Synthesis of new N-heterocyclic carbene-Pd-PEPPSI complexes and investigation of their catalytic efficiencies in some coupling reactions

1,4-Diaryl-1H-imidazoles prepared from the dehydration of 1,4-diaryl-2,5-dihydro-1H-imidazolin 3-oxides were treated with benzyl bromide to prepare the corresponding imidazolium bromides. The reaction of imidazolium salts with Pd(OAc)2 in the presence of KBr and pyridine in acetonitrile gave a new series of air-stable NHC-Pd-PEPPSI type complexes (4a-g). The new complexes bearing unsymmetrical NHC ligand, with a phenyl group in the backbone, were characterized by means of 1H NMR, 13C NMR, FT-IR spectroscopy and HRMS analysis. The molecular and crystal structure of the complexes were determined by single-crystal X-ray diffraction method. X-ray studies show that the molecular structure adopts slightly distorted square planar geometry around the palladium (II) center. The absolute structure was refined as an inversion twin. The new NHC-Pd-PEPPSI complexes were screened for their catalytic activity in the Heck-Mizoroki and Suzuki-Miyaura cross-coupling reactions. The high yields of the Heck-Mizoroki reactions between the various aryl/hetaryl bromides and styrenes in DMF in the presence of NaOAc as a base with low catalys loadings (%0.5) manifested that the new catalysts are highly efficient. The reactions also occur in aqueous media (DMF-H2O) in high yields. Similarly, the Suzuki-Miyaura reaction between various aryl/hetaryl bromides and phenylboronic acids came off in excellent yields in DMF-H2O in the presence of K2CO3 as a base with low catalys loadings (%0.1) at room temperature.