Visible light amination/Smiles cascade: access to phthalazine derivatives

Electrochemical cascade migratory versus ortho-cyclization of 2-alkynylbenzenesulfonamides

Efficient control over several possible reaction pathways of free radicals is the chemical basis of their highly selective transformations. Among various competing reaction pathways, sulfonimidyl radicals generated from the electrolysis of 2-alkynylbenzenesulfonamides undergo cascade migratory or ortho-cyclization cyclization selectively. It is found that the incorporation of an extra 2-methyl substituent biases the selective migration of the acyl- over vinyl-linker of the key spirocyclic cation intermediate and thus serves as an enabling handle to achieve the synthetically interesting yet under-investigated cascade migratory cyclization of spirocyclic cations.

Synthesis of substituted benzylboronates by light promoted homologation of boronic acids with N-sulfonylhydrazones

The synthesis of benzylboronates by photochemical homologation of boronic acids with N-tosylhydrazones under basic conditions is described. The reaction involves the photolysis of the N-tosylhydrazone salt to give a diazoalkane followed by the geminal carboborylation of the diazoalkane. Under the mild reaction conditions, the protodeboronation of the unstable benzylboronic acid is circumvented and the pinacolboronates can be isolated after reaction of the benzylboronic acid with pinacol. The metholodogy has been applied to the reactions of alkylboronic acids with N-tosylhydrazones of aromatic aldehydes and ketones, and to the reactions of arylboronic acids with N-tosylhydrazones of aliphatic ketones. Moreover, the employment of the DBU/DIPEA bases combination allows for homogeneous reactions which have been adapted to photochemical continuous flow conditions. Additionally, the synthetic versatility of boronates enables their further transformation via Csp3-C or Csp3-X bond forming reactions converting this methodology into a novel method for the geminal difunctionalization of carbonyls via N-tosylhydrazones.

Photocatalytic Intramolecular Alkene Hydroamination of N-Alkoxy Ureas: An Approach to Imidazolinones

Imidazolinones were obtained in good yields by intramolecular hydroamination of N-alkoxy ureas in the presence of an organic photocatalyst and an inorganic base. In this reaction, the N-alkoxy urea anion generated by deprotonation undergoes photocatalyzed single-electron-transfer oxidation to generate the corresponding radical, which cyclizes to afford the imidazolinone ring. This new protocol grants access to an array of complex molecules containing a privileged imidazolinone core.

Blue Light Irradiated Metal-, Oxidant-, and Base-Free Cross-Dehydrogenative Coupling of C(sp2)-H and N-H Bonds: Amination of Naphthoquinones with Amines

Herein, we report a blue-light-driven amination of C(sp²)-H bond of naphthoquinones and quinones with the N-H bond of primary and secondary amines for the synthesis of 2-amino-naphthoquinones and 2-amino-quinones. The coupling of naphthoquinones with a wide array of aliphatic, aromatic, chiral, primary, and secondary amines having electron donating (-CH₃, -OCH₃, -SCH₃), withdrawing (-F, -Cl, -Br, -I), and CO₂H, -OH, -NH₂ groups with acidic protons selectively occurred to afford C-N coupled 2-amino-naphthoquinones in 60-99% yields and hydrogen gas as a byproduct in methanol solvent without using any additional reagents, additives, and oxidant under the blue light irradiation. Mechanistic insight by DFT computation, controlled experiments, kinetic isotopic effect, and substitution effect of the substrates suggest that the reaction proceeds by radical pathway in which naphthoquinone forms a highly oxidizing naphthoquinonyl biradical upon irradiation of blue light (457 nm). Consequently, electron transfer from electron-rich amine to an oxidizing naphthoquinonyl biradical leads to a naphthoquinonyl radical anion and aminyl radical cation, followed by proton transfer and delocalization leading to a carbon-centered naphthoquinonyl radical. The cross-coupling of naphthoquinonyl carbon-centered and aminyl nitrogen radicals forms a C-N bond, with subsequent elimination of hydrogen gas (which was also confirmed by GC-TCD), affording 2-amino-1,4-naphthoquinone under metal-, reagent-, base-, and oxidant-free conditions.

Visible-Light-Induced Diradical-Mediated ipso-Cyclization towards Double Dearomative [2+2]-Cycloaddition or Smiles-Type Rearrangement

When mono-radical ipso-cyclization of aryl sulfonamides tend to undergo Smiles-type rearrangement through aromatization-driven C-S bond cleavage, diradical-mediated cyclization must perform in a distinct reaction pathway. It is interesting meanwhile challenging to tune the rate of C-S bond cleavage to achieve a chemically divergent reaction of (hetero) aryl sulfonamides in a visible-light induced energy transfer (EnT) reaction pathway involving diradical species. Herein a chemically divergent reaction based on the designed indole-tethered (hetero)arylsulfonamides is reported which involves a diradical-mediated ipso-cyclization and a controllable cleavage of an inherent C-S bond. The combined experimental and computational results have revealed that the cleavage of the C-S bond in these substrates can be controlled by tuning the heteroaryl moieties: a) If the (hetero)aryl is thienyl, furyl, phenanthryl, etc., the radical coupling of double dearomative diradicals (DDDR) precedes over C-S bond cleavage to afford cyclobutene fused indolines by double dearomative [2+2]-cycloaddition; b) if the (hetero)aryl is phenyl, naphthyl, pyridyl, indolyl etc., the cleavage of C-S bond in DDDR is favored over radical coupling to afford biaryl products.

Concise Synthesis of Spirocyclic Dihydrophthalazines through Spiroannulation Reactions of Aryl Azomethine Imines with Cyclic Diazo Compounds

Spiroannulation reactions are fundamental and invaluable for the synthesis of spirocyclic compounds. Presented herein are novel cascade reactions of aryl azomethine imines with cyclic diazo compounds leading to the formation of spirocyclic dihydrophthalazine derivatives. Based on experimental mechanistic studies, the formation of the title products is believed to go through azomethine imine-assisted cylcometalation, Rh-carbene formation through dediazonization, and migratory insertion followed by reductive elimination and azomethine imine ring opening. Control experiments revealed that air acts as an effective and sustainable co-oxidant to facilitate the cascade reaction. In general, this concise synthesis of the unprecedented spirocyclic dihydrophthalazine derivatives has advantages such as easily accessible substrates, good functional group compatibility, mild reaction conditions, high efficiency and selectivity, and excellent atom-economy. In addition, the value of this protocol is underlined by its ready scalability and divergent derivation of products.

Photoredox synthesis of 6- and 7-membered ring scaffolds via N-centered radicals

N-Containing heterocycles are important scaffolds due to their ubiquitous presence in bioactive compounds. Their synthesis has been considered as an important research field. In this work we report the access to 6- and 7-membered rings via a photoinduced strategy. To our knowledge, this work represents the first exemple of photo-induced 7-endo-trig cyclization with N-centered radicals.

Controlled Photochemical Synthesis of Substituted Isoquinoline-1,3,4(2H)-triones, 3-Hydroxyisoindolin-1-ones, and Phthalimides via Amidyl Radical Cyclization Cascade

We report a controlled radical cyclization cascade of isoquinoline-1,3,4(2H)-triones, 3-hydroxyisoindolin-1-ones, and phthalimides from o-alkynylated benzamides by metal-free photoredox catalyzed amidyl N-centered radical addition to the C-C triple bond using the proton-coupled electron transfer (PCET) process under mild reaction conditions. A time tunable synthesis of 3-hydroxyisoindolin-1-ones and phthalimides via β-carbonyl-C(sp³) bond cleavage was also achieved under visible light irradiation. A mechanistic rationale for the radical cyclization cascade is supported by various control and quenching experiments as well as EPR studies.

Recent Advances in the Smiles Rearrangement: New Opportunities for Arylation

The Smiles rearrangement has undergone a renaissance in recent years providing new avenues for non-canonical arylation techniques in both the radical and polar regimes. This short review will discuss recent applications of the reaction (from 2017 to late 2021), including its relevance to areas such as heterocycle synthesis and the functionalization of alkenes and alkynes as well as glimpses at new directions for the field.

1 Introduction

2 Polar Smiles Rearrangements

3 Radical Smiles: Alkene and Alkyne Functionalization

4 Radical Smiles: Rearrangements via C–X Bond Cleavage

5 Radical Smiles: Miscellaneous Rearrangements

6 Conclusions

Visible light-induced N-radical 5-exo/6-endo cyclization of alkenyl amides: facile access to isoindolinones/isoquinolinones

An efficient N-centered radical intramolecular cyclization reaction of alkenyl amides induced by visible light was described. In this process, an alkenyl amide underwent 5-exo/6-endo cyclization to selectively yield two critical alkaloid structures, namely isoindolinones and isoquinolinones.

Controlled microwave-assisted reactions: A facile synthesis of polyfunctionally substituted phthalazines as dual EGFR and PI3K inhibitors in CNS SNB-75 cell line

Brain tumors are stubborn cancers with poor prognosis and disappointing survival rates. Targeted cancer therapeutics with higher efficacy and lower resistance are highly demanded. An efficient one-pot synthesis of polyfunctionalized phthalazines derivatives was developed by reacting ethyl 1-aryl-5-cyano-1,6-dihydro-4-methyl-6-oxo-3-pyridazine-carboxylates with cinnamonitrile derivatives and the cycloaddition reaction of thieno[3,4-d]pyridazines with activated double or triple bond systems under controlled microwave heating with high yields. The resultant synthesized phthalazines (5a-e, 9 and 13) were tested for their in vitro anti-cancer activities by using in vitro one dose assay at National Cancer institute, USA. Only phthalazine (5b) showed broad spectrum anti-tumor activity against most tested cancer cell lines from all subpanels with mean % GI = 22.61. Interestingly, all tested compounds showed varying growth inhibitory activity against a particular cell line, CNS SNB-75 cell line, but (5b) exhibited the highest growth inhibitory activity against CNS-SNB-75 cell line with (GI% = 108.81) and (IC₅₀ = 3.703 ± 0.2) compared to erlotinib; (IC₅₀ = 12.5 ± 0.68). It caused Pre-G1 apoptosis and growth arrest at S phase. It also increased percentage of the total apoptotic cells in CNS-SNB-75 cell line (39.26%) compared to control cells (2.17%) in the annexin V-FITC experiment. It revealed pronounced EGFR inhibitory activity (IC₅₀ = 47.27 ± 2.41 ng/mL) compared to erlotinib (IC₅₀ = 30.7 ± 1.56 ng/mL). It also inhibited the different PI3K isoforms α, β, γ and δ (with IC₅₀ of 4.39, 13.6, 12.5 and 3.11 μg/mL, respectively compared to LY294002 (with IC₅₀ of 12.7, 8.57, 6.89 and 5.7 μg/mL, respectively). It also caused significant lower protein expression levels of pPI3K, AKT, pAKT and Bcl2 and higher protein expression levels of BAX, Casp3 and Casp9 when compared to untreated cells. Conclusion: Phthalazine (5b) may be an effective, convenient and safe anti-cancer agent acting via proapoptotic and dual EGFR and PI3K kinase inhibitory actions in CNS SNB-75 cell line.

Recent developments in the green synthesis of biologically relevant cinnolines and phthalazines

Both cinnolines and phthalazines are heterocyclic compounds which have a wide range of biological activities and pharmacological profiles. This work represents the recent advances in the green synthesis of cinnolines and phthalazines as 1,2 and 2,3-diazanaphalenes were cited. The docking studies and mode of action for key scaffolds were also reported.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Aryl Transfer Strategies Mediated by Photoinduced Electron Transfer

Radical aryl migrations are powerful techniques to forge new bonds in aromatic compounds. The growing popularity of photoredox catalysis has led to an influx of novel strategies to initiate and control aryl migration starting from widely available radical precursors. This review encapsulates progress in radical aryl migration enabled by photochemical methods─particularly photoredox catalysis─since 2015. Special attention is paid to descriptions of scope, mechanism, and synthetic applications of each method.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

As the "chemical chameleon", sulfonyl-containing compounds and their variants have been merged with various types of reactions for the efficient construction of diverse molecular architectures by taking advantage of their incredible reactive flexibility. Currently, their involvement in radical transformations, in which the sulfonyl group typically acts as a leaving group via selective C-S, N-S, O-S, S-S, and Se-S bond cleavage/functionalization, has facilitated new bond formation strategies which are complementary to classical two-electron cross-couplings via organometallic or ionic intermediates. Considering the great influence and synthetic potential of these novel avenues, we summarize recent advances in this rapidly expanding area by discussing the reaction designs, substrate scopes, mechanistic studies, and their limitations, outlining the state-of-the-art processes involved in radical-mediated desulfonylation and related transformations. With a specific emphasis on their synthetic applications, we believe this review will be useful for medicinal and synthetic organic chemists who are interested in radical chemistry and radical-mediated desulfonylation in particular.

Radical-mediated rearrangements: past, present, and future

Rearrangement reactions, one of the most significant transformations in organic chemistry, play an irreplaceable role in improving synthetic efficiency and molecular complexity. Concomitant cleavage and reconstruction of chemical bonds can display the great artistry and the glamour of synthetic chemistry. Over the past century, ionic rearrangement reactions, in particular those involving cationic pathways, have represented most of the research. Alongside the renaissance of radical chemistry, radical-mediated rearrangements have recently seen a rapid increase of attention from the chemical community. Many new radical rearrangements that extensively reveal the migratory behaviour of functional groups have been unveiled in the last decade. This Review provides a comprehensive perspective on the area from the past to present achievements, and brings up the prospects that may inspire colleagues to develop more useful synthetic tools based on radical rearrangements.

Truce-Smiles Rearrangements by Strain Release: Harnessing Primary Alkyl Radicals for Metal-Free Arylation

The ring-opening of 3-aminocyclobutanone oximes enables easy generation of primary alkyl radicals, capable of undergoing an unprecedented strain-release, desulfonylative radical Truce-Smiles rearrangement, providing divergent access to valuable 1,3 diamines and unnatural β-amino acids. Characterized by mild conditions and wide scope of migrating species, this protocol allows the modular assembly of sp³ -aryls under transition metal-free, room-temperature conditions.

Cerium photocatalyzed radical smiles rearrangement of 2-aryloxybenzoic acids

We report herein a cerium photocatalyzed aryl migration from an aryl ether to a carboxylic acid group through radical-Smiles rearrangement. This operationally simple protocol utilizes inexpensive CeCl3 as a photocatalyst and converted a variety of 2-aryloxybenzoic acids into aryl-2-hydroxybenzoates in good yields.

Photoredox Generation of N-Centered Hydrazonyl Radicals Enables the Construction of Dihydropyrazole-Fused gem-Difluoroalkenes

An efficient visible-light-promoted N-radical-mediated tandem radical cyclization/defluorinated alkylation of β,γ-unsaturated hydrazones, and α-trifluoromethyl alkenes is described. This protocol provides a general and effective route to synthesize various dihydropyrazole-fused gem-difluoroalkenes at moderate to excellent yields under redox-neutral, metal-free, and mild conditions.

Photochemical radical cyclization reactions with imines, hydrazones, oximes and related compounds

Photochemical reactions are a key method to generate radical intermediates. Often under these conditions no toxic reagents are necessary. During recent years, photo-redox catalytic reactions considerably push this research domain. These reaction conditions are particularly mild and safe which enables the transformation of poly-functional substrates into complex products. The synthesis of heterocyclic compounds is particularly important since they play an important role in the research of biologically active products. In this review, photochemical radical cyclization reactions of imines and related compounds such as oximes, hydrazones and chloroimines are presented. Reaction mechanisms are discussed and the structural diversity and complexity of the products are presented. Radical intermediates are mainly generated in two ways: (1) electronic excitation is achieved by light absorption of the substrates. (2) The application of photoredox catalysis is now systematically studied for these reactions. Recently, also excitation of charge transfer complexes has been studied in this context from many perspectives.

Photocatalyzed cycloaromatization of vinylsilanes with arylsulfonylazides

Sila-molecules have recently attracted attention due to their promising applications in medical and industrial fields. Compared with all-carbon parent compounds, the different covalent radius and electronegativity of silicon from carbon generally endow the corresponding sila-analogs with unique biological activity and physicochemical properties. Vinylsilanes feature both silyl-hyperconjugation effect and versatile reactivities, developing vinylsilane-based Smiles rearrangement will therefore provide an efficient platform to assemble complex silacycles. Here we report a practical Ir(III)-catalyzed cycloaromatization of ortho-alkynylaryl vinylsilanes with arylsulfonyl azides for delivering naphthyl-fused benzosiloles under visible-light photoredox conditions. The combination of experiments and density functional theory (DFT) energy profiles reveals the reaction mechanism involving α-silyl radical Smiles rearrangement.

Arene-fused siloles have attracted interest due to their promising applications in electronic and optoelectronic devices. Here, the authors report Ir(III)-catalyzed cycloaromatization of ortho-alkynylaryl vinylsilanes with arylsulfonyl azides via α-silyl radical Smiles rearrangement for accessing naphthyl-fused benzosiloles under visible-light photoredox conditions.

Triarylmethanes and their Medium-Ring Analogues by Unactivated Truce-Smiles Rearrangement of Benzanilides

Intramolecular nucleophilic aromatic substitution (Truce–Smiles rearrangement) of the anions of 2‐benzyl benzanilides leads to triarylmethanes in an operationally simple manner. The reaction succeeds even without electronic activation of the ring that plays the role of electrophile in the S_NAr reaction, being accelerated instead by the preferred conformation imposed by the tertiary amide tether. The amide substituent of the product may be removed or transformed into alternative functional groups. A ring‐expanding variant (n to n+4) of the reaction provided a route to doubly benzo‐fused medium ring lactams of 10 or 11 members. Hammett analysis returned a ρ value consistent with the operation of a partially concerted reaction mechanism.

Copper-catalyzed [3 + 2]/[3 + 2] carboannulation of dienynes and arylsulfonyl chlorides enabled by Smiles rearrangement: access to cyclopenta[a]indene-fused quinolinones

A radical Smiles rearrangement strategy for allowing an unprecedented [3 + 2]/[3 + 2] carboannulation of dienynes with arylsulfonyl chlorides using cheap copper catalysis is described.

Photoredox-neutral alkene aminoarylation for the synthesis of 1,4,5,6-tetrahydropyridazines

A mild and redox-neutral protocol is developed for the synthesis of 1,4,5,6-tetrahydropyridazines via photoredox catalysis.

A Cascade Reaction of Michael Addition and Truce-Smiles Rearrangement to Synthesize Trisubstituted 4-Quinolone Derivatives

A novel transition-metal-free cascade reaction to synthesize 4-quinolone derivatives has been demonstrated. Michael addition and Truce-Smiles rearrangement are included in this protocol, providing a broad scope of 4-quinolones in moderate-to-excellent yields. This work serves as an example of the use of sulfonamides through Truce-Smiles rearrangement to build heterocyclic compounds under mild conditions.

Synthetic and medicinal chemistry of phthalazines: Recent developments, opportunities and challenges

Fused diaza-heterocycles constitute the core structure of numerous bioactive natural products and effective therapeutic drugs. Among them, phthalazines have been recognized as remarkable structural leads in medicinal chemistry due to their wide application in pharmaceutical and agrochemical industries. Accessing such challenging pharmaceutical agents/drug candidates with high chemical complexity through synthetically efficient approaches remains an attractive goal in the contemporary medicinal chemistry and drug discovery arena. In this review, we focus on the recent developments in the synthetic routes towards the generation of phthalazine-based active pharmaceutical ingredients and their biological potential against various targets. The general reaction scope of these innovative and easily accessible strategies was emphasized focusing on the functional group tolerance, substrate and coupling partner compatibility/limitation, the choice of catalyst, and product diversification. These processes were also accompanied by the mechanistic insights where deemed appropriate to demonstrate meaningful information. Moreover, the rapid examination of the structure-activity relationship analyses around the phthalazine core enabled by the pharmacophore replacement/integration revealed the generation of robust, efficient, and more selective compounds with pronounced biological effects. A large variety of in silico methods and ADME profiling tools were also employed to provide a global appraisal of the pharmacokinetics profile of diaza-heterocycles. Thus, the discovery of new structural leads offers the promise of improving treatments for various tropical diseases such as tuberculosis, leishmaniasis, malaria, Chagas disease, among many others including various cancers, atherosclerosis, HIV, inflammatory, and cardiovascular diseases. We hope this review would serve as an informative collection of structurally diverse molecules enabling the generation of mature, high-quality, and innovative routes to support the drug discovery endeavors.

Mechanistic insights into Smiles rearrangement. Focus on π-π stacking interactions along the radical cascade

The synthesis of new arene and heteroarene scaffolds of therapeutic interest has generated a renewed interest in the domino radical cyclisation-Smiles. In this work we present a detailed mechanistic investigation of the radical version of a cascade involving a desulfonative Smiles rearrangement on an aromatic ring bearing a sulfonamide linker. Competing routes have been explored to characterize the molecular mechanism of the studied reaction. The knowledge gained from previous experimental observations is explained through the energy profile obtained by means of quantum mechanical calculations. This study answers questions about the rate determining step and the type of mechanism involved (two-step or concerted). Supplementary rate constant calculations as well as quantum molecular dynamics support experimental observations. An IGM-δg analysis performed along the reaction path unveils and quantifies an intramolecular π-π stacking interaction accelerating the reaction. This novel post processing IGM-δg tool based on the electron density, turns out to be useful to monitor and quantify specific intramolecular weak interactions along a reaction path from wave functions. From this mechanistic investigation it turns out that Smiles rearrangement here takes place in two steps rather than in a direct intramolecular radical substitution. Furthermore, we show that chain length effects must be taken into account in the functionalization of new sulfonylated derivatives subjected to this radical cascade, given their influence in the reaction rate.

When Light Meets Nitrogen-Centered Radicals: From Reagents to Catalysts

Nitrogen-centered radicals (NCRs) are a versatile class of highly reactive species that have a longer history than the classical carbon-based radicals in synthetic chemistry. Depending on the N-hybridization and substitution patterns, NCRs can serve as electrophiles or nucleophiles to undergo various radical transformations. Despite their power, progress in nitrogen-radical chemistry is still slow compared with the popularity of carbon radicals, and their considerable synthetic potential has been largely underexplored, which is, as concluded by Zard, mainly hampered by "a dearth of convenient access to these species and a lack of awareness pertaining to their reactivity".Over the past decade, visible-light photoredox catalysis has been established as a powerful toolbox that synthetic chemists can use to generate a diverse range of radical intermediates from native organic functional groups via a single electron transfer process or energy transfer under mild reaction conditions. This catalytic strategy typically obviates the need for external stoichiometric activation reagents or toxic initiators and often enables traditionally inaccessible ionic chemical reactions. On the basis of our long-standing interest in nitrogen chemistry and catalysis, we have emphasized the use of visible-light photoredox catalysis as a tactic to discover and develop novel methods for generating NCRs in a controlled fashion and synthetic applications. In this Account, we describe our recent advances in the development of visible-light-driven photoredox-catalyzed generation of NCRs and their synthetic applications.Inspired by the natural biological proton-coupled electron transfer (PCET) process, we first developed a strategy of visible-light-driven photoredox-catalyzed oxidative deprotonation electron transfer to activate the N-H bonds of hydrazones, benzamides, and sulfonamides to give the corresponding NCRs under mild reaction conditions. With these reactive species, we then achieved a range of 5-exo and 6-endo radical cyclizations as well as cascade reactions in a highly regioselective manner, providing access to a variety of potentially useful nitrogen heterocycles. To further expand the repertoire of possible reactions of NCRs, we also revealed that iminyl radicals, derived from O-acyl cycloalkanone oxime esters, can undergo facile ring-opening C-C bond cleavage to give cyanoalkyl radicals. These newly formed radical species can further undergo a variety of C-C bond-forming reactions to allow the synthesis of diverse distally functionalized alkyl nitriles. Stimulated by these studies, we further developed a wide variety of visible-light-driven copper-catalyzed radical cross-coupling reactions of cyanoalkyl radicals. Because of their inherent highly reactive and transient properties, the strategy of heteroatom-centered radical catalysis is still largely underexplored in organic synthesis. Building on our understanding of the fundamental chemistry of NCRs, we also developed for the first time the concept of NCR covalent catalysis, which involves the use of in situ-photogenerated NCRs to activate allyl sulfones, vinylcyclopropanes, and N-tosyl vinylaziridines. This catalytic strategy has thus enabled efficient difunctionalization of various alkenes and late-stage modification of complex biologically active molecules.In this Account, we describe a panoramic picture of our recent contributions since 2014 to the development and application of the visible-light-driven photoredox systems in the field of NCR chemistry. These studies provide not only efficient methods for the synthesis of functionally rich molecules but also some insight into the exploration of new reactivity or reaction modes of NCRs.

Tandem Construction of Indole-Fused Phthalazines from (2-Alkynylbenzylidene)hydrazines under Metal-Free Conditions

An efficient approach to invent diversely substituted indole-fused phthalazines from in situ formed (2-alkynylbenzylidene)hydrazines under metal-free conditions via selective radical cyclization has been developed. Notably, this 6-exo-dig addition-cyclization tandem procedure proceeds under air atmosphere and shows a broad substrate suitability, as well as avoids harmful byproducts, which complies with the concept of green synthesis.

Ynamide Smiles Rearrangement Triggered by Visible-Light-Mediated Regioselective Ketyl-Ynamide Coupling: Rapid Access to Functionalized Indoles and Isoquinolines

In the past decades, significant advances have been made on radical Smiles rearrangement. However, the eventually formed radical intermediates in these reactions are limited to the amidyl radical, except for the few examples initiated by a N-centered radical. Here, a novel and practical radical Smiles rearrangement triggered by photoredox-catalyzed regioselective ketyl-ynamide coupling is reported, which represents the first radical Smiles rearrangement of ynamides. This method enables facile access to a variety of valuable 2-benzhydrylindoles with broad substrate scope in generally good yields under mild reaction conditions. In addition, this chemistry can also be extended to the divergent synthesis of versatile 3-benzhydrylisoquinolines through a similar ketyl-ynamide coupling and radical Smiles rearrangement, followed by dehydrogenative oxidation. Moreover, such an ynamide Smiles rearrangement initiated by intermolecular photoredox catalysis via addition of external radical sources is also achieved. By control experiments, the reaction was shown to proceed via key ketyl radical and α-imino carbon radical intermediates.

Visible light-mediated Smiles rearrangements and annulations of non-activated aromatics

Novel and efficient visible light-mediated Smiles rearrangements and annulations progressing via a radical-cation intermediate catalytically generated with an acridinium salt.

A visible light-mediated, decarboxylative, desulfonylative Smiles rearrangement for general arylethylamine syntheses

A decarboxylative, desulfonylative Smiles rearrangement is reported for the synthesis of a wide range of biologically relevant arylethylamines, including fluorinated phenylethylamines, heterocyclic amphetamines and an unnatural amino acid.

Visible-light-promoted N-centered radical generation for remote heteroaryl migration

An efficient visible-light-mediated organocatalyzed N–H heteroarylation was accomplished via remote heteroaryl ipso-migration.