Silica-supported bismuth(III) chloride as a new recyclable heterogeneous catalyst for the Paal–Knorr pyrrole synthesis

Modular Synthesis of Tetrasubstituted Pyrroles through a Four-Component Cyclization Strategy Using Ammonium Salt as the Nitrogen Source

A four-component synthesis of tetrasubstituted pyrroles was developed under metal-free conditions. The pyrrole ring was formed in one pot through [2 + 1 + 1 + 1] condensation using ammonium salt as the nitrogen source. In this strategy, 1,4-naphthoquinones and maleimides were used as the versatile C2 fragments to provide substituted benzo[f]isoindole-4,9-diones and pyrrolo[3,4-c]pyrrole-1,3-diones, respectively. This work is highlighted by using ammonium salt as the nitrogen source, readily available starting materials and multibond formation (two C-C and two C-N bonds) in a single operation.

Mild and Efficient Copper-Catalyzed Synthesis of Trisubstituted Pyrroles

A sustainable and time economic approach has been developed for the synthesis of polysubstituted pyrroles using copper iodide as a catalyst. The reaction proceeded through imine formation followed by cyclization with alkyne-Cu intermediate, which was supported by control experiments studies. The newly formed substituted pyrroles were obtained in excellent yields with high regioselectivity under mild conditions.

Design and Exploration of Catalytic Activity of Two-Dimensional Surface-Engineered Graphene Oxide Nanosheets in the Transannulation of N-Heterocyclic Aldehydes or Ketones with Alkylamines

In this work, pharmaceutically and biologically important compounds containing imidazo[1,5-a]pyridine nuclei have been synthesized via transannulation of N-heteroaryl aldehydes or ketones with alkylamines using a graphene oxide-supported copper catalyst. The nanocatalyst was fabricated by the covalent immobilization of 4-aminoantipyrine onto an amine-functionalized graphene oxide nanosupport followed by its metallation with copper acetate. Structural analysis by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction demonstrates that the two-dimensional sheet-like structure of graphene oxide is maintained even after the chemical modifications, whereas XPS revealed crucial information related to elemental composition and surface electronic states of the metal present in the catalyst. Apart from this, Fourier transform infrared spectroscopy helped in identifying the degree of oxidation and the presence of oxygenated groups in graphene oxide nanocomposites. As a heterogeneous catalyst, this graphene oxide-supported copper complex showed moderate to good catalytic activity in the C(sp3)-H bond activation/amination of a variety of substrates. This superior catalytic performance originated from the unique 2-dimensional structure of graphene oxide-based material which provided space between graphitic overlayers due to appropriate positioning of metal on their basal planes, decreasing the diffusion resistances of reactant surfaces, thus making it function as a nanoreactor. More importantly, this nanomaterial could be recovered easily and reused repeatedly by simple washing without chemical treatment with no appreciable loss in its catalytic activity, showing good potential for increasing the overall turnover number of this synthetically useful catalyst.

A mild and efficient method for the synthesis of pyrroles using MIL-53(Al) as a catalyst under solvent-free sonication

A highly efficient method for the synthesis of pyrroles using MIL-53(Al) as a catalyst has been developed under solvent-free sonication. This reaction has a broad substrate scope and high yields were obtained within a short reaction time. Remarkably, no additional additives and volatile organic solvent are required for this method and the MIL-53(Al) could be recovered and reused several times without significant drop-off in catalytic activity.

Catalytic activity of MIL-53(Al) in the synthesis of pyrroles under solvent-free sonication.

New nano-Fe3O4-supported Lewis acidic ionic liquid as a highly effective and recyclable catalyst for the preparation of benzoxanthenes and pyrroles under solvent-free sonication

A novel magnetic nanomaterial-immobilized Lewis acidic ionic liquid was successfully synthesized by the covalent embedding of 3-(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium chlorozincate (ii) ionic liquid to the surface of Fe₃O₄ nanoparticles. The material was then characterized by FT-IR, SEM, TEM, TGA, ICP-OES, Raman, and EDS. Its performance as a new-generation Lewis acidic catalyst was also examined on the ultrasound-mediated synthesis of benzoxanthenes and pyrroles. Upon completion, the catalyst was simply recovered by an external magnet for multiple reuses without significant lessening of catalytic performance.

A novel magnetically separable catalyst can be used as a green solid Lewis acid catalyst in the synthesis of benzoxanthenes and pyrroles under solvent-free sonication.

A green and efficient method for the synthesis of pyrroles using a deep eutectic solvent ([CholineCl][ZnCl2]3) under solvent-free sonication

A green and efficient method for Paal–Knorr pyrrole synthesis has been developed under mild conditions.

Base-promoted intramolecular cyclization of N-alkyl, N-propargylic β-enaminones for the synthesis of polysubstituted pyrroles

An efficient, mild, metal-free, base-mediated intramolecular cyclization of N-alkyl, N-propargylic β-enaminones has been realized for the generation of polysubstituted pyrrole derivatives.

Synthesis of two distinct pyrrole moiety-containing arenes from nitroanilines using Paal–Knorr followed by an indium-mediated reaction

Synthesis of arenes substituted with two different substituted-pyrrole moieties was investigated.

Catalytic activity of the nanoporous MCM-41 surface for the Paal–Knorr pyrrole cyclocondensation

The investigation of different oxide surfaces revealed that nanoporous silica (MCM-41) had the best catalytic activity for Paal–Knorr pyrrole synthesis. Despite the same composition, MCM-41 proved to be more effective than SiO2 itself, probably due to a significantly higher surface area of the SiO2 nanopores. The important features of this “clean” solvent-free protocol are the ease of recovery and the reuse of the catalyst for several cycles, operational simplicity, and easy product isolation and purification.

Mechanism of the Paal–Knorr reaction: the importance of water mediated hemialcohol pathway

The mechanism of the Paal–Knorr reaction was explored using quantum chemical methods. Hydronium ion catalysed hemialcohol pathway has been established as the preferred mechanistic route for the Paal–Knorr formation of furan, pyrrole and thiophene.