tert-Butyl Hydroperoxide (TBHP)-Initiated Vicinal Sulfonamination of Alkynes: A Radical Annulation toward 3-Sulfonylindoles

Copper-Catalyzed and 1,3-Sulfonyl Migration Enabled Installation of Azaindoles in the Periphery of Aryl Rings: Synthesis of Sulfonylated Pyrrolo[2,3-b]quinolines and Investigation of Antimalarial Potency

Herein, an atom efficient and one-pot protocol, offering a series of sulfonylated pyrrolo[2,3-b]quinolines via C-N, C-C, and C-S bond formation, has been developed with an inexpensive copper catalyst. Notably, the reaction proceeds via a double-annulation reaction followed by a 1,3-sulfonyl migration sequence. Moreover, the method is applicable to a broad range of 2-carbonylanilines. Furthermore, synthetic applications and the scale-up reaction highlight the utility potential of this protocol. In addition, the antimalarial property of sulfonylated pyrrolo[2,3-b]quinolines showed parasite inhibition without cytotoxic effects in mammalian cells.

Laccase Mediated One-Pot, Green Synthesis of Thiazoles, β-Keto Sulfones and Imidazopyridines

We report the first in situ reaction of the β-haloketones obtained from laccase catalysed oxidation of secondary alcohol 2-halo phenylethanol's in present study. To the best of our knowledge, this is the first ever fusion of laccase catalysed oxidation reaction with green organic synthetic reaction. The methodology employs molecular oxygen to oxidize secondary alcohol in biphasic medium by laccase from T. giganteum AGHP, to obtain β-haloketones. This research provides environmentally benign access to thiazole, β-ketosulfone and imidazopyridine derivatives in good to very good yields with wide functional group tolerance.

EDA Complex-Promoted Cascade Cyclization of Alkynes Enabling the Rapid Assembly of 3-Sulfonylindoles and Vinyl Sulfone Oxindoles

Herein, we disclose a photoinduced radical cascade cyclization of alkynes with sulfinates via a novel EDA complex for the synthesis of various 3-sulfonylindoles and vinyl sulfone oxindoles, which are crucial motifs in medicinal and biological chemistry. The reaction proceeds under mild, photocatalyst- and transition-metal-free conditions, featuring operational simplicity, broad substrate scope, and easy scalability. Mechanistic studies reveal that the reaction is initiated with a photoinduced intermolecular charge transfer from sulfinates to N-sulfonamides, generating a sulfonyl radical followed by an N-centered radical, thus enabling the cascade cyclization process.

Recent advances in the radical cascade reaction for constructing nitrogen heterocycles using azides as radical acceptors

Due to the high conversion properties, azide compounds are widely utilized in organic synthesis. For instance, azide compounds readily release nitrogen to form a new N-C bond when they function as radical acceptors for the active intermediates in the reaction. Over the past decade, strategies employing azides as radical acceptors to construct nitrogen heterocycles have been extensively developed. This approach has emerged as a crucial method for synthesizing nitrogen heterocycles. Therefore, this paper provides a review of the research advancements in tandem cyclization reactions using azides as radical acceptors, summarizing the process of reaction design, exploration, reasoning of the mechanism, and prospects for further research of these reactions.

Oxidant-Assisted Sulfonylation/Cyclization Cascade Synthesis of Alkylsulfonylated Oxindoles via the Insertion of SO2

An oxidant-assisted tandem sulfonylation/cyclization of electron-deficient alkenes with 4-alkyl-substituted Hantzsch esters and Na2S2O5 for the preparation of 3-alkylsulfonylated oxindoles under mild conditions in the absence of a photocatalyst and transition metal catalyst is established. The mechanism studies show that the alkyl radicals, which come from the cleavage of the C-C bond in 4-substituted Hantzsch esters under oxidant conditions, subsequently undergo the in situ insertion of sulfur dioxide to generate the crucial alkylsulfonyl radical intermediates. This three-component reaction provides an efficient and facile route for the construction of alkylsulfonylated oxindoles and avoids the use of highly toxic alkylsulfonyl chlorides or alkylsulfonyl hydrazines as alkylsulfonyl sources.

Microwave-accelerated and benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with cyclic alkanes under metal-free conditions

A novel and efficient method for preparing exocyclic indan derivatives, with this method involving benzoyl peroxide (BPO)-initiated cyclization of 1,5-enynes having cyano groups with simple cyclic alkanes under microwave irradiation, has been developed. The presented approach showed advantages of simple conditions, an environmentally friendly protocol, good functional-group tolerance, and high yields of products.

Palladium-catalyzed radical cascade cyanoalkylsulfonylation/cyclization of 3-arylethynyl-[1,1'-biphenyl]-2-carbonitriles with cyclobutanone oxime esters and DABSO

A palladium-catalyzed radical cascade cyanoalkylsulfonylation/cyclization of 3-arylethynyl-[1,1'-biphenyl]-2-carbonitriles with DABCO·(SO₂)₂ and cyclobutanone oxime esters via cleavage of a C-C single bond and insertion of SO₂ was described. A series of cyanoalkylsulfone-containing cyclopenta[gh]phenanthridines were obtained in moderate-to-good yields, thus featuring mild reaction conditions, a broad substrate scope, and a high functional group tolerance.

Visible-Light-Induced Multicomponent Cascade Cycloaddition of N-Propargyl Aromatic Amines, Cyclobutanone Oxime Esters, and K2S2O5: Access to Cyanoalkylsulfonylated Quinolines

A visible-light-induced cascade cyanoalkylsulfonylation/cyclization/aromatization of N-propargyl aromatic amines with K₂S₂O₅ and cyclobutanone oxime esters for the construction of cyanoalkylsulfonylated quinolines is developed. This cascade transformation features mild reaction conditions, a broad substrate scope, and excellent functional group compatibility, providing a convenient route toward cyanoalkylsulfonylated quinolines via the formation of a C-C bond and two C-S bonds in one step.

Metal-Free Sulfonylative Spirocyclization of Indolyl-ynones via Insertion of Sulfur Dioxide: Access to Sulfonated Spiro[cyclopentenone-1,3'-indoles]

A three-component sulfonylative spirocyclization of indolyl ynones with aryldiazonium salts and a sulfur dioxide surrogate of DABCO·(SO₂)₂ has been developed, providing a range of sulfonated spiro[cyclopentenone-1,3'-indoles] in moderate to good yields. This transformation was initiated by an in situ generated arylsulfonyl radical and proceeded efficiently under metal-free conditions, involving a radical-induced dearomative cascade cyclization accompanied by the insertion of sulfur dioxide. This protocol provides an efficient and convenient method to access sulfonated spiroindolenines, and tolerant various functional groups.

A photoredox- or metal-catalyzed three-component cyanoalkylsulfonylation reaction of 1-(allyloxy)-2-(1-arylvinyl)benzenes, potassium metabisulfite, and cycloketone oxime esters: synthesis of cyanoalkylsulfonylated benzoxepines

A visible light photocatalyzed or metal-catalyzed three-component cyanoalkylsulfonylation of 1-(allyloxy)-2-(1-arylvinyl)benzenes, potassium metabisulfite and cyclobutanone oxime esters is developed.

Metal-free alkynylsulfonylation of vinylarenes

A metal-free two-component alkynylsulfonylation of vinylarenes with aryl alkynylsulfones to afford various β-sulfonyl alkynes in moderate to excellent yields under mild conditions is developed.

A visible-light photoredox-catalyzed four-component reaction for the construction of sulfone-containing quinoxalin-2(1H)-ones

A visible-light photoredox-catalyzed four component reaction of quinoxalin-2(1H)-ones, alkenes, aryldiazonium, and sodium metabisulfite leading to sulfone-containing quinoxalin-2(1H)-ones has been developed.

Cobalt-catalyzed C8–H sulfonylation of 1-naphthylamine derivatives with sodium sulfinates

A facile and efficient protocol for cobalt-catalyzed regioselective C8–H sulfonylation of 1-naphthylamine derivatives with sodium sulfinates was developed to afford sulfonylated naphthylamines in moderate to good yields.

NBS-assisted palladium-catalyzed bromination/cross-coupling reaction of 2-alkynyl arylazides with KSCN: an efficient method to synthesize 3-thiocyanindoles

An efficient NBS-assisted palladium-catalyzed bromination/cross-coupling synthesis of 3-thiocyanindoles from 2-alkynyl arylazides with KSCN has been described.

Disulfonation of terminal alkynes for 1,2-bisulfonylethenes

A highly efficient and operationally simple method for the synthesis of 1,2-disulfonylethenes involving a hypervalent iodineI(iii) reagent to promote disulfonation of terminal alkynes has been developed. This protocol provides a facile and practical pathway to selectively access (E)-1,2-disulfonylethenes that features good functional group compatibility, easily available starting materials, excellent stereoselectivity, and good yields.

TBPB-initiated cascade cyclization of 3-arylethynyl-[1,1'-biphenyl]-2-carbonitriles with sulfinic acids: access to sulfone-containing cyclopenta[gh]phenanthridines

A novel TBPB-initiated cascade cyclization of 3-arylethynyl-[1,1'-biphenyl]-2-carbonitriles with sulfinic acids via C-S, C-C and C-N bond formation for the synthesis of 3-sulfonated cyclopenta[gh]phenanthridines under metal-free conditions has been developed. This protocol features mild conditions, good functional group tolerance and a broad substrate scope. By using this protocol, a variety of potentially bioactive 3-sulfonated cyclopenta[gh]phenanthridines were facilely synthesized via direct annulation.

Synthesis of indoles from alkynes and a nitrogen source under metal-free conditions

This review deals with the methodologies published in the last ten years, which used metal-free conditions to prepare indoles starting from alkynes and nitrogen compounds.

Copper and manganese co-mediated cascade aza-Michael addition/cyclization and azidation of 1,3-enynes: regioselective synthesis of fully substituted azido pyrroles

A Cu and Mn co-mediated aerobic oxidative cyclization and azidation reaction of 1,3-enynes with amines and trimethylsilyl azide has been developed to synthesize fully substituted azido pyrroles.

Silver-Catalyzed Nitration/Annulation of α-Alkynyl Arylols toward 3-Nitrated Benzofurans

A silver-catalyzed nitration/annulation of α-alkynyl arylols is reported by using tert-butyl nitrite (TBN) as a NO₂ radical precursor, from which a set of 3-nitrated benzofurans were synthesized with moderate to good yields. This transformation initiated by an in situ generated NO₂ radical proceeds efficiently under mild and neutral redox conditions, which provides a new pathway toward the 3-nitrobenzofuran framework via catalytic difunctionalization of internal alkynes.

Thio radical-induced denitrogenative annulation of 1-azido-2-isocyanoarenes to construct 2-thiolated benzimidazoles

A method for the synthesis of 2-thiolated benzimidazoles is described starting from thiols and 1-azido-2-isocyanoarenes. The isocyano group works as an acceptor of various thio radicals, followed by denitrogenative annulation of the resulting imidoyl radical intermediates to the azido group, with nitrogen loss as the only process involving high bond-forming efficiency. The one-pot method for the synthesis of these products with high functional group tolerance in the benzimidazole-based ring is not available in previous literature.

Iodine-mediated aminosulfonylation of alkenyl sulfonamides with sulfonyl hydrazides: synthesis of sulfonylmethyl piperidines, pyrrolidines and pyrazolines

An aminosulfonylation of alkenyl sulfonamides was reported. Using iodine as the catalyst, TBHP as the oxidant, and sulfonyl hydrazides as the sulfonyl radical sources, a variety of sulfonylmethyl piperidines, pyrrolidines and pyrazolines were obtained in moderate to excellent yields.

Synthesis of Sulfonylated Lactams by Copper-Mediated Aminosulfonylation of 2-Vinylbenzamides with Sodium Sulfinates

Treatment of 2-vinylbenzamide derivatives with sulfinate sodium in the presence of Cu(NO₃)₂·3H₂O led to an intra/intermolecular aminosulfonylation reaction to produce sulfonylated lactams in moderate to good yields. The developed method features the easily available and stable sulfone reagents, ease of operation, and a broad functional group tolerance.

Photoredox-catalyzed cascade annulation of N-propargylindoles with sulfonyl chlorides: access to 2-sulfonated 9H-pyrrolo[1,2-a]indoles

A photoredox-catalyzed cascade radical reaction of N-propargylindoles and sulfonyl chlorides to 2-sulfonated 9H-pyrrolo[1,2-a]indoles under external oxidant-free conditions was developed.

The hydrogen atom transfer reactivity of sulfinic acids

Sulfinic acids (RSO2H) have a reputation for being difficult reagents due to their facile autoxidation. Nevertheless, they have recently been employed as key reagents in a variety of useful radical chain reactions. To account for this paradox and enable further development of radical reactions employing sulfinic acids, we have characterized the thermodynamics and kinetics of their H-atom transfer reactions for the first time. The O-H bond dissociation enthalpy (BDE) of sulfinic acids was determined by radical equilibration to be ∼78 kcal mol-1; roughly halfway between the RS-H BDE in thiols (∼87 kcal mol-1) and RSO-H BDE in sulfenic acids (∼70 kcal mol-1). Regardless, RSH, RSOH and RSO2H have relatively similar inherent H-atom transfer reactivity to alkyl radicals (∼106 M-1 s-1). Counter-intuitively, the trend in rate constants with more reactive alkoxyl radicals follows the reaction energetics: ∼108 M-1 s-1 for RSO2H, midway between thiols (∼107 M-1 s-1) and sulfenic acids (∼109 M-1 s-1). Importantly, since sulfinic and sulfenic acids are very strong H-bond donors (αH2 ∼ 0.63 and 0.55, respectively), their reactivity is greatly attenuated in H-bond accepting solvents, whereas the reactivity of thiols is largely solvent-independent. Efforts to measure rate constants for the reactions of sulfinic acids with alkylperoxyl radicals were unsuccessful. Computations predict these reactions to be surprisingly slow; ∼1000-times slower than for thiols and ∼10 000 000-times slower than for sulfenic acids. On the other hand, the reaction of sulfinic acids with sulfonylperoxyl radicals - which propagate sulfinic acid autoxidation - is predicted to be almost diffusion-controlled. In fact, the rate-determining step in sulfinic acid autoxidation, and the reason they can be used for productive chemistry, is the relatively slow reaction of propagating sulfonyl radicals with O2 (∼106 M-1 s-1).

Recent advances in radical transformations of internal alkynes

This review highlights the recent progress in the radical transformation of internal alkynes and focuses on the reaction mechanisms by carbon/heteroatom-centered triggered additions, and offers a comprehensive overview on the existing procedures and employed methodologies.

Eosin Y photoredox catalyzed net redox neutral reaction for regiospecific annulation to 3-sulfonylindoles via anion oxidation of sodium sulfinate salts

An eosin Y photoredox catalyzed net redox neutral process for 3-sulfonylindoles via the anionic oxidation of sodium sulfinate salts and its radical cascade cyclization with 2-alkynyl-azidoarenes was developed with visible light as a mediator.

Direct synthesis of 3-acylbenzothiophenes via the radical cyclization of 2-alkynylthioanisoles with α-oxocarboxylic acids

A radical cascade cyclization strategy has been developed to provide a direct route to access 3-acylbenzothiophenes from simple chemical feedstocks.

Silver-Catalyzed Stereoselective Aminosulfonylation of Alkynes

A silver-catalyzed intermolecular aminosulfonylation of terminal alkynes with sodium sulfinates and TMSN3 is reported. This three-component reaction proceeds through sequential hydroazidation of the terminal alkyne and addition of a sulfonyl radical to the resultant vinyl azide. The method enables the stereoselective synthesis of a wide range of β-sulfonyl enamines without electron-withdrawing groups on the nitrogen atom. These enamines are found to be suitable for a variety of further transformations.