Insights into the mechanism of 3d transition-metal-catalyzed directed C(sp3)-H bond functionalization reactions

The growing interest in the catalytic activity of earth-abundant 3d transition-metals has led to the development of new and more sustainable methods for C-H bond functionalization reactions. However, this is an emerging field which involves considerable mechanistic complexity as the mode of action of 3d transition metals differs markedly from the well-studied mechanisms of precious metals. In this review, we present an overview of the research efforts in Ni-, Cu-, Fe- and Co-catalyzed directed C(sp3)-H bond functionalization reactions, covering design principles and mechanistic discussions, along with potential applications and limitations. To conclude, the unresolved challenges and future viewpoints are highlighted. We aspire for this review to serve as a relevant and valuable reference for researchers in this swiftly progressing field, helping to inspire the development of more original and innovative strategies.

Elucidation of Antimicrobials and Biofilm Inhibitors Derived from a Polyacetylene Core

The development of new antibiotics with unique mechanisms of action is paramount to combating the growing threat of antibiotic resistance. Recently, based on inspiration from natural products, an asymmetrical polyacetylene core structure was examined for its bioactivity and found to have differential specificity for different bacterial species based on the substituents around the conjugated alkyne. This research further probes the structural requirements for bioactivity through a systematic synthesis and investigation of new compounds with variable carbon chain length, alkynyl subunits, and alcohol substitution. Furthermore, the research examines the activity of the new compounds towards the inhibition of biofilm formation. Overall, several key new polyyne compounds have been identified in both decreasing bacterial viability and in disrupting pre-formed biofilms. These properties are key in the fight against bacterial infections and will be helpful in the further development of new antibiotic agents.

Platinum Elimination from Bis(triethylsilylpolyynyl) Complexes (n-Bu2C(CH2PPh2)2)Pt((C≡C)nSiEt3)2 (n=2, 3) and Macrocycles Comprised of Four L2Pt Corners and Four C≡CC≡CC≡C Linkers; An Approach to Cyclo[24]carbon

The overarching goal of this study is to effect the elimination of platinum from adducts with cis -C≡C-Pt-C≡C- linkages, thereby generating novel conjugated polyynes. Thus, the bis(hexatriynyl) complex trans-(p-tol3P)2Pt((C≡C)3H)2 is treated with 1,3-diphosphines R2C(CH2PPh2)2 to generate (R2C(CH2PPh2)2)2Pt((C≡C)3H)2 (14; R=c, n-Bu; e, p-tolCH2). These condense with the diiodide complexes R2C(CH2PPh2)2PtI2 (9 a,c) in the presence of CuI (cat.) and excess HNEt2 to give the title macrocycles [(R2C(CH2PPh2)2)Pt(C≡C)3]4 (16 c,e) as adducts of the byproduct [H2NEt2]+ I- (30-66 %). DOSY NMR experiments establish that this association is maintained in solution, but NaOAc removes the ammonium salt. The bis(triethylsilylpolyynyl) complexes (n-Bu2C(CH2PPh2)2)Pt((C≡C)nSiEt3)2 (n=2, 3) are synthesized analogously to 14 c. They react with I2 at rt to give mainly the diiodide complex 9 c and the coupling product Et3Si(C≡CC≡C)nSiEt3. The possibility of competing reactions giving IC≡C species is investigated. Analogous reactions of the Pt4C24 macrocycle 16 c also give 9 c, but no sp 13C NMR signals or mass spectrometric Cx z+ ions (x=24-100) could be detected. It is proposed that some cyclo[24]carbon is generated, but then rapidly converts to other forms of elemental carbon. No cyclotetracosane (C24H48) is detected when this sequence is carried out in the presence of PtO2 and H2.

Total Synthesis of Icumazole A Using a Modified Cadiot-Chodkiewicz Coupling

The first total synthesis of myxobacteria metabolite icumazole A (1) is reported. Key steps in the route include an organocatalyzed asymmetric self-aldol reaction followed by an acetate aldol reaction to form the stereotriad present in the oxazole moiety, an intramolecular Diels-Alder reaction to form the isochromanone, and an acetylide addition and selective methylation. The final steps involved a high-yielding modified Cadiot-Chodkiewicz coupling and stereoselective reduction to secure the Z,Z-diene and afford 1.

Air Tolerant Cadiot-Chodkiewicz and Sonogashira Cross-Couplings

Cadiot-Chodkiewicz cross-couplings generate an unsymmetric buta-1,3-diyne by way of a Cu(I)-catalyzed coupling between a terminal alkyne and a 1-haloalkyne. Despite their widespread use, Cadiot-Chodkiewicz reactions are plagued by the generation of symmetric buta-1,3-diyne side products, formed through competing: (a) formal reductive homo-coupling of the 1-haloalkyne and (b) oxidative (Glaser-Hay/Eglinton) homo-coupling of the terminal alkyne. To overcome this issue, a large excess of one of the two reacting alkynes is commonly deployed, and difficult separations of cross- and homo-coupled products are often encountered. Here, we demonstrate that the use of ascorbate as a reductant leads to a suppression of these unwanted side reactions, hence permitting excellent yields with a roughly stoichiometric ratio of reactants. The procedure also avoids an inert gas atmosphere and uses a sustainable solvent. A similar approach is effective for cross-couplings involving a Pd(0)/Pd(II) catalytic cycle, with air tolerant Sonogashira couplings also established.

A Simple and Practical Bis-N-Heterocyclic Carbene as an Efficient Ligand in Cu-Catalyzed Glaser Reaction

Conjugated diyne derivatives are important scaffolds in modern organic synthetic chemistry. Using the Glaser reaction involves the coupling of terminal alkynes which can efficiently produce conjugated diyne derivatives, while the use of a stoichiometric amount of copper salts, strong inorganic base, and excess oxidants is generally needed. Developing an environmentally friendly and effective method for the construction of symmetrical 1,3-diynes compounds by Glaser coupling is still highly desirable. In this study, we present an economical method for the production of symmetric diynes starting from various terminal acetylenes in a Glaser reaction. A simple and practical bis-N-heterocyclic carbene ligand has been introduced as efficient ligands for the Cu-catalyzed Glaser reaction. High product yields were obtained at 100 °C for a variety of substrates including aliphatic and aromatic terminal alkynes and differently substituted terminal alkynes including the highly sterically hindered substrate 2-methoxy ethynylbenzene or 2-trifluoromethyl ethynylbenzene and a series of functional groups, such as trifluoromethyl group, ester group, carboxyl group, and nitrile group. The established protocol is carried out in air under base-free condition and is operationally simple. These research work suggest that bis-N-heterocyclic carbene could also an appealing ligand for Glaser reaction and provide a reference for the preparation of symmetric 1,3-diynes in industrial filed.

De Novo Synthesis of Polysubstituted 3-Hydroxypyridines Via “Anti-Wacker”-Type Cyclization

We report an efficient method to prepare polysubstituted 3-hydroxypyridines from amino acids, propargyl alcohols, and arylboronic acids. The process involves Pd(0)-catalyzed anti-selective arylative cyclizations of N-propargyl-N-tosyl-aminoaldehydes with arylboronic acids (“anti-Wacker”-type cyclization), oxidation of the resulting 5-substituted-3-hydroxy-1,2,3,6-tetrahydropyridines to 3-oxo derivatives, and elimination of p-toluenesulfinic acid. This method provides diverse polysubstituted 3-hydroxypyridines, whose hydroxy group can be further substituted by a cross-coupling reaction via a triflate.

On-DNA Alkyne Iodination and Acetylenic Coupling as a Useful Tool for DEL Synthesis

1-Iodoalkynes and 1,3-diynes are versatile chemical intermediates and pharmaceutically valuable ingredients. In this study, copper mediated on-DNA alkyne iodination and Cadiot-Chodkiewicz coupling are developed for the first time. This generates diverse, systematic, and unprecedented topographic structural features, which could be invaluable as molecular recognition agents for drug discovery in DEL screening.

Advances in Polyynes to Model Carbyne

The formation and study of molecules that model the sp-hybridized carbon allotrope, carbyne, is a challenging field of synthetic physical organic chemistry. The target molecules, oligo- and polyynes, are often the preferred candidates as models for carbyne because they can be formed with monodisperse lengths as well as defined structures. Despite a simple linear structure, the synthesis of polyynes is often far from straightforward, due in large part to a highly conjugated framework that can render both precursors and products highly reactive, i.e., kinetically unstable. The vast majority of polyynes are formed as symmetrical products from terminal alkynes as precursors via an oxidative, acetylenic homocoupling reaction based on the Glaser, Eglinton-Galbraith, and Hay reactions. These reactions are very efficient for the synthesis of shorter polyynes (e.g., hexaynes and octaynes), but yields often drop dramatically as a function of length for longer derivatives, usually starting with the formation of decaynes. The most effective approach to circumvent unstable precursors and products has been through the incorporation of sterically demanding end groups that serve to "protect" the polyyne skeleton. This approach was arguably identified in the early 1950s by Bohlmann and co-workers with the synthesis of tBu-end-capped polyynes. During the next 50 years, a polyyne with 14 contiguous alkyne units remained the longest isolated derivative until 2010, when the record was extended to 22 alkyne units. The record length was broken again in 2020, when a polyyne consisting of 24 alkynes was isolated and characterized. Beyond polyynes, there have been several reports describing the potential synthesis of carbyne, but conclusive characterization and proof of structure have been tenuous. The sole example of synthetic carbyne arises from synthesis within carbon nanotubes, when chains of thousands of sp carbon atoms have been linked to form polydisperse samples of carbyne. Thus, model compounds for carbyne, the polyynes, remain the best means to examine and predict the experimental structure and properties of this carbon allotrope.This Account will discuss the general synthesis of polyynes using homologous series of polyynes with up to 10 alkyne units as examples (decaynes). The limited number of specific syntheses of series with longer polyynes will then be presented and discussed in more detail based on end groups. The monodisperse polyynes produced from these synthetic efforts are then examined toward providing our best extrapolations for the expected characteristics for carbyne based on ¹³C NMR spectroscopy, UV-vis spectroscopy, X-ray crystallography, and Raman spectroscopy.

Synthesis and Reactivity of a Terminal 1-Alkynyl Triazene

1-Alkynyl triazenes are versatile reagents in synthetic organic chemistry, but the structural diversity of this compound class has so far been limited. Herein, we describe the synthesis of a terminal 1-alkynyl triazene. Subsequent functionalization allows the preparation of 1-alkynyl triazenes with a range of functional groups including esters, alcohols, cyanides, phosphonates, and amides. Furthermore, the terminal 1-alkynyl triazene can be used for the synthesis of di- and triynes and for the preparation of (hetero)aromatic triazenes in metal-catalyzed cyclization reactions.

Effects of Structural Variations on Antibacterial Properties for Conjugated Diynes Generated through Glaser Hay Couplings

Antibiotic resistance is a growing problem facing global societies today. Many new antibiotics are derivatized versions of already existing antibiotics, which allows for antibiotic resistance to arise. To combat this issue, new antibiotics with different core structures need to be elucidated. Asymmetrical polyacetylenes have been isolated from natural products and they have previously been demonstrated to exhibit antimicrobial and antibacterial activity; however, their synthetic preparation has not made them easily amenable to rapid derivatization for SAR studies. Using a combination of solution and solid-supported chemistries, an array of diynes inspired by a known natural product were prepared and assessed for antibacterial activity. Ultimately, several compounds were identified with improved activity in bacterial viability assays. Moreover, some compounds were discovered that displayed a degree of specificity for E. coli over P. fluorescens and vice versa. These new compounds show promise, and further investigation is needed to pinpoint the specific structural components that elicit biological activity.

Iron-Catalyzed Cadiot-Chodkiewicz Coupling with High Selectivity in Water under Air

An iron-based catalytic system was developed for the cross-coupling of 1-bromoalkynes with terminal alkynes to selectively generate unsymmetrical 1,3-butadiynes in water under air. It was found that a combination of 1-bromoalkynes derived from less acidic terminal alkynes with more acidic counterparts would greatly enhance yields and selectivity for unsymmetrical 1,3-butadiynes. The reaction was also applicable for the synthesis of unsymmetrical 1,3,5-hexatriynes through coupling of 1-bromoalkynes and trimethylsilyl-protected 1,3-butadiynes in a one-pot manner.

Migratory insertion of copper-allenylidene from propargyl ester

The highly efficient copper-catalyzed homo-dimerization and cross-coupling of propargyl esters have been developed. Various 1-en-3,5-diynes, [5]cumulenes and 1,3-diynes were successfully furnished via the copper-allenylidene intermediates with moderate to excellent yields. Migratory insertion is proposed as the key step to achieve the selectivity at the carbene carbon of the copper-allenylidene.

Cobalt-Catalyzed Glaser-type Homocoupling Reaction

A highly efficient cobalt-catalyzed homocoupling of terminal alkynes with di-tert-butyldiaziridinone as the oxidant has been developed. The protocol tolerates a wide array of terminal alkynes, both activated and unactivated alkynes, to afford the corresponding conjugated 1,3-diynes. The mild reaction conditions further allow late-stage homocoupling of alkynes derived from complex natural products.

De novo Assembly of the Benzenoid Ring as a Core Strategy for Synthesis of the Isoindolinone Natural Products Isohericerin, Erinacerin A, and Sterenin A

Here we describe the use of the hexadehydro-Diels-Alder (HDDA) reaction for the de novo construction of the isoindolinone scaffold and its application to the synthesis of the title natural products. The key isoindolinone-forming HDDA reaction involved an unprecedented substrate motif in which an amide carbonyl group was conjugated to the 4π 1,3-diyne component. In addition, a dimethylsilyl (-SiMe₂H) substituent was exploited to trigger a Fleming-Tamao-Kumada oxidation for the installation of an essential phenolic hydroxyl group.

Trapping of Terminal Platinapolyynes by Copper(I) Catalyzed Click Cycloadditions; Probes of Labile Intermediates in Syntheses of Complexes with Extended sp Carbon Chains, and Crystallographic Studies

The silylated hexatriynyl complex trans-(C₆ F₅ )(p-tol₃ P)₂ Pt(C≡C)₃ SiEt₃ (PtC₆ TES) is converted in situ to PtC₆ H (wet n-Bu₄ N⁺ F^- , THF) and cross coupled with the diyne H(C≡C)₂ SiEt₃ (HC₄ TES; CuCl/TMEDA, O₂ ) to give PtC₁₀ TES (71 %). This sequence is repeated twice to afford PtC₁₄ TES (65 %) and then PtC₁₈ TES (27 %). An analogous series of reactions starting with PtC₈ TES gives PtC₁₂ TES (60 %), then PtC₁₆ TES (43 %), and then PtC₂₀ TES (17 %). Similar cross couplings with H(C≡C)₂ Si(i-Pr)₃ (HC₄ TIPS) give PtC₁₂ TIPS (68 %), PtC₁₄ TIPS (68 %), and PtC₁₆ TIPS (34 %). The trialkylsilyl species (up to PtC₁₈ TES) are converted to 3+2 "click" cycloadducts or 1,4-disubstituted 1,2,3-triazoles trans-(C₆ F₅ )(p-tol₃ P)₂ Pt(C≡C)_n-1 C=CHN(CH₂ C₆ H₅ )N=N (29-92 % after workups). The most general procedure involves generating the terminal polyynes PtC_x H (wet n-Bu₄ N⁺ F^- , THF) in the presence of benzyl azide in DMF and aqueous CuSO₄ /ascorbic acid. All of the preceding complexes are crystallographically characterized and the structural and spectroscopic properties analyzed as a function of chain length. Two pseudopolymorphs of PtC₂₀ TES are obtained, both of which feature molecules with parallel sp carbon chains in a pairwise head/tail packing motif with extensive sp/sp van der Waals contacts.

One-Pot Dual C-C Coupling Reaction via Site Selective Cascade Formation by PdII -Cryptate of an Amino-Ether Heteroditopic Macrobicycle

Selectivity of aryl iodo over ethynyl iodo toward the Suzuki cross coupling reaction is explored by utilizing a palladium complex of amino-ether heteroditopic macrobicycle. Subsequently, unreacted ethynyl iodide undergoes homocoupling reaction in the same catalytic atmosphere, thereby representing a cascade dual C-C coupling reaction. Furthermore, this approach is extended for novel one-pot synthesis of unsymmetrical 1,3-diynes.

Silver-catalysed C–H bond activation: a recent review

Transition metal catalysed C–H activations are efficient, simple, mild, cost-effective and stereoselective, and many of them are environmentally sustainable transformations.

Progress and prospects in copper-catalyzed C-H functionalization

Copper-catalyzed C-H functionalization is becoming a significant area in organic chemistry. Copper is now widely used as a catalyst in organic synthesis as it is inexpensive and not very toxic. Functionalization of C-H bonds to construct wide varieties of organic compounds has received much attention in recent times. This review focuses on the recent advances in Cu-catalyzed C-H functionalization and covers literature from 2018-2020.

Intramolecular Nicholas Reactions in the Synthesis of Heteroenediynes Fused to Indole, Triazole, and Isocoumarin

The applicability of an intramolecular Nicholas reaction for the preparation of 10-membered O- and N-enediynes fused to indole, 1,2,3-triazole, and isocoumarin was investigated. The general approach to acyclic enediyne precursors fused to heterocycles includes inter- and intramolecular buta-1,3-diyne cyclizations with the formation of iodoethynylheterocycles, followed by Sonogashira coupling. The nature of both a heterocycle and a nucleophilic group affects the possibility of a 10-membered ring closure by the Nicholas reaction. Among oxacycles, an isocoumarin-fused enediyne was obtained. In the case of O-enediyne annulated with indole, instead of the formation of a 10-membered cycle, BF₃-promoted addition of an OH-group to the proximal triple bond at the C3 position afforded dihydrofuryl-substituted indole. For 1,2,3-triazole-fused analogues, using NH-Ts as a nucleophilic functional group allowed obtaining 10-membered azaenediyne, while the substrate with a hydroxyl group gave only traces of the desired 10-membered oxacycle. An improved method for the deprotection of Co-complexes of cyclic enediynes using tetrabutylammonium fluoride in an acetone/water mixture and the investigation of the 10-membered enediynes' reactivity in the Bergman cyclization are also reported. In the solid state, all synthesized iodoethynylheterocycles were found to be involved in halogen bond (XB) formation with either O or N atoms as XB acceptors.