Redox Cyclization Research Articles

Synthesis of α‐NiS Decorated Fe3S4 Nanohybrid Composite and Its Heterogeneous Fenton Catalysis for Dye Degradation

AbstractGreigite (Fe3S4) and millerite (NiS) are important transition metal sulfides that exhibit light‐driven Fenton catalytic activity. In absence of light, they show limited activity. To utilize these properties without relying on light, we synergized these sulfides to prepare a composite Fe3S4–NiS (FSNS). Under fixed reaction conditions, the composite demonstrated excellent Fenton activity, degrading 93% of fast green dye (FG) and 98.1% of eriochrome black T dye (EBT) within a short reaction time. The mechanism involves electron transfer from the reduced sulfur state (S2−) in both NiS and Fe3S4 to the higher oxidation states of the metal ions viz. Fe3+and Ni3+ to facilitate the redox cyclization in the Fenton process. The catalyst showed high stability for five cyclical tests by recovering it from solution with an external magnet.

Iodine‐Promoted Cascade Redox Cyclization to Access 2‐Arylbenzothiazoles Using Elemental Sulfur

ABSTRACTA straightforward and efficient method has been developed to access a variety of benzothiazole derivatives via cascade reductive annulation. Iodine mediated, one‐pot three‐component reaction of o‐chloronitroarenes, aryl aldehydes, and elemental sulfur effectively produce benzothiazoles. Moreover, the metal‐free strategy allows the facile synthesis of diverse 2‐substituted benzothiazoles through multiple carbon–heteroatom bonds in good yields. The present protocol features a greener approach, readily accessible reagents, broad substrate scope, high product yields, and operational simplicity.

Palladium mediated domino heck redox cyclization of aromatic homoallylic alcohols to methylindanones

Palladium mediated domino heck redox cyclization of aromatic homoallylic alcohols to methylindanones

Unified short syntheses of oxygenated tricyclic aromatic diterpenes by radical cyclization with a photoredox catalyst

The biomimetic two-phase strategy employing polyene cyclization and subsequent oxidation/substitution is an effective approach for divergent syntheses of [6-6-6]-tricyclic diterpenes. However, this strategy requires lengthy sequences for syntheses of oxygenated tricyclic aromatic abietane/podocarpane diterpenes owing to the many linear oxidation/substitution steps after cyclization. Here, we present a new synthetic route based on a convergent reverse two-phase strategy employing a reverse radical cyclization approach, which enabled the unified short syntheses of four aromatic abietane/podocarpane diterpenes and the divergent short syntheses of other related diterpenes. Oxygenated and substituted precursors for cyclization were convergently prepared through Friedel-Crafts acylation and rhodium-catalyzed site-selective iodination. Radical redox cyclization using an iridium photoredox catalyst involving neophyl rearrangement furnished the thermodynamically favored 6-membered ring preferentially. (±)-5,6-Dehydrosugiol, salvinolone, crossogumerin A, and Δ5-nimbidiol were synthesized in only 8 steps. An oxygenated cyclized intermediate was also useful for divergent derivatization to sugiol, ferruginol, saprorthoquinone, cryptomeriololide, and salvinolone.

Synthesis of 2-Arylbenzothiazoles from Nitrobenzenes, Benzylamines, and Elemental Sulfur via Redox Cyclization

AbstractA sustainable advanced synthetic method was developed based on redox cyclization for the synthesis of 2-arylbenzothiazoles in good to excellent yields from readily available nitrobenzenes, benzylamines, and elemental sulfur without the use of transition-metal catalysts. This method is remarkable: nitro group reduction, a benzylamine redox reaction, sulfuration, condensation, and cyclization, all proceed in a single step to generate a heterocycle. It is also highly atom-economical and does not require any external oxidizing or reducing agents.

Gold-catalyzed redox cycloisomerization/nucleophilic addition/reduction: direct access to 2-phosphoryl indolin-3-ones.

An efficient gold(I)-catalyzed redox cycloisomerization/nucleophilic addition/reduction reaction of o-nitroalkynes with various H-phosphorus oxides is established. Through the intramolecular redox cyclization of o-nitroalkynes and subsequent intermolecular nucleophilic addition/reduction with no external reactant, a variety of arylphosphoryl and alkylphosphoryl indolin-3-ones with high functional-group compatibility are obtained in moderate to good yields. Mechanistic studies suggest that phosphorus nucleophiles mediate the cleavage of the N-O bond as a reductant.

Intramolecular redox cyclization reaction access to cinnolines from 2-nitrobenzyl alcohol and benzylamine via intermediate 2-nitrosobenzaldehyde.

A transition-metal-free intramolecular redox cyclization reaction for the synthesis of cinnolines has been developed from 2-nitrobenzyl alcohol and benzylamine. Mechanistic investigations disclosed the involvement of a key intramolecular redox reaction, followed by condensation, azo isomerization to hydrazone, cyclization, and aromatization to form the desired products. Notably, the formation of intermediate 2-nitrosobenzaldehyde and (E)-2-(2-benzylidenehydrazineyl) benzaldehyde plays an important role in this transformation.

Gold-Catalysed Nitroalkyne Cycloisomerization - Synthetic Utility.

The gold-catalysed intramolecular redox cyclization of o-alkynylnitrobenzens documented by Professors Naoki Asao and Yoshinori Yamamoto is an important discovery that has opened two complementary research domains. Advancing this cyclization with other metals as well as developing new methods around the products that result from this reaction is one aspect that has seen growing interest. On the other hand, the idea of generating α-oxo gold carbenes via oxygen transfer to alkynes has established another important aspect in gold-catalysis. In this account, we will be dealing with the first aspect, which revolves around the internal redox cyclization of nitroalkynes (trivially called as nitroalkyne cycloisomerization), focusing mainly on the gold-complexes and the synthetic methods developed around it from our group and from other groups, and also providing the details of similar transformations documented with other metals so that the complementary reactivity/diversity of these transformations could be appreciated.

Sulfur-Promoted Redox Cyclization of 2,2′-Dinitrodiphenyl Disulfides and Benzylamines

AbstractThis study aims to develop green and sustainable advanced synthetic methods based on redox cyclization. Elemental sulfur is shown to promote the efficient synthesis of 2-substituted benzothiazoles from 2,2′-dinitrodiphenyl disulfides and benzylamines via redox cyclization in the absence of transition-metal catalysts and solvents. The 2-substituted benzothiazoles are obtained in good to excellent yields. This synthetic methodology is highly atom-economical and does not require any external oxidizing and/or reducing agents.

A copper-catalyzed approach for the synthesis of asymmetrical disubstituted 1,2,4-thiadiazoles via elemental sulfur-mediated decarboxylative redox cyclization

The variety of asymmetrical disubstituted 1,2,4-thiadiazoles are smoothly prepared by copper-catalyzed approach, which employed arylacetic acids and amidines as substrates, and elemental sulfur to mediate decarboxylative redox cyclization. The advantages of this method are simple, efficient, and ligand-free. In addition, this method can provide products in moderate to good yields.

Redox Cyclization of Amides and Sulfonamides with Nitrous Oxide for Direct Synthesis of Heterocycles.

Herein we report a redox cyclization of amides and sulfonamides with nitrous oxide (N2O) for the direct synthesis of heterocycles. Various amides and sulfonamides could undergo directed ortho metalation (DoM) by treatment with BuLi, and the lithium intermediate could be trapped by N2O gas to achieve redox cyclization. N2O serves as a N-atom donor to mediate the intramolecular coupling of lithium species toward heterocycle formation with free external oxidant. This protocol offers a direct synthesis of heterocycles with features of readily available starting materials, simple operation, and a broad substrate scope.

Elemental Sulfur-Mediated Decarboxylative Redox Cyclization ­Reaction: Copper-Catalyzed Synthesis of 2-Substituted Benzo­thiazoles

A S8-mediated directed decarboxylative redox-cyclization strategy for the synthesis of 2-substituted benzothiazoles from o-iodoanilines, arylacetic acids, and elemental sulfur catalyzed by cheap copper metal has been developed. This reaction is operationally simple, ligand-free, compatible with a wide range of functional groups, and provides the desired products in good to excellent yields. In addition, a gram-scale experiment was carried out to furnish PMX 610, an antitumor drug.

An Unconventional Redox Cross Claisen Condensation-Aromatization of 4-Hydroxyprolines with Ketones.

Reaction of α-amino acids, particularly prolines and their derivatives with carbonyl compounds via decarboxylative redox process, is a viable strategy for synthesis of structurally diverse nitrogen centered heterocyclics. In these processes, the decarboxylation is the essential driving force for the processes. The realization of the redox process without decarboxylation may offer an opportunity to explore new reactions. Herein, we report the discovery of an unprecedented redox Claisen-type condensation aromatization cascade reaction of 4-substituted 4-hydroxyproline and its esters with unreactive ketones. We found that the use of propionic acid as a catalyst and a co-solvent can change the reaction course. The commonly observed redox decarboxylation and aldol condensation reactions are significantly minimized. Moreover, unreactive ketones can effectively participate in the Claisen condensation reaction. The new reactivity enables a redox cyclization via an unconventional Claisen-type condensation reaction of in situ formed enamine intermediates from ketone precursors with 4-substituted 4-hydroxyproline and its esters as electrophilic acylation partners. Under the reaction conditions, the cascade process proceeds highly regio- and stereoselectively to afford highly synthetically and biologically valued cis-2,3-dihydro-1H-pyrrolizin-1-ones with a broad substrate scope in efficient 'one-pot' operation, whereas such structures generally require multiple steps.

Nazarov Cyclization/Internal Redox Cyclization Sequence for the Synthesis of N-Heterocyclic Bridged Ring Systems.

A 1,6 conjugate addition/Nazarov electrocyclization/internal redox cyclization sequence was developed. Various 5-hydroxycyclopentenones were made through the 1,6-conjugate addition initiated Nazarov reaction with excellent diastereoselectivities. Under thermal conditions, these underwent a through-space 1,5-hydride-transfer/ring-closure reaction to form bridged bicyclic N-heterocyclic compounds with up to four stereogenic centers. It was also possible to convert simple acyclic dienyl diketones into the bicyclo[3.2.1] products in a one-pot process (with a solvent switch).

ChemInform Abstract: Formic Acid as a Sustainable and Complementary Reductant: An Approach to Fused Benzimidazoles by Molecular Iodine‐Catalyzed Reductive Redox Cyclization of o‐Nitro‐t‐anilines.

AbstractMolecular iodine proves to be an excellent catalyst for the reductive redox cyclization of (o‐dialkylamino)nitrobenzenes and related educts furnishing benzimidazole derivatives in good yield.

Formic acid as a sustainable and complementary reductant: an approach to fused benzimidazoles by molecular iodine-catalyzed reductive redox cyclization of o-nitro-t-anilines

Molecular iodine was found to be an excellent catalyst for reductive redox cyclization ofo-nitro-t-anilines1into fused tricyclic or 1,2-disubtituted benzimidazoles2.

Diverse Applications of Nitrones for the Synthesis of Heterocyclic Compounds

AbstractAlthough best known for their [3+2]‐dipolar cycloaddition reactivity and use in the preparation of isoxazolines and isoxazolidines, nitrones are versatile reagents that undergo a variety of transformations for the synthesis of a diverse array of heterocyclic compounds. The breadth of heterocycle synthesis using nitrone reagents includes: 1) stepwise [3+3]‐cycloaddition reactions of nitrones with vinyl diazoacetates, transition‐metal‐coordinated cycloisomerization intermediates, trimethylene methanes, and cyclopropane diesters to form dihydro‐ and tetrahydro‐1,2‐oxazines; 2) internal redox cyclization reactions of alkyne‐tethered nitrones to provide access to azabicyclooctanes, isoindoles, aminoindanones, and isoquinolones; 3) electrocyclizations of in situ generated N‐allenylnitrones to form pyridines and azetidine‐N‐oxides, as well as metal‐catalyzed cyclizations and rearrangements of nitrones to form azepine‐N‐oxides, spirocyclic isoxazolines, and α,β‐epoxyimines; and 4) the use of [3+2]‐cycloaddition reactions of nitrones to trigger cascade reactions for the formation of tetrahydrooxazepines, dihydrocarbazoles, and benzoindolizines. This Focus Review aims to highlight these diverse applications of nitrones for the synthesis of heterocycles to emphasize the broad utility of these reactive intermediates and to inspire their further development as important synthons beyond the scope of traditional [3+2]‐dipolar cycloaddition reactions.

ChemInform Abstract: Elemental Sulfur Mediated Decarboxylative Redox Cyclization Reaction of o‐Chloronitroarenes and Arylacetic Acids.

Abstract2‐substituted benzothiazoles are prepared by the title reaction under metal‐, oxidant‐, and solvent‐free condition.

Elemental sulfur mediated decarboxylative redox cyclization reaction of o-chloronitroarenes and arylacetic acids.

A decarboxylative redox cyclization strategy has been developed for the synthesis of 2-substituted benzothiazoles by the reaction of o-chloronitroarenes and arylacetic acids in the presence of elemental sulfur/N-methylmorpholine under metal- and solvent-free conditions.

ChemInform Abstract: Cascade Nitration/Cyclization of 1,7‐Enynes with tBuONO and H2O: One‐Pot Self‐Assembly of Pyrrolo[4,3,2‐de]quinolinones.

Here we describe the one-pot construction of the pyrrolo[4,3,2-de]quinolinone scaffold by a cascade nitration/cyclization sequence of 1,7-enynes with tBuONO and H2O. The cascade proceeds through alkene nitration, 1,7-enyne 6-exo-trig cyclization, CH nitrations, and redox cyclization, and exhibits excellent functional group tolerance. The mechanism was investigated using in situ high-resolution mass spectrometry (HR-MS).