Late-Stage Heteroarylation of Hetero(aryl)sulfonium Salts Activated by α-Amino Alkyl Radicals

Ruthenium(II)-Catalyzed Late-Stage Incorporation of N-Aryl Triazoles and Tetrazoles with Sulfonium Salts via C-H Activation

The late-stage functionalization of active pharmaceutical ingredients is a key challenge in medicinal chemistry. Furthermore, N-aryl triazoles and tetrazoles are important structural motifs with the potential to boost the activity of diverse drug molecules. Using easily accessible dibenzothiophenium salts for the ruthenium-catalyzed C-H arylation, these scaffolds were introduced into a variety of bioactive compounds. Our methodology uses cost-efficient ruthenium, KOAc as a mild base and gives access to a plethora of highly decorated triazole and tetrazole containing drug derivatives.

Palladium/norbornene-catalyzed diversified trifunctionalization of aryl-thianthreniums

A novel Catellani-type conversion is reported using aryl-thianthreniums (aryl-TTs) instead of aryl halides. Three classes of ortho-dual C-H functionalization involving alkylation, amination, and deuterated methylation and five types of ipso-operation including alkenylation, cyanation, methylation, hydrogenation, and alkynylation all proceed well in this procedure. In this conversion, aryl-TTs exhibit satisfactory reactivity and feature the advantage that the leaving TT unit can be recovered. More strikingly, this finding represents a new chemistry conversion of aryl-TTs, wherein contiguous tri-functionalization in a single chemical manipulation is realized.

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Photocatalytic Functionalization of Dehydroalanine-Derived Peptides in Batch and Flow

Unnatural amino acids, and their synthesis by the late-stage functionalization (LSF) of peptides, play a crucial role in areas such as drug design and discovery. Historically, the LSF of biomolecules has predominantly utilized traditional synthetic methodologies that exploit nucleophilic residues, such as cysteine, lysine or tyrosine. Herein, we present a photocatalytic hydroarylation process targeting the electrophilic residue dehydroalanine (Dha). This residue possesses an α,β-unsaturated moiety and can be combined with various arylthianthrenium salts, both in batch and flow reactors. Notably, the flow setup proved instrumental for efficient scale-up, paving the way for the synthesis of unnatural amino acids and peptides in substantial quantities. Our photocatalytic approach, being inherently mild, permits the diversification of peptides even when they contain sensitive functional groups. The readily available arylthianthrenium salts facilitate the seamless integration of Dha-containing peptides with a wide range of arenes, drug blueprints, and natural products, culminating in the creation of unconventional phenylalanine derivatives. The synergistic effect of the high functional group tolerance and the modular characteristic of the aryl electrophile enables efficient peptide conjugation and ligation in both batch and flow conditions.

Stereospecific syn-dihalogenations and regiodivergent syn-interhalogenation of alkenes via vicinal double electrophilic activation strategy

Whereas the conventional anti-dihalogenation of alkenes is a valuable synthetic tool with highly predictable stereospecificity, the restricted reaction mechanism makes it challenging to alter the diastereochemical course into the complementary syn-dihalogenation process. Only a few notable achievements were made recently by inverting one of the stereocenters after anti-addition using a carefully designed reagent system. Here, we report a conceptually distinctive strategy for the simultaneous double electrophilic activation of the two alkene carbons from the same side. Then, the resulting vicinal leaving groups can be displaced iteratively by nucleophilic halides to complete the syn-dihalogenation. For this purpose, thianthrenium dication is employed, and all possible combinations of chlorine and bromine are added onto internal alkenes successfully, particularly resulting in the syn-dibromination and the regiodivergent syn-bromochlorination.

Electrocatalytic Formal C(sp2)-H Alkylations via Nickel-Catalyzed Cross-Electrophile Coupling with Versatile Arylsulfonium Salts

Producing sp3-hybridized carbon-enriched molecules is of particular interest due to their high success rate in clinical trials. The installation of aliphatic chains onto aromatic scaffolds was accomplished by nickel-catalyzed C(sp2)-C(sp3) cross-electrophile coupling with arylsulfonium salts. Thus, simple non-prefunctionalized arenes could be alkylated through the formation of aryldibenzothiophenium salts. The reaction employs an electrochemical approach to avoid potentially hazardous chemical redox agents, and importantly, the one-pot alkylation proved also viable, highlighting the robustness of our approach.

Reactivity of α-diazo sulfonium salts: rhodium-catalysed ring expansion of indenes to naphthalenes

In the presence of catalytic amounts of the paddlewheel dirhodium complex Rh2(esp)2, α-diazo dibenzothiophenium salts generate highly electrophilic Rh-coordinated carbenes, which evolve differently depending on their substitution pattern. Keto-moieties directly attached to the azomethinic carbon promote carbene insertion into one of the adjacent C-S bonds, giving rise to highly electrophilic dibenzothiopyrilium salts. This intramolecular pathway is not operative when the carbene carbon bears ester or trifluoromethyl substituents; in fact, these species react with olefins delivering easy to handle cyclopropyl-substituted sulfonium salts. When indenes are the olefins of choice, the initially formed cyclopropyl rings smoothly open with concomitant departure of dibenzothiophene, enabling access to a series of 2-functionalized naphthalenes.

Base-Promoted Ring-Opening Hydroxylation of Cyclic Sulfonium Salts

Herein, we reported a general strategy for the synthesis of sulfur-containing primary alcohol derivatives by base-promoted ring-opening hydroxylation of cyclic sulfonium salts. A variety of sulfonium salts were successfully transformed into the desired hydroxylated products in moderate to excellent yields with good functional group tolerance. Moreover, the one-pot synthesis, scale-up reaction, and late-stage functionalization of complex molecules demonstrated the practicability of this synthetic protocol in the field of synthetic chemistry.

Reductive Radical-Polar Crossover Enabled Carboxylative Alkylation of Aryl Thianthrenium Salts with CO2 and Styrenes

Carboxylic functionalities are among the pivotal groups in bioactive molecules and in the synthesis of new lead compounds because of their unique character in the formation of hydrogen bonds and the possibility of constructing molecular complexes via amide couplings. We adopt the reductive radical-polar crossover strategy to introduce carboxyalkyl groups into arenes with styrenes and CO2 via thianthrenium salts. This protocol exhibits excellent potential as a straightforward and modular platform for site-selective carboxylative derivation of bioactive molecules.

A General Electron Donor-Acceptor Photoactivation Platform of Diaryliodonium Reagents: Arylation of Heterocycles

We report a photoredox system comprising sodium iodide, triphenyl phosphine, and N,N,N',N'-tetramethylethylenediamine (TMEDA) that can form a self-assembled tetrameric electron donor-acceptor (EDA) complex with diaryliodonium reagents (DAIRs) and furnish aryl radicals upon visible light irradiation. This practical mode of activation of DAIRs enables arylation of an array of heterocycles under mild conditions to provide the respective heteroaryl-(hetero)aryl assembly in moderate to excellent yields. Detailed mechanistic investigations comprising photophysical and DFT studies provided insight into the reaction mechanism.

Cobalt-catalyzed aminoalkylative carbonylation of alkenes toward direct synthesis of γ-amino acid derivatives and peptides

γ-Amino acids and peptides analogues are common constituents of building blocks for numerous biologically active molecules, pharmaceuticals, and natural products. In particular, γ-amino acids are providing with better metabolic stability than α-amino acids. Herein we report a multicomponent carbonylation technology that combines readily available amides, alkenes, and the feedstock gas carbon monoxide to build architecturally complex and functionally diverse γ-amino acid derivatives in a single step by the implementation of radical relay catalysis. This transformation can also be used as a late-stage functionalization strategy to deliver complex, advanced γ-amino acid products for pharmaceutical and other areas.

Site-selective carbonylation of arenes via C(sp2)-H thianthrenation: direct access to 1,2-diarylethanones

Herein, a new reaction for the site-selective carbonylation of arenes via C(sp²)-H thianthrenation under mild conditions has been developed. With low loadings of palladium catalysts, various desired 1,2-diarylethanones are produced in good yields. This strategy also enables the late-stage modification of complex molecules, which was previously challenging with similar carbonylative Negishi-type reactions.

Palladium-Catalyzed Carbonylative Synthesis of Diaryl Ketones from Arenes and Arylboronic Acids through C(sp2)-H Thianthrenation

The development of mild methodology for converting inert C-H bonds to value-added molecules has been an attractive research topic during the last few decades as it offers efficient preparation. Meanwhile, diaryl ketones hold potent applications in antitumor drugs, the agrochemical industry, and synthetic chemistry. Herein, we report versatile palladium-catalyzed carbonylative cross-coupling reactions of aryl thianthrenium salts with arylboronic acids. Arenes were transformed site selectively via C(sp²)-H thianthrenation, and various desired diaryl ketones were produced in good to excellent yields.

Late-stage C-H Functionalization of Tryptophan-Containing Peptides with Thianthrenium Salts: Conjugation and Ligation

Bioorthogonal late-stage diversification of structurally complex peptides bears enormous potential for drug discovery and molecular imaging, among other applications. Herein, we report on a palladium-catalyzed C-H arylation of tryptophan-containing peptides with readily accessible and modular arylthianthrenium salts. Under exceedingly mild reaction conditions, the late-stage diversification of structurally complex peptides was accomplished. The tunability and ease of preparation of arylthianthrenium salts allowed the expedient stitching of tryptophan-containing peptides with drug, natural product, and peptidic scaffolds by forging sterically congested biaryl linkages. The robustness of the palladium catalysis regime was reflected by the full tolerance of a plethora of sensitive and coordinating functional groups. Hence, our manifold enabled efficient access to highly decorated, labelled, conjugated, and ligated linear and cyclic peptides.

Synthesis of Arynes via Formal Dehydrogenation of Arenes

Arynes offer immense potential for diversification of benzenoid rings, which occur in pharmaceuticals, agrochemicals, and liquid crystals. However, accessing these high-energy intermediates requires synthetic precursors, which involve either harsh conditions or multistep syntheses. The development of alternative methods to access arynes using simpler substrates and milder conditions is necessary for a more streamlined approach. Here, we describe a two-step formal dehydrogenation of simple arenes to generate arynes at a remote position relative to traditionally reactive groups, e.g., halides. This approach is enabled by regioselective installation and ejection of an "onium" leaving group, and we demonstrate the compatibility of simple arenes (20 examples) and arynophiles (8 examples). Moreover, through direct comparison, we show that our formal dehydrogenation method is both more functional group tolerant and efficient in generating arynes than the current state-of-the-art aryne precursors. Finally, we show that aryne intermediates offer opportunities for regioselective C-H amination that are distinct from other methods.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Photoredox-catalyzed coupling of aryl sulfonium salts with CO2 and amines to access O-aryl carbamates

An efficient photoredox-catalyzed three-component coupling reaction of aryl sulfonium salts, carbon dioxide and amines has been developed for the first time. This reaction provides a new strategy for the synthesis of a range of valuable O-aryl carbamates from readily available arenes via a site-selective thianthrenation/carbamoyloxylation two-step process. Mild conditions, broad substrate scope and good functional group tolerance are the features of the transformation. The synthetic utility of the method was demonstrated by the late-stage modification of bioactive molecules and pharmaceuticals.

Direct Deaminative Functionalization

Selective functional group interconversions in complex molecular settings underpin many of the challenges facing modern organic synthesis. Currently, a privileged subset of functional groups dominates this landscape, while others, despite their abundance, are sorely underdeveloped. Amines epitomize this dichotomy; they are abundant but otherwise intransigent toward direct interconversion. Here, we report an approach that enables the direct conversion of amines to bromides, chlorides, iodides, phosphates, thioethers, and alcohols, the heart of which is a deaminative carbon-centered radical formation process using an anomeric amide reagent. Experimental and computational mechanistic studies demonstrate that successful deaminative functionalization relies not only on outcompeting the H-atom transfer to the incipient radical but also on the generation of polarity-matched, productive chain-carrying radicals that continue to react efficiently. The overall implications of this technology for interconverting amine libraries were evaluated via high-throughput parallel synthesis and applied in the development of one-pot diversification protocols.

Organothianthrenium salts: synthesis and utilization

Organothianthrenium salts are a class of compounds containing a positively charged sulfur atom and a neutral sulfur atom. Over the past years, organothianthrenium salts have been emerging as attractive precursors for a myriad of transformations to forge new C-C and C-X bonds due to their unique structural characteristics and chemical behaviors. The use of the thianthrenation strategy selectively transforms C-H, C-O, and other chemical bonds into organothianthrenium salts in a predictable manner, providing a straightforward alternative for regioselective functionalizations for arenes, alkenes, alkanes, alcohols, amines and so on through diverse reaction mechanisms under mild conditions. In this review, the preparation of different organothianthrenium salts is summarized, including aryl, alkenyl and alkyl thianthrenium salts. Moreover, the utilization of organothianthrenium salts in different catalytic processes and their synthetic potentials are also discussed.

Meerwein-type Bromoarylation with Arylthianthrenium Salts

Herein, we report a photocatalyzed Meerwein-type bromoarylation of alkenes with stable arylthianthrenium salts, formed by site-selective C-H thianthrenation. This protocol can be applied to late-stage functionalization of a variety of biomolecules that are difficult to access by other aryl coupling reagents. Halogen introduction allows for a variety of follow-up transformations, affording numerous biologically active skeletons.

Synthesis of Polycyclic Hetero-Fused 7-Deazapurine Heterocycles and Nucleosides through C-H Dibenzothiophenation and Negishi Coupling

A new approach for synthesizing polycyclic heterofused 7-deazapurine heterocycles and the corresponding nucleosides was developed based on C–H functionalization of diverse (hetero)aromatics with dibenzothiophene-S-oxide followed by the Negishi cross-cooupling with bis(4,6-dichloropyrimidin-5-yl)zinc. This cross-coupling afforded a series of (het)aryl-pyrimidines that were converted to fused deazapurine heterocycles through azidation and thermal cyclization. The fused heterocycles were glycosylated to the corresponding 2′-deoxy- and ribonucleosides, and a series of derivatives were prepared by nucleophilic substitutions at position 4. Four series of new polycyclic thieno-fused 7-deazapurine nucleosides were synthesized using this strategy. Most of the deoxyribonucleosides showed good cytotoxic activity, especially for the CCRF-CEM cell line. Phenyl- and thienyl-substituted thieno-fused 7-deazapurine nucleosides were fluorescent, and the former one was converted to 2′-deoxyribonucleoside triphosphate for enzymatic synthesis of labeled oligonucleotides.

Two Paths to Oxidative C-H Amination Under Basic Conditions: A Theoretical Case Study Reveals Hidden Opportunities Provided by Electron Upconversion

Traditionally, cross-dehydrogenative coupling (CDC) leads to C-N bond formation under basic and oxidative conditions and is proposed to proceed via a two-electron bond formation mediated by carbenium ions. However, the formation of such high-energy intermediates is only possible in the presence of strong oxidants, which may lead to undesired side reactions and poor functional group tolerance. In this work we explore if oxidation under basic conditions allows the formation of three-electron bonds (resulting in "upconverted" highly-reducing radical-anions). The benefit of this "upconversion" process is in the ability to use milder oxidants (e. g., O₂ ) and to avoid high-energy intermediates. Comparison of the two- and three-electron pathways using quantum mechanical calculations reveals that not only does the absence of a strong oxidant shut down two-electron pathways in favor of a three-electron path but, paradoxically, weaker oxidants react faster with the upconverted reductants by avoiding the inverted Marcus region for electron transfer.

Highly Efficient and para-Selective C-H Functionalization of Polystyrene Providing a Versatile Platform for Diverse Applications

Postpolymerization modification of polystyrene (PS) can afford numerous value-added materials with different functions and applications, but it has been hampered by the lack of efficient methods. We report herein a highly efficient and para-selective conversion of the C-H bonds of the aromatic ring of PS into diverse functional groups using a combination of thianthrenation and thio-Suzuki-Miyaura coupling reaction. Notably, the thianthrenation efficiency of PS is as high as 99% and the degree of thianthrenation can be conveniently controlled using stoichiometric tuning of the amount of thianthrene-S-oxide added, resulting in 24-99 mol % thianthrenation. In the subsequent thio-Suzuki-Miyaura coupling reaction, 18 functionalized PS containing various functional groups (-CH₂OH, -OMe, -SMe, -OTBS, -CH₃, -NHBoc, -OCOMe, -CHO, -COMe, -Si(Me)₃, etc.) were successfully prepared with a high degree of functionalization (64-99 mol %). The obtained functionalized PS can be readily converted into diverse functional materials, including solid-phase synthesis resins, aggregation-induced emission fluorophores, as well as ionomer binders and ion-exchange membranes for energy conversion devices. This method imparts diverse functionality onto PS with extremely high efficiency and selectivity, providing a versatile platform to transform existing commodity PS plastics into high-performance materials.

Palladium-Catalyzed Synthesis of Esters from Arenes through C-H Thianthrenation

The efficient palladium-catalyzed synthesis of esters from readily available arenes has been developed. These C-H bond esterifications were achieved relying on the regioselective thianthrenation to generate the aryl-TT salts, which were treated as reactive electrophilic substrates to couple with phenol formate and N-hydroxysuccinimide (NHS) formate giving access to phenol esters and NHS esters, respectively, in the absence of carbon monoxide. A wide range of functional esters could be prepared with high efficiency under this redox-neutral palladium-catalytic condition. Late-stage functionalization and investigations of synthetic applications demonstrated the potential application of the established platform and these products.

Palladium-Catalyzed Synthesis of C-1 Deuterated Aldehydes from (Hetero) Arenes Mediated by C (sp2)-H Thianthrenation

A palladium-catalyzed deuterated formylation of aryl sulfonium salts is prepared conveniently from readily available arenes, which enables the expedient synthesis of a series of structurally diverse C-1 deuterated aldehydes with 96%-99% deuterium incorporation. The easy to handle and cost-effective DCOONa provides a deuterium source, which can be introduced onto the formyl units with excellent selectivity under the palladium-catalytic redox neutral conditions. This catalytic route can accomplish the direct late-stage C-H functionalization of bioactive molecules and natural product derivatives assisted by C (sp²)-H thianthrenation. Moreover, on the basis of this practical approach, several deuterated drugs and analogues could be prepared with excellent levels of deuterium incorporation.

Recent Advances in Thianthrenation/Phenoxathiination Enabled Site-Selective Functionalization of Arenes

Site-selective functionalization of simple arenes remains a paramount challenge due to the similarity of multiple C–H bonds in the same molecule with similar steric environment and electronic properties. Recently, the site-selective thianthrenation/phenoxathiination of arenes has become an attractive solution to reach this challenging goal and it has been applied in the late-stage functionalization of various bioactive molecules. This short review aims to summarize recent advances in the site-selective C–H functionalization of arenes via aryl thianthrenium salts, as well as mechanistic insights in the remarkable site-selectivity obtained in thianthrenation step.

1 Introduction

2 Site-Selective Thianthrenation of Arenes and Mechanistic Insight

3 Thianthrenation-Enabled Site-Selective Functionalization of Arenes

3.1 Thianthrenation-Enabled C(sp 2)–C Bond Formation Reaction

3.2 Thianthrenation-Enabled C(sp 2)–X Bond Formation Reaction

4 Conclusion and Outlook

Visible-light-induced defluorinative carbonylative coupling of alkyl iodides with α-trifluoromethyl substituted styrenes

A visible-light-mediated defluorinative carbonylative cross-coupling of alkyl iodides with α-trifluoromethyl styrenes has been developed. The reaction occurs at room temperature under blue light irradiation, and various gem-difluoroalkenes were obtained in moderate to good yields. Synthetic transformations of the obtained product were performed as well.

Persulfate-nitrogen doped graphene mixture as an oxidant for the synthesis of 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters and nitrite

A series of 3-nitro-4-aryl-2H-chromen-2-ones in good yields have directly been obtained from aryl alkynoate esters and nitrite by employing a mixture of K₂S₂O₈-nitrogen doped graphene as an oxidant in a watery medium at room temperature. A plausible mechanism for the reaction is also reported. It reveals that the product is formed through a cascade of nitro radical addition, spirocyclization, and ester migration. When compared to known methods for the synthesis of 3-nitro-4-aryl-2H-chromen-2-ones from aryl alkynoate esters, this protocol is environmentally friendly, sustainable, practical and energy efficient and does not use a harmful nitro source. Furthermore, nitrogen doped graphene used in this approach can be easily recovered and reused at least four times without losing its activity.

Visible-light-catalyzed C-H arylation of (hetero)arenes via arylselenonium salts

A novel photo-induced C-H arylation of (hetero)arenes has been developed. Aryl selenonium salts as an aryl source led to the arylation of aromatic (hetero)cyclic compounds via C-Se bond activation under blue LED irradiation. The method simply utilizes the safe and clean energy source and yields a range of site-selective biphenyl or bi-heterocyclic products in medium to good yields. Furthermore, the borylation and Sonogashira coupling of aryl selenonium salts proceed in good yields as well. From the results, it is shown that selenonium salts are more reactive than sulfonium salts.

Sustainable Thioetherification via Electron Donor-Acceptor Photoactivation Using Thianthrenium Salts

The synthesis of sulfides has been widely studied because this functional subunit is prevalent in biomolecules and pharmaceuticals, as well as being a useful synthetic platform for further elaboration. Thus, various methods to build C-S bonds have been developed, but typically they require the use of precious metals or harsh conditions. Electron donor-acceptor (EDA) complex photoactivation strategies have emerged as versatile and sustainable ways to achieve C-S bond formation, avoiding challenges associated with previous methods. This work describes an open-to-air, photoinduced, site-selective C-H thioetherification from readily available reagents via EDA complex formation that tolerates a wide range of different functional groups. Moreover, C(sp² )-halogen bonds remain intact using this protocol, allowing late-stage installation of the sulfide motif in various bioactive scaffolds, while allowing yet further modification through more traditional C-X bond cleavage protocols. Additionally, various mechanistic investigations support the envisioned EDA complex scenario.

A general electron donor-acceptor complex for photoactivation of arenes via thianthrenation

General photoactivation of electron donor–acceptor (EDA) complexes between arylsulfonium salts and 1,4-diazabicyclo[2.2.2]octane with visible light or natural sunlight was discovered. This practical and efficient mode enables the production of aryl radicals under mild conditions, providing an unrealized opportunity for two-step para-selective C–H functionalization of complex arenes. The novel mode for generating aryl radicals via an EDA complex was well supported by UV-vis absorbance measurements, nuclear magnetic resonance titration experiments, and density functional theory (DFT) calculations. The method was applied to the regio- and stereo-selective arylation of various N-heterocycles under mild conditions, yielding an assembly of challengingly linked heteroaryl–(hetero)aryl products. Remarkably, the meaningful couplings of bioactive molecules with structurally complex drugs or agricultural pharmaceuticals were achieved to display favorable in vitro antitumor activities, which will be of great value in academia or industry.

General photoactivation of EDA complexes between arylsulfonium salts and 1,4-diazabicyclo[2.2.2]octane was discovered. This practical mode enables the generation of aryl radicals for C–H functionalization of arenes.

Sulfonium cations as versatile strongly π-acidic ligands

More than a century old, sulfonium cations are still intriguing species in the landscape of organic chemistry. On one hand they have found broad applications in organic synthesis and materials science, but on the other hand, while isoelectronic to the ubiquitous tertiary phosphine ligands, their own coordination chemistry has been neglected for the last three decades. Here we report the synthesis and full characterization of the first Rh(i) and Pt(ii) complexes of sulfonium. Moreover, for the first time, coordination of an aromatic sulfonium has been established. A thorough computational analysis of the exceptionally short S–Rh bonds obtained attests to the strongly π-accepting nature of sulfonium cations and places them among the best π-acceptor ligands available today. Our calculations also show that embedding within a pincer framework enhances their π-acidity even further. Therefore, in addition to the stability and modularity that these frameworks offer, our pincer complexes might open the way for sulfonium cations to become powerful tools in π-acid catalysis.

Back to the scene: while isolobal to the ubiquitous tertiary phosphines, sulfonium cations as ligands were neglected for decades. This work revives the coordination chemistry of these species showing their potential as ligands for π-acid catalysis.

Thianthrenium-Enabled Sulfonylation via Electron Donor-Acceptor Complex Photoactivation

Sulfone-containing compounds are prevalent building blocks in pharmaceuticals and other biomolecules, and they serve as key intermediates in the synthesis of complex scaffolds. During the past decade, several methods have been developed to access sulfones. These strategies, however, require the use of strong reaction conditions, limiting their substrate scope. Recently, visible light-mediated transformations have emerged as novel platforms to access unprecedented structural motifs. This report demonstrates a thianthrenium-enabled sulfonylation via intra-complex charge transfer to generate transient aryl- and persistent sulfonyl radicals that undergo selective coupling to generate alkyl- and (hetero)aryl sulfones under ambient conditions. Importantly, this strategy allows retention of halide handles, presenting a complementary approach to transition metal-mediated photoredox couplings. Furthermore, this sulfonylation allows high functional group tolerance and is amenable to late-stage functionalization of complex biomolecules. Mechanistic investigations support the intermediacy of electron donor-acceptor (EDA) complexes.

Torsional strain inversed chemoselectivity in a Pd-catalyzed atroposelective carbonylation reaction of dibenzothiophenium

A palladium-catalyzed enantioselective ring-opening/carbonylation of cyclic diarylsulfonium salts is reported. In comparison to thioethers, the sulfonium salts displayed high reactivity and enabled the reaction to be performed under mild conditions (room temperature). The steric repulsion of the two non-hydrogen substituents adjacent to the axis led cyclic diarylsulfonium salts to be distorted, which enabled the ring-opening reaction to proceed with significant preference for breaking the exocyclic C-S bond.

Application of α-Aminoalkyl Radicals as Reaction Activators

α-Aminoalkyl radicals are easily accessible through multiple pathways from various precursors. Apart from their utilization as nitrogen-containing building blocks, they have recently been used as halogen atom abstraction reagents or single-electron reductants to transform organic halides or sulfonium salts into their corresponding highly reactive radical species. Benefiting from the richness of various halides and the diverse reactivity of radical intermediates, new transformations of halides and sulfonium salts have been developed. This short review summarizes this emerging chemistry that uses α-aminoalkyl radicals as the reaction activators.

1 Introduction

2 Activation of Halides as Halogen-Atom Transfer Agents

2.1 Addition to Unsaturated Bonds

2.1.1 Addition to C=C Bonds

2.1.2 Addition to C=O Bonds

2.2 Substitution Reactions

2.2.1 Deuteration

2.2.2 Olefination

2.2.3 Allylation

2.2.4 Aromatic Substitution

2.2.5 Amination

3 Activation of Sulfonium Salts as Single-Electron Reductants

4 Conclusion and Outlook

A Perspective on Late-Stage Aromatic C-H Bond Functionalization

Late-stage functionalization of C–H bonds (C–H LSF) can provide a straightforward approach to the efficient synthesis of functionalized complex molecules. However, C–H LSF is challenging because the C–H bond must be functionalized in the presence of various other functional groups. In this Perspective, we evaluate aromatic C–H LSF on the basis of four criteria—reactivity, chemoselectivity, site-selectivity, and substrate scope—and provide our own views on current challenges as well as promising strategies and areas of growth going forward.

Metal-free allylic C-H nitrogenation, oxygenation, and carbonation of alkenes by thianthrenation

Selective functionalization of allylic C–H bonds into other chemical bonds is among the most straightforward and attractive, yet challenging transformations. Herein, a transition-metal-free protocol for direct allylic C–H nitrogenation, oxygenation, and carbonation of alkenes by thianthrenation was developed. This operationally simple protocol allows for the unified allylic C–H amination, esterification, etherification, and arylation of vinyl thianthrenium salts. Notably, the reaction furnishes multialkyl substituted allylic amines, ammonium salts, sulfonyl amides, esters, and ethers in good yields. The reaction proceeds under mild conditions with excellent functional group tolerance and could be applied to late-stage allylation of natural products, drug molecules and peptides with excellent chemoselectivity.

Diverse functionalizations of allylic C–H bonds of alkenes by thianthrenation have been demonstrated, featuring Z-selectivity to afford multi-alkyl substituted allylic esters, thioesters, ethers, amines, amides and arenes under metal-free conditions.

Indole-ynones as Privileged Substrates for Radical Dearomatizing Spirocyclization Cascades

Indole-ynones have been established as general substrates for radical dearomatizing spirocyclization cascade reactions. Five distinct and varied synthetic protocols have been developed─cyanomethylation, sulfonylation, trifluoromethylation, stannylation and borylation─using a variety of radical generation modes, ranging from photoredox catalysis to traditional AIBN methods. The simple and easily prepared indole-ynones can be used to rapidly generate diverse, densely functionalized spirocycles and have the potential to become routinely used to explore radical reactivity. Experimental and computational investigations support the proposed radical cascade mechanism and suggest that other new methods are now primed for development.

Regioselective and late-stage polyfluoroarylation of arenes with diverse fluoroaryl nucleophiles via Pd-catalysis

Dehydrogenative coupling of arenes with fluoroaryl nucleophiles enables the efficient synthesis of fluoroaromatic compounds from readily available starting materials.

Alkylsulfonium salts for the photochemical desulphurizative functionalization of heteroarenes

A metal-free organic photoredox-catalyzed alkylation of heteroarenes using alkylsulfonium salts as alkylation reagents has been developed.

18F-Fluorination: Challenge and Opportunity for Organic Chemists

18F-fluorination is an important and growing field in organic synthesis that has attracted many chemists in the recent past. Here we present our own, biased perspective with a focus on our own chemistry that evaluates recent advances in the field and provides our opinion on the challenges for the development of new chemistry, so that it may have an impact on imaging. We hope that the manuscript will provide a useful guide to chemists to develop reliable and robust reaction chemistry suitable for radiofluorination to have a real impact on human health.

High Site Selectivity in Electrophilic Aromatic Substitutions: Mechanism of C-H Thianthrenation

The introduction of thianthrene as a linchpin has proven to be a versatile strategy for the C–H functionalization of aromatic compounds, featuring a broad scope and fast diversification. The synthesis of aryl thianthrenium salts has displayed an unusually high para regioselectivity, notably superior to those observed in halogenation or borylation reactions for various substrates. We report an experimental and computational study on the mechanism of aromatic C–H thianthrenation reactions, with an emphasis on the elucidation of the reactive species and the nature of the exquisite site selectivity. Mechanisms involving a direct attack of arene to the isolated O-trifluoracetylthianthrene S-oxide (TT⁺-TFA) or to the thianthrene dication (TT²⁺) via electron transfer under acidic conditions are identified. A reversible interconversion of the different Wheland-type intermediates before a subsequent, irreversible deprotonation is proposed to be responsible for the exceptional para selectivity of the reaction.

Visible-Light-Mediated C-I Difluoroallylation with an α-Aminoalkyl Radical as a Mediator

Herein, we report a protocol for direct visible-light-mediated C-I difluoroallylation reactions of α-trifluoromethyl arylalkenes with alkyl iodides at room temperature with an α-aminoalkyl radical as a mediator. The protocol permits efficient functionalization of various α-trifluoromethyl arylalkenes with cyclic and acyclic primary, secondary, and tertiary alkyl iodides and is scalable to the gram level. This mild protocol uses an inexpensive mediator and is suitable for late-stage functionalization of complex natural products and drugs.

Late-Stage Heteroarylation of Hetero(aryl)sulfonium Salts Activated by α-Amino Alkyl Radicals

We report a late‐stage heteroarylation of aryl sulfonium salts through activation with α‐amino alkyl radicals in a mechanistically distinct approach from previously reported halogen‐atom transfer (XAT). The new mode of activation of aryl sulfonium salts proceeds in the absence of light and photoredox catalysts, engaging a wide range of hetarenes. Furthermore, we demonstrate the applicability of this methodology in synthetically useful cross‐coupling transformations.