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.

Bromine reacts with substituted methyl phenyl sulphides and isopropyl phenyl sulphide in 2 : 1 methanol-water (v/v) to give the corresponding sulphoxides. The reaction follows second-order kinetics, first order in bromine and first order in sulphide. The reaction rate is increased by the presence of NaOAc and decreased by addition of NaBr in solution, The reaction is very sensitive to structural factors. It is favoured by electron-donating substituents in the phenyl ring (p-= -3.2). On the other hand isopropyl phenyl sulphide reacts about 30 times more slowly than methyl phenyl sulphide. A mechanism involving preliminary fast formation of a brornosulphonium cation followed by its rate-determining hydrolysis is suggested.

Dialkyl sulphides reduce aryl methyl sulphoxides in aqueous methanol in the presence of ca. 4 mol dm-3 HCI. Ring substitution in phenyl methyl sulphoxides moderately affects their reactivity towards dibutyl sulphide, the overall effect resulting in a Hammett p value of -1.6. This parameter is a measure of the effect of structural changes on a protonation pre-equilibrium, the formation of chlorosulphonium ion, and its subsequent partitioning between reduction and return t o reactants by the action of water. Changing the alkyl moiety of the sulphides induces moderate reactivity changes suggesting a compensation of opposite polar and steric effects. Sulphoxides with electron-releasing groups display the highest selectivity towards dialkyl sulphides. When a large concentration of sulphide is used, both racemization of chiral sulphoxide and l80 exchange with the aqueous solvent are suppressed and this suggests that the species ArR'SCI' is a common intermediate for these processes and for the reduction reaction. These and other kinetic findings indicate that the reduction step involves the displacement at the ArR'SCI' chlorine of a sulphide by a sulphide molecule rather than by chloride ion and rules out, for the racemization too, the hypothesis involving a free halogen intermediate.

Alkyl aryl sulphoxides are reduced by dialkyl sulphides in 2 : 1 methanol-water in the presence of ~M -H C I . The kinetics of the reaction between p-chlorophenyl methyl sulphoxide and dibutyl sulphide and the basic properties of the sulphoxide have been investigated in detail. Reaction orders, acid catalysis, and comparison of the reduction rate with that of the loss of optical activity by the (+)-R-sulphoxide indicate a predominant role for the chlorosulphonium cation. A mechanism is suggested in which the changes of the oxidation states occur through the reaction between the sulphide and this intermediate.THE chemistry of sulphoxides has been the subject of a number of mechanistic studies in these laboratories. We were attracted by a reaction of some theoretical and synthetic interest, namely oxygen transfer from sulphoxides to organic sulphides. This reaction has been reported to occur 3-s particularly well in the presence of HCl,9-11 other acids such as H,SO, or HClO, being much less effective catalysts ; the reaction is strongly dependent on the structure of the reactants. The only related mechanistic study so far publisheds deals with intramolecular oxygen migration from o-alkylthiophenyl aryl sulphoxides to a sulphide sulphur in 70--80y0 H,S04. In the light of our earlier hypothesis,2 our interest was stimulated by the idea that catalysis by HC1 of the oxygen exchange reaction is mechanistically related to the effective oxidising and halogenating abilities of the sulphoxide-hydrogen halide system.l2-I5We present here a kinetic study of the HCI catalysed reaction between 9-chlorophenyl methyl sulphoxide (Ia) and dibutyl sulphide (IId), an equilibrium which because it is almost completely shifted towards the products and is virtually irreversible under suitable concentration conditions, offers considerable advantages for kinetic investigation. RESULTSProducts.-Sulphides and sulphoxides were identified and determined by g.1.c.; other products beside those of Taken from the doctoral Thesis of A. Box7i0, University' of Padua, 1973. 2 Some of the results have been presented by G. Modena, 5th

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