Rhodium(III)- and Iridium(III)-Catalyzed C7 Alkylation of Indolines with Diazo Compounds

Rh(III)-Catalyzed C7-Alkylation of Isatogens with Malonic Acid Diazoesters

Rh(III)-catalyzed C7-alkylation of isatogens (indolin-3-one N-oxides) with malonic acid diazoesters has been developed. This strategy utilizes oxygen anion on the N-oxide group of isatogens as a directing group and successfully achieves the synthesis of a series of C7-alkylated isatogens with moderate to good yields (48-86% yields). Moreover, the N-oxides of isatogens can not only serve as the simple directing group for C7-H bond cleavage but also be deoxidized for easy removal.

Cobalt(III)-Catalyzed Directed C-7 Selective C-H Alkynylation of Indolines with Bromoalkynes

A cobalt(III)-catalyzed directed C-7 alkynylation of indolines with easily accessible bromoalkynes has been developed. The reaction has a broad substrate scope with excellent yields and represents a powerful route to the synthesis of 7-alkynyl-substituted indolines. In addition, the reaction can be extended to the coupling of N-aryl 7-azaindoles, highlighting the synthetic practicability of the strategy.

Cu/SaBox-Catalyzed Photoinduced Coupling of Acylsilanes with Alkynes

The transition metal-catalyzed cross-coupling reaction with Fischer metal carbene intermediates bearing an electron-rich alkoxyl or siloxyl group remains a big challenge due to the lack of readily available corresponding carbene precursors. Herein, we report the coupling of alkynes with the Fischer-type copper carbene species bearing a α-siloxyl group, which could be in situ generated from acylsilanes catalytically under photoirradiation and redox-neutral conditions. The side-arm modified bisoxazoline (SaBox) ligands prove to be crucial for this coupling reaction, which provides the corresponding alkynyl alcohol in high yields with remarkable heterocycle tolerance and broad substrate scope.

Rh(III)-Catalyzed ortho C-H functionalization of aromatic amides with bis(phenylsulfonyl)diazomethane and α-diazosulfones

A Rh(III)-catalyzed migratory insertion of bis(phenylsulfonyl) carbene and α-sulfonyl carbenes into ortho C-H bonds of aryl amides has been developed. The products were obtained with moderate to excellent yields under mild reaction conditions. A reaction mechanism was proposed based on the control experiments and previous studies. Diverse desulfonylation transformations of the products were achieved.

Rhodium-catalysed decarbonylative C(sp2)-H alkylation of indolines with alkyl carboxylic acids and carboxylic anhydrides under redox-neutral conditions

We developed a rhodium-catalysed decarbonylative C(sp²)-H alkylation method for indolines. This reaction facilitates the use of alkyl carboxylic acids and their anhydrides as a cheap, abundant and non-toxic alkyl source under redox-neutral conditions, featuring the introduction of a primary alkyl chain, which cannot be addressed by previous radical-mediated decarboxylative reaction. Through a mechanistic investigation, we revealed that an initially formed C-7 acylated indoline was transformed into the corresponding alkylated indoline via a decarbonylation process.

Water-Mediated ortho-Carboxymethylation of Aryl Ketones under Ir(III)-Catalytic Conditions: Step Economy Total Synthesis of Cytosporones A-C

An expeditious Ir(III)-catalyzed carboxymethylation of aryl ketone with diazotized Meldrum's acid has been developed in aqueous medium. Flavanone and chromanone were also found to be facile substrates with the developed catalytic system. Mechanistic studies revealed the active catalytic species and the role of water in the reaction process as hydroxy and proton sources. Employing the developed method, total synthesis of cytosporone A was achieved in two steps and that of cytosporones B-C was achieved in three steps starting from resorcinol.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Catalytic Atroposelective C7 Functionalisation of Indolines and Indoles

Axially chiral atropisomeric compounds are widely applied in asymmetric catalysis and medicinal chemistry. In particular, axially chiral indole- and indoline-based frameworks have been recognised as important heterobiaryl classes because they are the core units of bioactive natural alkaloids, chiral ligands and bioactive compounds. Among them, the synthesis of C7-substituted indole biaryls and the analogous indoline derivatives is particularly challenging, and methods for their efficient synthesis are in high demand. Transition-metal catalysis is considered one of the most efficient methods to construct atropisomers. Here, we report the enantioselective synthesis of C7-indolino- and C7-indolo biaryl atropisomers by means of C-H functionalisation catalysed by chiral RhJasCp complexes.

Rhodium(III)-Catalyzed Sequential C-H Activation and Cyclization from N-Methoxyarylamides and 3-Diazooxindoles for the Synthesis of Isochromenoindolones

An efficient synthetic method for structurally various isochromenoindolones has been demonstrated through Rh(III)-catalyzed C-H activation followed by a cyclization reaction of N-methoxyarylamides with 3-diazooxindoles. The sequential reaction involves the streamlined formation of C-C and C-O bonds in one pot. The present method provides a broad range of isochromenoindolones as a new privileged scaffold in moderate to good yields with the release of methoxyamine and molecular nitrogen and has the benefits of a broad substrate scope and good functional group tolerance.

Aromatic C-H Methylation and Other Functionalizations via the Rh(III)-Catalyzed Migratory Insertion of Bis(phenylsulfonyl)carbene and Subsequent Transformations

The Rh(III)-catalyzed migratory insertion of bis(phenylsulfonyl)carbene into aromatic C-H bonds has been developed. A variety of bis(phenylsulfonyl)methyl derivatives were prepared with good yields under mild conditions. The methylated products were readily obtained after reductive desulfonylation. Furthermore, the diverse transformations of bis(phenylsulfonyl)methyl to trideuteriomethyl, aldehyde, and other functional groups were demonstrated.

Pd-Catalyzed C-H Halogenation of Indolines and Tetrahydroquinolines with Removable Directing Group

Pd-catalyzed directing-group-assisted regioselective halogenations to C7 of indolines and C8 of tetrahydroquinolines were achieved in good to excellent yields. The practicality and utility of the developed method have been illustrated by various functional group transformations such as arylation, alkenylation, cyanation, and silylation utilizing the installed synthetic handle. The concise synthesis of primaquine, an antimalarial drug, and formal syntheses of two bioactive natural products, hippadine and pratosine, have also been demonstrated.

Tandem Carbenoid C-H Functionalization/Conia-ene Cyclization of N-Propargyl Indoles Generates Pyrroloindoles under Cooperative Rh(II)/Zn(II) Catalysis

The decomposition of diazodicarbonyl compounds in the presence of various metal catalysts has become a reliable method for the functionalization of indoles via carbenoid intermediates. Exploiting the nucleophilic reactivity of the in situ generated malonic ester product formed, we herein report a tandem C-H functionalization/Conia-ene cyclization of N-alkyne tethered indoles. This double functionalization of diazodicarbonyls generates a range of pyrrolo[1,2-a]-, pyrido[1,2-a]-, and azepino[1,2-a]indole products with good synthetic efficiency.

Rhodium-catalyzed coupling of arenes and fluorinated α-diazo diketones: synthesis of chromones

Rh(iii)-Catalyzed synthesis of chromones using arenes and fluorinated diazo reagents via C–H activation, carbene insertion, and uncatalyzed intramolecular nucleophilic aromatic substitution.

Cu(II)-Mediated Cross-Dehydrogenative Coupling of Indolines with Sulfonamides, Carboxamides, and Amines

A facile and efficient Cu-mediated protocol for the cross-dehydrogenative coupling of indoline with sulfonamides, carboxamides, and anilines is reported. The reaction takes place through Cu-mediated C7-H activation via a 6-membered metallacycle to afford the amide and amine derivatives in good yields with a wide range of functional group tolerance. The importance of the protocol has been demonstrated by synthesizing the antiproliferative agent, ER-67836.

[2 + 1 + 1] Assembly of spiro β-lactams by Rh(ii)-catalyzed reaction of diazocarbonyl compounds with azirines/isoxazoles

A domino synthesis of dispiro-fused N-vinyl β-lactams from diazo-Meldrum's acid and 2H-azirines or 5-alkoxyisoxazoles via the "2-azabuta-1,3-diene formation/Staudinger ketene-imine cycloaddition" sequence is described. The Rh2(Piv)4-catalyzed formation of the 2-azabuta-1,3-diene intermediates in the first stage of the reaction sequence proceeds via addition of the rhodium carbenoid to the azirines/isoxazoles and provides the first example of the generation of iminium-type ylides from diazo-Meldrum's acid. This methodology was extended to monospiro-β-lactams, which were synthesized in two steps using acyclic α-diazocarbonyl compounds in the stage of the formation of 2-azabuta-1,3-dienes. The method allows for the rapid assemblage of the carbon part of the azetidin-2-one system from diazo compounds only and affords spiro- and dispiro-β-lactams in moderate yields. A bromine atom in the 2-bromoalkenyl moiety of the synthesized β-lactams can be easily substituted by hydrogen under catalytic hydrogenation conditions or by 2-pyridyl-substituent via the Stille cross-coupling reaction.

Directing-Group-Free C7-Alkylations of N-Alkylindoles Mediated by Cationic Zirconium Complexes: Role of Brønsted Acid for Catalytic Manifold

Highly position selective alkylations of N-alkylindoles at C7-positions have been enabled by cationic zirconium complexes. The strategy provides a straightforward access to install alkyl groups at C7-positions of indoles without a complex directing group. Mechanistic studies provided support for the importance of Brønsted acids in the catalytic manifold.

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.

Transition-metal-catalyzed site-selective C7-functionalization of indoles: advancement and future prospects

The advancement and future prospects of transition-metal-catalyzed auxiliary assisted regioselective C7-functionalization of indoles/indolines are covered in this article.

Ir(iii)-Catalyzed [4 + 2] cyclization of azobenzene and diazotized Meldrum's acid for the synthesis of cinnolin-3(2H)-one

The first report on Ir(iii)-catalyzed C–H alkylation/cyclization of azobenzene with diazotized Meldrum's acid was developed for the synthesis of cinnoline-3(2H)-one-4-carboxylic acid and its ester derivative under mild conditions.

Catalyst-controlled selective mono-/dialkylation of 2-aryl-4(3H)-quinazolinones

Catalyst-controlled selective mono-/dialkylation of 2-aryl-4(3H)-quinazolinones with α-diazotized Meldrum's acid has been achieved successfully via a metal carbene migratory insertion process.

Rhodium(iii)-catalyzed sulfonamide directed ortho C–H carbenoid functionalization via metal carbene migratory insertion

A rhodium(iii)-catalyzed sulfonamide directed ortho C–H carbenoid functionalization has been developed with good yields.

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.

Rh(III)-Catalyzed C-C Coupling of Diverse Arenes and 4-Acyl-1-sulfonyltriazoles via C-H Activation

4-Acyl-1-sulfonyltriazoles act as versatile carbene reagents in Cp*Rh(III)-catalyzed ortho-selective coupling with arenes via C-H activation. The coupling led to olefination with possible cyclization, depending on the nature of the arene.

RhIII -Catalyzed Direct C8-Arylation of Quinoline N-Oxides using Diazonaphthalen-2(1H)-ones: A Practical Approach towards 8-aza BINOL

An efficient Rh^III -catalyzed redox-neutral method for the direct C8-arylation of quinoline N-oxides using diazonaphthalen-2(1H)-one as coupling partner has been demonstrated. The developed method is simple, scalable and straightforward with a wide range of substrate scope. The applicative potential was extended with a late-stage functionalization and straightforward synthesis of 8-azaBINOL derivative. A plausible reaction pathway was proposed after carrying out preliminary control studies.

Synthesis of TMPA Derivatives through Sequential Ir(III)-Catalyzed C-H Alkylation and Their Antidiabetic Evaluation

The synthesis and antidiabetic evaluation of ethyl 2-[2,3,4-trimethoxy-6-(1-octanoyl)phenyl]acetate (TMPA) and its structural analogs are described. The construction of TMPA derivatives has been successfully achieved in only two steps, which involve the iridium(III)-catalyzed α-alkylation of acetophenones with alcohols and the ketone-directed iridium(III)- or rhodium(III)-catalyzed redox-neutral C-H alkylation of α-alkylated acetophenones using Meldrum's diazo compounds. This synthetic protocol efficiently provides a range of TMPA derivatives with site selectivity and functional group compatibility. In addition, the site-selective demethylation of TMPA derivative affords the naturally occurring phomopsin C in good yield. Moreover, all synthetic compounds were screened for in vitro adenosine 5'-monophosphate-activated protein kinase (AMPK) activation using HepG2 cells. Furthermore, TMPA (5ac) and 5cd showing the most potent AMPK activation were treated for the in vivo antidiabetic experiment. Notably, our synthetic compound 5cd was found to display the powerful antidiabetic effect, stronger than that of metformin and TMPA (5ac).