Abstract
Cross-coupling reactions are essential synthetic tools for the synthesis of polymers, natural products, agrochemicals, and pharmaceuticals. The most well-known and widely used cross-coupling reactions are the Mizoroki-Heck, Stille, Suzuki-Miyaura, Negishi, and Buchwald–Hartwig cross-coupling reactions, and palladium(II) complexes are generally used as catalyst precursors. Due to the significant role that these reactions play in organic synthesis, the search for metal complexes which can catalyse the reactions efficiently while maintaining high chemo- and regio-selectivities is ongoing. New ferrocenyl- and adamantyl-imine monodentate as well as N^N and N^P chelating ligands (L1 to L7) were successfully synthesized. Synthesis of the ligands was achieved via Schiff-base condensation reactions of acetyl ferrocene and adamantyl ketone with hydrazine monohydrate followed by a second Schiff base reaction, with appropriate aldehydes (2-pyridine carboxaldehyde and 2-diphenylphosphino benzaldehyde), or phosphination with chlorodiphenylphosphine (L2). The ligands were then used to prepare new palladium(II) complexes (C1 to C7) by reacting L1 to L7 with [PdCl2(MeCN)2]. All the ligands and complexes were fully characterized using common spectroscopic and analytical techniques such as 1H NMR and 13C NMR spectroscopy, FT-IR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray diffraction (for C7). Cyclic voltammetry was used to study the electrochemical properties of the ferrocenyl imine ligands (L1, L2 and L4) and ferrocenylimine palladium(II) complexes (C1, C2 and C4). Both the ferrocenyl and adamantyl palladium(II) complexes (C1 to C7) were active catalyst precursors for Mizoroki-Heck and Suzuki-Miyaura carbon-carbon (C-C) cross-coupling reactions. In particular, ferrocenyl N^P chelating palladium(II) complexes (C2 and C3) showed the best activity in Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions. Catalyst precursors bearing an adamantyl moiety required slightly higher temperatures for optimal performance. At higher temperatures the adamantyl palladium(II) catalyst precursors showed enhanced catalytic activity, and this was observed for both N^N and N^P bidentate systems. The adamantyl palladium(II) (pre)catalysts (C6 and C7) showed remarkable activity in Mizoroki-Heck coupling reactions involving substrates with substituents on the olefin and aryl halide (including 4-Cl, 4-CH3, -CO2Me and -CO2Et), while the ferrocenyl palladium(II) catalyst precursors demonstrated moderate catalytic activity. Both ferrocenyl and adamantyl...
M.Sc. (Chemistry)