Deuteration of N,N’-dialkylimidazolium salts: A problem with the 2-CH acidity

Research output: Conference materialsPoster

Abstract

It is well known in the chemical literature that various N,N'-dialkylimidazolium salts ("ionic liquids" or ILs) display acidic character at their C-2 site, to the extent that they can be deprotonated at C-2 by a strong base (e.g., KOBu-t, NaH, or LDA) to give the so-called Arduengo carbenes[1] (Scheme). Success, and stability, in their case presumably comes from the fortuitous presence of the neighboring iminium nitrogen, able to accommodate an electron pair from the C=N double bond. This removes the + and - charges, and the C-2 is left as a neutral carbon atom with a valence of two and a pair unshared valence electrons, i.e., a carbene.

Another well known property of 2-unsubstituted N,N'-dialkylimidazolium salts is the fact that they readily undergo, in a matter of minutes, H/D exchange at C-2, at ambient temperature, requiring only D2O (or CH3OD) but no basic or acidic reagents or catalysts other than D2O2. With the Arduengo reaction in view, it has apparently been tempting3 to consider the very facile H/D exchange as just an additional example of deprotonation at C-2, despite the apparent absence of a base to pick up the proton.
We feel that other mechanistic routes should be considered for the 2-H/D exchange in N,N'-dialkylimidazolium salts, and will describe here the results of our efforts towards that target.


References:
1. A.J. Arduengo, III Acc. Chem. Res. 32 (1999) 913; M.J. Earle, K.R. Seddon Patent application US2003/0186803.
2. R. Giernoth and D. Bankmann Eur. J. Org. Chem. 2008 2881; S.T. Handy and M. Okello J. Org. Chem. 70 (2005) 1915.
3. O. Holloczki, D. Gerhard, K. Massone, L. Szarvas, B. Nemeth, T. Vespremi and L. Nyulaszi New J. Chem. 34 (2010) 3004.
Original languageEnglish
Publication statusPublished - Jun 2014
MoE publication typeNot Eligible
EventTetrahedron Symposium - London, United Kingdom
Duration: 24 Jun 201427 Jun 2014

Conference

ConferenceTetrahedron Symposium
CountryUnited Kingdom
CityLondon
Period24/06/201427/06/2014

Cite this