Generation of N-heterocyclic carbenes (NHC) in aqueous solutions of zwitterionic imidazolium surfactant micelles

Abstract

N-heterocyclic carbenes (NHCs) are important compounds that are often prepared and used in water free environment. Herein we investigate transient formation of a NHC in the headgroup of zwitterionic imidazolium sulfonate surfactants of the type 3-(1-alkyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (ImS3-n) in water. As demonstrated by a series of 1H NMR experiments, the formation of the NHC reactive species is accelerated above the critical micelle concentration of these surfactants due to an increased local concentration of the hydroxide or deuteroxide ions (OH−/OD−) responsible for deprotonation of C(2)-H. The rate of NHC formation can be modulated by the nature and concentration of the anions of added salts, reflecting the selectivity of anion binding to the zwitterionic micelle surface. Analogous anion binding affects are observe in the NMR chemical shifts of the C(2)-H signal of the imidazolium headgroup of the surfactants. The NHCs generated at the surface of ImS3-n micelles promote the selective oxidation of 4-nitrobenzaldehyde to 4-nitrobenzoic acid, a reaction known to be catalyzed by nucleophilic carbenes. Below the cmc or in the presence of a short-chain non-micelle-forming analogue, 3-(1-methyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (ImS3-1), no selective oxidation of 4-nitrobenzaldehyde takes place. The results demonstrate the importance of the micellar microenvironment for controlling the reactivity and selectivity of chemical reactions involving NHC species in aqueous media and can ultimately lead to greener catalytic systems.