Chapter

Electronic Effects in Organic Chemistry

Volume 351 of the series Topics in Current Chemistry pp 1-24

Date:

Carbenes from Ionic Liquids

  • Oldamur HollóczkiAffiliated withWilhelm-Ostwald Institute of Physical and Theoretical Chemistry, University of Leipzig
  • , László NyulásziAffiliated withDepartment of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics Email author 

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Abstract

In the last decade an explosive development has been observed in the fields of both ionic liquids (ILs) as potential chemically inert solvents with many possible technical applications, and N-heterocyclic carbenes (NHCs) as catalysts with superb performance. Since the cations of many ILs can be deprotonated by strong bases yielding NHCs, this two fields are inherently connected. It has only recently been recognized that some of the commonly used basic anions of the ILs (such as acetate) are able to deprotonate azolium cations. While the resulting NHC could clearly be observed in the vapor phase, in the liquid – where the mutual electrostatic interactions within the ion network stabilize the ion pairs – the neutral NHC cannot be detected by commonly used analytical techniques; however, from these ionic liquids NHCs can be trapped, e.g., by complex formation, or more importantly these ILs can be directly used as catalysts, since the NHC content is sufficiently large for these applications. Apart from imidazole-2-ylidenes, the formation of other highly reactive neutral species (“abnormal carbenes,” 2-alkylideneimidazoles, pyridine-ylidenes or pyridinium-ylides) is feasible in highly basic ionic liquids. The cross-fertilizing overlap between the two fields may provide access to a great advance in both areas, and we give an overview here on the results published so far, and also on the remaining possibilities and challenges in the concept of “carbenes from ionic liquids.”

Keywords

1-Ethyl-3-methylimidazolium acetate Acid–base equilibria Basic ionic liquids Carbene Catalysis CO2 capture DFT calculations Hydrogen bonding Ionic liquids Mass spectroscopy N-Heterocyclic carbenes Organocatalysis Photoelectron spectroscopy Proton transfer Synthesis Vaporization mechanism