Wanzlick balance

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A Wanzlick equilibrium is a chemical equilibrium reaction between a stable carbene and its dimer .

In the 1960s, Hans-Werner Wanzlick and Eberhard Schikora suspected that it should be possible to produce a carbene with the dihydroimidazol-2-ylidene structure by vacuum pyrolysis of a 2-trichloromethyldihydroimidazole derivative with elimination of chloroform .

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

Both believed that a carbene produced in this way was to a small extent in equilibrium with its dimer. They concluded this from reactivity studies, since the reaction with an electrophile gave the product of the electrophilic substitution. The dimer would not be capable of this reaction.

The hypothesis of the carbene-dimer equilibrium was investigated by David M. Lemal and HE Winberg . They heated a mixture of two differently substituted tetraaminoethylenes.

Crossed dimer study

No crossed dimers could be found in these experiments, which, according to Wanzlick's hypothesis, should have been the case. They rather suggest that there is no carbene-dimer equilibrium in the case of the dihydroimidazol-2-ylidene derivatives.

Lemal proposed an alternative mechanism for the results of Wanzlick, taking into account the reactivity of the electron-rich tetraaminoethylenes and not that of the stable carbenes, which at that time only existed as a hypothesis.

Lemal's mechanism for the reaction of tetraaminoethylene with electrophiles. A catalytic amount of EX produces the dimer (purple). This assumes that the dimer is more stable than the carbene. EX stands for an acid or a metal salt such as LiCl

Lemal believed that a tetraaminomethylene, and not the carbene, reacts with the electrophile EX via a cationic intermediate. He suggested that the cation dissociates into a carbene and a salt. The free carbene can then dimerize to the starting compound or react with the electrophile. Both routes lead to the same reaction products, the dihydroimidazolium salt.

In 1999 M. Denk examined the cross experiment again and was able to observe the crossed dimer. These results prompted Lemal to repeat his 1964 experiments. It was possible to reproduce the results of Denk, but only in deutero- THF as solvent. It was again not possible to detect the crossed product in toluene and an electrophile trap.

In 1999 and 2000, Lemal and Hahn observed an equilibrium between dibenzotetraazafulvalene derivatives and its carbenes. This observation led Böhm and Herrmann to come to the conclusion in 2000 that a Wanzlick equilibrium exists between tetraaminoethylene and the corresponding carbene, which Kirmse confirmed in 2010.

Equilibrium between dibenzotetraazafulvalene and the corresponding carbene

In further investigations by Alder it could be shown that unhindered diaminocarbenes form a dimer under acid catalysis as observed by Lemal. Excess acid forms a dimeric salt.

Individual evidence

  1. ↑ Hans-Werner Wanzlick, E. Schikora: “A new approach to carbene chemistry” in Angewandte Chemie 1960 , 72 , 494 doi : 10.1002 / anie.19600721409 .
  2. ^ A b H. W. Wanzlick, E. Schikora: A nucleophilic carbene in Chemical Reports 1960 , 94 , 2389-2393 doi : 10.1002 / cber.19610940905
  3. ↑ HW Wanzlick: “Aspects of Nucleophilic Carbene Chemistry” in Angew. Chem. Int. Ed. Engl. 1962 , 1 , 75 doi : 10.1002 / anie.196200751 .
  4. ↑ a b D. M. Lemal, RA Lovald, KI Kawano: “Tetraaminoethylenes. The Question of Dissociation "in J. Am. Chem. Soc. 1964 , 86 , 2518 doi : 10.1021 / ja01066a044
  5. ^ HE Winberg, JE Carnahan, DD Coffman, M. Brown: "Tetraaminoethylenes" in J. Am. Chem. Soc. 1965 , 87 , 2055 doi : 10.1021 / ja01087a040
  6. ^ TA Taton, P. Chen: A Stable Tetraazafulvalene. In: Angew. Chem. Int. Ed. Engl. 1996 , 35 , 1011 doi : 10.1002 / anie.199610111
  7. ↑ Michael K. Denk, Ken Hatanoa, Martin Maa: Nucleophilic carbenes and the wanzlick equilibrium: A reinvestigation in Tetrahedron Letters 1999 , 40 , 2057-2060 doi : 10.1016 / S0040-4039 (99) 00164-1
  8. Jump up ↑ Yufa Liu, David M. Lemal: Concerning the Wanzlick equilibrium in Tetrahedron Letters 2000 , 41 , 599-602 doi : 10.1016 / S0040-4039 (99) 02161-9
  9. ↑ Yufa Liu, Patrick E. Lindner, David M. Lemal: Thermodynamics of a diaminocarbenes-tetraaminoethylene Equilibrium . J. Am. Chem. Soc. 1999 , 121 , 10626-10627 doi : 10.1021 / ja9922678
  10. ^ F. Ekkehardt Hahn, Lars Wittenbecher, Duc Le Van, Roland Fröhlich: Evidence for an Equilibrium between an N-heterocyclic Carbene and Its Dimer in Solution . Angewandte Chemie International Edition 2000 , 39 , 541-544. doi : 10.1002 / (SICI) 1521-3773 (20000204) 39: 3 <541 :: AID-ANIE541> 3.0.CO; 2-B .
  11. ↑ Volker PW Boehm, Wolfgang A. Herrmann: The Wanzlick Equilibrium Angew. Chem. 2000 , 39 , 4036-4038 doi : 10.1002 / 1521-3773 (20001117) 39:22 <4036 :: AID-ANIE4036> 3.0.CO; 2-L
  12. ^ Kirmse, W .: The Beginnings of N-Heterocyclic Carbenes . Angewandte Chemie International Edition 2010 , 49 , 8798–8801. doi : 10.1002 / anie.201001658
  13. ^ Roger W. Alder, Leila Chaker, François PV Paolini: Bis (diethylamino) carbene and the mechanism of dimerization for simple diaminocarbenes. In: Chem. Commun. 2004 , 2172-2173 doi : 10.1039 / b409112d .