Deuteration of imidazolium ionic liquids: A problem with the 2-CH acidity

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]. 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 H/D exchange at C-2, at ambient temperature, requiring only D2O (or deuterated alcohols) but no basic or acidic reagents or catalysts other than D2O [2]. With the Arduengo reaction in view, it has apparently been tempting [3] 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. Arduengo, III, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361-363; 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. 2005, 70, 1915.
3. O. Holloczki, D. Gerhard, K. Massone, L. Szarvas, B. Nemeth, T. Vespremi and L. Nyulaszi New J. Chem. 2010, 34, 3004.