Rhodium-Catalyzed Asymmetric Hydrogenation and Transfer Hydrogenation of 1,3-Dipolar Nitrones

Owing to their distinctive 1,3-dipolar structure, the catalytic asymmetric hydrogenation of nitrones to hydroxylamines has been a formidable and longstanding challenge, characterized by intricate enantiocontrol and susceptibility to N-O bond cleavage. In this study, the asymmetric hydrogenation and transfer hydrogenation of nitrones were accomplished with a tethered TsDPEN-derived cyclopentadienyl rhodium(III) catalyst (TsDPEN: p-toluenesulfonyl-1,2-diphenylethylene-1,2-diamine), the reaction proceeds via a novel 7-membered cyclic transition state, producing chiral hydroxylamines with up to 99 % yield and >99 % ee. The practical viability of this methodology was underscored by gram-scale catalytic reactions and subsequent transformations. Furthermore, mechanistic investigations and DFT calculations were also conducted to elucidate the origin of enantioselectivity.

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.

Rhodium-Catalyzed C(sp2)-O Cross Couplings of Diazo Quinones with Phenols to Construct Diaryl Ethers

The diaryl ether represents a prevalent structural motif found in numerous biologically active molecules. Herein, we describe a dirhodium-catalyzed C(sp2)-O cross coupling reaction between diazo quinones and phenols for the construction of diaryl ethers in moderate to high yields. The reaction proceeds under mild and neutral conditions and is tolerant of various functional groups. The synthetic method has been successfully applied to the concise synthesis of a Navl.7 inhibitor.

Synthesis of spiropyrans and arylquinones via Ru(II)-catalyzed condition-controlled coupling of 3-aryl-2H-benzoxazinones with benzoquinones

Ru(II)-catalyzed condition-controlled divergent coupling between 3-aryl-2H-benzoxazin-2-ones and benzoquinones has been realized under operationally simple conditions, affording a series of structurally stable spiropyrans and valuable arylquinones. The potential of this method is also demonstrated by scale-up synthesis and derivatization. Additionally, an unprecedented cycloruthenated complex has been identified as a key intermediate.

Rhodium-Catalyzed Asymmetric C-H Functionalization Reactions

This review summarizes the advancements in rhodium-catalyzed asymmetric C-H functionalization reactions during the last two decades. Parallel to the rapidly developed palladium catalysis, rhodium catalysis has attracted extensive attention because of its unique reactivity and selectivity in asymmetric C-H functionalization reactions. In recent years, Rh-catalyzed asymmetric C-H functionalization reactions have been significantly developed in many respects, including catalyst design, reaction development, mechanistic investigation, and application in the synthesis of complex functional molecules. This review presents an explicit outline of catalysts and ligands, mechanism, the scope of coupling reagents, and applications.

Rh(II)-Catalyzed Synthesis of N-Aryl 2-pyridone Using 2-Oxypyridine and Diazonaphthoquinone Via 1,6-Benzoyl Migratory Rearrangement

A Rh(II)-catalyzed simple and efficient synthesis of N-arylated 2-pyridone derivatives is described using 2-oxypyridine and diazonaphthoquinone as coupling partners. The reaction proceeds through the insertion of the nitrogen atom of the 2-oxypyridine derivative into quinoid carbene and subsequent 1,6-benzoyl migratory rearrangement. The reaction is broadened with sufficient scope and has the potential to offer axially chiral N-arylated 2-pyridone derivatives under suitable asymmetric conditions.

Rhodium-Catalyzed N-Arylation of 2-Pyridones Enabled by 1,6-Acyl Migratory Rearrangement of 2-Oxypyridines

An efficient rhodium-catalyzed dearomative rearrangement of 2-oxypyridines with quinone diazides has been developed for the direct synthesis of N-arylated pyridones, in which a novel 1,6-O-to-O rather than 1,4-O-to-C acyl rearrangement has been achieved under mild reaction conditions.

Rh(III)-Catalyzed Spiroannulation Reaction of N-Aryl Nitrones with 2-Diazo-1,3-indandiones: Synthesis of Spirocyclic Indole-N-oxides and Their 1,3-Dipolar Cycloaddition with Maleimides

An efficient strategy for the preparation of spirocyclic indole-N-oxide compounds through a Rh(III)-catalyzed [4 + 1] spiroannulation reaction of N-aryl nitrones with 2-diazo-1,3-indandiones as C1 synthons under extremely mild conditions is presented. From this reaction, 40 spirocyclic indole-N-oxides were easily obtained in up to 98% yield. In addition, the title compounds could be successfully used for the construction of structurally intriguing maleimide-containing fused polycyclic scaffolds via a diastereoselective 1,3-dipolar cycloaddition reaction with maleimides.

Rhodium-catalyzed enantioselective C-H alkynylation of sulfoxides in diverse patterns: desymmetrization, kinetic resolution, and parallel kinetic resolution

Rhodium-catalyzed enantioselective C-H alkynylation of achiral and racemic sulfoxides is disclosed with alkynyl bromide as the alkynylating reagent. A wide range of chiral sulfoxides have been constructed in good yield and excellent enantioselectivity (up to 99% ee, s-factor up to > 500) via desymmetrization, kinetic resolution, and parallel kinetic resolution under mild reaction conditions. The high enantioselectivity was rendered by the chiral cyclopentadienyl rhodium(iii) catalyst paired with a chiral carboxamide additive. The interactions between the chiral catalyst, the sulfoxide, and the chiral carboxylic amide during the C-H bond cleavage offer the asymmetric induction, which is validated by DFT calculations. The chiral carboxamide functions as a base to promote C-H activation and offers an additional chiral environment during the C-H cleavage.

Palladium/Amino Acid Co-catalyzed Atroposelective C-H Olefination to Access Tetra-Ortho-Substituted Atropisomers Featuring 2,2'-Difluoro-1-biaryl Scaffolds

Despite the great advancement in atroposelective synthesis in the past decades, the enantioselective synthesis of 2,2'-difluoro-1-biaryls is unprecedented. Herein, a palladium and chiral amino acid catalyzed atroposelective C-H olefination to construct the axially chiral 2,2'-difluoro-1-biaryls is reported. A variety of polyfluoro-substituted biaryls were forged under mild conditions in good yields with excellent enantioselectivity (up to 99% ee). The potential application was demonstrated by a gram-scale synthesis and synthetic transformations.

Atroposelective Synthesis of Axially Chiral C2-Arylindoles via Rhodium-Catalyzed Asymmetric C-H Bond Insertion

A highly efficient rhodium-catalyzed formal C-H insertion reaction between indoles and 1-diazonaphthoquinones has been established, providing a novel protocol for the atroposelective synthesis of axially chiral C2-arylindoles (up to 99:1 er) under mild reaction conditions. Typically, only 1 mol % of Rh₂(S-PTTL)₄ is used and the chelation group is not needed for this conversion.

Synthesis of N-Arylindoles from 2-Alkenylanilines and Diazonaphthalen-2(1H)-ones through Simultaneous Indole Construction and Aryl Introduction

In this paper, an efficient synthesis of N-arylindoles through the cascade reaction of 2-alkenylanilines with diazonaphthalen-2(1H)-ones is presented. Mechanistically, this reaction involves the generation of a Ru-carbene complex from diazonaphthalen-2(1H)-one, followed by carbene N-H bond insertion with 2-alkenylaniline, intramolecular cyclization, and oxidative aromatization. In this reaction, the Ru(II) complex acts as a multifunctional catalyst to promote not only the carbene formation but also the intramolecular cyclization and the dehydrogenative aromatization. Meanwhile, air acts as a green and cost-effective oxidant. To our knowledge, this is the first example in which N-arylindoles were synthesized through simultaneous introduction of the N-aryl unit and construction of the indole scaffold. Notable advantages of this method include readily accessible and halide-free substrates, additive-free reaction conditions, good efficiency, excellent atom economy, and compatibility with diverse functional groups. In addition, the utility of the product thus obtained was showcased by its diverse structural transformations.

Synthesis of spiropyrans via the Rh(III)-catalyzed annulation of 3-aryl-2H-benzo[b][1,4]oxazines with diazo ketoesters

Rh(III)-Catalyzed 1 : 2 coupling of 3-aryl-2H-benzo[b][1,4]oxazines with α-diazo-β-ketoesters has been realized for the mild synthesis of spiropyrans. The reaction proceeded via twofold C-H activation followed by unusual [3+3] and [4+2] annulation with decent functional group tolerance. Moreover, a pyranoid-skeleton intermediate was isolated as a key intermediate as a result of monoalkylation and enol oxygen annulation, which offers direct mechanistic insight.

A combination of polarity reversal, Diels-Alder cycloaddition and skeletal remodeling to access pyridine-fused nitrones

An unprecedented cascade strategy consisting of polarity reversal, normal electron-demand Diels-Alder cycloaddition and skeletal remodeling was developed to construct novel pyridine-fused nitrones in up to 82% yield. The key to the success was the umpolung process, which transformed the electron-deficient 3-nitropyridinium ring into a reactive, π-extended cyclic nitroalkene, serving as a rarely reported hetero-diene to participate in normal Diels-Alder cycloadditions.

Electrooxidative palladium- and enantioselective rhodium-catalyzed [3 + 2] spiroannulations

Despite indisputable progress in the development of electrochemical transformations, electrocatalytic annulations for the synthesis of biologically relevant three-dimensional spirocyclic compounds has as of yet not been accomplished. In sharp contrast, herein, we describe the palladaelectro-catalyzed C-H activation/[3 + 2] spiroannulation of alkynes by 1-aryl-2-naphthols. Likewise, a cationic rhodium(iii) catalyst was shown to enable electrooxidative [3 + 2] spiroannulations via formal C(sp3)-H activations. The versatile spiroannulations featured a broad substrate scope, employing electricity as a green oxidant in lieu of stoichiometric chemical oxidants under mild conditions. An array of spirocyclic enones and diverse spiropyrazolones, bearing all-carbon quaternary stereogenic centers were thereby accessed in a user-friendly undivided cell setup, with molecular hydrogen as the sole byproduct.

Synthesis of Spiro[benzo[d][1,3]oxazine-4,4'-isoquinoline]s via [4+1+1] Annulation of N-Aryl Amidines with Diazo Homophthalimides and O2

Synthesis of spiro[benzo[d][1,3]oxazine-4,4'-isoquinoline]s through a unique [4+1+1] annulation of N-aryl amidines with diazo homophthalimides and O₂ is presented. This unprecedented spirocyclization reaction features readily obtainable substrates, structurally and pharmaceutically attractive products, a cost-free and clean oxygen source, sustainable reaction medium, tolerance of a broad spectrum of functional groups, and an interesting reaction mechanism based on sequential C(sp²)-H/C(sp³)-H bond cleavage and oxygen insertion.

Rh(III)-catalyzed C-H annulation of sulfoxonium ylides with iodonium ylides towards isocoumarins

The direct synthesis of isocoumarin skeletons has been realized through the Rh(III)-catalyzed [3 + 3] annulation of sulfoxonium ylides with iodonium carbenes. The synthetic protocol was constructed efficiently with broad functional group tolerance and mild reaction conditions. This reaction can be formally viewed as the result of C-H activation, carbene insertion and nucleophilic addition processes. Furthermore, the further conversions of the product and gram-scale reactions were also demonstrate to support the effectiveness of the synthesis protocol.

Palladium-Catalyzed Migratory Insertion of Carbenes and C-C Cleavage of Cycloalkanecarboxamides

A palladium catalyzed reaction of cycloalkanecarboxamides and diazomalonates or bis(phenylsulfonyl)diazomethane has been developed. The reaction proceeds via carbene migratory insertion and cascade C-C cleavage pathways. Cycloalkanecarboxamides with four to seven membered rings are applicable in the transformation. A series of ring opening products were prepared with moderate yields. The finding provides valuable clues for the development of new reactions involving carbene migratory insertion and the cleavage of unstrained C(sp³)-C(sp³) bonds.

Cobalt-Catalyzed Enantioselective C-H Arylation of Indoles

Atropoisomeric (hetero)biaryls are scaffolds with increasing importance in the pharmaceutical and agrochemical industries. Although it is the most obvious disconnection to construct such compounds, the direct enantioselective C-H arylation through the concomitant induction of the chiral information remains extremely challenging and uncommon. Herein, the unprecedented earth-abundant 3d-metal-catalyzed atroposelective direct arylation is reported, furnishing rare atropoisomeric C2-arylated indoles. Kinetic studies and DFT computation revealed an uncommon mechanism for this asymmetric transformation, with the oxidative addition being the rate- and enantio-determining step. Excellent stereoselectivities were reached (up to 96% ee), while using an unusual N-heterocyclic carbene ligand bearing an essential remote substituent. Attractive dispersion interactions along with positive C-H---π interactions exerted by the ligand were identified as key factors to guarantee the excellent enantioselection.

Ruthenium-catalyzed reaction of diazoquinones with arylamines to synthesize diarylamines

The diarylamine scaffold is common in bioactive molecules. Herein, we report a Ru(ii)-catalyzed C–N cross-coupling reaction of diazoquinones with arylamines, which provides access to a range of functionalized diarylamines in 43–97% yields.

Rh(iii)-catalyzed substrate-dependent oxidative (spiro)annulation of isoquinolones with diazonaphthoquinones: selective access to new spirocyclic and oxepine-fused polycyclic compounds

An efficient protocol for the selective synthesis of novel isoquinolone-containing spirocyclic and oxepine-fused polycyclic compounds via rhodium(iii)-catalyzed (spiro)annulation of NH-isoquinolones with diazonaphthalen-2(1H)-ones is reported.

Rhodium-catalyzed reaction of diazoquinones with allylboronates to synthesize allylphenols

A rhodium-catalyzed reaction of diazoquinones and allylboronates was developed, which provides access to a range of substituted allylphenols under mild conditions.

Hydrogen Bond Assisted Central-to-Spiro Chirality Transfer and Central-to-Axial Chirality Conversion: Asymmetric Synthesis of Spirocycles

The asymmetric construction of chiral spiroenones bearing both axial and spiro-central chirality has been established for the first time by a central-to-spiro chirality transfer and a central-to-axial chirality conversion from chiral 2,3-diarylbenzoindolines. Mechanistic studies indicate the hydrogen bonds play important roles in this process, providing an efficient strategy for enantioselective construction of spirocyclic backbones via simultaneously controlling spiro-central and axial chirality in one operation.

Construction of C-C Axial Chirality via Asymmetric Carbene Insertion into Arene C-H Bonds

By using diazonaphthoquinones and anilines as key reagents and through a point-to-axis chiral transfer strategy, the atroposelective synthesis via asymmetric C(sp² )-H bond insertion reaction of arenes has been realized under rhodium catalysis, providing the resulting biaryl atropisomers in moderate to excellent yields with good enantiomeric ratios (up to 99:1). Further elaboration indicates this type of axially biaryl scaffold may have promising potentials in developing novel chiral ligands.

Recent Advances on Transition-Metal-Catalyzed Asymmetric C–H Arylation Reactions

Transition-metal-catalyzed asymmetric C–H functionalization has become a powerful strategy to synthesize complex chiral molecules. Recently, catalytic enantioselective C–H arylation has attracted great interest from organic chemists to construct aryl-substituted chiral compounds. In this short review, we highlight recent advances in asymmetric C–H arylation from 2019 to late 2021, including enantioselective C(sp2)–H arylation to construct axial or planar chiral compounds, and enantioselective C(sp3)–H arylation to introduce central chirality via desymmetrization of the methyl group or methylene C–H activation. These processes proceed with palladium, rhodium, iridium, nickel, or copper catalysts, and utilize aryl halides, boron, or diazo derivatives as arylation reagents.

1 Introduction

2 Transition-Metal-Catalyzed Asymmetric C(sp2)–H Arylation

2.1 Chelation-Assisted Asymmetric C(sp2)–H Arylation for the Construction of Atropisomer

2.2 Chelation-Assisted Asymmetric C(sp2)–H Arylation for the Construction of Planar Chiral Compounds

2.3 Chelation-Assisted Asymmetric C(sp2)–H Arylation and Axial-to-Central Chirality Transfer for the Construction of Spirocycles

2.4 Other Asymmetric C(sp2)–H Arylation Reactions

3 Transition-Metal-Catalyzed Asymmetric C(sp3)–H Arylation

3.1 Chelation-Assisted Enantioselective C(sp3)–H Arylation through Desymmetrization

3.2 Chelation-Assisted Enantioselective Methylene C(sp3)–H Aryl­ation

3.3 Other Asymmetric C(sp3)–H Arylations

4 Conclusion and Outlook

Transition-Metal-Catalyzed C-H Bond Functionalization of Arenes/Heteroarenes via Tandem C-H Activation and Subsequent Carbene Migratory Insertion Strategy

The past decade has witnessed tremendous developments in transition-metal-catalyzed C-H bond activation and subsequent carbene migratory insertion reactions, thus assisting in the construction of diverse arene/heteroarene scaffolds. Various transition-metal catalysts serve this purpose and provide efficient pathways for an easy access to substituted heterocycles. A brief introduction to metal-carbenes has been provided along with key mechanistic pathways underlying the coupling reactions. The purpose of this review is to provide a concise knowledge about diverse directing group-assisted coupling of varied arenes/heteroarenes and acceptor-acceptor/donor-acceptor diazo compounds. The review also highlights the synthesis of various carbocycles and fused heterocycles through diazo insertion pathways, via C-C, C-N and C-O bond forming reactions. The mechanism usually involves a C-H activation process, followed by diazo insertion leading to subsequent coupling.

Stereoselective copper-catalyzed heteroarene C-H functionalization/Michael-type annulation cascade with α-diazocarbonyls

A stereoselective, copper-catalyzed, arene C(sp²)-H functionalization/Michael-type annulation reaction involving α-diazocarbonyl compounds has been developed. The method features low catalyst loadings, high yields, and excellent regio and stereoselectivity, in the synthesis of various heteroaromatic frameworks by employing indoles as the arene partner.

Rhodium-Catalyzed and Chiral Zinc Carboxylate-Assisted Allenylation of Benzamides via Kinetic Resolution

Enantioenriched allenes are important building blocks. While they have been accessed by other coupling methodologies, enantioenriched allenes have been rarely obtained via C-H activation. In this work, kinetic resolution of tertiary propargyl alcohols as an allenylating reagent has been realized via rhodium(III)-catalyzed C-H allenylation of benzamides. The reaction proceeded efficiently under mild conditions, and both the allenylated products and the propargyl alcohols were obtained in high enantioselectivities with an s-factor of up to 139. The resolution results from bias of the two propargylic substituents and is assisted by a chiral zinc carboxylate additive.

CpxM(iii)-catalyzed enantioselective C-H functionalization through migratory insertion of metal-carbenes/nitrenes

Cp^xM(iii)-catalyzed enantioselective C-H functionalization reactions have progressed rapidly using either chiral cyclopentadienyl ligands or appropriate chiral carboxylic acids. In this context, highly reactive carbene and nitrene precursors can serve as effective C-H coupling partners, providing a straightforward and efficient approach to access chiral molecules. In this review, we highlight the developments in Cp^xM(iii)-catalyzed enantioselective C-H functionalization reactions through migratory insertion of metal-carbenes/nitrenes by employing chiral Cp^xM(iii) complexes or achiral Cp^xM(iii) complexes combined with chiral carboxylic acids.

Rhodium(iii)-catalyzed asymmetric [4+1] spiroannulations of O-pivaloyl oximes with α-diazo compounds

Chiral Rh^III catalysts can catalyze the asymmetric [4+1] spiroannulation of O-pivaloyl oximes with α-diazo homophthalimides under redox-neutral and acid/base-neutral conditions, leading to formation of chiral spirocyclic imines as a result of C-H activation and N-O cleavage. The reaction proceeded with high efficiency and features broad substrate scope, mild reaction conditions, and high to excellent enantioselectivities.

Atropo-Enantioselective Oxidation-Enabled Iridium(III)-Catalyzed C-H Arylations with Aryl Boronic Esters

Atropo-enantioselective biaryl coupling through C-H bond functionalization is an emerging technology allowing direct construction of axially chiral molecules. This approach is largely limited to electrophilic coupling partners. We report a highly atropo-enantioselective C-H arylation of tetralone derivatives paired with aryl boronic esters as nucleophilic components. The transformation is catalyzed by chiral cyclopentadienyl (Cpx ) iridium(III) complexes and enabled by oxidatively enhanced reductive elimination from high-valent cyclometalated Ir-species.

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.

Rhodium-Catalyzed C-H Activation-Based Construction of Axially and Centrally Chiral Indenes through Two Discrete Insertions

Reported herein is asymmetric [3+2] annulation of arylnitrones with different classes of alkynes catalyzed by chiral rhodium(III) complexes, with the nitrone acting as an electrophilic directing group. Three classes of chiral indenes/indenones have been effectively constructed, depending on the nature of the substrates. The coupling system features mild reaction conditions, excellent enantioselectivity, and high atom-economy. In particular, the coupling of N-benzylnitrones and different classes of sterically hindered alkynes afforded C-C or C-N atropochiral pentatomic biaryls with a C-centered point-chirality in excellent enantio- and diastereoselectivity (45 examples, average 95.6 % ee). These chiral center and axis are disposed in a distal fashion and they are constructed via two distinct migratory insertions that are stereo-determining and are under catalyst control.

Oxygen-Linked Cyclopentadienyl Rhodium(III) Complexes-Catalyzed Asymmetric C-H Arylation of Benzo[h]quinolines with 1-Diazonaphthoquinones

Chiral cyclopentadienyl rhodium (CpRh) complex-catalyzed asymmetric C-H functionalization reactions have witnessed a significant progress in organic synthesis. In sharp contrast, the reported chiral Cp ligands are limited to C-linked Cp and are often synthetically challenging. To address these issues, we have developed a novel class of tunable chiral cyclopentadienyl ligands bearing oxygen linkers, which were efficient catalysts for C-H arylation of benzo[h]quinolines with 1-diazonaphthoquinones, affording axially chiral heterobiaryls in excellent yields and enantioselectivity (up to 99 % yield, 98.5:1.5 er). Mechanistic studies suggest that the reaction is likely to proceed by electrophilic C-H activation, and followed by coupling of the cyclometalated rhodium(III) complex with 1-diazonaphthoquinones.