Pyrylium Salts: Selective Reagents for the Activation of Primary Amino Groups in Organic Synthesis

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling

A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

Synthesis of Collidine from Dinitrogen via a Tungsten Nitride

The synthesis of pyridines from dinitrogen in homogeneous solution is known to be challenging considering that an N2 cleavage step needs to be combined with two N-C coupling steps. Herein, a tungsten complex bearing a tailor-made 2,2'-(tBu2As)2-substituted tolane ligand scaffold was shown to split N2 to afford the corresponding tungsten nitride, which is not the case for the corresponding (iPr2As)2-substituted derivative. The former nitride was then reacted with 2,4,6-trimethylpyrylium triflate, which led to the formation of a tungsten oxo complex, along with collidine. Over the course of this reaction, the O atom of the pyrylium starting material was replaced with an N atom via a hitherto unprecedented skeletal editing process.

Visible-Light-Induced Photoredox 1,2-Dialkylation of Styrenes with α-Carbonyl Alkyl Bromides and Pyridin-1-ium Salts

A visible-light-driven photoredox dialkylation of styrenes with α-carbonyl alkyl bromides and pyridin-1-ium salts for the synthesis of polysubstituted 1,4-dihydropyridines is reported. This reaction enables the formation of two new C(sp3)-C(sp3) bonds in a single reaction step and provides a strategy that employs pyridin-1-ium salts as the functionalized alkylating reagents via dearomatization to directly trap the resulting alkyl radicals from radical addition of alkenes and then terminate the alkene dialkylation.

2,2'-Biquinoline-Based Recyclable Electroauxiliaries for the Generation of Alkyl Radicals via C-C Bond Cleavage

Alkyl radical precursors are essential for a wide variety of photocatalytic and 3d-metal-catalyzed C-C bond forming reactions. Neutral organic heterocycles as electroauxiliaries such as 4-alkyl Hantzsch esters have become reliable tools for alkyl radical formation. Here we show that 2,2'-biquinoline-derived alkyl-substituted dihydroquinolines act as competent radical precursors with the ability to form primary, secondary and tertiary alkyl radicals. Hydroalkylation of benzalmalononitriles and N-Boc protected diazenes has been achieved through copper catalysis under mild conditions of 50 °C with good to very good yields of up to 85 %. Furthermore, the dihydroquinolines' reactivity towards a denitrative alkylation of nitroolefins such as β-nitrostyrene was discovered. Most importantly, the released biquinoline can be recycled, which greatly improves the overall atom-economy of these alkyl radical precursors in comparison to previous N-heterocyclic electroauxiliaries.

Harnessing Sulfur(VI) Fluoride Exchange Click Chemistry and Photocatalysis for Deaminative Benzylic Arylation

While among the most common functional handles present in organic molecules, amines are a widely underutilized linchpin for C-C bond formation. To facilitate C-N bond cleavage, large activating groups are typically used but result in the generation of stoichiometric amounts of organic waste. Herein, we report an atom-economic activation of benzylic primary amines relying on the Sulfur(VI) Fluoride Exchange (SuFEx) click chemistry and the aza-Ramberg-Bäcklund reaction. This two-step sequence allows the high-yielding generation of 1,2-dialkyldiazenes from primary amines via loss of SO2. Excitation of the diazenes with blue light and an Ir photocatalyst affords radical pairs upon expulsion of N2, which can be coaxed into the formation of C(sp3)-C(sp2) bonds upon diffusion and capture by a Ni catalyst. This arylative strategy relying on a traceless click approach was harnessed in a variety of examples and its mechanism was investigated.

Catalytic photochemical enantioselective α-alkylation with pyridinium salts

We have developed a chiral amine catalyzed enantioselective α-alkylation of aldehydes with amino acid derived pyridinium salts as alkylating reagents. The reaction proceeds in the presence of visible light and in the absence of a photocatalyst via a light activated charge-transfer complex. We apply this photochemical stereoconvergent process to the total synthesis of the lignan natural products (-)-enterolactone and (-)-enterodiol. Mechanistic studies support the ground-state complexation of the reactive components followed by divergent charge-transfer processes involving catalyst-controlled radical chain and in-cage radical combination steps.

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.

Mechanistic insights into reductive deamination with hydrosilanes catalyzed by B(C6F5)3: A DFT study

Selective defunctionalization of synthetic intermediates is a valuable approach in organic synthesis. Here, we present a theoretical study on the recently developed B(C6F5)3/hydrosilane-mediated reductive deamination reaction of primary amines. Our computational results provide important insights into the reaction mechanism, including the active intermediate, the competing reactions of the active intermediate, the role of excess hydrosilane, and the origin of chemoselectivity. Moreover, the study on the substituent effect of hydrosilane indicated a potential way to improve the efficiency of the reductive deamination reaction.

Site-Selective Synthesis of N-Benzyl 2,4,6-Collidinium Salts by Electrooxidative C-H Functionalization

N-alkylpyridinium salts are versatile pseudohalides for SET-mediated cross couplings. However, the common 2,4,6-triphenylpyridinium salt is plagued by poor atom economy and high cost of synthesis. Thus, there is a growing need for more practical scaffolds and innovative strategies for pyridinium salt formation. Herein, we report the synthesis of benzylic 2,4,6-collidinium salts via electrooxidative C-H functionalization. This method provides a complementary approach to tradtional strategies relying on substitution and condensation of prefunctionalized substrates.

Traceless Benzylic C-H Amination via Bifunctional N-Aminopyridinium Intermediates

C-H amination reactions provide the opportunity to streamline the synthesis of nitrogen-containing organic small molecules. The impact of intermolecular C-H amination methods, however, is currently limited the frequent requirement for the amine precursors to bear activating groups, such as N-sulfonyl substituents, that are both challenging to remove and not useful synthetic handles for subsequent derivatization. Here, we introduce traceless nitrogen activation for C-H amination-which enables application of selective C-H amination chemistry to the preparation of diverse N-functionalized products-via sequential benzylic C-H N-aminopyridylation followed by Ni-catalyzed C-N cross-coupling with aryl boronic acids. Unlike many C-H amination reactions that provide access to protected amines, the current method installs an easily diversifiable synthetic handle that serves as a lynchpin for C-H amination, deaminative N-N functionalization sequences.

N-Aminopyridinium reagents as traceless activating groups in the synthesis of N-Aryl aziridines

N-functionalized aziridines, which are both useful intermediates and important synthetic targets, can be envisioned as arising from the addition of nitrenes (i.e., NR fragments) to olefinic substrates. The exceptional reactivity of most nitrenes, in particular with respect to unimolecular decomposition, prevents general application of nitrene-transfer to the synthesis of N-functionalized aziridines. Here we demonstrate N-aryl aziridine synthesis via 1) olefin aziridination with N-aminopyridinium reagents to afford N-pyridinium aziridines followed by 2) Ni-catalyzed C-N cross-coupling of the N-pyridinium aziridines with aryl boronic acids. The N-pyridinium aziridine intermediates also participate in ring-opening chemistry with a variety of nucleophiles to afford 1,2-aminofunctionalization products. Mechanistic investigations indicate aziridine cross-coupling proceeds via a noncanonical mechanism involving initial aziridine opening promoted by the bromide counterion of the Ni catalyst, C-N cross-coupling, and finally aziridine reclosure. Together, these results provide new opportunities to achieve selective incorporation of generic aryl nitrene equivalents in organic molecules.

Enantioselective Deaminative Alkylation of Amino Acid Derivatives with Unactivated Olefins

Herein, we report the first Ni-catalyzed enantioselective deaminative alkylation of amino acid and peptide derivatives with unactivated olefins. Key for success was the discovery of a new sterically encumbered bis(oxazoline) ligand backbone, thus offering a de novo technology for accessing enantioenriched sp³-sp³ linkages via sp³ C-N functionalization. Our protocol is distinguished by its broad scope and generality across a wide number of counterparts, even in the context of late-stage functionalization. In addition, an enantioselective deaminative remote hydroalkylation reaction of unactivated internal olefins is within reach, thus providing a useful entry point for forging enantioenriched sp³-sp³ centers at remote sp³ C-H sites.

Desulfurative Ni-Catalyzed Reductive Cross-Coupling of Benzyl Mercaptans/Mercaptoacetates with Aryl Halides

The C-S activation and sulfur removal from native thiols is challenging, which limits their application as feedstock materials in organic synthesis despite their natural abundance. Herein, we introduce a per-/polyfluoroaryl moiety, which serves as a redox-active scaffold, into sp³-hybridized thiols to activate the C-S bond. Using a Ni catalyst with MgBr₂ as an additive, the S group can be removed to yield an aliphatic radical that can react with an aryl halide in a reductive cross-coupling.

Deaminative chlorination of aminoheterocycles

Selective modification of heteroatom-containing aromatic structures is in high demand as it permits rapid evaluation of molecular complexity in advanced intermediates. Inspired by the selectivity of deaminases in nature, herein we present a simple methodology that enables the NH₂ groups in aminoheterocycles to be conceived as masked modification handles. With the aid of a simple pyrylium reagent and a cheap chloride source, C(sp²)‒NH₂ can be converted into C(sp²)‒Cl bonds. The method is characterized by its wide functional group tolerance and substrate scope, allowing the modification of >20 different classes of heteroaromatic motifs (five- and six-membered heterocycles), bearing numerous sensitive motifs. The facile conversion of NH₂ into Cl in a late-stage fashion enables practitioners to apply Sandmeyer- and Vilsmeier-type transforms without the burden of explosive and unsafe diazonium salts, stoichiometric transition metals or highly oxidizing and unselective chlorinating agents.

Tunable and Practical Homogeneous Organic Reductants for Cross-Electrophile Coupling

The syntheses of four new tunable homogeneous organic reductants based on a tetraaminoethylene scaffold are reported. The new reductants have enhanced air stability compared to current homogeneous reductants for metal-mediated reductive transformations, such as cross-electrophile coupling (XEC), and are solids at room temperature. In particular, the weakest reductant is indefinitely stable in air and has a reduction potential of -0.85 V versus ferrocene, which is significantly milder than conventional reductants used in XEC. All of the new reductants can facilitate C(sp²)-C(sp³) Ni-catalyzed XEC reactions and are compatible with complex substrates that are relevant to medicinal chemistry. The reductants span a range of nearly 0.5 V in reduction potential, which allows for control over the rate of electron transfer events in XEC. Specifically, we report a new strategy for controlled alkyl radical generation in Ni-catalyzed C(sp²)-C(sp³) XEC. The key to our approach is to tune the rate of alkyl radical generation from Katritzky salts, which liberate alkyl radicals upon single electron reduction, by varying the redox potentials of the reductant and Katritzky salt utilized in catalysis. Using our method, we perform XEC reactions between benzylic Katritzky salts and aryl halides. The method tolerates a variety of functional groups, some of which are particularly challenging for most XEC transformations. Overall, we expect that our new reductants will both replace conventional homogeneous reductants in current reductive transformations due to their stability and relatively facile synthesis and lead to the development of novel synthetic methods due to their tunability.

Electroreductive Cross-Coupling of Trifluoromethyl Alkenes and Redox Active Esters for the Synthesis of Gem-Difluoroalkenes

An electroreductive access to gem-difluoroalkenes has been developed through the decarboxylative/defluorinative coupling of N-hydroxyphtalimides esters and α-trifluoromethyl alkenes. The electrolysis is performed under very simple reaction conditions in an undivided cell using cheap carbon graphite electrodes. This metal-free transformation features broad scope with good to excellent yields. Tertiary, secondary as well as primary alkyl radicals could be easily introduced. α-aminoacids L-aspartic and L-glutamic acid-derived redox active esters were good reactive partners furnishing potentially relevant gem-difluoroalkenes. In addition, it has been demonstrated that our electrosynthetic approach toward the synthesis of gem-difluoroalkenes could use an easily prepared Kratitsky salt as alkyl radical precursor via a deaminative/defluorinative carbofunctionalization of trifluoromethylstyrene.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Substituent Effects on the Basicity of Patriscabrin A and Lettucenin A: Evolution Favors the Aromatic?

Basicities for derivatives of patriscabrin A and lettucenin A were calculated with density functional theory. A significant correlation is observed between the basicity and Hammett σ parameters. Protonation increases the aromatic character of the cyclic moieties of each natural product. The naturally occurring structures are predicted to be the most aromatic.

Primary Sulfonamide Functionalization via Sulfonyl Pyrroles: Seeing the N-Ts Bond in a Different Light

Despite common occurrence in molecules of value, methods for transforming sulfonamides are distinctly lacking. Here we introduce easy-to-access sulfonyl pyrroles as synthetic linchpins for sulfonamide functionalization. The versatility of the sulfonyl pyrrole unit is shown by generating a variety of products through chemical, electrochemical and photochemical pathways. Preliminary results on the direct functionalization of primary sulfonamides are also provided, which may lead to new modes of activation.

α-Chiral Amines via Thermally Promoted Deaminative Addition of Alkylpyridinium Salts to Sulfinimines

A deaminative reaction of Katritzky alkylpyridinium salts and sulfinimines has been developed to deliver enantiopure α-chiral amines. The success of this method relied on the discovery of a thermally promoted deamination via single-electron transfer of an anion-π complex of the alkylpyridinium cation with potassium carbonate. This method boasts excellent diastereoselectivity over the α-stereocenter as well as broad functional group and heterocycle tolerance.

Nickel-Catalyzed Kumada Cross-Coupling Reactions of Benzylic Sulfonamides

Herein, we report a Kumada cross-coupling reaction of benzylic sulfonamides. The scope of the transformation includes acyclic and cyclic sulfonamide precursors that cleanly produce highly substituted acyclic fragments. Preliminary data are consistent with a stereospecific mechanism that allows for a diastereoselective reaction.

Nickel-catalyzed deaminative Sonogashira coupling of alkylpyridinium salts enabled by NN2 pincer ligand

Alkynes are amongst the most valuable functional groups in organic chemistry and widely used in chemical biology, pharmacy, and materials science. However, the preparation of alkyl-substituted alkynes still remains elusive. Here, we show a nickel-catalyzed deaminative Sonogashira coupling of alkylpyridinium salts. Key to the success of this coupling is the development of an easily accessible and bench-stable amide-type pincer ligand. This ligand allows naturally abundant alkyl amines as alkylating agents in Sonogashira reactions, and produces diverse alkynes in excellent yields under mild conditions. Salient merits of this chemistry include broad substrate scope and functional group tolerance, gram-scale synthesis, one-pot transformation, versatile late-stage derivatizations as well as the use of inexpensive pre-catalyst and readily available substrates. The high efficiency and strong practicability bode well for the widespread applications of this strategy in constructing functional molecules, materials, and fine chemicals.

Alkynes are amongst the most valuable functional groups in organic chemistry, however, the preparation of alkyl-substituted alkynes still remains elusive. Here the authors show a nickel-catalyzed deaminative Sonogashira coupling of alkylpyridinium salts.

α-Active Pyrylium Salt 2,4,5-Triphenylpyrylium for Improved Mass Spectrometry-Based Detection of Peptides

Pyrylium salts are considered efficient chemical tags for amino groups. However, the apparent steric selectivity of pyrylium salts limits their application in the field of chemical labeling, especially during the labeling of sterically hindered compounds like amino acids, peptides, and proteins. Herein, we have investigated the effects of the α-substitution of pyrylium salts on their reactivity. We have also investigated the mechanism of nucleophilic reactions with pyrylium salts and further proposed that the reactivity of pyrylium salts mainly depends on the position and type of their substituents. A series of pyrylium salts were synthesized, and a highly active α-monosubstituted pyrylium salt, 2,4,5-triphenylpyrylium, was developed for efficient chemical labeling. All of the 15 amino acids studied were efficiently labeled under optimized reaction conditions. The 2,4,5-triphenylpyrylium salt was highly efficient in comparison to the previously reported 2,4,6-triphenylpyrylium salt developed for lysine-specific modifications. Furthermore, we successfully used 2,4,5-triphenylpyrylium salt for the hydrophobic labeling of peptides and protein hydrolysates. The most striking observation was that the ionization efficiency of short-chain multilabeled peptides in mixed samples, after derivatization, increased by up to 60 times. The increase in ionization efficiency gradually decreased with increasing peptide chain length. During the "soft" collision-induced dissociation (CID) process, the peptide was tagged at the N-terminus with 2,4,5-triphenylpyrylium, producing abundant a-type ions and b-type ions (Δ = 28), which eases the peptide resequencing process and assists in cracking the peptide codes. Moreover, 2,4,5-triphenylpyrylium has been utilized for the proteomic analysis of HeLa cell digests. In addition, 215 additional proteins were identified in the labeled products and the coverage of most proteins was improved.

Donor-Acceptor Complex Enables Cascade Radical Cyclization of N-Arylacrylamides with Katritzky Salts

Cascade radical cyclization of N-arylacrylamides is an attractive method to prepare 3,3-disubstituted oxindoles. As the reported methods often require additives and/or photocatalysts, we herein report an additive- and photocatalyst-free deaminative strategy for their synthesis under mild conditions, enabled by photoactivation of an electron donor-acceptor (EDA) complex. DFT studies indicated that the involvement of an explicit xylene solvent molecule can greatly enhance the photoactivity of the EDA complex between N-arylacrylamides and Katritzky salts.

Merging Photoredox/Nickel Catalysis for Cross-Electrophile Coupling of Aziridines with Pyridin-1-ium Salts via Dearomatization

Merging photoredox/nickel catalysis enabling the cross-electrophile coupling of aziridines with pyridin-1-ium salts involving dearomatization for the synthesis of β-(1,4-dihydropyridin-4-yl)-ethylamines, especially including bioactive motif-based analogues, is described. This method allows incorporation of a 1,4-dihydropyridin-4-yl group and formation a N-H amino group to construct highly valuable β-(1,4-dihydropyridin-4-yl)-ethylamine frameworks in a single step through the C2-N bond regioselective cleavage and dearomatization alkylation cascades with precise regioselectivity and excellent functional group tolerance, and represents an appealing cross-electrophile coupling strategy to accomplish transformations between two electrophiles, including aziridines and pyridin-1-ium salts, by avoiding prefunctionalization.

Continuous-Flow Synthesis of Pyrylium Tetrafluoroborates: Application to Synthesis of Katritzky Salts and Photoinduced Cationic RAFT Polymerization

Katritzky salts have emerged as effective alkyl radical sources upon metal- or photocatalysis. These are typically prepared from the corresponding triarylpyrylium ions, in turn an important class of photocatalysts for small molecules synthesis and photopolymerization. Here, a flow method for the rapid synthesis of both pyrylium and Katrizky salts in a telescoped fashion is reported. Moreover, several pyrylium salts were tested in the photoinduced RAFT polymerization of vinyl ethers under flow and batch conditions.

Photoinduced α-Alkenylation of Katritzky Salts: Synthesis of β,γ-Unsaturated Esters

β,γ-Unsaturated esters are building blocks in biologically important compounds, pharmaceuticals, and natural products. Because the current synthetic methods often require transition-metal catalysts or lack general variants, we herein describe a simple NaI-involved photoinduced deaminative alkenylation for their synthesis in the absence of photocatalysts and additives. The density functional theory study unveils that the electrostatic interaction of NaI with Katritzky salts is the key to forming the photoactive electron donor-acceptor complex, thus leading to the alkyl radicals for the alkenylation.

Reaction Acceleration at Solid/Solution Interfaces: Katritzky Reaction Catalyzed by Glass Particles

The Katritzky reaction in bulk solution at room temperature is accelerated significantly by the surface of a glass container compared to a plastic container. Remarkably, the reaction rate is increased by more than two orders of magnitude upon the addition of glass particles with the rate increasing linearly with increasing amounts of glass. A similar phenomenon is observed when glass particles are added to levitated droplets, where large acceleration factors are seen. Evidence shows that glass acts as a "green" heterogeneous catalyst: it participates as a base in the deprotonation step and is recovered unchanged from the reaction mixture. Reaction acceleration at two separate interfaces is recognized in this study: i) air/solution phase acceleration, as is well known in microdroplets; ii) solid/solution phase, where such acceleration appears to be a new phenomenon.

Visible light induced deaminative alkylation of difluoroenoxysilanes: a transition metal free strategy

Visible-light-promoted deaminative alkylation of difluoroenoxysilanes utilizing Hantzsch ester as a catalyst or through substrate-induced pathway have been demonstrated.

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

Deaminative carbonylative thioesterification of activated alkylamines with thiophenols under transition-metal-free conditions

A transition-metal-free radical carbonylation of activated alkylamines with thiophenols has been successfully developed. Various thioesters were selectively produced with moderate to good yields.

Late stage C-H functionalization via chalcogen and pnictogen salts

Late-stage functionalization (LSF) of heteroarenes can dramatically accelerate SAR studies by enabling the installation of functional groups that would otherwise complicate a synthetic sequence. Although heteroaryl halides and boronic esters have well-established chemistries for LSF, alternatives that enable site-selective C-H functionalization are highly attractive. Recently, three unrelated cationic groups (phosphonium, pyridinium, and thianthrenium), which can replace C-H bonds late stage, have been identified as precursors to various functional groups. This review will discuss the synthesis and application of these three salts with an emphasis on their use for LSF and application to medicinal chemistry.

Copper-Catalyzed Deaminative Difluoromethylation

The difluoromethyl group (CF₂ H) is considered to be a lipophilic and metabolically stable bioisostere of an amino (NH₂ ) group. Therefore, methods that can rapidly convert an NH₂ group into a CF₂ H group would be of great value to medicinal chemistry. We report herein an efficient Cu-catalyzed approach for the conversion of alkyl pyridinium salts, which can be readily synthesized from the corresponding alkyl amines, to their alkyl difluoromethane analogues. This method tolerates a broad range of functional groups and can be applied to the late-stage modification of complex amino-containing pharmaceuticals.

Photocatalytic deaminative benzylation and alkylation of tetrahydroisoquinolines with N-alkylpyrydinium salts

A ruthenium-catalyzed photoredox coupling of substituted N-aryltetrahydroisoquinolines (THIQs) and different bench-stable pyridinium salts was successfully developed to give fast access to 1-benzyl-THIQs. Furthermore, secondary alkyl and allyl groups were also successfully introduced via the same method. Additionally, the typically applied N-phenyl group in the THIQ substrate could be replaced by the cleavable p-methoxyphenyl (PMP) group and successful N-deprotection was demonstrated.