Synthesis of (Z)-Enediynes via Stereoinvertive Nucleophilic Substitution of (E)-Sulfonylethenes with Arylethynide, and Their Aggregation-Induced Optical Properties

(Z)-Enediynes were successfully synthesized from a trio of terminal ethynes through consecutive three-step reactions: iodosulfonylation of ethyne with I2/PhSO2Na, followed by ethynylations of iodo and sulfonyl moieties of the resulting iodosulfonylethene via Sonogashira-Hagihara coupling and nucleophilic addition-elimination, respectively. The molecular structure of the obtained (Z)-enediyne was fully characterized by X-ray crystal structure analysis, revealing that the nucleophilic substitution of (E)-sulfonylethene with arylethynide underwent a selective stereoinversion. The (Z)-enediynes exhibited photoluminescence in both the solution and solid states (crystals and powders). Ph2N-substituted derivatives showed remarkable solvatofluorochromism, and upon replacing the solvent from toluene to acetonitrile, the emission color changed from blue to yellow.

Scalable deoxygenative alkynylation of alcohols via flow photochemistry

Internal alkynes are often contained in bioactive pharmaceuticals and crucial intermediates in material sciences, yet their production methods are often limited and challenging, necessitating the development of more efficient and versatile synthetic routes. Here we report a method of deoxygenative alkynylation of alcohols via flow photochemistry. Formation of N-heterocyclic carbene-alcohol adducts undergoes oxidation by a photocatalyst, generating alkyl radicals. These radicals are subsequently trapped by an alkynylation agent, yielding the desired alkyne. Compared to batch reactions, the strategy using flow photochemistry is practical and efficient to complete the reaction in relatively short time with good yields. A wide range of functional groups were tolerated. The broad application of this method for alkyne synthesis in industry settings is anticipated, supported by the potential in late-stage functionalization of biomolecules and gram-scale synthesis.

Palladium-catalyzed cross-coupling of arylcarboxylic acid 2-pyridyl esters with terminal alkynes

In the presence of Na2CO3, the combination of PdCl2(dppf), dppp and CuI catalyzes the decarbonylative coupling of arylcarboxylic acid 2-pyridyl esters with terminal alkynes to afford 1,2-disubstituted acetylenes. (Hetero)aryl, alkyl, and silylacetylenes and various electron-donating and -withdrawing group-substituted arylcarboxylic acid 2-pyridyl esters can be used in this transformation, with a range of functional groups showing compatibility.

Halogenated monopyridinium oximes are less effective in reactivation of phosphylated cholinesterases than bisquaternary oximes

Mono-quaternary pyridinium oximes derived from K-oximes K027, K048 and K203 were designed, synthesized and evaluated for the reactivation of organophosphate-inhibited cholinesterases. The incorporation of the halogen atoms to the structure decreased the pKa value of the oxime group resulting in an increased formation of oximate necessary for reactivation. The stability and pKa values were found to be similar to analogous bis-quaternary compounds. Some mono-quaternary oximes resulted as relatively strong inhibitors of human acetylcholinesterase. Nevertheless, the reactivation ability of mono-quaternary oximes for organophosphate-inhibited cholinesterases was lower compared to their bis-quaternary analogues. These results were further confirmed by the determination of reactivation kinetics, when in some cases novel compounds showed improvement reactivation compared to the tested standards, but no improvement to bis-quaternary K-oximes. A computational study investigated reactivation process for K027, and its two analogues for VX-inhibited AChE. This study revealed slight differences between reactivation of mono-quaternary and bis-quaternary oximes. Abbreviations: 2-PAM, pralidoxime; AChE, acetylcholinesterase; ACN, acetonitrile; ATCI, acetylcholine iodide; BChE, butyrylcholinesterase; BTCI, butyrylcholine iodide; Bu3SnSnBu3, bis(tributyltin) Et2O, diethyl ether; ChEs, cholinesterases; CNS, central nervous system; DAD, diode array detector; DIBAL-H, diisobutylaluminium hydride; DMF, dimethylformamide; DMSO, dimethyl sulfoxide; DTNB, 5,5́-dithiobis-2-nitrobenzoic acid; Et3N, triethylamine; EtOAc, ethyl acetate; EWG, electron withdrawing group; HI-6, asoxime; hrAChE, human recombinant acetylcholinesterase; hrBChE, human recombinant butyrylcholinesterase; hrChEs, human recombinant cholinesterases; HPLC, high-performance liquid chromatography; HRMS, high-resolution mass spectrometry; KD, dissociation constant; kr, first-order reactivation rate constant; kr2, second-order reactivation rate constant; LüH-6, obidoxime; MeOH, methanol; MM, molecular mechanics; MMC-4, methoxime; m.p., melting point; NCIs, non-covalent interactions; NEDPA, 4-nitrophenyl ethyl dimethylphosphoramidate; NEMP, 4-nitrophenyl ethyl methylphosphonate; NIMP, isopropyl methylphosphonate; NMR, nuclear magnetic resonance spectroscopy; OPs, organophosphates; PBS, phosphate-buffered saline; Pd(dppf)Cl2.CH2Cl2, [1,1'-Bis(diphenylphosphino)ferrocene]dichloropalladium(II) in complex with dichloromethane; pKa, negative decimal logarithm of the dissociation constant; POX, paraoxon; PPh3, triphenylphosphine; QM, quantum mechanics; rt, room temperature; SN2, bimolecular nucleophilic substitution; SNAc, nucleophilic acyl substitution; THF, tetrahydrofuran; TMC-4, trimedoxime; TNB, 5-thio-2-nitrobenzoic acid; UHPLC, ultra high-performance liquid chromatography; UV, ultraviolet; UV-VIS, ultraviolet-visible.

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 of the monomeric counterpart of Marinomycin A and B

The synthesis of polyketide natural products has been a captivating pursuit in organic chemistry, with a particular focus on selectively introducing 1,3-polyol units. Among these natural products, Marinomycins A-D have garnered substantial interest due to their exceptional structural features and potent cytotoxicity. In this paper, we present a novel approach for synthesising the monomeric counterparts of Marinomycin A and B. Our method employs a previously established iterative cycle in conjunction with a standardised polyketide building block. Through this strategy, we showcase a promising pathway towards total and partial syntheses of these intriguing natural products.

Stereoselective polar radical crossover for the functionalization of strained-ring systems

Radical-polar crossover of organoborates is a poweful tool that enables the creation of two C-C bonds simultaneously. Small ring systems have become essential motifs in drug discovery and medicinal chemistry. However, step-economic methods for their selective functionalization remains scarce. Here we present a one-pot strategy that merges a simple preparation of strained organoboron species with the recently popularized polar radical crossover of borate derivatives to stereoselectively access tri-substituted azetidines, cyclobutanes and five-membered carbo- and heterocycles.

Tetrabutylammonium Bromide (TBAB)-Promoted, Pd/Cu-Catalyzed Sonogashira Coupling of N-Tosyl Aryltriazenes

Sonogashira coupling of N-tosyl aryltriazenes is reported to offer arylalkynes in yields up to 92% with the aid of tetrabutylammonium bromide (TBAB) as a dual activator for both the palladium catalyst and aryltriazenes. Common functional groups could be well tolerated, although large electronic effects from alkynes were observed. TBAB-assisted oxidative addition of palladium(0) to aryltriazene instead of in situ formed arylhalide has been proposed to initiate the catalytic cycle.

Total Synthesis of Putative Melognine

Total synthesis of melognine was accomplished. A 10-membered cyclic alkyne was prepared via an intramolecular SN2 reaction of a nosylamide. Enyne metathesis of the cyclic alkyne under an atmosphere of ethylene afforded a 1,3-diene. Intramolecular cycloaddition of a nitrone and an azomethine ylide with the 1,3-diene moiety constructed the characteristic highly fused skeleton. Further transformation, including ring-closing metathesis, resulted in the synthesis of melognine, whose NMR spectra did not match the reported data. Close inspection of the spectra of melognine in the literature suggested that the structure of melognine might be identical with that of a known alkaloid, melodinine L.

Pd@HKUST-1@Cu(II)/CMC composite bead as an efficient synergistic bimetallic catalyst for Sonogashira cross-coupling reactions

We fabricated an efficient Pd@HKUST-1@Cu(II)/CMC composite bead catalyst through an innovative strategy based on the unique properties of metal-organic frameworks (MOFs) and carboxymethylcellulose (CMC). In this strategy, HKUST-1 MOFs were grown in-situ on the surface of micrometer-sized Cu-based CMC beads (Cu(II)/CMC), then Pd(II) ions were incorporated into the pores of the MOF and further be partially reduced to Pd(0) NPs, which is an active species for oxidative addition with aryl halides in Sonogashira reactions. The micron-sized Cu(II)/CMC beads were formed through inter/intramolecularly crosslinking facilitated by Cu(II) ions, which was achieved by the metathesis of Cu(II) with numerous carboxylic groups of CMC. Such Cu(II)/CMC bead offers many Cu(II) ions as interaction sites for in-situ nucleation and growth of HKUST-1 MOFs. The architecture and composition of the prepared Pd@HKUST-1@Cu(II)/CMC composite were fully verified by various techniques such as FTIR, XRD, TGA, BET, XPS, SEM, TEM, EDX, and elemental mapping analysis. This novel composite bead was applied as an efficient and reusable heterogeneous Pd/Cu bimetallic catalyst for Sonogashira reactions, decarbonylative Sonogashira reaction, and Sonogashira cyclization tandem reactions. The catalyst is readily isolated by simple filtration, and can be reused for five consecutive runs with retaining its activity and structural integrity.

Total Synthesis of (-)-Enigmazole B

Total synthesis of (-)-enigmazole B was achieved for the first time. Highlights of the present synthesis include an olefin cross-metathesis and hemiacetalization/intramolecular oxa-Michael addition sequence for accessing an (E)-configured enol tosylate, a Sonogashira cross-coupling to assemble all the carbon atoms of the target natural product, a remarkably chemo- and regioselective Au-catalyzed intramolecular alkyne hydroalkoxylation for the construction of the dihydropyran ring, and a Yamaguchi macrolactonization to close the 18-membered macrolactone skeleton.

Directing group assisted para-selective C-H alkynylation of unbiased arenes enabled by rhodium catalysis

Regioselective C-H alkynylation of arenes via C-H activation is challenging yet a highly desirable transformation. In this regard, directing group assisted C(sp2)-H alkynylation of arenes offers a unique opportunity to ensure precise regioselectivity. While the existing methods are mainly centered around ortho-C-H alkynylation and a few for meta-C-H alkynylation, the DG-assisted para-selective C-H alkynylation is yet to be reported. Herein we disclose the first report on Rh-catalyzed para-C-H alkynylation of sterically and electronically unbiased arenes. The para-selectivity is achieved with the assistance of a cyano-based directing template and the selectivity remained unaltered irrespective of the steric and electronic influence of the substituents. The post-synthetic modification of synthesized para-alkynylated arenes is also demonstrated. The mechanistic intricacies of the developed protocol are elucidated through experimental and computational studies.

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.

Recent advances in the syntheses of guaianolides

Sesquiterpene lactones, especially guaianolides representing a bigger class of natural products, have served as appealing candidates for total synthesis due to their varied bio- and pharmaceutical activities. This tutorial review delineates the creative efforts of many researchers in the total syntheses of different complex guaianolides recently published in the literature. Many of the syntheses display meticulous interplay between new methods and the ingenuity of strategies achieved through well-planned routes. In some cases, the Chiron approach has come in quite handy, wherein the structural features and stereochemistry of select molecules could map well with naturally available starting materials. A few catalytic methods like diastereoselective aldol reaction, enediyne or dienyne metathesis, or photochemical methods have been efficiently used. This compilation also aims to enhance the diversity space based on these natural products and further interest in the sustainable total synthesis of this class of compounds and related molecules.

Room-Temperature, Copper-Free, and Amine-Free Sonogashira Reaction in a Green Solvent: Synthesis of Tetraalkynylated Anthracenes and In Vitro Assessment of Their Cytotoxic Potentials

The multifold Sonogashira coupling of a class of aryl halides with arylacetylene in the presence of an equivalent of Cs₂CO₃ has been accomplished using a combination of Pd(CH₃CN)₂Cl₂ (0.5 mol %) and cataCXium A (1 mol %) under copper-free and amine-free conditions in a readily available green solvent at room temperature. The protocol was used to transform several aryl halides and alkynes to the corresponding coupled products in good to excellent yields. The rate-determining step is likely to involve the oxidative addition of Ar-X. The green protocol provides access to various valuable polycyclic aromatic hydrocarbons (PAHs) with exciting photophysical properties. Among them, six tetraalkynylated anthracenes have been tested for their anticancer properties on the human triple-negative breast cancer (TNBC) cell line MDA-MB-231 and human dermal fibroblasts (HDFs). The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was performed to find out the IC₅₀ concentration and lethal dose. The compounds being intrinsically fluorescent, their cellular localization was checked by live cell fluorescence imaging. 4',6-Diamidino-2-phenylindole (DAPI) and propidium iodide (PI) staining was performed to check apoptosis and necrosis, respectively. All of these studies have shown that anthracene and its derivatives can induce cell death via DNA damage and apoptosis.

Copper iodide nanoparticles (CuI NPs): an efficient catalyst for the synthesis of alkynyl esters

An environmentally benign protocol for the synthesis of alkynyl esters, by the cross-coupling of diazoacetate with various substituted alkynes under neat reaction conditions, has been described. Copper iodide nanoparticles (CuI NPs) were found to promote the Sonogashira-type coupling to afford the corresponding alkynyl esters in good yields. The CuI nanoparticles were characterized by PXRD, FESEM, EDAX, and Raman techniques. The developed methodology has several advantages such as a broad substrate scope, less reaction time, atom economy, avoidance of an additive/base/solvent, and enhanced values of green chemistry. The catalyst was recycled up to threefold without the loss of its catalytic activity.

Unlocking Regiodivergence in PdII- and RhIII-Mediated Site-Selective C-H Bond Alkynylation of Imidazopyridines

An efficient Pd^II- and Rh^III-controlled site-selective C-H bond alkynylation of imidazopyridines using (bromoethynyl)triisopropylsilane is disclosed. The divergent methodology allows straightforward access to a wide range of products alkynylated at the C3 and ortho positions. This strategy is suggestive of a practical platform that can be suitable for late-stage diversification and may assist in the design of more selective and complementary catalytic systems.

Nickel-palladium bimetallic nanoparticles supported on multi-walled carbon nanotubes; versatile catalyst for Sonogashira cross-coupling reactions

We have developed an efficient method to generate highly active nickel-palladium bimetallic nanoparticles supported on multi-walled carbon nanotubes (Ni-Pd/MWCNTs) by dry mixing of the nickel and palladium salts utilizing the mechanical energy of a ball-mill. These nanoparticles were successfully employed in Sonogashira cross-coupling reactions with a wide array of functionalized aryl halides and terminal alkynes under ligand and copper free conditions using a Monowave 50 heating reactor. Notably, the concentration of palladium can be lowered to a minimum amount of 0.81% and replaced by more abundant and less expensive nickel nanoparticles while effectively catalyzing the reaction. The remarkable reactivity of the Ni-Pd/MWCNTs catalyst toward Sonogashira cross-coupling reactions is attributed to the high degree of the dispersion of Ni-Pd nanoparticles with small particle size of 5-10 nm due to an efficient grinding method. The catalyst was easily removed from the reaction mixture by centrifugation and reused several times with minimal loss of catalytic activity. Furthermore, the concentration of catalyst in Sonogashira reactions can be reduced to a minimum amount of 0.01 mol% while still providing a high conversion of the Sonogashira product with a remarkable turnover number (TON) of 7200 and turnover frequency (TOF) of 21 600 h^-1. The catalyst was fully characterized by a variety of spectroscopic techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).

Cu/Oxalic Diamide-Catalyzed Coupling of Terminal Alkynes with Aryl Halides

N¹-(2,6-Dimethylphenyl)-N²-(pyridin-2-ylmethyl)oxalamide (DMPPO) was revealed to be a more effective ligand for copper-catalyzed coupling reaction of (hetero)aryl halides with 1-alkynes than previously reported ones. Only 3 mol % CuCl and DMPPO are required to make the coupling complete at 100 °C (for bromides) and 80 °C (for iodides). Both (hetero)aryl and alkyl substituted 1-alkynes worked well under these conditions, leading to the formation of internal alkynes in great diversity.

Aryl Radical Enabled, Copper-Catalyzed Sonogashira-Type Cross-Coupling of Alkynes with Alkyl Iodides

Despite the success of Sonogashira coupling for the synthesis of arylalkynes and conjugated enynes, the engagement of unactivated alkyl halides in such reactions remains historically challenging. We report herein a strategy that merges Cu-catalyzed alkyne transfer with the aryl radical activation of carbon-halide bonds to enable a general approach for the coupling of alkyl iodides with terminal alkynes. This unprecedented Sonogashira-type cross-coupling reaction tolerates a broad range of functional groups and has been applied to the late-stage cross-coupling of densely functionalized pharmaceutical agents as well as the synthesis of positron emission tomography tracers.

Novel Highly Efficient Green and Reusable Cu(II)/Chitosan-Based Catalysts for the Sonogashira, Buchwald, Aldol, and Dipolar Cycloaddition Reactions

In this study, new Cu(II)/chitosan-based systems were designed via (i) the treatment of chitosan with sodium sulfate (1a) or sodium acetate (1b); (ii) the coating of 1a or 2a with a sodium hyaluronate layer (2a and 2b, correspondingly); (iii) the treatment of a cholesterol–chitosan conjugate with sodium sulfate (3a) or sodium acetate (3b); and (iv) the succination of 1a and 1b to afford 4a and 4b or the succination of 2a and 2b to yield 5a and 5b. The catalytic properties of the elaborated systems in various organic transformations were evaluated. The use of copper sulfate as the source of Cu2+ ions results in the formation of nanoparticles, while the use of copper acetate leads to the generation of conventional coarse-grained powder. Cholesterol-containing systems have proven to be highly efficient catalysts for the cross-coupling reactions of different types (e.g., Sonogashira, Buchwald–Hartwig, and Chan–Lam types); succinated systems coated with a layer of hyaluronic acid are promising catalysts for the aldol reaction; systems containing inorganic copper(II) salt nanoparticles are capable of catalyzing the nitrile-oxide-to-nitrile 1,3-dipolar cycloaddition. The elaborated catalytic systems efficiently catalyze the aforementioned reactions in the greenest solvent available, i.e., water, and the processes could be conducted in air. The studied catalytic reactions proceed selectively, and the isolation of the product does not require column chromatography. The product is separated from the catalyst by simple filtration or centrifugation.

Total Synthesis of Puberuline C

Puberuline C (1) is an architecturally complex C₁₉-diterpenoid alkaloid with a unique ring fusion pattern. The 6/7/5/6/6/6-membered rings (ABCDEF-rings) contain one tertiary amine and six oxygen functionalities, and possess 12 contiguously aligned stereocenters, three of which are quaternary. These structural features of 1 make its chemical construction exceptionally challenging. Here, we disclose the first total synthesis of 1. The synthesis was accomplished from 2-cyclohexenone (9) by integrating radical cascade and Mukaiyama aldol reactions as the key transformations. A double Mannich reaction fused the A- and E-rings, and Sonogashira coupling attached the C-ring, efficiently leading to ACE-rings with the requisite 19 carbons of 1. The chemically stable tertiary chloride of the ACE-ring structure was then transformed to the corresponding bridgehead radical, which participated in the simultaneous cyclization of the B- and F-rings via a highly organized radical cascade process. This unusual step installed five contiguous stereocenters, including two quaternary carbons, without damaging the preexisting multiple polar functionalities. Subsequently, the intramolecular Mukaiyama aldol reaction between silyl enol ether and acetal was realized by applying a combination of SnCl₄ and ZnCl₂, forging the last remaining D-ring of the hexacycle. Finally, 3 was elaborated into 1 through regio- and stereoselective functionalizations of the BCD-rings. Our novel radical-based strategy achieved the total synthesis of 1 in 32 total steps from simple 9, demonstrating the power of the radical cascade reaction to streamline the assembly of highly complex molecules.

Expanding the Synthetic Toolbox through Metal-Enzyme Cascade Reactions

The combination of metal-, photo-, enzyme-, and/or organocatalysis provides multiple synthetic solutions, especially when the creation of chiral centers is involved. Historically, enzymes and transition metal species have been exploited simultaneously through dynamic kinetic resolutions of racemates. However, more recently, linear cascades have appeared as elegant solutions for the preparation of valuable organic molecules combining multiple bioprocesses and metal-catalyzed transformations. Many advantages are derived from this symbiosis, although there are still bottlenecks to be addressed including the successful coexistence of both catalyst types, the need for compatible reaction media and mild conditions, or the minimization of cross-reactivities. Therefore, solutions are here also provided by means of catalyst coimmobilization, compartmentalization strategies, flow chemistry, etc. A comprehensive review is presented focusing on the period 2015 to early 2022, which has been divided into two main sections that comprise first the use of metals and enzymes as independent catalysts but working in an orchestral or sequential manner, and later their application as bionanohybrid materials through their coimmobilization in adequate supports. Each part has been classified into different subheadings, the first part based on the reaction catalyzed by the metal catalyst, while the development of nonasymmetric or stereoselective processes was considered for the bionanohybrid section.

The Alkyne Zipper Reaction: A Useful Tool in Synthetic Chemistry

The alkyne zipper reaction is an internal-to-terminal alkyne isomerization reaction with many interesting applications in synthetic chemistry, as it constitutes an efficient means of achieving acetylene functionalization. A review of its applications in synthesis processes is presented in this paper, with a brief overview of the mechanistic features of the alkyne zipper reaction, as well as a brief overview of its future potential.

Stepwise synthesis of 2,6-difunctionalized ethyl pyrazolo[1,5-a]pyrimidine-3-carboxylate via site-selective cross-coupling reactions: experimental and computational studies

A variety of novel disubstituted 2-(alknyl, aryl and arylamine)-6-alkynylpyrazolo[1,5-a]pyrimidine derivatives was prepared via sequential site-selective cross-coupling reactions from 2,6-dibromopyrazolo[1,5-a]pyrimidine 3. The regio-controlled Sonogashira-type coupling of 3 with a wide range of terminal alkynes proceeded smoothly with excellent selectivity in favor of the C6-position through careful adjustment of the coupling conditions, followed by the subsequent introduction of alkynyl, aryl or arylamine groups at the C2-position via the Sonogashira, Suzuki-Miyaura and Buchwald-Hartwig coupling reactions, respectively. These promising results allow for further use and diversification of the chemically and biologically interesting pyrazolo[1,5-a]pyrimidine scaffold. In addition, computational studies were conducted to provide explanations for the origin of regioselectivity.

Copper-catalyzed deacetonative Sonogashira coupling

A convenient Pd- and phosphine-free protocol for assembling internal alkynes from tertiary propargyl alcohols and (het)aryl halides has been developed. The proposed tandem approach includes the base-promoted retro-Favorskii fragmentation followed by Cu-catalyzed C(sp)-C(sp²) cross-coupling. The use of inexpensive reagents (e.g. a catalyst, additives, a base, and a solvent) and good functional group tolerance make the procedure practical and cost-effective. The synthetic utility of the method was demonstrated by a smooth alkynylation of vinyl iodides derived from natural steroidal hormones.

Assessing the environmental benefit of palladium-based single-atom heterogeneous catalysts for Sonogashira coupling

The Pd-Cu catalysed Sonogashira coupling of terminal alkynes and aryl halides is a cornerstone synthetic strategy for C-C bond formation. Homogeneous organometallic systems conventionally applied are typically not reusable and require efficient downstream Pd removal steps for product purification, making it challenging to fully recover the precious metal. A holistic cradle-to-gate life cycle assessment (LCA) unveils that process footprint can be improved up to two orders of magnitude from repeated catalyst reuse. New classes of heterogeneous catalysts based on isolated metal atoms (single-atom catalysts, SACs) demonstrate promising potential to synergise the benefits of solid and molecular catalysts for efficient Pd utilisation. Here we show that using Pd atoms anchored on nitrogen-doped carbon permits full recovery of the metal and reuse of the catalyst over multiple cycles. A hybrid process using the Pd-SAC with a homogeneous CuI cocatalyst is more effective than a fully heterogeneous analogue based on a bimetallic Pd-Cu SAC, which deactivates severely due to copper leaching. In some scenarios, the LCA-based metrics demonstrate the footprint of the hybrid homogeneous-heterogeneous catalytic process is leaner than the purely homogeneous counterpart already upon single reuse. Combining LCA with experimental evaluation will be a useful guide to the implementation of solid, reusable catalysts for sustainable organic transformations.

Pillar[n]arene-Mimicking/Assisted/Participated Carbon Nanotube Materials

The recent progress in pillar[n]arene-assisted/participated carbon nanotube hybrid materials were initially summarized and discussed. The molecular structure of pillar[n]arene could serve different roles in the fabrication of attractive carbon nanotube-based materials. Firstly, pillar[n]arene has the ability to provide the structural basis for enlarging the cylindrical pillar-like architecture by forming one-dimensional, rigid, tubular, oligomeric/polymeric structures with aromatic moieties as the linker, or forming spatially "closed", channel-like, flexible structures by perfunctionalizing with peptides and with intramolecular hydrogen bonding. Interestingly, such pillar[n]arene-based carbon nanotube-resembling structures were used as porous materials for the adsorption and separation of gas and toxic pollutants, as well as for artificial water channels and membranes. In addition to the art of organic synthesis, self-assembly based on pillar[n]arene, such as self-assembled amphiphilic molecules, is also used to promote and control the dispersion behavior of carbon nanotubes in solution. Furthermore, functionalized pillar[n]arene derivatives integrated carbon nanotubes to prepare advanced hybrid materials through supramolecular interactions, which could also incorporate various compositions such as Ag and Au nanoparticles for catalysis and sensing.

Metal–Organic Framework Derived Cobalt Oxide Supported Bimetallic Pd/Cu Nanoparticles for Efficient Catalysis of the Sonogashira Reaction under Aerobic Conditions

A bimetallic Pd/Cu catalyst supported on a metal-organic-framework-derived cobalt oxide was prepared and characterized by SEM, EDS, XRD, XPS, and ICP-OES analyses. The catalyst promoted the Sonogashira reaction of aryl iodides with terminal alkynes at a low loading of the palladium (0.032 mol%) and copper species (0.012 mol%) to give the corresponding disubstituted alkynes in moderate to good yields. When the catalyst was recovered by using an external magnetic field, its catalytic activity decreased slightly in a second cycle.

Pd-Catalyzed Cross-Couplings: On the Importance of the Catalyst Quantity Descriptors, mol % and ppm

This Review examines parts per million (ppm) palladium concentrations in catalytic cross-coupling reactions and their relationship with mole percentage (mol %). Most studies in catalytic cross-coupling chemistry have historically focused on the concentration ratio between (pre)catalyst and the limiting reagent (substrate), expressed as mol %. Several recent papers have outlined the use of “ppm level” palladium as an alternative means of describing catalytic cross-coupling reaction systems. This led us to delve deeper into the literature to assess whether “ppm level” palladium is a practically useful descriptor of catalyst quantities in palladium-catalyzed cross-coupling reactions. Indeed, we conjectured that many reactions could, unknowingly, have employed low “ppm levels” of palladium (pre)catalyst, and generally, what would the spread of ppm palladium look like across a selection of studies reported across the vast array of the cross-coupling chemistry literature. In a few selected examples, we have examined other metal catalyst systems for comparison with palladium.

Palladium-Catalyzed Sonogashira Coupling of a Heterocyclic Phosphonium Salt with a Terminal Alkyne

An efficient Sonogashira coupling of a heterocyclic phosphonium salt with a terminal alkyne via C-P bond cleavage was developed. The reactions proceeded smoothly in the presence of palladium catalyst, copper(I) iodide, and N,N-diisopropylethylamine (DIPEA) in N-methyl-2-pyrrolidone (NMP) at 100 °C for 12 h, producing the corresponding alkynyl-substituted pyridine, quinoline, pyrazine, and quinoxaline in moderate to good yields with wide substrate scope and broad functional group tolerance. In addition, gram-scale synthesis could also be achieved, and the reaction could be applied to the functionalization of alkyne-containing complex molecules derived from sugars and pharmaceutical and naturally occurring products (e.g., estrone, d-galactopyranose, menthol, and ibuprofen).

Synthesis of (Deoxy)difluoromethylated Phosphines by Reaction of R2P(O)H with TMSCF3 and Their Application in Cu(I) Clusters in Sonogashira Coupling

R₂PCF₂H ligands and their R₂P(O)CF₂H precursors were synthesized from R₂P(O)H with TMSCF₃ by simply modulating the H₂O concentration via deoxydifluoromethylation and difluoromethylation. The air sensitive R₂PCF₂H phosphines can be stabilized in Cu(I) clusters as ligands. Within these Cu(I) clusters, the Sonogashira cross-coupling reaction can proceed fast and efficiently using terminal alkynes and aryl iodides within 15 min at room temperature under air to give a variety of diaryl(alkyl)acetylenes in good yields (49 examples, yields of ≤99%). Six of the internal alkynes present in drug precursors can be obtained using this protocol in good yields. The mechanism is proposed on the basis of control experiments.

Directing group assisted rhodium catalyzed meta-C-H alkynylation of arenes

Site-selective C-H alkynylation of arenes to produce aryl alkynes is a highly desirable transformation due to the prevalence of aryl alkynes in various natural products, drug molecules and in materials. To ensure site-selective C-H functionalization, directing group (DG) assisted C-H activation has been evolved as a useful synthetic tool. In contrast to DG-assisted ortho-C-H activation, distal meta-C-H activation is highly challenging and has attracted significant attention in recent years. However, developments are majorly focused on Pd-based catalytic systems. In order to diversify the scope of distal meta-C-H functionalization, herein we disclosed the first Rh(i) catalyzed meta-C-H alkynylation protocol through the inverse Sonogashira coupling reaction. The protocol is compatible with various substrate classes which include phenylacetic acids, hydrocinnamic acids, 2-phenyl benzoic acids, 2-phenyl phenols, benzyl sulfonates and ether-based scaffolds. The post-synthetic modification of meta-alkynylated arenes is also demonstrated through DG-removal as well as functional group interconversion.

Palladium-Catalyzed Alkynylation of Enones with Alkynylsilanes via C-C Bond Activation

We report herein the synthesis of 1,3-enynes via palladium-catalyzed cross-coupling between enone derivatives and alkynylsilanes. The employment of an appropriate pyridine-oxazoline ligand is the key to the C-C cleavage and the high E/Z stereoselectivity. This protocol features broad substrate scope and wide functional-group tolerance, affording the desired products in moderate-to-good yields. Late-stage diversification of natural product β-ionone further demonstrated the synthetic utility of this protocol.

Strain-release arylations for the bis-functionalization of azetidines

The addition of nucleophilic organometallic species onto in situ generated azabicyclobutanes enables the selective formation of 3-arylated azetidine intermediates through strain-release. Single pot strategies were further developed for the N-arylation of resulting azetidines, employing either S_NAr reactions or Buchwald-Hartwig couplings.

Selective Para-C-H Alkynylation of Aniline Derivatives by Pd/S,O-Ligand Catalysis

Herein, we report a nondirected para-selective C-H alkynylation of aniline derivatives by a Pd/S,O-ligand-based catalyst. The reaction proceeds under mild conditions and is compatible with a variety of substituted anilines. The scalability and further derivatizations of the alkynylated products have been also demonstrated.

Synthesis of Alkyne-Substituted Dihydropyrrolones as Bacterial Quorum-Sensing Inhibitors of Pseudomonas aeruginosa

The Quorum-sensing system in Pseudomonas aeruginosa is responsible for the pathogenicity and the production of virulence factors and biofilm formation. Dihydropyrrolones were previously found to act as inhibitors of QS-dependent bacterial phenotypes. In this study, a range of dihydropyrrolone (DHP) analogues was synthesized via the lactone-lactam conversion of lactone intermediates followed by the formation of novel acetylene analogues of dihydropyrrolones from brominated dihydropyrrolones via Sonogashira coupling reactions in moderate to high yields. Upon biological testing, the most potent compounds, 39–40 and 44, showed higher bacterial quorum-sensing inhibitory (QSI) activity against P. aeruginosa reporter strain at 62.5 µM. Structure–activity relationship studies revealed that di-alkynyl substituent at the exocyclic position of DHPs possessed higher QSI activities than those of mono-alkynyl DHPs. Moreover, a hexyl-substituent at C3 of DHPs was beneficial to QSI activity while a phenyl substituent at C4 of DHPs was detrimental to QSI activity of analogues.

Inverting Conventional Chemoselectivity in the Sonogashira Coupling Reaction of Polyhalogenated Aryl Triflates with TMS-Arylalkynes

A newly developed phosphine ligand with a C2-cyclohexyl group on the indole ring was successfully applied in a chemoselective Sonogashira coupling reaction with excellent chemoselectivity, affording an inversion of the conventional chemoselectivity order of C-Br > C-Cl > C-OTf. This study also provided an efficient approach to the synthesis of polycyclic aromatic hydrocarbons (PAHs) and the natural product analogue trimethyl-selaginellin L by merging of chemoselective Sonogashira and Suzuki-Miyaura coupling reactions.

Efficient access to materials-oriented aromatic alkynes via the mechanochemical Sonogashira coupling of solid aryl halides with large polycyclic conjugated systems

Sonogashira coupling represents an indispensable tool for the preparation of organic materials that contain C(sp)-C(sp2) bonds. Improving the efficiency and generality of this methodology has long been an important research subject in materials science. Here, we show that a high-temperature ball-milling technique enables the highly efficient palladium-catalyzed Sonogashira coupling of solid aryl halides that bear large polyaromatic structures including sparingly soluble substrates and unactivated aryl chlorides. In fact, this new protocol provides various materials-oriented polyaromatic alkynes in excellent yield within short reaction times in the absence of bulk reaction solvents. Notably, we synthesized a new luminescent material via the mechanochemical Sonogashira coupling of poorly soluble Vat Red 1 in a much higher yield compared to those obtained using solution-based conditions. The utility of this method was further demonstrated by the rapid synthesis of a fluorescent metal-organic framework (MOF) precursor via two sequential mechanochemical Sonogashira cross-coupling reactions. The present study illustrates the great potential of Sonogashira coupling using ball milling for the preparation of materials-oriented alkynes and for the discovery of novel functional materials.

Photoinduced inverse Sonogashira coupling reaction

A transition-metal and photocatalyst-free, photoinduced inverse Sonogashira coupling reaction was developed. Under visible-light irradiation, the excited state iodoalkyne acted as an “alkynyl radical synthetic equivalent”.

An oxygen-bridged bimetallic [Cu–O–Se] catalyst for Sonogashira cross-coupling

Oxygen bridged bimetallic CuSeO3·2H2O catalyst is used for Sonogashira cross-coupling under ligand free condition. Catalyst is free from palladium up to 0.2 ppm.