Auto-Tandem Catalysis in the Synthesis of Substituted Quinolines from Aldimines and Electron-Rich Olefins: Cascade Povarov−Hydrogen-Transfer Reaction

Turning on Singlet Oxygen Generation by Outer-Sphere Microenvironment Modulation in Porphyrinic Covalent Organic Frameworks for Photocatalytic Oxidation

Singlet oxygen (1 O2 ) is ubiquitously involved in various photocatalytic oxidation reactions; however, efficient and selective production of 1 O2 is still challenging. Herein, we reported the synthesis of nickel porphyrin-based covalent organic frameworks (COFs) incorporating functional groups with different electron-donating/-withdrawing features on their pore walls. These functional groups established a dedicated outer-sphere microenvironment surrounding the Ni catalytic center that tunes the activity of the COFs for 1 O2 -mediated thioether oxidation. With the increase of the electron-donating ability of functional groups, the modulated outer-sphere microenvironment turns on the catalytic activity from a yield of nearly zero by the cyano group functionalized COF to an excellent yield of 98 % by the methoxy group functionalized one. Electronic property investigation and density-functional theory (DFT) calculations suggested that the distinct excitonic behaviors attributed to the diverse band energy levels and orbital compositions are responsible for the different activities. This study represents the first regulation of generating reactive oxygen species (ROS) based on the strategy of outer-sphere microenvironment modulation in COFs.

Direct Access to Quinone-Fused 5-Substituted-1,4-Benzodiazepine Scaffolds from Azidoquinones with/without [1,2]-Azide-Nitrogen Migration: Mechanistic Insights

Seven-membered nitrogen heterocycles have a strong influence in drug discovery due to their inherent 3D character, which allows the ability to explore a vast conformational space with a biological target. Notably, the privileged 1,4-benzodiazepine scaffold is dominant in treating the central nervous system due to its binding affinity with the GABAA receptor. Herein, we report a protocol for the transformation of azidoquinones to p-quinone fused 5-substituted-1,4-benzodiazepines (p-QBZDs) from InCl3-catalyzed intermolecular tandem cycloannulation of azidoquinones with amines and aldehydes. Detailed mechanistic studies reveal that the EDA complex between azidoquinones and InCl3 is crucial in determining the reaction pathway. In the absence of EDA complex formation, the reaction proceeds via the intermediacy of 2,3-bridged-2H-azirine followed by regiospecific addition of an amine to C═N/ring opening/cyclization to deliver p-QBZD with 1,2-azide-nitrogen migration. In the case of EDA complex formation, the reaction proceeds through regioselective aza-Michael addition/nitrene insertion with aldehyde and subsequent cyclization to deliver p-QBZD and p-quinone fused imidazole as a secondary product without 1,2-azide-nitrogen migration. This protocol provides straightforward access to redox-active quinone embedded 5-substituted-1,4-benzodiazepines from azidoquinones with diverse substrate scopes that would find potential applications in medicinal chemistry and drug discovery.

Three-Component Synthesis of Quinoline-4-carboxylic Acids Based on Doebner Hydrogen-Transfer Reaction

The Doebner hydrogen-transfer reaction has been developed for the synthesis of substituted quinolines from anilines possessing electron-withdrawing groups, which are known to give products in low yields when used in the conventional Doebner reaction. This reaction can be applied to not only anilines having electron-withdrawing groups but also those having electron-donating groups and can be used in the large-scale synthesis of bioactive molecules.

Constructing chemical stable 4-carboxyl-quinoline linked covalent organic frameworks via Doebner reaction for nanofiltration

Covalent linkages are the key component of covalent organic frameworks (COFs). The development of stable and functional linkages is essential to expand the COFs family and broaden their application prospects. In this work, we report the synthesis of crystalline and chemical stable 4-carboxyl-quinoline linked COFs (QL-COFs) via Doebner reactions in both one-pot (OP) and post-synthetic modification (PSM) methods. Both methods can be universally applied to most of the reported imine COFs family via bottom-up construction or linkage conversion. Owing to the contractive pore size, more hydrophilic structure and better chemical stability than the conventional imine COFs endowed by 4-carboxyl-quinoline linkage, QL-COFs are supposed to possess a wider application range. We further demonstrate the nanofiltration membrane (NFM) based on QL-COF exhibited a desirable separation capacity with high rejection for small dye molecules (> 90%), high water permeance (850 L m⁻² h⁻¹ MPa⁻¹) and tolerance of extreme conditions (1 M HCl/NaOH), which were benefitted from the enhanced properties of QL-COFs. Additionally, efficient ion sieving properties were also achieved by QL-COF membrane. We anticipate that this work opens up a way for the construction of robust and functional COFs materials for practical applications.

Covalent linkages are the key component of covalent organic frameworks (COFs) and the development of stable and functional linkages is essential to broaden their application prospects. Here, the authors report the synthesis of crystalline and ultrastable 4-carboxyl-quinoline linked COFs via Doebner reactions in both one-pot and post-synthetic modification methods and demonstrate application of the COFs in nanofiltration.

Fused-Ring-Linked Covalent Organic Frameworks

The development of linkage chemistry in the research area of covalent organic frameworks (COFs) is fundamentally important for creating robust structures with high crystallinity and diversified functionality. We reach herein a new level of complexity and controllability in linkage chemistry by achieving the first synthesis of fused-ring-linked COFs. A series of bicyclic pyrano[4,3-b]pyridine COFs have been constructed via a cascade protocol involving Schiff-base condensation, intramolecular [4 + 2] cycloaddition, and dehydroaromatization. With a broad scope of Brønsted or Lewis acids as the catalyst, the designed monomers, that is, O-propargylic salicylaldehydes and multitopic anilines, were converted into the fused-ring-linked frameworks in a one-pot fashion. The obtained COFs exhibited excellence in terms of purity, stability, and crystallinity, as comprehensively characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction, high-resolution transmission electron microscopy, and so on. Specifically, the highly selective formation (>94%) of pyrano[4,3-b]pyridine linkage was verified by quantitative NMR measurements combined with ¹³C-labeling synthesis. Moreover, the fused-ring linkage possesses fully locked conformation, which benefits to the high crystallinity observed for these COFs. Advancing the linkage chemistry from the formation of solo bonds or single rings to that of fused rings, this study has opened up new possibilities for the concise construction of sophisticated COF structures with high controllability.

Rh(III)-Catalyzed Synthesis of Indazolo[2,3-a]quinolines: Vinylene Carbonate as C1 and C2 Building Blocks

A rhodium-catalyzed cyclization of azobenzenes and vinylene carbonate via C-H bond activation to construct indazolo[2,3-a]quinolines has been developed. This protocol offers an efficient method for synthesis of the titled products in good yields with broad functional group tolerance. In this reaction, three C-C bonds and C-N bond are formed in one pot, and vinylene carbonate (VC) acts as C1 and C2 synthons as well as "vinylene transfer" agent and acylation reagent in the construction of target-fused heterocycles. Moreover, the products exhibit favorable fluorescence properties, which indicate their potential application as fluorescent materials and biosensors.

The Povarov Reaction: A Versatile Method to Synthesize Tetrahydroquinolines, Quinolines and Julolidines

The multicomponent Povarov reaction represents a powerful approach for the construction of substances containing N-heterocyclic frameworks. By using the Povarov reaction, in addition to accessing tetrahydroquinolines, quinolines and julolidines in a single step, it is possible to form the following new bonds: two Csp 3–Csp 3 and one Csp 3–Nsp 3, two Csp 2–Csp 2 and one Csp 2–Nsp 2, and four Csp 3–Csp 3 and two Csp 3–Nsp 1, respectively. This short review discusses the main features of the Povarov reaction, including its mechanism, the reaction scope by employing different catalysts and substrates, as well as stereoselective versions.

1 Introduction

2 Mechanism of the Povarov Reaction

3 Tetrahydroquinolines

4 Quinolines

5 Julolidines

6 Concluding Remarks

One-Pot Synthesis of Triazatriphenylene Using the Povarov Reaction

The Povarov reaction combines aromatic amines, aldehydes, and alkynes in a single step and is regarded as an annulative π-extension reaction of aromatic amines. In this study, the Povarov reaction was investigated as an efficient tool for the synthesis of aza-polycyclic aromatic hydrocarbons via multiple π-extensions. The double Povarov reaction of 1,4-diaminobenzene yielded the 4,7-phenanthroline derivative as the major product, regardless of the steric repulsion in the product. The site selectivity mainly depended on the HOMO distribution of the intermediate rather than the steric factor. Based on these insights, a 1,5,9-triazatriphenylene derivative was synthesized via a triple Povarov reaction. The structures of the synthesized compounds were unambiguously determined by single-crystal X-ray diffraction analysis. The triazatriphenylene derivative formed a smooth and stable thin film upon vacuum vapor deposition and served as a hole-blocking material in organic light-emitting diodes.

Synthesis of Robust MOFs@COFs Porous Hybrid Materials via an Aza-Diels-Alder Reaction: Towards High-Performance Supercapacitor Materials

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have attracted enormous attention in recent years. Recently, MOF@COF are emerging as hybrid architectures combining the unique features of the individual components to enable the generation of materials displaying novel physicochemical properties. Herein we report an unprecedented use of aza-Diels-Alder cycloaddition reaction as post-synthetic modification of MOF@COF-LZU1, to generate aza-MOFs@COFs hybrid porous materials with extended π-delocalization. A a proof-of-concept, the obtained aza-MOFs@COFs is used as electrode in supercapacitors displaying specific capacitance of 20.35 μF cm^-2 and high volumetric energy density of 1.16 F cm^-3 . Our approach of post-synthetic modification of MOFs@COFs hybrids implement rational design for the synthesis of functional porous materials and expands the plethora of promising application of MOFs@COFs hybrid porous materials in energy storage applications.

Helquats as Promoters of the Povarov Reaction: Synthesis of 1,2,3,4-Tetrahydroquinoline Scaffolds Catalyzed by Helicene-Viologen Hybrids

Helquats (HQs) are structurally linked to helicenes and viologens, and they represent an attractive field of research in chemistry and medicinal chemistry. In the present work they were used as catalysts for the synthesis of 1,2,3,4-tetrahydroquinolines in good yields by the Povarov reaction. The substrate scope and the capability of different helquats to promote Povarov reactions are demonstrated. Studies to elucidate mechanistic details revealed that helquats act as single-electron transfer oxidants through a cation-radical mechanism. The screening of the catalytic activity of HQs confirmed that an active HQ must have a LUMO energy below -8.67 eV and the standard redox potential higher (less negative) than -1.2 V vs. the ferrocene/ferrocenium redox couple.

Visible-light-induced photoxidation-Povarov cascade reaction: synthesis of 2-arylquinoline through alcohol and N-benzylanilines under mild conditions via Ag/g-C3N4 nanometric semiconductor catalyst

Ag/g-C₃N₄ nanometric semiconductor catalyzed cascade reaction for the synthesis of 2-arylquinoline through alcohol and N-benzylanilines under visible light irradiation.

Micellar Brønsted Acid Mediated Synthesis of DNA-Tagged Heterocycles

The translation of well-established molecular biology methods such as genetic coding, selection, and DNA sequencing to combinatorial organic chemistry and compound identification has made extremely large compound collections, termed DNA-encoded libraries, accessible for drug screening. However, the reactivity of the DNA imposes limitations on the choice of chemical methods for encoded library synthesis. For example, strongly acidic reaction conditions must be avoided because they damage the DNA by depurination, i.e. the cleavage of purine bases from the oligomer. Application of micellar catalysis holds much promise for encoded chemistry. Aqueous micellar dispersions enabled compound synthesis under often appealingly mild conditions. Amphiphilic block copolymers covalently functionalized with sulfonic acid moieties in the lipophilic portion assemble in water and locate the Brønsted catalyst in micelles. These acid nanoreactors enabled the reaction of DNA-conjugated aldehydes to diverse substituted tetrahydroquinolines and aminoimidazopyridines by Povarov and Groebke-Blackburn-Bienaymé reactions, respectively, and the cleavage of tBoc protective groups from amines. The polymer micelle design was successfully translated to the Cu/Bipyridine/TEMPO system mediating the oxidation of DNA-coupled alcohols to the corresponding aldehydes. These results suggest a potentially broad applicability of polymer micelles for encoded chemistry.

Theoretical Insight into the Mechansim and Origin of Ligand-Controlled Regioselectivity in Homogenous Gold-Catalyzed Intramolecular Hydroarylation of Alkynes

This work aims at understanding the mechanism and regioselectivity in ligand-controlled gold-catalyzed divergent intramolecular hydroarylation of alkynes reported by Jiang et al. ( J. Am. Chem. Soc. 2016 , 138 , 5218 - 5221 ). Focusing on a representative alkyne, N-propargyl-N-tosylaniline, we conducted a detailed computational study on the ortho- and para-position hydroarylation of the alkyne catalyzed by gold(I) catalysts with different ligands. Both the ortho- and para-position hydroarylation reactions are found to follow a similar three-stage mechanism: electrophilic cyclization, proton loss, and protiodeauration. The initial electrophilic cyclization was identified as the rate- and regiochemistry-determining step. With the flexible electron-deficient phosphite ligand, the ortho-position cyclization is identified as the energetically more favorable pathway, while with the rigid electron-abundant phosphine (Xphos) ligand, the dominant pathway turns to the para-position cyclization. The theoretical results are in good agreement with the experimental observations. The π-π interaction between alkynyl phenyl and the directing acylamino group are found to be mainly responsible for the observed ortho-selectivity, while a combination of favorable noncovalent CH···π interaction and steric repulsion between Xphos ligand and alkynyl group contributes to the observed exclusive para-selectivity. The present calculations provide deeper insight into the mechanism and origin of regioselectivity of the title reaction.

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.

Regioselective three-component synthesis of 2,3-disubstituted quinolines via the enaminone modified Povarov reaction

The enaminone modified three-component Povarov reactions enable the regioselective synthesis of 2,3-disubstituted quinolines.

A concise synthesis of quinolinium, and biquinolinium salts and biquinolines from benzylic azides and alkenes promoted by copper(ii) species

​A copper promoted multiple aza-[4 + 2] cycloaddition reactions between benzylic azides and alkenes to form four different products including quinolinium and biquinolinium cations, biquinolines and quinolines are described.

Understanding the stereoselectivity in Brønsted acid catalysed Povarov reactions generating cis/trans CF3-substituted tetrahydroquinolines: a DFT study

Stereochemistry in the Brønsted acid catalysed Povarov reactions.

Metal-free domino one-pot protocols for quinoline synthesis

Metal-free domino one-pot protocols for quinoline synthesis have been reviewed.