Facile and Regioselective Deuteration of C2-Alkylated Imidazolium Salts in the Presence of Cesium Carbonate.

Thirteen imidazolium iodides bearing benzyl, mesityl, or 2,6-diisopropylphenyl substituents on their nitrogen atoms, and C1-C4 alkyl chains on their C2 carbon atom were readily deuterated with D2O as a cheap and non-toxic deuterium source in the presence of Cs2CO3, a weak, innocuous, inorganic base. The isotopic exchange proceeded quickly and efficiently under mild, aerobic conditions to afford a range of aNHC and NHO precursors regioselectively labeled on their C2α exocyclic position and/or C4=C5 heterocyclic backbone. A "carbene-free" mechanism was postulated, in which the carbonate anion acts as a catalyst to activate an exocyclic, acidic C-H bond and ease a deuterium transfer from D2O to the imidazolium salt in a concerted fashion.