Rhodium(III)-catalyzed cascade oxidative annulation reactions of aryl imidazolium salts with alkynes involving multiple C-H bond activation.

Abstract

The cascade oxidative annulation reactions of aryl imidazolium salts with alkynes proceed efficiently in the presence of [Cp*RhCl2]2 and Cu(OAc)2·H2O to give substituted imidazo[1,2-a]-quinolinium salts and benzo[ij]imidazo[2,1,5-de]quinolizinium salts. The reactions were through the normal and abnormal N-heterocyclic carbene (NHC)-directed cyclometalation, alkyne insertion into the Rh-C bond, and reductive elimination of alkenyl and NHC ligands. The reactions are highly regioselective with unsymmetrical alkynes and can be achieved stepwise by controlling the reaction conditions. This provides a new application of NHCs as directing groups and substrates in the synthesis of fused N-heterocyclic compounds. The N-substituting group of the benzo[ij]imidazo[2,1,5-de]quinolizinium salts could be removed successfully with pyridine to afford benzo[ij]imidazo[2,1,5-de]quinolizines in excellent yields. Moreover, some of the benzo[ij]imidazo[2,1,5-de]quinolizinium salts exhibit intense fluorescence which might be useful in organic electronic materials.