Aerobic Catalytic Cross-Dehydrogenative Coupling of Furans with Indoles Provides Access to Fluorophores with Large Stokes Shift

Sustainability in chemical processes is a crucial aspect in contemporary chemistry with sustainable catalysis as a vital parameter of the same. There has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions. Furan is a versatile heterocycle of natural origin used for multiple applications. However, it has scarcely been used in cross-dehydrogenative coupling. In this work, we have explored the cross-dehydrogentive coupling of furans with indoles using commonly available, inexpensive FeCl3  ⋅ 6H2 O (<0.25 $/g) as catalyst in the presence of so called 'ultimate oxidant' - oxygen, without the need for any external ligand or additive. The reactions were found to be scalable and to work even under partially aqueous conditions. This makes the reaction highly economical, practical, operationally simple and sustainable. The methodology provides direct access to π-conjugated short oligomers consisting of furan, thiophene and indole. These compounds were found to show interesting fluorescence properties with remarkably large Stokes shift (up to 205 nm). Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by nucleophilic trapping by furan.

Cu(ii)-catalyzed aerobic oxidative coupling of furans with indoles enables expeditious synthesis of indolyl-furans with blue fluorescence

With the purpose of incorporating sustainability in chemical processes, there has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions and processes. In this work, we have explored the atom-economic oxidative coupling between two important electron-rich heterocycles - indoles and furans - using commonly available, inexpensive metal catalyst CuCl2·2H2O (<0.25$ per g) to develop an expeditious synthesis of indolyl-furans. Moreover, the reaction proceeded well in the presence of the so-called 'ultimate oxidant' - air, without the need for any external ligand or additive. The reaction was found to be scalable and to work even under partially aqueous conditions. This makes the methodology highly economical, practical, operationally simple and sustainable. In addition, the methodology provides direct access to novel indole-furan-thiophene (IFT)-based electron-rich π-conjugated systems, which show green-yellow fluorescence with large Stokes shift and high quantum yields. Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by the nucleophilic attack by furan.

Selective Cross-Dehydrogenative Coupling of Various Acyclic Enamides with Heteroarenes via Rh(III)-Catalyzed C-H Activation

The developed methodology describes an efficient Rh(III)-catalyzed oxidative C-H/C-H cross-coupling between acyclic enamides and heteroarenes. This cross dehydrogenative coupling (CDC) reaction offers advantages, including excellent regioselectivity and stereoselectivity, good functional group compatibility, and a broad substrate scope. Mechanistically, Rh(III)-catalyzed β-C(sp2)-H activation of acyclic enamides is proposed to be the critical step.

Copper-catalysed dehydrogenative self-coupling/cyclization of 5-aminopyrazoles: synthesis and photophysical study of pyridazines

An interesting self-coupling/cyclization of 5-aminopyrazoles is revealed, which provides a variety of pyridazine cores in reasonable yields. In this reaction, C(sp²)-C(sp²) and N-N bond formation occurs simultaneously in one reaction vessel. The photophysical properties of the synthesized compounds were also studied and some of them exhibited fluorescence properties with good quantum yields. A radical mediated reaction mechanism is proposed with the help of control experiments.

Transition metal-catalyzed C-H/C-C activation and coupling with 1,3-diyne

This review provides a broad overview of the recent developments in the field of transition metal-catalyzed C-H/C-C bond activation and coupling with 1,3-diyne for assembling alkynylated heterocycles, bis-heterocycles, and 1,3-enynes. Transition metal-catalyzed inert bond (C-H/C-C) activation has been the focus of attention among synthetic chemists in recent times. Enormous developments have taken place in C-H/C-C bond activation chemistry in the last two decades. In recent years the use of 2π-unsaturated units as coupling partners for the synthesis of heterocycles through C-H/C-C bond activation and annulation sequence has received immense attention. Among the unsaturated units employed for assembling heterocycles, the use of 1,3-diynes has garnered significant attention due to its ability to render bis-heterocycles in a straightforward manner. The C-H bond activation and coupling with 1,3-diyne has been very much explored in recent years. However, the development of strategies for the use of 1,3-diynes in the analogous C-C bond activation chemistry is less explored. Earlier methods employed to assemble bis-heterocycle used heterocycles that were preformed and pre-functionalized via transition metal-catalyzed coupling reactions. The expensive pre-functionalized halo-heterocycles and sensitive and expensive heterocyclic metal reagents limit its broad application. However, the transition metal-catalyzed C-H activation obviates the need for expensive heterocyclic metal reagents and pre-functionalized halo-heterocycles. The C-H bond activation strategy makes use of C-H bonds as functional groups for effecting the transformation. This renders the overall synthetic sequence both step and cost economic. Hence, this strategy of C-H activation and subsequent reaction with 1,3-diyne could be used for the larger-scale synthesis of chemicals in the pharmaceutical industry. Despite these advances, there is still the possibility of exploration of earth-abundant and cost-effective first-row transition metals (Ni, Cu, Mn. Fe, etc.) for the synthesis of bis-heterocycles. Moreover, the Cp*-ligand-free, simple metal-salt-mediated synthesis of bis-heterocycles is also less explored. Thus, more exploration of reaction conditions for the Cp*-free synthesis of bis-heterocycles is called for. We hope this review will inspire scientists to investigate these unexplored domains.

Boron Trifluoride Etherate Promoted Regioselective 3-Acylation of Indoles with Anhydrides

An efficient, high-yielding and scalable procedure for the regioselective 3-acylation of indoles with anhydrides promoted by boron trifluoride etherate under mild conditions was reported. This novel protocol provided a simple way to prepare 3-(benzofuran-2-yl) indole in three steps.

Expedient Ni-catalyzed C-H/C-H cross-dehydrogenative coupling of aryl amides with azoles

A nickel-catalyzed C-H heteroarylation of arenes has been described using a removable oxazoline-aniline derived directing group. Utilization of inexpensive nickel(II)-catalyst, substrate scope, functional group diversity and late-stage functionalization of xanthine-derived...

Synthesis of Unsymmetrical Biheteroarenes via Dehydrogenative and Decarboxylative Coupling: a Decade Update

The design and development of robust and efficient methods for installing one heterocycle with another is endowed as a ubiquitous and powerful synthetic strategy to access complex organic biheterocycles in recent days due to their pervasive applications in medicinal as well as material chemistry. This perspective presents an overview on the recent findings and developments for the synthesis of unsymmetrical biheteroarenes via dehydrogenative and decarboxylative couplings with literature coverage mainly extending from 2011 to 2021. For simplification of the readers, the article has been subcategorized based on the catalysts used in the reactions.

Recent advances in rhodium-catalyzed C(sp2)-H (hetero)arylation

Arylation is a common behaviour in organic synthesis for the construction of complex structures, especially the biaryls. Among those reported arylation procedures, transition-metal-catalyzed direct C(sp²)-H arylation has been rapidly developed in recent decades and has become a reliable alternative to traditional cross-coupling procedures using organometallic reagents. Great achievements in rhodium-catalyzed C(sp²)-H arylation have been witnessed during the last decade. Aryl halides, simple arenes, aryl boronic acids, arylsilanes, aryl aldehyde, aryl carboxylic acid, diazides, etc. were successfully utilized as arylating reagents under rhodium-catalyzed conditions. In this review, recent achievements in rhodium-catalyzed arylations through C(sp²)-H bond activation were summarized together with the mechanism discussions.

Metalated Porous Phenanthroline-Based Polymers as Efficient Heterogeneous Catalysts for Regioselective C-H Activation of Heteroarenes

Direct C-H bond activation of heterocycles as a step-economical and environmentally friendly approach to build the heterobiaryls motifs is highly attractive, but it still has a challenge to design and prepare a cheap and regioselective heterogeneous catalyst. To tackle this challenge, we have introduced Ni species into a porous phenanthroline-based organic polymer donated as POP-Phen@Ni. This heterogeneous catalyst shows excellent catalytic performances in regioselective C-H activation of heterocycles, even better than those of the corresponding homogenous catalyst. H/D exchange experiments show that the lithium bis(trimethylsilyl)amide (LiHMDS), a base added in the reaction, play a very important role during the reaction processes. We believe that this heterogeneous catalyst would open a new door for design of heterogeneous catalysts to efficiently catalyze the regioselective C-H activation of heterocycles.

RhIII -Catalyzed C6-Selective Oxidative C-H/C-H Crosscoupling of 2-Pyridones with Thiophenes

A rhodium(III)-catalyzed C6-selective dehydrogenative cross-coupling of 2-pyridones with thiophenes was developed for the synthesis of 6-thiophenyl pyridin-2(1H)-one derivatives. In this reaction, the excellent site selectivity was controlled by the 2-pyridyl directing group on the nitrogen of the pyridone ring. Control experiments indicated that the N-pyridyl was essential for the transformation. To the best of our knowledge, this procedure is the first successful example of the direct C6 heteroarylation of 2-pyridones with electron-rich thiophene derivatives. 4-Pyridone was also used as substrate to generate the corresponding C2 heteroarylated product. Moreover, this pyridyl directing group was readily removable to generate the biheteroaryl structures with a free N-H group.

Construction of 2-(2-Arylphenyl)azoles via Cobalt-Catalyzed C-H/C-H Cross-Coupling Reactions and Evaluation of Their Antifungal Activity

Although compounds with a 2-(2-arylphenyl) benzoxazole motif are biologically important, there are only a few methods for synthesizing them. Herein, we report an efficient method for synthesis of such compounds by means of cobalt-catalyzed C-H/C-H cross-coupling reactions. This method has a broad substrate scope and good tolerance for sensitive functional groups. In addition, we demonstrate that introducing a heteroarene moiety to biphenyl compounds enhanced their antifungal activity.

Copper(I)/DDQ-Mediated Double-Dehydrogenative Diels-Alder Reaction of Aryl Butenes with 1,4-Diketones and Indolones

A copper(I)/DDQ-mediated double-dehydrogenative Diels-Alder (DDDA) reaction of simple butenes with 1,4-diketones and indolones has been established for the first time. This strategy is based on a tandem double-dehydrogenation/Diels-Alder reaction from nonprefunctionalized starting materials, in which both a diene and dienophile were in situ generated via activation of fourfold inert C(sp³)-H bonds in one catalytic system.

Rhodium-Catalyzed Atroposelective Oxidative C-H/C-H Cross-Coupling Reaction of 1-Aryl Isoquinoline Derivatives with Electron-Rich Heteroarenes

Rhodium(III)-catalyzed enantioselective oxidative C-H/C-H cross-coupling reaction between two arenes is disclosed. With the combination of a chiral CpRh(III) complex and a chiral carboxylic acid additive, the direct coupling reactions between 1-aryl isoquinoline derivatives and electron-rich heteroarenes such as thiophenes, furans, benzothiophenes, and benzofurans are realized via a double C-H functionalization process. A series of axially chiral compounds are obtained in excellent yields and enantioselectivities (up to 99% yield and 99% ee). Mechanistic studies suggest that both C-H bond cleavages may not be the turnover-limiting step.

Indium controlled regioselective 1,4-alkylarylation of 1,3-dienes with α-carbonyl alkyl bromides and N-heterocycles

A selective indium-promoted silver-mediated intermolecular oxidative 1,4-alkylarylation of 1,3-dienes with α-carbonyl alkyl bromides and N-heterocycles was reported.

Oligothiophene Synthesis by a General C-H Activation Mechanism: Electrophilic Concerted Metalation-Deprotonation (eCMD)

Oxidative C-H/C-H coupling is a promising synthetic route for the streamlined construction of conjugated organic materials for optoelectronic applications. Broader adoption of these methods is nevertheless hindered by the need for catalysts that excel in forging core semiconductor motifs, such as ubiquitous oligothiophenes, with high efficiency in the absence of metal reagents. We report a (thioether)Pd-catalyzed oxidative coupling method for the rapid assembly of both privileged oligothiophenes and challenging hindered cases, even at low catalyst loading under Ag- and Cu-free conditions. A combined experimental and computational mechanistic study was undertaken to understand how a simple thioether ligand, MeS(CH₂)₃SO₃Na, leads to such potent reactivity toward electron-rich substrates. The consensus from these data is that a concerted, base-assisted C-H cleavage transition state is operative, but thioether coordination to Pd is associated with decreased synchronicity (bond formation exceeding bond breaking) versus the "standard" concerted metalation-deprotonation (CMD) model that was formalized by Fagnou in direct arylation reaction. Enhanced positive charge build-up on the substrate results from this perturbation, which rationalizes experimental trends strongly favoring π-basic sites. The term electrophilic CMD (eCMD) is introduced to distinguish this mechanism from the standard model, even though both mechanisms locate in a broad concerted continuum. More O'Ferrall-Jencks analysis further suggests eCMD should be a general mechanism manifested by many metal complexes. A preliminary classification of complexes into those favoring eCMD or standard CMD is proposed, which should be informative for studies toward tunable catalyst-controlled reactivity.

4,6-Dihydroxysalicylic Acid-Catalyzed Oxidative Condensation of Benzylic Amines and Aromatic Ketones for the Preparation of 2,4,6-Trisubstituted Pyridines and Its Application to Metal-Free Synthesis of G-Quadruplex Binding Ligands

4,6-Dihydroxysalicylic acid was activated under air to catalyze the one-pot oxidative condensation reaction of benzylamines with acetophenones in the presence of BF₃·Et₂O, affording 2,4,6-trisubstituted pyridines in yields of 59-91%. During this metal-free oxidative condensation reaction, the benzylamines not only provided the aryl moiety at the 4-position of the pyridines but also acted as the nitrogen donor. This method can be applied to the metal-free synthesis of G-quadruplex binding ligands by the sequential addition of 4-chlorobutyryl chloride and pyrrolidine to the reaction system of the 2,4,6-trisubstituted pyridine synthesis.

Rhodium-Catalyzed C(sp2 )- or C(sp3 )-H Bond Functionalization Assisted by Removable Directing Groups

In recent years, transition-metal-catalyzed C-H activation has become a key strategy in the field of organic synthesis. Rhodium complexes are widely used as catalysts in a variety of C-H functionalization reactions because of their high reactivity and selectivity. The availability of a number of rhodium complexes in various oxidation states enables diverse reaction patterns to be obtained. Regioselectivity, an important issue in C-H activation chemistry, can be accomplished by using a directing group to assist the reaction. However, to obtain the target functionalized compounds, it is also necessary to use a directing group that can be easily removed. A wide range of directed C-H functionalization reactions catalyzed by rhodium complexes have been reported to date. In this Review, we discuss Rh-catalyzed C-H functionalization reactions that are aided by the use of a removable directing group such as phenol, amine, aldehyde, ketones, ester, acid, sulfonic acid, and N-heteroaromatic derivatives.

Suzuki–Miyaura Coupling Using Monolithic Pd Reactors and Scaling-Up by Series Connection of the Reactors

The space integration of the lithiation of aryl halides, the borylation of aryllithiums, and Suzuki–Miyaura coupling using a Pd catalyst supported by a polymer monolith flow reactor without using an intentionally added base was achieved. To scale up the process, a series connection of the monolith Pd reactor was examined. To suppress the increase in the pressure drop caused by the series connection, a monolith reactor having larger pore sizes was developed by varying the temperature of the monolith preparation. The monolithic Pd reactor having larger pore sizes enabled Suzuki–Miyaura coupling at a higher flow rate because of a lower pressure drop and, therefore, an increase in productivity. The present study indicates that series connection of the reactors with a higher flow rate serves as a good method for increasing the productivity without decreasing the yields.

Direct C–H/C–H cross-coupling of benzimidates with heteroarenes to access biheteroaryl-2-carbonitriles

A rhodium(iii)-catalyzed oxidative C–H/C–H cross-coupling of benzimidates with various heteroarenes has been devloped to directly furnish biheteroaryl-2-carbonitriles.

Rhodium-catalyzed biheteroaryl-2-carbonitrile synthesis via double C–H activation

Rhodium(iii)-catalyzed double C–H activation and in situ dealcoholization to generate biheteroaryl-2-carbonitriles have been developed via a CDC mechanism, in which benzimidates act as both directing groups and the precursors of nitrile groups.

Metal-catalyzed regiodivergent organic reactions

This review discusses metal-catalysed regiodivergent additions, allylic substitutions, CH-activation, cross-couplings and intra- or intermolecular cyclisations.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Alkali metal complexes as efficient catalysts for hydroboration and cyanosilylation of carbonyl compounds

We report here reactions between the N-adamantyliminopyrolyl ligand 2-(AdN[double bond, length as m-dash]CH)-C4H3NH (L-H) and alkali metal hexamethyldisilazides [MN(SiMe3)2] (M = Li, Na and K) to afford the dimeric [{2-(AdN[double bond, length as m-dash]CH)-C4H3NLi(THF)}2] (1), [{2-(AdN[double bond, length as m-dash]CH)-C4H3N}{Na(THF)1.5}2] (2) and polymeric [{2-(AdN[double bond, length as m-dash]CH)-C4H3NK(THF)}n] (3) complexes at ambient temperature. A one-pot reaction between L-H, [KN(SiMe3)2] and alkaline earth metal diiodide (AeI2) in a 2 : 2 : 1 molar ratio, which resulted in the formation of a heteroleptic Ae metal complex [κ2-{2-(AdN[double bond, length as m-dash]CH)-C4H3N}2Ae(THF)2] [Ae = Mg (4), Ca (5)], is also reported. The solid-state structures of complexes 1, 3 and 4 were established through single-crystal X-ray diffraction analysis. The alkali and alkaline earth metal complexes 1-5 were utilised as precatalysts for the catalytic hydroboration of pinacolborane (HBpin) with aldehydes and ketones, and potassium complex 3 was identified as a competent catalyst under mild conditions. Additionally, cyanosilylation of carbonyl compounds was explored with trimethylsilyl cyanide and aldehydes/ketones, using the alkali metal precatalyst 3 under mild conditions. In both catalytic processes, the potassium catalyst 3 exhibited high tolerance towards a number of functional groups.

Rhodium-catalyzed ortho-heteroarylation of phenols: directing group-enabled switching of the electronic bias for heteroaromatic coupling partner

The directed oxidative C-H/C-H cross-coupling reactions between a functionalized arene and a heteroarene typically exhibit an electronic bias for the heteroaromatic coupling partner. Disclosed herein is a conception of directing group enabled-switching of the electronic bias for coupling partner from the electron-deficient to electron-rich heteroarene, demonstrating that the modification of the directing group may match the latent reactivity of heteroarene substrates caused by the distinctly different electronic nature. In this work, we develop a Rh(iii)-catalyzed ortho-heteroarylation of phenols with greatly important electron-rich heteroarenes such as benzothiophene, benzofuran, thiophene, furan and pyrrole via two-fold C-H activation, which presents broad substrate scopes of both phenols and electron-rich heteroarenes and shows the advantage of tolerance of reactive functional groups, especially halogen. This work also provides a new strategy for the construction of π-conjugated furan-fused heteroacenes prevalent in materials science in dramatically simplified procedures, which makes the protocol highly applicable.

Metal-Free Oxidative Cross Coupling of Indoles with Electron-Rich (Hetero)arenes

A new method for the synthesis of bi-heteroaryls is reported, based on the umpolung of indoles with benziodoxol(on)e hypervalent iodine reagents (IndoleBX). The oxidative coupling of IndoleBX with an equimolar amount of electron-rich benzenes, indoles, pyrroles, and thiophenes proceeded under mild transition-metal-free conditions. Functionalized non-symmetrical bi-indolyl heterocycles were accessed efficiently. Introduction of a new type of C2-substituted indole benziodoxole reagents further allowed extending the scope of the reaction to NH unprotected and C3-alkylated indoles. The obtained bi-heterocycles are important building blocks in synthetic and medicinal chemistry, and could be easily transformed into more complex heterocyclic systems.

Iridium-Catalyzed Annulation Reactions of Thiophenes with Carboxylic Acids: Direct Evidence for a Heck-type Pathway

The functionalization of thiophenes is a fundamental and important reaction. Herein, we disclose iridium-catalyzed one-pot annulation reactions of (benzo)thiophenes with (hetero)aromatic or α,β-unsaturated carboxylic acids, which afford thiophene-fused coumarin-type frameworks. Dearomatization reactions of 2-substituted thiophenes with α,β-unsaturated carboxylic acids deliver various thiophene-containing spirocyclic products. The occurrence of two interconnected reactions provides direct evidence for a Heck-type pathway. The mechanistic scenario described herein is distinctly different from the S_E Ar and concerted metalation-protodemetalation (CMD) pathways encountered in the well-described oxidative C-H/C-H cross-coupling reactions of thiophenes with other heteroarenes.

Rhodium-catalyzed C-C coupling reactions via double C-H activation

Various rhodium-catalyzed double C-H activations are reviewed. These powerful strategies have been developed to construct C-C bonds, which might be widely embedded in complex aza-fused heterocycles, polycyclic skeletons and heterocyclic scaffolds. In particular, rhodium(iii) catalysis shows good selectivity and reactivity to functionalize the C-H bond, generating reactive organometallic intermediates in most of the coupling reactions. Generally, intermolecular, intramolecular and multi-component coupling reactions via double C-H activations with or without heteroatom-assisted chelation are discussed in this review.

Indole- and Pyrrole-BX: Bench-Stable Hypervalent Iodine Reagents for Heterocycle Umpolung

The one-step synthesis of the bench-stable hypervalent iodine reagents IndoleBX and PyrroleBX using mild Lewis acid catalyzed conditions is reported. The new reagents are stable up to 150 °C and were applied in the C-H arylation of unactivated arenes using either rhodium or ruthenium catalysts. A broad range of heterocyclic systems of high interest for synthetic and medicinal chemistry was accessed in high yields. The developed C-H functionalization could not be achieved using reported reagents or methods, highlighting the unique reactivity of Indole- and Pyrrole-BX.