Stability and organocatalytic efficiency of n-heterocyclic carbenes electrogenerated in organic solvents from imidazolium ionic liquids

The kinetic of degradation of 1-butyl-3-methylimidazole-2-ylidene (selected as model N-heterocyclic carbene - NHC), generated in organic solvents by cathodic reduction of the parent 1-butyl-3-methylimidazolium salts BMIm-X, was studied by a simple voltammetric analysis. The effect of NHC degradation rate on the efficiency of an organocatalyzed reaction (the synthesis of γ-butyrolactone from cinnamaldehyde and trifluoromethylacetophenone) was investigated. The nature of the solvent and of the anion X- have a remarkable effect on the stability of the NHC, the bis(trifluoromethylsulfonyl) imide anion being the best for a long lasting carbene (while acetonitrile seems to be the worst solvent). The role of X- has been related to a competition between NHC and X-, in the hydrogen bonding interaction with BMIm+. The higher the stabilization of NHC by hydrogen bond, the lower its degradation rate. These hydrogen bonding interactions, previously reported in pure BMIm-X, seem to be operative even in organic solvents containing BMIm-X at low concentrations (c < 0.1 mol L-1). The effect of the nitrogen alkyl substituents on the degradation of NHC (and thus on its efficiency as organocatalyst) is also pointed out.