One-Pot Synthesis of Highly Substituted Polyheteroaromatic Compounds by Rhodium(III)-Catalyzed Multiple CH Activation and Annulation

Transition Metal-Catalyzed Dual C-H Activation/Annulation Reactions Involving Internal Alkynes

Recently, transition metal-catalyzed ortho-C-H bond activation/annulations involving two internal alkyne molecules have been extensively used to synthesize highly substituted polycyclic aromatic scaffolds. Such reactions have emerged as a powerful atom and step-economical strategy for the assembly of multifunctional bioactive molecules. In this context, we focused on the recent achievements of dual C-H bond activation/annulations, as well as functionalization reactions involving diaryl/alkyl alkynes.

Alkaline-Metal-Promoted Divergent Synthesis of 1-Aminoisoquinolines and Isoquinolines

Alkaline-metal-promoted divergent syntheses of 1-aminoisoquinolines and isoquinolines have been reported involving 2-methylaryl aldehydes, nitriles, and LiN(SiMe3)2 as reactants. In addition, the three-component reaction of 2-methylaryl nitriles, aldehydes, and LiN(SiMe3)2 has been developed to furnish 1-aminoisoquinolines. This protocol features readily available starting materials, excellent chemoselectivity, broad substrate scope, and satisfactory yields.

Silver-Free C-H Activation: Strategic Approaches towards Realizing the Full Potential of C-H Activation in Sustainable Organic Synthesis

The activation of carbon-hydrogen bonds is considered as one of the most attractive techniques in synthetic organic chemistry because it bears the potential to shorten synthetic routes as well as to produce complementary product scopes compared to traditional synthetic strategies. However, many current methods employ silver salts as additives, leading to stoichiometric metal waste and thereby preventing the full potential of C-H activation to be exploited. Therefore, the development of silver-free protocols has recently received increasing attention. Mechanistically, silver can serve various roles in C-H activation and thus, avoiding the use of silver requires different approaches based on the role it serves in a given process. In this Review, we present the comparison of silver-based and silver-free methods. Focusing on the strategic approaches to develop silver-free C-H activation, we provide the reader with the means to develop sustainable methods for C-H activation.

Pd(II)-Catalyzed Synthesis of Furo[2,3-b]pyridines from β-Ketodinitriles and Alkynes via Cyclization and N-H/C Annulation

A Pd(II)-catalyzed synthesis of furopyridines has been developed from β-ketodinitriles and alkynes via an unusual N-H/C annulation. The participation of both the nitrile groups and the concurrent construction of furan and pyridine rings through the formation of C-C, C═C, C-O, C-N, and C═N bonds are the important features. The synthetic applicability is further demonstrated through a series of postsynthetic alterations.

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.

Rh-Catalyzed [4 + 2] Annulation with a Removable Monodentate Structure toward Iminopyranes and Pyranones by C-H Annulation

The Rh-catalyzed reactions of N-pyridinyl enaminones with internal alkynes leading to the synthesis of iminopyranes via a key C-H bond activation and subsequent tautomeric O-H bond cleavage are reported. Moreover, the pyridine ring in the amino group acts as an auxiliary monodentate site for this annulation and can be easily removed by a simple hydrolysis to afford pyranones.

Multiple annulations of inert C(sp2)-H bonds with alkynes

Transition-metal catalyzed directing group (DG) assisted annulation of inert C-H bonds leads to the formation of complex molecular frameworks from readily accessible substrates. Thus, multiple annulation of less functionalized substrates with unsaturated species leads to the construction of structurally diverse fused poly(hetero)cycles. The directed inert C(arene)-H bond activation and the mode of TM-migration in this process could enabled obatining L-type [involves DG heteroatom, o-C(arene)-H bond, and C(arene)-H bond of aryl-motif in alkyne], Y-type [involves two heteroatoms of the DG and o-,o'-C(arene)-H bonds], and B-type [involves o-C(arene)-H bond and m-C(arene)-H bond] π-extended annulation products. The coordination preference of the DG heteroatom makes the transformation chemo- and regio-selective. This article underlines the conceptual development of unsymmetrical multiple annulation of arene C(sp²)-H bonds with alkynes, which is exceedingly appealing and highly important.

Me3Al-mediated domino nucleophilic addition/intramolecular cyclisation of 2-(2-oxo-2-phenylethyl)benzonitriles with amines; a convenient approach for the synthesis of substituted 1-aminoisoquinolines

A simple and efficient protocol for the construction of 1-aminoisoquinolines was achieved by treating 2-(2-oxo-2-phenylethyl)benzonitriles with amines in the presence of Me₃Al. The reaction proceeds via a domino nucleophilic addition with subsequent intramolecular cyclisation. This method provides a wide variety of substituted 1-aminoisoquinolines with good functional group tolerance. Furthermore, the synthetic utility of this protocol was demonstrated in the successful synthesis of the antitumor agent CWJ-a-5 in gram scale.

Advances on the Merger of Electrochemistry and Transition Metal Catalysis for Organic Synthesis

Synthetic organic electrosynthesis has grown in the past few decades by achieving many valuable transformations for synthetic chemists. Although electrocatalysis has been popular for improving selectivity and efficiency in a wide variety of energy-related applications, in the last two decades, there has been much interest in electrocatalysis to develop conceptually novel transformations, selective functionalization, and sustainable reactions. This review discusses recent advances in the combination of electrochemistry and homogeneous transition-metal catalysis for organic synthesis. The enabling transformations, synthetic applications, and mechanistic studies are presented alongside advantages as well as future directions to address the challenges of metal-catalyzed electrosynthesis.

Synthesis of 1-Benzyl-, 1-Alkoxyl-, and 1-Aminoisoquinolines via Rhodium(III)-Catalyzed Aryl C-H Activation and Alkyne Annulation

One-pot syntheses of 1-benzyl-, 1-alkoxyl-, and 1-alkylamino- isoquinolines through automatic directing group (DG^auto)-assisted, rhodium(III)-catalyzed aryl C-H activation and annulation with internal alkynes were developed. The reactions affording 1-benzylisoquinolines involve a cascade oximation of diarylacetylenes with hydroxylamine, forming aryl benzyl ketone oxime, and oxime-assisted rhodium(III)-catalyzed aryl C-H activation and followed annulation with another molecule of diarylacetylene in a one-pot manner. The formation of 1-alkoxyl/amino isoquinolines includes the addition of nucleophilic alcohols or amines to aryl nitriles, imine-assisted rhodium-catalyzed aryl C-H activation, and subsequent alkyne annulation.

Pd(II)-Catalyzed alkyne annulation through allylic isomerization: synthesis of spiro-cyclopentadiene pyrazolones

The Pd(ii)-catalyzed activation of Csp2-H bond and double alkyne annulation which proceeds via allylic isomerization is reported for the first time. This reaction of antipyrines with alkynes provides an efficient synthetic route for the biologically important spiro-cyclopentadiene pyrazolones. In the presence of Lawesson's reagent, this Pd(ii)-catalyzed annulation reaction affords another spiro-cyclopentadiene pyrazolone which displays very good fluorescence properties.

Harnessing sulfur and nitrogen in the cobalt(iii)-catalyzed unsymmetrical double annulation of thioamides: probing the origin of chemo- and regio-selectivity

An unconventional cobalt(iii)-catalyzed one-pot domino double annulation of aryl thioamides with unactivated alkynes is presented. Sulfur (S), nitrogen (N), and o,o'-C-H bonds of aryl thioamides are involved in this reaction, enabling access to rare 6,6-fused thiopyrano-isoquinoline derivatives. A reverse 'S' coordination over a more conventional 'N' coordination of thioamides to the Co-catalyst specifically regulates the formation of four [C-C and C-S at first and then C-N and C-C] bonds in a single operation, a concept which is uncovered for the first time. The power of the N-masked methyl phenyl sulfoximine (MPS) directing group in this annulation sequence is established. The transformation is successfully developed, building a novel chemical space of structural diversity (56 examples). In addition, the late-stage annulation of biologically relevant motifs and drug candidates is disclosed (17 examples). The preliminary photophysical properties of thiopyrano-isoquinoline derivatives are discussed. Density functional theory (DFT) studies authenticate the participation of a unique 6π-electrocyclization of a 7-membered S-chelated cobaltacycle in the annulation process.

Rh(III)-Catalyzed C-H Olefination Cascades to Divergently Construct Diverse Polyheterocycles by Tuning Manipulations of Directing Groups

Inspired by the diversity created by nature, organic chemists have been using a divergent strategy to improve the synthetic efficiency of diverse molecules. Transition-metal-catalyzed C-H functionalization has become one of the most straightforward, powerful, and atom-economical methods to construct complex scaffolds. However, C-H activation initiated divergent transformation to prepare diverse molecules is still limited. To address this challenge, we herein developed Rh(III)-catalyzed C-H olefination/annulation reaction cascades to divergently construct diverse polyheterocycles by tuning manipulations of directing groups (DGs). Up to 9 distinct scaffolds were creatively synthesized under simple conditions with good functional group tolerance, chemo-, and regioselectivity. Such a versatile strategy and its extension may encourage researchers to discover more promising manipulations of DGs for transition-metal-catalyzed C-H bond activation, making diverse available targets and materials that would have been previously out of range.

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.

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(II)-Catalyzed Double Annulation of Quinones: Step-Economical Access to Valuable Bioactive Compounds

Double ruthenium(II)-catalyzed alkyne annulations of quinones were accomplished. Thus, a strategy is reported that provides step-economical access to valuable quinones with a wide range of applications. C-H/N-H activations for alkyne annulations of naphthoquinones provided challenging polycyclic quinoidal compounds by forming four new bonds in one step. The singular power of the thus-obtained compounds was reflected by their antileukemic activity.

Directing Group Assisted Unsymmetrical Multiple Functionalization of Arene C-H Bonds

Multiple C-H bond functionalizations promptly install diverse groups on the molecular framework and consequently fabricate complex molecular entities. This review briefly surveys the conceptual development of directing group assisted unsymmetrical multiple functionalization of arene C(sp² )-H bonds, which is exceedingly appealing and highly important.

Heterogeneous gold(III)-catalyzed tandem cyclization of 2-alkynylbenzamides with ammonium acetate toward 1-aminoisoquinolines

Heterogeneous tandem cyclization of 2-alkynylbenzamides with ammonium acetate is achieved in acetonitrile at 85 °C using a magnetic nanoparticles-immobilized bipy-gold(III) complex and AgSbF6 as catalysts to afford a variety of 1-aminoisoquinoline derivatives in moderate to high yields. This heterogeneous gold catalyst can be easily recovered from the reaction mixture by simply applying an external magnetic field and can be recycled at least seven times without any apparent loss of catalytic activity.

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.

Electrochemical Access to Aza-Polycyclic Aromatic Hydrocarbons: Rhoda-Electrocatalyzed Domino Alkyne Annulations

Nitrogen-doped polycyclic aromatic hydrocarbons (aza-PAHs) have found broad applications in material sciences. Herein, a modular electrochemical synthesis of aza-PAHs was developed via a rhodium-catalyzed cascade C-H activation and alkyne annulation. A multifunctional O-methylamidoxime enabled the high chemo- and regioselectivity. The isolation of two key rhodacyclic intermediates made it possible to delineate the exact order of three C-H activation steps. In addition, the metalla-electrocatalyzed multiple C-H transformation is characterized by unique functional group tolerance, including highly reactive iodo and azido groups.

Palladium(ii)-catalyzed vinylic geminal double C-H activation and alkyne annulation reaction: synthesis of pentafulvenes

The first transition-metal-catalyzed vinylic geminal double C(sp²)-H activation and di-substituted alkyne annulation reaction is reported. This palladium(ii)-catalyzed, amide directed reaction of vinylic compounds with di-substituted alkynes offers an efficient synthetic path to pentafulvenes, which are very important compounds because of their bioactivity and interesting optical properties. A FeCl₃-mediated transformation of pentafulvenes to fluorescent cyclopenta[b]quinolines is also developed.

Annulation of imidazo[1,2-a]pyridines under metal-free conditions

Synthesis of benzo[a]imidazo[5,1,2-cd]indolizines from 2-arylimidazo[1,2-a]pyridines and benzyne precursors under metal-free conditions has been described.

Rhodium-catalyzed oxidative annulation of 1H-indazoles with alkynes for the synthesis of indazolo[3,2-a]isoquinolines via C–H bond functionalization

A Rh(iii)-catalyzed oxidative annulation of 1H-indazoles with internal alkynes via C–C and C–N coupling for the preparation of highly functionalized indazolo[3,2-a]isoquinolines is disclosed.

Double annulation of ortho- and peri-C–H bonds of fused (hetero)arenes to unusual oxepino-pyridines

Direct difunctionalization of chemically distinct ortho- and peri-C–H bonds of fused hetero(arenes) is illustrated through an unusual one-pot domino {[4 + 2] & [5 + 2]} double annulation with alkynes for the first time.

Recent Advancements in Transition-Metal-Catalyzed One-Pot Twofold Unsymmetrical Difunctionalization of Arenes

Transition-metal-catalyzed direct C-H bond activation reactions have been embraced as a powerful synthetic tool to access diverse functionalized arenes. However, site-selective incorporation of multiple distinct functionalities in an arene has always been a formidable challenge. Recent efforts from the synthetic community have disclosed a few dynamic synthetic approaches to fabricate multifunctionalized arenes in one-pot using a single catalytic system. These reports manifested the immense potential of such approaches to expedite contemporary organic synthesis towards building molecular complexity. In this minireview, we have illustrated the recent progress in this area, highlighting the contribution from several synthetic chemists including our group.

Construction of Pyranoisoquinolines via Ru(II)-Catalyzed Unsymmetrical Double Annulation of N-Methoxybenzamides with Unactivated Alkynes

A ruthenium (Ru)-catalyzed double annulation of easily accessible N-methoxybenzamide derivatives with unactivated alkynes for the synthesis of unusual 6,6-fused pyranoisoquinolines is described. Both ortho-C-H bonds of arenes and the N- and O-moieties of N-methoxybenzamides are involved in the construction of four [(C-C)-(C-N) and (C-C)-(C-O)] bonds in one step under single catalytic conditions. The unsymmetrical annulation of N-methoxybenzamides with two distinct alkynes is also demonstrated. The oxidizable directing group N-methoxyamine (NHOMe) assists the unsymmetrical double annulations of arenes [that use both N- and O-heteroatoms] in a single operation. This synthetic method features excellent substrate scope and tolerates a wide range of functional groups. Peripheral modification of pyranoisoquinolines for the construction of complex heterocyclic compounds is also demonstrated.

Orchestrated catalytic double rollover annulation: rapid access to N-enriched cationic and neutral PAHs

Disclosed herein is a rhodium(iii)-catalyzed novel one-step back-to-back double rollover annulation on pyridine and pyrazine backbones leading to structurally and optoelectronically diverse class of nicely decorated multi-ring-fused, extensively π-conjugated, N-enriched PAH molecules by virtue of orchestrated quadruple C–H activation events.

Iridium-Catalyzed, Silyl-Directed, peri-Borylation of C-H Bonds in Fused Polycyclic Arenes and Heteroarenes

peri-Disubstituted naphthalenes exhibit interesting physical properties and unique chemical reactivity, due to the parallel arrangement of the bonds to the two peri-disposed substituents. Regioselective installation of a functional group at the position peri to 1-substituted naphthalenes is challenging due to the steric interaction between the existing substituent and the position at which the second one would be installed. We report an iridium-catalyzed borylation of the C-H bond peri to a silyl group in naphthalenes and analogous polyaromatic hydrocarbons. The reaction occurs under mild conditions with wide functional group tolerance. The silyl group and the boryl group in the resulting products are precursors to a range of functional groups bound to the naphthalene ring through C-C, C-O, C-N, C-Br and C-Cl bonds.

An Orchestrated Unsymmetrical Annulation Episode of C(sp2)-H Bonds with Alkynes and Quinones: Access to Spiro-isoquinolones

A nontrivial Ru-catalyzed one-pot sequential oxidative coupling of a (hetero)arene/vinylic/chromene system with alkyne and quinone is presented; the methyl phenyl sulfoximine (MPS) directing group is vital. This cyclization forms four (two C-C and two C-N) bonds in a single operation and produces unusual spiro-fused-isoquinolones with a broad scope. The release of phenyl methyl sulfoxide makes the MPS group transformable. A deuterium scrambling study sheds light on the reaction path.