Cobalt(III)-catalyzed synthesis of indazoles and furans by C-H bond functionalization/addition/cyclization cascades

Innovative cascade reaction for 2H-indazole derivative synthesis

In the realm of synthetic organic chemistry, by using a one-pot sequential combination of MCR, it is possible to manufacture chemical commodities (fine chemicals, agrochemicals, and pharmaceutical substances) that enhance our quality of life while generating less waste materials and increasing economic advantages. With this motivation, using a "one-pot" method with multiple components, we present a relatively simple way to make stereoselective substitute 2H-indazole analogues for this study. Firstly, functionalised 3-bromo-4-((methylthio)methyl) derivatives were produced using DMSO as both a carbon source and a solvent, in conjunction with TMSOTf as the Lewis acid promoter. These derivatives were then utilised in the synthesis of 2-H-indazole derivatives with an up to 80% yield using t-Bu3PHBF4 as the ligand and Cs2CO3 as the base, in the presence of a Pd catalyst at 100°C in an airtight tube. The phenyl ring is endowed with an electron-releasing group situated at position C-6, which efficiently synthesises several 2-H-indazol derivatives with cost-efficient and noteworthy yields by using this method. A comparative analysis of a number of halogen derivatives was also undertaken, using a variety of solvents that were classified according to their halogen group. To confirm the structures of the synthesised target compounds, spectrometric analysis (1H NMR, 13C NMR, and LCMS) was performed.

Ru(II)-Catalyzed Divergent C-H Alkynylation Cascade with Bifunctional α-Alcohol Haloalkynes

Native functionality directed the C-H activation cascade to enable rapid construction of molecular complexity, featuring step-economy and synthetic efficiency. Herein, by exploiting bifunctional α-alcohol haloalkynes, we developed Ru(II)-catalyzed carboxylic acid, amine, and amide assisted divergent C-H alkynylation and annulation cascade, affording polyfunctional heterocycles. Significantly, a bilateral aryl C-H polycyclization cascade of azobenzenes was achieved using the versatile haloalkynes.

Cobalt-Catalyzed Enantio- and Regioselective C(sp3 )-H Alkenylation of Thioamides

An enantioselective cobalt-catalyzed C(sp3 )-H alkenylation of thioamides with but-2-ynoate ester coupling partners employing thioamide directing groups is presented. The method is operationally simple and requires only mild reaction conditions, while providing alkenylated products as single regioisomers in excellent yields (up to 85 %) and high enantiomeric excess [up to 91 : 9 enantiomeric ratio (er), or up to >99 : 1 er after a single recrystallization]. Diverse downstream derivatizations of the products are demonstrated, delivering a range of enantioenriched constructs. Extensive computational studies using density functional theory provide insight into the detailed reaction mechanism, origin of enantiocontrol, and the unusual regioselectivity of the alkenylation reaction.

Visible Light-Mediated Heterodifunctionalization of Alkynylazobenzenes for 2H-Indazole Synthesis

We disclose the heterodifunctionalization of alkynylazobenzenes promoted exclusively by visible light in the absence of any transition metal and/or photocatalyst. This reaction features excellent regioselectivity on a broad variety of substrates with perfect atom economy. Alcohols, carboxylic acids, thiols, amides, heterocycles, and even water are suitable substrates for the promotion of the oxyamination, sulfenoamination, and diamination reactions. In this manner, biologically active indazole scaffolds can be rapidly assembled from alkyne feedstocks.

Indazolin-3-ylidenes (Indy*): easily accessible, sterically-hindered indazole-derived N-heterocyclic carbenes and their application in gold catalysis

Sterically-hindered N-heterocyclic carbenes (NHCs) with functionalized N-wingtips are a pivotal class of ligands in organic synthesis. Herein, we report the first class of sterically-hindered N-heterocyclic carbenes based on the indazole framework. These ligands combine the strong σ-donation of the carbene center due to the carbene placement at the C3-indazole position with the sterically-hindered and flexible N-substitution with the versatile 2,6-bis(diphenylmethyl)aryl moiety that extends beyond the metal centre for the first time in non-classical N-heterocyclic carbenes. The ligands are readily accessible by the rare Cadogan indazole synthesis of sterically-hindered N-aryl-1-(2-nitrophenyl)methanimines. Steric and electronic characterization as well as catalytic studies in the synthesis of oxazolines are described. Considering the unique properties of indazole-derived carbenes, we anticipate that this class of compounds will find broad application in organic synthesis and catalysis.

Co(III)-Catalyzed C6-Selective C-H Activation/Pyridine Migration of 2-Pyridones with Propiolates

A versatile Co(III)-catalyzed C6-selective C-H activation/pyridine migration of 2-pyridones with available propiolates as coupling partners was demonstrated. This method features high atom economy, excellent regioselectivity, and good functional group tolerance by employing an inexpensive Co(III) catalyst under mild reaction conditions. Moreover, gram-scale synthesis and late-stage modifications of pharmaceuticals were performed to prove the effectiveness of these synthetic approaches.

Site-Selective C8-Alkylation of Quinolines with Cyclopropanols: Merging C-H/C-C Bond Activation

The site-selective C8-alkylation of quinolines has been accomplished using cyclopropyl alcohols as the alkylating agents and N-oxide as a weak chelating group in the presence of Co(III) catalysis via merging C-H/C-C bond activation. The use of cyclopropanol as the alkyl source, Co catalysis, substrate scope, HRMS analysis of the reaction intermediate, and late-stage mutation of drug molecules/natural products are the important practical features.

A Scalable and Metal-Free Synthesis of Indazoles from 2-Aminophenones and In Situ Generated De-Boc-Protected O-Mesitylsulfonyl Hydroxylamine Derivatives

A one-pot metal-free protocol to access indazoles from easily available 2-aminophenones and hydroxylamine derivatives has been achieved. The reaction is operationally simple, mild, and insensitive to air and moisture. A broad range of indazoles were prepared in good to excellent yield (up to 97% yield), and the reaction displayed a broad functional group tolerance. The reaction was performed at gram scale, and its synthetic application was exhibited through the rapid and efficient preparation of bioactive molecule YC-3 and FDA-approved drug axitinib.

Metal-Free Synthesis of 2H-Indazole Skeletons by Photochemistry or Thermochemistry

A simple and tuned synthesis of a 2H-indazole skeleton under metal-free conditions was developed. Under visible-light irradiation at room temperature, 2-((aryl/alkyl/H)ethynyl))aryltriazenes reacted with arylsulfinic acids to afford 3-functionalized 2H-indazoles without extra photocatalyst via an electron donor-acceptor complex. In the presence of arylsulfinic acid, 2-(ethynyl)aryltriazenes underwent an intramolecular oxidation/cyclization to provide 2H-indazole-3-carbaldehydes at 50 °C in air.

Synthesis of 2,4,6-Trisubstituted Pyrimidines through Copper-Catalyzed [4 + 2] Annulation of α,β-Unsaturated Ketoximes with Activated Nitriles

The copper-catalyzed [4 + 2] annulation of α,β-unsaturated ketoximes with activated nitriles for the rapid construction of 2,4,6-trisubstituted pyrimidines in moderate to good yields has been developed. The reaction features synthetic simplicity, good functional group tolerance, and gram-scale applicability. A plausible mechanism is proposed based on mechanistic investigations.

Zn(II)-Catalyzed Multicomponent Sustainable Synthesis of Pyridines in Air

Herein, we report a Zn(II)-catalyzed solvent-free sustainable synthesis of tri- and tetra-substituted pyridines using alcohols as the primary feedstock and NH₄OAc as the nitrogen source. Using a well-defined air-stable Zn(II)-catalyst, 1a, featuring a redox-active tridentate azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^a), a wide variety of unsymmetrical 2,4,6-substituted pyridines were prepared by three-component coupling of primary and secondary alcohols with NH₄OAc. Catalyst 1a is equally compatible with the four-component coupling. Unsymmetrical 2,4,6-substituted pyridines were also prepared via a four-component coupling of a primary alcohol with two different secondary alcohols and NH₄OAc. A series of tetra-substituted pyridines were prepared up to 67% yield by coupling primary and secondary alcohols with 1-phenylpropan-1-one or 1,2-diphenylethan-1-one and NH₄OAc. The 1a-catalyzed reactions also proceeded efficiently upon replacing the secondary alcohols with the corresponding ketones, producing the desired tri- and tetra-substituted pyridines in higher yields in a shorter reaction time. A few control experiments were performed to unveil the mechanistic aspects, which indicates that the active participation of the aryl-azo ligand during catalysis enables the Zn(II)-complex to act as an efficient catalyst for the present multicomponent reactions. Aerial oxygen acts as an oxidant during the Zn(II)-catalyzed dehydrogenation of alcohols, producing H₂O and H₂O₂ as byproducts.

Photoinduced Cobalt-Catalyzed Desymmetrization of Dialdehydes to Access Axial Chirality

Enantioselective metallaphotoredox catalysis, which combines photoredox catalysis and asymmetric transition-metal catalysis, has become an effective approach to achieve stereoconvergence under mild conditions. Although many impressive synthetic approaches have been developed to access central chirality, the construction of axial chirality by metallaphotoredox catalysis still remains underexplored. Herein, we report two visible light-induced cobalt-catalyzed asymmetric reductive couplings of biaryl dialdehydes to synthesize axially chiral aldehydes (60 examples, up to 98% yield, >19:1 dr, and >99% ee). This protocol shows good functional group tolerance, broad substrate scope, and excellent diastereo- and enantioselectivity.

Cobalt(III)-catalyzed asymmetric ring-opening of 7-oxabenzonorbornadienes via indole C-H functionalization

Asymmetric ring-opening of 7-oxabenzonorbornadienes is achieved via Co-catalyzed indole C-H functionalization. The utilization of chiral Co-catalyst consisting of a binaphthyl-derived trisubstituted cyclopentadienyl ligand resulted in high yields (up to 99%) and excellent enantioselectivity (>99% ee) for the target products with tolerance for diverse functional groups. Opposite diastereoselectivities are obtained with chiral Co-catalyst or Cp*CoI₂CO. Combined experimental and computational studies suggest β-oxygen elimination being the selectivity-determining step of the reaction. Meanwhile, the reactions of 7-azabenzonorbornadiene could also be executed in a diastereodivergent manner.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Three-component carboformylation: α-quaternary aldehyde synthesis via Co(iii)-catalysed sequential C-H bond addition to dienes and acetic formic anhydride

All carbon α-quaternary aldehydes are prepared via Co(iii)-catalysed sequential C-H bond addition to dienes and acetic formic anhydride, representing a rare example of intermolecular carboformylation. A wide range of internally substituted dienes containing diverse functionality can be employed in this reaction, affording complex α-quaternary aldehydes that would not be accessible via hydroformylation approaches. Mechanistic investigations, including control reactions and deuterium labeling studies, establish a catalytic cycle that accounts for formyl group introduction with an uncommon 1,3-addition selectivity to the conjugated diene. Investigations into the role of the uniquely effective additive Proton Sponge® were also conducted, leading to the observation of a putative, intermediate Co(i) tetramethylfulvene complex at low temperatures via NMR spectroscopy. The synthetic utility of the aldehyde products is demonstrated by various transformations, including proline-catalysed asymmetric aldol addition, reductive amination, and the asymmetric synthesis of amines using tert-butanesulfinamide technology.

Synthesis of Furans via Rhodium(III)-Catalyzed Cyclization of Acrylic Acids with α-Diazocarbonyl Compounds

An efficient protocol for the synthesis of furans through Rh(III)-catalyzed vinyl C-H activation from acrylic acids and α-diazocarbonyl compounds has been developed. The reaction features broad functional group tolerance and affords a series of furans in moderate to good yields. Moreover, no additives such as copper or silver salts are required. Some control experiments are performed to give insight into the mechanism of this cascade transformation and the decarbonylation process is involved in the formation of the furan product.

Recent Strategies in Transition-Metal-Catalyzed Sequential C–H Activation/Annulation for One-Step Construction of Functionalized Indazole Derivatives

Designing new synthetic strategies for indazoles is a prominent topic in contemporary research. The transition-metal-catalyzed C–H activation/annulation sequence has arisen as a favorable tool to construct functionalized indazole derivatives with improved tolerance in medicinal applications, functional flexibility, and structural complexity. In the current review article, we aim to outline and summarize the most common synthetic protocols to use in the synthesis of target indazoles via a transition-metal-catalyzed C–H activation/annulation sequence for the one-step synthesis of functionalized indazole derivatives. We categorized the text according to the metal salts used in the reactions. Some metal salts were used as catalysts, and others may have been used as oxidants and/or for the activation of precatalysts. The roles of some metal salts in the corresponding reaction mechanisms have not been identified. It can be expected that the current synopsis will provide accessible practical guidance to colleagues interested in the subject.

C-H bond activation and sequential addition to two different coupling partners: a versatile approach to molecular complexity

Sequential multicomponent C-H bond addition is a powerful approach for the rapid, modular generation of molecular complexity in a single reaction. In this approach, C-H bonds are typically added across π-bonds or π-bond isosteres, followed by subsequent coupling to another type of functionality, thereby forming two σ-bonds in a single reaction sequence. Many sequential C-H bond addition reactions have been developed to date, including additions across both conjugated and isolated π-systems followed by coupling with reactants such as carbonyl compounds, cyanating reagents, aminating reagents, halogenating reagents, oxygenating reagents, and alkylating reagents. These atom-economical reactions transform ubiquitous C-H bonds under mild conditions to more complex structures with a high level of regiochemical and stereochemical control. Surprising connectivities and diverse mechanisms have been elucidated in the development of these reactions. Given the large number of possible combinations of coupling partners, there are enormous opportunities for the discovery of new sequential C-H bond addition reactions.

Synthesis of Pyranones: Ru-Catalyzed Cascade Reaction via Vinylic C-H Addition to Glyoxylate

The efficient synthesis of pyranones is presented by a three-component cascade reaction from readily available acrylic acids, ethyl glyoxylate, and p-toluenesulfonamide under ruthenium catalysis. For the first time, the nucleophilic addition of the vinylic C-H bond of acrylic acids across aldehyde is achieved, and the intramolecular cyclization as well as subsequent second insertion to aldehyde form the substituted butenolides. The elimination of sulfonamides occurs at higher temperature to give the pyranones.

Stereoselective Synthesis of Allenyl Alcohols by Cobalt(III)-Catalyzed Sequential C-H Bond Addition to 1,3-Enynes and Aldehydes

An efficient and stereoselective CoIII -catalyzed sequential C-H bond addition to 1,3-enynes and aldehydes is disclosed. This transformation represents the first example of sequential C-H bond additions to 1,3-enynes and a second coupling partner and provides the first example of preparing allenes by C-H bond addition to 1,3-enynes. A wide range of aldehydes, C-H bond substrates and 1,3-enynes with large substituents on the alkynes are effective substrates. The allenyl alcohol products can be further converted to dihydrofurans with high stereoselectivity either in situ or under Ag-mediated cyclization conditions. The allenyl silyl group can also be transferred to the adjacent alcohol by a Brook rearrangement. Moreover, a mechanism for the transformation is proposed supported by X-ray structural characterization of a cobaltacycle intermediate.

Copper-catalyzed [4+2] oxidative annulation of α,β-unsaturated ketoxime acetates with ethyl trifluoropyruvate

A novel route for tandem C-N/C-O formation via copper-catalyzed [4+2] oxidative annulation of α,β-unsaturated ketoxime acetates with ethyl trifluoropyruvate to synthesize valuable trifluoromethyl-containing 2H-1,3-oxazines in moderate to good yields is developed. This procedure represents the first [4+2] oxidative annulation of oxime derivatives with activated CO bonds and provides an alternative route towards functionalized 2H-1,3-oxazines.

Photoassisted Cobalt-Catalyzed Asymmetric Reductive Grignard-Type Addition of Aryl Iodides

Grignard addition is one of the most important methods used for syntheses of alcohol compounds and has been known for over a hundred years. However, research on asymmetric catalysis relies on the use of organometallic nucleophiles. Here, we report the first visible-light-induced cobalt-catalyzed asymmetric reductive Grignard-type addition for synthesizing chiral benzyl alcohols (>50 examples, up to 99% yield, and 99% ee). This methodology has the advantages of mild reaction conditions, good functionality tolerance, excellent enantiocontrol, the avoidance of mass metal wastes, and the use of precious metal catalysts. Kinetic realization studies suggested that migratory insertion of an aryl cobalt species into the aldehyde was the rate-determining step of the reductive addition reaction.

Rh(III)-Catalyzed Synthesis of Indazolo[2,3-a]quinolines: Vinylene Carbonate as C1 and C2 Building Blocks

A rhodium-catalyzed cyclization of azobenzenes and vinylene carbonate via C-H bond activation to construct indazolo[2,3-a]quinolines has been developed. This protocol offers an efficient method for synthesis of the titled products in good yields with broad functional group tolerance. In this reaction, three C-C bonds and C-N bond are formed in one pot, and vinylene carbonate (VC) acts as C1 and C2 synthons as well as "vinylene transfer" agent and acylation reagent in the construction of target-fused heterocycles. Moreover, the products exhibit favorable fluorescence properties, which indicate their potential application as fluorescent materials and biosensors.

Looking deep into C-H functionalization: the synthesis and application of cyclopentadienyl and related metal catalysts

Metal catalyzed C-H functionalization offers a versatile platform for methodology development and a wide variety of reactions now exist for the chemo- and site-selective functionalization of organic molecules. Cyclopentadienyl-metal (CpM) complexes of transition metals and their correlative analogues have found widespread application in this area, and herein we highlight several key applications of commonly used transition-metal Cp-type catalysts. In addition, an understanding of transition metal Cp-type catalyst synthesis is important, particularly where modifications to the catalyst structure are required for different applications, and a summary of this aspect is given.

Synthesis of 3C-alkylated active methylene substituted 2H-indazole derivatives via sequential ring opening of donor–acceptor cyclopropanes and reductive cyclization reaction

A facile synthesis of 3C-alkylated active methylene substituted 2H-indazoles from o-nitrocyclopropanes and primary arylamines catalyzed by Ni(ClO4)2.6H2O/SnCl2.2H2O is reported.

The regioselectivity of the sulfonylation of tetrazoles: a theoretical view

DFT calculations were performed to reveal the regioselectivity for the sulfonylation of tetrazoles.

Cobalt(III)-catalyzed C-H amidation of N,N-dialkyl thiobenzamides by sulfur coordination

An efficient inexpensive cobalt(III)-catalyzed intermolecular amidation of N,N-dialkyl thiobenzamides with 1,4,2-dioxazol-5-ones via C-H bond activation is described. The reaction proceeds with high functional group tolerance under external oxidant free conditions, providing a straightforward approach for the direct modification of thioamide derivatives, which are prevalent organic motifs found in vital biological and pharmaceutical molecules.

Cobalt and Iron Stabilized Ketyl, Ketiminyl and Aldiminyl Radical Anions

Carbonyl and iminyl based radical anions are reactive intermediates in a variety of transformations in organic synthesis. Herein, the isolation of ketyl, and more importantly unprecedented ketiminyl and aldiminyl radical anions coordinated to cobalt and iron complexes is presented. Insights into the electronic structure of these unusual metal bound radical anions is provided by X-Ray diffraction analysis, NMR, IR, UV/Vis and Mössbauer spectroscopy, solid and solution state magnetometry, as well as a by a detailed computational analysis. The metal bound radical anions are very reactive and facilitate the activation of intra- and intermolecular C-H bonds.

Cobalt-Catalyzed Vinylic C-H Addition to Formaldehyde: Synthesis of Butenolides from Acrylic Acids and HCHO

A carboxyl-assisted C-H functionalization of acrylic acids with formaldehyde to give butenolides is described. It is the first time that the addition of an inert vinylic C-H bond to formaldehyde has been achieved via cobalt-catalyzed C-H activation. The unique reactivity of the cobalt species was observed when compared with related Rh or Ir catalysts. γ-Hydroxymethylated butenolides were produced by the treatment of Na₂CO₃ after the catalytic reaction in one pot.

C-H bond functionalization by high-valent cobalt catalysis: current progress, challenges and future perspectives

Over the last decade, high-valent cobalt catalysis has earned a place in the spotlight as a valuable tool for C-H activation and functionalization. Since the discovery of its unique reactivity, more and more attention has been directed towards the utilization of cobalt as an alternative to noble metal catalysts. In particular, Cp*Co(III) complexes, as well as simple Co(II) and Co(III) salts in combination with bidentate chelation assistance, have been extensively used for the development of novel transformations. In this review, we have demonstrated the existing trends in the C-H functionalization methodology using high-valent cobalt catalysis and highlighted the main challenges to overcome, as well as perspective directions, which need to be further developed in the future.

The copper-catalyzed synthesis of dihydrooxazoles from α,β-unsaturated ketoximes and activated ketones

The first copper-catalyzed [3+2]-type condensation reaction of α,β-unsaturated ketoximes with activated ketones has been described for the synthesis of dihydrooxazoles, especially trifluoromethyl-decorated dihydrooxazoles. Notable features of this method include its broad substrate scope, good functional group tolerance, and simple operation.

Cp*Co(III)-catalyzed C-H amination/annulation cascade of sulfoxonium ylides with anthranils for the synthesis of indoloindolones

Cp*Co(iii)-catalyzed [4+1] annulation of sulfoxonium ylides with anthranils has been developed for the synthesis of indole-indolone scaffolds. The dual functionality of anthranils was exploited, wherein the nitrogen has been used for C-H amination and the aldehyde group was utilized in the subsequent intramolecular aldol condensation to furnish the corresponding annulated products.

Synthesis of (2H)-Indazoles and Dihydrocinnolinones through Annulation of Azobenzenes with Vinylene Carbonate under Rh(III) Catalysis

The Rh(III)-catalyzed C-H functionalization and subsequent intramolecular cyclization between azobenzenes and vinylene carbonate is described herein. Depending on the electronic property of azobenzenes, this transformation results in the formation of (2H)-indazoles or dihydrocinnolin-4-ones through the generation of ortho-alkylated azo-intermediates followed by decarboxylation. Surprisingly, vinylene carbonate acts as an acetaldehyde or acetyl surrogate to enable the [4 + 1] or [4 + 2] annulation reaction. This transformation is characterized by its mild reaction conditions, simplicity, and excellent functional group compatibility.

Cp*CoIII-Catalyzed C(7)-H Bond Annulation of Indolines with Alkynes

An efficient protocol for the synthesis of biologically essential pyrroloquinolinones has been developed under Cp*Co^III catalysis, which involves a cascade reaction of C(7)-H alkenylation with alkynes followed by nucleophilic addition. A wide variety of internal alkynes including enyne, diyne, and ynamide and more challenging terminal alkynes were successfully employed for the annulation in good to excellent yield with high regioselectivity.