Ru(II)-Catalyzed C–H Functionalization of N-Hydroxyoximes with 1,3-Diynes Unveils a Regioselective Disparity

Rh(III)-Catalyzed [4 + 2] Annulation and Dehydrogenative Annulation of N-Chloroimines with Maleimides

We herein report a Rh(III)-catalyzed C-H bond coupling of N-chloroimines with maleimides, in which the [4 + 2] annulation and dehydrogenative annulation processes can be selectively achieved by simply adjusting the reaction conditions. This protocol is compatible with various functional groups, shows exquisite selectivity, and presents a concise synthetic procedure to respective products in moderate to good yields. With all these merits, this strategy may be applicable in the construction of related azaheterocyclic skeletons.

Silver-Catalyzed Markovnikov Addition Hydrofunctionalization of Terminal Alkynes: Synthesis of N-Enoxyimides and Oximes

N-Enoxyimides are versatile and valuable synthetic synthons in modern organic synthesis and catalysis. Herein, a silver-catalyzed hydrooxyimidation of terminal alkynes that enables the synthesis of N-enoxyimides, has been demonstrated. The protocol features are simple, step- and atom-efficient, scalable, and exhibit a broad scope of functional group tolerance under mild conditions. A novel hydrolysis of the N-enoxyimides unexpectedly provides the corresponding aromatic oximes in 96% yield, showing the potential application of this strategy.

Pragmatic Access to Hybrid Quinoxaline Scaffold Mediated by Elemental Sulfur Enabling Actualization to π-Extended and Aza-Annulated Heterocyclic Units

A metal-free approach for synthesizing hybrid quinoxaline derivatives from sulfoxonium ylide and a 1,5-bis-nucleophilic N-heterocycle mediated by elemental sulfur is presented to illuminate the [5+1] cascade cyclization sequence. Large-scale synthesis and postsynthetic functionalizations for the annulative π-extension and intramolecular aza-annulation reactions reveal the potential utility and actualize the fabricated approach.

Modular Assembly of Pyrrolo[3,4-c]isoquinolines through Rh-Catalyzed Cascade C-H Activation/Annulation of O-Methyl Aryloximes with Maleimides

An efficient and practical strategy for the construction of pyrrolo[3,4-c]isoquinolines via Rh(III)-catalyzed cascade C-H activation and subsequential annulation process from easily available O-methyl aryloximes and maleimides has been disclosed. This facile protocol does not require any inert atmosphere protection with good efficiency in a low loading of catalyst and exhibits good functional group tolerance and broad substrate scope. Notably, the as-prepared products show potential photophysical properties.

Deciphering the Origin of Regioselectivity in Ru(II)-Catalyzed C-H Annulation of N-Chlorobenzamides with 1,3-Diynes

Understanding the reaction mechanism and origin of regioselectivity in transition metal-catalyzed C-H activation/annulation reactions with 1,3-diynes has remained an intriguing challenge. In this article, to establish the mechanism and decipher the origin of regioselectivity, we report a detailed computational density functional theory-based mechanistic investigation on the recently developed Ru(II)-catalyzed [4 + 2] annulation of N-chlorobenzamides with 1,3-diynes for the synthesis of 3-alkynylated isoquinolone derivatives. Our calculations reveal a redox-neutral pathway for the annulation reaction. The stepwise analysis of the reaction channels indicates the migratory insertion step and the concerted reductive elimination/oxidative addition of the Ru(p-cymene) moiety to form the N-C bond leading to the 3-alkynylated product to be the selectivity- and rate-determining steps, respectively. Finally, the distortion/interaction analysis using the activation-strain model suggests the steric effect as the determining factor for the observed regioselectivity for the formation of the 3-alkynylated product. Overall, the computationally obtained key insights into the catalytic mechanism and the origin of regioselectivity in the C-H activation/annulation reaction can be used as a guide to rationally design and develop novel transformation strategies for heterocycle synthesis.

Rh(iii)-catalyzed sp3/sp2-C-H heteroarylations via cascade C-H activation and cyclization

The development of an efficient strategy for facile access to quinoline-based bis-heterocycles holds paramount importance in medicinal chemistry. Herein, we describe a unified approach for accessing 8-(indol-3-yl)methyl-quinolines by integrating Cp*Rh(iii)-catalyzed C(sp3)-H bond activation of 8-methylquinolines followed by nucleophilic cyclization with o-ethynylaniline derivatives. Remarkably, methoxybiaryl ynones under similar catalytic conditions delivered quinoline tethered spiro[5.5]enone scaffolds via a dearomative 6-endo-dig C-cyclization. Moreover, leveraging this method for C8(sp2)-H bond activation of quinoline-N-oxide furnished biologically relevant oxindolyl-quinolines. This reaction proceeds via C(sp2)-H bond activation, regioselective alkyne insertion, oxygen-atom-transfer (OAT) and intramolecular nucleophilic cyclization in a cascade manner. One C-C, one C-N and one C[double bond, length as m-dash]O bond were created with concomitant formation of a quaternary center.

Mechanistic insights into an NH4OAc-promoted imine dance in Rh-catalysed multicomponent double C-H annulations through an N-retention/exchange dual channel

Developing new and understanding multicomponent reactions (MCRs) is an appealing but challenging task. Herein, Rh(iii)-catalyzed multicomponent double C-H annulations of cyclic diimines (or diketones and acetone), alkynes, and ammonium acetate to assemble functionalized 1,1'-biisoquinolines and C-bridged 1,1'-bisisoquinolines with controllable 14N/15N editing in one shot has been developed. Through a combination of isotopic-labeling (2H, 18O, and 15N) experiments, crystallography, and time-dependent ESI-MS, the reaction process was studied in detail. Ammonium acetate accounts for two rounds of Hofmann elimination and iminization, thus leading to an unprecedented imine dance, cyclic imine → N-alkenyl imine → NH imine. The N-alkenyl imine can immediately guide a C-H annulation (N-retention channel), and some of it is converted into NH-imine to trigger another annulation (N-exchange channel). The channels and 15N ratios can be regulated by the reaction mode and acidity. Moreover, the resulting 1,1'-biisoquinolines are a privileged ligand scaffold which is exemplified herein by a hydrazine-iodine exchange reaction to form drug-like benzo[c]cinnolines.

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.

Transition metal-catalyzed C-H/C-C activation and coupling with 1,3-diyne

This review provides a broad overview of the recent developments in the field of transition metal-catalyzed C-H/C-C bond activation and coupling with 1,3-diyne for assembling alkynylated heterocycles, bis-heterocycles, and 1,3-enynes. Transition metal-catalyzed inert bond (C-H/C-C) activation has been the focus of attention among synthetic chemists in recent times. Enormous developments have taken place in C-H/C-C bond activation chemistry in the last two decades. In recent years the use of 2π-unsaturated units as coupling partners for the synthesis of heterocycles through C-H/C-C bond activation and annulation sequence has received immense attention. Among the unsaturated units employed for assembling heterocycles, the use of 1,3-diynes has garnered significant attention due to its ability to render bis-heterocycles in a straightforward manner. The C-H bond activation and coupling with 1,3-diyne has been very much explored in recent years. However, the development of strategies for the use of 1,3-diynes in the analogous C-C bond activation chemistry is less explored. Earlier methods employed to assemble bis-heterocycle used heterocycles that were preformed and pre-functionalized via transition metal-catalyzed coupling reactions. The expensive pre-functionalized halo-heterocycles and sensitive and expensive heterocyclic metal reagents limit its broad application. However, the transition metal-catalyzed C-H activation obviates the need for expensive heterocyclic metal reagents and pre-functionalized halo-heterocycles. The C-H bond activation strategy makes use of C-H bonds as functional groups for effecting the transformation. This renders the overall synthetic sequence both step and cost economic. Hence, this strategy of C-H activation and subsequent reaction with 1,3-diyne could be used for the larger-scale synthesis of chemicals in the pharmaceutical industry. Despite these advances, there is still the possibility of exploration of earth-abundant and cost-effective first-row transition metals (Ni, Cu, Mn. Fe, etc.) for the synthesis of bis-heterocycles. Moreover, the Cp*-ligand-free, simple metal-salt-mediated synthesis of bis-heterocycles is also less explored. Thus, more exploration of reaction conditions for the Cp*-free synthesis of bis-heterocycles is called for. We hope this review will inspire scientists to investigate these unexplored domains.

Rh(III)-Catalyzed C-H Annulation of Sulfoxonium Ylides and 1,3-Diynes: A Rapid Access to Alkynyl-1-Naphthol Derivatives

An effective redox-neutral strategy to synthesize aryl/alkynyl and alkyl/alkynyl substituted 1-naphthol derivatives has been efficaciously developed by Rh(III)-catalyzed [4+2]-annulation of sulfoxonium ylides and 1,3-diynes with excellent regio- and chemoselectivity. Subsequently, the same strategy was extended to furnish various unsymmetrical binaphthol motifs in one-pot manner. Interestingly, the TMS-derived 1,3-diyne predominantly delivered the 3-alkynyl-1-naphthol via desilylation pathway. The salient features such as traceless directing group, broad substrate scope, good functional group tolerance, and operationally simple conditions made the present protocol more valuable and appealing.

1,3-Diynes: A Versatile Precursor in Transition-Metal Catalyzed (Mediated) C-H Functionalizations

Transition metal-catalyzed C-H functionalization of diverse arenes with alkyne units has attracted enormous attention for decades since they provide straightforward access to various functionalization/annulations, which are commonly present in bioactive compounds and natural products. Recently, conjugated alkynes (1,3-diynes) have been utilized as key coupling partner in many C-H activation reactions due to their versatile characteristic properties. The presence of two C≡C bonds in conjugated 1,3-diyne brings the new diversity in synthetic transformations, such as chemo-, regioselective pathways, mono-bis functionalizations, cascade annulations, etc. Herein, we summarized the latest developments in the realm of transition-metal-catalyzed C-H functionalizations of diverse arenes with 1,3-diynes. Moreover, we highlighted the diverse transformations, conditions, mechanisms and applications of the corresponding reaction in detail.

Ru(II)-Catalyzed [4 + 2]-Annulation and Arylation of 1,4-Naphthoquinones

Naphthoquinones form the core of a variety of drugs and natural products. As a result, the conjugation of 1,4-naphthoquinones with organic building blocks would offer a facile strategy toward scaffolds of biological interest. In this regard, we hereby report a Ru(II)-catalyzed [4 + 2] annulation of 1,4-naphthoquinones with benzoic acids to afford various naphthoquinone lactones. Additionally, ketone directed arylation of naphthoquinones using acetophenones under Ru(II)-catalysis was also illustrated. The feedstock availability of these precursors allowed access to a large library of naphthoquinone derivatives in good to excellent yields under fairly mild conditions. The practicality of these protocols was justified by carrying out a gram scale synthesis and further functionalizations. Also, preliminary mechanistic studies were carried out to probe the reaction mechanism.

Selective Synthesis of Pyrazolonyl Spirodihydroquinolines or Pyrazolonyl Spiroindolines under Aerobic or Anaerobic Conditions

Presented herein is a condition-controlled selective synthesis of pyrazolonyl spirodihydroquinolines or pyrazolonyl spiroindolines through formal [5 + 1] or [4 + 1] spiroannulation of 2-alkenylanilines with diazopyrazolones. Mechanistically, the formation of the title products involves initial generation of a pyrazolonyl spiro-fused seven-membered ruthenacycle species serving as a key intermediate through Ru(II)-catalyzed C-H/N-H bonds metalation, carbene formation, and its migratory insertion. When the reaction is carried out under air, the key intermediate undergoes reductive elimination to afford spirodihydroquinoline. When the reaction is run under argon, the key intermediate undergoes protonation and intramolecular nucleophilic addition to furnish spiroindoline. This work provides an atom-economical protocol for the effective functionalization of alkenyl C(sp²)-H bond, allowing rapid and selective assembly of valuable spiroscaffolds with a broad range of substrates.

Regioselective Dichotomy in Ru(II)-Catalyzed C-H Annulation of Aryl Pyrazolidinones with 1,3-Diynes

Herein, we present a substrate-controlled regiodivergent strategy for the selective synthesis of C3 or C2-alkynylated indoles via ruthenium-catalyzed [3 + 2]-annulation of readily available pyrazolidinones and 1,3-diynes. Remarkably, C3-alkynylated indoles were obtained in good yields when 1,4-diarylbuta-1,3-diynes were employed as the coupling partners. On the other hand, dialkyl-1,3-diynes led to the selective formation of C2-alkynylated indoles. The key features of the strategy are the operationally simple conditions and external-oxidant-free, broad-scope, and substrate-switchable indole synthesis. Scale-up reactions and further transformations expanded the synthetic utility of the protocol.

Diverse reactivity of alkynes in C-H activation reactions

Alkynes occupy a prominent role as a coupling partner in the transition metal-catalysed directed C-H activation reactions. Due to low steric requirements and linear geometry, alkynes can effectively coordinate with metal d-orbitals. This makes alkynes one of the most successful coupling partners in terms of the number of useful transformations. Remarkably, by changing the reaction conditions and transition-metals from 5d to 3d, the pattern of reactivity of alkynes also changes. Due to the varied reactivity of alkynes, such as alkenylation, annulation, alkylation, and alkynylation, they have been extensively used for the synthesis of valuable organic molecules. Despite enormous explorations with alkynes, there are still a lot more possible ways by which they can be made to react with M-C bonds generated through C-H activation. Practically there is no limit for the creative use of this approach. In particular with the development of new high and low valent first-row metal catalysts, there is plenty of scope for this chemistry to evolve as one of the most explored areas of research in the coming years. Therefore, a highlight article about alkynes is both timely and useful for synthetic chemists working in this area. Herein, we have highlighted the diverse reactivity of alkynes with various transition metals (Ir, Rh, Ru, Pd, Mn, Fe, Co, Ni, Cu) and their applications, along with some of our thoughts on future prospects.

Dual Photoredox Cobalt Catalyzed [4+1] Annulation and C-H Alkoxylation

Herein, we developed two distinct pyridine N-oxide directed C-H activation protocols to achieve [4+1] annulation and alkoxylation of benzamide derivatives by merging Co-catalysis with visible light photoredox catalysis. The protocols deliver the respective products in good yields under facile conditions at room temperature. The use of cheap photocatalyst coupled with molecular oxygen bypassing the need of stoichiometric oxidants forms the chief highlight of the work. The protocols are scalable and the products could be further modified. Additionally, preliminary studies were carried out to probe the reaction mechanism.

Synthesis of Indenone Derivatives by Rh(III)-Catalyzed C-H Functionalization of Sulfoxonium Ylides with 1,3-Diynes

The transition-metal-catalyzed C-H functionalization of sulfoxonium ylides with alkynes formally participates in [4 + 2] annulations to deliver the naphthol scaffolds. In contrast, herein we disclose the first Rh(III)-catalyzed C-H activation, followed by redox-neutral [3 + 2] annulation of sulfoxonium ylides with 1,3-diynes, which delivers the alkynated indenone derivatives. This protocol features a good functional group tolerance, a broad substrate scope, moderate to excellent yields, and mild reaction conditions. The reaction mechanism was supported through ESI-HRMS by characterizing key intermediates in the catalytic cycle.

Copper-mediated synthesis of fullerooxazoles from [60]fullerene and N-hydroxybenzimidoyl cyanides

A novel and efficient copper-mediated [3 + 2] heteroannulation reaction of [60]fullerene with N-hydroxybenzimidoyl cyanides has been developed for the synthesis of fullerooxazoles. A possible reaction mechanism involving unique C-CN and N-OH bond cleavages and subsequent C-OH bond formation for N-hydroxybenzimidoyl cyanides is proposed to explain the generation of fullerooxazoles. In addition, the formed fullerooxazoles can be further electrochemically transformed into amidated 1,2-hydrofullerenes.

Catalyst-Controlled C-H Transformation of Pyrazolidinones with 1,3-Diynes for Highly Selective Synthesis of Functionalized Bisindoles and Indoles

An efficacious synthetic solution to offer functionalized bisindoles and indoles has been developed based on catalyst-controlled C-H functionalization of pyrazolidinones and 1,3-diynes with highly selective control of both chemoselectivity and regioselectivity. This straightforward pathway conquers chemo- and regioselectivity challenges concerning the use of 1,3-diynes.

Cobalt(III)-Catalyzed Chemo- and Regioselective [4 + 2]-Annulation of Aromatic Sulfoxonium Ylides with 1,3-Diynes

Air-stable, highly abundant, and cost-effective Co(III)-catalyzed redox-neutral [4 + 2]-annulation of aromatic sulfoxonium ylides with 1,3-diynes providing useful substituted 1-naphthol derivatives in a regioselective manner is described. Further, the prepared 1-naphthols having internal alkyne were converted into useful polycarbocyclic molecules and spiro-dienone derivatives in good-to-excellent yields. A possible reaction mechanism involving ortho C-H activation as a key step was proposed and supported by deuterium labeling and kinetic isotope labeling studies.

Palladium-catalyzed selective C-C bond cleavage and stereoselective alkenylation between cyclopropanol and 1,3-diyne: one-step synthesis of diverse conjugated enynes

The stereoselective synthesis of 1,3-enynes from 1,3-diynes is demonstrated by palladium-catalyzed selective C–C bond cleavage of cyclopropanol. Exclusive formation of mono-alkenylated adducts was achieved by eliminating the possibility of di-functionalization with high stereoselectivity. Indeed, this protocol worked very well with electronically and sterically diverse substrates. Several studies, including deuterium labeling experiments and intermolecular competitive experiments, were carried out to understand the mechanistic details. The atomic-level mechanism followed in the catalytic process was also validated using DFT calculations, and the rate-controlling states in the catalytic cycle were identified. Furthermore, preliminary mechanistic investigations with radical scavengers revealed the non-involvement of the radical pathway in this transformation.

Palladium-catalyzed tandem activation and functionalization of readily accessible cyclopropanols have been demonstrated to access valuable conjugated enynes from 1,3-diynes with high stereo-selectivity.

The annulation of N-hydroxyoximes and 1,3-diyne to synthesize alkynylated isoquinolines regioselectively catalyzed by ruthenium: a theoretical study

The mechanisms of the regioselective annulation of N-hydroxyoximes and 1,3-diyne to synthesize alkynylated isoquinolines by using catalyst [RuCl2(p-cymene)]2 have been theoretically investigated with the aid of density functional theory (DFT)...

DFT study on ruthenium-catalyzed N-methylbenzamide-directed 1,4-addition of the ortho C–H bond to maleimide via C–H/C–C activation

B3LYP-D3a+IDSCRF/tzp-dkh(-dfg) calculations indicate that CO as a directing group is much more favourable than the N–H group, and the real active catalyst is an ionic type with one [SbF6]− group.

Ruthenium(II)-catalyzed C-H activation and (4+2) annulation of aromatic hydroxamic acid esters with allylic amides

A (4+2) annulation under Ru(II)-catalysis is reported using aromatic hydroxamic acid esters as the oxidizing directing group and allylic amides as unactivated olefin coupling partners, delivering a wide variety of aminomethyl isoquinolinones in good to excellent yields. This annulation is distinctive as allylic congeners typically result in allylation and not the annulation. Late-stage derivatization of a bioactive synthetic bile acid has been showcased.

Rh(III)-Catalyzed Stereoselective C-C Bond Cleavage of ACPs with N-Phenoxyacetamides: The Critical Role of the Nucleophilic Directing Group

Rh(III)-catalyzed redox-neutral chemodivergent coupling of N-phenoxyacetamides and alkylidenecyclopropanes (ACPs) has been documented. The reaction proceeds via C-H activation, regioselective migratory insertion and stereoselective β-carbon elimination followed by β-hydride elimination, resulting in o-dienylation of phenols in nonpolar solvents, whereas [3 + 2]-annulation leading to dihydrobenzofurans was realized in polar fluorinated solvents. It was observed that the nucleophilic directing group controls the elimination of β-carbon and so plays a vital role for achieving high stereoselectivities. The synthetic utility of the dienylation and annulation was demonstrated by carrying out gram scale reactions and further derivatization.

Ru(II)-catalyzed allenylation and sequential annulation of N-tosylbenzamides with propargyl alcohols

We hereby report Ru(ii)-catalyzed C(sp2)-H allenylation of N-tosylbenzamides to access multi-substituted allenylamides. Furthermore, the allenylamides were converted to the corresponding isoquinolone derivatives via base mediated annulation. The current protocol features low catalyst loading, mild reaction conditions, high functional group compatibility and desired scalability. The unique functionality of the afforded allenes allowed further transformations to expand the practicality of the protocol.

One-Pot Tandem Access to Phenothiazine Derivatives from Acetanilide and 2-Bromothiophenol via Rhodium-Catalyzed C-H Thiolation and Copper-Catalyzed C-N Amination

A one-pot and step economic reaction involving Rh(III)-catalyzed C-H thiolation and relay Cu(II)-catalyzed C-N amination of acetanilide and 2-bromothiophenol is reported here, with several valuable phenothiazine products obtained. This synthesis protocol proceeds from easily starting materials, demonstrating high atom economy, broad substrate scope, and good yield. Furthermore, the directing group can be easily eliminated, and chlorpromazine is provided in a large scale; thus this synthesis protocol could be utilized to construct phenothiazine scaffolds.

Rh(iii)-Catalyzed [3 + 2]/[4 + 2] annulation of acetophenone oxime ethers with 3-acetoxy-1,4-enynes involving C–H activation

A novel, synthetically simple, selective rhodium(iii)-catalyzed [3 + 2]/[4 + 2] annulation cascade reaction to construct complex azafluorenone frameworks has been developed.

The C-H Activation/Bidirecting Group Strategy for Selective Direct Synthesis of Diverse 1,1'-Biisoquinolines

Multidentate ligands are highly important but difficult to access. Herein we disclose an atom- and step-economic synthesis of highly substituted 1,1'-biisoquinolines by a C-H activation/bididirecting group strategy. Through rational design of a bididirecting group to "N-OH + N-OAc", selective unsymmetrical diannulation with two different alkynes in a one-pot reaction has been achieved for the first time to access unsymmetrical biisoquinolines. Moreover, the resultant biisoquinolines show tunable photoluminescence and serve as aggregation-induced emission (AIE) systems.

Synthesis of [60]fullerene-fused dihydrobenzooxazepinesviathe palladium-catalyzed oxime-directed C–H bond activation and subsequent electrochemical functionalization

The palladium-catalyzed heteroannulation of [60]fullerene with oximes affords the unprecedented seven-membered [60]fullerene-fused dihydrobenzooxazepines, which can be further derivatized electrochemically.