Microwave-assisted synthesis of 3-nitroindoles from N-aryl enamines via intramolecular arene-alkene coupling

(Indol-3-yl)(DMIX)Iodonium Salts: Novel Electrophilic Indole Reagents

A new umpolung approach to the C3-H functionalization of indoles with diverse nucleophiles based on the intermediate formation of I(III) reagents is described. The 3,5-dimethylisoxazol-4-yl auxiliary allows for selective indole transfer under catalyst-free conditions, which was impossible using previously reported reagents. Combining the mildness of transition-metal-free conditions and the high reactivity of hypervalent iodine reagents, this protocol tolerates various functional groups and provides access to indoles that are difficult to prepare conventionally.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

δ-Keto Aminoacrylonitriles and δ-Keto Aminoenones from 1-Pyrrolines, Cyanoacetylenes, and Acetylenic Ketones

The ring-opening/functionalization of 1-pyrrolines by cyano­acetylenes or acetylenic ketones (20–80 °C, MeCN, H2O) affords δ-keto aminoacrylonitriles and δ-keto aminoenones, mostly as the Z-isomers, in up to 85% yields. The synthesis involves C(2)–N bond cleavage in the intermediate hemiaminal resulting from the intermediate 1,3(4)-dipolar 1-pyrroline/acetylene complexes and water.

A novel domino protocol for three-component synthesis of new dibenzo[e,g]indoles: flexible intramolecular charge transfers

New and electronically interesting 2-aryl-3-nitrodibenzo[e,g]indoles were synthesized effectively via a hitherto unreported three-component domino reaction under catalysis of a superparamagnetic nano-composite.

Synthesis of 3-nitroindoles by sequential paired electrolysis

3-Nitroindoles are synthetically versatile intermediates but current methods for the preparation hinder their widespread application. Herein, we report that nitroenamines undergo electrochemical cyclisation to 3-nitroindoles in the presence of potassium iodide. Detailed control experiments and cyclic voltammogram studies infer the reaction proceeds via a sequential paired electrolysis process, beginning with anodic oxidation of iodide (I^-) to the iodine radical (I˙), which facilitates cyclisation of the nitroenamine to give a 3-nitroindolinyl radical. Cathodic reduction and protonation generates a 3-nitroindoline that upon oxidation forms the 3-nitroindole.

Copper-catalyzed, N-auxiliary group-controlled switchable transannulation/nitration initiated by nitro radicals: selective synthesis of pyridoquinazolones and 3-nitroindoles

Within the scope of nitration reactions, the efficiency of sensitive heteroaromatics such as indoles is often eroded by various competitive oxidative decomposition pathways.

Benefits and applications of microwave-assisted synthesis of nitrogen containing heterocycles in medicinal chemistry

Nitrogen containing heterocycles are of immense research interest because they are often found as naturally occurring bioactive compounds.

Tandem Ullmann-Goldberg Cross-Coupling/Cyclopalladation-Reductive Elimination Reactions and Related Sequences Leading to Polyfunctionalized Benzofurans, Indoles, and Phthalanes

On exposure to a combination of Cu[I]- and Pd[0]-based catalysts, compounds such as 1 and 7 engage in tandem Ullmann-Goldberg cross-coupling and cyclopalladation-reductive elimination reactions to give benzofurans such as 8. Related reactions involving hetero-Michael additions of o-halogenated phenols or anilines to propiolates and the Pd[0]-catalyzed cyclization of the resulting conjugates provide, in a one-pot process, alternately functionalized benzofurans, indoles, or phthalanes.

One-pot synthesis of thiazino[2,3,4-hi]indole derivatives through a tandem oxidative coupling/heteroannulation process

A novel and expedient one-pot synthesis of thiazino[2,3,4-hi]indole derivatives was developed through a tandem oxidative coupling/heteroannulation process.

Oxidative Nitration of Styrenes for the Recycling of Low-Concentrated Nitrogen Dioxide in Air

The oxidative nitration of styrenes in ethyl acetate represents a metal-free, environmentally friendly, and sustainable technique to recover even low concentrations of NO2 in air. Favorable features are that the product mixture comprising nitroalcohols, nitroketones, and nitro nitrates simplifies at lower concentrations of NO2 . Experiments in a miniplant-type 10 L wet scrubber demonstrated that the recycling technique is well applicable on larger scales at which initial NO2 concentrations of >10 000 ppm were reliably reduced to less than 40 ppm.

Expedient one-pot synthesis of indolo[3,2-c]isoquinolines via a base-promoted N-alkylation/tandem cyclization

A transition metal-free, one-pot protocol has been developed for the synthesis of 11H-indolo[3,2-c]isoquinolin-5-amines via the atom economical annulation of ethyl (2-cyanophenyl)carbamates and 2-cyanobenzyl bromides. This method proceeds via sequential N-alkylation and base-promoted cyclization. Optimization data, substrate scope, mechanistic insights, and photoluminescence properties are discussed.

Synthesis of α-enamino esters via Cu(ii)-promoted dehydrogenation of α-amino acid esters: application to the synthesis of polysubstituted pyrroles

α-Enamino esters are synthesized from α-amino acid esters using Cu(ii)-promoted dehydrogenation. Application of the α-enamino esters to the synthesis of polysubstituted pyrroles is achieved through oxidative coupling with alkynes under Cu(ii)-promoted conditions.

A facile approach for the synthesis of novel 1-oxa- and 1-aza-flavonyl-4-methyl-1H-benzo[d][1,3]oxazin-2(4H)-ones by microwave enhanced Suzuki–Miyaura coupling using bidentate chromen-4-one-based Pd(ii)–diimine complex as catalyst

A facile access to novel 1-oxa- and 1-aza-flavonyl-4-methyl-1H-benzo[d][1,3]oxazin-2(4H)-ones using microwave assisted Suzuki coupling of bromo flavones with boronic acids using a novel catalytic system (C) under aerobic condition is reported.

Palladium-catalysed carbonylative α-arylation of nitromethane

A palladium-catalysed approach to α-nitroketones via carbonylative α-arylation of nitromethane is presented, thus providing easy access to key intermediates and important heterocycles.

Stereoselective intermolecular nitroaminoxylation of terminal aromatic alkynes: trapping alkenyl radicals by TEMPO

The vinyl radical is one of the most unstable organic radicals. It is demonstrated that a nitro radical attacks phenylacetylene and makes the phenyl ring deconjugated with a double bond so that the resulting vinyl radical may be stabilized by delocalization to the phenyl ring's π orbital and easily trapped by TEMPO. It is noteworthy that all desired products were obtained in moderate to good yields in an (E)-configuration.

Palladium-loaded Renewable Polymer as a Green Heterogeneous Catalyst for Cross-coupling Reactions under Microwave Irradiation

A new palladium catalyst was prepared by immobilising ligand 2, 2′-dipyridylamine on the backbone of an acidic rosin polymer from gum rosin, on to which palladium(II) was bound via coordination. The catalyst at a low loading of 0.2 mol% was found to be highly effective for Suzuki–Miyaura coupling reactions of aryl halides and arylboronic acids under microwave irradiation in the presence of 1 equiv. of Na2CO3, affording excellent yields of the corresponding biaryls. Moreover, the catalyst exhibited very good recyclability over three cycles.

Cascade nitrosation and addition-elimination of nitroacetanilides for the highly efficient synthesis of 1,4,2,5-dioxadiazine derivatives

A domino nitrosation and addition-elimination of nitroacetanilides with NaNO2 and H2SO4 has been developed to synthesize a variety of 1,4,2,5-dioxadiazine-3,6-dicarboxamides in excellent yields. The substrate scope can be extended to aryl nitromethyl ketones. A cascade reaction mechanism is proposed and the conjugated aryl moiety is considered to help stabilize the aci-nitroso species, the key intermediates in the cascade reaction. The methodology is practical and efficient because it avoids the purification of the intermediates. The nitroacetanilides were prepared from nitroacetic acid and various anilines employing DCC-DMAP as coupling reagents, and this protocol also possesses advantages like easy handling and high yields.

A chemoselective route to β-enamino esters and thioesters

Conditions were developed for syntheses of β-enamino esters, thioesters, and amides. These reactions involve hydroxybenzotriazole derivatives in buffered media. Illustrative syntheses of some heterocyclic systems are given, including some related to protein–protein interface mimics.

An efficient microwave-assisted Suzuki reaction using a new pyridine-pyrazole/Pd(II) species as catalyst in aqueous media

A new pyridine-pyrazole N–N ligand has been conveniently synthesized and characterized by ¹H-, ¹³C-NMR, IR spectroscopies, HRMS and X-ray single-crystal crystallography analyses. The ligand adds to palladium(II) under basic conditions to give high yields of an air-stable and water-soluble complex that was fully characterized by NMR and HRMS. The complex was investigated as a catalyst for the Suzuki reaction in aqueous media under microwave irradiation. The compound proved to be an effective catalyst.