N1 -Selective Oxidative CN Coupling of Azoles with Pyrroles Using a Hypervalent Iodine Reagent

Green Halogenation of Indoles with Oxone-Halide

Oxidative functionalization of indoles is one of the most widely used approaches to exploit the synthetic utility of indoles. In continuation of our research interest in the green oxidation of indoles, we further explore the oxidation of indoles with oxone-halide and discover that the protecting group on the nitrogen of indoles plays a decisive role in controlling the pathways of indole oxidation with oxone-halide. An electron-withdrawing group on the nitrogen of indoles (N-EWG) enables C2 halogenation with stoichiometric halide, while C3 halogenation could be selectively achieved by using stoichiometric halide without dependence on the electronic property of the protecting group on the indole nitrogen. Different from our previous results obtained by using catalytic halide, these findings lead to the development of an environmentally friendly, efficient, and mild protocol for access to 2- or 3-haloindoles (chloro and bromo). As compared to the previous synthetic methods for 2-/3-haloindoles, our method exploits the in situ-generated reactive halogenating species from oxone-halide for halogenation of indoles and thus eliminates the use of stoichiometric halogenating agents and the production of toxic and hazardous organic byproducts derived from oxidants.

Copper-Catalyzed Azide-Alkyne Cycloaddition of Hydrazoic Acid Formed In Situ from Sodium Azide Affords 4-Monosubstituted-1,2,3-Triazoles

We report a copper-catalyzed cycloaddition of hydrogen azide (hydrazoic acid, HN3) with terminal alkynes to form 4-substituted-1H-1,2,3-triazoles in a sustainable manner. Hydrazoic acid was formed in situ from sodium azide under acidic conditions to react with terminal alkynes in a copper-catalyzed reaction. Using polydentate N-donor chelating ligands and mild organic acids, the reactions were realized to proceed at room temperature under aerobic conditions in a methanol-water mixture and with 5 mol % catalyst loadings to afford 4-substituted-1,2,3-triazoles in high yields. This method is amenable on a wide range of alkyne substrates, including unprotected peptides, showing diverse functional group tolerance. It is applicable for late-stage functionalization synthetic strategies, as demonstrated in the synthesis of the triazole analogue of losartan. The preparation of orthogonally protected azahistidine from Fmoc-l-propargylglycine was realized on a gram scale. The hazardous nature of hydrazoic acid has been diminished as it forms in situ in <6% concentrations at which it is safe to handle. Reactions of distilled solutions of hydrazoic acid indicated its role as a reactive species in the copper-catalyzed reaction.

Copper-iodine Co-catalyzed C−H Aminoalkenylation of Indoles via Temperature-controlled Selectivity Switch: Facile Synthesis of 2-Azolyl-3-alkenylindoles

An efficient copper-iodine co-catalyzed 2,3-difunctionalization of indoles with azoles and phenols via temperature-controlled selectivity switch has been developed for the green synthesis of 2-azolyl-3-alkenylindoles. The strategy involves the simultaneous establishment...

Electro-Oxidative C-N Bond Formation through Azolation of Indole Derivatives: An Access to 3-Substituent-2-(Azol-1-yl)indoles

A practical protocol to synthesize 3-substituent-2-(azol-1-yl)indole derivatives has been developed via an electrochemical oxidative cross coupling process under mild conditions. This electro-oxidative C-N bond formation strategy tolerates a range of functional groups and is amenable to gram scale synthesis. Moreover, this method was applied to the late-stage functionalization of bioactive molecules.

Cross-Dehydrogenative N-N Coupling of Aromatic and Aliphatic Methoxyamides with Benzotriazoles

Nitrogen-nitrogen bond containing motifs are ubiquitous in bioactive compounds and organic materials. However, intermolecular hetero-selective N-H/N-H oxidative coupling reactions remain very challenging and largely unexplored. Here, we report an unprecedented, simple and hetero-selective cross-dehydrogenative N-N coupling of amides and benzotriazoles, utilizing only a hypervalent iodine species as the terminal oxidant. The scope and mechanistic investigations are discussed.

Aerobic Oxidative C-H Azolation of Indoles and One-Pot Synthesis of Azolyl Thioindoles by Flavin-Iodine-Coupled Organocatalysis

The aerobic oxidative cross-coupling of indoles with azoles driven by flavin-iodine-coupled organocatalysis has been developed for the green synthesis of 2-(azol-1-yl)indoles. The coupled organocatalytic system enabled the one-pot three-component synthesis of 2-azolyl-3-thioindoles from indoles, azoles, and thiols in an atom-economical manner by utilizing molecular oxygen as the only sacrificial reagent.

Metal-Free Oxidative Cross-Coupling Reaction of Heteroaromatic and Related Compounds

The biary unit having heteroatom as important scaffolds widely exist in a large number of biologically active compounds and functional organic molecules. Since the cross-coupling is a useful synthetic method for constructing biaryl and heterobiaryl structures, the development of novel cross-coupling methods has been studied intensively. The oxidative biaryl coupling reaction of aromatic compounds having heteroatoms is an attractive method since they do not require the prefunctionalization of arenes. This report describes recent advances in hypervalent iodine(III) induced metal-free synthesis of biaryls having heteroatoms.

Late stage functionalization of heterocycles using hypervalent iodine(iii) reagents

Late stage functionalization (LSF) through direct X-H manipulations (X = C, N) enables synthetic chemists to accelerate the diversification of natural products, agrochemicals and pharmaceuticals allowing rapid access to novel bioactive molecules without resorting to arduous de novo synthesis. LSF does not only allow tapping of the hitherto unexplored chemical space but also renders the synthetic sequence more straightforward, atom economical and cost-effective. In this regard, the recent decade has witnessed the emergence of hypervalent iodine(iii) reagents as a powerful synthetic tool owing to their easy availability, mild reaction conditions, remarkable oxidizing properties and high functional group tolerance. Iodine(iii) reagents have tremendous applications in the regio- and chemo-selective late-stage functionalization of a diverse variety of heterocycles through an exciting range of transformations, such as oxidative amination, cross-dehydrogenative coupling (CDC), fluoroalkylation, azidation, halogenation and oxidation. The present review, classified according to the types of synthetic methods involved, encompasses all these recent developments in the field of transition-metal-free iodine(iii)-catalyzed/mediated direct functionalizations of heterocycles with representative examples and insightful mechanistic discussions.

Metal-Free Oxidative Cross Coupling of Indoles with Electron-Rich (Hetero)arenes

A new method for the synthesis of bi-heteroaryls is reported, based on the umpolung of indoles with benziodoxol(on)e hypervalent iodine reagents (IndoleBX). The oxidative coupling of IndoleBX with an equimolar amount of electron-rich benzenes, indoles, pyrroles, and thiophenes proceeded under mild transition-metal-free conditions. Functionalized non-symmetrical bi-indolyl heterocycles were accessed efficiently. Introduction of a new type of C2-substituted indole benziodoxole reagents further allowed extending the scope of the reaction to NH unprotected and C3-alkylated indoles. The obtained bi-heterocycles are important building blocks in synthetic and medicinal chemistry, and could be easily transformed into more complex heterocyclic systems.

Stabilized pyrrolyl iodonium salts and metal-free oxidative cross-coupling

Pyrrole-aryl derivatives are important due to their unique biological activities in medicinal chemistry. We now report a new oxidative biaryl coupling for pyrroles and indoles toward various arenes using a hypervalent iodine reagent and an appropriate stabilizer for pyrrolyl iodonium intermediates. The reactions readily provide a variety of desired coupling products in good yields.