Carbamates: A Directing Group for Selective C-H Amidation and Alkylation under Cp*Co(III) Catalysis

Direct Synthesis of Benzothiazoles and Benzoxazoles from Carboxylic Acids Utilizing (o-CF3PhO)3P as a Coupling Reagent

A general and efficient method for the direct synthesis of benzothiazoles and benzoxazoles from carboxylic acids with 2-aminobenzenethiols or 2-aminophenols using (o-CF3PhO)3P as a simple coupling reagent has been developed. Diverse benzothiazoles and benzoxazoles were synthesized in moderate to excellent yields. And the gram-scale preparation of benzothiazole and benzoxazole also proceeded smoothly under the mild conditions. Moreover, a plausible reaction mechanism was discussed, with (o-CF3PhO)3P and its hydrolysis product (o-CF3PhO)2P(O)H contributing to the formation of the target products as an amide synthesis coupling agent and a cyclization reaction promoter, respectively.

The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

Regioselective Substitution of BINOL

1,1'-Bi-2-naphthol (BINOL) has been extensively used as the chirality source in the fields of molecular recognition, asymmetric synthesis, and materials science. The direct electrophilic substitution at the aromatic rings of the optically active BINOL has been developed as one of the most convenient strategies to structurally modify BINOL for diverse applications. High regioselectivity has been achieved for the reaction of BINOL with electrophiles. Depending upon the reaction conditions and substitution patterns, various functional groups can be introduced to the specific positions, such as the 6-, 5-, 4-, and 3-positions, of BINOL. Ortho-lithiation at the 3-position directed by the functional groups at the 2-position of BINOL have been extensively used to prepare the 3- and 3,3'-substituted BINOLs. The use of transition metal-catalyzed C-H activation has also been explored to functionalize BINOL at the 3-, 4-, 5-, 6-, and 7-positions. These regioselective substitutions of BINOL have allowed the construction of tremendous amount of BINOL derivatives with fascinating structures and properties as reviewed in this article. Examples for the applications of the optically active BINOLs with varying substitutions in asymmetric catalysis, molecular recognition, chiral sensing and materials are also provided.

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.

Annulative π-Extension by Cp*Co(III)-Catalyzed Ketone-Directed peri-Annulation: An Approach to Access Fused Arenes

A masked-bay-region selective first-row transition-metal Cp*Co(III)-catalyzed annulative π-extension of arene-derived ketones is achieved to afford K-region-functionalized benzo[e]pyrenes, benzotetraphenes, and pyrenes. Comprehensive density functional theory studies buttress the mechanistic pathway comprising key steps like peri-C-H activation, alkyne 1,2-migratory insertion, and nucleophilic attack toward ketone, this attack being the rate-determining step. In addition, π-conjugated 1,1'-bipyrenes, potential photocatalyst pyrene-quinones, and putative n-type semiconductor cyano group-containing dibenzo[de,qr]tetracenes are also accessed.

Metal-Free Selective Ortho-C-H Amidation of Hypervalent(III) Iodobezenes with N-Methoxy Amides under Mild Conditions

A metal-free selective ortho-C-H amidation of aryl iodines(III) with the use of N-methoxy amides as aminating reagents under mild conditions is described here. In the protocol, excellent chemoselectivity and high regioselectivity were obtained. Notably, the iodine substituent rendered the amidation product suitable to be used for further elaboration.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

Iridium-Catalyzed Direct Ortho-C-H Amidation of α-Ketoesters with Sulfonyl Azides Using a Transient Directing Group Strategy

Direct C-H amidation of α-ketoesters was accomplished using various organic azides as the amino source through the combination of transient directing group strategy and iridium catalysis. Excellent functional group tolerance and wide substrate scope were explored under simple and mild conditions. Importantly, it was found that the steric hindrance of the ester moiety played a pivotal role for the reaction efficacy. In addition, the reaction could be enlarged to gram scale, and several useful heterocycles were readily constructed via one-step late-stage derivatization.

Site- and Stereoselective C(sp3 )-H Borylation of Strained (Hetero)Cycloalkanols Enabled by Iridium Catalysis

Transition metal-catalyzed site- and stereoselective C-H activation of strained (hetero)cycloalkanes remains a formidable challenge. We herein report a carbamate-directed iridium-catalyzed asymmetric β-C(sp³ )-H borylation of cyclopropanol derivatives. A variety of densely functionalized cyclopropanols were obtained in good enantioselectivities via desymmetrization and kinetic resolution. In addition, site-selective C(sp³ )-H borylation of methine groups furnished α-borylated (hetero)cycloalkanols in moderate to good yields. The synthetic utility of the method was further shown in a gram-scale synthesis and diverse downstream transformations of borylated products.

Carbonyl-Assisted Iridium-Catalyzed C-H Amination Using 2,2,2-Trichloroethoxycarbonyl Azide

The carbonyl-directed, mono C-H amination of arenes has been achieved using [Cp*Ir(III)Cl₂]₂ as the catalyst and 2,2,2-trichloroethoxycarbonyl (Troc) azide as an aminating reagent. The amination proceeds smoothly with a variety of arylcarbonyl compounds, including alkyl and vinyl arylketones, secondary and tertiary aryl amides, and acetyl indoles. The resulting ortho-TrocNH arylcarbonyl compounds are easily transformed to the corresponding free arylamines, aryl carbamates, or aryl ureas. Taking advantage of the electrophilic nature of both Troc and carbonyl groups in ortho-TrocNH arylcarbonyl compounds, the subsequent cyclization with dinucleophilic reagents has also been demonstrated. This provides an efficient strategy for the construction of aryl-fused N-heterocycles.

Cp*CoIII-catalyzed formal [4 + 2] cycloaddition of 2-phenyl-1H-imidazoles to afford imidazo[1,2-c]quinazoline derivatives

A synthetic protocol based on Cp*Co^III-catalyzed C-H amidation/annulation of 2-aryl-1H-imidazoles with 1,4,2-dioxazol-5-ones was developed to give imidazo[1,2-c]quinazoline derivatives with broad substrate scope in moderate to good yields. The method has good prospects of application in the synthesis of imidazo[1,2-c]quinazoline drugs.

Thioether-directed Rh(III)-catalyzed peri-selective acyloxylation of arenes

A thioether directed acyloxylation of arenes has been realized via Cp*Rh(III)-catalyzed C-H activation and subsequent coupling with carboxylic acids. This new method showed high functional group compatibility and broad substrate scope. Primary mechanistic studies have been conducted and a tentative reaction mechanism was proposed. It represents the first example of a thioether-directed Cp*Rh(III)-catalyzed C(sp²)-H acyloxylation reaction.

Cp*Co(III)-catalyzed thiocarbamate directed C−H aminocarbonyl-ation, amination, and cascade annulation of pyrroles

Cobalt(III)-catalyzed thiocarbamate directed aminocarbonylation and amination of C−H bond are described to access diverse amides. Biologically relevant pyrrolo[1,2-c]imidazoles were readily accessed via one-pot intramolecular cyclization at thiocarbamoyl directing group. Notably,...

Cp*CoIII-catalyzed C2-alkylation of indole derivatives with substituted cyclopropanols

A general and efficient Cp*CoIII-catalyzed C2-alkylation of N-pyridylindoles has been achieved utilizing cyclopropanols as an alkylating reagent.

Iridium(iii)-catalyzed two-fold C–H alkylation of BINOLs with allyl alcohols

Ir(iii)-Catalyzed C–H alkylation of BINOL units has been well examined by using allyl alcohols as coupling partners.

One-Pot Construction of Indolo[2,3-b]quinoxalines through Ruthenium-Catalyzed Ortho C-H Bond Functionalization of 2-Arylquinoxalines with Sulfonyl Azides

The synthesis of N-substituted indolo[2,3-b]quinoxalines has been developed through a Ru(II)-catalyzed ortho C-H functionalization of 2-arylquinoxalines with sulfonyl azides and further oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in one pot. This double C-N bond formation strategy provides a new efficient route for the preparation of a series of biologically relevant 6H-indolo[2,3-b]quinoxaline derivatives in up to 94% yield, suggesting a broad substrate scope applicability. The preliminary mechanistic studies reveal that the sequential C-N bond formations proceed through the formation of a five-membered ruthenacyclic intermediate in the first step and a radical mechanism in the second step.

Regioselective C-H Functionalization of the Six-Membered Ring of the 6,5-Fused Heterocyclic Systems: An Overview

The regioselective C-H functionalization of the five-membered ring of the 6,5-fused heterocyclic systems is nowadays well documented due to its high reactivity compared to the six-membered ring. So, developing new procedures of C-H functionalization of the six-membered ring "by thinking out of the box" is extremely challenging, which explains the limited number of reports published to date. This review paper aims to highlight advances achieved in this emerging chemistry research and discusses recently reported methods.

Pd-catalyzed cross-electrophile Coupling/C-H alkylation reaction enabled by a mediator generated via C(sp3)-H activation

Transition-metal-catalyzed cross-electrophile C(sp²)–(sp³) coupling and C–H alkylation reactions represent two efficient methods for the incorporation of an alkyl group into aromatic rings. Herein, we report a Pd-catalyzed cascade cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides. Methoxy and benzyloxy groups, which are ubiquitous functional groups and common protecting groups, were utilized as crucial mediators via primary or secondary C(sp³)–H activation. The reaction provides an innovative and convenient access for the synthesis of alkylated phenol derivatives, which are widely found in bioactive compounds and organic functional materials.

A cascade Pd-catalyzed cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides has been developed by using an ortho-methoxy or benzyloxy group as a mediator via C(sp³)–H activation.

O-Directed C-H functionalization via cobaltacycles: a sustainable approach for C-C and C-heteroatom bond formations

This review focuses on providing comprehensive highlights of the recent advances in the field of cobalt-catalysed C-H functionalization and related synthetic concepts, relying on these through oxygen atom coordination. In recent years, 3d transition metal (Fe, Co, Cu & Ni) catalysed C-H functionalization reactions have received immense attention on account of its higher abundance and low cost, as compared to noble metals such as Ir, Rh, Ru and Pd. Among the first-row transition metals, cobalt is one of the extensively used metals for sustainable synthesis due to its unique reactivity towards the functionalization of inert C-H bonds. The functionalization of the inert C-H bond necessitates a proximal directing group. In this context, strongly coordinating nitrogen atom directed C-H functionalizations have been well explored. Nevertheless, strongly coordinating nitrogen-containing scaffolds, such as pyridine, pyrimidine, and 8-aminoquinoline, have to be installed and removed in a separate process. In contrast, C-H functionalizations through weakly coordinating atoms, such as oxygen, are largely underdeveloped. Since the oxygen atom is a part of many readily available functional groups, such as aldehydes, ketones, carboxylic acids, and esters, it could be used as directing groups for selective C-H functionalization reactions without any modification. Thus, the use of 3d transition metals, such as cobalt, along with weakly coordinating (oxygen) directing groups for C-H functionalization reactions are more sustainable, especially for the large-scale production of pharmaceuticals in industries. During the last decade, notable progress has been made using this concept. Nonetheless, almost all the reports are restricted to the formation of C-C and C-N bond. Therefore, there is a wide scope for developing this area for the formation of other bonds, such as C-X (halogens), C-B, C-S, and C-Se.

Cp*Ir(iii)- and Cp*Rh(iii)-catalyzed C(sp2)–H amination of arenes using thioethers as directing groups

A mild and selective Cp*Ir(iii)- and Cp*Rh(iii)-catalyzed direct C(sp²)–H amination of arenes and three types of nitrene precursor reagents is reported, with the assistance of a thioether directing group.

Electrochemical C-H Amidation of Heteroarenes with N-Alkyl Sulfonamides in Aqueous Medium

The construction of C-N bonds by free radical reactions represents a powerful synthetic approach for direct C-H amidations of arenes or heteroarenes. Developing efficient and more environmentally friendly synthetic methods for C-H amidation reactions remains highly desirable. Herein, metal-free electrochemical oxidative dehydrogenative C-H amidations of heteroarenes with N-alkylsulfonamides have been accomplished. The catalyst- and chemical-oxidant-free C-H amidation features an ample scope and employs electricity as the green and sole oxidant. A variety of heteroarenes, including indoles, pyrroles, benzofuran and benzothiophene, thereby underwent this C(sp² )-H nitrogenation. Cyclic voltammetry studies and control experiments provided evidence for nitrogen-centered radicals being directly generated under metal-free electrocatalysis.

Co(III)-Catalyzed C-H Amidation of Nitrogen-Containing Heterocycles with Dioxazolones under Mild Conditions

A cobalt(III)-catalyzed C-8 selective C-H amidation of quinoline N-oxide using dioxazolone as an amidating reagent under mild conditions is disclosed. The reaction proceeds efficiently with excellent functional group compatibility. The utility of the current method is demonstrated by gram scale synthesis of C-8 amide quinoline N-oxide and by converting this amidated product into functionalized quinolines. Furthermore, the developed catalytic method is also applicable for C-7 amidation of N-pyrimidylindolines and ortho-amidation of benzamides.

Rapid Synthesis of Alkenylated BINOL Derivatives via Rh(III)-Catalyzed C-H Bond Activation

Modification of BINOL units has been well examined via Rh-catalyzed C-H activation and functionalization reactions by using ester carbonyls as directing groups and alkenes as coupling partners. The one-pot strategy was an efficient protocol for the rapid synthesis of BINOL derivatives with retention of optical purity.