Kinetic acidity of supramolecular imidazolium salts -: Effects of substituent, preorientation, and counterions on H/D exchange rates

The deprotonation of imidazolium salts to N-heterocyclic carbenes is often a decisive step in modem catalytic reactions. Therefore, we studied the H/D exchange of the (CH)-H-2 of 15 imidazolium-substituted calix[4]arenes and 11 nonmacro-cyclic model compounds in methanol/water (97:3). The influence of the counterion, substitution directly on the imidazolium unit or on the preorientating calixarene backbone could be studied. The observed exchange rates might give a rational for the suitability of the imidazolium salts as precursors in the Suzuki coupling.