Photoinduced C(sp3)-H sulfination empowers the direct and chemoselective introduction of the sulfonyl group

Mild Strategy for the Preparation of Alkyl Sulfonyl Fluorides from Alkyl Bromides and Alcohols Using Photoredox Catalysis and Flow Chemistry

Facile access to sp3-rich scaffolds containing a sulfonyl fluoride group is still limited. Herein, we describe a mild and scalable strategy for the preparation of alkyl sulfonyl fluorides from readily available alkyl bromides and alcohols using photoredox catalysis. This approach is based on halogen atom transfer (XAT), followed by SO2 capture and fluorination. The method features mild conditions enabling fast access to high-value derivatives and has been scaled up to 5 g using a continuous stirred tank reactor cascade.

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.

Highly Selective C(sp3)-H Bond Oxygenation at Remote Methylenic Sites Enabled by Polarity Enhancement

A detailed study on the C(sp3)-H bond oxygenation reactions with H2O2 catalyzed by the [Mn(OTf)2(TIPSmcp)] complex at methylenic sites of cycloalkyl and 1-alkyl substrates bearing 19 different electron-withdrawing functional groups (EW FGs) was carried out. Oxidations in MeCN were compared to the corresponding ones in the strong hydrogen bond donating (HBD) solvents 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and nonafluoro tert-butyl alcohol (NFTBA). Formation of the products deriving from oxygenation at the most remote methylenic sites was observed, with yields, product ratios (PR) for oxygenation at the most remote over the next methylenic sites, and associated site-selectivities that significantly increased going from MeCN to HFIP and NFTBA. Unprecedented site-selectivities were obtained in the oxidation of cyclohexyl, cycloheptyl, cyclooctyl, 1-pentyl, 1-hexyl, and 1-heptyl substrates, approaching >99%, >99%, 90%, >99%, 93%, and 88% (PR >99, >99, 9.4, >99, 14, and 7.5) with cyclohexyl-2-pyridinecarboxylate, cycloheptyl-2-pyridinecarboxylate, cyclooctyl-4-nitrobenzenesulfonamide, 1-pentyl-3,5-dinitrobenzoate, 1-hexyl-3,5-dinitrobenzoate, and 1-heptyl-3,5-dinitrobenzoate, respectively. The results are rationalized on the basis of a polarity enhancement effect via synergistic electronic deactivation of proximal methylenic sites imparted by the EWG coupled to solvent HB. Compared to previous procedures, polarity enhancement provides the opportunity to tune site-selectivity among multiple methylenes in different substrate classes, extending the strong electronic deactivation determined by native EWGs by two carbon atoms. This study uncovers a simple procedure for predictable, high-yielding, and highly site-selective oxidation at remote methylenes of cycloalkyl and 1-alkyl substrates that occurs under mild conditions, with a large substrate scope, providing an extremely powerful tool to be implemented in synthetically useful procedures.

One-Pot Synthesis of Sulfonamides from Unactivated Acids and Amines via Aromatic Decarboxylative Halosulfonylation

The coupling of carboxylic acids and amines to form amide linkages is the most commonly performed reaction in the pharmaceutical industry. Herein, we report a new strategy that merges these traditional amide coupling partners to generate sulfonamides, important amide bioisosteres. This method leverages copper ligand-to-metal charge transfer (LMCT) to convert aromatic acids to sulfonyl chlorides, followed by one-pot amination to form the corresponding sulfonamide. This process requires no prefunctionalization of the native acid or amine and extends to a diverse set of aryl, heteroaryl, and s-rich aliphatic substrates. Further, we extend this strategy to the synthesis of (hetero)aryl sulfonyl fluorides, which have found utility as "click" handles in chemical probes and programmable bifunctional reagents. Finally, we demonstrate the utility of these protocols in pharmaceutical analogue synthesis.

Photoredox Catalysis-Enabled Sulfination of Alcohols and Bromides

Sulfinates are important lynchpin intermediates in pharmaceutical production; however, their synthesis via photoredox catalysis is challenging because of their facile oxidation. We herein disclose a photocatalytic strategy for the direct conversion of alcohols and alkyl bromides into alkyl sulfinates. These transformations are enabled by the utilization of easily oxidized radical precursors─namely, alcohol N-heterocyclic carbene adducts and N-adamantyl aminosupersilane─that facilitate efficient synthesis of the oxidatively labile sulfinate products. A broad range of functional groups are amenable to the reported transformations, providing rapid access to sulfonamides, sulfonyl halides, sulfones, and sulfonic acids. The utility of these methods is further demonstrated via the late-stage diversification of natural products and drugs into pharmaceutically relevant sulfonamides and "clickable" sulfonyl fluorides. In summary, this work illustrates the potential of novel radical precursors to expand the breadth of photoredox transformations.

Radical approaches to C-S bonds

Organosulfur functionalities are ubiquitous in nature, pharmaceuticals, agrochemicals, materials and flavourants. Historically, these moieties were introduced almost exclusively using ionic chemistry; however, radical-based methods for the installation of sulfur-based functional groups have recently come to the fore. These radical methods have enabled their late-stage introduction into complex molecules, avoiding the need to preserve labile organosulfur moieties through multistep synthetic sequences. Here, we discuss homolytic C-S bond-forming processes, with a particular emphasis on radical substitution approaches to sulfide, disulfide and sulfinyl products, and the use of sulfur dioxide and its surrogates to build sulfonyl products. We also highlight the mechanistic considerations that we hope will guide further development of radical-based strategies compatible with the various organosulfur moieties that feature in modern chemistry.

Recent advances in the synthesis of cyclic sulfinic acid derivatives (sultines and cyclic sulfinamides)

The chemistry of cyclic sulfinic acid derivatives (sultines and cyclic sulfinamides) was underdeveloped for a long time due to their inaccessibility. Considering the importance of cyclic sulfinate esters and amides in the fields of chemistry, pharmaceutical science, and material science, synthesis strategies involving cyclic sulfinic acid derivatives have been paid more attention in recent years, and have been widely used in the synthesis of sulfur-containing compounds such as sulfoxides, sulfones, sulfinates and thioethers. Despite the impressive improvements that have been made in last twenty years with the new strategies, to date, no reviews have been published, to the best of our knowledge, dealing with the preparation of cyclic sulfinic acid derivatives. This review summarizes the latest advances in the development of new synthesis methods to access cyclic sulfinic acid derivatives in the last two decades. The synthetic strategies are reviewed by highlighting their product diversity, selectivity and applicability, and the mechanistic rationale is presented where possible. We wish to bring readers a comprehensive understanding of the state-of-play of cyclic sulfinic acid derivative formation and make a contribution to future research.

Photoinduced Radical Sulfinylation of C(sp3)-H Bonds with Sulfinyl Sulfones

A direct C(sp³)-H sulfinylation reaction of alkanes with sulfinyl sulfones via decatungstate photocatalysis is reported. The sulfinyl sulfones generated in situ from sulfinates in the presence of an acylating reagent were able to trap the alkyl radicals that were produced via the photoinduced direct hydrogen atom transfer of alkanes, leading to a range of sulfoxides. This radical sulfinylation process provides an efficient and concise method for the synthesis of sulfoxides from abundant alkanes under mild conditions. Using the same strategy, aldehydes can also be transferred to the corresponding sulfoxides via decarbonylative sulfinylation.

Photocatalytic synthesis of alkyl-alkyl sulfones via direct C(sp3)-H bond functionalization

We report a highly efficient one-pot, three-component strategy for the construction of alkyl-alkyl sulfones through a photoinduced TBADT-catalyzed C(sp³)-H sulfonylation of unactivated hydrocarbon compounds. A wide range of commercially available hydrocarbon compounds and bioactive molecules can be successfully applied to the catalytic system, affording the corresponding alkyl-alkyl sulfones in good to excellent yields (>50 examples, up to 87% yield).

Decarboxylative Sulfinylation Enables a Direct, Metal-Free Access to Sulfoxides from Carboxylic Acids

The intermediate oxidation state of sulfoxides is central to the plethora of their applications in chemistry and medicine, yet it presents challenges for an efficient synthetic access, limiting the structural diversity of currently available sulfoxides. Here, we report a data-guided development of direct decarboxylative sulfinylation that enables the previously inaccessible functional group interconversion of carboxylic acids to sulfoxides in a reaction with sulfinates. Given the broad availability of carboxylic acids and the growing synthetic potential of sulfinates, the direct decarboxylative sulfinylation is poised to improve the structural diversity of synthetically accessible sulfoxides. The reaction is facilitated by a kinetically favored sulfoxide formation from the intermediate sulfinyl sulfones, despite the strong thermodynamic preference for the sulfone formation, unveiling the previously unknown and chemoselective radicalophilic sulfinyl sulfone reactivity.

Photo-organocatalytic Synthesis of β-Keto Sulfonyl Fluorides via Radical Fluorosulfonylation of Vinyl Acetates

A metal-free synthesis of useful β-keto sulfonyl fluorides has been established via radical fluorosulfonylation of ketone-derived vinyl acetates under photoredox organocatalysis by using 1-fluorosulfonyl benzoimidazolium (FABI) as the fluorosulfonyl radical source and oxygen-doped anthanthrene (ODA) as the photocatalyst. A series of aryl and alkyl β-keto sulfonyl fluorides as well as cyclic analogues can be readily obtained in moderate to high yields from widely available ketone starting materials.

Tricomponent Decarboxysulfonylative Cross-Coupling Facilitates Direct Construction of Aryl Sulfones and Reveals a Mechanistic Dualism in the Acridine/Copper Photocatalytic System

Dual catalytic systems involving photocatalytic activation and transition metal-catalyzed steps have enabled innovative approaches to the construction of carbon-carbon and carbon-heteroatom bonds. However, the mechanistic complexity of the dual catalytic processes presents multiple challenges for understanding of the roles of divergent catalytic species that can impede the development of future synthetic methods. Here, we report a dual catalytic process that enables the previously inaccessible, broad-scope, direct conversion of carboxylic acids to aromatic sulfones-centrally important carbonyl group bioisosteric replacements and synthetic intermediates-by a tricomponent decarboxysulfonylative cross-coupling with aryl halides. Detailed mechanistic and computational studies revealed the roles of the copper catalyst, base, and halide anions in channeling the acridine/copper system via a distinct dual catalytic manifold. In contrast to the halide-free decarboxylative conjugate addition that involves cooperative dual catalysis via low-valent copper species, the halide counteranions divert the decarboxysulfonylative cross-coupling with aryl halides through a two-phase, orthogonal relay catalytic manifold, comprising a kinetically coupled (via antithetical inhibitory and activating roles of the base in the two catalytic cycles), mechanistically discrete sequence of a photoinduced, acridine-catalyzed decarboxylative process and a thermal copper-catalyzed arylative coupling. The study underscores the importance of non-innocent roles of counteranions and key redox steps at the interface of catalytic cycles for enabling previously inaccessible dual catalytic transformations.

Rapid Access to Aliphatic Sulfonamides

Herein, we report a mild, rapid, straightforward method for visible-light-mediated sulfonamide ethylation to afford a diverse array of compounds with C(sp³)-sulfonamide skeletons. The method relies on inexpensive, abundant, commercially available primary, secondary, and tertiary alkyl carboxylic acids and alkyl iodides as substrates. The method has a broad substrate scope and potential utility for late-stage functionalization of natural products and synthetic medicines and can be expected to facilitate rapid structural diversification of bioactive molecules.

Three-component synthesis of arylsulfonyl-substituted indolo[2,1-a]isoquinolinones and benzimidazo-[2,1-a]isoquinolin-6(5H)-ones by SO2 insertion and radical cascade cyclization

An efficient arylsulfonylation/cyclization of 2-aryl-N-methacryloyl indoles with potassium metabisulfite and aryldiazonium tetrafluoroborates was developed. A series of variously substituted arylsulfonyl indolo[2,1-a]isoquinolin-6(5H)-ones were formed in moderate to good yields via utilization of the nature abundant inorganic salt potassium metabisulfite as a SO₂ surrogate. Additionally, this three-component protocol can also be employed for the synthesis of arylsulfonyl-substituted benzimidazo-[2,1-a]isoquinolin-6(5H)-ones.

Functional group divergence and the structural basis of acridine photocatalysis revealed by direct decarboxysulfonylation

The reactivity of the sulfonyl group varies dramatically from nucleophilic sulfinates through chemically robust sulfones to electrophilic sulfonyl halides-a feature that has been used extensively in medicinal chemistry, synthesis, and materials science, especially as bioisosteric replacements and structural analogs of carboxylic acids and other carbonyls. Despite the great synthetic potential of the carboxylic to sulfonyl functional group interconversions, a method that can convert carboxylic acids directly to sulfones, sulfinates and sulfonyl halides has remained out of reach. We report herein the development of a photocatalytic system that for the first time enables direct decarboxylative conversion of carboxylic acids to sulfones and sulfinates, as well as sulfonyl chlorides and fluorides in one step and in a multicomponent fashion. A mechanistic study prompted by the development of the new method revealed the key structural features of the acridine photocatalysts that facilitate the decarboxylative transformations and provided an informative and predictive multivariate linear regression model that quantitatively relates the structural features with the photocatalytic activity.

Electronic control over site-selectivity in hydrogen atom transfer (HAT) based C(sp3)-H functionalization promoted by electrophilic reagents

The direct functionalization of C(sp³)-H bonds represents one of the most investigated approaches to develop new synthetic methodology. Among the available strategies for intermolecular C-H bond functionalization, increasing attention has been devoted to hydrogen atom transfer (HAT) based procedures promoted by radical or radical-like reagents, that offer the opportunity to introduce a large variety of atoms and groups in place of hydrogen under mild conditions. Because of the large number of aliphatic C-H bonds displayed by organic molecules, in these processes control over site-selectivity represents a crucial issue, and the associated factors have been discussed. In this review article, attention will be devoted to the role of electronic effects on C(sp³)-H bond functionalization site-selectivity. Through an analysis of the recent literature, a detailed description of the HAT reagents employed in these processes, the associated mechanistic features and the selectivity patterns observed in the functionalization of substrates of increasing structural complexity will be provided.

Construction of sulfonated spiro[5,5]trienones from sulfur dioxide via iron-catalyzed dearomative spirocyclization of biaryls

An iron-catalyzed dearomative spirocyclization of biaryl ynones with sodium metabisulfite and cycloketone oxime esters is developed. By using sodium metabisulfite as the source of sulfur dioxide, this approach enables the...