Ali R. Siamaki

Palladium nanoparticles supported on carbon nanotubes from solventless preparations: versatile catalysts for ligand-free Suzuki cross coupling reactions

Palladium nanoparticles supported on single- or multi-walled carbon nanotubes (Pd/SWCNT and Pd/MWCNT) were prepared by a rapid, solventless method that does not require reducing agents or electric current. The method involves a straightforward process using dry mixing of a precursor Pd salt (e.g., palladium acetate) with carbon nanotubes at ambient temperature by ball-milling (mechanochemical route) or with subsequent annealing at 300 °C (thermal route) in an inert atmosphere. The Pd/MWCNT sample with Pd nanoparticle size of 1–3 nm and uniform dispersion prepared by mechanochemical ball-milling at room temperature [designated as (Pd/MWCNT)M] displayed remarkable catalytic activity towards Suzuki cross coupling reactions with a high turn over number (TON) of 7250 and turn over frequency (TOF) of 217500 h−1. These nanoparticles were characterized by a variety of techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Additionally, the (Pd/MWCNT)M sample was successfully employed in Suzuki cross coupling reactions with a wide variety of functionalized substrates.

Palladium-Catalyzed Carbonylative Cross-Coupling with Imines: A Multicomponent Synthesis of Imidazolones

The palladium-catalyzed coupling of imines, chloroformates, organotin reagents, and carbon monoxide leads to the one-pot formation of ketocarbamates in good yields. These products can further be converted to highly substituted imidazolones via a cyclocondensation reaction. Overall, this methodology provides an alternative approach to imidazolones from five simple and readily available building blocks via a one-pot, multicomponent process.

Selective N-Chelation-Directed C–H Activation Reactions Catalyzed by Pd(II) Nanoparticles Supported on Multiwalled Carbon Nanotubes

N-Chelation-directed C-H activation reactions that utilize the Pd(II)/Pd(IV) catalytic cycle have been previously reported. To date, these reactions employ only homogeneous palladium catalysts. The first use of a solid-supported Pd(II) catalyst [Pd(II) nanoparticles on multiwalled carbon nanotubes, Pd(II)/MWCNT] to carry out N-chelation-directed C-H to C-O, C-Cl, and C-Br transformations is reported. The results presented demonstrate that the solid-supported Pd(II)/MWCNT catalyst can effectively catalyze C-H activation reactions using the Pd(II)/Pd(IV) catalytic cycle.

A palladium-catalysed multicomponent coupling approach to conjugated poly(1,3-dipoles) and polyheterocycles

Conjugated polymers have emerged over the past several decades as key components for a range of applications, including semiconductors, molecular wires, sensors, light switchable transistors and OLEDs. Nevertheless, the construction of many such polymers, especially highly substituted variants, typically involves a multistep synthesis. This can limit the ability to both access and tune polymer structures for desired properties. Here we show an alternative approach to synthesize conjugated materials: a metal-catalysed multicomponent polymerization. This reaction assembles multiple monomer units into a new polymer containing reactive 1,3-dipoles, which can be modified using cycloaddition reactions. In addition to the synthetic ease of this approach, its modularity allows easy adaptation to incorporate a range of desired substituents, all via one-pot reactions.

A convergent approach to the total synthesis of telmisartan via a Suzuki cross-coupling reaction between two functionalized benzimidazoles.

A direct and efficient total synthesis has been developed for telmisartan, a widely prescribed treatment for hypertension. This approach brings together two functionalized benzimidazoles using a high-yielding Suzuki reaction that can be catalyzed by either a homogeneous palladium source or graphene-supported palladium nanoparticles. The ability to perform the cross-coupling reaction was facilitated by the regio-controlled preparation of the 2-bromo-1-methylbenzimidazole precursor. This convergent approach provides telmisartan in an overall yield of 72% while circumventing many issues associated with previously reported processes.

A flow-based synthesis of telmisartan

A highly efficient continuous synthesis has been developed for telmisartan, the active ingredient in the antihypertensive drug, Micardis. This synthetic route employs a convergent strategy that requires no intermediate purifications or solvent exchanges. The key step in the reaction scheme is a Suzuki cross-coupling reaction between two functionalized benzimi-dazoles that is catalyzed by a solid-supported Pd catalyst. This flow-based approach utilizes a tubular reactor system coupled with a plug flow packed bed cartridge unit that produces telmisartan in an 81% isolated yield.

The Effect of Graphene on Catalytic Performance of Palladium Nanoparticles Decorated with Fe3O4, Co3O4, and Ni (OH)2: Potential Efficient Catalysts Used for Suzuki Cross—Coupling

In this research, we report a scientific investigation of an efficient method used for the synthesis of highly active Palladium Nanoparticles decorated with Fe3O4, Co3O4, and Ni (OH)2 Supported on Graphene as Potential Efficient Catalysts for Suzuki Cross—Coupling. Pd/Fe3O4 nanoparticles supported on graphene nanosheets (Pd/Fe3O4/G) showed an excellent catalytic activity for Suzuki coupling reactions and recycled for up to four times without loss of catalytic activity. An efficient magnetic catalyst has been successfully synthesized using a simple, reproducible fast and reliable method using microwave irradiation conditions. The prepared catalysts are magnetic as in case of iron and cobalt oxides which is an advantage in the separation process of catalyst from the reaction medium via applying a strong external magnetic field. The synthesis approach is based on the Microwave (MW)-assisted simultaneous reduction of palladium and ferric nitrates in the presence of graphene oxide (GO) nanosheets using hydrazine hydrate as the reducing agent. The results provide a fundamental understanding of the system variables by comparing the catalytic activity and recyclability of different catalysts with different properties. The most active and recyclable catalyst was Pd–Fe3O4—supported on graphene which offers several added advantages including recyclability of up to seven times, mild reaction conditions, and short reaction times in an environmentally benign solvent system. Furthermore, the magnetic properties imparted by the Fe3O4 component of the catalyst enables the catalyst to be easily isolated and recycled, thus greatly simplifying the ability to purify the reaction products and increasing the economic value of the catalyst. The utility of these magnetic catalysts towards Suzuki cross coupling reaction was also demonstrated. The high activity and recyclability of these catalysts are attributed to a strong catalyst-support interaction where the defect sites in the reduced GO nanosheets act as nucleation centers for anchoring the Pd and Fe3O4 nanoparticles thus minimizing the potential of their agglomeration and the subsequent decrease in the catalytic activity.Graphical Abstract

The continuous synthesis and application of graphene supported palladium nanoparticles: a highly effective catalyst for Suzuki-Miyaura cross-coupling reactions

Abstract An efficient, sustainable, and continuous method for the preparation of graphene supported palladium nanoparticles (Pd/G) has been developed using microwave irradiation as a heating source for the metal deposition process. The Pd/G produced from this method was effective in Suzuki-Miyaura cross-coupling reactions with a broad range of substrates. When incorporated into a packed bed flow reactor, this ligand free catalyst system continued to demonstrate high reaction conversions with limited catalyst leaching in the reaction mixture (347 ppb palladium).