Pummerer rearrangement

The usage of α-acyl sulfoxides and Lewis acids, such as TiCl4 and SnCl4, allow the reaction to proceed at lower temperatures (0 °C).

[5] Thionyl chloride can be used in place of acetic anhydride to trigger the elimination for forming the electrophilic intermediate and supplying chloride as the nucleophile to give an α-chloro-thioether:[6] Other anhydrides and acyl halides can give similar products.

The intermediate is so electrophilic that even neutral nucleophiles can be used, including aromatic rings with electron donating groups such as 1,3-benzodioxole:[8] It is possible to perform the rearrangement using selenium in the place of sulfur.

[9] When a substituent on the α position can form a stable carbocation, this group rather than the α-hydrogen atom will eliminate in the intermediate step.

[10] This reaction type is demonstrated below with a set of sulfoxides and trifluoroacetic anhydride (TFAA): The organic group "R2" shown in the diagram above on the bottom right is the methyl violet carbocation, whose pKR+ of 9.4 is not sufficient to out-compete loss of H+ and therefore a classical Pummerer rearrangement occurs.