Synthesis and Structural Characterization of Monophosphine-Cyclopalladated Ferrocenylpyrimidine Complexes and Reusable Catalytic System for Amination of Hindered Aryl Chlorides in PEG-400

Abstract

A new ferrocene-based ligand, 4,6-dimethyl-2-pyrimidinylferrocene (1), was conveniently prepared via the coupling reaction of chloromercuriferrocene and 4,6-dimethyl-2-iodopyrimidine, and its monophosphine-palladacycle complexes 3−6 were also readily obtained from the cyclopalladation reactions and bridge-splitting reactions. These compound have been fully characterized by 1H NMR, 13C{1H} NMR, IR, ESI-MS, and elemental analysis. Additionally, their detailed structures have been determined by X-ray single-crystal diffraction, and many types of intramolecular and intermolecular hydrogen bonds are found to exist in the crystals of these palladacycles. The catalytic activity of these air- and moisture-stable palladacycles was evaluated in the Buchwald−Hartwig amination involving a range of sterically hindered aryl chlorides. 5 and 6 were found to be very efficient for this reaction. Typically, using 1 mol % of catalyst in the presence of 2 equiv of KtOBu as base in PEG-400 [poly(ethylene glycol-400)] at 120 °C provided coupling products in excellent yields. Moreover, the 6/PEG-400 system could be recycled and reused three times without any loss of catalytic activity.