Photochemical intermolecular dearomative cycloaddition of bicyclic azaarenes with alkenes

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.

Light-enabled intramolecular [2 + 2] cycloaddition via photoactivation of simple alkenylboronic esters

The photoactivation of organic molecules via energy transfer (EnT) catalysis is often limited to conjugated systems that have low-energy triplet excited states, with simple alkenes remaining an intractable challenge. The ability to address this limitation, using high energy sensitizers, would represent an attractive platform for future reaction design. Here, we disclose the photoactivation of simple alkenylboronic esters established using alkene scrambling as a rapid reaction probe to identify a suitable catalyst and boron motif. Cyclic voltammetry, UV-vis analysis, and control reactions support sensitization, enabling an intramolecular [2 + 2] cycloaddition to be realized accessing 3D bicyclic fragments containing a boron handle.

Transforming 2D azolium salts to 3D caged tertiary amines via stereoselective dearomative cascade annulation

Three-dimensional fused-ring frameworks, especially those incorporating heteroatoms, are fundamental to expanding chemical space and unlocking unique properties critical for drug discovery and functional materials, yet their synthesis remains a formidable challenge. Herein, we report for the first time the union of two distinct azolium salts as an efficient synthetic platform to access tertiary amine-caged frameworks under mild conditions. The strategy combines the masked nucleophilic and electrophilic properties of isoquinolinium and pyridinium salts, and avails double dearomatization guided inverse electron demand (4 + 2) or (3 + 2) annulation in a highly regio- and diastereoselective manner to construct the nitrogen caged motifs. Our methodology creates two new rings and four new bonds in a single operation and transforms flat-aromatic compounds into structurally unprecedented three-dimensional architectures with contiguous stereocenters in very high yields. DFT studies have shed light on the reaction mechanism, indicating that the annulation step is rate-limiting, with (4 + 2) annulation proceeding stepwise and (3 + 2) annulation following a concerted pathway.

Supramolecular Gel Impede Oxygen Permeation and Foster Air-Sensitive Intermolecular Dearomative [4+2] Cycloaddition of Naphthalenes by Visible Light Energy Transfer Catalysis

The dearomative cycloaddition reaction provides a framework for transforming flat aromatic compounds into sp3-rich 3D molecular structures. Nonetheless, the instability of the triplet excited state of the photocatalyst and substrates in the presence of triplet quencher oxygen has so far necessitated an oxygen-free environment. Here, in this study, we illustrate that efficient intermolecular dearomative [4+2] cycloaddition via visible light energy-transfer catalysis can be conducted in the presence of air by employing readily assembled organogel networks as reaction platforms, producing outcomes akin to those under inert conditions. However, these reactions are completely suppressed in a homogeneous solution exposed to air. Our design has enabled the triplet-sensitized chemical reactions, confined within the segregated solvent pools amidst the nanofibers under aerobic conditions.

Enantioselective dearomative ortho-cycloaddition transformation of unactivated arenes by cage-confined visible-light photocatalysis

Photoinduced dearomatization of arenes is a powerful strategy in organic synthesis to disrupt the stable aromaticity; however, the asymmetric dearomatization photocatalysis of unactivated arenes remains highly challenging and rare. Herein we demonstrate an enzyme-mimicking cage-confined visible-light asymmetric photocatalysis method for intramolecular dearomative cycloaddition with electron-deficient β-aryl enones. Owing to the multi-functional synergy of chirality, energy transfer, and host-guest interactions in the confined microenvironments, the self-assembled chiral cage-photoreactor could pre-organize the arenes and activate the β-aryl enones to give stereoselectively fused cyclobutanes through visible-light induced [2 + 2] ortho-cycloaddition. Notably, the competing transformation to stable [4 + 2] cycloadducts has been inhibited, producing thermodynamically unfavorable [2 + 2] cycloadducts with excellent regio-, diastereo-, and enantioselectivities.

Visible light-mediated dearomative spirocyclization/imination of nonactivated arenes through energy transfer catalysis

Aromatic compounds serve as key feedstocks in the chemical industry, typically undergoing functionalization or full reduction. However, partial reduction via dearomative sequences remains underexplored despite its potential to rapidly generate complex three-dimensional scaffolds and the existing dearomative strategies often require metal-mediated multistep processes or suffer from limited applicability. Herein, a photocatalytic radical cascade approach enabling dearomative difunctionalization through selective spirocyclization/imination of nonactivated arenes is reported. The method employs bifunctional oxime esters and carbonates to introduce multiple functional groups in a single step, forming spirocyclic motifs and iminyl functionalities via N-O bond cleavage, hydrogen-atom transfer, radical addition, spirocyclization, and radical-radical cross-coupling. The reaction constructs up to four bonds (C-O, C-C, C-N) from simple starting materials. Its broad applicability is demonstrated on various substrates, including pharmaceuticals, and it is compatible with scale-up under flow conditions, offering a streamlined approach to synthesizing highly decorated three-dimensional frameworks.

Photocatalytic Hydrogenation of Quinolines to Form 1,2,3,4-Tetrahdyroquinolines Using Water as the Hydrogen Atom Donor

The design of a sequential process combining hydrogenation and a subsequent stereomutation is an attractive strategy for the stereoselective reduction of cyclic disubstituted π-systems to access the thermodynamically more stable trans isomer, which would be the minor compound considering a kinetically controlled cis hydrogenation process. Herein, we demonstrate stereoselective photocatalytic phosphine-mediated quinoline reductions with water as the hydrogen atom source under mild conditions to afford the corresponding 1,2,3,4-tetrahydroquinolines with complete selectivity towards reduction of the heteroaromatic part. The method shows broad functional group tolerance and provides access to trans-2,3-disubstituted tetrahydroquinolines with moderate to excellent diastereoselectivity. These trans isomers are not readily obtained using established methods, as transition-metal-catalyzed regioselective quinoline hydrogenations provide the corresponding cis-2,3-disubstituted isomers with high selectivity. Mechanistic studies reveal that the hydrogenation of the 2,3-disubstituted quinolines proceeds through a cascade process comprising an initial cis selective photocatalytic hydrogenation of the heteroarene core of the quinoline, followed by a trans selective photoisomerization.

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.

Synthesis of thiopyran derivatives via [4 + 2] cycloaddition reactions

In this review, we provide a comprehensive overview of the synthesis of thiopyran family compounds via cycloaddition reactions, with examples spanning from the year 2000 to the present. We have categorized the [4 + 2] cycloaddition processes using several criteria, particularly distinguishing between intermolecular and intramolecular types based on the Diels-Alder partners. Additionally, from a mechanism standpoint, we differentiate between concerted and stepwise [4 + 2] processes, offering an analysis of these mechanisms based on the current literature.

Photochemical Dual Radical Coupling of Carboxylates with Alkenes/Heteroarenes via Diradical Equivalents

Carboxylate diradical intermediates, with α-carbon and carboxylic oxygen acting as reactive radical centers, represent a highly attractive and long-sought species in reaction design and synthesis. However, capturing these intermediates for diradical coupling reactions poses a formidable challenge due to their inherent instability and spontaneous decarboxylation. Here, we addressed this challenge by temporarily masking the carboxylate oxygen radical reactivity via a photocleavable dynamic oxygen-iodine bond. This approach effectively prevents unwanted decarboxylation and enables the controlled utilization of the carboxylate oxygen radical in forming new bonds. Carboxylates and alkenes/heteroarenes, among the most readily available raw materials, can now seamlessly couple via radical pathways to form γ-butyrolactones, which are common motifs found in numerous natural products and bioactive molecules. Ionic reaction pathways via traditional intermediates of carboxylates are ruled out based on experimental studies and density functional theory (DFT) calculations. This strategy overcomes the substrate limitations of traditional methods, significantly expanding the range of applicable alkenes/heteroarenes. Our method allows transforming carboxylates and alkenes via new reaction modes to diverse products and offers new insights into developing di- and multiradical equivalents for unprecedented reactions and synthetic designs.

Dearomative Skeletal Editing of Benzenoids via Diradical

Dearomative skeletal editing of benzenoids represents a promising yet challenging strategy for the rapid construction of high-value carbon frameworks from readily accessible starting materials. Büchner reaction is a unique type of expansive skeletal editing that transforms benzenoids into functionalized cycloheptatrienes. However, due to challenges in compatibility and selectivity, achieving seamless integration of this reaction with dearomative cycloaddition within a unified system remains undeveloped. Here, we demonstrated an energy-transfer-induced intermolecular dearomative skeletal editing reaction of benzenoids with a range of electronically diverse alkynes. This protocol employed N-acylimines as diradical precursors to efficiently construct various structurally diverse polycyclic frameworks in high chemo-, regio-, and diastereoselectivities that have been previously inaccessible. The challenges related to general reactivity and selectivity issues were circumvented through the smooth merging of photoinduced skeletal editing with dearomative cycloaddition. Experimental and computational studies were performed to support the diradical mechanism and interpret the origins of the observed chemo-, regio-, and diastereoselectivities.

Reimagining Dearomatization: Arenophile-Mediated Single-Atom Insertions and π-Extensions

ConspectusDearomatization of simple aromatics serves as one of the most direct strategies for converting abundant chemical feedstocks into three-dimensional value-added products. Among such transformations, cycloadditions between arenes and alkenes have historically offered effective means to construct complex polycyclic architectures. However, traditionally harsh conditions, such as high-energy UV light irradiation, have greatly limited the scope of this transformation. Nevertheless, recent progress has led to the development of visible-light-promoted dearomative photocycloadditions with expanded scope capable of preparing complex bicyclic structures.A fundamentally distinct approach to dearomative photocycloadditions involves the visible-light activation of arenophiles, which undergo para-photocycloaddition with various aromatic compounds to produce arene-arenophile cycloadducts. While only transiently stable and subject to retro-cycloaddition, further functionalization of the photocycloadducts has allowed for the development of a wide collection of dearomatization methodologies that access products orthogonal to existing chemical and biological processes. Central to this strategy was the observation that arene-arenophile photocycloaddition reveals a π-system that can be functionalized through traditional olefin chemistry. Coupled with subsequent [4 + 2]-cycloreversion of the arenophile, this process acts to effectively isolate a single π-system from an aromatic ring. We have developed several transformations that bias this methodology to perform dearomative single-atom insertion and π-extension reactions to prepare unique products that cannot be prepared easily through traditional means.Through the application of a dearomative epoxidation, we were able to develop a method for the epoxidation of arenes and pyridines to arene-oxides and pyridine-oxides, respectively. Notably, when this arenophile chemistry is applied to polycyclic arenes, photocycloaddition reveals a π-system transposed from the site of native olefinic reactivity, enabling unique site-selectivity for dearomative functionalization. As a result, we were able to perform a single-atom insertion of oxygen into polycyclic (aza)arenes to prepare 3-benzoxepines. When applying this strategy in the context of cyclopropanations, we were able to accomplish a dearomative cyclopropanation of polycyclic (aza)arenes which yield benzocycloheptatrienes upon cycloreversion. Notably, while the Buchner ring expansion is a powerful method for the direct single-atom insertion of carbon into arenes, the corresponding cyclopropanation of polycyclic arenes does not yield ring-expanded products. Furthermore, this strategy could be utilized for the synthesis of novel nanographenes through the development of an M-region annulative π-extension (M-APEX) reaction. Traditionally, methods for π-extension rely on the native reactivity of polycyclic aromatics at the K- and bay-region. However, photocycloaddition of polycyclic aromatics with arenophiles acts as a strategy to activate the M-region for further reactivity. As a result, arenophile-mediated dearomative diarylation, followed by cycloreversion, could deliver π-extended nanographenes with exclusive M-region site selectivity.

Diversity-Generating Skeletal Editing Transformations

ConspectusSkeletal editing, as a synthetic tool, offers the unique potential to selectively and efficiently modify the core skeleton of a target molecule at a late-stage. The main benefit of such transformations is the rapid exploration of the chemical space around lead compounds without necessitating a de novo synthesis for each new molecule. However, many skeletal editing transformations are inherently restricted to generating a single product from a single starting compound, limiting the potential for diversification, a concept central to expediting structure-activity relationship (SAR) investigations. In this Account, we describe our efforts to develop novel skeletal editing transformations in which a modification to the central motif of a molecule is performed simultaneously with the incorporation of additional functionality that can be easily varied through a judicious choice of the reagents. Specifically, we successfully developed an α-iodonium diazo-based carbynyl radical equivalent reagent that, under photoredox conditions, could facilitate the ring-expansion of indene scaffolds while enabling the insertion of over ten different functionalized carbon atoms into the corresponding naphthalene products. This concept was later extended to the design of an atomic carbon equivalent reagent that could promote mild and selective Ciamician-Dennstedt-type indole ring-expansion reactions, while simultaneously installing an oxime ester handle that could undergo further functionalization. Furthermore, we highlight recent work from our group on multiple-atom insertion reactions, namely, the development of a photocatalyzed De Mayo reaction for the ring-expansion of cyclic ketones and a photocatalyzed dearomative ring-expansion of thiophenes via small-ring insertion. In both of these cases, multiple products can be potentially accessed from a single starting material upon variation of the insertion reagent. The diversity-generating skeletal editing strategy could also be applied to single-atom transmutation, as demonstrated by the development of a nitrogen-to-functionalized carbon atom transmutation reaction to convert pyridine to benzene rings. Here, the desired transformation was achieved via a sequence of pyridine ring-opening, Horner-Wadsworth-Emmons (HWE) olefination, and ring-closure, with a judicious choice of the HWE reagent allowing the installation of a wide variety of versatile functional groups. Finally, an energy transfer-mediated quinoline ring-contraction is discussed, specifically with reference to the ways in which it does and does not fit the criteria of a skeletal editing reaction. Although formal atom deletion transformations are typically restricted to single products from each discrete substrate, this [2 + 2] cycloaddition/rearrangement cascade also involves the incorporation of an alkene into the molecule and introduces a point of variation that can be exploited for diversity generation. We hope to not only highlight the transformations reported herein but also inspire further research into this synthetic strategy to access new classes of skeletal editing transformations that, through rapid diversity generation, provide the potential to expedite SAR investigations.

Palladium-Catalyzed Dearomative para-/ortho-Cycloaddition Cascades of N-Allylanilines with 1,4-Enynes and CO via Skeletal Reorganization

A selectivity-control approach for palladium-catalyzed dearomative para-/ortho-cycloaddition cascades of aromatic compounds with 1,4-enynes and CO via a skeletal reorganization process to produce polycycle-fused bicyclo[2.2.2]octenes is reported. This mechanistically novel process depends on a skeletal reorganization that consists of a sequence of dearomative [4 + 2] para-cycloaddition, 3,3-Cope rearrangement, and carbon-carbon bond activation/[4 + 2] cycloaddition.

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.

Energy Transfer (EnT) Catalysis of Non-Symmetrical Borylated Dienes: Origin of Reaction Selectivity in Competing EnT Processes

Energy transfer catalysis (EnT) has had a profound impact on contemporary organic synthesis enabling the construction of higher in energy, complex molecules, via efficient access to the triplet excited state. Despite this, intermolecular reactivity, and the unique possibility to access several reaction pathways via a central triplet diradical has rendered control over reaction outcomes, an intractable challenge. Extended chromophores such as non-symmetrical dienes have the potential to undergo [2+2] cycloaddition, [4+2] cycloaddition or geometric isomerisation, which, in combination with other mechanistic considerations (site- and regioselectivity), results in chemical reactions that are challenging to regulate. Herein, we utilise boron as a tool to probe reactivity of non-symmetrical dienes under EnT catalysis, paying particular attention to the impact of boron hybridisation effects on the target reactivity. Through this, a highly site- and regioselective [2+2] cycloaddition was realised with the employed boron motif effecting reaction efficiency. Subtle modifications to the core scaffold enabled a [4+2] cycloaddition, while a counterintuitive regiodivergence was observed in geometric isomerisation versus [2+2] cycloaddition. The observed reactivity was validated via a mechanistic investigation, determining the origin of regiodivergence and reaction selectivity in competing EnT processes.

Photoredox-Catalyzed Decarboxylation of Oxetane-2-Carboxylic Acids and Unique Mechanistic Insights

Oxetanes are valuable motifs in medicinal chemistry applications, with demonstrated potential to serve as bioisosteres for an array of functional groups. Through the visible-light-mediated photoredox hydrodecarboxylation of 2-aryl oxetane 2-carboxylic acids this work enables access to the products of a [2+2]-photocycloaddition between alkenes and aryl aldehydes without the challenges associated with a traditional UV-light-mediated Paternò-Büchi reaction. Investigation into the hydrodecarboxylation mechanism reveals substrate-dependent modes of initiation under the conditions reported herein. Divergence in diastereomeric outcome is observed, with mechanistic probes elucidating key hydrogen-bonding and steric interactions.

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.

Transforming an azaarene into the spine of fusedbicyclics via cycloaddition-induced scaffold hopping of 5-Hydroxypyrazoles

Skeleton editing for heteroarenes, especially pyrazoles, is challenging and remains scarce because these non-strained aromatics exhibit inert reactivities, making them relatively inactive for performing a dearomatization/cleavage sequence. Here, we disclose a cycloaddition-induced scaffold hopping of 5-hydroxypyrazoles to access the pyrazolopyridopyridazin-6-one skeleton through a single-operation protocol. By converting a five-membered aza-arene into a five-unit spine of a 6/6 fused-bicyclic, this work unlocks a ring-opening reactivity of the pyrazole core that involves a formal C = N bond cleavage while retaining the highly reactive N-N bond in the resulting product. A [4 + 2] cycloaddition of a temporarily dearomatized 5-hydroxypyrrole with an in situ generated aza-1,3-diene, followed by oxidative C-N bond cleavage, constitutes the domino pathway. A library of pyrazolopyridopyridazin-6-ones, which are medicinally relevant nitrogen-atom-rich tricyclics, is obtained efficiently from readily available materials.

Preparing Ruthenium Complex-Contained DaTp COFs via π-π Interactions for Visible-Light-Driven Photocatalytic Hydrogen Peroxide Production

Hydrogen peroxide (H2O2) is a crucial energy carrier with growing significance in sustainable energy systems. Covalent organic frameworks (COFs) have recently emerged as promising materials for efficient H2O2 photosynthesis, while transition-metal complexes are recognized for their efficacy as molecular photocatalysts in H2O2 production. This study introduces a novel π-π interaction strategy to immobilize ruthenium complexes into COFs, using DaTp COF as a model system. This approach significantly enhances the photocatalytic activity for H2O2 production, achieving an initial rate of 3276 μmol g-1 h-1 without using scavengers under visible-light irradiation (λ > 420 nm). Notably, incorporating ruthenium complexes optimizes the oxygen reduction reaction pathways, shifting from a less efficient four-electron process to a more efficient two-electron process. Density functional theory calculations further reveal that ruthenium complexes not only broaden the light absorption spectrum of the COF but also increase water affinity, directly contributing to H2O2 generation. These findings offer a strategic framework for designing and enhancing COFs in H2O2 photosynthesis applications.

Dynamic Vertical Triplet Energies: Understanding and Predicting Triplet Energy Transfer

A computational approach for modeling and predicting triplet energy sensitization of organic molecules is described, which involves sampling the instantaneous, vertical energy gaps over molecular vibrational motions. This approach provides new theoretical support for the hot-band mechanism of energy transfer, in which the energy difference between donor and acceptor can be lessened by geometric distortions. We demonstrate excellent predictive performance against experimental triplet energies, with R2 = 0.97 and a mean absolute error (MAE) of 1.7 kcal/mol, for a collection of 24 small organic molecules, whereas a static, adiabatic description performs significantly worse (R2 = 0.51, MAE = 9.5 kcal/mol). Using this approach, it is possible to quantitatively predict the correct E/Z-isomerism of alkenes under energy transfer, for which adiabatic calculations predict the wrong outcome.

Arylthianthrenium Salts for Triplet Energy Transfer Catalysis

Sigma bond cleavage through electronically excited states allows synthetically useful transformations with two radical species. Direct excitation of simple aryl halides to form both aryl and halogen radicals necessitates UV-C light, so undesired side reactions are often observed and specific equipment is required. Moreover, only aryl halides with extended π systems and comparatively low triplet energy are applicable to synthetically useful energy transfer catalysis. Here we show the conceptual advantages of arylthianthrenium salts (ArTTs) for energy transfer catalysis with high energy efficiency compared to conventional aryl (pseudo)halides and their utility in arylation reactions of ethylene. The fundamental advance is enabled by the low triplet energy of ArTTs that may originate in large part from the electronic interplay between the distinct sulfur atoms in the tricyclic thianthrene scaffold, which is not accessible in either simple (pseudo)halides or other conventional sulfonium salts.

Supramolecular Catalyzed Cascade Reduction of Azaarenes Interrogated via Data Science

Catalysis of multicomponent transformations requires controlled assembly of reactants within the active site. Supramolecular scaffolds possess synthetic microenvironments that enable precise modulation over noncovalent interactions (NCIs) engaged by reactive, encapsulated species. While molecular properties that describe the behavior of single guests in host cavities have been studied extensively, multicomponent transformations remain challenging to design and deploy. Here, simple univariate regression and threshold analyses are employed to model reactivity in a cascade reduction of azaarenes catalyzed by water-soluble metal organic cages. Yield and stereoselectivity models help deduce unknown mechanisms of reactivity by the multicomponent, host-guest complexes. Furthermore, a comprehensive model is established for NCIs driving stereoselectivity in the reported host-guest adducts.

Visible-light-mediated deoxygenative transformation of 1,2-dicarbonyl compounds through energy transfer process

Through the energy transfer process, mild transformations can be achieved that are often difficult to realize under thermodynamic conditions. Herein, a visible-light-driven deoxygenative coupling of 1,2-dicarbonyl compounds for C-O, C-S, and C-N bonds construction is developed via triplet state 1,2-dicarbonyls, affording a wide range of α-functionalized ketones/esters under transition-metal and external photocatalyst free conditions. The usefulness of this method is demonstrated by gram-scale synthesis, late-stage functionalization of various carboxylic acid drugs, and the synthesis of natural products and drug molecules.

Organic photosensitized aziridination of alkenes

We report a new TADF-catalyzed aziridination of alkenes under visible light. In this protocol, a free triplet nitrene is in situ generated from the commercially available tosyl azide by energy transfer of the excited photocatalyst 4DPAIPN. Our finding enables the smooth installation of the strained aziridine ring into a remarkably wide scope of alkenes and pharmaceutical-derived olefins and natural products, as well as the synthesis of sitagliptin. This metal-free method provides a new opportunity for the late-stage modification of complex molecules or synthesis of nitrogen-containing pharmaceuticals.

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.

Lewis-Acid-Promoted Visible-Light-Mediated C(sp3)-H Bond Functionalization of Arylvinylpyridines via Diradical Hydrogen Atom Transfer

A visible-light-induced intramolecular diradical-mediated hydrogen atom transfer (DHAT) of primary, secondary, and tertiary C(sp3)-H bonds and subsequent cyclization is described. This transformation is enabled by triplet energy transfer upon Lewis acid coordination to alkyl-substituted arylvinylpyridines and gives access to a variety of benzocyclobutenes (>40 examples, 32-96% yield). Notably, tri- and tetrasubstituted olefins with tertiary C(sp3)-H bonds effectively delivered sterically hindered products with adjacent all-carbon quaternary centers. Mechanistic evidence and density functional theory (DFT) calculations suggest that Lewis acid coordination was crucial for the success by modulating the reactivity of the diradical intermediates to unlock a challenging carbon-to-carbon DHAT and subsequent cyclization with a rather low barrier, which allows the functionalization of benzylic C(sp3)-H bonds to construct otherwise inaccessible benzocyclobutenes.

Direct photochemical intramolecular [4 + 2] cycloadditions of dehydrosecodine-type substrates for the synthesis of the iboga-type scaffold and divergent [2 + 2] cycloadditions employing micro-flow system

Photocyclisation reactions offer a convenient and versatile method for constructing complex polycyclic scaffolds, particularly in the synthesis of natural products. While the [2 + 2] photocycloaddition reaction is well-established and extensively reported, the [4 + 2] counterpart via direct photochemical means remains challenging and relatively unexplored. In this work, we devised the rapid assembly of the iboga-type scaffold through photochemical intramolecular Diels-Alder reaction using a common biomimetic dehydrosecodine-type intermediate having vinyl indole and dihydropyridine (DHP) sub-units. Exploiting a micro-flow system, the medicinally important iboga-type scaffold was obtained up to 77% yield under mild, neutral conditions at room temperature. This study demonstrated the site-selective activation of the DHP moiety by direct UV-LED irradiation, eliminating the need for external photocatalysts or photosensitisers and showing good tolerance to a wide range of stabilised dehydrosecodine-type substrates. By adjusting the spatial arrangement of the DHP ring and the vinyl indole group, this versatile photochemical approach efficiently facilitates both [4 + 2] and [2 + 2] cyclisations, assembling architecturally complex multicyclic scaffolds. Precise photoactivation of the DHP subunit, generating short-lived biradical species, enabled the new way of harnessing the hidden but innately pre-encoded reactivity of the polyunsaturated dehydrosecodine-type intermediate. These photo-mediated [4 + 2] cyclisation and divergent [2 + 2] cycloadditions are distinct from biosynthetic processes, which are mainly mediated through concerted thermal cycloadditions.

Ruthenium-Catalyzed Interrupted Transfer Hydrogenation: An Approach for Reductive Functionalization of Quinolinium and Napthyridinium Salts

Until now, a myriad of effective approaches have emerged for the functionalization of N-heteroaryl C-H bonds. In contrast, dearomatization and construction of fused heterocycles from activated heteroarenes is still a subject to explore. In this work, we present a refined approach for both dearomatization of N-heteroarenes and the synthesis of fused heterocycles from activated heteroarenes ruthenium catalysis using paraformaldehyde along with additive and base. Notably, quinolinium salts with a hydrogen at the C-4 position yield a methoxymethyl-substituted fused cyclic product through the Thorpe Ingold effect. An innovative aspect of this research is the dual functionality of paraformaldehyde as both a hydride donor and electrophile, utilizing readily available feedstock chemicals. A broad range of electron withdrawing and donating substituents was tolerable under standardized reaction conditions.

Photocatalytic PPh3-Mediated Synthesis of C3-Functionalized Indoles via Radical Annulation of Nitroarenes and Alkenes

Oxidatively generated phosphine radical cations are reactive intermediates that can be used for the generation of carbon and heteroatom centered radicals via deoxygenation processes. Such P-radical cations can readily be generated via single electron transfer oxidation using a redox catalyst. Cheap and commercially available nitroarenes are ideal nitrogen sources for the construction of organic amines and N-containing heterocycles. Activation of nitroarenes with phosphines has been achieved in the ionic mode, which requires specially designed P-nucleophiles and high temperatures. Herein, we report an alternative mode of nitro activation that proceeds via a radical process. The radical strategy leads to open shell intermediates that show interesting unexplored reactivity. This is documented by the development of an economic and highly efficient synthesis of valuable indole derivatives through photocatalytic PPh3-mediated annulation of nitroarenes with alkenes showing large functional group tolerance. The method allows room-temperature activation of nitroarenes and a double C-H bond functionalization of alkenes is achieved to provide rapid access to C3-functionalized indoles, which are key structural components of diverse natural and drug molecules. Experimental mechanistic studies that are further supported by DFT calculations indicate that a nitrosoarene radical cation plays a key role in the annulation process.

Dearomative Intramolecular meta-Thermocycloadditions of Benzene Rings via Wheland Intermediates

Dearomative cycloadditions are powerful synthetic transformations utilizing aromatic compounds for cycloaddition reactions. They have been extensively applied to the synthesis of biologically relevant compounds not only because of the complexity generated from simplicity but also the atom- and step-economy. For the most studied yet challenging benzene ring systems, ortho- and para-cycloadditions have been realized both photochemically and thermally, while the meta-cycloadditions are still limited to the photochemical processes tracing back to the 1960s. Herein, we for the first time realized the thermal cycloadditions of benzene rings with alkenes in a meta fashion via Wheland intermediates. A broad spectrum of readily available C(sp2)-rich aniline-tethered enynes were transformed into C(sp3)-rich 3D complex polycyclic architectures simply by stirring in TFA. Moreover, the reaction could be performed in gram-scales and the products could be diversely elaborated.

Three-dimensional saturated C(sp3)-rich bioisosteres for benzene

Benzenes, the most ubiquitous structural moiety in marketed small-molecule drugs, are frequently associated with poor 'drug-like' properties, including metabolic instability, and poor aqueous solubility. In an effort to overcome these limitations, recent developments in medicinal chemistry have demonstrated the improved physicochemical profiles of C(sp3)-rich bioisosteric scaffolds relative to arenes. In the past two decades, we have witnessed an exponential increase in synthetic methods for accessing saturated bioisosteres of monosubstituted and para-substituted benzenes. However, until recent discoveries, analogous three-dimensional ortho-substituted and meta-substituted biososteres have remained underexplored, owing to their ring strain and increased s-character hybridization. This Review summarizes the emerging synthetic methodologies to access such saturated motifs and their impact on the application of bioisosteres for ortho-substituted, meta-substituted and multi-substituted benzene rings. It concludes with a perspective on the development of next-generation bioisosteres, including those within novel chemical space.

Charge-recombinative triplet sensitization of alkenes for DeMayo-type [2 + 2] cycloaddition

Synthetic photochemistry has undergone significant development, largely owing to the development of visible-light-absorbing photocatalysts (PCs). PCs have significantly improved the efficiency and precision of cycloaddition reactions, primarily through energy or electron transfer pathways. Recent research has identified photocatalysis that does not follow energy- or electron-transfer formalisms, indicating the existence of other, undiscovered photoactivation pathways. This study unveils an alternative route: a charge-neutral photocatalytic process called charge-recombinative triplet sensitization (CRTS), a mechanism with limited precedents in synthetic chemistry. Our investigations revealed CRTS occurrence in DeMayo-type [2 + 2] cycloaddition reactions catalyzed by indole-fused organoPCs. Our mechanistic investigations, including steady-state and transient spectroscopic analyses, electrochemical investigations, and quantum chemical calculations, suggest a mechanism involving substrate activation through photoinduced electron transfer, followed by charge recombination, leading to substrate triplet state formation. Our findings provide valuable insights into the underlying photocatalytic reaction mechanisms and pave the way for the systematic design and realization of innovative photochemical processes.

Visible light-induced chemoselective 1,2-diheteroarylation of alkenes

Visible-light photocatalysis has evolved as a powerful technique to enable controllable radical reactions. Exploring unique photocatalytic mode for obtaining new chemoselectivity and product diversity is of great significance. Herein, we present a photo-induced chemoselective 1,2-diheteroarylation of unactivated alkenes utilizing halopyridines and quinolines. The ring-fused azaarenes serve as not only substrate, but also potential precursors for halogen-atom abstraction for pyridyl radical generation in this photocatalysis. As a complement to metal catalysis, this photo-induced radical process with mild and redox neutral conditions assembles two different heteroaryl groups into alkenes regioselectively and contribute to broad substrates scope. The obtained products containing aza-arene units permit various further diversifications, demonstrating the synthetic utility of this protocol. We anticipate that this protocol will trigger the further advancement of photo-induced alkyl/aryl halides activation.

Visible light-mediated aza Paternò-Büchi reaction of acyclic oximes and alkenes to azetidines

The aza Paternò-Büchi reaction is a [2+2]-cycloaddition reaction between imines and alkenes that produces azetidines, four-membered nitrogen-containing heterocycles. Currently, successful examples rely primarily on either intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò-Büchi reactions, it is essential to extend the reaction to acyclic imine equivalents. Here, we report that matching of the frontier molecular orbital energies of alkenes with those of acyclic oximes enables visible light-mediated aza Paternò-Büchi reactions through triplet energy transfer catalysis. The utility of this reaction is further showcased in the synthesis of epi-penaresidin B. Density functional theory computations reveal that a competition between the desired [2+2]-cycloaddition and alkene dimerization determines the success of the reaction. Frontier orbital energy matching between the reactive components lowers transition-state energy (ΔGǂ) values and ultimately promotes reactivity.

Carbene-Assisted Arene Ring-Opening

Despite the significant achievements in dearomatization and C-H functionalization of arenes, the arene ring-opening remains a largely unmet challenge and is underdeveloped due to the high bond dissociation energy and strong resonance stabilization energy inherent in aromatic compounds. Herein, we demonstrate a novel carbene assisted strategy for arene ring-opening. The understanding of the mechanism by our DFT calculations will stimulate wide application of bulk arene chemicals for the synthesis of value-added polyconjugated chain molecules. Various aryl azide derivatives now can be directly converted into valuable polyconjugated enynes, avoiding traditional synthesis including multistep unsaturated precursors, poor selectivity control, and subsequent transition-metal catalyzed cross-coupling reactions. The simple conditions required were demonstrated in the late-stage modification of complex molecules and fused ring compounds. This chemistry expands the horizons of carbene chemistry and provides a novel pathway for arene ring-opening.

Stereoselective polar radical crossover for the functionalization of strained-ring systems

Radical-polar crossover of organoborates is a poweful tool that enables the creation of two C-C bonds simultaneously. Small ring systems have become essential motifs in drug discovery and medicinal chemistry. However, step-economic methods for their selective functionalization remains scarce. Here we present a one-pot strategy that merges a simple preparation of strained organoboron species with the recently popularized polar radical crossover of borate derivatives to stereoselectively access tri-substituted azetidines, cyclobutanes and five-membered carbo- and heterocycles.

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.

Catalytic asymmetric [4 + 2] dearomative photocycloadditions of anthracene and its derivatives with alkenylazaarenes

Photocatalysis through energy transfer has been investigated for the facilitation of [4 + 2] cycloaddition reactions. However, the high reactivity of radical species poses a challenging obstacle to achieving enantiocontrol with chiral catalysts, as no enantioselective examples have been reported thus far. Here, we present the development of catalytic asymmetric [4 + 2] dearomative photocycloaddition involving anthracene and its derivatives with alkenylazaarenes. This accomplishment is achieved by utilizing a cooperative photosensitizer and chiral Brønsted acid catalysis platform. Importantly, this process enables the activation of anthracene substrates through energy transfer from triplet DPZ, thereby initiating a precise and stereoselective sequential transformation. The significance of our work is highlighted by the synthesis of a diverse range of pharmaceutical valuable cycloadducts incorporating attractive azaarenes, all obtained with high yields, ees, and drs. The broad substrate scope is further underscored by successful construction of all-carbon quaternary stereocenters and diverse adjacent stereocenters.

EnTdecker - A Machine Learning-Based Platform for Guiding Substrate Discovery in Energy Transfer Catalysis

Due to the magnitude of chemical space, the discovery of novel substrates in energy transfer (EnT) catalysis remains a daunting task. Experimental and computational strategies to identify compounds that successfully undergo EnT-mediated reactions are limited by their time and cost efficiency. To accelerate the discovery process in EnT catalysis, we herein present the EnTdecker platform, which facilitates the large-scale virtual screening of potential substrates using machine-learning (ML) based predictions of their excited state properties. To achieve this, a data set is created containing more than 34,000 molecules aiming to cover a vast fraction of synthetically relevant compound space for EnT catalysis. Using this data predictive models are trained, and their aptitude for an in-lab application is demonstrated by rediscovering successful substrates from literature as well as experimental validation through luminescence-based screening. By reducing the computational effort needed to obtain excited state properties, the EnTdecker platform represents a tool to efficiently guide substrate selection and increase the experimental success rate for EnT catalysis. Moreover, through an easy-to-use web application, EnTdecker is made publicly accessible under entdecker.uni-muenster.de.

Enantioselective [2π + 2σ] Cycloadditions of Bicyclo[1.1.0]butanes with Vinylazaarenes through Asymmetric Photoredox Catalysis

Here we present a highly enantioselective [2π + 2σ] photocycloaddition of bicyclo[1.1.0]butanes (BCBs). The reaction uses a variety of vinylazaarenes as partners and is catalyzed by a polycyclic aromatic hydrocarbon (PAH)-containing chiral phosphoric acid as a bifunctional chiral photosensitizer. A wide array of pharmaceutically important bicyclo[2.1.1]hexane (BCH) derivatives have been synthesized with high yields, enantioselectivity, and diastereoselectivity. In addition to the diverse 1-ketocarbonyl-3-substituted BCBs, α/β-substituted vinylazaarenes are compatible with such an unprecedented photoredox catalytic pathway, resulting in the successful assembly of an all-carbon quaternary stereocenter or two adjacent tertiary stereocenters on the product.

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.

Valence-isomer selective cycloaddition reaction of cycloheptatrienes-norcaradienes

The rapid and precise creation of complex molecules while controlling multiple selectivities is the principal objective in synthetic chemistry. Combining data science and organic synthesis to achieve this goal is an emerging trend, but few examples of successful reaction designs are reported. We develop an artificial neural network regression model using bond orbital data to predict chemical reactivities. Actual experimental verification confirms cycloheptatriene-selective [6 + 2]-cycloaddition utilizing nitroso compounds and norcaradiene-selective [4 + 2]-cycloaddition reactions employing benzynes. Additionally, a one-pot asymmetric synthesis is achieved by telescoping the enantioselective dearomatization of non-activated benzenes and cycloadditions. Computational studies provide a rational explanation for the seemingly anomalous occurrence of thermally prohibited suprafacial [6 + 2]-cycloaddition without photoirradiation.

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.

Regioselective Diels-Alder Reactions of Anthracenes with Olefins via Visible Light Photocatalysis in a Homogeneous Solution

Diels-Alder cycloaddition of anthracene with olefin is achieved in a homogeneous solution via energy transfer under visible light. A series of substrates including electroneutral styrene derivatives can be successfully converted into the corresponding cycloadducts in a head-to-head orientation with high to excellent yields. The high ortho-regioselectivity, mild condition, and broad substrate scope enable promising advances in organic transformation.

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.

Oxidative Dearomatization of Pyridines

Dearomatization of pyridines is a well-established synthetic approach to access piperidines. Although remarkably powerful, existing dearomatization processes have been limited to the hydrogenation or addition of carbon-based nucleophiles to activated pyridiniums. Here, we show that arenophile-mediated dearomatizations can be applied to pyridines to directly introduce heteroatom functionalities without prior substrate activation. The arenophile platform in combination with olefin oxidation chemistry provides access to dihydropyridine cis-diols and epoxides. These previously elusive compounds are now readily accessible and can be used for the downstream preparation of diversely functionalized piperidines.

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.

Visible-Light-Promoted Tandem Skeletal Rearrangement/Dearomatization of Heteroaryl Enallenes

Access to complex three-dimensional molecular architectures via dearomatization of ubiquitous aryl rings is a powerful synthetic tool, which faces, however, an inherent challenge to overcome energetic costs due to the loss of aromatic stabilization energy. Photochemical methods that allow one to populate high-energy states can thus be an ideal strategy to accomplish otherwise prohibitive reaction pathways. We present an original dearomative rearrangement of heteroaryl acryloylallenamides that leads to complex fused tricycles. The visible-light-promoted method occurs under mild conditions and tolerates a variety of functional groups. According to DFT modeling used to rationalize the outcome of the cascade, the reaction involves a sequential [2+2] allene-alkene photocycloaddition, which is followed by a selective retro- [2+2] step that paves the way for the dearomatization of the heteroaryl partner. This scenario is original with respect to the reported photochemical reactivity of similar substrates and thus holds promise for ample future developments.

Photochemical Reduction of Quinolines with γ-Terpinene

The saturation of aromatic scaffolds is valuable for the synthesis of complex rings. Herein, we demonstrate a process for photochemical dearomative reduction of quinolines. The process involves capture of a quinoline excited state with γ-terpinene. Importantly, the reaction is chemoselective as other easily reduced functionalities such as halogens or alkenes do not undergo reduction. The mechanism of the reaction has also been investigated. Finally, the generality of the approach towards other substrates is demonstrated.