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.^{[1] [2]} In this reaction, sulfur is reduced while adjacent carbon is oxidized.

The Pummerer rearrangement

Several reviews have been published.^{[3] [4] [5]}

The usage of α-acyl sulfoxides and Lewis acids, such as TiCl₄ and SnCl₄, allow the reaction to proceed at lower temperatures (0 °C).^[6]

Mechanism

The mechanism of the Pummerer rearrangement begins with the acylation of the sulfoxide (1 and 2). Compound 3 undergoes elimation to produce the sulfonium ion 4. Acetate adds to the sulfonium ion to give the final product 5.

The mechanism of the Pummerer rearrangement

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.^[7]

Variations

The sulfonium ion can be trapped by both intramolecular and intermolecular nucleophiles forming carbon-carbon bonds and carbon-heteroatom bonds. For example, thionyl chloride can be used to generate and trap the sulfonium ion:^[8]

Example of the Pummerer rearrangement using thionyl chloride

Likewise, nucleophiles (such as veratrole ^[9]) can also be used.

Example of the Pummerer rearrangement using veratrole

Pummerer fragmentation

When the α-organic residue can form a very good leaving group this group and not the α-hydrogen atom will eliminate in the intermediate step in a Pummerer fragmentation ^[10]. This reaction type is demonstrated below with a set of sulfoxides and the anhydride of trifluoroacetic acid (TFAA):

Pummerer fragmentation

The organic group on the right is the moderately methyl violet carbocation with pK_R+ = 9.4 leading to a classical Pummerer rearrangement. The reaction on the left is a fragmentation because the leaving group with pK_R+ = 9.4 is particularly stable.

See also

• Organosulfur chemistry

References

1. ^ Pummerer, R. Ber. 1909, 42, 2282.
2. ^ Pummerer, R. Ber. 1910, 43, 1401.
3. ^ De Lucchi, O.; Miotti, U.; Modena, G. Org. React. 1991, 40, 157-184. (Review)
4. ^ Padwa, A.; Gunn, D. E., Jr.; Osterhout, M. H. Synthesis 1997, 1353-1377. (Review)
5. ^ Padwa, A.; Rur, S. K.; Danca, M. d.; Ginn, J. D.; Lynch, S. M. Synlett 2002, 851-862. (Review)
6. ^ Stamos, I. K. Tetrahedron Lett. 1986, 27, 6261.
7. ^ Meffre, P.; Durand, P.; Le Goffic, F. Organic Syntheses, Coll. Vol. 10, p.562 (2004); Vol. 76, p.123 (1999). (Article)
8. ^ Kosugi, H.; Watanabe, Y.; Uda, H. Chem. Lett. 1989, 1865.
9. ^ Ishibashi, H. et al. Chem. Pharm. Bull. 1989, 37, 3396.
10. ^ Pummerer fragmentation vs. Pummerer rearrangement: a mechanistic analysis Benoît Laleu, Marco Santarem Machado and Jérôme Lacour Chem. Commun., 2006, 2786 - 2788, doi:10.1039/b605187a