Copper-catalyzed Sonogashira reactions: advances and perspectives since 2014

Organosulphur and organoselenium compounds as ligands for catalytic systems in the Sonogashira coupling

Sonogashira coupling is a reaction of aryl/vinyl halides with terminal alkynes. It is used for the synthesis of conjugated enynes. Generally, copper (Cu) is required as a mediator for this reaction. It requires a long reaction time, high catalyst loading, or expensive ligands. Recently, homogeneous, heterogeneous, and nanocatalysts have been developed using organosulphur and organoselenium compounds as building blocks. Preformed complexes of metals with organosulphur and organoselenium ligands are used for homogeneous catalysis. Heterogeneous catalytic systems have also been developed using Cu, Pd, and Ni as metals. The nanocatalytic systems (synthesized using such ligands) include copper selenides and stabilized palladium(0) nanospecies. This article aims to cover the developments in the field of the processes and techniques used so far to generate catalytically relevant organic ligands having sulphur or selenium donor sites, the utility of such ligands in the syntheses of homogeneous, heterogeneous, and nanocatalytic systems, and critical analysis of their application in the catalysis of this coupling reaction. The results of catalysis are analyzed in terms of the effects of the S/Se donor, halogen atom of aryl halide, the effect of the presence/absence of electron-withdrawing or electron-donating groups or substituents on the aromatic ring of haloarenes/substituted phenylacetylenes, as well as the position (ortho or para) of the substitution. Substrate scope is discussed for all the kinds of catalysis. The supremacy of heterogeneous and nanocatalytic systems indicates promising future prospects.

Synthesis and Optical Properties of Binaphthyl Derivatives with Comprehensive Introduction of Phenylethynyl Groups

In this study, compounds with phenylethynyl (PE) groups introduced at all of the possible positions of the methylene-bridged structure of the 1,1'-bi-2-naphthol backbone (3-PE to 8-PE) were synthesized. Compounds with four or six phenylethynyl groups (3,6-PE, 4,6-PE, 5,6-PE, 6,7-PE, and 3,4,6-PE) were also synthesized. The key reaction for the synthesis of these compounds was the Sonogashira reaction using halogen scaffolds. The new transformation methods include (1) selective bromination of the 5-position of the binaphthyl skeleton and (2) bromination of the 6-position and then iodination of the 4-position, followed by the Sonogashira reaction of iodine at the 4-position and lithiation and protonation of bromine at the 6-position. The optical properties of the compounds were evaluated. The extension of the π system greatly differed depending on the position of the phenylethynyl group. 4-PE, 4,6-PE, and 3,4,6-PE, in which the phenylethynyl groups were introduced in the extended direction of the naphthalene linkage axis, showed longer absorption and emission wavelengths and higher fluorescence quantum yields than the other compounds. In circularly polarized luminescence measurements, 7-PE showed a relatively large glum value, an interesting finding that reverses the sense.

Cross-Coupling Between Aryl Halides and Aryl Alkynes Catalyzed by an Odd Alternant Hydrocarbon

Catalytic cross-coupling between aryl halides and alkynes is considered an extremely important organic transformation (popularly known as the Sonogashira coupling) and it requires a transition metal-based catalyst. Accomplishing such transformation without any transition metal-based catalyst in the absence of any external stimuli such as heat, photoexcitation or cathodic current is highly challenging. This work reports transition-metal-free cross-coupling between aryl halides and alkynes synthesizing a rich library of internal alkynes without any external stimuli. A chemically double-reduced phenalenyl (PLY)-based molecule with the super-reducing property was employed for single electron transfer to activate aryl halides generating reactive aryl radicals, which subsequently react with alkyne. This protocol covers not only various types of aryl, heteroaryl and polyaryl halides but also applies to a large variety of aromatic alkynes at room temperature. With a versatile substrate scope successfully tested on more than 75 entries, this radical-mediated pathway has been explained by several control experiments. All the key reactive intermediates have been characterized with spectroscopic evidence. Detailed DFT calculations have been instrumental in portraying the mechanistic pathway. Furthermore, we have successfully extended this transition-metal-free catalytic strategy for the first time towards solvent-free cross-coupling between solid aryl halide and alkyne substrates.

Protocol for Sonogashira coupling of alkynes and aryl halides via nickel catalysis

Alkynes are widely present in natural products and pharmaceutical compounds. Here, we present a protocol for nickel-catalyzed cross-coupling of terminal alkynes with aryl iodides or bromides for constructing a C(sp2)-C(sp) bond. We describe steps for reagent preparation, reaction setup, purification process, and product characterization. We also detail procedures for obtaining a single crystal of 6-(phenylethynyl)-1-(phenylsulfonyl)-1H-indole (3b). The application of this protocol is limited to aryl bromide and iodide. For complete details on the use and execution of this protocol, please refer to Chen et al.1.

Theoretical study of copper hydride complexes catalyzing terminal alkyne hydroalkylation for C(sp2)-C(sp3) bond formation

This study applies Density Functional Theory (DFT) to theoretically investigate the reaction mechanism of a copper complex catalyst facilitating the reaction between a terminal alkyne and α-bromo amide, enabling the formation of E-alkenes through C(sp2)-C(sp3) coupling. Initially, the study explores the reaction mechanism, identifying the predominant reaction pathway and the rate-determining step. Next, we discuss the addition reaction mode of copper hydride with terminal alkynes, determining the causes of regional and stereoselectivity. Subsequently, the reaction mechanism between the alkenyl copper intermediate and α-bromo amide is examined, including the discussion of alkyl fragment activation and introduction methods. Furthermore, the role of NHC ligands in catalyzing the single electron transfer process for C-Br bond activation is investigated. Finally, we analyze and discuss the reasons for the high energy barrier of the non-radical pathway. These investigations not only deepen our understanding of the reaction mechanisms of terminal alkynes and α-bromo amide catalyzed by copper but also provide valuable guidance for the future design of more efficient catalysts and reaction conditions.

[Advances in synthesis methods and applications of microporous organic networks for sample preparation]

Sample pretreatment is an essential step in chromatographic analysis. Solid phase extraction (SPE) is a widely used sample pretreatment method. In SPE, the quality of the adsorbent directly affects the adsorption and enrichment efficiency of the target compounds as well as the sensitivity and selectivity of the pretreatment and subsequent analysis. Therefore, the selection and development of adsorbents has become a research hotspot. Microporous organic networks (MONs) are a novel type of covalent organic materials that are synthesized by the Sonogashira reaction of aromatic alkynes and aromatic halides. These networks have the advantages of modifiable structures, large specific surface areas, high porosity, and simple synthesis methods. This paper reviews the synthesis and functional modification methods of MONs, with an emphasis on their applications in sample pretreatment. Future development trends are also prospected. In terms of synthesis, the preparation methods for MON-based materials have progressed from reflux and solvothermal synthesis to room-temperature synthesis, the conditions of which tend to be milder and more efficient. In terms of functional modification, the introduction of macromolecules and active groups (including amino, hydroxyl, and carboxylic groups) can increase the selectivity and active sites of MON-based materials. The combination of MONs with Fe3O4, SiO2, and metal organic frameworks yields core-shell-structured MONs. Furtherly, they can be calcined and etched to form porous carbon structures or hollow multilayer materials. Functionalized MONs and their composite materials have multiple interaction mechanisms (e. g., hydrogen bonding, hydrophobic, electrostatic, and π-π interactions) with various target compounds, thereby realizing their efficient extraction. MONs can be used as adsorbent materials in SPE, Solid phase microextraction, dispersed solid phase extraction, magnetic solid phase extraction, and other pretreatment methods. When combined with chromatography and chromatography-mass spectrometry, MONs achieve good adsorption effects and high sensitivity, demonstrating the application potential of these materials in sample pretreatment.

Triazole-Appended Glycohybrid/CuI-Catalyzed C-C Cross-Coupling of Aryl/Heteroaryl Halides with Alkynyl Sugars

This report describes a convenient method for the Cu(I)-catalyzed Sonogashira cross-coupling reaction of aryl/heteroaryl halides and alkynyl sugars in the presence of a 1,2,3-triazole-appended glycohybrid as a biocompatible ligand. The Sonogashira cross-coupling products were exclusively formed without the Glaser-Hay homocoupling reaction in the presence of a glycosyl monotriazolyl ligand at 120 °C. However, the Glaser-Hay homocoupling products were obtained at 60-70 °C in the presence of bis-triazolyl-based macrocyclic glycohybrid ligand L8. The glycosyl triazole ligands were synthesized via the CuI/DIPEA-mediated regioselective CuAAC click reaction, and a series of glycohybrids of glucose, mannose, and galactose alkynes including glycosyl rods were developed in good yields. The developed glycohybrids have been well characterized by various spectroscopic techniques, such as nuclear magnetic resonance, high-resolution mass spectrometry, and single-crystal X-ray data of L3. The protocol works well with the heteroaryl and naphthyl halides, and the mechanistic approach leads to CuI/ligand-assisted oxidative coupling. The coupling protocol has notable features, including low catalytic loading, cost-effectiveness, biocompatible nature, and a wide substrate scope.

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.