Electrochemical Iodoamination of Indoles Using Unactivated Amines

Electrochemical Annulation of p-Alkoxy or p-Hydroxy Anilines with Enaminones for Selective Indole and Benzofuran Synthesis

The green and sustainable electrochemical oxidative cyclization of enaminones with p-methoxy/p-hydroxy anilines has been developed, enabling the regioselective synthesis of indoles and benzofurans. The methods are of significant advantage not only due to the mild and metal-free sustainable electrochemical conditions, but also because of the employment of easily available and stable aniline substrates to replace the unstable quinoneimide for a similar annulation in previous work.

Electrochemical Three-Component C-H Functionalization of Indoles with Sodium Bisulfite and Alcohols to Access Indole-Containing Sulfonate Esters

Herein, an efficient electrochemical three-component C-H functionalization of indoles with sodium bisulfite and alcohols is described, providing a sustainable and convenient synthetic route for the construction of structurally valuable indole-containing sulfonate esters in moderate to good yields. This protocol proceeds in an undivided cell without any metal catalysts or oxidants, features a broad substrate scope, and has an excellent functional group tolerance. Preliminary mechanistic studies suggest that a radical-radical pathway may be involved in this three-component reaction system.

Recent advances in the electrochemical functionalization of N-heterocycles

Nitrogen-containing heterocyclic cores are of immense importance due to their high abundance in naturally occurring or synthetic molecules having wide applications in different fields of basic and applied sciences. The functionalities introduced in an N-heterocyclic core play an important role in regulating the physiochemical behavior of the particular N-heterocycles to alter their chemical and biological reactivity. Suitably functionalized N-heterocycles demonstrate their widespread applications in pharmaceuticals, agronomy, materials sciences, synthetic chemistry, pigments, etc. During the last decade, electrochemistry has emerged as a sustainable alternative to conventional synthetic approaches by minimizing reagent uses and chemical waste. Synthetic chemists have extensively utilized the tool to functionalize N-heterocycles. This is evidenced by the appearance of more than a hundred methods on the topic over recent years, signifying the importance of the synthetic area. This review is focused on the accumulation of synthetic methods based on the electrochemical functionalization of N-heterocycles developed over the recent decade. Literature reports on the C-/N-H-functionalization and functional modifications of N-heterocycles that are accessible through the available search engines are included in the review. Relevant mechanistic details in support of the reported reactions are discussed to present a clear picture of the reaction pathways. The review aims to provide a clear picture of the possible pathways of electron transfer, the electrochemical behavior of different N-heterocyclic cores, functionalization reagents, and the chemical processes that occur during the electrochemical functionalization/modification of N-heterocycles.

Iodine-mediated oxidative triple functionalization of indolines with azoles and diazonium salts

We report an innovative synthetic strategy for the generation of polysubstituted indoles from indolines, aryldiazonium salts, and azoles. The methodology encompasses an electrophilic substitution reaction affording C5-indoline intermediates which undergo an iodine-mediated oxidative transformation coupled with C-H functionalization to yield the indole derivatives.

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.

Green One-Pot Syntheses of 2-Sulfoximidoyl-3,6-dibromo Indoles Using N-Br Sulfoximines as Both Brominating and Sulfoximinating Reagents

A green one-pot 2,3,6-trifunctionalization of N-alkyl/aryl indoles was achieved by adding three equivalents of N-Br sulfoximine to the indole solution. A variety of 2-sulfoximidoyl-3,6-dibromo indoles were prepared with 38-94% yields using N-Br sulfoximines as both brominating and sulfoximinating reagents. Based on the results of controlled experiments, we propose that a radical substitution involving 3,6-dibromination and 2-sulfoximination occurs in the reaction process. This is first time that 2,3,6-trifunctionalization of indole in one pot has been achieved.

Copper-Catalyzed Intermolecular Cross-dehydrogenative C-N Coupling at Room Temperature via Remote Activating Group Enabled Radical Relay Strategy

Employing N-fluorobenzenesulfonimide (NFSI) as a nitrogen-centered radical (NCR) precursor, an intermolecular C(sp²)-N coupling on heteroarenes or substituted benzenes with remote activated aniline derivatives via copper catalyzed N-N radical relay strategy at room temperature is developed. Good to excellent yields are acquired, and no ligand or additive is required. Reaction scope investigation and preliminary mechanistic studies demonstrate that the remote activating strategy and delicate control on the reactivities of active NCR species are essential to guarantee satisfactory chemo- and site-selectivity.

Halogen Bond Motifs in Cocrystals of N,N,O and N,O,O Acceptors Derived from Diketones and Containing a Morpholine or Piperazine Moiety

In this study, we investigate the halogen bond acceptor potential of oxygen and nitrogen atoms of morpholine and piperazine fragments when they are peripherally located on N,O,O or N,N,O acceptor molecules. We synthesized four acceptor molecules derived from either acetylacetone or benzoylacetone and cocrystallized them with 1,4-diiodotetrafluorobenzene and 1,3,5-triiodotrifluorobenzene. This resulted in eight cocrystals featuring different topicities and geometric dispositions of donor atoms. In all cocrystals, halogen bonds are formed with either the morpholinyl oxygen atom or the terminal piperazine nitrogen atom. The I···O_morpholine halogen bonds feature lower relative shortening values than I···N_terminal, I···O_carbonyl, and I···N_proximal halogen bonds. The N and O halogen bond acceptor sites were evaluated through calculations of molecular electrostatic potential values.

An organophotoredox-catalyzed C(sp2)-N cross coupling reaction of cyclic aldimines with cyclic aliphatic amines

An organophotocatalyzed C(sp2)-H/N-H cross-dehydrogenative coupling of cyclic aldimines with aliphatic amines has been developed, which represents the first example of visible-light-induced C-H amination of N-sulfonylated imines. This methodology enables the streamline assembly of amine derivatives via radical mediated C-N bond formation. The current protocol features transition-metal-free, mild conditions, good functional group tolerance and good yields.

Electrochemically facilitated oxidative C–H amination enables access to fluorescent N9-fused tricyclic xanthines

An electrochemically enabled intramolecular C−H amination route for accessing a broad range of fluorescent N9-fused tricyclic xanthines with various substitution patterns under simple, green, and mild condition is developed.