Acid-Free Intermolecular Hydroarylation of Acetylene Catalyzed by Dicationic Palladium(II) and Platinum(II) Ethylene Complexes

Four dicationic palladium and platinum ethylene complexes of the type [M(PNP)(C₂H₄)]X₂ (M=Pd, Pt; X=BF₄, SbF₆, PNP=2,6-bis(diphenylphosphinomethyl)pyridine) were studied as pre-catalysts for the intermolecular hydroarylation of acetylene under acid-free conditions. The palladium complex [Pd(PNP)(C₂H₄)](SbF₆)₂ was found to be the most active catalyst for the addition of pentamethylbenzene to acetylene at room temperature. In a ³¹P NMR spectroscopic study the impact of the counter anion on the rate determining step was demonstrated. Various reaction parameters were screened to optimize the catalytic efficiency. The presence of small amounts of water were beneficial and increased the reaction rate. Water acts as co-catalyst assisting in proton transfer during the catalytic reaction. After optimization of the reaction conditions, a benchmark for the palladium(II) catalyzed hydroarylation of acetylene was achieved with TON 200 at room temperature in 24 h under acid-free conditions. However, this catalytic system has a very limited substrate scope.