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. The complex cis‐[Pd(L)(PPh3)(H2O)2](SO3CF3)2, in which L is an amido‐functionalized N‐heterocyclic carbene (NHC) ligand, has been prepared. This complex was 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.