Wanzlick equilibrium

The Wanzlick equilibrium is a chemical equilibrium between a relatively stable carbene compound and its dimer.

History

Original conjecture

In 1960, H.-W. Wanzlick and E. Schikora proposed that carbenes derived from dihydroimidazol-2-ylidene were generated by vacuum pyrolysis of 2-trichloromethyl dihydroimidazole derivatives, with the loss of chloroform.[1][2]

Wanzlick's mechanism for the reaction of dihydroimidazol-2-ylidene with electrophiles

Wanzlick and Schikora believed that once prepared these carbenes existed in an unfavourable equilibrium with their corresponding dimers. This assertion was based on reactivity studies which they believed showed that the free carbene reacted with electrophiles (E-X). The dimer (a substituted tetraaminoethylene) was believed to be inactive to the electrophiles (E-X), and thought to merely act as a stable carbene reservoir.[3]

Conjecture challenged

Wanzlick’s hypothesis of a carbene-dimer equilibrium was tested by D. Lemal and others,[4] and separately by H. Winberg and others.[5] They heated together two tetraaminoethylene derivatives with different N-aryl substitututions:

Dimer cross-over experiment

This reaction did not produce a mixed dimeric product, and accordingly indicated that a 'carbene-dimer equilibrium' did not exist for these dihydroimidazol-2-ylidene derivatives.

Lemal[4] proposed an alternative mechanism to account for the reactions observed by Wanzlick[3] by considering the reactivity of the electron rich tetraaminoethylenes — and not the stable carbenes, which at the time were still hypothetical.[2]

Lemal's mechanism for the reaction of tetraaminoethylene with electrophiles. In conditions of excess E-X, the salt (blue) is formed. In conditions of catalytic E-X, the dimer (purple) will be formed. However, this is based on the assumption that the dimer is more stable than the carbene; but this assumption has been questioned.[6] E-X may be an acid or even a metal salt e.g. LiCl.

Lemal believed that the tetraaminoethylene, not the carbene, reacted with the electrophile (E-X) to generate a transient cationic species. He proposed that this cation then dissociated into the free carbene plus the resultant salt. The free carbene could then either re-dimerise (regenerating the tetraaminoethylene starting material) or react with E-X (as Wanzlick originally predicted), with either route eventually giving the same reaction product, the dihydroimidazolium salt.

Conjecture confirmed

In 1999 M. Denk reinvestigated the cross-over experiments and did find mixed dimers thus supporting the Wanzlick equilibrium.[7] This prompted Lemal to repeat his 1964 experiments. He was able to confirm Denk's findings but only with deuterated tetrahydrofuran (THF) as a solvent. With toluene and added KH as an electrophile quencher, however, the cross-over product was again not observed by Lemal.[8]

In 1999 Lemal [9] and Hahn [10] both investigated an equilibrium between a dibenzotetraazafulvalene derivative and its carbene, which led Böhm & Herrmann to conclude in 2000 that "the Wanzlick equilibrium between a tetraaminoethylene and its corresponding carbene did exist" after all.,[11] a notion confirmed in 2010 by Kirmse [12]

Further analysis

More recent work by Alder and others[13] has shown that unhindered diaminocarbenes form dimers by acid catalysed dimerisation as shown in the Lemal reaction above. In excess acid conditions the dimer forms the salt.

References

  1. ↑ Wanzlick Hans-Werner, Schikora E (1960). "Ein neuer Zugang zur Carben-Chemie". Angewandte Chemie 72 (14): 494. doi:10.1002/ange.19600721409.
  2. 1 2 Wanzlick H. W., Schikora E. (1960). "Ein nucleophiles Carben". Chemische Berichte 94 (9): 2389–2393. doi:10.1002/cber.19610940905.
  3. 1 2 H. W. Wanzlick (1962). "Aspects of Nucleophilic Carbene Chemistry". Angew. Chem. Int. Ed. Engl. 1 (2): 75–80. doi:10.1002/anie.196200751.
  4. 1 2 D. M. Lemal, R. A. Lovald, and K. I. Kawano (1964). "Tetraaminoethylenes. The Question of Dissociation". J. Am. Chem. Soc. 86 (12): 2518–2519. doi:10.1021/ja01066a044.
  5. ↑ H. E. Winberg, J. E. Carnahan, D. D. Coffman, and M. Brown (1965). "Tetraaminoethylenes". J. Am. Chem. Soc. 87 (9): 2055–2056. doi:10.1021/ja01087a040.
  6. ↑ T. A. Taton and P. Chen (1996). "A Stable Tetraazafulvalene". Angew. Chem. Int. Ed. Engl. 35 (9): 1011–1013. doi:10.1002/anie.199610111.
  7. ↑ Denk Michael K., Hatanoa Ken, Maa Martin (1999). "Nucleophilic carbenes and the wanzlick equilibrium: A reinvestigation". Tetrahedron Letters 40 (11): 2057–2060. doi:10.1016/S0040-4039(99)00164-1.
  8. ↑ Liu Yufa, Lemal David M (2000). "Concerning the Wanzlick equilibrium". Tetrahedron Letters 41 (5): 599–602. doi:10.1016/S0040-4039(99)02161-9.
  9. ↑ Liu Yufa, Lindner Patrick E., Lemal David M. (1999). "Thermodynamics of a Diaminocarbene−Tetraaminoethylene Equilibrium". J. Am. Chem. Soc. 121 (45): 10626–10627. doi:10.1021/ja9922678.
  10. ↑ Hahn F. Ekkehardt, Wittenbecher Lars, Le Van Duc, Fröhlich Roland (2000). "Evidence for an Equilibrium between an N-heterocyclic Carbene and Its Dimer in Solution". Angewandte Chemie International Edition 39 (3): 541–544. doi:10.1002/(sici)1521-3773(20000204)39:3<541::aid-anie541>3.0.co;2-b.
  11. ↑ Böhm Volker P. W., Herrmann Wolfgang A. (2000). "The Wanzlick Equilibrium". Angewandte Chemie 39 (22): 4036–4038. doi:10.1002/1521-3773(20001117)39:22<4036::AID-ANIE4036>3.0.CO;2-L.
  12. ↑ Kirmse W (2010). "The Beginnings of N-Heterocyclic Carbenes". Angewandte Chemie International Edition 49 (47): 8798–8801. doi:10.1002/anie.201001658.
  13. ↑ Roger W. Alder; Leila Chaker; François P. V. Paolini (2004). "Bis(diethylamino)carbene and the mechanism of dimerisation for simple diaminocarbenes". Chem. Commun. (19): 2172–2173. doi:10.1039/b409112d. PMID 15467857.
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