Nickel Meets Aryl Thianthrenium Salts: Ni(I)-Catalyzed Halogenation of Arenes

Copper-Catalyzed Difluoromethylation of Alkenyl Thianthrenium Salts

We have developed a novel and straightforward protocol that facilitates the transformation of alkenylsulfonium salts leading to the direct synthesis of E-difluoromethylated alkenes. The success of this method relies on the use of copper catalysis and Vicic-Mikami reagent (DMPU)2Zn(CF2H)2. These mild protocols offer the advantage of selectively synthesizing either aromatic or aliphatic difluoromethylated alkenes. Furthermore, our methodology extends to the perfluoroalkylation of alkenylsulfonium salts. Notably, this approach is conducive to large-scale synthesis and holds promise for diverse applications.

BINAP-Accelerated Copper-Mediated Photochemical Late-Stage Trifluoromethylation of Arenes

Site-selective and late-stage C-H trifluoromethylation (including pentafluoroethylation) of arylthianthrenium salts using in situ generated CuCF3 from Ruppert-Prakash reagent under visible light was found to be significantly accelerated by a catalytic amount of rac-BINAP, allowing the reaction to complete within 15 min under very mild conditions without using any additional photoredox catalysts. The process showed a broad substrate scope, high efficiency, and excellent regioselectivity, which holds the promise to accelerate the discovery of fluorinated medicinal compounds.

Migratory Aryl Cross-Coupling

A fundamental property of cross-coupling reactions is regiospecificity, meaning that the site of bond formation is determined by the leaving group's location on the electrophile. Typically, achieving a different substitution pattern requires the synthesis of a new, corresponding starting-material isomer. As an alternative, we proposed the development of cross-coupling variants that would afford access to multiple structural isomers from the same coupling partners. Here, we first demonstrate that a bulky palladium catalyst can facilitate the efficient, reversible transposition of aryl halides by temporarily forming metal aryne species. Despite the nearly thermoneutral equilibrium governing this process, combining it with the gradual addition of a suitable nucleophile results in dynamic kinetic resolution of the isomeric intermediates and high yields of unconventional product isomers. The method accommodates a range of oxygen- and nitrogen-centered nucleophiles and tolerates numerous common functional groups. A Curtin-Hammett kinetic scheme is supported by computational and experimental data, providing a general mechanistic framework for extending this migratory cross-coupling concept.

Recent Applications of Sulfonium Salts in Synthesis and Catalysis

The use of sulfonium salts in organic synthesis has experienced a dramatic increase during the last years that can arguably be attributed to three main factors; the development of more direct and efficient synthetic methods that make easily available sulfonium reagents of a wide structural variety, their intrinsic thermal stability, which facilitates their structural modification, handling and purification even on large scale, and the recognition that their reactivity resembles that of hypervalent iodine compounds and therefore, they can be used as replacement of such reagents for most of their uses. This renewed interest has led to the improvement of already existing reactions, as well as to the discovery of unprecedented transformations; in particular, by the implementation of photocatalytic protocols. This review aims to summarize the most recent advancements on the area focusing on the work published during and after 2020. The scope of the methods developed will be highlighted and their limitations critically evaluated.

A Paramagnetic Nickel-Zinc Hydride Complex

Reaction of a molecular zinc-hydride [{(ArNCMe)2CH}ZnH] (Ar=2,6-di-isopropylphenyl) with 0.5 equiv. of [Ni(CO)Cp]2 led to the isolation of a nickel-zinc hydride complex containing a bridging 3-centre,2-electron Ni-H-Zn interaction. This species has been characterized in the solid-state by single crystal X-ray diffraction. DFT calculations are consistent with its formulation as a σ-complex derived from coordination of the zinc-hydride to a paramagnetic nickel(I) fragment. Continuous-wave and pulse EPR experiments suggest that this species is labile in solution. Further experiments show that in the presence of catalytic quantities of nickel(I) precursors, zinc-hydride bonds can undergo either H/D-exchange with D2 or dehydrocoupling to form Zn-Zn bonds. In combination, the data support the activation and functionalisation of zinc-hydride bonds at nickel(I) centres.

Photoredox-Catalyzed Three-Component Construction of Aryl Sulfonyl Fluoride Using KHF2: Late-Stage Drug Fluorosulfonylation

Aryl sulfonyl fluorides are prominently featured in organic synthesis and medicinal chemistry. Herein, a metal-free photoredox-catalyzed three-component assembly of aryl sulfonyl fluoride via aryl sulfonyl ammonium salt intermediate has been reported. A variety of structurally diverse aryl sulfonyl fluorides were synthesized rapidly from dibenzothiophenium (DBT) salts under mild conditions by using KHF2 as the fluorine source. Notably, this methodology can be employed as an efficient and sustainable approach for late-stage drug fluorosulfonylation. Good yields and broad functionality tolerance were the features of this methodology. Moreover, the derivatization of aryl sulfonyl fluoride molecules was also demonstrated to showcase its synthetic utility.

Palladium-Catalyzed Cross-Electrophile Couplings of Aryl Thianthrenium Salts with Aryl Bromides via C-S Bond Activation

We report here a step-economic and cost-effective cross-electrophile coupling of aryl thianthrenium salts with widely available aryl bromides, which proceeded effectively via C-S bond activation at ambient temperature in THF in the presence of a palladium catalyst, magnesium turnings, and lithium chloride to enable the facile assembly of a wide array of structurally diverse biaryls in modest to good yields with good functional group compatibility. In addition, the gram-scale reaction could also be realized.

Alkylation of Glycine Derivatives through a Synergistic Single-Electron Transfer and Halogen-Atom Transfer Process

Here, we present a versatile method for forming C(sp3)-C(sp3) bonds, enabling the synthesis of a range of natural and non-natural amino acids. This approach utilizes readily available glycine derivatives and alkyl iodides, combining single-electron transfer and halogen-atom transfer processes. The utility of this step-economic and redox-economic C(sp3)-C(sp3) bond formation is further highlighted in the late-stage site-selective modifications of the glycine residue in short peptides.

Hydrodeuteroalkylation of Unactivated Olefins Using Thianthrenium Salts

Isotopically labeled alkanes play a crucial role in organic and pharmaceutical chemistry. While some deuterated methylating agents are readily available, the limited accessibility of other deuteroalkyl reagents has hindered the synthesis of corresponding products. In this study, we introduce a nickel-catalyzed system that facilitates the synthesis of various deuterium-labeled alkanes through the hydrodeuteroalkylation of d2-labeled alkyl TT salts with unactivated alkenes. Diverging from traditional deuterated alkyl reagents, alkyl thianthrenium (TT) salts can effectively and selectively introduce deuterium at α position of alkyl chains using D2O as the deuterium source via a single-step pH-dependent hydrogen isotope exchange (HIE). Our method allows for high deuterium incorporation, and offers precise control over the site of deuterium insertion within an alkyl chain. This technique proves to be invaluable for the synthesis of various deuterium-labeled compounds, especially those of pharmaceutical relevance.

Discovery of N-X anomeric amides as electrophilic halogenation reagents

Electrophilic halogenation is a widely used tool employed by medicinal chemists to either pre-functionalize molecules for further diversity or incorporate a halogen atom into drugs or drug-like compounds to solve metabolic problems or modulate off-target effects. Current methods to increase the power of halogenation rely on either the invention of new reagents or activating commercially available reagents with various additives such as Lewis or Brønsted acids, Lewis bases and hydrogen-bonding activators. There is a high demand for new reagents that can halogenate otherwise unreactive compounds under mild conditions. Here we report the invention of a class of halogenating reagents based on anomeric amides, taking advantage of the energy stored in the pyramidalized nitrogen of N-X anomeric amides as a driving force. These robust halogenating methods are compatible with a variety of functional groups and heterocycles, as exemplified on over 50 compounds (including 13 gram-scale examples and 1 flow chemistry scale-up).

Thianthrene/TfOH-catalyzed electrophilic halogenations using N-halosuccinimides as the halogen source

Organohalides are vital organic building blocks with applications spanning various fields. However, direct halogenation of certain neutral or unreactive substrates by using solely the regular halogenating reagents has proven challenging. Although various halogenation approaches via activating halogenating reagents or substrates have emerged, a catalytic system enabling broad substrate applicability and diverse halogenation types remains relatively underexplored. Inspired by the halogenation of arenes via thianthrenation of arenes, here we report that thianthrene, in combined use with trifluoromethanesulfonic acid (TfOH), could work as an effective catalytic system to activate regular halogenating reagents (NXS). This new protocol could accomplish multiple types of halogenation of organic compounds including aromatics, olefins, alkynes and ketones. The mechanism study indicated that a highly reactive electrophilic halogen thianthrenium species, formed in situ from the reaction of NXS with thianthrene in the presence of TfOH, was crucial for the efficient halogenation process.

Palladium-Catalyzed Hiyama-Type Coupling of Thianthrenium and Phenoxathiinium Salts

Here, we demonstrate palladium-catalyzed Hiyama-type cross-coupling reactions of aryl thianthrenium or phenoxathiinium salts. By employing stable and inexpensive organosilanes, the arylation, alkenylation, and alkynylation were realized in high efficiency using commercially available Pd(tBu3P)2 as the catalyst, thus providing a reliable method for preparation of biaryls, styrenes, and aryl acetylenes with a broad functional group tolerance under mild conditions. Given the accessibility of aryl thianthrenium or phenoxathiinium salts from simple arenes in a remarkable regioselective fashion, this protocol also provides an attractive approach for the late-stage modification of complex bioactive scaffolds.

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.

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.

Metallaphotoredox deuteroalkylation utilizing thianthrenium salts

Deuterium labeling compounds play a crucial role in organic and pharmaceutical chemistry. The synthesis of such compounds typically involves deuterated building blocks, allowing for the incorporation of deuterium atoms and functional groups into a target molecule in a single step. Unfortunately, the limited availability of synthetic approaches to deuterated synthons has impeded progress in this field. Here, we present an approach utilizing alkyl-substituted thianthrenium salts that efficiently and selectively introduce deuterium at the α position of alkyl chains through a pH-dependent HIE process, using D2O as the deuterium source. The resulting α-deuterated alkyl thianthrenium salts, which bear two deuterium atoms, exhibit excellent selectivity and deuterium incorporation in electrophilic substitution reactions. Through in situ formation of isotopically labelled alkyl halides, these thianthrenium salts demonstrate excellent compatibility in a series of metallaphotoredox cross-electrophile coupling with (hetero)aryl, alkenyl, alkyl bromides, and other alkyl thianthrenium salts. Our technique allows for a wide range of substrates, high deuterium incorporation, and precise control over the site of deuterium insertion within a molecule such as the benzyl position, allylic position, or any alkyl chain in between, as well as neighboring heteroatoms. This makes it invaluable for synthesizing various deuterium-labeled compounds, especially those with pharmaceutical significance.

Photoinduced Regioselective Fluorination and Vinylation of Remote C(sp3)-H Bonds Using Thianthrenium Salts

Herein, a photoredox-driven practical protocol for fluorinated alkene synthesis using easily accessible and modular thianthrenium salts with electron-withdrawing alkynes or propargyl alcohols is reported. Vinyl radical intermediates, formed by the reaction between the alkyl or trifluoromethyl thianthrenium salts and electronically diverse alkynes, can mediate the key 1,5-HAT process of regioselective C(sp3)-H fluorination and vinylation. This protocol provides straightforward access to structurally diverse trifluoromethyl- or distally fluoro-functionalized alkene products in 21-79% yields with a broad substrate range under mild photocatalytic conditions.

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.

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.

Chemo-divergent Cyano Group Migration: Involving Elimination and Substitution of the Key α-Thianthrenium Cyano Species

Herein, we report a light-driven, radical-type cyano migration in the absence of a photocatalyst, enabling a chemo-divergent synthesis of (Z)-alkenyl nitriles and ketones. Trifluoromethyl thianthrenium salt (TT-CF3+OTf-) plays multiple roles: (a) absorbing light to generate trifluoromethyl radicals to initiate the reaction and (b) forming α-thianthrenium cyano species by in situ capture of TT• +. (Z)-Alkenyl nitriles were formed through the elimination of thianthrenium salts, and aryl ketones were obtained via the nucleophilic substitution of thianthrenium salts.

Synthesis of Nickel(I)-Bromide Complexes via Oxidation and Ligand Displacement: Evaluation of Ligand Effects on Speciation and Reactivity

Nickel's +1 oxidation state has received much interest due to its varied and often enigmatic behavior in increasingly popular catalytic methods. In part, the lack of understanding about NiI results from common synthetic strategies limiting the breadth of complexes that are accessible for mechanistic study and catalyst design. We report an oxidative approach using tribromide salts that allows for the generation of a well-defined precursor, [NiI(COD)Br]2, as well as several new NiI complexes. Included among them are complexes bearing bulky monophosphines, for which structure-speciation relationships are established and catalytic reactivity in a Suzuki-Miyaura coupling (SMC) is investigated. Notably, these routes also allow for the synthesis of well-defined monomeric t-Bubpy-bound NiI complexes, which has not previously been achieved. These complexes, which react with aryl halides, can enable previously challenging mechanistic investigations and present new opportunities for catalysis and synthesis.

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.