Double C-H Functionalization to Construct Polycyclic Heteroarenes Catalyzed by an Ionic Salt of a Pd Complex with an N-Heterocyclic Carbene Ligand.

A new ionic salt, cis-[Pd(L)(PPh3)(H2O)2](SO3CF3)2 (A), in which L is an amido-functionalized N-heterocyclic carbene (NHC) ligand, has been prepared by the salt metathesis reaction between [Pd(L)(PPh3)Cl2] and AgOTf in wet acetonitrile. It was successfully characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction analysis. Complex A was found to be effective in catalyzing the double C–H functionalization of various nitrogen- and sulfur-containing heteroarenes to form fused polycyclic heteroaromatic compounds with alkynes in the presence of Cu(OAc)2 and tetra-n-butylammonium bromide. Indoles, thiophenes, imidazo[1,2-a]pyridine, and imidazoles were used as substrates. In particular, intriguing imidazo[5,1,2-cd]indolizine compounds could be obtained under the conditions employed.