Visible-Light-Induced Dearomatizations

Visible-light-induced dearomative 1,4-carbamoylpyridinylation of nonactivated naphthalenes

A visible-light-induced dearomative 1,4-carbamoylpyridinylation of nonactivated naphthalenes is described. The protocol provides rapid access to a series of pyridinylated spiro 1,2-dihydronaphthalenes in moderate yields by using naphthyl-substituted oxamic acids and 4-cyanopyridines as substrates through radical-radical cross-coupling followed by base-mediated alkene tautomerization. In addition, this method enabled late-stage functionalization of several drug derivatives, demonstrating the practical utility of this reaction.

Divergent Construction of Cyclobutane-Fused Pentacyclic Scaffolds via Double Dearomative Photocycloaddition

Cyclobutane-fused polycyclic scaffolds are structurally interesting cores in natural product synthesis and drug discovery. The construction of these skeletons often requires elaborate synthetic effort and gives low efficiency. We herein demonstrated the divergent construction of various cyclobutane-fused 2D/3D pentacyclic scaffolds by a photocatalytic intermolecular double dearomative cycloaddition of arenes. These skeletons, typically unattainable under thermal conditions, could be accessed with exclusive diastereoselectivity under mild photochemical conditions. Combined experimental and computational mechanistic studies elucidate that the reaction proceeds through a cascade sequence involving photocatalytic 1,4-hydroalkylation, alkene isomerization, and [2 + 2] cycloaddition via an intertwined single electron transfer (SET)/energy transfer (EnT) nature. This protocol provided a divergent synthetic approach for constructing (pseudo)-dimeric cyclobutane-fused 2D/3D pentacyclic scaffolds. The visible light induced intermolecular double dearomative cycloaddition between naphthalenes and benzothiophenes was also realized, providing indispensable methods for unprecedented structurally diverse polycyclic molecules that were difficult to access by conventional transformations.

Ligand-Controlled Regioselective Dearomative Vicinal and Conjugate Hydroboration of Quinolines

A dearomative strategy to regioselectively modify arenes using a "diene" synthon within aromatic rings provides access to highly functionalized heterocycles from abundant aromatic feedstocks and represents an alternative synthetic approach besides traditional cross-coupling and C-H functionalization methodologies. In this study, we present an efficient method for selectively introducing boron onto quinolines through dearomative hydroboration using easily accessible and stable phosphine-ligated borane complexes. The vicinal 5,6- and conjugate 5,8-hydroborated products could be obtained regioselectively by modifying the phosphine ligand. Drawing inspiration from diverse organoboron transformations, these borane building blocks were diversified by a range of downstream functionalizations, providing modular pathways for the skeletal modifications of quinolines to access a variety of challenging functionalized heterocycles.

Intramolecular ortho Photocycloaddition of 4-Substituted 7-(4'-Alkenyloxy)-1-indanones and Ensuing Reaction Cascades

4-Substituted 7-(4'-alkenyloxy)-1-indanones were prepared from the respective substituted aryl propanoic acids and subjected to UV-A irradiation (λ = 350 or 366 nm). While the 4-chloro compound was directly converted at λ = 366 nm into a pentacyclic product (47% yield) by a three-photon cascade process, the oxygenated substrates reacted in trifluoroethanol at λ = 350 nm by a two-photon cascade, involving an ortho photocycloaddition, a thermal disrotatory ring opening, and a [4π] photocyclization (six examples, 67-82% yield). An ensuing photochemical di-π-methane rearrangement of the latter products was achieved by irradiation at λ = 350 nm in toluene (five examples, 36-70% yield). The diastereoselectivity of the reaction was probed employing a chiral 1-indanone with a stereogenic center at carbon atom C3. 1-Indanones with a 4-hexenyloxy side chain [(E)- or (Z)-configured] at carbon atom C7 served to interrogate the stereospecifity of the reaction.

Atom-efficient chlorinative dearomatization of naphthol, quinolinol, and isoquinolinol derivatives using trichloroisocyanuric acid (TCCA)

A variety of dearomatized compounds have been prepared in moderate to excellent yields from planar scaffolds using trichloroisocyanuric acid (TCCA) as an atom-economical chlorinating agent. The method tolerates a broad range of functionalities and can take place in several green and/or sustainable solvents. Twenty-one examples of 1,1-dichlorinated products of dearomatized 2-naphthols and analogous heteroarenes (quinolinols, isoquinolinols, and quinazolinol) are reported along with five examples of monochlorinated dearomatized products. The utility of the 1,1-dichloronaphthalenone product as a reactive intermediate species is demonstrated in a two-step, one-pot reaction carried out in a green solvent. In a mechanistic investigation, the coordination of the chlorinating agent to the hydroxy substituent of the planar scaffold prior to chlorine transfer is implicated.

Chemodivergent dearomatization of benzene-linked O-oxime esters via EnT-induced radical cross-coupling

Radical-mediated dearomatization strategies offer a blueprint for building value-added and synthetically valuable three-dimensional skeletons from readily available aromatic starting materials. Herein, we report a novel strategy by leveraging benzene-linked O-oxime esters as triply functionalized precursors to form two distinct persistent radicals under a chemodivergent pathway. These radicals then couple with a cyclohexadienyl radical for either carboamination or carbo-aminoalkylation. Remarkably, a series of 4-(2-aminoethyl)anilines derivatives featuring all-carbon quaternary centers, along with the formation of four different types of chemical bonds, are efficiently constructed through a unique rearomatization cascade in the carboamination. Importantly, employing DMPU as the hydrogen atom transfer (HAT) donor strategically diverts the reaction pathway from the C-N bond formation towards the C-C bond formation. Our mechanistic explorations support a sequential HAT/energy transfer (EnT)/HAT cascade as the key stage for carbo-aminoalkylation involving the N-center iminyl radical. Significantly, this work demonstrates the elegant expansion of divergent C-N and C-C bond formation using the imine moiety within O-oxime esters as the bifunctional reagent, and it broadens the chemical space of both benzenes and O-oxime esters in radical-mediated transformations.

Boron Enabled Directed [2+2]- and Dearomative [4+2]-Cycloadditions Initiated by Energy Transfer

A strategy for the photosensitized [2+2]-cycloaddition between styrenyl dihaloboranes and unactivated allylamines to access cyclobutylboronates with control of stereochemistry and regiochemistry is presented. The success of the reaction relies on the temporary coordination between in situ generated dihaloboranes and amines under mild reaction conditions. In addition, cyclobutanes with varying substitution patterns have been prepared using N-heterocycles as directing group. Manipulation of the C-B bond allows for the synthesis of a diverse class of cyclobutanes from simple precursors. Moreover, these reactions lead to the synthesis of complex amines and heteroaromatic compounds, which have significant utility in medicinal chemistry. Finally, a dearomative [4+2]-cycloaddition of naphthalenes using a boron-enabled temporary tethering strategy has also been uncovered to synthesize complex 3-dimensional borylated building blocks.

Complexity-Building Exhaustive Dearomatization of Benzenoid Aromatics within an ESIPT-Initiated Three-Step Photochemical Cascade

Dearomative cycloadditions offer rapid access to complex 3D molecular architectures, commonly via a sp2-to-sp3 rehybridization of two atoms of an aromatic ring. Here we report that the 6e π-system of a benzenoid aromatic pendant could be exhaustively depleted within a single photochemical cascade. An implementation of this approach involves the initial dearomative [4+2] cycloaddition of the Excited State Intramolecular Proton Transfer (ESIPT)-generated azaxylylene, followed by two consecutive [2+2] cycloadditions of auxiliary π moieties strategically positioned in the photoprecursor. Such photochemical cascade fully dearomatizes the benzenoid aromatic ring, saturating all six sp2 atoms to yield a complex sp3-rich scaffold with high control of its 3D molecular shape, rendering it a robust platform for rapid systematic mapping of underexplored chemical space. Significant growth of molecular complexity-starting with a modular synthesis of photoprecursors from readily available building blocks-is quantified by Böttcher score calculations.

Dearomative hydroamination of heteroarenes catalyzed by the phenolate photocatalyst

Dearomative functionalization of heteroarenes offers an attractive and sustainable approach for the rapid construction of complex 3D heterocyclic scaffolds from planar structures. Despite progress in this field, dearomative amination of heteroarenes via a radical anion intermediate remains a challenge. Here, we report a photoredox-catalyzed dearomative hydroamination of heteroarenes with hydrazodiformates under mild and transition-metal-free reaction conditions. Various benzofurans and benzothiophenes can efficiently participate in this transformation. A series of mechanistic experiments revealed that heteroaryl radical anions are the crucial intermediates, generated through photo-induced electron transfer between the excited phenolate photocatalyst and heteroarenes.

Construction of fused heterocycles by visible-light induced dearomatization of nonactivated arenes

A diverse array of fused [6-6-5] tricyclic heterocycles has been synthesized via the dimerization and dearomative cyclization of benzene derivatives under visible light irradiation. The initiation of the cascade process is likely from aryloxy radicals, engendered through proton-coupled electron transfer by the photoexcited vinylidene ortho-quinone methide (VQM) and a Brønsted base.

Dearomative Construction of 2D/3D Frameworks from Quinolines via Nucleophilic Addition/Borate-Mediated Photocycloaddition

Dearomative construction of multiply-fused 2D/3D frameworks, composed of aromatic two-dimensional (2D) rings and saturated three-dimensional (3D) rings, from readily available quinolines has greatly contributed to drug discovery. However, dearomative cycloadditions of quinolines in the presence of photocatalysts usually afford 5,6,7,8-tetrahydroquinoline (THQ)-based polycycles, and dearomative access to 1,2,3,4-THQ-based structures remains limited. Herein, we present a chemo-, regio-, diastereo-, and enantioselective dearomative transformation of quinolines into 1,2,3,4-THQ-based 6-6-4-membered rings without any catalyst, through a combination of nucleophilic addition and borate-mediated [2+2] photocycloaddition. Detailed mechanistic studies revealed that the photoexcited borate complex, generated from quinoline, organolithium, and HB(pin), accelerates the cycloaddition and suppresses the rearomatization that usually occurs in conventional photocycloaddition. Based on our mechanistic analysis, we also developed further photoinduced cycloadditions affording other types of 2D/3D frameworks from isoquinoline and phenanthrene.

Tribromide enabled step-up generation of spirolactams from esters employing oxidative dearomatization of arenols

Synthetically challenging spirolactams were developed in good yields and regio-selectivity through a step-up oxidative dearomatization of easily accessible arenols tethered to esters in the presence of quaternary ammonium tribromide as an economic oxidant and amines. The reaction mechanism associated with this unprecedented dearomative lactamisation has been forecasted with a series of controlled experiments and DFT studies.

Catalytic Asymmetric Dearomative [2 + 2] Photocycloaddition/Ring-Expansion Sequence of Indoles with Diversified Alkenes

Developing novel strategies for catalytic asymmetric dearomatization (CADA) reactions is highly valuable. Visible light-mediated photocatalysis is demonstrated to be a powerful tool to activate aromatic compounds for further synthetic transformations. Herein, a catalytic asymmetric dearomative [2 + 2] photocycloaddition/ring-expansion sequence of indoles with simple alkenes was reported, providing a facile access to enantioenriched cyclopenta[b]indoles with good to high yields and enantioselectivities by means of chiral lanthanide photocatalysis. This protocol exhibited a broad substrate scope and good functional group tolerance, as well as potential applications in the synthesis of bioactive molecules. Mechanistic studies, including control experiments, UV-vis absorption spectroscopy, emission spectroscopy, and DFT calculations, were carried out, shedding insights into the reaction mechanism and the origin of enantioselectivity.

Electrophile-Arene Affinity: An Energy Scale for Evaluating the Thermodynamics of Electrophilic Dearomatization Reactions

Rational design and development of organic reactions are lofty goals in synthetic chemistry. Quantitative description of the properties of molecules and reactions by physical organic parameters plays an important role in this regard. In this Article, we report an energy scale, namely, electrophile-arene affinity (EAA), for evaluating the thermodynamics of electrophilic dearomatization reactions, a class of important transformations that can rapidly build up molecular complexity and structural diversity by converting planar aromatic compounds into three-dimensional cyclic molecules. The acquisition of EAA data can be readily achieved by theoretically calculating the enthalpy changes (ΔH) of the hypothetical reactions of various (cationic) electrophiles with aromatic systems (taking the 1-methylnaphthalen-2-olate ion as an example in this study). Linear correlations are found between the calculated ΔH values and established physical organic parameters such as the percentage of buried volume %VBur (steric effect), Hammett's σ or Brown's σ+ (electronic effect), and Mayr's E (reaction kinetics). Careful analysis of the ΔH values leads to the rational design of a dearomative alkynylation reaction using alkynyl hypervalent iodonium reagents as the electrophiles.

Umpolung-Enabled Divergent Dearomative Carbonylations

Although dearomative functionalizations enable the direct conversion of flat aromatics into precious three-dimensional architectures, the case for simple arenes remains largely underdeveloped owing to the high aromatic stabilization energy. We herein report a dearomative sequential addition of two nucleophiles to arene π-bonds through umpolung of chromium-arene complexes. This mode enables divergent dearomative carbonylation reactions of benzene derivatives by tolerating various nucleophiles in combination with alcohols or amines under CO-gas-free conditions, thus providing modular access to functionalized esters or amides. The tunable synthesis of 1,3- or 1,4-cyclohexadienes as well as the construction of carbon quaternary centers further highlight the versatility of this dearomatization. Diverse late-stage modifications and derivatizations towards synthetically challenging and bioactive molecules reveal the synthetic utility. A possible mechanism was proposed based on control experiments and intermediate tracking.

Organocatalytic Dearomatization of 5-Aminopyrazoles: Synthesis of 4-Hydroxypyrazolines

Dearomatization is a fundamental chemical reaction that affords complex three-dimensional heterocyclic frameworks. We disclose the first organocatalytic dearomatization of 5-aminopyrazoles, which yields a range of structurally diversified C4-hydroxylated pyrazolines with yields of ≤95% in <1.5 h at room temperature. This catalytic process is achieved using in situ-generated hypervalent iodine. The method also yields a spirolactone via an intramolecular dearomatization process. Furthermore, we demonstrate that substrate-directed reduction of the resulting iminopyrazoline leads to 4,5-difunctionalized pyrazoline as a single diastereomer. Mechanistic studies suggest that the reaction proceeds through a dearomatized cationic intermediate.

Gd(III)-Catalyzed Regio-, Diastereo-, and Enantioselective [4 + 2] Photocycloaddition of Naphthalene Derivatives

Catalytic asymmetric dearomatization (CADA) reactions have evolved into an efficient strategy for accessing chiral polycyclic and spirocyclic scaffolds from readily available planar aromatics. Despite the significant developments, the CADA reaction of naphthalenes remains underdeveloped. Herein, we report a Gd(III)-catalyzed asymmetric dearomatization reaction of naphthalene with a chiral PyBox ligand via visible-light-enabled [4 + 2] cycloaddition. This reaction features application of a chiral Gd/PyBox complex, which regulates the reactivity and selectivity simultaneously, in excited-state catalysis. A wide range of functional groups is compatible with this protocol, giving the highly enantioenriched bridged polycycles in excellent yields (up to 96%) and selectivity (up to >20:1 chemoselectivity, >20:1 dr, >99% ee). The synthetic utility is demonstrated by a 2 mmol scale reaction, removal of directing group, and diversifications of products. Preliminary mechanistic experiments are performed to elucidate the reaction mechanism.

Photochemical dearomative skeletal modifications of heteroaromatics

Dearomatization has emerged as a powerful tool for rapid construction of 3D molecular architectures from simple, abundant, and planar (hetero)arenes. The field has evolved beyond simple dearomatization driven by new synthetic technology development. With the renaissance of photocatalysis and expansion of the activation mode, the last few years have witnessed impressive developments in innovative photochemical dearomatization methodologies, enabling skeletal modifications of dearomatized structures. They offer truly efficient and useful tools for facile construction of highly complex structures, which are viable for natural product synthesis and drug discovery. In this review, we aim to provide a mechanistically insightful overview on these innovations based on the degree of skeletal alteration, categorized into dearomative functionalization and skeletal editing, and to highlight their synthetic utilities.

Enantioselective Intramolecular ortho Photocycloaddition Reactions of 2-Acetonaphthones

2-Acetonaphthones, which bear an alkenyl group tethered to its C1 carbon atom via an oxygen atom, were found to undergo an enantioselective intramolecular ortho photocycloaddition reaction. A chiral oxazaborolidine Lewis acid leads to a bathochromic absorption shift of the substrate and enables an efficient enantioface differentiation. Visible light irradiation (λ=450 nm) triggers the reaction which is tolerant of various groups at almost any position except carbon atom C8 (16 examples, 53-99 % yield, 80-97 % ee). Consecutive reactions were explored including a sensitized rearrangement to tetrahydrobiphenylenes, which occurred with full retention of configuration. Evidence was collected that the catalytic photocycloaddition occurs via triplet intermediates, and the binding mode of the acetonaphthone to the chiral Lewis acid was elucidated by DFT calculations.

Photo-induced intramolecular dearomative [5 + 4] cycloaddition of arenes for the construction of highly strained medium-sized-rings

Medium-sized-ring compounds have been recognized as challenging synthetic targets in organic chemistry. Especially, the difficulty of synthesis will be augmented if an E-olefin moiety is embedded. Recently, photo-induced dearomative cycloaddition reactions that proceed via energy transfer mechanism have witnessed significant developments and provided powerful methods for the organic transformations that are not easily realized under thermal conditions. Herein, we report an intramolecular dearomative [5 + 4] cycloaddition of naphthalene-derived vinylcyclopropanes under visible-light irradiation and a proper triplet photosensitizer. The reaction affords dearomatized polycyclic molecules possessing a nine-membered-ring with an E-olefin moiety in good yields (up to 86%) and stereoselectivity (up to 8.8/1 E/Z). Detailed computational studies reveal the origin behind the favorable formation of the thermodynamically less stable isomers. Diverse derivations of the dearomatized products have also been demonstrated.

Tunable C-H functionalization and dearomatization enabled by an organic photocatalyst

C-H functionalization and dearomatization constitute fundamental transformations of aromatic compounds, which find wide applications in various research areas. However, achieving both transformations from the same substrates with a single catalyst by operating a distinct mechanism remains challenging. Here, we report a photocatalytic strategy to modulate the reaction pathways that can be directed toward either C-H functionalization or dearomatization under redox-neutral or net-reductive conditions, respectively. Two sets of indoles and indolines bearing tertiary alcohols are divergently furnished with good yields and high selectivity. The key to success is the introduction of isoazatruxene ITN-2 as a novel photocatalyst (PC), which outperforms the commonly used PCs. The ready synthesis and high modulability of isoazatruxene type PCs indicate their great application potential.

Regioselective Dearomative Amidoximation of Nonactivated Arenes Enabled by Photohomolytic Cleavage of N-nitrosamides

Dearomative spirocyclization reactions represent a promising means to convert arenes into three-dimensional architectures; however, controlling the regioselectivity of radical dearomatization with nonactivated arenes to afford the spirocyclizative 1,2-difunctionalization other than its kinetically preferred 1,4-difunctionalization is exceptionally challenging. Here we disclose a novel strategy for dearomative 1,2- or 1,4-amidoximation of (hetero)arenes enabled by direct visible-light-induced homolysis of N-NO bonds of nitrosamides, giving rise to various highly regioselective amidoximated spirocycles that previously have been inaccessible or required elaborate synthetic efforts. The mechanism and origins of the observed regioselectivities were investigated by control experiments and density functional theory calculations.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Energy transfer photocatalysis: exciting modes of reactivity

Excited (triplet) states offer a myriad of attractive synthetic pathways, including cycloadditions, selective homolytic bond cleavages and strain-release chemistry, isomerizations, deracemizations, or the fusion with metal catalysis. Recent years have seen enormous advantages in enabling these reactivity modes through visible-light-mediated triplet-triplet energy transfer catalysis (TTEnT). This tutorial review provides an overview of this emerging strategy for synthesizing sought-after organic motifs in a mild, selective, and sustainable manner. Building on the photophysical foundations of energy transfer, this review also discusses catalyst design, as well as the challenges and opportunities of energy transfer catalysis.

Photoredox-Enabled Dearomative [2π + 2σ] Cycloaddition of Phenols

Dearomative photocycloaddition of monocyclic arenes is an appealing strategy for comprehending the concept of "escape from flatland". This brings the replacement of readily available planar aromatic hydrocarbon units with a 3D fused bicyclic core with sp3-enriched carbon units. Herein, we outline an intermolecular approach for the dearomative photocycloaddition of phenols. In order to circumvent the ground-state aromaticity and to construct conformationally restrained building blocks, bicyclo[1.1.0]butanes were chosen as coupling partners. This dearomative approach renders straightforward access to a bicyclo[2.1.1]hexane unit fused to a cyclic enone moiety, which further contributed as a synthetic linchpin for postmodifications. Mechanistic experiment advocates for a plausible onset from both the reactants, depending on the redox potential.

Diversification of Simple Arenes into Complex (Amino)cyclitols

Highly oxygenated cyclohexanes, including (amino)cyclitols, are featured in natural products possessing a notable range of biological activities. As such, these building blocks are valuable tools for medicinal chemistry. While de novo synthetic strategies have provided access to select compounds, challenges including stereochemical density and complexity have hindered the development of a general approach to (amino)cyclitol structures. This work reports the use of arenophile chemistry to access dearomatized intermediates which are amenable to diverse downstream transformations. Practical guidelines were developed for the synthesis of natural and non-natural (amino)cyclitols from simple arenes through a series of strategic functionalization events.

Synthesis of Pyrroloindolines via N-Heterocyclic Carbene Catalyzed Dearomative Amidoacylation of Indole Derivatives

Pyrroloindoline is a privileged heterocyclic motif that is widely present in many natural products and pharmaceutical compounds. Herein, we report an amidyl radical-mediated dearomatization for synthesizing a series of pyrroloindolines via N-heterocyclic carbene catalysis. In this organocatalytic process, the Breslow enolate served as both a single electron donor and an acyl radical equivalent to assemble C3a-acyl pyrroloindolines with a broad substrate scope. Sequential reduction of the indole derivatives provided the analogues of (±)-desoxyeseroline, which exhibited potential anticancer activity.

Dearomative Ring-Fused Azafulleroids and Carbazole-Derived Metallofullerenes: Reactivity Dictated by Encapsulation in a Fullerene Cage

Herein, we report divergent additions of 2,2'-diazidobiphenyls to C60 and Sc3 N@Ih -C80 . In stark contrast to that of the previously reported bis-azide additions, the unexpected cascade reaction leads to the dearomative formation of azafulleroids 2 fused with a 7-6-5-membered ring system in the case of C60 . In contrast, the corresponding reaction with Sc3 N@Ih -C80 switches to the C-H insertion pathway, thereby resulting in multiple isomers, including a carbazole-derived [6,6]-azametallofulleroid 3 and a [5,6]-azametallofulleroid 4 and an unusual 1,2,3,6-tetrahydropyrrolo[3,2-c]carbazole-derived metallofullerene 5, whose molecular structures have been unambiguously determined by single-crystal X-ray diffraction analyses. Among them, the addition type of 5 is observed for the first time in all reported additions of azides to fullerenes. Furthermore, unexpected isomerizations from 3 to 5 and from 4 to 5 have been discovered, providing the first examples of the isomerization of an azafulleroid to a carbazole-derived fullerene rather than an aziridinofullerene. In particular, the isomerism of the [5,6]-isomer 4 to the [5,6]-isomer 5 is unprecedented in fullerene chemistry, contradicting the present understanding that isomerization generally occurs between [5,6]- and [6,6]-isomers. Control experiments have been carried out to rationalize the reaction mechanism. Furthermore, representative azafulleroids have been applied in organic solar cells, thereby resulting in improved power conversion efficiencies.

Photosensitized [4+2]- and [2+2]-Cycloaddition Reactions of N-Sulfonylimines

The synthesis of polycyclic compounds is of high interest due to the prevalence of these motifs in drugs and natural products. Herein, we report on the stereoselective construction of 3D bicyclic scaffolds and azetidine derivatives by modulation of N-sulfonylimines to achieve either [4+2]- or [2+2]-cycloaddition reactions. The utility of the method was established by further modulation of the product. Mechanistic studies are also included, which support reaction via Dexter energy transfer.

Construction of a Bicyclo[2.2.2]octene Skeleton via a Visible-Light-Mediated Radical Cascade Reaction of Amino Acid Derivatives with N-(2-Phenyl)benzoyl Groups

Bridged polycyclic ring systems constitute the core structures of numerous natural products and biologically active molecules. We found that simple biphenyl substrates derived from amino acids participate in a radical cascade reaction under visible light irradiation in the presence of [Ir{dF(CF₃)ppy}₂(dtbpy)]PF₆ to enable the direct construction of bicyclo[2.2.2]octene structures. Isotopic labeling experiments suggested that intramolecular hydrogen atom transfer is involved in the cascade processes.

Aqueous Flavin Photoredox Catalysis Drives Exclusive C3-Alkylation of Indolyl Radical Cations in Route to Unnatural Tryptophan Mimetics

One way to build chemical diversity into indoles is to oxidize them to indolyl radical cations (Ind•+). These intermediates can accept new functional groups across C2-C3 bonds or independently at C2. Less encountered is selective diversification at C3, a position plagued by competing dearomative side reactions. We disclose an aqueous photoredox-catalyzed method for transforming Ind•+ into C3-substituted tryptophan mimetics that uses water as a transient protecting group to guide site-selective C3 alkylation.

Brønsted Acid-Promoted Intermolecular Dearomative Photocycloaddition of Bicyclic Azaarenes with Olefins under Aerobic Conditions

Herein, we present a simplified reaction protocol for the dearomatization of bicyclic azaarenes via photochemical cycloaddition with alkenes using an Ir(III) photosensitizer, trifluoroacetic acid (TFA), dichloroethane, and a blue light-emitting diode. An efficient protonation of azaarenes with TFA enhances the reactivity of triplet azaarene toward olefins, enabling the photocycloaddition under aerobic conditions. The protocol applies to a broad range of substrates. Control experiments indicate a strong correlation between the degree of protonation of azaarene and the product yield.

Photoinduced Vinylogous Dearomatization

The first vinylogous dearomatization is reported. Under a photoinduced platform, various benzothiophenes functionalized by ketones at the 3-position could react with 3-methylenechroman-4-ones efficiently, leading to a variety of valuable products that contain the pharmaceutically significant chromones and 2,3-dihydrobenzo[b]thiophenes concurrently. The transformations were revealed to experience hydrogen-atom transfer, dearomatization, olefin migration, and radical cross coupling.

Formation of Isolable Dearomatized [4 + 2] Cycloadducts from Benzenes, Naphthalenes, and N-Heterocycles Using 1,2-Dihydro-1,2,4,5-tetrazine-3,6-diones as Arenophiles under Visible Light Irradiation

We report that the dearomative [4 + 2] cycloaddition between 1,2-dihydro-1,2,4,5-tetrazine-3,6-diones (TETRADs) and benzenes, naphthalenes, or N-heteroaromatic compounds under visible light irradiation affords the corresponding isolable cycloadducts. Several synthetic transformations including transition-metal-catalyzed allylic substitution reactions using the isolated cycloadducts at room temperature or above were demonstrated. Computational studies revealed that the retro-cycloaddition of the benzene-TETRAD adduct proceeds via an asynchronous concerted mechanism, while that of the benzene-MTAD adduct (MTAD = 4-methyl-1,2,4-triazoline-3,5-dione) proceeds via a synchronous mechanism.

Visible-Light Promoted Intramolecular para-Cycloadditions on Simple Aromatics

Dearomative cycloadditions are a powerful tool to access a large chemical space exploiting simple and ubiquitous building blocks. The energetic burden due to the loss of aromaticity has however greatly limited their synthetic potential. We devised a general intramolecular method that overcomes these limitations thanks to the photosensitization of allenamides. The visible-light-promoted process gives complex [2.2.2]-(hetero)-bicyclooctadienes at room temperature, likely through the stabilization of transient (bi)radicals by naphthalene. The reaction tolerates several valuable functionalities, offering a convenient handle for a myriad of applications, including original isoindoles and metal complexes.

Photoredox-Enabled Dearomatization of Protected Anilines: Access to Cyclohexadienone Imines with Contiguous Quaternary Centers

A photoredox-enabled alkylative dearomatization of protected anilines is reported. Under Ir catalysis and light irradiation, an N-carbamoyl-protected aniline and an α-bromocarbonyl compound could be simultaneously activated, and the two resulting radical species then recombine with each other to afford a dearomatized cyclohexadienone imine as the major product. A series of such imines with contiguous quaternary carbon centers were prepared, which can be further converted into cyclohexadienones, cyclohexadienols, and cyclohexyl amines.

Dearomatization of Biaryls through Polarity Mismatched Radical Spirocyclization

Dearomatization reactions involving radical cyclizations can facilitate the synthesis of complex polycyclic systems that find applications in medicinal chemistry and natural product synthesis. Here we employ redox-neutral photocatalysis to affect a radical spirocyclization that transforms biaryls into spirocyclic cyclohexadienones under mild reaction conditions. In a departure from previously reported methods, our work demonstrates the polarity mismatched addition of a nucleophilic radical to an electron rich arene, and allows the regioselective synthesis of 2,4- or 2,5-cyclohexadienones with broad functional group tolerance. By transforming biaryls into spirocycles, our methodology accesses underexplored three-dimensional chemical space, and provides an efficient means of creating quaternary spirocenters that we apply to the first synthesis of the cytotoxic plant metabolite denobilone A.

Uncanonical Semireduction of Quinolines and Isoquinolines via Regioselective HAT-Promoted Hydrosilylation

Heterocycles are the backbone of modern medical chemistry and drug development. The derivatization of "an olefin" inside aromatic rings represents an ideal approach to access functionalized saturated heterocycles from abundant aromatic building blocks. Here, we report an operationally simple, efficient, and practical method to selectively access hydrosilylated and reduced N-heterocycles from bicyclic aromatics via a key diradical intermediate. This approach is expected to facilitate complex heterocycle functionalizations that enable access to novel medicinally relevant scaffolds.

Visible-Light-Induced Cascade Difunctionalization of Indoles Enabled by the Synergy of Photoredox and Photoexcited Ketones: Direct Access to Alkylated Pyrrolophenanthridones

Herein, we describe a methodology to construct polycyclic pyrrolophenanthridones with an (amino)alkyl side chain that involves visible-light-induced decarboxylative radical addition for the intermolecular dearomatization of indoles and subsequent photoinduced C(sp²)-X bond activation via photoexcited ketones for an intramolecular cyclization cascade. Carboxylic acids serve both as a radical source toward indole dearomatization and as reductants to initiate an electron transfer with photoexcited N-acylindole derivatives in the reaction toward pyrrolophenantridone skeletons, which occurs under mild reaction conditions with good functional group tolerance.

Synthesis of complex bicyclic scaffolds by intermolecular photosensitized dearomative cycloadditions of activated alkenes and naphthalenes

The rapid buildup of molecular complexity from simple precursors is a key goal in organic chemistry. One strategy to achieve this is through a dearomative cycloaddition wherein a 2D arene and alkene is converted to a 3D structure. In many cases this type of reactivity has been achieved with photochemistry. Despite the prospect of such a reaction, most known variants are intramolecular, which greatly limits the scope of chemical space that can be accessed. Intermolecular variants are known but are generally limited to heterocyclic systems such as indoles or quinolines. Herein, a method for intermolecular dearomative cycloaddition of simple naphthalenes with alkenes is presented. The reactions operate by a photoinduced sensitization of the arene. The bridged bicyclic products are generated with control of regiochemistry and function for a range of alkenes. In addition, the products can serve as useful intermediates as demonstrated in the synthesis of a biologically active benzazapine analog. Mechanistic studies are also included, which support reaction via a triplet excited state and that the selectivity can be rationalized by spin-density calculations.

Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions

While 2-alk-ω-enyloxy-sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ=366 nm) or under sensitizing conditions (λ=420 nm, 2.5 mol% thioxanthen-9-one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44-99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3-position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regio- and diastereoselectivity (2 examples, 58-79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò-Büchi reaction (2 examples, 95-98 % yield).

Photochemical Dearomative Cycloadditions of Quinolines and Alkenes: Scope and Mechanism Studies

Photochemical dearomative cycloaddition has emerged as a useful strategy to rapidly generate molecular complexity. Within this context, stereo- and regiocontrolled intermolecular para-cycloadditions are rare. Herein, a method to achieve photochemical cycloaddition of quinolines and alkenes is shown. Emphasis is placed on generating sterically congested products and reaction of highly substituted alkenes and allenes. In addition, the mechanistic details of the process are studied, which revealed a reversible radical addition and a selectivity-determining radical recombination. The regio- and stereochemical outcome of the reaction is also rationalized.

Energy-Transfer-Enabled Dearomative Cycloaddition Reactions of Indoles/Pyrroles via Excited-State Aromatics

Exploring the enormous chemical space through an expedient building-up of molecular diversity is an important goal of organic chemistry. The development of synthetic methods toward molecules with unprecedented structural motifs lays the foundation for wide applications ranging from pharmaceutical chemistry to materials science. In this regard, the dearomatization of arenes has been recognized as a unique strategy since it provides novel retrosynthetic disconnections for various spiro or fused polycyclic molecules with increased saturation and stereoisomerism. However, inherent thermodynamic challenges are associated with dearomatization processes. The disruption of the aromaticity of arene substrates usually requires large energy inputs, which makes harsh conditions necessary for many ground-state dearomatization reactions. Therefore, further expansion of the scope of dearomatization reactions remains a major problem not fully solved in organic chemistry.The past decade has witnessed tremendous progress on photocatalytic reactions under visible light. Particularly, reactions via an energy transfer mechanism have unlocked new opportunities for dearomatization reactions. Mediated by appropriately chosen photosensitizers, aromatic substrates can be excited. This kind of precise energy input might make feasible some dearomatization reactions that are otherwise unfavorable under thermal conditions because of the significant energy increases of the substrates. Nevertheless, the lifetimes of key intermediates in energy-transfer-enabled reactions, such as excited-state aromatics and downstream biradical species, are quite short. How to regulate the reactivities of these transient intermediates to achieve exclusive selectivity toward a certain reaction pathway among many possibilities is a crucial issue to be addressed.Since 2019, our group has reported a series of visible-light-induced dearomative cycloaddition reactions for indole and pyrrole derivatives. It was found that the aromatic units in substrates can be excited under the irradiation of visible light in the presence of a suitable photosensitizer. These excited aromatics readily undergo various [m + n] cycloaddition reactions with appropriately tethered unsaturated functionalities including alkenes, alkynes, N-alkoxy oximes, (hetero)arenes, and vinylcyclopropanes. The reactions yield polycyclic indolines and pyrrolines with highly strained small- and/or medium-sized rings embedded, some of which possess unique bridge- or cagelike topologies. Systematic mechanistic studies confirmed the involvement of an energy transfer process. Density functional theory (DFT) calculations revealed the correlation between the substrate structure and the excitation efficiency, which accelerated the optimization of the reaction parameters. Meanwhile, DFT calculations demonstrated the competition between kinetically and thermodynamically controlled pathways for the open-shell singlet biradical intermediates, which allowed the complete switches from [2 + 2] cycloaddition to 1,5-hydrogen atom transfer in reactions with N-alkoxy oximes and to [4 + 2] cycloaddition in reactions with naphthalene. Furthermore, ab initio molecular dynamics (AIMD) simulations uncovered post-spin crossing dynamic effects, which determine the regioselectivity for the open-shell singlet biradical recombination step in the reactions of pyrrole-derived vinylcyclopropanes.An increasing number of scientists have joined in the research on visible-light-induced dearomative cycloaddition reactions and contributed to more elegant examples in this area. The visible-light-induced dearomatization reaction via energy transfer mechanism, although still in its infancy, has exhibited great potential in the synthesis of molecules that can hardly be accessed by other methods. We believe that future development will further push the boundary of organic chemistry and find applications in the synthesis of functional molecules and related disciplines.

Highly congested spiro-compounds via photoredox-mediated dearomative annulation cascade

Photo-mediated radical dearomatization involving 5-exo-trig cyclizations has proven to be an important route to accessing spirocyclic compounds, whereas 6-exo-trig spirocyclization has been much less explored. In this work, a dearomative annulation cascade is realized through photoredox-mediated C-O bond activation of aromatic carboxylic acids to produce two kinds of spirocyclic frameworks. Mechanistically, the acyl radical is formed through oxidation of triphenylphosphine and subsequent C-O bond cleavage, followed by a 6-exo-trig cyclization/SET/protonation sequence to generate the spiro-chromanone products in an intramolecular manner. Furthermore, the protocol was extended to more challenging intermolecular tandem sequences consisting of C-O bond cleavage, radical addition to an alkene substrate, and 5-exo-trig cyclization to yield complex spirocyclic lactams.