Copper-Catalyzed Arylsulfonylation and Cyclizative Carbonation of N-(Arylsulfonyl)acrylamides Involving Desulfonative Arrangement toward Sulfonated Oxindoles

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.

A general alkene aminoarylation enabled by N-centred radical reactivity of sulfinamides

Arylethylamines are popular structural elements in bioactive molecules but are often made through a linear series of synthetic steps. A modular protocol to assemble arylethylamines from alkenes in one step would represent a useful advance in discovery chemistry, though current limitations preclude a generally applicable method. In this work we disclose an aminoarylation of alkenes using aryl sulfinamide reagents as bifunctional amine and arene donors. This reaction features excellent regioselectivity and diastereoselectivity on a variety of activated and unactivated substrates. Using a weakly oxidizing photocatalyst, a nitrogen radical is generated under mild conditions and adds to an alkene to form a new C-N bond. A desulfinylative aryl migration event known as a Smiles-Truce rearrangement follows to form a new C-C bond. In this manner, arylethylamines can be rapidly assembled from abundant alkene feedstocks. Moreover, chiral information from the sulfinamide can be transferred via rearrangement to a new carbon stereocentre in the product, thus advancing the development of traceless asymmetric alkene difunctionalization.

Recent advances in ring-opening of cyclobutanone oximes for capturing SO2, CO or O2via a radical process

Cyclobutanone oximes and their derivatives are pivotal core structural motifs in organic chemistry. Iminyl-radical-triggered C-C bond cleavage of cyclobutanone oximes delivers an efficient strategy to produce stable distal cyano-substituted alkyl radicals, which can capture SO2, CO or O2 to form cyanoalkylsulfonyl radicals, cyanoalkylcarbonyl radicals or cyanoalkoxyl radicals under mild conditions. In the past several years, cyanoalkylsulfonylation/cyanoalkylcarbonyaltion/cyanoalkoxylation has attracted a lot of interest. In this updated report, the strategies for trapping SO2, CO or O2via iminyl-radical-triggered ring-opening of cyclobutanone oximes are summarized.

A three-component reaction of cyclobutanone oxime esters, sulfur dioxide and N-alkyl-N-methacryloyl benzamides

A three-component reaction of cyclobutanone oxime esters, DABCO·(SO2)2 and N-alkyl-N-methacryloyl benzamides is described. This reaction proceeds without the addition of any oxidant or transition metal, affording sulfonyl-containing isoquinoline-1,3-(2H,4H)-diones in moderate to good yields. Various functional groups are tolerated well in this transformation. Mechanistic studies suggest that a radical pathway is involved, including β-scission, sulfur dioxide insertion, and intramolecular cyclization processes.

Annulative Aminosulfonylation of Unactivated Alkenes for Accessing Pyrazolines via Multicomponent SO2 Insertion

A direct arylsulfonylation of β,γ-unsaturated hydrazones method, in which sulfonated pyrazolines are accessed by a three-component reaction of β,γ-unsaturated hydrazones, DABSO, and aryldiazonium tetrafluoroborates, has been developed without external oxidants or catalysts. This transformation is triggered by the formation of arylsulfonyl radicals in situ from the reaction of aryldiazonium tetrafluoroborates and DABSO, and is enabled by controllable generation of C center radical, in which DABSO was utilized as the sulfone source and an oxidant in this radical-mediated cascaded reaction. A wide range of substrates can be applied in this process to afford pyrazolines in good yield, and it is amenable for gram-scale synthesis.

2-Oxindole and related heterocycles: synthetic methodologies for their natural products and related derivatives

Natural goods, medications, and pharmaceutically active substances all contain substituted oxindoles. Generally, the C-3 stereocenter of the substituents of oxindoles and their absolute arrangement have a substantial impact on the bioactivity of these substances. In this case, the desire for contemporary probe and drug-discovery programs for the synthesis of chiral compounds using desirable scaffolds with high structural diversity further drives research in this field. Also, the new synthetic techniques are generally simple to apply for the synthesis of other similar scaffolds. Herein, we review the distinct approaches for the synthesis of diverse useful oxindole scaffolds. Specifically, the research findings on the naturally existing 2-oxindole core and a variety of synthetic compounds having a 2-oxindole core are discussed. We present an overview of the construction of oxindole-based synthetic and natural products. In addition, the chemical reactivity of 2-oxindole and its related derivatives in the presence of chiral and achiral catalysts are thoroughly discussed. The data compiled herein provides broad information related to the bioactive product design, development, and applications of 2-oxindoles and the reported techniques will be helpful for the investigation of novel reactions in the future.

Construction of β-Amino Sulfones from Sodium Metabisulfite via a Radical 1,4-Amino Migration

A three-component reaction of alkenyl-tethered oxime ethers, sodium metabisulfite, and aryldiazonium tetrafluoroborates under mild conditions is developed. This reaction proceeds at room temperature without any oxidants or additives, affording β-amino sulfones with good functional group tolerance through aminosulfonylation of unactivated alkene. Mechanistic studies show that this transformation undergoes a radical process, including radical trapping with sulfur dioxide and radical 1,4-amino migration.

Accessing chiral sulfones bearing quaternary carbon stereocenters via photoinduced radical sulfur dioxide insertion and Truce-Smiles rearrangement

From the viewpoint of synthetic accessibility and functional group compatibility, photoredox-catalyzed sulfur dioxide insertion strategy enables in situ generation of functionalized sulfonyl radicals from easily accessible starting materials under mild conditions, thereby conferring broader application potential. Here we present two complementary photoinduced sulfur dioxide insertion systems to trigger radical asymmetric Truce-Smiles rearrangements for preparing a variety of chiral sulfones that bear a quaternary carbon stereocenter. This protocol features broad substrate scope and excellent stereospecificity. Aside from scalability, the introduction of a quaternary carbon stereocenter at position β to bioactive molecule-derived sulfones further demonstrates the practicality and potential of this methodology.

Photoinduced Desaturation of Amides by Palladium Catalysis

A photoinduced palladium-catalyzed desaturation method that is suitable for converting the linear amides to their α,β-unsaturated counterparts is reported. The reaction does not require strong base/acid or sulfur/selenium and oxidant reagents and can be carried out at room temperature through a simple one-step operation. The protocol exhibits great scalability and functional group tolerance. The reaction mechanism has been investigated through deuterium labeling experiments, radical clock, radical capture, and kinetic studies. Mechanistic studies suggested a radical pathway involving aryl/alkyl Pd-radical intermediates.

A metal-free four-component sulfonylation, Giese cyclization, selenylation cascade via insertion of sulfur dioxide

We hereby report a highly regio- and diastereoselective arylsulfonylation-radical cyclization-selenylation cascade of alkynyl cyclohexadienones for the facile synthesis of highly functionalized dihydrochromenones. The protocol utilizes aryldiazonium salts as aryl partners and DABSO as a benign SO₂ source and also as a redox mediator. Additionally, we also developed a visible light mediated protocol wherein diaryliodonium salts were used as the aryl partners at room temperature.

Aryldiazonium Salts and DABSO: a Versatile Combination for Three-Component Sulfonylative Cross-Coupling Reactions

A combination of aryldiazonium salts and DABSO provides a unique opportunity for sulfonylative multicomponent cross-coupling reactions. Here, a copper-catalyzed three-component cross-coupling of aryldiazonium salts, DABSO with arylboronic acids to obtain medicinally relevant unsymmetrical diarylsulfones is disclosed. Interestingly, a catalyst-free approach for the synthesis of arylvinylsulfones from the corresponding vinyl boronic acid or vinyl halides is explored under basic condition. Tethered aryldiazonium salts provided the corresponding annulated alkylvinylsulfones via alkene difunctionalization under the same transition metal-free condition. Mechanistically, these multicomponent reactions proceed through a single electron pathway by the formation of arylsulfonyl radical as a key intermediate.

Applications of DABSO as an SO2 Gas Surrogate in Organic Synthesis

1,4-Diazabicyclo[2.2.2]octane bis(sulfur dioxide), DABCO.SO2, or DABSO, a bench-stable colorless solid, is industrially produced by the reaction of DABCO with the condensed and bubbled sulfur dioxide gas at low temperatures. However,...

Access to chiral β-sulfonyl carbonyl compounds via photoinduced organocatalytic asymmetric radical sulfonylation with sulfur dioxide

An organocatalytic enantioselective radical reaction of potassium alkyltrifluoroborates, DABCO·(SO₂)₂ and α,β-unsaturated carbonyl compounds under photoinduced conditions is developed, which provides an efficient pathway for the synthesis of chiral β-sulfonyl carbonyl compounds in good yields with excellent enantioselectivity (up to 96% ee). Aside from α,β-unsaturated carbonyl compounds with auxiliary groups, common chalcone substrates are also well compatible with this organocatalytic system. This method proceeds through an organocatalytic enantioselective radical sulfonylation under photoinduced conditions, and represents a rare example of asymmetric transformation involving sulfur dioxide insertion.

A photoinduced organocatalytic enantioselective radical reaction is developed, affording chiral β-sulfonyl carbonyl compounds in good yields with excellent enantioselectivity (up to 96% ee).

A photocatalytic radical relay reaction of 2-methylthiolated phenylalkynones and potassium metabisulfite

The generation of methylsufonyl-containing thioflavones through a radical relay reaction of methylthiolated phenylalkynones and potassium metabisulfite in the presence of sodium methylsulfinate under visible light irradiation is developed.

Generation of Heteroaryl-Substituted Sulfonyl Compounds from Sulfur Dioxide via Remote Heteroaryl ipso-Migration

The generation of heteroaryl-substituted sulfonyl compounds via a catalyst-, base-, and additive-free three-component reaction of heteroaryl-substituted tertiary alcohols, aryldiazonium tetrafluoroborates, and DABCO·(SO₂)₂ under mild conditions is developed. Various functional groups are tolerated well in this transformation, and a broad substrate scope is demonstrated. A preliminary mechanistic investigation shows that this reaction undergoes a radical process, including the insertion of sulfur dioxide, sulfonyl radical addition to unactivated alkene, and remote heteroaryl ipso-migration.

Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

As the "chemical chameleon", sulfonyl-containing compounds and their variants have been merged with various types of reactions for the efficient construction of diverse molecular architectures by taking advantage of their incredible reactive flexibility. Currently, their involvement in radical transformations, in which the sulfonyl group typically acts as a leaving group via selective C-S, N-S, O-S, S-S, and Se-S bond cleavage/functionalization, has facilitated new bond formation strategies which are complementary to classical two-electron cross-couplings via organometallic or ionic intermediates. Considering the great influence and synthetic potential of these novel avenues, we summarize recent advances in this rapidly expanding area by discussing the reaction designs, substrate scopes, mechanistic studies, and their limitations, outlining the state-of-the-art processes involved in radical-mediated desulfonylation and related transformations. With a specific emphasis on their synthetic applications, we believe this review will be useful for medicinal and synthetic organic chemists who are interested in radical chemistry and radical-mediated desulfonylation in particular.

Mechanochemical synthesis of aromatic sulfonamides

A three-component Pd-catalysed aminosulfonylation reaction of K₂S₂O₅ and amine with aryl bromides or aromatic carboxylic acids was developed. This strategy was developed to utilise mechanical energy and accommodate primary as well as secondary aliphatic and aromatic amines to provide a new shortcut to a wide range of sulfonamides. Studies on the scope and limitations of the reaction indicated its tolerance of a vast range of functional groups and many structural patterns. The reactions were scaled up to gram quantities.

N-Methylation of Amines with Methanol in the Presence of Carbonate Salt Catalyzed by a Metal-Ligand Bifunctional Ruthenium Catalyst [(p-cymene)Ru(2,2'-bpyO)(H2O)]

A ruthenium complex [(p-cymene)Ru(2,2'-bpyO)(H₂O)] was found to be a general and efficient catalyst for the N-methylation of amines with methanol in the presence of carbonate salt. Moreover, a series of sensitive substituents, such as nitro, ester, cyano, and vinyl groups, were tolerated under present conditions. It was confirmed that OH units in the ligand are crucial for the catalytic activity. Notably, this research exhibited the potential of metal-ligand bifunctional ruthenium catalysts for the hydrogen autotransfer process.

Photoinduced intramolecular carbosulfonylation of alkynes: access to sulfone-containing dibenzazepines from sulfur dioxide

A visible-light-driven three-component carbosulfonylation of terminal alkynes, DABCO (SO₂)₂ and aryldiazonium tetrafluoroborates is developed, leading to sulfone-containing dibenzazepines in moderate to good yields under mild conditions.

A multi-component reaction of electron-rich arenes, potassium metabisulfite, aldehydes and aryldiazonium tetrafluoroborates

A multi-component reaction of electron-rich arenes, potassium metabisulfite, aldehydes and aryldiazonium tetrafluoroborates under mild conditions is reported, leading to a range of (arylsulfonyl)methylbenzenes in moderate to good yields.

Iminyl radical initiated sulfonylation of alkenes with rongalite under photoredox conditions

A photoredox-catalyzed reaction of oximes, rongalite and electrophiles is accomplished, affording pyrrole-substituted aliphatic sulfones or sulfonamides in moderate to good yields.

A metal-free reaction of sulfur dioxide, cyclopropanols and electron-deficient olefins

The importance of γ-keto sulfones in medicinal chemistry and organic synthesis is known. An efficient route to γ-keto sulfones via a metal-free reaction of cyclopropanols, sulfur dioxide and electron-deficient olefins is achieved. This reaction proceeds smoothly under mild conditions without the need of catalyst, oxidant or additive. A plausible mechanism is proposed, which occurs through a γ-keto sulfinate intermediate generated in situ from the reaction of cyclopropanol with sulfur dioxide. The γ-keto sulfinate intermediate would be trapped by the electron-deficient olefin, resulting in the formation of γ-keto sulfones. Various functional groups in the cyclopropanols and electron-deficient olefins are compatible in this transformation.

Direct Synthesis of Sulfonylated Spiro[indole-3,3'-pyrrolidines] by Silver-Mediated Sulfonylation of Acrylamides Coupled with Indole Dearomatization

A dearomative tandem spiro-cyclization reaction of N-[(1H-indol-3-yl)methyl]methacrylamide derivatives with sulfinate sodium in the presence of AgNO₃ and K₂CO₃ is reported, which produced sulfonylated spiro[indole-3,3'-pyrrolidines] in medium to excellent yields. The characteristics of this transformation contain good functional group tolerance and ease of operation.

Visible-light-induced intramolecular radical cascade of α-bromo-N-benzyl-alkylamides: a new strategy to synthesize tetracyclic N-fused indolo[2,1-a]isoquinolin-6(5H)-ones

Polycyclic indole scaffolds are ubiquitous in pharmaceuticals and natural products and in materials science. Here, we present a visible-light-initiated intramolecular aryl migration/desulfonylation/cyclization cascade reaction for the synthesis of tetracyclic indolo[2,1-a]isoquinolin-6(5H)-ones. This protocol not only exhibited a wide substrate scope but also provided a mild route to access a variety of tetracyclic N-fused indoles.

Photoredox-catalyzed sulfonylation of difluoroenoxysilanes with the insertion of sulfur dioxide

A photoredox-catalyzed three-component reaction of aryldiazonium tetrafluoroborates with sodium metabisulfite and 2,2-difluoro enol silyl ethers is described, providing α,α-difluoro-β-ketosulfones in moderate to good yields under mild conditions.

Dual Photoredox/Nickel-Catalyzed Three-Component Carbofunctionalization of Alkenes

The potential of merging photoredox and nickel catalysis to perform multicomponent alkene difunctionalizations under visible-light irradiation is demonstrated here. Secondary and tertiary alkyl groups, as well as sulfonyl moieties can be added to the terminal position of the double bond with simultaneous arylation of the internal carbon atom in a single step under mild reaction conditions. The process, devoid of stoichiometric additives, benefits from the use of bench-stable and easy-to-handle reagents, is operationally simple, and tolerates a wide variety of functional groups.

Photoredox-Catalyzed Sulfonylation of O-Acyl Oximes via Iminyl Radicals with the Insertion of Sulfur Dioxide

A multicomponent sulfonylation of O-acyl oximes via iminyl radicals with the insertion of sulfur dioxide under photoredox catalysis is achieved. This multicomponent reaction of O-acyl oximes, potassium metabisulfite, alkenes, and nucleophiles under visible-light irradiation is efficient, giving rise to a range of sulfones in moderate to good yields. A broad reaction scope is presented with good functional group compatibility.

Multicomponent reactions (MCRs): a useful access to the synthesis of benzo-fused γ-lactams

Benzo-fused γ-lactam rings such as isoindolin-2-ones and 2-oxindoles are part of the structure of many pharmaceutically active molecules. They can be often synthesized by means of multicomponent approaches and recent contributions in this field are summarized in this review. Clear advantages of these methods include the efficiency in saving raw materials and working time. However, there is still a need of new catalytic systems to allow the enantioselective preparation of these heterocycles by multicomponent reactions.