Methoxycarbonylation of higher olefins catalysed by (pyrazolyl-ethyl)-pyridine palladium(II) complexes.
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Methoxycarbonylation belongs to the family of carbonylation reactions that are efficiently catalysed by homogeneous palladium catalysts to give ester products. To date, achieving cost effect, stable and selective catalysts still remains a major challenge in ligand design and catalyst development. Reactions of 2-[1-(3,5-dimethylpyrazol-1-yl)ethyl]pyridine (L1) and 2-[1-(3,5-diphenylpyrazol-1-yl)ethyl]pyridine (L2) with the [Pd(COD)Cl2] or [Pd(COD)MeCl] produced novel palladium(II) complexes [Pd(L1)ClMe] (C1), [Pd(L1)Cl2] (C2), [Pd(L2)ClMe] (C3), and [Pd(L2)Cl2] (C4) in low to satisfactory yields. The characterization of these compounds involved 1H- and 13C-NMR spectroscopy, infrared spectroscopy, elemental analysis (for compounds), mass spectrometry, and single crystal X-ray crystallography of compounds L2, C1, C2 and C4. The solid state structures of all the complexes were mononuclear and showed ligands L1 and L2 binding in bidentate coordination modes. Catalytic investigations of the palladium(II) complexes C1-C4 indicated that all the complexes gave active catalysts in the methoxycarbonylation of olefins. Catalyst C2 containing two auxiliary coordinating chloride ligands and methyl substituents on the ligand structure was the most active in the methoxycarbonylation reaction. Reactions using HCl, an acid promoter produced the highest activity while p-TsOH gave no catalytic activity. Furthermore, changing the phosphine used from non-chelating to chelating resulted in decreased catalytic activities. Changes in catalyst concentration, temperature, pressure, solvent, time and substrate also influenced the regioselectivity and catalytic activities.