Regioselective Palladium-Catalyzed Decarboxylative Cross-Coupling Reaction of Alkenyl Acids with Coumarins: Synthesis of 3-Styrylcoumarin Compounds

Coumarins Synthesis and Transformation via C–H Bond Activation—A Review

For several decades, coumarins have attracted considerable attention due to the fact of their application in diverse fields such as medical science and biomedical research as well as several industrial branches. Recently, many compounds containing the coumarin moiety have been intensively studied, mainly due to the fact of their biological activities such as antitumor, antioxidative, anti-HIV, vasorelaxant, antimicrobial, and anticancer. They are also widely used as fluorescent dyes and probes because of their great structural flexibility and large fluorescent quantum yields. For this reason, numerous attempts have been made to develop new and more practical methods for the synthesis of these compounds. This review aims at providing a comprehensive overview of coumarin synthesis methods by direct C–H bond activation in order to demonstrate the current state-of-the-art methods as well as the current limitations.

Phosphine-Mediated Morita-Baylis-Hillman-Type/Wittig Cascade: Access to E-Configured 3-Styryl- and 3-(Benzopyrrole/furan-2-yl) Quinolinones

A phosphine-mediated, well-designed Morita-Baylis-Hillman-type/Wittig cascade for the rapid assembly of a quinolinone framework from benzaldehyde derivatives is developed for the first time. By rationally combining I₂/NIS-mediated cyclization, biologically relevant 3-(benzopyrrole/furan-2-yl) quinolinones were facilely synthesized in a one-pot process by starting from 3-styryl-quinolinones bearing an o-hydroxy/amino group, significantly expanding the chemical space of this privileged skeleton. Further utility of this protocol is illustrated by successfully performing this transformation in a catalytic manner through in situ reduction of phosphine oxide by phenylsilane.

Bulky selenium ligand stabilized trans-palladium dichloride complexes as catalyst for silver-free decarboxylative coupling of coumarin-3-carboxylic acids

This report describes synthesis of three new trans -palladium dichloride complexes of bulky selenium ligands. These complexes possess a Cl-Pd-Cl rotor spoke attached to a Se-Pd-Se axle. The new ligands and palladium complexes ( C1 - C3 ) were characterized with the help of NMR, HRMS, UV-Vis., IR, and elemental analysis. The single crystal structure of metal complex C2 confirmed a square planer geometry of complex with trans -orientation. The X-ray structure revealed intramolecular secondary interactions (SeCH---Cl) between chlorine of PdCl 2 and CH 2 proton of selenium ligand. Variable temperature NMR data shows coalescence of diastereotopic protons, which indicates pyramidal inversion of selenium atom at elevated temperature. The relaxed potential energy scan of C2 suggests a rotational barrier of ~12.5 kcal/mol for rotation of chlorine atom through Cl-Pd-Cl rotor. The complex C3 possess dual intramolecular secondary interactions (OCH 2 ---Cl and SeCH 2 ---Cl) with stator ligand. Molecular rotor C2 was found to be most efficient catalyst for the decarboxylative Heck-coupling under mild reaction conditions. The protocol is applicable to a broad range of substrates with large functional group tolerance and low catalyst loading (2.5 mol %). The mechanism of decarboxylative Heck-coupling reaction was investigated through experimental and computational studies. Importantly the reaction works under silver-free conditions which reduces the cost of overall protocol. Further, the catalyst also worked for decarboxylative arylation and decarboxylative Suzuki-Miyaura coupling reactions with good yields of the coupled products.

Pd(II)-Catalyzed Fujiwara-Moritani Reactions for the Synthesis and Functionalization of Substituted Coumarins

Highly substituted coumarins, privileged and versatile scaffolds for bioactive natural products and fluorescence imaging, are obtained via a Pd(II)-catalyzed direct C-H alkenylation reaction (Fujiwara-Moritani reaction), which has emerged as a powerful tool for the construction and functionalization of heterocyclic compounds because of its chemical versatility and its environmental advantages. Thus, a selective 6-endo cyclization led to 4-substituted coumarins in moderate yields. Selected examples have been further functionalized in C3 through a second intermolecular C-H alkenylation reaction to give coumarin-acrylate hybrids, whose fluorescence spectra have been measured.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Access to 4-Alkenylated Coumarins via Ruthenium-Catalyzed Olefinic C-H Alkenylation of Coumarins with Modifiable and Removable Directing Groups

The ruthenium-catalyzed activation of the C4 position of coumarins for coupling with acrylates was described using modifiable ketone as a directing group. The alkenylation reaction provided a direct approach to prepare previously inaccessible 4-alkenylated coumarins with operational simplicity and high atom-economy. This protocol also worked well with coumarin-3-carboxylic acids to unveil a rare instance of a tandem alkenylation/decarboxylation reaction. The potential value of this approach was further highlighted by the efficient synthesis of several heterocyclic fused coumarin derivatives.

Metal-free regioselective C-H chalcogenylation of coumarins/(hetero)arenes at ambient temperature

A novel, practical and metal-free approach for the regioselective selenation of coumarins employing (bis(trifluoroacetoxy)iodo)benzene (PIFA) at room temperature is presented. The developed method is suitable for a wide substrate scope and affords 3-selenyl coumarins in good to excellent yields with high selectivity. A radical mechanism is proposed for this new transformation. Furthermore, the application of sulfenylation with coumarines and selenation with other (hetero)arenes in this transformation is successful.

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.

Rhodium(i)-catalyzed C6-selective C-H alkenylation and polyenylation of 2-pyridones with alkenyl and conjugated polyenyl carboxylic acids

A versatile Rh(i)-catalyzed C6-selective decarbonylative C-H alkenylation of 2-pyridones with readily available, and inexpensive alkenyl carboxylic acids has been developed. This directed dehydrogenative cross-coupling reaction affords 6-alkenylated 2-pyridones that would otherwise be difficult to access using conventional C-H functionalization protocols. The reaction occurs with high efficiency and is tolerant of a broad range of functional groups. A wide scope of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, are amenable to this transformation and no addition of external oxidant is required. Mechanistic studies revealed that (1) Boc2O acts as the activator for the in situ transformation of the carboxylic acids into anhydrides before oxidative addition by the Rh catalyst, (2) a decarbonylation step is involved in the catalytic cycle, and (3) the C-H bond cleavage is likely the turnover-limiting step.

An efficient light on–off one-pot method for the synthesis of 3-styryl coumarins from aryl alkynoates

An efficient one-pot stepwise method to synthesize 3-styryl-4-arylcoumarins from simple alkynoates is demonstrated.

A transition-metal-free fast track to flavones and 3-arylcoumarins

A highly regioselective and transition-metal free one-pot arylation of chromenones with arylboronic acids has been achieved employing K₂S₂O₈. The procedure consists of a sequence of some reactions including an arylation/decarboxylation cascade and proceeds well in aqueous media to afford biologically interesting flavones and 3-arylcoumarins. This method exhibited excellent selectivity and functional group tolerance under mild conditions. The reaction also showed perfect efficacy for the preparation of styryl coumarins.

Peroxy mediated Csp2–Csp3 dehydrogenative coupling: regioselective functionalization of coumarins and coumarin-3-carboxylic acids

Regioselective functionalization of coumarins/coumarin carboxylic acids at C-3 via activation of Csp³–H bonds of ethers under metal-free conditions is developed.

Rhodium(i)-catalysed decarbonylative direct C–H vinylation and dienylation of arenes

Rh(i)-Catalyzed decarbonylative direct C–H bond vinylation and dienylation of arenes with acrylic acid and (E)-penta-2,4-dienoic acid have been developed.

Benzocoumarin-Styryl Hybrids: Aggregation and Viscosity Induced Emission Enhancement

Two benzo[h]chromen-3-yl)ethylidene) malononitrile styryl hybrid dyes are synthesized and characterized by NMR and elemental analysis. One is based on nitrogen donor and other on oxygen (3b and 3b respectively). Dyes are low emissive in the solution but dramatically showed increase in emission intensity in aggregates form in the THF (tetrahydrofuran) /water system. Dyes are also sensitive to viscosity and showed increased emission intensity in the DCM:PEG 400 system and DMF:PEG 400 system respectively. Dyes 3a and 3b showed higher viscosity sensitivity constant (0.67 and 0.39 respectively) in DMF:PEG 400 system compared to DCM:PEG 400 (0.47 and 0.21 respectively) system which is contrary to the traditional concept of FMRs. Results shows that lowering of twisted intramolecular charge transfer (TICT) and increase in intramolecular charge transfer (ICT) in the excited state could be the reason for such behavior in the aggregate and highly viscous state. This study may provide the new insights into the field of AIEE and FMR research of such hybrid molecules.

Silver-catalyzed decarboxylative C(sp2)–C(sp3) coupling reactions via a radical mechanism

A silver catalyzed decarboxylative C(sp²)–C(sp³) coupling of vinylic carboxylic acids with alcohols, alkylbenzenes, cycloalkanes and cyclic ethers was developed by using DTBP as an oxidant.

Copper-catalyzed decarboxylative cross-coupling of cinnamic acids and ACCN via single electron transfer

A novel and convenient method for the synthesis of β,γ-unsaturated nitriles using ACCN (1,1′-azobis(cyclohexane-1-carbonitrile)) as a cyano source was described.

Decarboxylative functionalization of cinnamic acids

Decarboxylative functionalization of α,β-unsaturated carboxylic acids is an emerging area that has been developed significantly in recent years.