Propargyl Alcohols as One-Carbon Synthons: Redox-Neutral Rhodium(III)-Catalyzed C–H Bond Activation for the Synthesis of Isoindolinones Bearing a Quaternary Carbon

Combinatorial synthesis of substituted pyrazolo-fused quinazolines by the Rh(III)-catalyzed [5 + 1] annulation of phenyl-1H-pyrazol-5-amine with alkynoates and alkynamides

A Rh(III)-catalyzed C-H activation/cyclization cascade was developed for the first divergent synthesis of pyrazolo[1,5-a]quinazolines through a [5 + 1] annulation reaction exclusively. The one-pot procedure is recognized for its broad substrate scope, functional group tolerance, and high atom economy. Mechanistic studies reveal the reaction pathway, addressing current limitations. Notably, this catalytic transition metal-assisted tandem annulation smoothly proceeds through the reaction of substituted phenyl-1H-pyrazol-5-amine with an alkyne ester or amide, where a one ring carbon is provided by the alkynoate building block. This transformation highlights the potential of transition metal-catalyzed methods for synthesizing diverse pyrazolo[1,5-a]quinazoline frameworks.

Rapid Synthesis of Benzimidazole-Fused Isoindoles by Rh(III)/Ru(II)-Catalyzed [4 + 1] Cascade C-H/N-H Annulation of 2-Arylbenzimidazoles with Alkynoates and Alkynamide

A Rh(III)-catalyzed [4 + 1] cyclization of 2-arylbenzimidazoles with alkynoates through C-H activation/ortho-alkenylation/intramolecular annulation cascade to obtain benzimidazole-fused isoindoles is reported. The reaction of the Rh catalyst and internal alkyne ester provides benzo[4,5]imidazo[2,1-a]isoindole acetate exclusively. Conversely, internal alkyne amide participates in the annulation process in the presence of a Ru catalyst to provide benzo[4,5]imidazo[2,1-a]isoindole acetamide. The alkyne acts as a C1 synthon and undergoes [4 + 1] cyclization rather than traditional [4 + 2] annulation.

Merging Rh-Catalyzed C-H Functionalization and Cascade Cyclization to Enable Propargylic Alcohols as Three-Carbon Synthons

Reported herein is a reactivity of propargyl alcohols as "Three-Carbon Synthons" in a Rh(III)-catalyzed C-H functionalization of acetanilides, leading to the synthesis of core structures of isocryptolepine, γ-carbolines, dihydrochromeno[2,3-b]indoles, and diindolylmethanes (DIM) derivatives. The transformation involves a rhodium(III)-catalyzed C-H functionalization and heteroannulation to yield indoles followed by a cascade cyclization with both external and internal nucleophiles to afford diverse products. The role of the hydroxy group, the key function of the silver additive, the origin of the reverse regioselectivity and the rate-determining step, are rationalized in conformity with the combination of experimental, noncovalent interaction analysis and DFT studies. This protocol is endowed with several salient features, including one-pot multistep cascade approach, exclusive regioselectivity, good functional group tolerance and synthesis of variety of molecular frameworks.

Rhodium-catalyzed aminoacylation of alkenes via carbonylative C–H activation toward poly(hetero)cyclic alkylarylketones

This work discloses the facile construction of polyheterocyclic alkylarylketones by the rhodium-catalyzed carbonylative aminoacylation of alkenes involving C–H activation, which provides molecules as candidates for the screening of antitumor agents.

Synthesis of 3-(2-oxopropyl)-2-arylisoindolinone derivatives via a three-component reaction of diaryliodonium salts with 2-formylbenzonitriles and phenylacetylenes

A copper-catalyzed multicomponent cascade cyclization using readily available substrates, including 2-formylbenzonitriles, phenylacetylenes, and diaryliodonium salts, is achieved. A broad reaction scope is presented with good functional group compatibility, giving rise to a range of 3-(2-oxopropyl)-2-arylisoindolinones in moderate to good yields.

Scalable (Enantioselective) Syntheses of Novel 3-Methylated Analogs of Pazinaclone, (S)-PD172938 and Related Biologically Relevant Isoindolinones

Herein, we report the application of an efficient and practical K₂CO₃ promoted cascade reaction of 2-acetylbenzonitrile in the synthesis of novel 3-methylated analogs of Pazinaclone and PD172938, belonging to isoindolinones heterocyclic class bearing a tetrasubstituted stereocenter. Organocatalytic asymmetric synthesis of the key intermediate and its transformation into highly enantioenriched 3-methylated analog of (S)-PD172938 was also developed. These achievements can be of particular interest also for medicinal chemistry, since the methyl group is a very useful structural modification in the rational design of new and more effective bioactive compounds.

Rh(III)-Catalyzed (4 + 1) Annulation of Pyrazol-3-ones with Alkynoates via Ortho-Alkenylation/Cyclization Cascade: Synthesis of Indazole-Fused Pyrazoles

A facile synthesis of novel indazole-fused pyrazoles from pyrazol-3-ones and alkynoate esters/amides via Rh(III)-catalyzed sequential C-H activation/ortho-alkenylation/intramolecular cyclization cascade is reported. The important characteristic of this method is that the resulting scaffold bearing quaternary carbon has been obtained through unusual [4 + 1] rather than expected [4 + 2] addition where alkynoate acts as a one-carbon unit.

Rh(III)-Catalyzed C-H Activation and [4+1+1] Sequential Cyclization Cascade to Give Highly Fused Indano[1,2-b]azirines

A Rh(III)-catalyzed C-H activation of α-keto oximes and a cyclization cascade with diazo compounds were developed to construct the highly fused indano[1,2-b]azirine frameworks in good yields with a broad range of substrates under mild reaction conditions. More intriguingly, a [4+1+1] sequential annulation cascade is demonstrated for the first time in this reaction and opened a new reaction mode for α-keto oximes. These fused indano[1,2-b]azirine derivatives could also be further transformed into intriguing privileged drug scaffolds.

Isoindolinone synthesis through Rh/Cu-catalyzed oxidative C-H/N-H annulation of N-methoxy benzamides with saturated ketones

The synthesis of isoindolinones from N-methoxy benzamides and saturated ketones via a bimetallic tandem catalytic annulation has been accomplished. The reaction is catalyzed by a Rh/Cu-cocatalytic system and proceeds via the combination of Cu-catalyzed dehydrogenation of ketones and Rh-catalyzed direct C-H functionalization with the assistance of the N-methoxy amide group which also acts as an oxidant to regenerate the Rh catalyst. This method shows good compatibility with a wide range of substrates and functional groups, and provides an alternative strategy to obtain diverse isoindolinones.

Redox-neutral rhodium(III)-catalyzed chemo- and regiospecific [4 + 1] annulation between benzamides and alkenes for the synthesis of functionalized isoindolinones

Herein, using electron-deficient alkenes embedded with an oxidizing function/leaving group as a rare and nontraditional C₁ synthon, we have achieved the redox-neutral Rh(III)-catalyzed chemo- and regioselective [4 + 1] annulation of benzamides for the synthesis of functionalized isoindolinones. This method features broad substrate scope, good to excellent yields, excellent chemo- and regioselectivity, good tolerance of functional groups and mild external-oxidant-free conditions.

Electrodimerization of N-Alkoxyamides for the Synthesis of Hydrazines

An efficient and valuable N-N dimerization reaction of N-alkoxyamides is reported under undivided electrolytic conditions. This electrochemical strategy provides a powerful way to access a wide range of advanced, highly functionalized hydrazines. Remarkably, an N-centered radical generated from the cleavage of the N-H bond under electrolytic conditions plays a crucial role in this transformation. Furthermore, various N-alkoxyamides bearing different substituents are suitable in this transformation, furnishing the corresponding hydrazines in up to 92% yield.

Easy synthesis of imidazo[1,5-a]indol-3-ones through Rh(III)-catalyzed C-H allenylation/annulation

A highly efficient and regioselective synthesis of imidazo[1,5-a]indol-3-ones has been developed via a sequential C-H allenylation/annulation starting from easily available N-methoxycarbamoyl indoles and propargyl alcohols, in which the propargyl alcohols served as a C1 synthon. This strategy displays excellent regioselectivity, high atom economy and tolerates a broad substrate scope.

Mechanistic Insights into the Dual Directing Group-Mediated C-H Functionalization/Annulation via a Hydroxyl Group-Assisted MIII-MV-MIII Pathway

The experimental investigations on the catalyst [Cp*Rh(OAc)₂ and Cp*Ir (OAc)₂)]-controlled [3 + 2] and [4 + 2] annulations of oximes with propargyl alcohols have been finished in our previous work and a supposed dual directing group-mediated reaction pathway has been deduced for the chemodivergent product synthesis. However, the detailed interaction modes of the dual directing groups binding with the corresponding metal center to achieve the above observed chemoselectivity remain unclear and even contradict. For instance, the calculational traditional dual direct coupling transition states suggested that both Cp*Rh(OAc)₂- and Cp*Ir(OAc)₂-catalyzed reactions would generate five-membered indenamines as the dominant products via [3 + 2] annulation. To address this concern, herein, systematic DFT calculations combined with proof-of-concept experiments have been carried out. Accordingly, a novel and more favorable M^III-M^V-M^III reaction mechanism, which involves an unprecedented HOAc together with a hydroxyl group-assisted reaction pathway in which the hydroxyl group acts as double effectors for the formation of M-O coordination and [MeO···H···O(CCH₃)O···H···O] bonding interactions, was deduced. Taken together, the present results would provide a rational basis for future development of the dual directing group-mediated C-H activation reactions.

Catalytic System-Controlled Divergent Reaction Strategies for the Construction of Diversified Spiropyrazolone Skeletons from Pyrazolidinones and Diazopyrazolones

A catalytic system-controlled divergent reaction strategy was here reported to construct four types of intriguing spiroheterocyclic skeletons from simple and readily available starting materials via a precise chemical bond activation/[n+1] annulation cascade. The tetraazaspiroheterocyclic and trizazspiroheterocyclic scaffolds could be independently constructed by a selective N-N bond activation/[n+1] annulation cascade, a C(sp² )-H activation/[4+1] annulation and a novel tandem C(sp² )-H/C(sp³ )-H bond activation/[4+1] annulation strategy, along with a broad scope of substrates, moderate to excellent yields and valuable transformations. More importantly, in these transformations, we are the first time to capture a N-N bond activation and a C(sp³ )-H bond activation of pyrazolidinones under different catalytic system.

Rh(III)-Catalyzed [5 + 1] Annulation of Indole-enaminones with Diazo Compounds To Form Highly Functionalized Carbazoles

A novel Rh(III)-catalyzed C-H activation/annulation cascade of indole-enaminones with diazo compounds was reported to construct diversely functionalized carbazole frameworks. The most notable characteristic is that this transformation could smoothly furnish a novel [5 + 1] cyclization product with good to excellent yields (up to 95%), accompanied by the thorough removal of acetyl and N,N-dimethyl groups of two substrates from the target products, rather than the normally expected [4 + 2] cyclization products.

Isoindolinone Synthesis via One-Pot Type Transition Metal Catalyzed C-C Bond Forming Reactions

Isoindolinone structure is an important privileged scaffold found in a large variety of naturally occurring as well as synthetic, biologically and pharmaceutically active compounds. Owing to its crucial role in a number of applications, the synthetic methodologies for accessing this heterocyclic skeleton have received significant attention during the past decade. In general, the synthetic strategies can be divided into two categories: First, direct utilization of phthalimides or phthalimidines as starting materials for the synthesis of isoindolinones; and second, construction of the lactam and/or aromatic rings by different catalytic methods, including C-H activation, cross-coupling, carbonylation, condensation, addition and formal cycloaddition reactions. Especially in the last mentioned, utilization of transition metal catalysts provides access to a broad range of substituted isoindolinones. Herein, the recent advances (2010-2020) in transition metal catalyzed synthetic methodologies via formation of new C-C bonds for isoindolinones are reviewed.

A Rh(iii)-catalyzed C–H activation/regiospecific annulation cascade of benzoic acids with propargyl acetates to unusual 3-alkylidene-isochromanones

We developed a new approach to synthesize isochromanones with benzoic acids and propargyl acetates, which introducing an unusual exocyclic C–C double bond at the 3-position with high regioselectivity and moderate to excellent yields.

Formation of diversified spiro-[imidazole-indene] derivatives from 2H-imidazoles: based on versatile propargyl alcohols

Rh(iii)-Catalyzed efficient cascade annulation for the regioselective construction of various spiro-[imidazole-indene] derivatives has been reported by utilizing versatile propargyl alcohols as coupling partners.

Rh(III)-Catalyzed Dual C-H Functionalization/Cyclization Cascade by a Removable Directing Group: A Method for Synthesis of Polycyclic Fused Pyrano[de]Isochromenes

An interesting Rh(III)-catalyzed dual C-H functionalization/cyclization cascade of azomethine imine with diazophosphonate by a removable directing group for the synthesis of highly fused pyrano[de]isochromene has been achieved. The transformation shows that the desired pyrano[de]isochromenes with two oxygen atoms on its core scaffold could be constructed with good to excellent yields (up to 86%) via a facile one-pot, multiple-step cascade reaction, along with broad generality and versatility.

Ruthenium-catalyzed cascade C-H activation/annulation of N-alkoxybenzamides: reaction development and mechanistic insight

A highly selective ruthenium-catalyzed C–H activation/annulation of alkyne-tethered N-alkoxybenzamides has been developed. In this reaction, diverse products from inverse annulation can be obtained in moderate to good yields with high functional group compatibility. Insightful experimental and theoretical studies indicate that the reaction to the inverse annulation follows the Ru(ii)–Ru(iv)–Ru(ii) pathway involving N–O bond cleavage prior to alkyne insertion. This is highly different compared to the conventional mechanism of transition metal-catalyzed C–H activation/annulation with alkynes, involving alkyne insertion prior to N–O bond cleavage. Via this pathway, the in situ generated acetic acid from the N–H/C–H activation step facilitates the N–O bond cleavage to give the Ru-nitrene species. Besides the conventional mechanism forming the products via standard annulation, an alternative and novel Ru(ii)–Ru(iv)–Ru(ii) mechanism featuring N–O cleavage preceding alkyne insertion has been proposed, affording a new understanding of transition metal-catalyzed C–H activation/annulation.

A highly selective ruthenium-catalyzed C–H activation/annulation through a pathway involving N–O bond cleavage prior to alkyne insertion is developed.

Regio- and Stereoselective Synthesis of the Core Structure of Hexahydrobenzo[c]phenanthridine Alkaloids via Redox-Neutral Cp*Rh(III)-Catalyzed C-H/N-H Annulation of Cyclic Alkenes with Benzamides

A highly stereo- and regioselective synthesis of the core skeleton of hexahydrobenzo[c]phenanthridine-type alkaloids is reported herein for the first time. A wide range of substrate scope, excellent functional group tolerance, and good to excellent yields were observed. This protocol gives very concise and efficient access to the core skeleton of chelidonine alkaloids as compared to the earlier approaches.

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.

Cp*Rh(III)-Catalyzed C-H Bond Difluorovinylation of Indoles with α,α-Difluorovinyl Tosylate

Unprecedented Rh(III)-catalyzed C-H bond difluorovinylation of indoles has been successfully developed, and this method provided an example of direct difluorovinylation reaction through C-H bond activation which was rarely reported. In this context, N-ethoxycarbamoyl served as the directing group and 2,2-difluorovinyl tosylates were used for the construction of difluorovinyl-substituted indoles. This method provided a practical strategy for difluorovinylation of indoles with moderate to good yields and is characterized by the broad synthetic utility, mild conditions, and high efficiency.