Cobalt-Catalyzed Cycloamination: Synthesis and Photophysical Properties of Polycyclic N-Heterocycles

2-Amino-1,4-naphthoquinone as a key precursor for naphthoquinone-fused N-heterocycles: synthetic approaches and mechanistic perspectives

2-Amino-1,4-naphthoquinone has gained significant attention as a key precursor for constructing naphthoquinone-fused N-heterocycles, a class of compounds with broad applications in medicinal chemistry, materials science, and organic synthesis. Its unique structure, characterized by a reactive C-3 position and an amino group, facilitates a wide range of reactions and synthetic strategies. This review provides a comprehensive overview of recent advancements (2012-2024) in utilizing 2-amino-1,4-naphthoquinone for the development of N-heterocyclic frameworks integrating synthetic approaches with mechanistic insights. It is organized based on the type of heterocyclic rings formed, leveraging the C,N-binucleophilic properties of 2-amino-1,4-naphthoquinone.

Recent advances in the transformation of maleimides via annulation

Over the past five years, maleimide scaffolds have gained considerable attention in organic synthesis for their role in forming cyclized molecules through annulation and C-H activation. As versatile and reactive coupling agents, maleimides have enabled the efficient synthesis of various cyclized products, including annulation, benzannulation, cycloaddition, and spirocyclization, with applications in medicinal chemistry, drug discovery, and materials science. Despite the extensive study of maleimide chemistry, certain reactions-such as cycloaddition-based annulation, photoannulation, and electrochemical transformations-remain underexplored despite their promising potential in the pharmaceutical and chemical industries. Recent advancements, such as photocatalysis and electrochemical methods, have expanded the utility of maleimides, providing more sustainable and selective approaches for synthesizing complex molecules. This review compiles research published between 2019 and 2024, highlighting the substrate scope, reaction diversity, and industrial relevance of maleimide-based annulation strategies. Additionally, we discuss emerging trends and future directions in maleimide chemistry, exploring opportunities for novel reaction pathways and broader applications in synthetic biology and materials science.

Pd/Ag-Cocatalyzed Merging Intramolecular Oxidative Coupling and Cascade [4 + 2] Cycloaddition: Synthesis and Photophysical Properties of Novel Polycyclic N-Heterocycles Fused Naphthoquinones

We report a step-economic strategy for the direct synthesis of novel polycyclic N-heterocycle-fused naphthoquinones by merging intramolecular oxidative coupling and cascade [4 + 2] cycloaddition. In the protocol, mechanistic investigations suggest that the cascade reaction involves the intermediate spiro polycyclic N-heterocycles and [4 + 2] cycloaddition processes. This protocol is featured with moderate to excellent yields, wide substrate scope, and divergent structures of products. In addition, the photophysical properties of the synthesized products were evaluated. These products exhibit interesting fluorescence properties, and surprisingly, the compounds have the ability to selectively recognize trifluoroacetic acid.

Base-Promoted Tandem Cyclization of 2-(Indol-3-yl)naphthoquinones with Benzamidines toward Polysubstituted Pyrimido[4,5-b]indoles

A novel and unique approach for the construction of polysubstituted pyrimido[4,5-b]indoles from 2-(indol-3-yl)naphthoquinones and benzamidines is described. Our strategy, promoted by an inorganic base, involves the ring opening and recyclization of naphthoquinone and produces three nitrogen heterocyclic rings via multiple C-N bond formations in one pot under transition-metal-free conditions.

Iron-Catalyzed Friedel-Crafts-type 3,5-Diacylation of Indoles

The exploration of remote functionalization of indoles is impeded by the inherently dominant reactivity intrinsic to the pyrrole moiety. Herein, we delineate a novel strategy facilitated by Lewis acid mediation, enabling the remote C-H functionalization, which culminates in the synthesis of an array of selectively functionalized indole derivatives, encompassing 3-trifluoroacetyl and 5-benzoyl motifs, utilizing trifluoroacetic anhydride and various acyl chlorides. Notably, the protocol exhibits versatility, as epitomized by the extension of C5-acylation to alkylation and sulfonation reactions. This methodology is distinguished by its exemplary regio- and chemo-selectivity, extensive substrate scope, commendable tolerance to a diverse array of functional groups, and the employment of comparatively mild reaction conditions.

Cobalt-Catalyzed Deacylative Ipso-C-C Bond Functionalization: An Approach toward Indole-Acyloins and Its Photophysical Studies

Selective functionalization of the indole-C3-C bond with aromatic/heteroaromatic 1,2-diketones has been uncovered for the first time. Cobalt catalyst was found to be an effective catalyst for this unusual transformation. This ipso-C-C bond functionalization occurred in the presence of easily available weakly coordinating groups such as ketone and ester. One of the salient features of this methodology is the in situ generation of water from hexafluoro-2-propanol which acts as a reactant for the removal of the pivaloyl/ester group in a deacylative manner. The plausible mechanism has been supported by DFT calculations. Moreover, photophysical studies show the potential utility of indole-C3-acyloin and indolo-fused carbazole, which could be used in photovoltaic and optoelectronic application.

Ruthenium(II)-Catalyzed Sequential C-H/N-H Alkene Annulation Cascade of Phenanthroimidazoles: Synthesis and Photophysical Studies

We report ruthenium(II)-catalyzed sequential C-H/N-H alkenylation cascade of phenanthroimidazole and alkenes to form novel phenanthroimidazoisoindol acrylates via dual C-H activation and aza-Michael reaction. The two nitrogen atoms of the imidazole ring act as directing groups for regioselective dual sequential ortho C-H activation. These polycyclic N-heterocycles were evaluated for their photophysical properties.

Synthesis of Spiro Polycyclic N-Heterocycles and Indolecarbazoles via Merging Oxidative Coupling and Cascade Palladium-Catalyzed Intramolecular Oxidative Cyclization

We report a step-economic strategy for the direct synthesis of spiro polycyclic N-heterocycles and indolecarbazole-fused naphthoquinones by merging oxidative coupling and cascade palladium-catalyzed intramolecular oxidative cyclization. In the protocol, bi-indolylnaphthoquinones were first synthesized by oxidative coupling of indoles and naphthoquinones. Subsequent cascade palladium-catalyzed intramolecular oxidative cyclization of bi-indolylnaphthoquinones gave spiro polycyclic N-heterocycles and indolecarbazoles. The intramolecular oxidative cyclization approach could also be realized by the presence or absence of iron catalysts under standard conditions. This protocol is featured with moderate to excellent yields, a wide substrate scope, and divergent structures of products.

Access to 5-Methyl-5H-naphtho[2,3-c]carbazole-8,13-dione Derivatives via Copper-Catalyzed Intramolecular Isomerization

Carbazole-fused quinones are important compounds for their potential pharmacological activities and photophysical properties. Here, a novel copper-catalyzed intramolecular isomerization process to access a new class of naphtho[2,3-c]carbazole-8,13-dione derivatives via a furan isomerization/γ-H elimination and β-C elimination/6π-electrocyclization/aromatization cascade is reported. Furthermore, the preliminary photophysical properties of the functional 5-methyl-5H-naphtho[2,3-c]carbazole-8,13-dione derivatives have been studied.

Iodine-Mediated C2,3-H Aminoheteroarylation of Indoles

A metal-free one-pot oxidative cross-dehydrogenation coupling reaction for the formation of C-N/C-C bonds at the C2,3-positions of indoles with azoles and quinoxalinones has been developed. The proposed method has several notable features, including metal-free catalysis, the use of N-H free indoles as substrates, ease of operation, mild reaction conditions, and compatibility with a wide range of substrates.

'On Water'-Promoted Three-Component Tandem Michael Addition/D-A Cycloaddition Reaction to Construct Polycyclic N-Heterocycles Derivatives

'On Water'-promoted the three-component tandem Michael addition/D-A cycloaddition reaction in 80 °C at 3 h has been developed without employing any catalyst and organic solvent. The process allows facile access to polycyclic N-heterocycles derivatives contain indole and maleimide from easily accessible starting materials in moderate to high yields (up to 91 %). Compared with conventional reaction conditions, this reaction not only improves the reaction efficiency and rate but also minimizes the side reaction.

B(C6F5)3-Catalyzed [4 + 2] Cyclization Strategy: Synthesis and Photophysical Properties of 5H-Naphtho[2,3-c]carbazole-8,13-dione Derivatives

In this paper, a series of novel carbazolequinones were efficiently obtained by a B(C₆F₅)₃-catalyzed [4 + 2] cyclization reaction. This protocol not only had a simple operation, broad substrate range, and high atomic economy, but also had a low catalyst loading and avoided using metal catalysts. In addition, we constructed diverse new carbazole-fused compounds under different reduction conditions. The results of photophysical characterization showed that the structure of carbazole-fused derivatives had a significant impact on the fluorescence properties.

Substrate-directed divergent synthesis of fused indole polycycles through Rh(II)-catalyzed cascade reactions of bis(diazo)indolin-2-ones

Herein, we report a substrate-directed diverse synthetic strategy toward two kinds of structurally intriguing fused indole polycycles through Rh(II)-catalyzed cascade reactions of bis(diazo)indolin-2-ones with enaminones. The subtle structural changes in enaminones lead to different reactivities. Cyclic enaminones produce indolo[2,3-b]indoles, whereas acyclic ones afford oxazolo[3,2-a]indoles.

Metal-Free Aminohalogenation of Quinones With Alkylamines and NXS at Room Temperature

A simple and practical strategy for intermolecular aminohalogenation of quinone with alkyl amines and NXS was developed, in which haloamines generated in situ were employed as bifunctional reagents. The reaction system is reliable, efficient and wide in substrate range, which is suitable for the two-fold aminochlorination of 1, 4-benzoquinones, large-scale reaction and late-stage modification of pharmaceuticals.

One-pot synthesis of indole-fused nitrogen heterocycles via the direct C(sp2)–H functionalization of naphthoquinones; accessibility for deep red emitting materials

A convenient one-pot, two-step methodology was developed for the transformation of readily available planar naphthoquinone derivatives into structurally complex indole-fused nitrogen heterocycles under aerobic conditions.

Cu(II)-Promoted Cascade Synthesis of Fused Imidazo-Pyridine-Carbonitriles

A Cu(II)-promoted synthesis of an aza-fused N-heterocycle having a benz-imidazopyridine scaffold is developed via an addition-cyclization reaction followed by an Ullmann-type C-N coupling between o-iodoanilines and γ-ketodinitriles. This protocol features a broad substrate scope, giving products in 32-84% yields. The compounds show excellent photoluminescence properties having two absorption maxima in the region between 270-280 and 338-350 nm and emission maxima in the range of 502-533 nm. The HOMO-LUMO energy gap of 3.49-3.57 eV was determined using Gaussian 09 at the B3LYP/6-31G (d, p) basis set level. We also demonstrated a few postsynthetic modifications.

(NH4)2S2O8-Promoted Direct C–C Coupling of Indoles with Quinones/Hydroquinones without Catalyst

An atom-economical and environmentally benign approach for the synthesis of 3-indolylquinones was achieved successfully via direct oxidative C–C coupling of quinones/hydroquinones with indoles using (NH4)2S2O8 in dichloroethane at 80 °C. The efficiency of this catalytic approach was established by a broad scope of substrates involving quinones and hydroquinones to give high yields (61–93%) of 3-indolylquinones. The present protocol is simple, practical, and show good functional group tolerance. In addition, the obtained 3-indolylnaphthoquinones were conducted to further transformation to synthesize 2-amino-3-indolylnaphthoquinone and polycyclic N-heterocycles, respectively.

Rapid Access to Benzimidazo[1,2-a]quinoline-Fused Isoxazoles via Pd(II)-Catalyzed Intramolecular Cross Dehydrogenative Coupling: Synthetic Versatility and Photophysical Studies

An efficient and atom-economical palladium-catalyzed intramolecular cross dehydrogenative coupling (CDC) reaction has been developed for the construction of highly π-conjugated benzimidazo[1,2-a]quinoline-fused isoxazole scaffolds using molecular oxygen as sole oxidant. The approach portrayed wide substrate scope with good functional group tolerance and depicted a useful tool for the generation of fluorescence active compounds with high quantum yield. Synthetic versatility of the method via Fe-catalyzed reductive isoxazole ring cleavage toward pyridine, pyrimidine, pyrazole fused heteropolycyclic compounds has been showcase.

Copper-catalyzed one-pot amine-alkylation of quinones with amines and alkanes

A copper-catalyzed one-pot amine-alkylation of quinones with amines and alkanes in the presence of di-tert-butyl peroxide (DTBP) was developed via a radical reaction process. Various alkanes and aromatic or aliphatic amines with diverse structures and electronic properties are suitable substrates, and the chirality of amines can be maintained for the transformation. This method has high step and atom economy for straightforward access to aminated and alkylated quinones from readily available starting materials.

t-BuOK mediated oxidative coupling amination of 1,4-naphthoquinone and related 3-indolylnaphthoquinones with amines

The transition-metal free amination of 1,4-naphthoquinone and related 3-indolylnaphthoquinones with amines, such as various (hetero)aromatic amine and aliphatic amine via t-BuOK-mediated oxidative coupling at room temperature has been developed. This reaction provides efficient access to the biologically important and synthetically useful 2-amino-1,4-naphthoquinones and 2-amino-3-indolylnaphthoquinones with good yields under mild conditions. The present protocol is simple, practical and shows good functional group tolerance. In addition, the obtained 2-amino-3-indolylnaphthoquinones were further transformed to synthesize polycyclic N-heterocycles.

Assembly of polycyclic N-heterocycles via copper-catalyzed cycloamination of indolylquinones and aromatic amines

The copper-catalyzed cycloamination of indolylquinones and various (hetero)aromatic amines under ligand-free conditions for the synthesis of polycyclic N-heterocycles has been developed. This method allows facile access to polycyclic N-heterocycles with the tolerance of chloride, bromide, amino, thio, etc. groups in moderate to high yields (60-89%).

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

B(C6F5)3-catalyzed three-component tandem reaction to construct novel polycyclic quinone derivatives: synthesis of a carbonate salt chromogenic chemosensor

A series novel polycyclic quinone derivatives were constructed providing a carbonate salt chromogenic chemosensor.