Conjugated mesoporous polyazobenzene–Pd(II) composite: A potential catalyst for visible-light-induced Sonogashira coupling

Photochemical Sonogashira coupling reactions: beyond traditional palladium-copper catalysis

Sonogashira coupling is one of the Nobel reactions discovered in 1975 to form a C-C bond using palladium and copper as co-catalysts. Incorporating alkyne functionalities either in macro or micro molecules by using this Sonogashira reaction has already proven its viability and relevance in the sphere of synthetic chemistry. While aiming for sustainable chemistry, in recent years, visible light photoredox catalysts have appeared as an advanced tool in this regard. In this review, we aim to portray a comprehensive summary of modern visible light photo redox catalyzed Sonogashira reaction, which will leave space for the readers to rethink alternative strategies to conduct the Sonogashira reaction. This review briefly describes the implementation of various metal-based nanomaterial photocatalysts, developing either copper or palladium-free photocatalytic methods, and organo-photolytic and bioinspired photocatalysts for the Sonogashira coupling reactions. Besides, this review also gives a concise overview of the mechanistic aspects and highlights selective examples for substrate scope.

Palladium Nanoparticle-Decorated Porous Metal-Organic-Framework (Zr)@Guanidine: Novel Efficient Catalyst in Cross-Coupling (Suzuki, Heck, and Sonogashira) Reactions and Carbonylative Sonogashira under Mild Conditions

A novel heterogeneous Zr-based metal-organic framework containing an amino group functionalized with nitrogen-rich organic ligand (guanidine), through a step-by-step post synthesis modification approach, was successfully modified by the stabilization of palladium metal nanoparticles on the prepared UiO-66-NH₂ support in order to synthesize the Suzuki-Murray, Mizoroki-Heck, and copper-free Sonogashira reactions and also the carbonylative Sonogashira reaction incorporating H₂O as a green solvent under mild conditions. This newly synthesized highly efficient and reusable UiO-66-NH₂@cyanuric chloride@guanidine/Pd-NPs reported catalyst has been utilized to increase anchoring palladium onto the substrate with the aim of altering the construction of the intended synthesis catalyst to form the C-C coupling derivatives. Several strategies, including X-ray diffraction, Fourier transform infrared, scanning electron microscopy, Brunauer-Emmett-Teller, transmission microscopy electron, thermogravimetric analysis, inductively coupled plasma, energy-dispersive X-ray, and elemental mapping analyzes, were used to indicate the successful preparation of the UiO-66-NH₂@cyanuric chloride@guanidine/Pd-NPs. In these reactions, the UiO-66-NH₂-supported Pd-NPs illustrated superior performances compared to their catalyst, revealing the benefits of providing nanocatalysts. As a result, the proposed catalyst is favorable in a green solvent, and also, the outputs are accomplished with good to excellent outputs. Furthermore, the suggested catalyst represented very good reusability with no remarkable loss in activity up nine sequential runs.

Palladium-Supported Polydopamine-Coated NiFe2O4@TiO2: A Sole Photocatalyst for Suzuki and Sonogashira Coupling Reactions under Sunlight Irradiation

The effective utilization of solar energy in synthetic organic chemistry has gained extensive attention owing to its enormous energy and environmentally benign nature. In this context, we designed and synthesized a magnetically retrievable, sole palladium (Pd)-supported polydopamine-coated core@shell (NiFe₂O₄@TiO₂) heterogeneous nanophotocatalyst for Suzuki and Sonogashira coupling reactions under sunlight irradiation. The synthesized catalyst was characterized by powder X-ray diffraction (PXRD), Fourier-transform infrared, UV-vis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometer analysis. The photocatalytic activity of the synthesized nanocatalyst under sunlight irradiation was assessed for both Suzuki and Sonogashira coupling reactions, where it worked excellently well with a high yield of the product up to 98 and 96%, respectively. Its efficacy was also investigated in the conversion of substituted substrates in both the coupling reactions into desired biaryls and diarylacetylenes. Unique features of the synthesized catalyst are (i) its effective performance for both the aforesaid coupling reactions under ambient reaction conditions for a short reaction time in polar protic solvents (ethanolic water/EtOH) with good yield without any byproduct, (ii) magnetic retrieval of the catalyst from the reaction mixture employing an external magnet is an added advantage, and (iii) the retrieved catalyst could potentially be reutilized for up to five consecutive runs without appreciable diminution of catalytic efficacy, and its stability was confirmed by inductively coupled plasma optical emission spectroscopy analysis and XRD.

A ‘Defective’ Conjugated Porous Poly-Azo as Dual Photocatalyst

A heterogeneous photocatalyst amenable to catalyze different chemical reactions is a highly enabling and sustainable material for organic synthesis. Herein we report the synthesis and characterization of an azobenzene-based organic π–conjugated porous polymer (AzoCPP) as heterogeneous dual photocatalyst manifesting net-oxidative bromination of arenes and dehydroxylation of boronic acids to corresponding phenols. Hierarchical porosity and high surface area of the nano-sized AzoCPP allowed superior catalyst-substrate contact during catalyses, whereas the inherent structural defect present in the CPP backbone resulted in low-energy sinks functioning as de facto catalytic sites. A combination of these two structure-property aspects of AzoCPP, in addition to the dielectric constant manipulation of the system, led to excellent catalytic performance. The protocols remained valid for a wide substrate scope and the catalyst was recycled multiple times without substantial loss in catalytic activity. With the aid of subsequent control experiments and analytical characterizations, mechanisms for each catalysis are proposed and duly corroborated.

Copper-free Sonogashira cross-coupling reactions: an overview

The Sonogashira reaction is a cross-coupling reaction of a vinyl or aryl halide with a terminal alkyne to form a C-C bond. In its original form, the Sonogashira reaction is performed with a palladium species as a catalyst while co-catalyzed by a copper species and a phosphine or amine. The reaction is conducted under mild conditions, i.e., room temperature, aqueous solutions, and the presence of mild bases. Undeniably, the Sonogashira reaction is among the most competent and efficient reactions widely used in organic synthesis. This named reaction has proved useful in many organic synthesis areas, including the synthesis of pharmaceuticals, heterocycles, natural products, organic compounds, complex molecules having biological activities, nanomaterials, and many more materials that we use in our daily lives. The presence of transition metals as a catalyst was indeed essential in the Sonogashira reaction. However, recently, the reaction has been successfully conducted without copper as a co-catalyst and phosphines or amines as bases. In this critical review, we have focused on developments in the Sonogashira reaction successfully performed in the absence of copper complexes, phosphines or amines, which could be of particular advantage in implementing green chemistry principles and making the reactions more achievable from an economic viewpoint.

Novel rapid room temperature synthesis of conjugated microporous polymer for metal-free photocatalytic degradation of fluoroquinolones

The existence of Fluoroquinolones (FQs), non-biodegradable pharmacophores, in the natural environment possesses a serious threat. We herein report a novel, rapid, room-temperature synthesis of semiconducting conjugated microporous polymer (CMP) for the decontamination of four second-generation FQs, Norfloxacin, Enrofloxacin, Ciprofloxacin, and Ofloxacin. The CMP demonstrated impressive gas uptake and FQ adsorption ability. Decreased HOMO-LUMO bandgap resulted in enhanced exciton pair generation on visible-light-illumination. Additionally, a high degree of photocurrent response and suitable redox potentials of the material conjointly endorsed its almost quantitative FQ-degradation efficiency. Ofloxacin showed the best removal efficiency with 0.061 and 0.207 min^-1 adsorption and degradation rate constants, respectively, one of the highest values reported. The CMP exhibited equipotent activity for other FQs as well. On increasing the concentration of the FQs or decreasing the incident photo-intensity, quantitative removal efficiencies are observed. Changing the pH of the medium from acidic to alkaline did not impart any change in catalytic activity as well. The reactive species involved viz. O₂^-, ¹O₂, etc. and their roles in the degradation process were determined through control and trapping experiments. A plausible in-depth mechanistic pathway was assessed from the FQ degradation intermediates, and the reactive catalytic species substantiating step-by-step break down of the antibiotic backbone.