Intramolecular C-H activation by alkylpalladium(II) complexes: insights into the mechanism of the palladium-catalyzed arylation reaction.

Abstract

The cyclization of [ArOCH2PdL2Cl] complexes proceeds at room temperature in CH3CN in the presence of base, such as KOPh or carbonate, to form palladacycles. The effect of substituents on the aryl moiety (p-MeO > H >p-NO2) is as expected for an electrophilic aromatic substitution by electrophilic Pd(II). The absence of isotopic effect is also consistent with this proposal. Cyclopalladation proceeds with bidentate ligands (dppf, COD and phen); although the C-H activation reactions are slower in these cases. The starting [ArOCH2Pd(PPh3)2Cl] and [ArOCH2Pd(PPh3)Cl]2 complexes were prepared by transmetalation of organostannanes [(ArOCH2)4Sn] with [Pd(PPh3)2Cl2] or [Pd(PPh3)Cl2]2, respectively. Cleavage of palladacycles with HCl also gave [ArOCH2PdL2Cl] complexes.