Palladium complexes of (Benzoimidazol-2-ylmethyl) amine ligands : synthesis and applications as catalysts in methoxycarbonylation of olefins.

Abstract

Compounds N-(1H-benzoimidazol-2-ylmethyl)-2-aniline(L1), N-(1H-benzoimidazol- 2-ylmethyl)-2-methoxyaniline (L2), N-(1H-benzoimidazol-2-ylmethyl)-2-thioaniline (L3) and N-(1H-benzoimidazol-2-ylmethyl)-2-bromoaniline (L4) were prepared by reaction of 2-(chloromethyl) benzimidazole with the appropriate aniline. Ligands L1– L4 were reacted with either [PdCl2(COD)] or [PdClMe(COD)] to form palladium complexes [PdCl2(L1)], [PdClMe(L1)], [PdCl2(L2)],[PdClMe(L2)], [PdCl2(L4)] and [PdClMe(L4)]. Treatment of the neutral complexes C1 – C6 with one equivalent of PPh3 in the presence of the halide abstractor NaBAr4 (Ar = 3,5-(CF3)2C6H3) led to the formation of the cationic species [(PdCl(L1)]Bar4, [(PdMe(L1)]Bar4, [(PdCl(L2)]BAr4, [(PdMe(L2)]BAr4 and [(PdMe(L4)]BAr4. All the new compounds were characterized by combination of 1H and 31P NMR spectroscopy, mass spectrometry, X-ray crystallography of complexes C8, C9 and C11 and elemental analysis (for complexes). The palladium complexes were investigated as catalyst in methoxycarbonylation of olefins. Methoxycarbonylation of 1-hexene using C2, C4, C6, C8-C11 was studied in order to investigate the effect of catalyst structure and PPh3 ligand on activity and selectivity. Complex C10 bearing electron donating OCH3 substituent on phenyl group was the most active, while the corresponding analogue containing the Br substituent on phenyl group was the least active. Complex C9 with Pd-Cl showed lower catalytic activity, compared complex C10 with Pd-Me. Addition of PPh3 to the

palladium complexes resulted in higher catalytic activities and high regioselectivity towards the linear esters. The activity also dependent in the identity of acid promoter where HCl gave highest active system while no activity was observed with p-TsOH. The effects of pressure, temperature, catalysts concentration, identity of the olefin and time was also investigated. Increasing the catalysts concentration, chain length and time resulted in higher catalytic activities and higher regioselectivity towards the branched ester.