Pummerer rearrangement
reaction in organic chemistry
The Pummerer rearrangement is an organic reaction whereby an alkyl sulfoxide rearranges to an α-acyloxy–thioether (monothioacetal-ester) in the presence of acetic anhydride.
The stoichiometry of the reaction is:
RS(O)CHR'2 + Ac2O → RSC(OAc)R'2 + AcOH
Synthetic implementation
Aside from acetic anhydride, trifluoroacetic anhydride and trifluoromethanesulfonic anhydride have been employed as activators. Common nucleophiles besides acetates are arenes, alkenes, amides, and phenols.
Many variations of the basic Pummerer rearrangement have been described:
additive Pummerer reactions
domino Pummerer reaction, whereby the C-S bond is cleaved often
asymmetric Pummerer reactions, which exploits the chirality of most sulfoxides
interrupted Pummerer reactions
The usage of α-acyl sulfoxides and Lewis acids, such as TiCl4 and SnCl4, allow the reaction to proceed at lower temperatures (0 °C).
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:
Other anhydrides and acyl halides can give similar products. Inorganic acids can also give this reaction. This product can be converted to aldehyde or ketone by hydrolysis.
Mechanism
The mechanism of the Pummerer rearrangement begins with the acylation of the sulfoxide (resonance structures 1 and 2) by acetic anhydride to give 3, with acetate as byproduct. The acetate then acts as a catalyst to induce an elimination reaction to produce the cationic-thial structure 4, with acetic acid as byproduct. Finally, acetate attacks the thial to give the final product 5.
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