Photoinduced C(sp3 )-N Bond Cleavage Leading to the Stereoselective Syntheses of Alkenes

Deconstructive annulation mediated one-pot synthesis of xanthene derivatives

Direct conversion of naphthoxazines to diverse xanthene derivatives was achieved under one-pot operation through deconstructive annulation methodology. Sequential oxidative C(sp3)-O/C(sp3)-N cleavage followed by intramolecular/intermolecular annulation reaction was carried out under aerobic reaction conditions. Mechanistic analyses performed on the substrate revealed that the C(sp3)-O bond cleavage supersedes the C(sp3)-N bond scission. The in situ generated Betti base intermediate through the C(sp3)-O cleavage was successfully isolated. Based on a molecular docking investigation, the intermolecular annulated products demonstrated good α-glucosidase inhibitory properties.

Metal-Organic Layers with Photosensitizer and Pyridine Pairs Activate Alkyl Halides for Photocatalytic Heck-Type Coupling with Olefins

Photochemical generation of alkyl radicals from haloalkanes often requires strong energy input from ultraviolet light or a strong photoreductant. Haloalkanes can alternatively be activated with nitrogen-based nucleophiles through a sequential SN2 reaction and single-electron reduction to access alkyl radicals, but these two reaction steps have opposite steric requirements on the nucleophiles. Herein, we report the design of Hf12 metal-organic layers (MOLs) with iridium-based photosensitizer bridging ligands and secondary-building-unit-supported pyridines for photocatalytic alkyl radical generation from haloalkanes. By bringing the photosensitizer and pyridine pairs in proximity, the MOL catalysts allowed facile access to the pyridinium salts from SN2 reactions between haloalkanes and pyridines and at the same time enhanced electron transfer from excited photosensitizers to pyridinium salts to facilitate alkyl radical generation. Consequentially, the MOLs efficiently catalyzed Heck-type cross-coupling reactions between haloalkanes and olefinic substrates to generate functionalized alkenes. The MOLs showed 4.6 times higher catalytic efficiency than the homogeneous counterparts and were recycled and reused without a loss of catalytic activity.

Multiple-cycle photochemical cascade reactions

Cascade reactions represent an efficient and economical synthetic approach, enabling the rapid synthesis of a wide array of structurally complex organic compounds. These compounds, previously inaccessible, can now be synthesized in a remarkably limited number of steps. Concurrently, the photochemical reactions of organic molecules have gained prominence as a potent strategy for accessing a diverse range of radical species and intermediates. This is achieved in a controlled manner under mild conditions. Owing to the relentless endeavors of chemists, significant strides have been made in the realm of photochemical cascade reactions. These advancements have facilitated the synthesis of novel molecular structures with high complexity, structures that are typically challenging to generate under thermal conditions. In this review, we comprehensively summarize and underscore the recent pivotal advancements in visible-light-induced cascade reactions. Our focus is on the elucidation of multiple photochemical catalytic cycles, emphasizing the catalytic activation modes and the types of reactions involved.

Isonitriles as Alkyl Radical Precursors in Visible Light Mediated Hydro- and Deuterodeamination Reactions

Herein, we report the use of isonitriles as alkyl radical precursors in light-mediated hydro- and deuterodeamination reactions. The reaction is scalable, shows broad functional group compatibility and potential to be used in late-stage functionalization. Importantly, the method is general for Cα -primary, Cα -secondary and Cα -tertiary alkyl isonitriles. For most examples, high yields were obtained through direct visible-light irradiation of the isonitrile in the presence of a silyl radical precursor. Interestingly, in the presence of an organic photocatalyst (4CzIPN) a dramatic acceleration was observed. In-depth mechanistic studies using UV/Vis absorption, steady-state and time-resolved photoluminescence, and transient absorption spectroscopy suggest that the excited state of 4CzIPN can engage in a single-electron transfer with the isonitrile.

Divergent Photosensitizer Controlled Reactions of 4-Hydroxycoumarins and Unactivated Olefins: Hydroarylation and Subsequent [2+2] Cycloaddition

Herein, we report a photoinduced approach for hydroarylation of unactivated olefins using 4-hydroxycoumarins as the arylating reagent. Key to the success of this reaction is the conversion of nucleophilic 4-hydroxycoumarins into electrophilic carbon radicals via photocatalytic arene oxidation, which not only circumvents the polarity-mismatch issue encountered under ionic conditions but also accommodates a broad substrate scope and inhibits side reactions that were previously observed. Moreover, divergent reactivity was achieved by changing the photocatalyst, enabling a subsequent [2+2] cycloaddition to deliver cyclobutane-fused pentacyclic products that are otherwise challenging to access in high yields and with high diastereoselectivity. Mechanistic studies have elucidated the mechanism of the reactions and the origin of the divergent reactivity.

Deaminative bromination, chlorination, and iodination of primary amines

The primary amino group has been seldom utilized as a transformable functionality in organic synthesis. Reported herein is a deaminative halogenation of primary amines using N-anomeric amide as the nitrogen-deletion reagent. Both aliphatic and aromatic amines are competent substrates for direct halogenations. The mildness and robustness of the protocol are evidenced by the successful reactions of several complex- and functional group-enriched bioactive compounds or drugs. Elaboration of the resulting products provides interesting analogues of drug molecules.

Beyond Retigabine: Design, Synthesis, and Pharmacological Characterization of a Potent and Chemically Stable Neuronal Kv7 Channel Activator with Anticonvulsant Activity

Neuronal Kv7 channels represent important pharmacological targets for hyperexcitability disorders including epilepsy. Retigabine is the prototype Kv7 activator clinically approved for seizure treatment; however, severe side effects associated with long-term use have led to its market discontinuation. Building upon the recently described cryoEM structure of Kv7.2 complexed with retigabine and on previous structure–activity relationship studies, a small library of retigabine analogues has been designed, synthesized, and characterized for their Kv7 opening ability using both fluorescence- and electrophysiology-based assays. Among all tested compounds, 60 emerged as a potent and photochemically stable neuronal Kv7 channel activator. Compared to retigabine, compound 60 displayed a higher brain/plasma distribution ratio, a longer elimination half-life, and more potent and effective anticonvulsant effects in an acute seizure model in mice. Collectively, these data highlight compound 60 as a promising lead compound for the development of novel Kv7 activators for the treatment of hyperexcitability diseases.

Redox activated amines in the organophotoinduced alkylation of coumarins

The coumarin core represents the quintessential scaffold of many natural products. While C-3 alkylation is easily achievable, effective greener strategies for C-4 alkylation have been less forthcoming. Herein, we report a metal-free photoinduced deaminative strategy for C-4 alkylation of coumarins using redox activated secondary and benzylic amine derived Katritzky pyridinium salts.

Recent Progress in Fragmentation of Katritzky Salts Enabling Formation of C-C, C-B, and C-S Bonds

Since their discovery in 1970s, Katritzky salts have emerged as one of the most important classes of building blocks for use in organic synthesis and drug discovery. These bulky pyridinium salts derived from alkylamine can readily generate alkyl radical and undergo a variety of organic transformation reactions such as alkylation, arylation, alkenylation, alkynylation, carbonylation, sulfonylation, and borylation. Through these transformations, complexed molecules bearing new C-C, C-B, or C-S bonds can be constructed in easy ways and in simple steps. This review aims to summarize recent advances in these versatile building blocks in well-classified categories. Representative examples and their reaction mechanisms are discussed. The hope is to provide the scientific community with convenient access to collective information and accelerate further research.

Recent advances in difunctionalization of alkenes using pyridinium salts as radical precursors

In this review, we summarise the recent applications of pyridinium salts in the radical-mediated difunctionalization of alkenes. Pyridinium salts are a privileged class of compounds that show great utility in natural products and synthetic chemistry. Various organic transformations of pyridinium salts, especially in radical chemistry, have been developed in recent years. We prepared this review based on the two distinguished properties of pyridinium salts in radical transformation: (1) pyridinium salts can easily undergo single electron reduction to deliver X radicals. (2) Pyridinium salts are highly electrophilic so that alkyl radical intermediates can easily add to the pyridine core. Based on the role of pyridinium salts in difunctionalization of alkenes, the main body of this review is divided into three parts: (1) using pyridinium salts as X transfer reagents. (2) Using pyridinium salts as novel pyridine transfer reagents. (3) Using pyridinium salts as bifunctional reagents (X and pyridine). The C2 and C4 selectivity during pyridylation is discussed in detail. We hope that this review will provide a comprehensive overview of this topic and promote the wider development and application of pyridinium salts.

Visible-Light-Promoted Unsymmetrical Phosphine Synthesis from Benzylamines

Herein, by applying visible-light photoredox catalysis, we have achieved the catalytic deaminative alkylation of diphenylphosphine and phenyl phosphine with benzylamine-derived Katritzky salts at room temperature. The use of Eosin Y as photoredox catalyst and visible light can largely promote the reaction. A series of unsymmetrical tertiary phosphines were successfully synthesized, including phosphines with three different substituents that are otherwise difficult to obtain.

Boron, silicon, nitrogen and sulfur-based contemporary precursors for the generation of alkyl radicals by single electron transfer and their synthetic utilization

Recent developments in the use of boron, silicon, nitrogen and sulfur derivatives in single-electron transfer reactions for the generation of alkyl radicals are described. Photoredox catalyzed, electrochemistry promoted or thermally-induced oxidative and reductive processes are discussed highlighting their synthetic scope and discussing their mechanistic pathways.

Dual Nickel/Photoredox-Catalyzed Deaminative Cross-Coupling of Sterically Hindered Primary Amines

We report a method to activate α-3° amines for deaminative arylation via condensation with an electron-rich aldehyde and merge this reactivity with nickel metallaphotoredox to generate benzylic quaternary centers, a common motif in pharmaceuticals and natural products. The reaction is accelerated by added ammonium salts. Evidence is provided in support of two roles for the additive: inhibition of nickel black formation and acceleration of the overall reaction rate. We demonstrate a robust scope of amine and haloarene coupling partners and show an expedited synthesis of ALK2 inhibitors.

Visible-Light-Promoted Cross-Coupling of N-Alkylpyridinium Salts and Nitrostyrenes

A stereoselective, denitrative cross-coupling of β-nitrostyrenes with N-alkylpyridinium salts for the preparation of functionalized styrenes has been developed. The visible-light-induced reaction proceeds without any catalyst at ambient temperature. Broad in scope and tolerant to multiple functional groups, the moderately yielding transformation is orthogonal to several traditional metal-catalyzed cross-couplings.

Deaminative Reductive Methylation of Alkylpyridinium Salts

Methyl groups can imbue valuable properties in organic molecules, often leading to enhanced bioactivity. To enable efficient installation of methyl groups on simple building blocks and in late-stage functionalization, a nickel-catalyzed reductive coupling of secondary Katritzky alkylpyridinium salts with methyl iodide was developed. When coupled with formation of the pyridinium salt from an alkyl amine, this method allows amino groups to be readily transformed to methyl groups with broad functional group and heterocycle tolerance.

Advances in the E → Z Isomerization of Alkenes Using Small Molecule Photocatalysts

Geometrical E → Z alkene isomerization is intimately entwined in the historical fabric of organic photochemistry and is enjoying a renaissance (Roth et al. Angew. Chem., Int. Ed. Engl. 1989 28, 1193-1207). This is a consequence of the fundamental stereochemical importance of Z-alkenes, juxtaposed with frustrations in thermal reactivity that are rooted in microscopic reversibility. Accessing excited state reactivity paradigms allow this latter obstacle to be circumnavigated by exploiting subtle differences in the photophysical behavior of the substrate and product chromophores: this provides a molecular basis for directionality. While direct irradiation is operationally simple, photosensitization via selective energy transfer enables augmentation of the alkene repertoire to include substrates that are not directly excited by photons. Through sustained innovation, an impressive portfolio of tailored small molecule catalysts with a range of triplet energies are now widely available to facilitate contra-thermodynamic and thermo-neutral isomerization reactions to generate Z-alkene fragments. This review is intended to serve as a practical guide covering the geometric isomerization of alkenes enabled by energy transfer catalysis from 2000 to 2020, and as a logical sequel to the excellent treatment by Dugave and Demange (Chem. Rev. 2003 103, 2475-2532). The mechanistic foundations underpinning isomerization selectivity are discussed together with induction models and rationales to explain the counterintuitive directionality of these processes in which very small energy differences distinguish substrate from product. Implications for subsequent stereospecific transformations, application in total synthesis, regioselective polyene isomerization, and spatiotemporal control of pre-existing alkene configuration in a broader sense are discussed.

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.

Phosphite mediated asymmetric N to C migration for the synthesis of chiral heterocycles from primary amines

A phosphite mediated stereoretentive C-H alkylation of N-alkylpyridinium salts derived from chiral primary amines was achieved. The reaction proceeds through the activation of the N-alkylpyridinium salt substrate with a nucleophilic phosphite catalyst, followed by a base mediated [1,2] aza-Wittig rearrangement and subsequent catalyst dissociation for an overall N to C-2 alkyl migration. The scope and degree of stereoretention were studied, and both experimental and theoretical investigations were performed to support an unprecedented aza-Wittig rearrangement-rearomatization sequence. A catalytic enantioselective version starting with racemic starting material and chiral phosphite catalyst was also established following our understanding of the stereoretentive process. This method provides efficient access to tertiary and quaternary stereogenic centers in pyridine systems, which are prevalent in drugs, bioactive natural products, chiral ligands, and catalysts.

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.

Deaminative metal-free reaction of alkenylboronic acids, sodium metabisulfite and Katritzky salts

A convenient and efficient approach to (E)-alkylsulfonyl olefins via a metal/light-free three-component reaction of alkenylboronic acids, sodium metabisulfite and Katritzky salts is described. This alkylsulfonylation proceeds smoothly with a broad substrate scope, leading to diverse (E)-alkylsulfonyl olefins in moderate to good yields. During the process, excellent functional group tolerance is observed and sodium metabisulfite is used as the source of sulfur dioxide. Mechanistic studies show that the alkyl radical generated in situ from Katritzky salt via a single electron transfer with alkenylboronic acid or DIPEA is the key step for providing an alkyl radical intermediate, which undergoes further alkylsulfonylation with sulfur dioxide.

Reframing primary alkyl amines as aliphatic building blocks

While primary aliphatic amines are ubiquitous in natural products, they are traditionally considered inert to substitution chemistry. This review highlights historical and recent advances in the field of aliphatic deamination chemistry which demonstrate these moieties can be harnessed as valuable C(sp3) synthons. Cross-coupling and photocatalyzed transformations proceeding through polar and radical mechanisms are compared with oxidative deamination and other transition metal catalyzed reactions.

Photoinduced Deaminative Coupling of Alkylpyridium Salts with Terminal Arylalkynes

A novel and simple Z-alkene synthesis by the photocatalyzed coupling reactions of alkylpyridium salts, which were prepared from primary amines, with terminal aryl alkynes at room temperature is reported here. A wide range of primary amines, which contain different functional groups, were tolerated under these conditions. The mild reaction conditions, broad substrate scope, functional group tolerance, and operational simplicity make this deaminative coupling reaction a valuable method in organic syntheses.

Organo-photoredox-Catalyzed Atom-Transfer Radical Substitution of Alkenes with α-Carbonyl Alkyl Halides

A light-driven atom-transfer radical substitution (ATRS) and carboesterification reaction of alkenes with alkyl halides has been developed using PTH as the organo-photoredox catalyst. Two types of products were obtained, depending on the additive and solvent used during the reaction. Primary, secondary, and tertiary alkyl halides reacted to give the ATRS products. This protocol has several advantages: it requires mild reaction conditions and a low catalyst loading and exhibits a broad substrate scope and good functional group tolerance. Mechanistic studies indicate that alkyl radicals might be generated as the key intermediates via photocatalysis, providing a new direction for ATRS reactions.

Tertiary Amines Acting as Alkyl Radical Equivalents Enabled by a P/N Heteroleptic Cu(I) Photosensitizer

An unprecedented exploration of tertiary amines as alkyl radical equivalents for cross-coupling with aromatic alkynes to access allylarenes has been achieved by a P/N heteroleptic Cu(I)-based photosensitizer under photoredox catalysis conditions. Mechanistic studies reveal that the reaction might undergo radical addition of in situ-generated α-amino radical intermediates to alkynes followed by 1,5-hydrogen transfer, C-N bond cleavage, and concomitant isomerization of the resulting allyl radical species.

Visible Light-Driven α-Alkylation of N-Aryl tetrahydroisoquinolines Initiated by Electron Donor-Acceptor Complexes

The visible light-driven α-alkylation of N-aryl tetrahydroisoquinolines was initiated through electron donor-acceptor complex photochemistry. The reaction can proceed smoothly without the addition of any photocatalysts, transition-metal catalysts, or additional oxidants. The proposed mechanism was supported by various mechanistic studies, and the reactive open-shell alkyl radicals were generally produced from an alkylamine and underwent radical coupling for alkylating a wide range of N-aryl tetrahydroisoquinolines.

Radical C-N Borylation of Aromatic Amines Enabled by a Pyrylium Reagent

Herein, we report a radical borylation of aromatic amines through a homolytic C(sp2 )-N bond cleavage. This method capitalizes on a simple and mild activation via a pyrylium reagent (Sc Pyry-OTf) thus priming the amino group for reactivity. The combination of terpyridine and a diboron reagent triggers a radical reaction which cleaves the C(sp2 )-N bond and forges a new C(sp2 )-B bond. The unique non-planar structure of the pyridinium intermediate, provides the necessary driving force for the aryl radical formation. The method permits borylation of a wide variety of aromatic amines indistinctively of the electronic environment.

Visible-Light-Promoted C(sp3 )-H Alkylation by Intermolecular Charge Transfer: Preparation of Unnatural α-Amino Acids and Late-Stage Modification of Peptides

Disclosed herein is the visible-light-promoted deaminative C(sp³ )-H alkylation of glycine and peptides using Katritzky salts as electrophiles. Simple reaction conditions and excellent functional-group tolerance provide a general strategy for the efficient preparation of unnatural α-amino acids and precise modification of peptides with unnatural α-amino-acid residues. Mechanistic studies suggest that visible-light-promoted intermolecular charge transfer within a glycine-Katritzky salt electron donor-acceptor (EDA) complex induces a single-electron transfer process without the assistance of photocatalyst.

Nickel-Catalyzed Deaminative Acylation of Activated Aliphatic Amines with Aromatic Amides via C-N Bond Activation

Deaminative functionalization of aliphatic primary amines has great synthetic utility. Herein, we describe a Ni-catalyzed reductive deaminative cross-electrophile coupling reaction between Katritzky salts and aromatic amides. This work provides examples of the synthesis of various ketones from alkylpyridinium salts, including both primary and secondary alkylamines. Given its mild reaction conditions and high functional group tolerance, this cross-coupling strategy is expected to be useful for late-stage functionalization of complex compounds.

Deaminative carbonylative coupling of alkylamines with styrenes under transition-metal-free conditions

Transition-metal-free deaminative carbonylation through C–N bonds activation via Katritzky salts has been successful developed. Various α,β-unsaturated ketones were obtained in moderate to good yields with alkylamines and styrenes as the substrates.

Fernandes V, Matsuo BT, C. Delgado JA, de Souza WC, Paixão MW. Photoinduced deaminative strategies: Katritzky salts as alkyl radical precursors

Primary amines are one of the most predominant functional groups found in organic molecules. This review covers the most recent developments on photocatalytic deaminative strategies by using Katritzky Salts as alkyl radical reservoirs.

Double C-N bond cleavages of N-alkyl 4-oxopiperidinium salts: access to unsymmetrical tertiary sulfonamides

In this paper, regiospecific, double intraannular C-N bond cleavages of N-alkyl 4-oxopiperidinium salts are presented. The reaction sequence involves a charge-transfer complex, in situ formed between sulfonyl chloride and N-methylmorpholine, which induces S-Cl bond homolysis of sulfonyl chloride, yielding a reactive sulfonyl radical that further induces the double C-N bond cleavages of N-alkyl 4-oxopiperidinium salt. The secondary amine thus produced was trapped by sulfonyl chloride to yield the desired sulfonamide product. The key feature of this protocol is that two intraannular C-N bonds of the 4-oxopiperidine ring are cleaved in one step under metal- and oxidant-free conditions.