Palladium(II)-Catalyzed C–H Bond Activation/C–C and C–O Bond Formation Reaction Cascade: Direct Synthesis of Coumestans

Controllable Chemoselectivity Cascade Reactions for the Synthesis of Phenanthrenols via Palladium-Catalyzed-Suzuki/Heck Reaction and Michael Addition

Palladium serves as a multi-functional catalyst which is controllable by tuning reaction conditions. This work demonstrated the utilization of a palladium catalyst for the synthesis of phenanthrenols by cascade palladium-catalyzed Suzuki/Heck reaction between chalcone and 2-bromophenylboronic acid, followed by Michael addition. The sequential reaction could be controlled by reactivity of the palladium catalyst in different solvents and concentrations of reagents. This protocol could be applied to a broad range of substrates to give products in low to good yields.

Palladium(II)-catalyzed annulation of N-methoxy amides and arynes: computational mechanistic insights and substituents effects

CONTEXT

The combined use of transition metal-catalyzed C-H activation with aryne annulation reactions has emerged as an important strategy in organic synthesis. In this study, the mechanisms of the palladium(II)-catalyzed annulation reaction of N-methoxy amides and arynes were computationally investigated by density functional theory. The role of methoxy amide as a directing group was elucidated through the calculation of three different pathways for the C-H activation step, showing that the pathway where amide nitrogen acts as a directing group is preferable. At the reductive elimination transition state, an unstable seven-membered ring is formed preventing the lactam formation. A substituent effect study based on an NBO analysis, Hammet, and using a More O'Ferall-Jenks plot indicates that the C-H activation step proceeds via an electrophilic concerted metalation-deprotonation (eCMD) mechanism. The results show that electron-withdrawing groups increase the activation barrier and contribute to an early Pd-C bond formation and a late C-H bond breaking when compared with electron-donating substituents. Our computational results are in agreement with the experimental data provided in the literature.

METHODS

All calculations were performed using Gaussian 16 software. Geometry optimizations, frequency analyses at 393.15 K, and IRC calculations were conducted at the M06L/Def2-SVP level of theory. Corrected electronic energies, NBO charges, and Wiberg bond indexes were computed at the M06L/Def2-TZVP//M06L/Def2-SVP level of theory. Implicit solvent effects were considered in all calculations using the SMD model, with acetonitrile employed as the solvent.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

Pd-Catalyzed Alkyne and Aryne Annulations: Synthesis and Photophysical Properties of π-Extended Coumarins

A Pd-catalyzed alkyne and aryne annulation strategy via C-H activation has been implemented for the synthesis of π-extended coumarins. This synthetic strategy provides a wide range of π-extended coumarins in moderate to good yields with good functional group compatibility. Photophysical properties of the synthesized π-extended coumarins have been evaluated, and some of them show interesting fluorescent properties. Three of the synthesized coumarins have been unambiguously established by a single-crystal XRD study.

Merging Metals and Strained Intermediates

Strained intermediates such as cyclic alkynes and allenes are most commonly utilized in nucleophilic additions and cycloadditions, but have seen increased use in a third area of reactivity: metal-mediated transformations. The merger of strained intermediates and metal catalysis has enabled rapid access to complex, polycyclic systems. Following a discussion of relevant landmark studies involving metals and strained intermediates, this article highlights recent advances in transition metal-mediated transformations from our laboratory. Specifically, this includes the use of arynes in the synthesis of decorated organometallic complexes, and the utilization of cyclic allenes to access enantioenriched heterocycles. Moreover, the broad applicability of such transformations, and exciting future areas of research are discussed.

Aryne Annulations for the Synthesis of Carbocycles and Heterocycles: An Updated Review

Carbocycles and heterocycles are the most relevant structural moieties, which find boundless applications in interdisciplinary areas. In this regard, the interesting chemistry of aryne is highly embraced by the researchers for the synthesis of structurally different polycyclic systems i.e. carbocycles and heterocycles. In this review, we intended to provide a comprehensive overview on annulative approach considering Kobayashi's aryne intermediate as the key reactive species with diverse nucleo-/electrophilic partners. The review highlights the different aryne annulation reactions adopted for the synthesis of carbocycles and heterocycles (covering articles mainly from middle of 20’s), showcasing their viability towards broad range of substrates bearing diverse functionalities.

Synthesis of Phenolic Coumestans via a Sequential Dehydrogenation/Oxa-Michael Addition Reaction of 2',4'-Dihydroxyl-3-arylcoumarins

A facile and practical approach for the synthesis of natural coumestans and derivatives starting from 2',4'-dihydroxyl-3-arylcoumarins has been developed. The process involved a seqential intramolecular dehydrogenation/oxa-Micheal reaction efficiently promoted by 1,8-diazabicyclo[5.4.0]undec-7-ene at 40 °C under metal- and ligand-free conditions with good functional group compatibility.

Pd-catalyzed annulation of imidazo[1,2-a]pyridines with coumarins and indoles: synthesis of benzofuran and indole fused heterocycles

Palladium-catalyzed annulation of imidazo[1,2-a]pyridines with coumarins provided benzofuran fused transannulated products in good to excellent yields via a decarbonylative approach, while imidazo[1,2-a]pyridines with N-methyl indoles in the presence of palladium and base yielded conjugated imidazopyridine fused indole derivatives. Additional experiments revealed that the presence of the phenyl ring at the C-2 position of imidazo[1,2-a]pyridines is essential for the annulation than the alkyl groups. Both transformations follow the ionic mechanism by Pd-catalyzed double C-H bond activation. All the annulated products were shown to exhibit high fluorescence characteristics and their photophysical properties were evaluated.

One-pot synthesis of 3-substituted-3-hydroxyindolin-2-ones: three component coupling of N-protected isatin, aryne precursor and 1,3-cyclodione under metal-free conditions

A transition-metal free synthetic strategy for the direct synthesis of 3-substituted-3-hydroxy-indolin-2-ones via three component coupling of N-protected isatin, aryne precursor, and 1,3-cyclodione.

Ruthenium Catalyzed Intramolecular C-X (X=C, N, O, S) Bond Formation via C-H Functionalization: An Overview

Ruthenium catalyzed C-H activation is well known for its high tolerance towards the functional group and broad applicability in organic synthesis and molecular sciences, with significant applications in pharmaceutical industries, material sciences, and polymer industry. In the last few decades, enormous progress has been observed with ruthenium-catalyzed C-H activation chemistry. Notably, the vast majority of the C-H functionalization known in the literature are intermolecular, although the intramolecular variant provides fascinating new structural facet starting from the simple molecular scaffolds. Intramolecular C-H functionalization is atom economical and step efficient, results in less formation of undesired products which is easy to purify. This has created a lot of interest in organic chemistry in developing new synthetic strategies for such functionalization. The focus of this review is to present the relatively unexplored intramolecular functionalization of C-H bonds into C-X (X=C, N, O, S) bonds utilizing versatile ruthenium catalysts, their scope, and brief mechanistic discussion.

Palladium-Catalyzed Weakly Coordinating Lactone-Directed C-H Bond Functionalization of 3-Arylcoumarins: Synthesis of Bioactive Coumestan Derivatives

A palladium-catalyzed highly regioselective ortho-selective C-H functionalization of 3-arylcoumarins has been developed. The method utilizes the weakly coordinating lactone as a directing group. The versatility of the strategy is highlighted by developing methodologies for alkenylation, halogenation, fluoroalkoxylation, and hydroxylation. Different functional groups were well tolerated, and functionalized coumarins were obtained in moderate to high yields. The method also showed good selectivity for monofunctionalization versus difunctionalization. The generated ortho-hydroxy derivatives were cyclized in the presence of DDQ, thus developing a simple and fast method for the synthesis of bioactive coumestan from 3-arylcoumarins.

A palladium-catalyzed cascade process for spirooxindole: an alternative way for the synthesis of spiro(indoline-3,2'-quinazolin)-2-ones

A palladium-catalyzed cascade strategy has been developed for one-pot synthesis of functionalized spiro(indoline-3,2'-quinazolin)-2-one derivatives from readily available starting materials. The reaction proceeds via C-C and two C-N bond formations in a single reaction operation. This method offers an attractive pathway for the synthesis of a broad range of spiro(indoline-3,2'-quinazolin)-2-ones in good yields.

Pd(II)-Catalyzed oxidative alkenylation of 4-hydroxycoumarin with maleimide via a C-H bond activation strategy

A Pd(ii)-catalyzed oxidative alkenylation of 4-hydroxycoumarins with maleimides for the synthesis of 4-hydroxy-3-maleimidecoumarins has been described. This methodology proceeds via C-H activation and C(sp2)-C(sp2) bond formation providing a series of alkenylated Heck-type products.

Silyl Tosylate Precursors to Cyclohexyne, 1,2-Cyclohexadiene, and 1,2-Cycloheptadiene

Transient strained cyclic intermediates have become valuable intermediates in modern synthetic chemistry. Although silyl triflate precursors to strained intermediates are most often employed, the instability of some silyl triflates warrants the development of alternative precursors. We report the syntheses of silyl tosylate precursors to cyclohexyne, 1,2-cyclohexadiene, and 1,2-cycloheptadiene. The resultant strained intermediates undergo trapping in situ to give cycloaddition products. Additionally, the results of competition experiments between silyl triflates and silyl tosylates are reported.

β-Carboline directed regioselective hydroxylation by employing Cu(OAc)2 and mechanistic investigation by ESI-MS

A β-carboline directed regioselective C-H activation protocol for hydroxylation has been developed by employing Cu(OAc)2, a cost-effective 3d metal. This fastidious reaction proceeds under microwave irradiation and furnishes exclusively ortho-hydroxylated products with moderate to good yields under greener reaction conditions. Gratifyingly, this approach retains considerable functional group tolerance and is feasible on both electron-rich and electron-deficient substrates. This protocol signifies monohydroxylation on β-carboline via C-H functionalization which leads into development of biologically relevant molecules. The reaction mechanism was confirmed by intercepting and characterizing all the proposed intermediates by ESI-QTOF-MS.

Safety Assessment of Benzyne Generation from a Silyl Triflate Precursor

Silyl triflate precursors to benzyne and related intermediates have emerged as valuable synthetic building blocks. However, data addressing the safety of employing these silyl triflate precursors are lacking. We report the calorimetric analysis of a typical Kobayashi procedure for forming and trapping benzyne using a silyl triflate precursor. Our findings suggest that, unlike benzenediazonium carboxylate precursors to benzyne, silyl triflates may be employed under mild conditions without severe concern for runaway reaction.

Transition metal-catalyzed coupling of heterocyclic alkenes via C–H functionalization: recent trends and applications

Heterocyclic alkenes and their derivatives are an important class of reactive feedstock and valuable synthons. This review highlights the transition-metal-catalyzed coupling of heterocyclic alkenes via a C–H functionalization strategy.

Direct synthesis of ortho-methylthio allyl and vinyl ethers via three component reaction of aryne, activated alkene and DMSO

A transition-metal free synthetic strategy for the direct synthesis of ortho-methylthio allyl and vinyl ethers via cascade three-component coupling of aryne, activated alkene and DMSO.

Palladium-catalyzed synthesis of [60]fullerene-fused furochromenones and further electrochemical functionalization

The palladium-catalyzed heteroannulation of [60]fullerene with 4-hydroxycoumarins affords [60]fullerene-fused furochromenones, which can be further derivatized via an electrochemical method to synthesize 1,2,3,4-adducts.

Copper-catalyzed intramolecular cross dehydrogenative coupling approach to coumestans from 2'-hydroxyl-3-arylcoumarins

A copper-catalyzed intramolecular cross dehydrogenative C-O coupling reaction of 2'-hydroxyl-3-arylcoumarins was developed. This protocol provided a facile and efficient strategy for the construction of natural coumestans and derivatives in moderate to high yields. This transformation exhibited good functional group compatibility and was amenable to substrates with free phenolic hydroxyl groups.

Cycloadditions of Oxacyclic Allenes and a Catalytic Asymmetric Entryway to Enantioenriched Cyclic Allenes

The chemistry of strained cyclic alkynes has undergone a renaissance over the past two decades. However, a related species, strained cyclic allenes, especially heterocyclic derivatives, have only recently resurfaced and represent another class of valuable intermediates. We report a mild and facile means to generate the parent 3,4-oxacyclic allene from a readily accessible silyl triflate precursor, and then trap it in (4+2), (3+2), and (2+2) reactions to provide a variety of cycloadducts. In addition, we describe a catalytic, decarboxylative asymmetric allylic alkylation performed on an α-silylated substrate, to ultimately permit access to an enantioenriched allene. Generation and trapping of the enantioenriched cyclic allene occurs with complete transfer of stereochemical information in a Diels-Alder cycloaddition through a point-chirality, axial-chirality, point-chirality transfer process.

Concise Approach to Cyclohexyne and 1,2-Cyclohexadiene Precursors

Silyl triflate precursors to cyclic alkynes and allenes serve as valuable synthetic building blocks. We report a concise and scalable synthetic approach to prepare the silyl triflate precursors to cyclohexyne and 1,2-cyclohexadiene. The strategy involves a retro-Brook rearrangement of an easily accessible cyclohexanone derivative, followed by triflation protocols. This simple, yet controlled, method should enable the further study of strained alkynes and allenes in chemical synthesis.

De novo synthesis of benzofurans via trifluoroacetic acid catalyzed cyclization/oxidative aromatization cascade reaction of 2-hydroxy-1,4-diones

A simple metal-free cyclization/oxidative aromatization reaction starting from 2-hydroxy-1,4-diones for the de novo synthesis of benzofurans was developed.

Aryne insertion into the PO bond: one-pot synthesis of quaternary phosphonium triflates

A novel transition-metal free synthetic strategy for the direct synthesis of quaternary phosphonium triflates via insertion of aryne into phosphine oxide.

A cascade process for the synthesis of ortho-formyl allyl aryl ethers and 2H-chromen-2-ol derivatives from arynes via trapping of o-quinone methide with an activated alkene

A transition-metal free synthetic strategy for the direct synthesis of ortho-formyl substituted allyl aryl ethers and 2H-chromen-2-ol derivatives via a cascade three-component coupling of arynes, activated alkene and N,N-dimethylformamide.

Transition-metal free C(sp2)–C(sp2) bond formation: arylation of 4-aminocoumarins using arynes as an aryl source

A mild, efficient and transition-metal free synthetic strategy has been developed for the α-arylation of 4-aminocoumarins using arynes as an aryl source. This synthetic strategy proceeds via C(sp²)–C(sp²) bond formation between 4-aminocoumarins and aryne precursors in a single step in the absence of a metal-catalyst.

Recent advances in intramolecular C–O/C–N/C–S bond formationviaC–H functionalization

This review presents the construction of C–X bonds (X = O/N/S) by using intramolecular C–H functionalization for the synthesis of heterocyclic compounds.

Arynes and Cyclic Alkynes as Synthetic Building Blocks for Stereodefined Quaternary Centers

We report a facile method to synthesize stereodefined quaternary centers from reactions of arynes and related strained intermediates using β-ketoester-derived substrates. The conversion of β-ketoesters to chiral enamines is followed by reaction with in situ generated strained arynes or cyclic alkynes. Hydrolytic workup provides the arylated or alkenylated products in enantiomeric excesses as high as 96%. We also describe the one-pot conversion of a β-ketoester substrate to the corresponding enantioenriched α-arylated product. Computations show how chirality is transferred from the N-bound chiral auxiliary to the final products. These are the first theoretical studies of aryne trapping by chiral nucleophiles to set new stereocenters. Our approach provides a solution to the challenging problem of stereoselective β-ketoester arylation/alkenylation, with formation of a quaternary center.

Aryne-based strategy in the total synthesis of naturally occurring polycyclic compounds

This review has outlined the strategies and tactics of using arynes in the total syntheses of polycyclic natural products.

A ruthenium-catalyzed alkenylation–annulation approach for the synthesis of indazole derivatives via C–H bond activation

A new approach to indazole derivatives through a Ru(ii)-catalyzed C–H activation–annulation reaction, which proceeds via C–C and C–N bond forming reactions, is reported.

One-pot two-step synthesis of 3-iodo-4-aryloxy coumarins and their Pd/C-catalyzed annulation to coumestans

An efficient protocol for the synthesis of various coumestans from the intramolecular annulation of 3-iodo-4-aryloxy coumarins through C–H activation has been developed.

Synthesis of pyrrolo[3,4-c]quinoline-1,3-diones: a sequential oxidative annulation followed by dehydrogenation and N-demethylation strategy

A synthetic strategy for pyrrolo[3,4-c]quinoline-1,3-diones via oxidative annulation followed by DDQ mediated dehydrogenation and N-demethylation.

2,3-Diaroyl benzofurans from arynes: sequential synthesis of 2-aroyl benzofurans followed by benzoylation

A cascade synthetic strategy for the direct synthesis of 2-aroyl benzofurans from aryne precursors has been described. The synthesized 2-aroyl benzofurans were further benzoylated for the synthesis of 2,3-diaroyl benzofurans.

Mechanistic studies on the palladium-catalyzed cross-dehydrogenative coupling of 4-phenoxy-2-coumarins: experimental and computational insights

Computational and experimental studies suggest that intramolecular dehydrogenative coupling of 4-phenoxy-2-coumarins proceeds via two concerted metallation-deprotonation (CMD) events.