Monophosphine-Cyclopalladated Ferrocenylpyrimidine Complex: Synthesis, Crystal Structure and Reusable Catalytic System for Coupling Reaction of Aryl Chlorides in PEG-400

Abstract

Since the first report on the use of palladacycles for the Suzuki and Heck reactions by Herrmann, Beller and coworkers in 1995, a wide variety of known and new palladacycles have been successfully used in the coupling reactions. Employing aryl chlorides for this reaction has been focused because aryl chlorides are cheaper and more available than their bromides and iodide counterparts. However, only a few palladacycles have proved to be efficient for the activation of notoriously unreactive aryl chlorides. In most cases the catalysts employed need to be used in relatively high loadings, and poor recovery, which negated the advantages associated with the use of aryl chlorides. PEGs [poly(ethylene glycol)] have clear advantages as a solvent in organic synthesis because they are cheap, steady, readily available, and nontoxic. Recently, liquid PEGs have been adopted as a new approach for catalyst recycling, in a broad range of catalytic organic reactions. We have also found biaryl phosphine adducts of cyclopalladated ferrocenylpyrimidine are very efficient for the amination of aryl chlorides in PEG-400. These adducts combine the stability induced by the presence of a palladacycle framework with the high activity commonly associated with phosphine ligands, and were far more active than the corresponding dimeric palladacycle. As a continuation of our interest in the synthesis and application of cyclopalladated complexes, we have prepared a new phosphine adduct of palladacycle 2 from the reaction of cyclopalladated ferrocenylpyrimidine dimer 1 with commercially available 2-dicyclohexylphosphanyl-2'-(N,N-dimethylamino)biphenyl (DCPAB) (Scheme 1) and examined its activity in the coupling reactions. Complex 2 is airand moisture-stable, both in solid state and in solution. It is very soluble in chloroform, dichloromethane and acetone, but insoluble in petroleum ether and n-hexane. The new complex was fully characterized by elemental analysis, IR, H NMR and ESI-MS. These spectra were well consistent with the title complex. Moreover, the molecular structure of 2 has been ascertained by means of X-ray studies. The molecule of 2 together with selected bond distances and angles is shown in Figure 1. The Pd atom is in a slightly distorted square-planar environment bonded to the phosphorus atom, the chlorine atom, the pyrimidinyl nitrogen atom and the carbon atom of the ferrocenyl moiety. The Pd–P bond length of 2 is similar to those of the related DCPAB-palladacycles, while the above bond length of 2 is longer than those of the related PPh3-palladacycles possibly due to the steric bulk of the DCPAB ligand. 16-18 In the crystal of 2 there exist two types of intermolecular C–H···Cl hydrogen bonds, which link the molecules into a 2D