Pummerer rearrangement
The Pummerer rearrangement is an organic reaction whereby an alkyl sulfoxide rearranges to an α-acyloxy–thioether in the presence of acetic anhydride.
[Image:Pummerer Rearrangement Scheme.png|center|300px|The Pummerer rearrangement]
The stoichiometry of the reaction is:
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
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:
[Image:Pummerer Ex ThionylChloride.png|center|400px|Example of the Pummerer rearrangement using thionyl chloride]
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 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.[Image:Pummerer Rearrangement Mechanism.png|center|600px|The mechanism of the Pummerer rearrangement]
The activated thial electrophile can be trapped by various intramolecular and intermolecular nucleophiles to form carbon–carbon bonds and carbon–heteroatom bonds.
The intermediate is so electrophilic that even neutral nucleophiles can be used, including aromatic rings with electron donating groups such as 1,3-benzodioxole:
[Image:Pummerer Ex Veratrole.png|center|400px|Example of the Pummerer rearrangement using veratrole]
It is possible to perform the rearrangement using selenium in the place of sulfur.
Pummerer fragmentation
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. This variation is called a Pummerer fragmentation. This reaction type is demonstrated below with a set of sulfoxides and trifluoroacetic anhydride :[Image:PummererFragmentation.png|center|400px|Pummerer fragmentation]
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. The reaction on the left is a fragmentation because the leaving group with pKR+ = 23.7 is particularly stable.