Dearomative Cascade Photocatalysis: Divergent Synthesis through Catalyst Selective Energy Transfer

Electric-field modulated energy transfer in phosphorescent material- and fluorescent material-codoped polymer light-emitting diodes

The excited-state energy transfer widely exists in mixed-material systems and devices. The modulation of an electric field on the energy transfer in photoluminescence has been demonstrated. However, to date, no studies on the electric-field modulation of the excited-state energy transfer in organic optoelectronic devices have been reported. Herein, we investigate the effect of an electric field on the energy transfer in the poly(N-vinylcarbazole) (PVK) thin films doped with iridium(iii)[bis(4,6-difluorophenyl)pyridinato-N,C2']-tetrakis(1-pyrazolyl)borate (Fir6) and 5,6,11,12-tetraphenylnaphthacene (rubrene) (PVK:Fir6:rubrene) and the corresponding light-emitting diodes. Combined with the Onsager model describing electric-field enhanced exciton dissociation, we find that the electric field increases the rate of Dexter energy transfer from Fir6 to rubrene in the films and the diodes. The voltage-dependent color shift in the PVK:Fir6:rubrene light-emitting diodes can be explained by the electric-field enhanced Dexter energy transfer from Fir6 to rubrene. Our findings are important for the control of energy transfer process in organic optoelectronic devices by an electric field for desirable applications.

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.

Multiple-cycle photochemical cascade reactions

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

Radical-Dearomative Generation of Cyclohexadienyl Pd(II) toward the 3D Transformation of Nonactivated Phenyl Rings

Traditional palladium-catalyzed dearomatization of (hetero)arenes takes place via an ionic pathway and usually requires elevated temperatures to overcome the energy barrier of the dearomative insertion step. Herein, a combination of the radical and two-electron pathways is disclosed, which enables room temperature dearomative 3D transformations of nonactivated phenyl rings with Pd(0) as the catalyst. Experimental results together with density functional theory (DFT) calculations indicate a versatile π-allyl Pd(II) species, cyclohexadienyl Pd(II), possibly is involved in the dearomatization. This species is generated by combining the cyclohexadienyl radical and Pd(I). The cyclohexadienyl Pd(II) provides chemoselective (carboamination and trieneylation), regioselective (1,2-carboamination), and diastereoselective (carbonyl-group directed face selectivity) conversions.

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.

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.

Construction of Bridged Benzazepines via Photo-Induced Dearomatization

Bridged benzazepine scaffolds, possessing unique structural and physicochemical activities, are widespread in various natural products and drugs. The construction of these skeletons often requires elaborate synthetic effort with low efficiency. Herein, we develop a simple and divergent approach for constructing various bridged benzazepines by a photocatalytic intermolecular dearomatization of naphthalene derivatives with readily available α-amino acids. The bridged motif is created via a cascade sequence involving photocatalytic 1,4-hydroaminoalkylation, alkene isomerization and cyclization. Interestingly, the diastereoselectivity can be regulated through different reaction modes in the cyclization step. Moreover, aminohydroxylation and its further bromination have also been demonstrated to access highly functionalized bridged benzazepines. Preliminary mechanistic studies have been performed to get insights into the mechanism. This method provides a divergent synthetic approach for construction of highly functionalized bridged benzazepines, which have been otherwise difficult to access.

Extent of carbon nitride photocharging controls energetics of hydrogen transfer in photochemical cascade processes

Graphitic carbon nitride is widely studied in organic photoredox catalysis. Reductive quenching of carbon nitride excited state is postulated in many photocatalytic transformations. However, the reactivity of this species in the turn over step is less explored. In this work, we investigate electron and proton transfer from carbon nitride that is photocharged to a various extent, while the negative charge is compensated either by protons or ammonium cations. Strong stabilization of electrons by ammonium cations makes proton-coupled electron transfer uphill, and affords air-stable persistent carbon nitride radicals. In carbon nitrides, which are photocharged to a smaller extent, protons do not stabilize electrons, which results in spontaneous charge transfer to oxidants. Facile proton-coupled electron transfer is a key step in the photocatalytic oxidative-reductive cascade - tetramerization of benzylic amines. The feasibility of proton-coupled electron transfer is modulated by adjusting the extent of carbon nitride photocharging, type of counterion and temperature.

Rejuvenation of dearomative cycloaddition reactions via visible light energy transfer catalysis

Dearomative cycloaddition is a powerful technique to access sp3-rich three-dimensional structural motifs from simple flat, aromatic feedstock. The building-up of unprecedentedly diverse polycyclic scaffolds with increased saturation and stereochemical information having various applications ranging from pharmaceutical to material sciences, is an essential goal in organic chemistry. However, the requirement of large energy inputs to disrupt the aromaticity of an arene moiety necessitates harsh reaction conditions for ground state dearomative cycloaddition. The photochemical requirement encompasses use of ultraviolet (UV) light to enable the reaction on an excited potential energy surface. The microscopic reversibility under thermal conditions and the use of high energy harmful UV irradiation in photochemical manoeuvres, however, constrain their widespread use from a synthetic point of view. In this context, the recent renaissance of visible light energy transfer (EnT) catalysis has become a powerful tool to initiate dearomative cycloaddition as a greener and more sustainable approach. The excited triplet state population is achieved by triplet energy transfer from the appropriate photosensitizer to the substrate. While employing mild visible light energy as fuel, the process leverages an enormous potential of excited state reactivity. The discovery of an impressive portfolio of organic and inorganic photosensitizers with a range of triplet energies facilitates visible light photosensitized dearomative cycloaddition of various substrates to form sp3-rich fused polycyclic architectures with diverse applications. The tutorial review comprehensively surveys the reawakening of dearomative cycloadditions via visible light-mediated energy transfer catalysis in the past five years. The progress ranges from intra- and intermolecular [2π + 2π] to [4π + 2π], and ends at intermolecular [2π + 2σ] cycloadditions. Furthermore, the review provides potential possibilities for future growth in the growing field of visible light energy transfer catalysis.

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

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

Enantioselective Synthesis of Cyclobutane Derivatives via Cascade Asymmetric Allylic Etherification/[2 + 2] Photocycloaddition

Chiral cyclobutane presents as a popular motif in natural products and biologically active molecules, and its derivatives have been extensively used as key synthons in organic synthesis. Herein, we report an efficient synthetic method toward enantioenriched cyclobutane derivatives. The reaction proceeds in a cascade fashion involving Ir-catalyzed asymmetric allylic etherification and visible-light induced [2 + 2] cycloaddition. Readily available branched allyl acetates and cinnamyl alcohols are directly used as the substrates under mild reaction conditions, providing a broad range of chiral cyclobutanes in good yields with excellent diastereo- and enantioselectivities (up to 12:1 dr, >99% ee). It is worth noting that all substrates and catalysts were simultaneously added without any separated step in this approach. The gram-scale reaction and diverse transformations of product further enhance the potential utility of this method.

Strain-Release Photocatalysis

The concept of strain in organic compounds is as old as modern organic chemistry and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate─with significant driving force─rigid aliphatic systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an additional dimension to strain-release processes by leveraging the energy of photons to create chemical complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis─with emphases on mechanisms, catalytic cycles, and current limitations─the unique chemical architectures that can be produced, and possible future directions.

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.

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

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

Visible-Light-Promoted Switchable Selective Oxidations of Styrene Over Covalent Triazine Frameworks in Water

Using photocatalytic oxidation to convert basic chemicals into high value compounds in environmentally benign reaction media is a current focus in catalytic research. The challenge lies in gaining controllability over product formation selectivity. We design covalent triazine frameworks as heterogeneous, metal-free, and recyclable photocatalysts for visible-light-driven switchable selective oxidation of styrene in pure water. Selectivity in product formation was achieved by activation or deactivation of the specific photogenerated oxygen species. Using the same photocatalyst, by deactivation of photogenerated H₂ O₂ , benzaldehyde was obtained with over 99 % conversion and over 99 % selectivity as a single product. The highly challenging and sensitive epoxidation of styrene was carried out by creating peroxymonocarbonate as an initial epoxidation agent in the presence of bicarbonate, which led to formation of styrene oxide with a selectivity up to 76 % with near quantitative conversion. This study demonstrates a preliminary yet interesting example for simple control over switchable product formation selectivity for challenging oxidation reactions of organic compounds in pure water.

Organophotocatalytic [2+2] Cycloaddition of Electron-Deficient Styrenes

A visible-light organophotocatalytic [2+2] cycloaddition of electron-deficient styrenes is described. Photocatalytic [2+2] cycloadditions are typically performed with electron-rich styrene derivatives or α,β-unsaturated carbonyl compounds, and with transition-metal-based catalysts. We have discovered that an organic cyanoarene photocatalyst is able to deliver high-value cyclobutane products bearing electron-deficient aryl substituents in good yields. A range of electron-deficient substituents are tolerated, and both homodimerisations and intramolecular [2+2] cycloadditions to fused bicyclic systems are available by using this methodology.

Photoredox aerobic oxidative cycliation of N-arylacrylamides with benzylalcohols

While the radical coupling/cyclization of N-arylacrylamides has been well established for the synthesis of functionalized oxindoles, in the present work, a visible-light-induced aerobic oxidative coupling/cyclization reaction of N-arylacrylamides with benzylalcohols has been developed. The combination of LiBr and benzophenone as the additive was found to be highly effective to enhance the catalytic efficacy. This protocol offers a mild alternative access to structurally valuable hydroxyalkyl oxindoles. Mechanistic studies reveal that the bromo radical in situ formed through the photoredox SET process under aerobic conditions enables efficient HAT to generate the key hydroxyalkyl radical intermediate.

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.

Divergent and Synergistic Photocatalysis: Hydro- and Oxoalkylation of Vinyl Arenes for the Stereoselective Synthesis of Cyclopentanols via a Formal [4+1]-Annulation of 1,3-Dicarbonyls

The controllable divergent reactivity of 1,3-dicarbonyls is described, which enables the efficient hydro- and oxoalkylation of vinyl arenes. Both reaction pathways are initiated through the formation of polarity-reversed C-centered-radical intermediates at the active methylene center of 1,3-dicarbonyls via direct photocatalytic C–H bond transformations. The oxoalkylation of alkenes is achieved under aerobic conditions via a Cu(II)-photomediated rebound mechanism, while the corresponding hydroalkylation becomes possible under a nitrogen atmosphere by the combination of 4CzIPN and a Brønsted base. The breadth of these divergent protocols is demonstrated in the late-stage modification of drugs and natural products and by the transformation of the products to a variety of heterocycles such as pyridines, pyrroles, or furans. Moreover, the two catalytic modes can be combined synergistically for the stereoselective construction of cyclopentanol derivatives in a formal [4+1]-annulation process.

Dearomative (3+2) Cycloadditions between Indoles and Vinyldiazo Species Enabled by a Red-Shifted Chromium Photocatalyst

A direct dearomative photocatalyzed (3+2) cycloaddition between indoles and vinyldiazo reagents is described. The transformation is enabled by the development of a novel oxidizing Cr^III photocatalyst, its specific reactivity attributed to increased absorptive properties over earlier Cr analogs and greater stability than Ru counterparts. A variety of fused indoline compounds are synthesized using this method, including densely functionalized ring systems that are feasible due to base-free conditions. Experimental insights corroborate a cycloaddition initiated by nucleophilic attack at C3 of the indole radical cation by the vinyldiazo species.

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.

Visible-Light Photocatalyzed peri-(3 + 2) Cycloadditions of Quinolines

Cycloaddition reactions─epitomized by the Diels-Alder reaction─offer an arguably unmatched springboard for achieving chemical complexity, often with excellent selectivity, in a modular single step. We report the synthesis of aza-acenaphthenes in a single step by an unprecedented formal peri-(3 + 2) cycloaddition of simple quinolines with alkynes. A commercially available iridium complex exerts a dual role of photosensitizer and photoredox catalyst, fostering a cyclization/rearomatization cascade. The initial energy-transfer phase leads to the acenaphthene skeleton, while the ensuing redox shuttling step leads to aromatization. We applied this technology to 8-substituted quinolines and phenanthrolines, which smoothly reacted with both terminal and internal alkynes with excellent levels of regio- and diastereoselectivity. Density functional theory calculations revealed the intertwined EnT/SET nature of the process and offered guiding design principles for the synthesis of new aza-acenaphthenes.

Facile access to fused 2D/3D rings via intermolecular cascade dearomative [2 + 2] cycloaddition/rearrangement reactions of quinolines with alkenes

Hybrid fused two-dimensional/three-dimensional (2D/3D) rings are important pharmacophores in drugs owing to their unique structural and physicochemical properties. Preparation of these strained ring systems often requires elaborate synthetic effort and exhibits low efficiency, thus representing a limiting factor in drug discovery. Here, we report two types of energy-transfer-mediated cascade dearomative [2 + 2] cycloaddition/rearrangement reactions of quinoline derivatives with alkenes, which provide a straightforward avenue to 2D/3D pyridine-fused 6−5−4−3- and 6−4−6-membered ring systems. Notably, this energy-transfer-mediated strategy features excellent diastereoselectivity that bypasses the general reactivity and selectivity issues of photochemical [2 + 2] cycloaddition of various other aromatics. Tuning the aza-arene substitutions enabled selective diversion of the iridium photocatalysed energy transfer manifold towards either cyclopropanation or cyclobutane-rearrangement products. Density functional theory calculations revealed a cascade energy transfer scenario to be operative.

Strain-Release [2π + 2σ] Cycloadditions for the Synthesis of Bicyclo[2.1.1]hexanes Initiated by Energy Transfer

Saturated bicycles are becoming ever more important in the design and development of new pharmaceuticals. Here a new strategy for the synthesis of bicyclo[2.1.1]hexanes is described. These bicycles are significant because they have defined exit vectors, yet many substitution patterns are underexplored as building blocks. The process involves sensitization of a bicyclo[1.1.0]butane followed by cycloaddition with an alkene. The scope and mechanistic details of the method are discussed.

Vinylcyclopropane [3+2] Cycloaddition with Acetylenic Sulfones Based on Visible Light Photocatalysis**

The first intermolecular visible light [3+2] cycloaddition reaction performed on a meta photocycloadduct employing acetylenic sulfones is described. The developed methodology exploits the advantages of combining UV and visible‐light in a two‐step sequence that provides a photogenerated cyclopropane which, through a strain‐release process, generates a new cyclopentane ring while significantly increasing the molecular complexity. Mechanistic studies and DFT calculations indicate an energy transfer pathway for the visible light‐driven reaction step. This strategy could be extended to simpler vinylcyclopropanes.

Visible-light induced dearomatization reactions

Dearomatization reactions provide rapid access to structurally complex three-dimensional molecules from simple aromatic compounds. Plenty of reports have demonstrated their utilities in the synthesis of natural products, medicinal chemistry, and materials science in the last decades. Recently, visible-light mediated photocatalysis has emerged as a powerful tool to promote many kinds of transformations. The dearomatization reactions induced by visible-light have also made significant progress during the past several years. This review provides an overview of visible-light induced dearomatization reactions classified based on the manner in which aromaticity is disrupted.

Radical-mediated alkene carboamination/dearomatization of arylsulfonyl-o-allylanilines via photoredox catalysis

A mild and redox-neutral protocol is developed for the synthesis of 1,4-cyclohexadiene-containing indoline-fused heterocycles via photoredox catalysis.

Radical boron migration of allylboronic esters

A photocatalyzed 1,3-boron shift of allylboronic esters is reported. The atom-switch acrobatics proceeds via cascade 1,2-boron migrations and Smiles type rearrangement to furnish a variety of terminally functionalized alkyl boronates.

Post-spin crossing dynamics determine the regioselectivity in open-shell singlet biradical recombination

Comprehensive computational studies reveal unique dynamic effects in a multi-spin-state reaction that determine the regioselectivity of a biradical recombination process.

Intermolecular dearomative [4 + 2] cycloaddition of naphthalenes via visible-light energy-transfer-catalysis

The dearomative cycloaddition reaction serves as a blueprint for creating sp³-rich three-dimensional molecular topology from flat-aromatic compounds. However, severe reactivity and selectivity issues make this process arduous. Herein, we describe visible-light energy-transfer catalysis for the intermolecular dearomative [4 + 2] cycloaddition reaction of feedstock naphthalene molecules with vinyl benzenes. Tolerating a wide range of functional groups, structurally diverse 2-acyl naphthalenes and styrenes could easily be converted to a diverse range of bicyclo[2.2.2]octa-2,5-diene scaffolds in high yields and moderate endo-selectivities. The late-stage modification of the derivatives of pharmaceutical agents further demonstrated the broad potentiality of this methodology. The efficacy of the introduced methods was further highlighted by the post-synthetic diversification of the products. Furthermore, photoluminescence, electrochemical, kinetic, control experiments, and density-functional theory calculations support energy-transfer catalysis.

Constructing 3D molecular scaffolds from aromatic hydrocarbons is challenging. Herein, we report dearomative [4 + 2] cycloaddition reaction of naphthalenes via visible-light EnT catalysis which overcomes issues of unfavorable thermodynamics, low yields, and selectivity.

Access to high value sp3-rich frameworks using photocatalyzed [2+2]-cycloadditions of γ-alkylidene-γ-lactams

By harnessing an energy transfer process, new photocatalyzed [2+2]-cycloadditions occurring between γ-alkylidene-γ-lactams and unsaturated substrates have been developed. The reaction mode is particularly powerful because it leads to the formation...

Radical-induced denitration of N-(p-nitrophenyl)propiolamides coupled with dearomatization: access to phosphonylated/trifluoromethylated azaspiro[4.5]-trienones

A robust dearomative denitration of nitroarene derivatives induced by a radical ipso-cyclization process has been developed, delivering valuable phosphonated or trifluoromethylated azaspiro[4.5]trienones with good functional group tolerance. This represents a convenient and powerful approach to activate nitroarenes in a radical manner.

Dynamic Kinetic Sensitization of β-Dicarbonyl Compounds-Access to Medium-Sized Rings by De Mayo-Type Ring Expansion

Herein, we present a photocatalyzed two-carbon ring expansion of β-dicarbonyl compounds with unactivated olefins that provides facile access to medium-sized rings. Selective sensitization of the substoichiometric enol tautomer enables reactivity of substrates incompatible with the classical De Mayo reaction conditions. Key to success is the identification of the metal-based sensitizer fac-[Ir(CF₃ -pmb)₃ ], which can be excited using common near-visible LEDs, and possesses a high triplet excited state energy of 73.3 kcal mol^-1 . This exactly falls in the range between the triplet energies of the enol and keto tautomer, thereby enabling a dynamic kinetic sensitization. Demonstrating the applicability of fac-[Ir(CF₃ -pmb)₃ ] as a photocatalyst in organic synthesis for the first time, we describe a two-step photocycloaddition-ring-opening cascade with β-ketoesters, -diketones, and -ketoamides. The mechanism has been corroborated by time-resolved spectroscopy, as well as further experimental and computational studies.

Photo-Induced Ruthenium-Catalyzed C-H Benzylations and Allylations at Room Temperature

The ruthenium‐catalyzed synthesis of diarylmethane compounds was realized under exceedingly mild photoredox conditions without the use of exogenous photocatalysts. The versatility and robustness of the ruthenium‐catalyzed C−H benzylation was reflected by an ample scope, including multifold C−H functionalizations, as well as transformable pyrazoles, imidates and sensitive nucleosides. Mechanistic studies were indicative of a photoactive cyclometalated ruthenium complex, which also enabled versatile C−H allylations.

Visible-Light Mediated Metal-Free 6π-Photocyclization of N-Acrylamides: Thioxanthone Triplet Energy Transfer Enables the Synthesis of 3,4-Dihydroquinolin-2-ones

An efficient thioxanthone-catalyzed triplet energy transfer process for the synthesis of 3,4-dihydroquinolin-2-ones via a 6π-photocyclization is reported. Featuring a rare example of a metal-free formal C(sp²)-H/C(sp³)-H arylation mediated by visible-light, this work hopes to inspire further interest in these small molecules as sustainable alternatives to existing transition-metal photocatalysts in related processes.