur with dialkyl disulfides R‘ ‘SSR‘ ‘ (R‘ ‘ = Me, Et and R‘ ‘ ≠ R) in CD2Cl2 at 25 °C to give S-R‘ ‘ substituted ions. The reaction rates are affected by the steric hindrance of the substituents at sulfur and at ring carbons. t-2,t-3-Di-tert-butyl-r-1-methylthiiranium hexachloroantimonate (2) does not react, and the t-2-tert-butyl-c-3-phenyl-r-1-methylthiiranium (5a) reacts about 100 times faster than the c-2,t-3-di-tert-butyl-r-1-methylthiiranium ion (1a). The analysis of the kinetic data suggests that the sulfonium sulfur undergoes attack by the disulfide in the ring plane from a direction that is parallel to the C−C ring bond. This is also the direction which ensures the maximum overlap with the LUMO of thiiranium or thiirenium ions (determined at the RHF/3-21G*//RHF/3-21G* level). The combined consideration of the approach modality and of the maximum orbital overlap suggests that the nucleophilic substitution at sulfonium sulfur is not an SN2-like reaction but occurs via an intermediate with episulfurane-like structure. The principle of microscopic reversibility will dictate that this is also the first intermediate in the electrophilic sulfenylation of unsaturated C−C bonds.

The ESIMS technique, combined with H-1 NMR evidence, provides a precise inventory of the catalytic Ti-IV precursors and the characterization of the reactive peroxometal complex involved in enantioselective sulfoxidation employing the Ti-IV-homochiral trialkanolamine-alkylhydroperoxide system