Alternative Synthetic Methods for PEPPSI-Type Palladium Complexes

Alternative procedures are reported for the preparation of PEPPSI-type palladium complexes (PEPPSI = pyridine-enhanced precatalyst preparation stabilization and initiation) in good to excellent yields. One method involves the reaction of [(NHC)Pd(acac)Cl] complexes (NHC = N-heterocyclic carbene, acac = acetylacetonate) with hydrohalides (HX; X = Cl, Br, I) of pyridine or 2,6-lutidine. Two other one-pot syntheses involve Pd(acac)(2), the azolium salt and pyridine hydrohalide (or 2,6-lutidine hydrohalide). The complexes were obtained as moisture- and air-stable solids and were characterized by H-1 and C-13 NMR spectroscopy, CHN analysis, and IR spectroscopy. The various synthetic procedures and the yields of the desired products are discussed. The X-ray structure of [1-(2,6-diisopropylphenyl)-3-(2,4,6-trimethylphenyl)imidazolin-2-ylidene]Pd(acac)Cl (4c) is also reported.