Cobalt-Catalyzed Cyclization of Aliphatic Amides and Terminal Alkynes with Silver-Cocatalyst

Synthesis of lactones and lactams via C(sp3)-H bond functionalization

The field of directing group-assisted, transition-metal-catalyzed functionalization has undergone a significant transformation, evolving from the use of auxiliary group attachment for the exploitation of native functional groups in novel organic reactions. In particular, coordination-assisted C(sp3)-H bond functionalization has revolutionized synthetic planning to build molecular complexity. Recently, the use of native directing groups in transition-metal-catalyzed reactions has allowed a step-economic process for increased access to biologically important lactones and lactams. Accordingly, lactones and lactams are unavoidable structural motifs with widespread presence in many biological and pharmaceutical arenas, encouraging researchers to access and modify their structures for improved biological properties. In this review, we showcase the diverse aspects of transition metal catalysis, biocatalysis, and photocatalytic C(sp3)-H bond functionalization to access lactones and lactams assisted by carboxylic acid and amines/amides with auxiliary or transient directing groups or unique ligands. This article also emphasizes the role of specially designed complexes, artificial metalloenzymes, and biocatalysts in assembling lactones and lactams. Besides, three-component reactions involving CO as a C1 synthon play a vital role in developing these heterocycles. Importantly, the crucial role of ligands in determining regioselectivity and enhancing enantioselectivity is discussed thoroughly. For better clarity, this review is divided into twelve sections based on the catalysts involved, with subsections categorized by the type of bond activation or formation. Overall, this review aims to inspire the growth of C(sp3)-H bond functionalization, leading to the integration of lactams and lactones in organics.

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.

Alkynyl Radicals, Myths and Realities

This Perspective deals with the organic chemistry of alkynyl radicals, a species that is ultimately still little known in the synthetic community. Starting with the first observations and characterizations of alkynyl radicals generated by various methodologies in the gas phase, we then particularly turned our attention to the implications of these highly reactive intermediates in organic synthesis and materials science. Mechanistic considerations have been provided, in particular, for the key steps of generating alkynyl radicals, which are mainly based on photochemical or thermal activation and single electron transfer processes. This Perspective should serve as a roadmap for the synthetic chemist in order to plan more reliably alkynylation reactions based on alkynyl radicals.

Transition Metal-Free Domino Hydroamination/Isomerization/Transamidation Sequence: An Entry to Trifluorinated γ-Lactams

A method for the construction of trifluorinated-5-methylenepyrrolidinone is reported. This strategy combines an acid-catalyzed two-carbon homologation process between ynamides and aldehydes, providing CF3-substituted dienes followed by a metal-free domino hydroamination/isomerization/transamidation sequence, delivering trifluorinated-5-methylenepyrrolidinone stereoselectively.

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.

Copper-Catalyzed Asymmetric Hydroboration Reaction of Novel Methylene Isoindolinone Compounds through Microwave Irradiation and Their Antileishmanial and Antitoxoplasma Activities

The aim of this study was devoted into molecular docking calculations to discover the potential antileishmania and antitoxoplasma activities of newly synthesized compounds obtained by applying a practical and simple method under microwave irradiation. All these compounds were tested in vitro for their biological activity against Leishmania major promastigotes, amastigotes, and Toxoplasma gondii tachyzoites. Compounds 2a, 5a, and 5e were the most active against both L. major promastigotes and amastigotes, with IC₅₀ values of less than 0.4 μM mL^-1. Compounds 2c, 2e, 2h, and 5d had a strong antitoxoplasma activity of less than 2.1 μM mL^-1 against T. gondii. We can conclude that aromatic methyleneisoindolinones are potently active against both L. major and T. gondii. Further studies for mode of action evaluation are recommended. Compounds 5c and 5b are the best drug candidates for antileishmania and antitoxoplasma due to their SI values being over 13. The docking studies of compounds 2a-h and 5a-e against pteridine reductase 1 and T. gondii enoyl acyl carrier protein reductase reveal that compound 5e may be an effective antileishmanial and antitoxoplasma drug discovery initiative.

Advances on the Merger of Electrochemistry and Transition Metal Catalysis for Organic Synthesis

Synthetic organic electrosynthesis has grown in the past few decades by achieving many valuable transformations for synthetic chemists. Although electrocatalysis has been popular for improving selectivity and efficiency in a wide variety of energy-related applications, in the last two decades, there has been much interest in electrocatalysis to develop conceptually novel transformations, selective functionalization, and sustainable reactions. This review discusses recent advances in the combination of electrochemistry and homogeneous transition-metal catalysis for organic synthesis. The enabling transformations, synthetic applications, and mechanistic studies are presented alongside advantages as well as future directions to address the challenges of metal-catalyzed electrosynthesis.

Transition-Metal-Catalyzed Annulations Involving the Activation of C(sp3 )-H Bonds

The selective functionalization of C(sp3 )-H bonds using transition-metal catalysis is among the more attractive transformations of modern synthetic chemistry. In addition to its inherent atom economy, such reactions open unconventional retrosynthetic pathways that can streamline synthetic processes. However, the activation of intrinsically inert C(sp3 )-H bonds, and the selection among very similar C-H bonds, represent highly challenging goals. In recent years there has been notable progress tackling these issues, especially with regard to the development of intermolecular reactions entailing the formation of C-C and C-heteroatom bonds. Conversely, the assembly of cyclic products from simple acyclic precursors using metal-catalyzed C(sp3 )-H bond activations has been less explored. Only recently has the number of reports on such annulations started to grow. Herein we give an overview of some of the more relevant advances in this exciting topic.

Cobalt-catalyzed direct α-hydroxymethylation of amides with methanol as a C1 source

Herein, we report a cobalt-catalyzed α-hydroxymethylation of amides with methanol under mild conditions. Using CoCl₂·6H₂O as an inexpensive and efficient catalyst, some important bioactive β-hydroxyamides were obtained in moderate to excellent yields. The developed method features a wide substrate scope and good functional group tolerance. In addition, anticholinergic tropicamide was easily synthesized in this way.

Cobalt-catalyzed C(sp3)-H bond functionalization to access indole derivatives

Herein, we develop an efficient method of cobalt-catalyzed C(sp3)-H bond functionalization to synthesize indole derivatives. The highlight of this protocol is accomplished by the sequential C-H activation. This “cobalt/ organic...

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Facile Synthesis of Alkylidene Phthalides by Rhodium-Catalyzed Domino C-H Acylation/Annulation of Benzamides with Aliphatic Carboxylic Acids

The Rh-catalyzed ortho-C(sp² )-H functionalization of 8-aminoquinoline-derived benzamides with aliphatic acyl fluorides generated in situ from the corresponding acids has been developed. This reaction initiated with 8-aminoquinoline-directed ortho-C(sp² )-H acylation, which was accompanied by subsequent intramolecular nucleophilic acyl substitution of amide group to produce alkylidene phthalides This approach exhibits high stereo-selectivity for Z-isomer products, and tolerates a variety of functional groups as well as aliphatic carboxylic acids with diverse structural scaffolds.

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.

Isoindolinone Synthesis via One-Pot Type Transition Metal Catalyzed C-C Bond Forming Reactions

Isoindolinone structure is an important privileged scaffold found in a large variety of naturally occurring as well as synthetic, biologically and pharmaceutically active compounds. Owing to its crucial role in a number of applications, the synthetic methodologies for accessing this heterocyclic skeleton have received significant attention during the past decade. In general, the synthetic strategies can be divided into two categories: First, direct utilization of phthalimides or phthalimidines as starting materials for the synthesis of isoindolinones; and second, construction of the lactam and/or aromatic rings by different catalytic methods, including C-H activation, cross-coupling, carbonylation, condensation, addition and formal cycloaddition reactions. Especially in the last mentioned, utilization of transition metal catalysts provides access to a broad range of substituted isoindolinones. Herein, the recent advances (2010-2020) in transition metal catalyzed synthetic methodologies via formation of new C-C bonds for isoindolinones are reviewed.

Metallaphotoredox-catalyzed C–H activation: regio-selective annulation of allenes with benzamide

We have developed an efficient annulation of benzamides with allenes using cobalt and photoredox dual catalysis under an oxygen atmosphere. The transformation features an alternative strategy for the regeneration of a cobalt catalyst with the aid of Eosin Y.

Cobalt-catalyzed C8–H sulfonylation of 1-naphthylamine derivatives with sodium sulfinates

A facile and efficient protocol for cobalt-catalyzed regioselective C8–H sulfonylation of 1-naphthylamine derivatives with sodium sulfinates was developed to afford sulfonylated naphthylamines in moderate to good yields.

Microfluidic synthesis of pyrrolidin-2-ones via photoinduced organocatalyzed cyclization of styrene, α-bromoalkyl esters and primary amines

A green and efficient reaction route for the synthesis of pyrrolidin-2-ones via photoinduced organocatalyzed three-component cyclization of styrene, tertiary α-bromoalkyl esters and primary amines in a microchannel reactor under visible light conditions has been developed. Moreover, the overall process can be carried out under mild conditions without using a metal. In addition, a reasonable reaction mechanism was proposed based on control experiments.

Direct β- and γ-C(sp3 )-H Alkynylation of Free Carboxylic Acids*

In this study we report the identification of a novel class of ligands for palladium-catalyzed C(sp3 )-H activation that enables the direct alkynylation of free carboxylic acid substrates. In contrast to previous synthetic methods, no introduction/removal of an exogenous directing group is required. A broad scope of acids including both α-quaternary and challenging α-non-quaternary can be used as substrates. Additionally, the alkynylation in the distal γ-position is reported. Finally, this study encompasses preliminary findings on an enantioselective variant of the title transformation as well as synthetic applications of the products obtained.

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.

Chemodivergent reactions

An important strategy for the efficient generation of diversity in molecular structures is the utilization of common starting materials in chemodivergent transformations. The most studied solutions for switching the chemoselectivity rely on the catalyst, ligand, additive, solvent, temperature, time, pressure, pH and even small modifications in the substrate. In this review article several processes have been selected such as inter- and intramolecular cyclizations, including carba-, oxa-, thia- and oxazacyclizations promoted mainly by Brønsted or Lewis acids, transition metals and organocatalysts, as well as radical reactions. Catalyst-controlled intra- and intermolecular cyclizations are mainly described to give five- and six-membered rings. Cycloaddition reactions involving (2+2), (3+2), (3+3), (4+1), (4+2), (5+2), (6+2) and (7+2) processes are useful reactions for the synthesis of cyclic systems using organocatalysts, metal catalysts and Lewis acid-controlled processes. Addition reactions mainly of carba- and heteronucleophiles to unsaturated conjugated substrates can give different adducts via metal catalyst-, Lewis acid- and solvent-dependent processes. Carbonylation reactions of amines and phenols are carried out via ligand-controlled transition metal-catalyzed multicomponent processes. Ring-opening reactions starting mainly from cyclopropanols, cyclopropenols and epoxides or aziridines are applied to the synthesis of acyclic versus cyclic products under catalyst-control mainly by Lewis acids. Chemodivergent reduction reactions are performed using dissolving metals, sodium borohydride or hydrogen transfer conditions under solvent control. Oxidation reactions include molecular oxygen under solvent control or using different dioxiranes, as well as chemodivergent palladium catalyzed cross-coupling reactions using boronic acids are applied to aromatic and allenic compounds. Other chemodivergent reactions such as alkylations and allylations under transition metal catalysis, dimerization of acetylenes, bromination of benzylic substrates, and A³-couplings are performed via catalyst- or reaction condition-dependent processes.

Cobalt-Catalyzed Oxidative C-H Activation: Strategies and Concepts

Inexpensive cobalt-catalyzed oxidative C-H functionalization has emerged as a powerful tool for the construction of C-C and C-Het bonds, which offers unique potential for transformative applications to modern organic synthesis. In the early stage, these transformations typically required stoichiometric and toxic transition metals as sacrificial oxidants; thus, the formation of metal-containing waste was inevitable. In contrast, naturally abundant molecular O₂ has more recently been successfully employed as a green oxidant in cobalt catalysis, thus considerably improving the sustainability of such transformations. Recently, a significant momentum was gained by the use of electricity as a sustainable and environmentally benign redox reagent in cobalt-catalyzed C-H functionalization, thereby preventing the consumption of cost-intensive chemicals while at the same time addressing the considerable safety hazards related to the use of molecular oxygen in combination with flammable organic solvents. Considering the unparalleled potential of the aforementioned approaches for sustainable green synthesis, this Review summarizes the recent progress in cobalt-catalyzed oxidative C-H activation until early 2020.

Mechanistic details of the cobalt-mediated dehydrogenative dimerization of aminoquinoline-directed benzamides

Key mechanistic features of the cobalt-mediated and aminoquinoline-directed dehydrogenative aryl-aryl coupling were investigated computationally and experimentally. A series of CoII and CoIII complexes relevant to the proposed reaction cycle have been synthesized and characterized. Stoichiometric reactions and electrochemical studies were used to probe the role of different additives in the reaction pathway. Computationally, three different mechanisms, such as charge neutral, anionic, and dimetallic were explored. It is shown that the mono-metallic anionic and charge neutral mechanisms are the most favorable ones, among which the former mechanism is slightly more encouraging and proceeds via the: (a) concerted-metalation-deprotonation (CMD) of the first benzamide C-H bond, (b) PivOH-to-PivO- rearrangement, (c) CMD of the second benzamide C-H bond, (d) C-C coupling, (e) product formation facilitated by the amide nitrogen re-protonation, and (f) catalyst regeneration. The rate-determining step of this multi-step process is the C-C coupling step. The computational studies suggest that the electronics of both the aryl-benzamide and pyridine fragments of the aminoquinoline-benzamide ligand control the efficiency of the reaction.

Insights into Cobalta(III/IV/II)-Electrocatalysis: Oxidation-Induced Reductive Elimination for Twofold C-H Activation

The merger of cobalt‐catalyzed C−H activation and electrosynthesis provides new avenues for resource‐economical molecular syntheses, unfortunately their reaction mechanisms remain poorly understood. Herein, we report the identification and full characterization of electrochemically generated high‐valent cobalt(III/IV) complexes as crucial intermediates in electrochemical cobalt‐catalyzed C−H oxygenations. Detailed mechanistic studies provided support for an oxidatively‐induced reductive elimination via highly‐reactive cobalt(IV) intermediates. These key insights set the stage for unprecedented cobaltaelectro two‐fold C−H/C−H activation.

Synthesis of 3-Hydroxymethyl Isoindolinones via Cobalt-Catalyzed C(sp2)-H Carbonylation of Phenylglycinol Derivatives

An efficient method for the synthesis of 3-hydroxymethyl isoindolinones via cobalt-catalyzed C(sp²)-H carbonylation of phenylglycinol derivatives using picolinamide as a traceless directing group is demonstrated. The reaction proceeds in the presence of a commercially available cobalt(II) tetramethylheptanedionate catalyst and employs DIAD as a "CO" surrogate. This synthetic route offers a broad substrate scope, excellent regioselectivity, and full preservation of the original stereochemistry. Besides, the developed method provides a pathway for accessing valuable enantiopure 3-substituted isoindolinone derivatives.

Cobaltaelectro-catalyzed oxidative allene annulation by electro-removable hydrazides

An efficient C-H/N-H functionalization with allenes was enabled via versatile electro-oxidative cobalt catalysis. Thus, electrochemical C-H activations were accomplished with high levels of chemoselectivity and regioselectivity in an operationally simple undivided cell setup. The user-friendly nature of this protocol was highlighted by excellent functional group tolerance, an electro-reductive removable hydrazide directing group and easy scalability. Experimental mechanistic studies were indicative of a facile BIES C-H cobaltation event.