N-heterocyclic carbenes

from Wikipedia, the free encyclopedia
Structural motif of the NHC

N -Heterocyclic carbenes or NHC for short(sometimes also Arduengo carbenes) are a group of organic chemical compounds . These are electron-rich, aromatic or unsaturated heterocyclic compounds that can form stable carbenes .

They were discovered by Anthony J. Arduengo in 1991. Its existence was suspected as early as 1960 by Hans-Werner Wanzlick .

Structure and stability

Overview of common NHCs

The high stability of NHCs compared to other carbenes is mainly caused by the -I and + M effects of the neighboring heteroatoms. This increases the electron density of the empty p orbital on carbon. In addition to nitrogen, sulfur or oxygen can also stabilize the carbene. NHCs with imidazole as the basic structure are additionally stabilized by their aromaticity.

education

Free carbenes are mostly obtained by the deprotonation of imidazolium salts or the corresponding cationic precursor compounds. Strong bases such as sodium hydride or alcoholates are typically used.

Dimerization

Some NHC dimerize to tetraaminoethylene - derivatives . They are in a chemical equilibrium , the so-called Wanzlick equilibrium , with the corresponding carbenes. Contrary to what was initially assumed, this balance does not play a role for most NHCs.

Wanzlick balance

application

NHCs play an important role as ligands for metal complexes. Due to their properties as strong sigma donors and their kinetic inertness , they represent a popular alternative for phosphines, especially for transition metal catalysis . Transition metal-NHC complexes are often more temperature-stable and less sensitive to oxidation than phosphine complexes.

A well-known example of the application of transition metal-NHC complexes in catalysis is the olefin metathesis - second generation catalyst by Robert H. Grubbs .

NHC as a ligand in a 2nd generation metathesis catalyst

Another important property is the ability of the NHC to form charge transfer complexes .

See also

literature

  • Entry on N-heterocyclic carbenes. In: Römpp Online . Georg Thieme Verlag, accessed on April 29, 2014.
  • Matthew N. Hopkinson, Christian Richter, Michael Schedler, Frank Glorius: An Overview of N-Heterocyclic Carbenes. In Nature 510, 2014, pp. 485-496, doi: 10.1038 / nature13384
  • F. Ekkehardt Hahn, Mareike C. Jahnke: Heterocyclic Carbenes - Synthesis and Coordination Chemistry. In: Angew. Chem. 120, 2008, pp. 3166-3216, doi: 10.1002 / anie.200703883 .
  • Silvia Díez-González, Nicolas Marion, Steven P. Nolan: Chem. Rev. 109, No. 8, 2009, pp. 3612-3676.
  • Silvia Díez-González (Ed.): N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools , RSC catalysis series, Royal Society of Chemistry 2010
  • T. Dröge, F. Glorius: The measure of all rings - N-heterocyclic carbenes. In: Angew. Chem. 122, 2010, pp. 7094-7107, doi: 10.1002 / anie.201001865 .
  • Dieter Enders, Oliver Niemeier, Alexander Henseler: Chemical Reviews. 107, No. 12, 2007, pp. 5606-5655.

Individual evidence

  1. ↑ AJ Arduengo III., RL Harlow, MJ Kline: A stable crystalline carbene, J. of the Am. Chem. Soc. Volume 113, 1991, 113, pp. 361-363.
  2. ↑ Volker PW Böhm, Wolfgang A. Herrmann: The Wanzlick Equilibrium. In: Angewandte Chemie . 2000, 39, pp. 4036-4038, doi : 10.1002 / 1521-3773 (20001117) 39:22 <4036 :: AID-ANIE4036> 3.0.CO; 2-L .
  3. ^ RH Grubbs: Handbook of Metathesis . Wiley-VCH, 2003.
  4. ^ RH Grubbs, Trnka ™: Ruthenium-Catalyzed Olefin Metathesis. In: Ruthenium in Organic Synthesis (S.-I. Murahashi, Ed.), Wiley-VCH, 2004. doi: 10.1002 / 3527603832.ch6 .