Photocatalytic Carboxylate to Sulfinamide Switching Delivers a Divergent Synthesis of Sulfonamides and Sulfonimidamides

Electrochemical Conversions of Sulfenamides into Sulfonimidoyl- and Sulfondiimidoyl Fluorides

The invention of versatile linkage agents provides the chemical basis for SuFEx chemistry. Sulfonimidoyl fluorides and sulfondiimidoyl fluorides are aza-isosteres of sulfonyl fluorides with diverse reactivity through the fine-tuning of N-substituents. However, limited synthetic approaches impede their wide applications in SuFEx chemistry. Herein, we develop a straightforward electrochemical strategy for sulfonimidoyl- and sulfondiimidoyl fluorides through sequential oxidations of the readily available sulfenamides via sulfinamide and iminosulfinamide intermediates, respectively. The previously rarely investigated (bis)-sulfondiimidoyl fluorides are now easily accessible and readily participate in SuFEx chemistry with diverse oxygen and nitrogen nucleophiles, macrocyclization, and polymerization.

Iron-Catalyzed Alkylative Aminosulfonylation of Alkenes and Alkynes via Radical-Anion Relay

A novel Fe-catalyzed alkylative aminosulfonylation of alkenes and alkynes with alkyl halides and O-Ts activated hydroxylamines by using Na2S2O4 as a reductant and sulfone source has been developed. The metal-electron-shuttle catalysis was discovered to be vital for the highly efficient generation of sulfonyl radicals and anions without requiring organometallic intermediates. This method provides efficient access to sulfonamides from easily available alkenes and Na2S2O4 and features a broad substrate scope.

Catalytic Enantioselective Reductive Arylation and Alkenylation of Sulfinylamines to Access Sulfinamides Enabled by Cobalt Catalysis

This study reports a cobalt-catalyzed method for the enantioselective reductive arylation and alkenylation of sulfinylamines, facilitating the efficient synthesis of enantiopure sulfinamides. By employing a cobalt/chiral diphosphine and tridentate ligand system, a range of diverse aryl and alkenyl iodides were successfully transformed into sulfinamides, achieving high yields and excellent enantioselectivity. This methodology underscores the utility of sulfinylamines as versatile electrophilic sulfur sources. Detailed mechanistic insights from density functional theory (DFT) calculations suggest that the key step involves migratory insertion into the sulfilimine group, which plays a crucial role in enantioselectivity. These findings offer a sustainable approach for the development of biologically relative enantiopure sulfur-containing compounds.

Photoinduced Late-Stage Radical Decarboxylative and Deoxygenative Coupling of Complex Carboxylic Acids and Their Derivatives

The simple and efficient conversion of carboxylic acids into structurally diverse organic molecules is highly desirable in chemical synthesis. This review covers recent developments in photocatalytic methodology for late-stage transformations of complex carboxylic acids and their derivatives enabled by radical decarboxylation and deoxygenation, highlighting some representative and significant contributions in this field. These advancements are categorized based on the reactivity patterns exhibited by the carboxylic acids. Several activation modes to generate alkyl or aryl radical intermediates during decarboxylation of carboxylic acids are presented, namely, single-electron transfer (SET) oxidation, ligand-to-metal charge transfer (LMCT), SET reduction, and energy transfer (EnT) processes. On the other hand, direct activation of C-O bonds in carboxylic acids mediated by phosphoranyl radicals has been discussed and illustrates their potential synthetic application for the synthesis of complex aldehydes, ketones and amides.

Photoredox-catalyzed deoxygenative radical transformation of alcohols to sulfinamides

Sulfinamides play a crucial role in organic synthesis and pharmaceuticals. In this study, we introduce a highly effective method for the deoxygenative radical addition to N-tritylsulfinylamine, which produces sulfinamides via photoredox catalysis. This method is compatible with a diverse array of functional groups and the resulting sulfonamides were achieved in moderate to high yields. Furthermore, the synthetic applications to access various sulfur(vi)-centered functional groups highlight the practicality of this approach.

Strategic Synthesis of Sulfinamides as Versatile S(IV) Intermediates

Sulfinamides constitute adaptable S(IV) intermediates with a sulfur stereocenter, having emerging interest in divergent synthesis of high-valent S(VI) functional bioisosteres. Recent years have witnessed the strategic development of mild and selective synthetic routes for highly functionalized sulfinamides, employing stable organometallic reagents, carbon-centered radical precursors, and other abundant coupling partners merged with various sulfur reagents in the arena of metal, photoredox, and organocatalysis. Furthermore, asymmetric metal and organocatalysis have enabled the stereoselective synthesis of enantioenriched sulfinamides. In this Perspective, we present the recent (2021 to present) advancement of various synthetic methods toward sulfinamides.

Photoredox-Catalyzed Aminolactonization of 2-Styrylbenzoic Acids with N-Aminopyridinium Salts to Access 4-Sulfonamino-3,4-dihydroisocoumarins

A photoredox-catalyzed aminolactonization of unsaturated carboxylic acids was achieved using N-aminopyridinium salts as the amino radical precursor. The transformation features mild conditions and a remarkably broad substrate scope, offering an efficient approach to construct a wide range of 4-sulfonamino 3,4-dihydroisocoumarins. Mechanistic studies indicate that the reaction proceeds via a distinctive N-aminopyridinium salt-promoted electrophilic amination of 2-styrylbenzoic acids.

Reductive sulfinylation by nucleophilic chain isomerization of sulfonylpyridinium

Sulfur-containing units are fundamental components widely found in bioactive compounds, prompting notable efforts toward developing synthetic methodologies for incorporating sulfur functionality into organic precursors. The synthesis of sulfinate esters and sulfinamides has garnered significant interest owing to their immense potential for applications, especially in drug development. However, most existing synthetic protocols suffer from some limitations. To address these challenges, we herein present a practical and efficient approach for the reductive sulfinylation of diverse nucleophiles with sulfonylpyridinium salts (SulPy) through the nucleophilic chain substitution, namely SNC reaction, which involves S(VI) to S(IV) nucleophilic chain isomerization process. These versatile sulfinylation reagents can be readily accessed from diverse commercially available resourses. The late-stage modification of complex molecules and the ability to rapidly synthesize numerous sulfinyl bioisosteres of various drugs highlights the utility of this protocol.

Iterative One-Carbon Homologation of Unmodified Carboxylic Acids

The one-carbon homologation of carboxylic acids is a valuable route to construct families of homologues, which play fundamental roles in chemistry and biology. However, known procedures are based on multistep sequences, use harsh conditions or are limited in scope. Thus, almost a century after the discovery of the original Arndt-Eistert homologation sequence, a general method to directly convert carboxylic acids into their corresponding homologues remains elusive. Exploiting the photoredox reactivity of nitroethylene, we disclose a practical visible-light-induced homologation of unmodified carboxylic acids. Iterations of the procedure reveal an exceptionally tunable strategy for the construction of inert carbon spacers, opening new opportunities in synthesis.

Conversion of Carboxylic Acids to Sulfonamide Bioisosteres via Energy Transfer Photocatalysis

More than 450 drugs containing a carboxylic acid functional group have been marketed worldwide. Herein, we report a concise and environmentally friendly organic photoinduced protocol for the interconversion of carboxylic acids into their bioisosteres. With this strategy, a variety of substrates, including alkyl, (hetero)aryl, and alkenyl acids, as well as various biologically relevant acids are successfully converted into primary sulfonamides.

Acridine/Lewis Acid Complexes as Powerful Photocatalysts: A Combined Experimental and Mechanistic Study

A class of in-situ generated Lewis acid (LA) activated acridine complexes is reported, which act as potent photochemical catalysts for the oxidation of a variety of protected secondary amines. Acridine/LA complexes exhibit tunable excited state reduction potentials ranging from +2.07 to 2.38 V vs. SCE. The ytterbium triflate complex of 3,6-di-t-butyl-9-mesitylacridine catalyzes a photochemical Giese-type reaction of Boc-protected secondary amines with challenging conjugate acceptors such as acrylates, that are inaccessible to the analogous acridinium (t-Bu-Mes-Acr⊕) catalyzed reaction. The mechanism of this reaction was investigated using a suite of physical organic probes including intramolecular 13C kinetic isotope effects (KIEs), variable time normalization analysis (VTNA) kinetics, determination of redox potentials, and computational studies. In the reaction catalyzed by t-Bu-Mes-Acr⊕, mechanistic studies are consistent with single-electron transfer (SET) from the ground-state reduced t-Bu-Mes-Acr• to the α-keto radical intermediate as the first irreversible step in the catalytic cycle. Intriguingly, we find that the reduced acridine/LA complexes are better ground state reductants (-0.72 to -0.74 V vs SCE) relative to t-Bu-Mes-Acr• (-0.59 V vs SCE) and predict that the increased substrate reactivity stems from a lower energy barrier for this key SET event.

Photocatalytic Decarboxylative Alkylation of Cyclic Imine-BF3 Complexes: A Modular Route to Functionalized Azacycles

Alkyl radicals generated via an acridine photocatalyzed decarboxylation reaction of feedstock carboxylic acids engage with a range of cyclic imine-BF3 complexes to provide α-functionalized azacycles in an operationally simple process. A three-component variant of this transformation incorporating [1.1.1]propellane as an additional reaction partner enables the synthesis of valuable bicyclopentane (BCP)-containing azacycles. Reactions exhibit good functional group compatibility, enabling late-stage modification of complex bioactive molecules.

Exploiting trans-Sulfinylation for the Synthesis of Diverse N-Alkyl Sulfinamides via Decarboxylative Sulfinamidation

Combining simple amines with the bench-stable sulfinylamine Tr-NSO allows in situ preparation of reactive alkyl sulfinylamines, which when combined with alkyl radicals generated by photocatalytic decarboxylation, provides N-alkyl sulfinamides. The reactions are broad in scope and tolerate a wide variety of functional groups on both the acid and amine components. The sulfinamide products are used to prepare a selection of challenging S(VI) products. The method provides a convenient way to use reactive and unstable alkyl sulfinylamines.

Metal-free introduction of primary sulfonamide into electron-rich aromatics

We report herein a direct and practical synthesis of arylsulfonamides from electron-rich aromatic compounds by using in situ generated N-sulfonylamine as the active electrophile. Substrates include derivatives of aniline, indole, pyrrole, furan, styrene and so on. The reaction proceeds under mild conditions and tolerates many sensitive functional groups such as alkyne, acetate, the trifluoromethoxy group or acetoxymethyl ester. Applications of this method for the construction of metal ion sensors and fluorogenic dye have been demonstrated, thus highlighting the potential of this method for probe development.

Zinc Promoted Cross-Electrophile Sulfonylation to Access Alkyl-Alkyl Sulfones

The transition metal-catalyzed multi-component cross-electrophile sulfonylation, which incorporates SO2 as a linker within organic frameworks, has proven to be a powerful, efficient, and cost-effective means of synthesizing challenging alkyl-alkyl sulfones. Transition metal catalysts play a crucial role in this method by transferring electrons from reductants to electrophilic organohalides, thereby causing undesirable side reactions such as homocoupling, protodehalogenation, β-hydride elimination, etc. It is worth noting that tertiary alkyl halides have rarely been demonstrated to be compatible with current methods owing to various undesired side reactions. In this work, a zinc-promoted cross-electrophile sulfonylation is developed through a radical-polar crossover pathway. This approach enables the synthesis of various alkyl-alkyl sulfones, including 1°-1°, 2°-1°, 3°-1°, 2°-2°, and 3°-2° types, from inexpensive and readily available alkyl halides. Various functional groups are well tolerated in the work, resulting in yields of up to 93%. Additionally, this protocol has been successfully applied to intramolecular sulfonylation and homo-sulfonylation reactions. The insights gained from this work shall be useful for the further development of cross-electrophile sulfonylation to access alkyl-alkyl sulfones.

Palladium-Catalyzed Addition of Aryl Halides to N-Sulfinylamines for the Synthesis of Sulfinamides

Sulfinamides are versatile, synthetically useful intermediates, and final motifs. Traditional methods to synthesize sulfinamides generally require substrates with preinstalled sulfur centers. However, these precursors have limited commercial availability, and the associated synthetic routes often require harsh reaction conditions and highly reactive reagents, thus severely limiting their application. Herein, we report the synthesis of sulfinamides from aryl and alkenyl (pseudo)halides and N-sulfinylamines, enabled by palladium catalysis. The reactions use mild conditions and are achieved without the use of highly reactive preformed organometallic reagents, resulting in transformations of broad generality and high functional group tolerance. In particular, substrates featuring protic and electrophilic functional groups can be used successfully. The modification of complex aryl cores and natural product derivatives demonstrates the utility of this method.

Asymmetric 2,3-Addition of Sulfinylamines with Arylboronic Acids Enabled by Nickel Catalysis

Sulfinamides have been widely used in organic synthesis, with research on their preparation spanning more than a century. Despite advancements in catalytic methodologies, creating sulfur stereocenters within these molecules remains a significant challenge. In this study, we present an effective and versatile method for synthesizing a diverse range of S-chirogenic sulfinamides through catalytic asymmetric aryl addition to sulfinylamines. By utilizing a nickel complex as a catalyst, this process exhibits impressive enantioselectivity and can incorporate various arylboronic acids at the sulfur position. The resulting synthetic sulfinamides are stable and highly adaptable, allowing for their conversion to a variety of sulfur-containing compounds. Our study also incorporates detailed experimental and computational studies to elucidate the reaction mechanism and factors influencing enantioselectivity.

Acridine photocatalysis enables tricomponent direct decarboxylative amine construction

Amines are centrally important motifs in medicinal chemistry and biochemistry, and indispensable intermediates and linchpins in organic synthesis. Despite their cross-disciplinary prominence, synthetic access to amine continues to rely on two-electron approaches based on reductions and additions of organometallic reagents, limiting their accessible chemical space and necessitating stepwise preassembly of synthetic precursors. We report herein a homogeneous photocatalytic tricomponent decarboxylative radical-mediated amine construction that enables modular access to α-branched secondary amines directly from the broad and structurally diverse chemical space of carboxylic acids in a tricomponent reaction with aldehydes and aromatic amines. Our studies reveal the key role of acridine photocatalysis acting in concert with copper and Brønsted acid catalytic processes in facilitating the previously inaccessible homogeneous photocatalytic reaction and provide a streamlined segue to a wide range of amines and nonproteinogenic α-amino acids.

Iron-Catalyzed Photochemical Synthesis of Sulfinamides from Aliphatic Hydrocarbons and Sulfinylamines

An iron-catalyzed photochemical sulfinamidation of hydrocarbons with N-sulfinylamines has been developed. The merger of ligand-to-metal charge transfer (LMCT) of FeCl3 with hydrogen atom transfer (HAT) process is the key for the generation of alkyl radicals from hydrocarbons, and the resultant alkyl radicals were readily trapped by N-sulfinylamines to produce structurally diverse sulfinamides. Contrary to traditional methods that inevitably use sensitive organometallic reagents and prefunctionalized substrates, our approach features simple operation and the wide availability of starting materials. Gratifyingly, the reaction is scalable, and the obtained sulfinamides can be conveniently converted to highly functionalized sulfur(VI) derivatives.

Multimodal Acridine Photocatalysis Enables Direct Access to Thiols from Carboxylic Acids and Elemental Sulfur

Development of photocatalytic systems that facilitate mechanistically divergent steps in complex catalytic manifolds by distinct activation modes can enable previously inaccessible synthetic transformations. However, multimodal photocatalytic systems remain understudied, impeding their implementation in catalytic methodology. We report herein a photocatalytic access to thiols that directly merges the structural diversity of carboxylic acids with the ready availability of elemental sulfur without substrate preactivation. The photocatalytic transformation provides a direct radical-mediated segue to one of the most biologically important and synthetically versatile organosulfur functionalities, whose synthetic accessibility remains largely dominated by two-electron-mediated processes based on toxic and uneconomical reagents and precursors. The two-phase radical process is facilitated by a multimodal catalytic reactivity of acridine photocatalysis that enables both the singlet excited state PCET-mediated decarboxylative carbon-sulfur bond formation and the previously unknown radical reductive disulfur bond cleavage by a photoinduced HAT process in the silane-triplet acridine system. The study points to a significant potential of multimodal photocatalytic systems in providing unexplored directions to previously inaccessible transformations.

Unifying N-Sulfinylamines with Alkyltrifluoroborates by Organophotoredox Catalysis: Access to Functionalized Alkylsulfinamides and High-Valent S(VI) Analogues

We describe an organophotoredox-catalyzed sp3 C-S coupling of N-sulfinylamines with bench-stable alkyltrifluoroborates as a latent nucleophilic counterpart en route to alkylsulfinamides in high efficiency. In contrast to the two-electron reactivity of traditional organometallic reagents, this catalytic method reports the single-electron process of an organometallic reagent with N-sulfinylamines in C-S coupling. This mild and scalable protocol offers operational simplicity and exceptional functional group compatibility, including ketone, ester, amide, nitrile, and halides, that is vulnerable to organolithium or Grignard reagents. Additionally, the sulfinamides are conveniently converted to a variety of important S(VI) compounds, like sulfonamides, sulfonimidamides, and sulfonimidates, among others.

Dual Acridine/Decatungstate Photocatalysis for the Decarboxylative Radical Addition of Carboxylic Acids to Azomethines

A concept for the dual use of acridine and tetrabutylammonium decatungstate photocatalysts in the reactions of carboxylic acids is proposed. Imines generated in situ from aldehydes and p-methoxyaniline, as well as other azomethines, were used as radical acceptors. The role of the decatungstate is believed to facilitate the turnover of the acridine photocatalyst by means of hydrogen atom transfer.

Synthesis of S(IV)-Stereogenic Chiral Thio-Oxazolidinones via Palladium-Catalyzed Asymmetric [3+2] Annulations

Organic molecules bearing chiral sulfur stereocenters exert a great impact on asymmetric catalysis and synthesis, chiral drugs, and chiral materials. Compared with acyclic ones, the catalytic asymmetric synthesis of thio-heterocycles has largely lagged behind due to the lack of efficient synthetic strategies. Here we establish the first modular platform to access chiral thio-oxazolidinones via Pd-catalyzed asymmetric [3+2] annulations of vinylethylene carbonates with sulfinylanilines. This protocol is featured by readily available starting materials, and high enantio- and diastereoselectivity. In particular, an unusual effect of a non-chiral supporting ligand on the diastereoselectivity was observed. Possible reaction mechanisms and stereocontrol models were proposed.

Photoredox-Catalyzed Preparation of Sulfones Using Bis-Piperidine Sulfur Dioxide - An Underutilized Reagent for SO2 Transfer

Sulfonyl groups are widely observed in biologically relevant molecules and consequently, SO2 capture is an increasingly attractive method to prepare these sulfonyl-containing compounds given the range of SO2 -surrogates now available as alternatives to using the neat gas. This, along with the advent of photoredox catalysis, has enabled mild radical capture of SO2 to emerge as an effective route to sulfonyl compounds. Here we report a photoredox-catalyzed cross-electrophile sulfonylation of aryl and alkyl bromides making use of a previously under-used amine-SO2 surrogate; bis(piperidine) sulfur dioxide (PIPSO). A broad selection of alkyl and aryl bromides were photocatalytically converted to their corresponding sulfinates and then trapped with various electrophiles in a one-pot multistep procedure to prepare sulfones and sulfonamides.

Visible-Light-Induced Decarboxylative Aminosulfonylation of (Hetero)aryl Carboxylic Oxime Esters

Sulfonamides are important structures in pharmaceuticals, agrochemicals, and organocatalysts, yet the rapid and benign synthesis of these compounds is still a great challenge. Herein we report a photoinduced method for synthesizing sulfonamides from (hetero)aryl carboxylic acid oxime esters. This reaction proceeds via one-pot cascade radical-radical cross-coupling by energy-transfer-mediated photocatalysis. A wide substrate scope including (hetero)aryl substrates and late-stage modification of pharmaceutical molecular entities reveal its generality.

Direct conversion of carboxylic acids to free thiols via radical relay acridine photocatalysis enabled by N-O bond cleavage

Carboxylic acids and thiols are basic chemical compounds with diverse utility and widespread reactivity. However, the direct conversion of unprotected acids to thiols is hampered due to a fundamental problem - free thiols are incompatible with the alkyl radicals formed on decarboxylation of carboxylic acids. Herein, we describe a concept for the direct photocatalytic thiolation of unprotected acids allowing unprotected thiols and their derivatives to be obtained. The method is based on the application of a thionocarbonate reagent featuring the N-O bond. The reagent serves both for the rapid trapping of alkyl radicals and for the facile regeneration of the acridine-type photocatalyst.

Synthesis of sulfinamides via photocatalytic alkylation or arylation of sulfinylamine

Sulfinamides are a versatile class of compounds that find applications in both organic synthesis and pharmaceuticals. Here we developed an efficient photocatalytic approach for the convenient preparation of sulfinamides. Commercially available potassium trifluoro(organo)borates and readily available sulfinyl amines are rationally used and converted to a series of alkyl or aryl sulfinamides in moderate to high yields. The reaction allows for the gram-scale preparation of sulfinamides. Moreover, sulfonimidamides, sulfonimidate esters and sulfonyl amides could be obtained in one pot.

Kinetically-driven reactivity of sulfinylamines enables direct conversion of carboxylic acids to sulfinamides

Sulfinamides are some of the most centrally important four-valent sulfur compounds that serve as critical entry points to an array of emergent medicinal functional groups, molecular tools for bioconjugation, and synthetic intermediates including sulfoximines, sulfonimidamides, and sulfonimidoyl halides, as well as a wide range of other S(iv) and S(vi) functionalities. Yet, the accessible chemical space of sulfinamides remains limited, and the approaches to sulfinamides are largely confined to two-electron nucleophilic substitution reactions. We report herein a direct radical-mediated decarboxylative sulfinamidation that for the first time enables access to sulfinamides from the broad and structurally diverse chemical space of carboxylic acids. Our studies show that the formation of sulfinamides prevails despite the inherent thermodynamic preference for the radical addition to the nitrogen atom, while a machine learning-derived model facilitates prediction of the reaction efficiency based on computationally generated descriptors of the underlying radical reactivity.

A Sulfur Monoxide Surrogate Designed for the Synthesis of Sulfoxides and Sulfinamides

Sulfur monoxide (SO) is a highly reactive species that cannot be isolated in bulk. However, SO can play a pivotal role as a fundamental building block in organic synthesis. Reported herein is the design and application of a sulfinylhydrazine reagent as an easily prepared sulfur monoxide surrogate. We show facile thermal SO transfer from this reagent to dienes where a reaction using a mechanistic probe suggests the generation of singlet SO. Combined with Grignard reagents and appropriate carbon or nitrogen electrophiles, the reagent serves as an effective "SO" donor to enable the one-pot, three-component synthesis of sulfoxides and sulfinamides.