Ligandless Palladium-Catalyzed Regioselective Direct C-H Arylation of Imidazo[1,2-a]imidazole Derivatives.

Herein a novel access to functionalizable 6-substituted imidazo[1,2-a]imidazole scaffolds is described. The reactivity of this heterobicyclic unit toward direct C-H arylation was studied, and conditions allowing regioselective arylation at position 3 were successfully developed. The practicability of this method is manifested by the ligandless conditions and low catalyst loading. The strategy is functional group tolerant and provides rapid access to a large variety of 3,6-di(hetero)arylated imidazo[1,2-a]imidazole derivatives. A second arylation at position 2 was then carried out, and a library of diversified 2,3,6-tri(hetero)arylated imidazo[1,2-a]imidazoles was generated in good yields. A one-pot, two-step procedure was finally developed.