In Situ Generated HypoIodite Activator for the C2 Sulfonylation of Heteroaromatic N-oxides

C-H Bond Functionalization of N-Heteroarenes Mediated by Selectfluor

Herein, recent developments for Selectfluor-mediated C-H functionalization of N-heteroarenes are described. This type of C-H bond activation is an attractive and competitive alternative to traditional methodologies, allowing the functionalization of a variety of chemical functions. In addition, Selectfluor is a more sustainable and economically accessible oxidant compared with expensive/toxic metals or hazardous peroxides. For a practical understanding, the current review classified systematically the reported strategies in four subsections as follows: (1) carbon-carbon formation, (2) carbon-nitrogen bond formation, (3) carbon-chalcogen bond, and (4) carbon-halogen bond formation. Mechanistic aspects and reaction conditions are fully discussed to provide an understanding of the aspects that govern C-H functionalization in N-heteroarenes mediated by Selectfluor.

Metal-Free Highly Regioselective 1,4-Sulfonyliodination of 1,3-Enynes

Herein, a novel, practical, and green synthetic method using readily available 1,3-enynes with sulfonyl hydrazides and I₂ through tert-butyl hydroperoxide (TBHP)-mediated 1,4-sulfonyliodination has been developed for synthesizing various tetrasubstituted allenyl iodides under metal-free conditions. Notably, the proposed method exhibits a broad substrate scope, operational simplicity, tolerance to air, high functional-group tolerance, satisfactory yields, and excellent regioselectivity as well as involves the use of cost-effective reagents such as green oxidants.

Copper-catalyzed C2 alkenylation of pyridine-N-oxides with alkynes

A general and expedient method to construct the scaffolds of 2-alkenylpyridines, through copper-catalyzed C2 alkenylation of pyridine-N-oxides with alkynes, has been disclosed. This protocol shows operational simplicity, good functional group compatibility and broad substrate scope.

Base-promoted synthesis of diarylsulfones from sulfonyl hydrazines and diaryliodonium salts

An efficient and concise method for the synthesis of diverse substituted sulfones was developed with high selectivity. Using n-PrOH as the solvent, diaryl sulfones are formed even on a gram scale via metal-free coupling from sulfonyl hydrazines with symmetrical or unsymmetrical diaryliodonium salts.

Synthesis and Characterization of 1,10-Phenanthroline-mono-N-oxides

N-oxides of N-heteroaromatic compounds find widespread applications in various fields of chemistry. Although the strictly planar aromatic structure of 1,10-phenanthroline (phen) is expected to induce unique features of the corresponding N-oxides, so far the potential of these compounds has not been explored. In fact, appropriate procedure has not been reported for synthesizing these derivatives of phen. Now, we provide a straightforward method for the synthesis of a series of mono-N-oxides of 1,10-phenanthrolines. The parent compounds were oxidized by a green oxidant, peroxomonosulfate ion in acidic aqueous solution. The products were obtained in high quality and at good to excellent yields. A systematic study reveals a clear-cut correlation between the basicity of the compounds and the electronic effects of the substituents on the aromatic ring. The UV spectra of these compounds were predicted by DFT calculations at the TD-DFT/TPSSh/def2-TZVP level of theory.

NaI/KI/NH4I and TBHP as powerful oxidation systems: use in the formation of various chemical bonds

In modern organic synthesis, the execution of reactions in the absence of expensive transition metals has received significant attention from the view-point of green chemistry and sustainable development. As a consequence, the combination of MI-TBHP as an oxidation system (M = Na, K, NH4) has opened a new avenue with significant impact for the succinct synthesis of complex heterocycle molecules via the construction of various chemical bonds [C-X (X = C, N, S, O), N-X (X = N, P) and S-N]. This comprehensive review article delineates the progress of recent developments in this emerging area, with an in-depth discussion on the substrate scope, limitations and proper mechanistic underpinnings. We hope this review will highlight the great potential of this MI-TBHP as a powerful oxidation system and inspire researchers to conduct further endeavors in this domain.

An update on the use of sulfinate derivatives as versatile coupling partners in organic chemistry

The use of sulfinic acids and their salts continues to be extensively developed in organic chemistry. This is attributable to their dual capacity for acting as nucleophilic or electrophilic reagents, as well as their ease of preparation and stability on storage. This report highlights the research accomplished since 2015 on this topic, updating a previous review published by our team in 2014.

Iodine-catalyzed convergent aerobic dehydro-aromatization toward benzazoles and benzazines

An iodine-catalyzed aerobic dehydro-aromatization has been developed, providing straightforward and efficient access to various benzoazoles and benzoazines. The present transition-metal-free protocol enables the dehydro-aromatization of tetrahydrobenzazoles and tetrahydroquinolines with molecular oxygen as the green oxidant, along with some other N-heterocycles. Hence, a broad range of heteroaromatic compounds are generated in moderate to good yields under facile reaction conditions.

An iodine-catalyzed aerobic dehydro-aromatization has been developed, providing a straightforward and efficient access to various benzoazoles and benzoazines.

Transition-metal- and oxidant-free three-component reaction of quinoline N-oxides, sodium metabisulfite and aryldiazonium tetrafluoroborates via a dual radical coupling process

A convenient and straightforward three-component transformation of quinoline N-oxides, sodium metabisulfite and aryldiazonium tetrafluoroborates has been developed, providing the target products in moderate to good yields. Compared with previous studies, the present methodology avoids the use of transition-metal catalysts and excess oxidants, providing a simple and practical alternative approach for the construction of 2-sulfonylquinolines. Control experiments indicate that a dual radical coupling process is responsible for this reaction.

TBHP/NH4I-Mediated Direct N-H Phosphorylation of Imines and Imidates

A direct and practical metal-free N-H phosphorylation has been achieved via the TBHP/NH₄I-mediated cross-dehydrogenative coupling (CDC) reactions between imines/imidates and P(O)H compounds. This transformation provides an efficient synthetic route to the construction of P-N bonds with good functional group compatibility, leading to the formation of N-phosphorylimines and N-phosphorylimidates in up to 95% yield (33 examples) under mild conditions.

Photoredox-catalyzed sulfonylation of alkyl iodides, sulfur dioxide, and electron-deficient alkenes

A photoredox-catalyzed sulfonylation of alkyl iodides, sulfur dioxide, and electron-deficient alkenes under mild conditions is achieved. This reaction proceeds through alkyl radicals formed in situ from alkyl iodides under visible light irradiation in the presence of a photoredox catalyst. The alkyl radical intermediates would react with sulfur dioxide leading to alkylsulfonyl radicals, which would be trapped by electron-deficient alkenes giving rise to alkyl sulfones. Various functional groups including nitro, halo, acetyl, sufonyl, and pyridinyl are all tolerated under the photoredox conditions.

Metal-free NaI/TBHP-mediated sulfonylation of thiols with sulfonyl hydrazides

A highly efficient sulfonylation of thiols has been achieved through the metal-free NaI/TBHP-mediated cross-coupling of sulfonyl hydrazides and thiols at room temperature. This method provides a convenient and practical route to thiosulfonates in 84-99% yields (39 examples) with wide functional group compatibility.

AgBF4-catalyzed deoxygenative C2-amination of quinoline N-oxides with isothiocyanates

A general, effective and convenient protocol for the direct synthesis of various 2-aminoquinolines (39 examples) through AgBF₄-catalyzed amination of quinoline N-oxides with isothiocyanates under base-, oxidant-free and mild conditions was developed.

Metal-free catalyzed arylsulfonylation of chloroquinoline with sodium arylsulfinates under microwave irradiation

An efficient protocol for the synthesis of 2-arylsulfonyl quinolines has been developed via a metal-free catalyzed cross-coupling reaction of chloroquinoline with sodium arylsulfinates in moderate-to-good yields under microwave irradiation. The reactions proceed with a wide range of substrates with good functional group tolerance.

Regioselective Reaction of Heterocyclic N-Oxides, an Acyl Chloride, and Cyclic Thioethers

Treatment of electron deficient pyridine N-oxides with 4-nitrobenzoyl chloride and a cyclic thioether in the presence of triethylamine leads to the corresponding 2-functionalized product in up to a 74% isolated yield. The transformation can also be accomplished with alternative nitrogen containing heterocycles, including quinolines, pyrimidines, and pyrazines. To expand the scope of the transformation, diisopropyl ether can be used as the reaction medium to allow for the use of solid thioether substrates.

Sulfinates and thiocyanates triggered 6-endocyclization ofo-alkynylisocyanobenzenes

An efficient synthesis of 2-sulfonyl- and 2-thiocyanato-3-substituted quinolines through sequential nucleophilic addition/cyclization ofo-alkynylisocyanobenzenes is described.

Metal-free C(sp3)–H bond sulfonyloxylation of 2-alkylpyridines and alkylnitrones

Direct transformation of an inert C(sp³)–H bond of 2-alkylpyridines and nitrones into a C–OSO₂R bond has been described.

Copper-mediated sulfonylation of aryl iodides and bromides with arylsulfonyl hydrazides in PEG-400

Sulfonylation using aryl iodides or bromides and arylsulfonyl hydrazides mediated by cupric acetate with polyethylene glycol (PEG-400) as an eco-friendly medium was successfully achieved.

Metal-free deoxygenative sulfonylation of quinoline N-oxides with sodium sulfinates via a dual radical coupling process

The first example of a metal- and reductant-free deoxygenative sulfonylation of quinoline N-oxides with sodium sulfinates via a dual radical coupling process is reported.

Exogenous-oxidant-free electrochemical oxidative C–H sulfonylation of arenes/heteroarenes with hydrogen evolution

This work describes an exogenous-oxidant-free radical C–H sulfonylation using an electrochemical oxidative protocol.

TsOH·H2O-mediated N-amidation of quinoline N-oxides: facile and regioselective synthesis of N-(quinolin-2-yl)amides

An operationally simple method with 100% atom economy was developed for the synthesis of various N-(quinolin-2-yl)amides via the TsOH·H₂O-mediated N-amidation of quinoline N-oxides using inexpensive and commercially available nitriles as the amidation reagents.

Iron-Catalysed Regioselective Arylsulfonylation of Quinoline N-Oxides with Sodium Arylsulfinates under Microwave Irradiation

An efficient protocol for one-pot deoxygenative and regioselective synthesis of 2-arylsulfonylquinolines has been developed via iron-catalysed cross-coupling reaction of quinoline N-oxides with sodium arylsulfinates in moderate to good yields under microwave irradiation. The reactions proceeded over a broad range of substrates with good regioselectivity and functional group tolerance.