Wagner–Meerwein rearrangement explained

A Wagner–Meerwein rearrangement is a class of carbocation 1,2-rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon.^{[1] [2]} They can be described as cationic [1,2]-sigmatropic rearrangements, proceeding suprafacially and with stereochemical retention. As such, a Wagner–Meerwein shift is a thermally allowed pericyclic process with the Woodward-Hoffmann symbol [_ω0_s + _σ2_s]. They are usually facile, and in many cases, they can take place at temperatures as low as –120 °C. The reaction is named after the Russian chemist Yegor Yegorovich Vagner; he had German origin and published in German journals as Georg Wagner; and Hans Meerwein.

Several reviews have been published.^{[3] [4] [5] [6] [7]}

The rearrangement was first discovered in bicyclic terpenes for example the conversion of isoborneol to camphene:

The story of the rearrangement reveals that many scientists were puzzled with this and related reactions and its close relationship to the discovery of carbocations as intermediates.^[8]

In a simple demonstration reaction of 1,4-dimethoxybenzene with either 2-methyl-2-butanol or 3-methyl-2-butanol in sulfuric acid and acetic acid yields the same disubstituted product,^[9] the latter via a hydride shift of the cationic intermediate:

Currently, there are works relating to the use of skeletal rearrangement in the synthesis of bridged azaheterocycles. These data are summarized in ^[10]

Plausible mechanisms of the Wagner–Meerwein rearrangement of diepoxyisoindoles:

The related Nametkin rearrangement, named after Sergey Namyotkin, involves the rearrangement of methyl groups in certain terpenes. In some cases the reaction type is also called a retropinacol rearrangement (see pinacol rearrangement).

Notes and References

1. Vagner, Ye. Ye. (Wagner, G.). In "Protokol zasedaniya Otdeleniya Khimii R. F. Khimicheskago Obshchestva. 9-go sentyabrya 1899 goda [Minutes of the meeting of the Chemistry Section of the Russian Physical-Chemical Society. 9th September 1899]" ]. J. Russ. Phys. Chem. Soc. [Zh. Russ. Fiz.-Khim. O-va.] . 1899 . 31 . 680–684 . Igor Igorevich Wagner .
2. Hans Meerwein . Über den Reaktionsmechanismus der Umwandlung von Borneol in Camphen; [Dritte Mitteilung über Pinakolinumlagerungen.] ]. . 1914 . 405 . 129–175 . Hans Meerwein . 10.1002/jlac.19144050202. 2 .
3. Popp, F. D. . McEwen, W. E. . Chem. Rev.. 1958. 58. 375. 10.1021/cr50020a004. Polyphosphoric Acids As a Reagent in Organic Chemistry. 2.
4. Acc. Chem. Res.. 1974. 7. 106–113. 10.1021/ar50076a002. Acid-catalyzed rearrangements of β,γ-unsaturated ketones. Robert L.. Cargill. Thomas E.. Jackson. Norton P.. Peet. David M.. Pond. 4.
5. Olah, G. A. . Acc. Chem. Res.. 1976. 9. 41–52. 10.1021/ar50098a001. Stable carbocations, 189. The σ-bridged 2-norbornyl cation and its significance to chemistry. 2. George Olah.
6. Hogeveen, H.; Van Krutchten, E. M. G. A. . Top. Curr. Chem.. 1979. 80. 89–124. 10.1007/BFb0050203. Wagner-meerwein rearrangements in long-lived polymethyl substituted bicyclo[3.2.0]heptadienyl cations. Topics in Current Chemistry. 3-540-09309-5.
7. Hanson, J. R. . Compr. Org. Synth.. 1991. 3. 705–719. 10.1016/B978-0-08-052349-1.00077-9. Wagner–Meerwein Rearrangements. 978-0-08-052349-1.
8. Birladeanu, L. . J. Chem. Educ.. 2000. 77. 858–863. 10.1021/ed077p858. The Story of the Wagner-Meerwein Rearrangement. 7. 2000JChEd..77..858B.
9. 10.1021/ed9000238. Carbocation Rearrangement in an Electrophilic Aromatic Substitution Discovery Laboratory. 2010. Polito. Victoria. Hamann. Christian S.. Rhile. Ian J.. Journal of Chemical Education. 87. 9. 969. 2010JChEd..87..969P.
10. Zubkov, F. I. . Zaytsev, V. P. . Nikitina, E. V. . Khrustalev, V. N. . Gozun, S. V. . Boltukhina, E. V. . Varlamov, A. V. . 10.1016/j.tet.2011.09.099. Skeletal Wagner–Meerwein rearrangement of perhydro-3a,6;4,5-diepoxyisoindoles. 2011. Tetrahedron. 67. 47. 9148.