Search for: All records

An efficient Suzuki cross-coupling reaction under continuous flow conditions was developed utilizing an immobilized solid supported catalyst consisting of bimetallic nickel–palladium nanoparticles (Ni–Pd/MWCNTs). In this process, the reactants can be continuously pumped into a catalyst bed at a high flow rate of 0.6 mL/min and the temperature of 130 °C while the Suzuki products are recovered in high steady-state yields for prolonged continuous processing. The catalyst was prepared by mechanical shaking of the appropriate nickel and palladium salts using ball-mill energy without the requirement of any solvent or reducing agent. This straightforward, facile, and simple method allows for bulk production of Ni–Pd/MWCNTs nanoparticles with a small particle size ideal for application in continuous flow cross-coupling catalysis. The as-prepared catalyst mostly contains nickel (7.9%) with a very small amount of palladium (0.81%) according to ICP-OES analysis. This remarkable immobilized catalyst can be used several times for different Suzuki reactions with a minimum loss of reactivity and no detectable leaching of the metal nanoparticles. Notably, by modifying the groups on both aryl halides and phenylboronic acids, the method provides access to a diverse array of the Suzuki products in flow with high steady-state yield, making it suitable for applications in industrial and pharmaceutical scales. Moreover, several spectroscopic techniques were employed to identify the structure and composition of the as-prepared Ni–Pd/MWCNTs nanoparticles before and after the reaction in flow such as transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), BET surface area (physisorption), and FTIR spectroscopy. 

We have developed an efficient method to generate highly active nickel–palladium bimetallic nanoparticles supported on multi-walled carbon nanotubes (Ni–Pd/MWCNTs) by dry mixing of the nickel and palladium salts utilizing the mechanical energy of a ball-mill. These nanoparticles were successfully employed in Sonogashira cross-coupling reactions with a wide array of functionalized aryl halides and terminal alkynes under ligand and copper free conditions using a Monowave 50 heating reactor. Notably, the concentration of palladium can be lowered to a minimum amount of 0.81% and replaced by more abundant and less expensive nickel nanoparticles while effectively catalyzing the reaction. The remarkable reactivity of the Ni–Pd/MWCNTs catalyst toward Sonogashira cross-coupling reactions is attributed to the high degree of the dispersion of Ni–Pd nanoparticles with small particle size of 5–10 nm due to an efficient grinding method. The catalyst was easily removed from the reaction mixture by centrifugation and reused several times with minimal loss of catalytic activity. Furthermore, the concentration of catalyst in Sonogashira reactions can be reduced to a minimum amount of 0.01 mol% while still providing a high conversion of the Sonogashira product with a remarkable turnover number (TON) of 7200 and turnover frequency (TOF) of 21 600 h −1 . The catalyst was fully characterized by a variety of spectroscopic techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).