Recent approaches for the synthesis of heterocycles from amidines via a metal catalyzed C-H functionalization reaction

Transition metal catalyzed C-H bond activation has become one of the most important tools for constructing new chemical bonds. Introducing directing groups to the substrates is the key to a successful reaction, these directing groups can also be further transformed in the reaction. Amidines with their unique structure and reactivity are ideal substrates for transition metal-catalyzed C-H transformations. This review describes the major advances and mechanistic investigations of the C-H activation/annulation tandem reactions of amidines until early 2024, focusing on metal-catalyzed C-H activation of amidines with unsaturated compounds, such as alkynes, ketone, vinylene carbonate, cyclopropanols and their derivatives. Meanwhile this manuscript also explores the reaction of amidines with different carbene precursors, for example diazo compounds, azide, triazoles, pyriodotriazoles, and sulfoxonium ylides as well as their own C-H bond activation/cyclization reactions. A bright outlook is provided at the end of the manuscript.

(3+2) Cycloaddition of Heteroaromatic N-Ylides with Sulfenes

A (3+2) cycloaddition of heteroaromatic N-ylides with sulfenes, which are generated in situ from sulfonyl chlorides, has been developed. A variety of ylides were transformed into the corresponding sulfone-embedded N-fused heterocycles in high yields. Hexafluoroisopropyl mesylate was demonstrated to be a suitable reactant for quinolinium ylides. Furthermore, this cycloaddition could be performed with an ylide prepared by a Cu-catalyzed ylide transfer reaction in a one-pot manner, extending the substrate scope to an unisolable ylide.

Cp*Rh(III)-catalyzed regioselective cyclization of aromatic amides with allenes

A new Cp*Rh(III)-catalyzed regioselective cyclization reaction of aromatic amides with allenes is reported. The use of allenyl derivatives bearing a directing-group assistant as a reaction promoter was the key to the success of this protocol. In this catalytic system, N-(pivaloyloxy)benzamide substrates react with allenes via Rh-σ-alkenyl intermediates, while N-(pivaloyloxy) indol substrates react via Rh-π-allyl intermediates. These reactions were characterized by mild reaction conditions, a broad substrate scope, and high functional-group compatibility to yield several high-value isoquinolinone and pyrimido[1,6-a]indol-1(2H)-one skeleton-containing compounds. The synthetic applications and primary mechanisms were also investigated.

Halogen cation-promoted and solvent-regulated electrophilic cyclization for the regioselective synthesis of 3-haloquinolines and 3-halospirocyclohexadienones

A novel approach for the production of halogen cations through the reaction of halogens with silver ions is described in this paper. On this basis, the regioselective synthesis of 3-haloquinolines and 3-halospirocyclohexadienones is realized through solvent regulation. The gram-scale reaction and the compatibility of complex substrates demonstrate the synthetic potential of this protocol, which will be an appealing strategy in organic synthesis.

Iodonium ylides: an emerging and alternative carbene precursor for C-H functionalizations

The metal-catalyzed successive activation and functionalization of arene/heteroarene is one of the most fundamental transformations in organic synthesis and leads to privileged scaffolds in natural products, pharmaceuticals, agrochemicals, and fine chemicals. Particularly, transition-metal-catalyzed C-H functionalization of arenes with carbene precursors via metal carbene migratory insertion has been well studied. As a result, diverse carbene precursors have been evaluated, such as diazo compounds, sulfoxonium ylides, triazoles, etc. In addition, there have been significant developments with the use of iodonium ylides as carbene precursors in recent years, and these reactions proceed with high efficiencies and selectivities. This review provides a comprehensive overview of iodonium ylides in C-H functionalizations, including the scope, limitations, and their potential synthetic applications.

Visible-light-promoted radical cascade alkylation/cyclization: access to alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-ones

A new protocol is herein described for the direct generation of alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-one derivatives by using Hantzsch esters as alkylation radical precursors using a photoredox/K₂S₂O₈ system. This oxidative alkylation of active alkenes involves a radical cascade cyclization process and a sequence of Hantzsch ester single electron oxidation, C-C bond cleavage, alkylation, arylation and oxidative deprotonation.

Recent Strategies in Nickel-Catalyzed C–H Bond Functionalization for Nitrogen-Containing Heterocycles

N-heterocycles are ubiquitous in natural products, pharmaceuticals, organic materials, and numerous functional molecules. Among the current synthetic approaches, transition metal-catalyzed C–H functionalization has gained considerable attention in recent years due to its advantages of simplicity, high atomic economy, and the ready availability of starting materials. In the field of N-heterocycle synthesis via C–H functionalization, nickel has been recognized as one of the most important catalysts. In this review, we will introduce nickel-catalyzed intramolecular and intermolecular pathways for N-heterocycle synthesis from 2008 to 2021.

Sc(OTf)3-Mediated [4 + 2] Annulations of N-Carbonyl Aryldiazenes with Cyclopentadiene to Construct Cinnoline Derivatives: Azo-Povarov Reaction

We disclose the first accomplishment of the azo-Povarov reaction involving Sc(OTf)₃-catalyzed [4 + 2] annulations of N-carbonyl aryldiazenes with cyclopentadiene in chloroform, in which N-carbonyl aryldiazenes act as 4π-electron donors. Hence, this protocol offers a rapid access to an array of cinnoline derivatives in moderate to good yields for substrates over a wide scope. The synthetic potential of the protocol was achieved by the gram-scale reaction and further derivatization of the obtained polycyclic product.

Recent Strategies in Transition-Metal-Catalyzed Sequential C–H Activation/Annulation for One-Step Construction of Functionalized Indazole Derivatives

Designing new synthetic strategies for indazoles is a prominent topic in contemporary research. The transition-metal-catalyzed C–H activation/annulation sequence has arisen as a favorable tool to construct functionalized indazole derivatives with improved tolerance in medicinal applications, functional flexibility, and structural complexity. In the current review article, we aim to outline and summarize the most common synthetic protocols to use in the synthesis of target indazoles via a transition-metal-catalyzed C–H activation/annulation sequence for the one-step synthesis of functionalized indazole derivatives. We categorized the text according to the metal salts used in the reactions. Some metal salts were used as catalysts, and others may have been used as oxidants and/or for the activation of precatalysts. The roles of some metal salts in the corresponding reaction mechanisms have not been identified. It can be expected that the current synopsis will provide accessible practical guidance to colleagues interested in the subject.

Remote Construction of N-Heterocycles via 1,4-Palladium Shift-Mediated Double C-H Activation

In the past years, Pd⁰‐catalyzed C(sp³)−H activation provided efficient and step‐economical methods to synthesize carbo‐ and heterocycles via direct C(sp²)−C(sp³) bond formation. We report herein that a 1,4‐Pd shift allows access to N‐heterocycles which are difficult to build via a direct reaction. It is shown that o‐bromo‐N‐methylanilines undergo a 1,4‐Pd shift at the N‐methyl group, followed by intramolecular trapping by C(sp²)−H or C(sp³)−H activation at another nitrogen substituent and remote C−C bond formation to generate biologically relevant isoindolines and β‐lactams. The product selectivity is influenced by the employed ligand, with NHCs favoring the product of remote C−C coupling against products arising from direct C−C coupling and N‐demethylation.

Pd(II)/Lewis acid catalyzed regioselective olefination of indole with dioxygen

Transition metal ion catalyzed indole olefination through C-H activation is a convenient protocol to synthesize versatile bioactive vinylindole compounds; however, in most cases, stoichiometric amounts of oxidants were necessary to accomplish the catalytic cycle. The present study describes a Pd(II)/LA (LA: Lewis acid) catalyzed indole olefination with dioxygen as the sole oxidant. The olefination reaction with electron-rich olefins proceeded smoothly through the pyrrolyl N-carboxamide group directed remote C-H activation at the C3 position of the indole with the Pd(II)/LA catalyst, whereas Pd(II) alone was a very sluggish catalyst under identical conditions. For the electron-deficient olefins, the directing N-carboxamide group was not essential for olefination with this Pd(II)/LA catalyst, demonstrating a different olefination pathway from that of electron-rich olefins. Remarkably, ¹H NMR kinetics disclosed that olefination proceeded much faster with electron-rich olefins than with electron-deficient ones.

Copper(i)-catalyzed synthesis of natural alkaloid tryptanthrin and its derivatives

A facile one-pot synthesis of natural alkaloid tryptanthrin and its derivatives from isatins catalyzed by CuI in the presence of KHCO3 has been developed.

RhIII-Catalyzed heteroarylation of N-2,6-difluorophenyl arylamides with heteroaryl boronate esters

An efficient strategy to aryl-heteroaryl formation via RhIII-catalyzed C–H heteroarylation of arenes with N-heterocyclic boronates has been disclosed.

Recent advances in transition metal-catalyzed heteroannulative difunctionalization of alkenes via C-H activation for the synthesis of heterocycles

Heterocyclic compounds are the fundamental structural motifs distributed in natural products, pharmaceuticals and biologically active compounds. Thus, there is increasing interest in the development of novel synthetic strategies for the...

Recent Advances in the Tandem Copper-Catalyzed Ullmann-Goldberg N-Arylation–Cyclization Strategies

N‒Aryl bond formation under copper catalysis has been playing a pivotal role and has been extensively used as a key step in the total syntheses of several therapeutic molecules. The...

Understanding Cu(II)-based systems for C(sp3)-H bond functionalization: insights into the synthesis of aza-heterocycles

Herein we report the synthesis of imidazo[1,5-a]pyridine heterocycles via a Cu(II)-mediated functionalization of α'-C(sp³)-H bonds of pyridinylaldimines and subsequent cyclization. This strategy exploits the inherent directing ability of heteroleptic aldimine and pyridine groups in the substrate yielding the C-H functionalization of α'-methylene groups in a regioselective fashion over distant methyl or methylene groups in β or γ positions. The observed correlation between the nature of the anionic ligands (halide vs. carboxylate) bonded to copper and the chemoselectivity of the C(sp³)-H activation process points to a concerted metalation-deprotonation pathway prior to cyclization to furnish the corresponding imidazo[1,5-a]pyridine derivative. This copper-mediated C(sp³)-H bond functionalization reaction works for a variety of substrates incorporating linear alkyl chains (from 3 to 12 carbon atoms), and good functional group tolerance (aryl, ether and ester groups). Cu-Catalyzed C(sp²)-H cyanation on the imidazole ring can then take place selectively under oxidative conditions.

Multiple Reaction Pathways of Eight-Membered Rhodacycles in Rh-Catalyzed Annulations of 2-Alkenyl Phenols/Anilides with Alkynes

Density functional theory calculations were performed to study the competing pathways of rhodacycle intermediates generated in Rh(III)-catalyzed annulations of 2-alkenyl phenols and 2-alkenyl anilides with alkynes. The results show that the multiple pathways of eight-membered rhodacycles can be subtly tuned to give specific cyclic products. The seven-membered oxacyclic and spirocyclic products from 2-alkenyl phenols are formed by favoring the pathway of dissociating the Rh-O bond of O-contained rhodacycles, which are followed by antarafacial nucleophilic attack. The indoline product from 2-alkenyl anilides is generated through the pathway of intramolecular olefin migratory insertion of the N-contained rhodacycle.

K2S2O8 activation by glucose at room temperature for the synthesis and functionalization of heterocycles in water

While persulfate activation at room temperature using glucose has primarily been focused on kinetic studies of the sulfate radical anion, the utilization of this protocol in organic synthesis is rarely demonstrated. We reinvestigated selected K₂S₂O₈-mediated known organic reactions that invariably require higher temperatures and an organic solvent. A diverse, mild functionalization and synthesis of heterocycles using the inexpensive oxidant K₂S₂O₈ in water at room temperature is reported, demonstrating the sustainability and broad scope of the method. Unlike traditional methods used for persulfate activation, the current method uses naturally abundant glucose as a K₂S₂O₈ activator, avoiding the use of higher temperature, UV light, transition metals or bases.

Azapyrene-based fluorophores: synthesis and photophysical properties

The latest methods for the preparation/functionalization of azapyrenes, and their photophysical/electrochemical properties and possible applications are overviewed.

Pd(ii)-Catalyzed enantioconvergent twofold C–H annulation to access atropisomeric aldehydes: a platform for diversity-oriented-synthesis

The first Pd(ii)-catalyzed atroposelective dual C–H annulative strategy for diverse synthesis of functionalized axially chiral biaryls was developed.