A new mononuclear nickel complex with 5,5′-dimethyl-2,2′-bipyridine: Synthesis, structural investigation and catalytic properties

Synthesis of propargylamine: pioneering a green path with non-conventional KA2 coupling approach

The KA2 coupling reaction is a well-explored and versatile method for forming C-C bonds in synthetic chemistry. It is composed of ketone, amine, and alkyne, which play a major role in the synthesis of propargylamines, known for their diverse biological activities and are used in treating neurogenetical disorders. The KA2 coupling is highly challenging due to the low reactivity of ketimines toward nucleophilic attacks with metal acetylide intermediates formed by activating the C-H bond of the alkyne. Despite predominant studies conducted on thermal conditions for KA2 coupling reactions, green and sustainable approaches like non-conventional methods still have a lot to achieve. This review article provides a comprehensive introduction to the non-conventional approach in the KA2 coupling reaction, outlining its mechanisms and exploring future aspects.

A novel core@double-shell three-layer structure with dendritic fibrous morphology based on Fe3O4@TEA@Ni-organic framework: a highly efficient magnetic catalyst in the microwave-assisted Sonogashira coupling reaction

In synthetic organic chemistry, the formation of carbon-carbon bonds is a significant and substantial reaction. As a result, developing a highly active magnetic heterogeneous catalyst with excellent performance is a very appealing technique for constructing C-C bonds in organic chemistry. The present study describes the fabrication of a novel and readily recoverable nickel-based metal-organic framework (MOF) for C-C bond formation through the Sonogashira coupling reaction. The efficient magnetic core-shell structure (Fe₃O₄@TEA@MOF) with a 3D dendritic fibrous morphology was successfully synthesized using a hydrothermal approach by immobilizing Ni-based MOF onto the Fe₃O₄@TEA core-shell structure. The fabrication of Fe₃O₄@TEA@MOF was confirmed by various analyses; Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray analysis (EDS), and elemental mapping confirmed the stepwise fabrication of catalyst. X-ray diffraction analysis (XRD) showed the crystalline nature of the catalyst. Field-emission scanning electron microscopy (FE-SEM) displayed the 3D dendritic fibrous morphology. Thermogravimetric analysis (TGA) and vibrating sample magnetometer analysis (VSM) showed the excellent thermal stability and magnetic properties of Fe₃O₄@TEA@MOF. The Brunauer-Emmett-Teller analysis (BET) found that the fabricated catalyst with a surface area of 36.2 m² g^-1, pore volume of 0.18 cm³ g^-1, and mean pore diameter of 20.38 nm belongs to mesoporous structures. In addition, the information from the inductively coupled plasma-optical emission spectroscopy (ICP-OES) about fresh and reused catalysts showed that the metal leaching amount is slight and about 1.98%. Other advantages of the Fe₃O₄@TEA@MOF catalyst can be mentioned as easily reusable for four runs and high performance (above 98%) in synthesizing diphenylacetylene from phenylacetylene, aryl halide, and cesium carbonate (as the base) under solvent-free and microwave conditions.

Nickel catalysts in Sonogashira coupling reactions

Nickel has emerged as a desirable substitute for palladium in Sonogashira coupling reactions due to its abundance, less toxicity and high catalytic activity. Ni complexes have been developed to catalyse C(sp)-C(sp2) and C(sp)-C(sp3) Sonogashira couplings that find applications in the synthesis and modifications of biologically relevant molecules. This review focuses on the catalytic potential and mechanistic details of various Ni complexes employed in the Sonogashira coupling. These include homogeneous catalytic systems with Ni-phosphorus and Ni-nitrogen catalysts, ligand-free catalysts, and carbonylative coupling strategies. Various heterogeneous catalytic systems using supported Ni complexes, Ni nanoparticles and Pd-Ni bimetallic catalysts have also been discussed. This is the first review reported so far dealing exclusively with Ni-catalysed Sonogashira coupling reactions. This review illustrates the current strategies and potential of Ni-catalysed Sonogashira coupling reactions in both homogeneous and heterogeneous systems, and covers the literature up to 2020.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.