Synthesis, characterization and catalytic activity of PEPPSI-type palladium-NHC complexes

Eight benzimidazolium salts (2a-h) with two nitrogen atoms substituted by various alkyl groups have been synthesized in high yields. The benzimidazolium salts were readily converted into the corresponding PEPPSItype palladium-NHC complexes (PEPPSI = pyridine-enhanced precatalyst preparation, stabilization, and initiation) (3a-h). The structures of all compounds were characterized by H-1 NMR, C-13 NMR, and IR spectroscopy as well as by elemental analysis techniques, which support the proposed structures. The catalytic activity of the PEPPSI-type palladium-NHC complexes was evaluated with respect to the direct C5-arylation of 2-substituted heteroaryl derivatives (thiophene, furan and thiazole) with various aryl bromides. This arylation occurs efficiently and selectively at the C5-position of the 2-substituted thiophene, furan and thiazole derivatives.