Rhodium(III)-Catalyzed Annulation between N-Sulfinyl Ketoimines and Activated Olefins: C–H Activation Assisted by an Oxidizing N–S Bond

Isatoic anhydride as a masked directing group and internal oxidant for Rh(III)-catalyzed decarbonylative annulation through C-H activation: insights from DFT calculations

Density functional theory calculations uncovered a new mechanism for the rhodium-catalyzed decarbonylative annulation of isatoic anhydride with alkynes, in which the acyloxy group formed from the N-H deprotonation and C-O bond cleavage of isatoic anhydride acts as the directing group to assist the ortho C-H activation. From the generated five-membered rhodacycle intermediate, the final aminoisocoumarin product could be formed by subsequent steps of alkyne insertion, reductive elimination, decarbonylation, and protonation. The isocyanate moiety contained in the annulation intermediate was uncovered as a novel internal oxidant for the reaction, which oxidizes the Rh(I) to Rh(III) by decarbonylation.

Ruthenium(II)-Catalyzed Annulation of Oximes with Maleimides: Synthesis of Pyrrolo[3,4-c]isoquinoline-1,3-diones

A series of pyrroloisoquinoline-1,3-diones has been synthesized using ruthenium(II) as the catalyst and oxygen as the oxidant in a straightforward manner. The reaction proceeds through a tandem C-C/C-N bond formation process between maleimides and ketoximes, providing a direct approach for the synthesis of the titled products. This operationally simple reaction procedure supplies suitable conditions for synthesizing diverse isoquinoline-based heterocycles with a range of functional groups in moderate to good yields and compatible with gram-scale synthesis. Furthermore, the compatibility of this reaction with oxygen as a green and environmentally friendly oxidant raises the importance of the present method.

Rh(III)-Catalyzed Switchable [4 + 1] and [4 + 2] Annulation of N-Aryl Pyrazolones with Maleimides: An Access to Spiro Pyrazolo[1,2-a]indazole-pyrrolidine and Fused Pyrazolopyrrolo Cinnolines

A rhodium(III)-catalyzed controllable [4 + 1] and [4 + 2] annulation of N-aryl pyrazolones with maleimides as C1 and C2 synthon has been explored for the synthesis of spiro[pyrazolo[1,2-a]indazole-pyrrolidines] and fused pyrazolopyrrolo cinnolines. The product selectivity was achieved through time-dependent annulation. The [4 + 1] annulation reaction involves sequential Rh(III)-catalyzed C-H alkenylation of N-aryl pyrazolone, followed by an intramolecular spirocyclization via aza-Michael-type addition to afford spiro[pyrazolo[1,2-a]indazole-pyrrolidine]. However, prolonged reaction time converts in situ formed spiro[pyrazolo[1,2-a]indazole-pyrrolidine] into fused pyrazolopyrrolocinnoline. This unique product formation switch proceeds via strain-driven ring expansion through a 1,2-shift of the C-C bond.

Recent Developments in Isoindole Chemistry

Isoindoles are highly reactive aromatic heterocycles that have a variety of important applications in areas such as medicine, analytical detection, and solar energy. Due to their highly reactive nature, isoindoles can be used to access their derivatives, which possess a diverse array of biological activities. However, their reactivity also makes isoindoles unstable and thus, difficult to prepare. Consequently, there has been a need for the development of novel methods that address some of the synthetic challenges and limitations, as well as reactions that utilize isoindoles to access potentially useful compounds. This review will give an overview of the novel reactions reported within the past decade (2012 to 2022) that involve 2H- and 1H-isoindoles and fused isoindoles as reactants, key intermediates, or products. This review is divided into two parts, with the first part focusing on the synthesis of isoindoles and the second part focusing on reactions of isoindoles. The scopes and limitations of the methods described therein will be discussed and the significance of their contributions to the literature will be highlighted. Similar reactions will also be compared.

1 Introduction

2 Synthesis of Isoindoles

2.1 Synthesis of 2H-Isoindoles

2.2 Synthesis of 1H-Isoindoles

3 Reactions of Isoindoles

3.1 Reactions of 2H-Isoindoles

3.2 Reactions of 1H-Isoindoles

4 Conclusions

Recent advances in transition-metal-catalyzed directed C-H alkenylation with maleimides

Transition-metal-catalyzed directed C-H alkenylation with maleimides has attracted much attention in recent years, as maleimide core moieties are present in various natural products and pharmaceuticals. In addition, these derivatives can be readily modified into biologically important compounds including succinimides, pyrrolidines and γ-lactams. The efficient chelation-assisted inert C-H bond activation strategy provides straightforward access to a wide array of structurally diverse molecules containing maleimide units. This review describes the major progress and mechanistic investigations on Heck-type reaction/cyclization of maleimides with organic molecules until early 2022.

Mild Three-Step Consecutive C-H Activations

Herein, the Rh-catalyzed consecutive C-H bond olefination/annulation/olefination cascade, tandemly directed by sulfonamide and ester groups, has been developed under mild conditions with the assistance of 1-adamantane carboxylic acid. A seven-membered metallacycle including an ester group was preferred to the five-membered one including a sulfonamide group for the third C-H activation. In this transformation, the Rh catalyst exhibits its high reactivity by catalyzing a triple C-H activation process with a low catalyst loading at 50 °C. This method can be applied in the construction of various pharmaceutical derivatives.

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.

Diastereoselective synthesis of chiral 3-substituted isoindolinones via rhodium(III)-catalyzed oxidative C-H olefination/annulation

A new method for the direct and stereoselective synthesis of 3-substituted isoindolinones via Rh(iii)-catalyzed chiral N-sulfinyl amide directed asymmetric [4 + 1] annulation of benzamides with acrylic esters has been developed. The reaction proceeded through an oxidative C-H olefination and a subsequent cyclization by intramolecular aza-Michael addition, producing a series of diastereoisomeric chiral isoindolinones (20 examples) in generally good yields with a dr value up to 5.5 : 1. The absolute configurations of the newly formed C-stereocenters in the major and minor diastereomers of the catalysis product have been determined by X-ray crystal diffraction analysis to be S and R, respectively. The separation of the major diastereoisomers from the catalysis products and subsequent removal of the N-sulfinyl chiral auxiliary afforded enantiomerically pure (S)-isoindolinones. The application of the obtained (S)-isoindolinones in the synthesis of several biologically active isoindolinones such as (S)-PD172938, (S)-pazinaclone and (S)-pagoclone is presented.

Silver(i)-catalyzed tandem reaction of enynones and 4-alkynyl isoxazoles: regioselective synthesis of highly functionalized 4H-furan[3,4-c]pyrroles

This work reports a silver(i)-catalyzed tandem reaction of enynones with 4-alkynyl isoxazoles.

Transition metal-catalyzed coupling of heterocyclic alkenes via C–H functionalization: recent trends and applications

Heterocyclic alkenes and their derivatives are an important class of reactive feedstock and valuable synthons. This review highlights the transition-metal-catalyzed coupling of heterocyclic alkenes via a C–H functionalization strategy.

Rhodium-Catalyzed Annulative Coupling of Isothiazoles with Alkynes through N-S Bond Cleavage

A Rh(III)-catalyzed annulative coupling of 3,5-diarylisothiazoles and alkynes is reported. The N-S bond in the isothiazole ring acts as an internal oxidant to regenerate the Rh(III) species in combination with an external Cu(II) oxidant, and the corresponding 1:2 coupling products are obtained. The remarkable difference in the reaction outcome between isothiazoles and the relevant isoxazoles has been investigated by DFT calculations, revealing that the relative stability of the enolate intermediates dictates the product selectivity.

Rh(III)-Catalyzed C–H Bond Activation for the Construction of Heterocycles with sp3-Carbon Centers

Rh(III)-catalyzed C–H activation features mild reaction conditions, good functional group tolerance, high reaction efficiency, and regioselectivity. Recently, it has attracted tremendous attention and has been employed to synthesize various heterocycles, such as indoles, isoquinolines, isoquinolones, pyrroles, pyridines, and polyheterocycles, which are important privileged structures in biological molecules, natural products, and agrochemicals. In this short review, we attempt to present an overview of recent advances in Rh(III)-mediated C–H bond activation to generate diverse heterocyclic scaffolds with sp3 carbon centers.

Rh(III)-Catalyzed Coupling of N-Chloroimines with α-Diazo-α-phosphonoacetates for the Synthesis of 2H-Isoindoles

We report herein the first use of N-chloroimines as effective synthons for directed C-H functionalization. Rh(III)-catalyzed coupling of N-chloroimines with α-diazo-α-phosphonoacetates allows for efficient dechlorinative/dephosphonative access to 2H-isoindoles. Further deesterification under Ni(II) catalysis enables the complete elimination of reactivity-assisting groups and full exposure of reactivity of C3 and N2 ring atoms for attaching structurally distinct appendages.

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.

Rhodium(III)-Catalyzed C-H Activation of α-Iminonitriles or α-Imino Esters and Cyclization with Acrylates to 2 H-Isoindoles

Rh(III) catalysts have played increasingly important roles in the activation of C-H bonds to build heterocyclic scaffolds. However, there are few reports on the more challenging synthesis of pharmaceutically important 2 H-isoindoles and fused 2 H-isoindoles. The process reported herein is an effective strategy to produce 2 H-isoindole or fused 2 H-isoindole derivatives via a Rh(III)-catalyzed transformation of α-iminonitriles or α-imino esters with acrylates.

Rhodium-catalyzed intramolecular cascade sequence for the formation of fused carbazole-annulated medium-sized rings by cleavage of C(sp2)-H/C(sp3)-H bonds

The rhodium(iii)-catalyzed intramolecular annulation of alkyne-tethered 3-(indol-3-yl)-3-oxopropanenitriles for the synthesis of fused carbazole scaffolds via C-H activation has been developed. A series of six-, seven-, and eight-membered hydroazepino[3,2,1-jk]carbazoles were achieved. This reaction proceeded under mild reaction conditions and with a broad substrate scope. The reaction involved sequential cleavage of C(sp2)-H/C(sp3)-H bonds and annulation with the tethered alkyne.

Impact of the Valence Charge of Transition Metals on the Cobalt- and Rhodium-Catalyzed Synthesis of Indenamines, Indenols, and Isoquinolinium Salts: A Catalytic Cycle Involving MIII/MV [M = Co, Rh] for [4 + 2] Annulation

Reaction mechanisms for the synthesis of indenamines, indenols, and isoquinolinium salts through cobalt- and rhodium-catalysis were investigated using density functional theory calculations. We found that the valence charge of transition metals dramatically influences the reaction pathways. Catalytic reactions involving lower-oxidation-state transition metals (M^I/M^III, M = Co and Rh) generally favor a [3 + 2] cyclization pathway, whereas those involving higher oxidation states (M^III/M^V) proceed through a [4 + 2] cyclization pathway. A catalytic cycle with novel M^III/M^V as a crucial species was successfully revealed for isoquinolinium salts synthesis, in which highly valent M^V was encountered not only in the [RhCp*]-catalysis but also in the [CoCp*]-catalysis.

Rh-Catalyzed regioselective C–H activation and C–C bond formation: synthesis and photophysical studies of indazolo[2,3-a]quinolines

Rh(iii)-Catalyzed oxidative annulation of 2-aryl-2H-indazoles with alkynes and their photophysical studies are reported with high quantum yields.

Mechanistic insight into the C7-selective C–H functionalization of N-acyl indole catalyzed by a rhodium complex: a theoretical study

The role of the additive AgNTf₂ and the origins of the reaction are clarified through our calculations.

Rhodium(III)‐Catalyzed Diastereoselective Synthesis of 1‐Aminoindanes via C−H Activation

The rhodium(III)‐catalyzed cross‐coupling reaction between N‐sulfonyl aldimines and various olefins such as maleimides, fumarates, maleates, α,β‐unsaturated ketones, acrylate and nitroalkenes is described. This transformation efficiently leads to the diastereoselective synthesis of pharmacologically privileged 1‐aminoindane derivatives via the C−H alkylation followed by subsequent intramolecular cyclization. Notably, single diastereomers in all cases were observed, and the relative stereochemistry of products was confirmed by the X‐ray crystallographic data.

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Mechanism of Rhodium-Catalyzed C-H Functionalization: Advances in Theoretical Investigation

Transition-metal-catalyzed cross-coupling has emerged as an effective strategy for chemical synthesis. Within this area, direct C-H bond transformation is one of the most efficient and environmentally friendly processes for the construction of new C-C or C-heteroatom bonds. Over the past decades, rhodium-catalyzed C-H functionalization has attracted considerable attention because of the versatility and wide use of rhodium catalysts in chemistry. A series of C-X (X = C, N, or O) bond formation reactions could be realized from corresponding C-H bonds using rhodium catalysts. Various experimental studies on rhodium-catalyzed C-H functionalization reactions have been reported, and in tandem, mechanistic and computational studies have also progressed significantly. Since 2012, our group has performed theoretical studies to reveal the mechanism of rhodium-catalyzed C-H functionalization reactions. We have studied the changes in the oxidation state of rhodium and compared the Rh(I)/Rh(III) catalytic cycle to the Rh(III)/Rh(V) catalytic cycle using density functional theory calculation. The development of advanced computational methods and improvements in computing power make theoretical calculation a powerful tool for the mechanistic study of rhodium chemistry. Computational study is able to not only provide mechanistic insights but also explain the origin of regioselectivity, enantioselectivity, and stereoselectivity in rhodium-catalyzed C-H functionalization reactions. This Account summarizes our computational work on rhodium-catalyzed C-H functionalization reactions. The mechanistic study under discussion is divided into three main parts: C-H bond cleavage step, transformation of the C-Rh bond, and regeneration of the active catalyst. In the C-H bond cleavage step, computational results of four possible mechanisms, including concerted metalation-deprotonation (CMD), oxidative addition (OA), Friedel-Crafts-type electrophilic aromatic substitution (S_EAr), and σ-complex assisted metathesis (σ-CAM) are discussed. Subsequent transformation of the C-Rh bond, for example, via insertion of CO, olefin, alkyne, carbene, or nitrene, constructs new C-C or C-heteroatom bonds. For the regeneration of the active catalyst, reductive elimination of a high-valent rhodium complex and protonation of the C-Rh bond are emphasized as potential mechanism candidates. In addition to detailing the reaction pathway, the regioselectivity and diastereoselectivity of rhodium-catalyzed C-H functionalization reactions are also commented upon in this Account. The origin of the selectivity is clarified through theoretical analysis. Furthermore, we summarize and compare the changes in the oxidation state of rhodium along the complete reaction pathway. The work described in this Account demonstrates that rhodium catalysis might proceed via Rh(I)/Rh(III), Rh(II)/Rh(IV), Rh(III)/Rh(V), or non-redox-Rh(III) catalytic cycles.

One‐pot Synthesis of Oxindoles through C−H Alkylation and Intramolecular Cyclization of Azobenzenes with Internal Olefins

The rhodium(III)‐catalyzed site‐selective C−H alkylation of azobenzenes and internal olefins, such as maleimides, maleates and fumarates, followed by reductive intramolecular cyclization is described. A cationic rhodium catalyst in the presence of acetic acid additive in dichloroethane solvent was found to be the optimal catalytic system for the construction of ortho‐alkylated azobenzenes, which smoothly underwent the intramolecular cyclization leading to the formation of C3‐functionalized oxindoles in the presence of zinc powder and acetic acid. The formed oxindole scaffold could be an important asset towards the development of novel bioactive compounds.

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C-H Activation and Alkyne Annulation via Automatic or Intrinsic Directing Groups: Towards High Step Economy

Direct transformation of carbon-hydrogen bond (C-H) has emerged to be a trend for construction of molecules from building blocks with no or less prefunctionalization, leading high atom and step economy. Directing group (DG) strategy is widely used to achieve higher reactivity and selectivity, but additional steps are usually needed for installation and/or cleavage of DGs, limiting step economy of the overall transformation. To meet this challenge, we proposed a concept of automatic DG (DG^auto ), which is auto-installed and/or auto-cleavable. Multifunctional oxime and hydrazone DG^auto were designed for C-H activation and alkyne annulation to furnish diverse nitrogen-containing heterocycles. Imidazole was employed as an intrinsic DG (DGⁱⁿ ) to synthesize ring-fused and π-extended functional molecules. The alkyne group in the substrates can also be served as DGⁱⁿ for ortho-C-H activation to afford carbocycles. In this account, we intend to give a review of our progress in this area and brief introduction of other related advances on C-H functionalization using DG^auto or DGⁱⁿ strategies.