Abstract
Carbon-carbon cross-coupling reactions are essential synthetic tools for synthesizing polymers, natural products, agrochemicals, and pharmaceuticals. Therefore, new catalysts that function with greater efficiency and functional group tolerance are being researched. We have prepared new ferrocenylimine monodentate N and P donor ligands and N^N and N^P bidentate chelating ligands (L1 to L4) employed in stabilizing palladium ions for application in Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions. The ferrocenylimine ligands were successfully synthesized by Schiff base condensation reactions of acetyl ferrocene with hydrazine monohydrate to afford ferrocenyl hydrazone (L1). Ligand L1 was further treated with aldehydes to give ferrocenyl(2-diphenylphosphino)imine (L3) and ferrocenyl(pyridyl)imine (L3), while phosphination of L1 with chlorodiphenylphosphine afforded L2. The ligands were used to prepare new palladium(II) complexes (C1 to C4) by complexation with [PdCl2(MeCN)2]. All the ligands and complexes were fully characterized using standard spectroscopic and analytical techniques, including 1H NMR and 13C NMR spectroscopy, FT-IR spectroscopy, mass spectrometry and elemental analysis. The complexes (C1 to C4) were tested for efficacies in catalyzing Mizoroki-Heck and Suzuki-Miyaura C-C cross-coupling reactions and proved to be suitable catalyst precursors. Ferrocenyl(2-diphenylphosphine)imino and ferrocenyl-methyl hydrazone palladium(II) complexes C2 and C3 showed the best activities at TONs of up to 201. The ferrocenyl palladium(II) (pre)catalysts demonstrated moderate activity in Mizoroki-Heck reactions involving substrates with substituents on the olefin and aryl halide (including 4-Cl, 4-CH3, -CO2Me and -CO2Et). Density Functional Theory was used to study the mechanism of the Mizoroki-Heck cross-coupling reactions and have led to confirmation of the widely accepted catalytic cycle. Catalyst precursors (C1 to C4) also displayed good activity and selectivity in Suzuki-Miyaura cross-coupling reactions, at 0.5 mol% catalyst loading, with good tolerance to functional groups present on the aryl halide and boronic acid substrates (such as 4-Cl, 4-CHO, 4-COOH, 3-NO2, 3,5-dimethoxy and 4-CH3).
Highlights
We aimed to Discussion contribute to this field by synthesizing ferrocenylimine ligands as new entries to the bulky
We investigated the electrochemical properties of ferrocenylimine palladium(II) complexes (C1, C2 and C4)
Good product conversions (>98%) were obtained when coupling iodobenze with methyl acrylate and ethyl acrylate, which proved that all the pre-catalysts C1–C4 have the ability to tolerate different functional groups such as esters
Summary
Iodides and triflates are activated by palladium catalysts and are often employed for the Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions. The application of dppf in Suzuki-Miyaura cross-coupling of less reactive and less costly substrates (such as chloroarenes) proved to be difficult [20]; there are numerous examples of successful Suzuki-Miyaura cross-coupling of aryl and heteroaryl iodides, triflates and bromides with boronic acids using PdL2 and 1.1 [21,22,23,24] type ligands (L = Cl or OAc). The air-stable and sterically hindered ferrocenyl ligand (1.2) has been reported to have successfully catalyzed C-O, C-N and C-C (Suzuki-Miyaura) cross-coupling reactions and good yields were obtained (78–98%) [28] (Figure 1).
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