Construction of Isoquinolone Scaffolds on DNA via Rhodium(III)-Catalyzed C-H Activation

Isoquinolone is one of the most common heterocyclic core structures in countless natural products and many bioactive compounds. Here, a highly efficient approach to synthesize isoquinolone scaffolds on DNA via rhodium(III)-catalyzed C-H activation has been described. This chemistry transformation is robust and has shown good compatibility with DNA, which is suitable for DNA-encoded library synthesis.

Transition Metal-Catalyzed C-H Activation/Annulation Approaches to Isoindolo[2,1-b]isoquinolin-5(7H)-ones

The isoindolo[2,1-b]isoquinolin-5(7H)-one scaffold is widely present in lots of bioactive natural products. Diverse types of strategies have been developed to construct this scaffold. Recently, transition metal-catalyzed C-H activation/annulation is emerging as a powerful and straightforward method to construct diverse polyheterocycles with high atom- and step-economy. It also has been employed for the synthesis of the isoindolo[2,1-b]isoquinolin-5(7H)-one scaffold. This review provides an introduction to recent advances for the preparation of isoindolo[2,1-b]isoquinolin-5(7H)-ones by using transition metal-catalyzed C-H activation/annulation. It will help researchers to find hidden opportunities and accelerate the discovery of novel transformations based on C-H activation/annulation.

Rh(III)-Catalyzed C-H Activation/Annulation of Benzohydroxamates and 2-Imidazolones: Access to Urea-Fused-Dihydroisoquinolone Scaffolds Reminiscent of Pyrrole Alkaloid Natural Products

A Rh(III)-catalyzed C-H activation/annulation with an imidazolone as alkene partner is reported to access dihydroisoquinolone-fused imidazolin-2-ones. These bicycles are reminiscent of scaffolds belonging to the pyrrole alkaloid family of natural products. This approach facilitates construction of a variety of urea-fused dihydroisoquinolone scaffolds including heterocyclic moieties.

Rhodium(III)-Catalyzed Asymmetric 1,2-Carboamidation of Alkenes Enables Access to Chiral 2,3-Dihydro-3-benzofuranmethanamides

Through the initial screening and further rational design of chiral cyclopentadienyl ligands, a chiral rhodium-catalyzed enantioselective 1,2-carboamidation of aromatic tethered alkenes was developed, enabling the asymmetric preparation of various chiral 2,3-dihydro-3-benzofuranmethanamides with an enantioenriched all-carbon quaternary center at the β position of amide. This robust transformation has a broad functional group tolerance, excellent enantioselectivities (up to 98.5:1.5 er), and a mild reaction conditions, releasing CO₂ as the single byproduct.

RhodiumIII-Catalyzed Remote Difunctionalization of Arenes Assisted by a Relay Directing Group

Rhodium-catalyzed diverse tandem twofold C-H bond activation reactions of para olefin-tethered arenes have been realized, with unsaturated reagents such as internal alkynes, dioxazolones, and isocyanates being the coupling partner as...

Recent advances in transition metal-catalyzed heteroannulative difunctionalization of alkenes via C-H activation for the synthesis of heterocycles

Heterocyclic compounds are the fundamental structural motifs distributed in natural products, pharmaceuticals and biologically active compounds. Thus, there is increasing interest in the development of novel synthetic strategies for the...

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

Rhodium-Catalyzed Redox-Neutral Olefination of Aryldiazenes with Acrylate Esters via C-H Activation and Transfer Hydrogenation

Rh(III)-catalyzed redox-neutral C-H olefination of aryldiazenecarboxylates has been realized using arylate esters as the olefinating reagents. This reaction proceeds under mild and redox-neutral conditions, resulting in integration of C-H activation and transfer hydrogenation. The chemoselectivity complements that of previously reported rhodium-catalyzed coupling of the same substrates.

Rhodium-Catalyzed One-Pot Access to N-Polycyclic Aromatic Hydrocarbons from Aryl Ketones through Triple C-H Bond Activations

A Rh-catalyzed pot and step economic synthesis of aza-polycyclic aromatic hydrocarbons (N-PAHs) from readily available aryl ketones and alkynes has been disclosed. Additionally, a novel synthetic application of the well-known aminating reagent hydroxylamine-O-sulfonic acid (HOSA) has been explored as an in situ redox-neutral directing group for the formation of N-PAHs via isoquinoline. Multiple bond formation in a single operation through a cascade of triple C-H bond activations is the beauty of this protocol. The challenging annulations of two different alkynes in a regioselective fashion have been demonstrated effectively. Mechanistic studies reveal that 3,4-diphenyl-1-methylisoquinoline is an active intermediate for this one-pot transformation.

Rh(iii)-catalyzed tandem annulative redox-neutral arylation/amidation of aromatic tethered alkenes

Transition-metal-catalyzed directed C–H functionalization has emerged as a powerful and straightforward tool to construct C–C bonds and C–N bonds. Among these processes, the intramolecular annulative alkene hydroarylation reaction has received much attention because this intramolecular annulation can produce more complex and high value-added structural motifs found in numerous natural products and bioactive molecules. Despite remarkable progress, these annulative protocols developed to date remain limited to hydroarylation and functionalization of one side of alkenes, thus largely limiting the structural diversity and complexity. Herein, we developed a rhodium(iii)-catalyzed tandem annulative arylation/amidation reaction of aromatic tethered alkenes to deliver a variety of 2,3-dihydro-3-benzofuranmethanamine derivatives bearing an all-carbon quaternary stereo center by employing 3-substituted 1,4,2-dioxazol-5-ones as an amidating reagent to capture the transient C(sp³)–Rh intermediate. Notably, by simply changing the directing group, a second, unsymmetrical ortho C–H amidation/annulation can be achieved to provide tricyclic dihydrofuro[3,2-f]quinazolinones in good yields.

A rhodium(iii)-catalyzed tandem annulative arylation/amidation reaction of aromatic tethered alkenes was developed to deliver a variety of 2,3-dihydro-3-benzofuranmethanamine derivatives.

Transition Metal-Catalyzed Intermolecular Cascade C-H Activation/Annulation Processes for the Synthesis of Polycycles

Polycycles are abundantly present in numerous advanced chemicals, functional materials, bioactive molecules and natural products. However, the strategies for the synthesis of polycycles are limited to classical reactions and transition metal-catalyzed cross-coupling reactions, requiring pre-functionalized starting materials and lengthy synthetic operations. The emergence of novel approaches shows great promise for the fields of organic/medicinal/materials chemistry. Among them, transition metal-catalyzed C-H activation followed by intermolecular annulation reactions prevail, due to their straightforward manner with high atom- and step-economy, providing rapid, concise and efficient methods for the construction of diverse polycycles. Several strategies have been developed for the synthesis of polycycles, relying on sequential multiple C-H activation/annulation, or combination of C-H activation/annulation and further interaction with a proximal group, or merger of C-H activation with a cycloaddition reaction, or in situ formation of the directing group. These are attractive, efficient, step- and atom-economic methods starting from commercially available materials. This Minireview will provide an introduction to transition metal-catalyzed C-H activation for the synthesis of polycycles, helping researchers to discover indirect connections and reveal hidden opportunities. It will also promote the discovery of novel synthetic strategies relying on C-H activation.

A New Class of C2 -Symmetric Chiral Cyclopentadienyl Ligand Derived from Ferrocene Scaffold: Design, Synthesis and Application

A new class of C₂ -symmetric, chiral cyclopentadienyl ligand based on planar chiral ferrocene backbone was developed. A series of its corresponding rhodium(I), iridium(I), and ruthenium(II) complexes were prepared as well. In addition, the rhodium(I) complexes were evaluated in the asymmetric catalytic intramolecular amidoarylation of olefin-tethered benzamides via C-H activation.

Computational Study on the Fate of Oxidative Directing Groups in Ru(II), Rh(III), and Pd(II) Catalyzed C-H Functionalization

Activation of C-H bonds assisted by a directing group is indispensable in organic synthesis. Among them, utilizing oxidative directing groups that can serve as an internal oxidant to drive the Mⁿ/Mⁿ⁺² catalytic cycle has recently become a promising strategy. A survey of published reactions involving N-alkoxyamides or N-acyloxyamides reveals that not all N-O bonds act as an internal oxidant. We have therefore systematically investigated the effect of the oxidative groups on a model reaction catalyzed by Ru(II), Rh(III), and Pd(II) complexes. DFT calculations show that N-methoxy and N-acyloxy groups oxidize Ru(II) to Ru(IV) and Rh(III) to Rh(V), but cannot oxidize a cyclo-Pd(II) intermediate to Pd(IV). The stability of the metal imido intermediate 7-M (M = Ru, Rh, and Pd) controls whether the oxidation occurs or not. N-Acyloxy groups show a more pronounced selectivity than N-methoxy to oxidize Ru(II) and Rh(III) species, while no distinctive effect is observed for Pd(II).

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.

Merging Cu-catalysed C–H functionalisation and intramolecular annulations: computational and experimental studies on an expedient construction of complex fused heterocycles

A copper-catalysed protocol for the synthesis of fused dihydrobenzofuran-isoquinolone compounds through an intramolecular annulation of readily accessible benzamide substrates is reported, along with a full DFT study into the mechanism.

Ene reactions of 2-borylated α-methylstyrenes: a practical route to 4-methylenechromanes and derivatives

4-Methylenechromanes were prepared via a three-step process from 2-borylated α-methylstyrenes. This sequence is based on a key glyoxylate-ene reaction catalyzed by scandium(iii) triflate. The resulting γ-hydroxy boronates, which cyclise to seven-membered homologues of benzoxaborole on silica gel, were cleanly oxidized with sodium perborate, and then cyclised under Mitsunobu conditions. Additionally, several further functional transformations of 4-methylenechromanes or their precursors were carried out to illustrate the synthetic potential of these intermediates.

Synthesis of Indole-Fused Polycyclics via Rhodium-Catalyzed Undirected C-H Activation/Alkene Insertion

A Rh(III)-catalyzed undirected C-H activation/alkene insertion to synthesize diversified indole-fused polycyclics has been developed. Intramolecular electrophilic cyclization generated a 3-indolyl rhodium species that went through an aryl-to-aryl 1,4-rhodium migration to realize the C-H activation. The subsequent [4 + 2] carboannulation or hydroarylation of alkenes could be achieved, respectively, by simply adjusting the additives of the reaction.

Rhodium(III)-Catalyzed C-H Activation: Ligand-Controlled Regioselective Synthesis of 4-Methyl-Substituted Dihydroisoquinolones

Rh-catalyzed C-H functionalization of O-pivaloyl benzhydroxamic acids with propene gas provides access to 4-methyl-substituted dihydroisoquinolones. Good to excellent levels of regioselectivity are achieved using [Cp^tRhCl₂]₂ as a precatalyst under optimized conditions. Thorough examination of aryl/heteroaryl O-pivaloyl hydroxamic acid substrates, ligand effects on C-H site selectivity, alkene scope, and demonstration of scale are discussed within.

Rhodium(III)-Catalyzed Intramolecular Olefin Hydroarylation of Aromatic Aldehydes Using a Transient Directing Group

A Rh(III)-catalyzed intramolecular olefin hydroarylation of aromatic aldehydes with a transient directing group has been described. The bidentate directing groups in situ generated from aromatic aldehydes and β-alanine could enable the subsequent C-H activation and hydroarylation with excellent site selectivities and high functional group compatibility. The further conversion of the aldehyde group showcased the broad application prospects of this methodology.

Rhodium(III)-Catalyzed Intramolecular Annulations of Acrylic and Benzoic Acids to Alkynes

Rh(III) catalysts can promote a formal (4 + 2) intramolecular oxidative annulation between acrylic or benzoic acid derivatives and alkynes. The reaction, which involves a C-H activation process, allows for a rapid assembly of appealing bicyclic pyran-2-ones and tricyclic isocoumarin derivatives in moderate to good yields. The α-pyrone moiety of the products provides for further manipulations to obtain relatively complex cyclic skeletons in a very simple manner.

Pd-Catalyzed Alkyne Insertion/C-H Activation/[4 + 2] Carboannulation of Alkenes to the Synthesis of Polycyclics

An unprecedented Pd-catalyzed alkyne insertion/C-H activation/intramolecular [4 + 2] carboannulation of alkenes has been reported. In this transformation, the C-H activation was triggered by an in situ generated alkenylpalladium species via the Pd-catalyzed cross-coupling reaction of aryl iodides and alkynes. Subsequently, the resulting five-membered C, C-palladacycle intermediates were added across the alkenes, providing a unique approach to access diversified polycyclics in good efficiency. Two new rings and three C-C bonds were formed in one pot.

Silver-promoted regioselective [4+2] annulation reaction of indoles with alkenes to construct dihydropyrimidoindolone scaffolds

An Ag^I-promoted regioselective [4+2] annulation reaction of indoles with alkenes has been established.

Palladium-catalyzed annulation of N-alkoxy benzsulfonamides with arynes by C–H functionalization: access to dibenzosultams

Palladium-catalyzed C–H/N–H functionalization of N-alkoxy benzsulfonamides with arynes provides dibenzosultams via C–C/C–N bond formation, accompanied by an unexpected N–O bond cleavage.

Rh(III)-Catalyzed [4 + 2] Self-Annulation of N-Vinylarylamides

An efficient rhodium(III)-catalyzed self-annulation of N-vinylarylamide has been developed. This reaction features a simple system and good reactivity with complete regioselectivity. The protocol provides easy access to an aminal incorporated dihydroisoquinolinone, which proved to be a versatile synthetic synthon. The kinetic isotope effect experiments showed that C-H activation is the rate-limiting step, and competition studies revealed the annulation exhibits a strong self-recognition mode. In addition, a seven-membered rhodacycle species was isolated and established as the key reaction intermediate.

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.

A tethering directing group strategy for ruthenium-catalyzed intramolecular alkene hydroarylation

We report a new catalyst design for N-heterocycle synthesis that utilizes an alkene-tethered amide moiety as a directing group for aromatic C-H activation. This tethering directing group strategy is demonstrated in a ruthenium-catalyzed intramolecular alkene hydroarylation with N-aryl acrylamides to form oxindole products.

Mild rhodium(iii)-catalyzed intramolecular annulation of benzamides with allylic alcohols to access azepinone derivatives

A mild intramolecular Rh(iii)-catalyzed annulation by C–H activation to access azepinone derivatives has been developed.

C–H bond cleavage occurring on a Rh(v) intermediate: a theoretical study of Rh-catalyzed arene azidation

Our theoretical calculations indicated that the oxidation of Rh(iii) to Rh(v) by PhI(OAc)OTs is a facile process. Subsequent electrophilic deprotonation was shown to occur from a Rh(v) intermediate rather than a Rh(iii) intermediate.

O-Substituted group-controlled selectivity in Rh(iii)-catalyzed coupling of benzamides with α,α-difluoromethylene alkynes: a computational mechanistic study

The selectivity mediated by the O-substituted groups depends on the N–O bond strength and the coordination of the O-substituted groups.