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[2π + 2π]‐photocycloadditions and their ability to trigger controlled and reversible photoligation through disparate wavelengths provide an attractive platform to unlock advanced functionalities in soft materials. Yet, among the limited amount of functional motifs enabling reversible photoreactions, cyclability is often overlooked due to poor reaction yield and orthogonality. In this study, the advantageous photocharacteristics of the previously underexplored N‐methyl‐quinolinone photoresponsive motif are leveraged to create a covalent gated system, enabling controlled formation and cleavage of covalent bonds on demand. A systematic evaluation of individual cycloadditions and reversions on the molecular scale, including reaction rates, conversions, and photoproducts, allows identification of the required conditions for generating controlled photoreactions with a remarkable degree of cyclability; while, maintaining high reaction yields. Ultimately, these controlled and cyclable reactions are translated to a macromolecular scale, showcasing a comparable performance in initiating reversible photoligation, as observed at the molecular level. In addition, it is also shown that this progressive methodology can be leveraged to gain a comprehensive understanding of cyclability and clarify the factors contributing to its decreasing yield. Overall, unlocking the potential of quinolinone derivatives through this step‐by‐step approach lays the foundation for the development of highly controlled and responsive polymer materials with unprecedented potential. [ABSTRACT FROM AUTHOR]

[32](1,4)Naphthalenophanes, bearing carbon-bridge chains (syn- and anti-NPs) and nitrogen-bridge chains (syn- and anti-ANPs), were synthesized, and their X-ray structures and photoreactions were investigated. The intramolecular separation distance between the naphthalene cores for ANPs was shorter than that for NPs, suggesting that intramolecular interactions between the naphthalene rings were more efficient for ANPs compared to NPs. Upon photoirradiation at 300 nm, anti-NP, syn-ANP and anti-ANP produced the corresponding intramolecular [π4s + π4s] cycloadducts, whereas syn-NP gave an unidentified complex product mixture. Quantum yields for the photo-consumption (ΦPC) of NPs and ANPs were evaluated to quantitatively compare their photoreactivity. The ΦPC values of ANPs were approximately two-fold higher than those of ANPs.Noteworthily, the ΦPC value of syn-ANP was estimated to be unity. Based on these results we discuss the effects of the alignments of the naphthalene cores (antivs. syn) and the bridging elements (C-bridge vs. N-bridge) on the photoreaction efficiencies of [32](1,4)naphthalenophanes. [ABSTRACT FROM AUTHOR]

In the current field of photopharmacology, molecular photoswitches are applied whose interactions with DNA can be triggered or controlled by light. And although several photochromic reactions have been shown to serve this purpose well, the reversible photocycloaddition and photocycloreversion reactions have been largely neglected. This absence of research is surprising because especially the photodimerization of a DNA ligand leads to products with significant change of the size and shape which, in turn, leads to strongly diminished or even suppressed DNA association. Therefore, photocycloaddition–cycloreversion sequences have a huge potential for the photoinduced, reversible deactivation and activation of ligand–DNA interactions, as will be shown with selected examples in this Concept Article. Specifically, heterostyryl and ‐stilbene derivatives are presented whose DNA–binding properties are efficiently switched in reversible [2+2] photocycloaddition reactions. In addition, the photocontrolled DNA–binding of anthracene derivatives and their heterocyclic benzo[b]quinolizinium analogues in a [4+4] photocycloaddition, as well as the use of this reaction as part of dual–mode switches in combination with redox‐active functionalities, are highlighted. Furthermore, examples of conjugates are provided, in which the photochromic unit is bound covalently to nucleic acids or proteins, such that the photocycloaddition reaction can be used for reversible photoinduced crosslinking, ligation, or inhibition of gene expression. [ABSTRACT FROM AUTHOR]

Photomechanical molecular crystals, actuated by solid-state photochemical reactions, manifest a spectrum of mechanical motions upon light exposure, underscoring their prospective integration into the next generation of intelligent materials and devices. Utilizing the solid-state photodimerization of naphthyl acrylic acid as a paradigm, this study delved into the interplay between crystal morphology and reaction dynamics on the photomechanical responses of molecular crystals. Distinct crystal forms—bulk, microrods, and microplates—were cultivated through tailored crystallization conditions. While bulk crystals of naphthyl acrylic acid (NA) underwent shattering and splintering upon UV light exposure, the microplate counterparts displayed unique cracking patterns with fissures yet retained their overall structural integrity. In contrast, NA microrods underwent pronounced bending under identical irradiation conditions. These phenomena are attributed to the efficient lattice reconfiguration stemming from the [2 + 2] cycloaddition photochemical reaction within the crystals. An intermediate fluorescence enhancement was observed across all crystal types upon light exposure. Collectively, our results underscore the pivotal role of crystal shape in dictating photomechanical behavior, thereby heralding novel strategies for developing advanced photomechanical materials. [ABSTRACT FROM AUTHOR]

Photoclick reactions combine the advantages offered by light‐driven processes and classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photo‐crosslinking, and protein labeling. Despite these advances, the dependency of most of the photoclick reactions on UV light poses a severe obstacle for their general implementation, as this light can be absorbed by other molecules in the system resulting in their degradation or unwanted reactivity. However, the development of a simple and efficient system to achieve bathochromically shifted photoclick transformations remains challenging. Here, we introduce triplet‐triplet energy transfer as a fast and selective way to enable visible light‐induced photoclick reactions. Specifically, we show that 9,10‐phenanthrenequinones (PQs) can efficiently react with electron‐rich alkenes (ERAs) in the presence of a catalytic amount (as little as 5 mol %) of photosensitizers. The photocycloaddition reaction can be achieved under green (530 nm) or orange (590 nm) light irradiation, representing a bathochromic shift of over 100 nm as compared to the classical PQ‐ERAs system. Furthermore, by combining appropriate reactants, we establish an orthogonal, blue and green light‐induced photoclick reaction system in which the product distribution can be precisely controlled by the choice of the color of light. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Solid-state structural transformation is an interesting methodology used to prepare various metal–organic frameworks (MOFs) that are challenging to prepare in direct synthetic procedures. On the other hand, solid-state [2 + 2] photoreactions are distinctive methodologies used for light-driven solid-state transformations. Meanwhile, most of these photoreactions explored are quantitative in nature, in addition to them being stereo-selective and regio-specific in manner. In this work, we successfully synthesized two photoreactive novel binuclear Zn(II) MOFs, [Zn2(spy)2(tdc)2] (1) and [Zn2(spy)4(tdc)2] (2) (where spy = 4-styrylpyridine and tdc = 2,5-thiophenedicarboxylate) with different secondary building units. Both MOFs are interdigitated in nature and are 2D and 1D frameworks, respectively. Both the compounds showed 100% and 50% photoreaction upon UV irradiation, as estimated from the structural analysis for 1 and 2, respectively. This light-driven transformation resulted in the formation of 3D, [Zn2(rctt-ppcb)(tdc)2] (3), and 2D, [Zn2(spy)2(rctt-ppcb)(tdc)2] (4) (where rctt = regio, cis, trans, trans; ppcb = 1,3-bis(4′-pyridyl)-2,4-bis(phenyl)cyclobutane), respectively. These solid-state structural transformations were observed as an interesting post-synthetic modification. Overall, we successfully transformed novel lower-dimensional frameworks into higher-dimensional materials using a solid-state [2 + 2] photocycloaddition reaction. [ABSTRACT FROM AUTHOR]

Keywords: cycloaddition; domino reaction; electrocyclic reactions; photochemistry; stereoselective synthesis; terpenoids EN cycloaddition domino reaction electrocyclic reactions photochemistry stereoselective synthesis terpenoids 1343 1355 13 07/11/23 20230725 NES 230725 Graph 1 Introduction This is a personal account on a reaction cascade that our group has been working on for roughly six years. [72] First, the xanthate I rac i - B 46 b was quantitatively generated from alcohol I rac i - B 45 b before treatment with azobisisobutyronitrile (AIBN) and phosphinic acid resulted in the desired de-oxygenated compound I rac i - B 47 b with a 96% yield. Along these lines, the use of compound I rac i - B 36 b , bearing a methyl group at the -position relative to the carbonyl group and two methyl groups on the alkene, as the starting material for the photochemical reaction cascade would lead to the desired intermediate I rac i - B 34i b . The authors suggested that the reaction proceeds via intermediate I rac i - B 9 b , obtained upon the initial I ortho i photocycloaddition, followed by a thermal disrotatory ring opening resulting in the cycloocta-1,3,5-triene derivative I rac i - B 10 b . [Extracted from the article]

Developing a new strategy to realize on‐demand photoreactions with distinguishable signals is highly desirable. Herein, by simply tuning the molecular state, tuned photoreactions with different dynamic signals based on a series of tridentate cyanostilbene molecules (TCS1‐5) are realized. The results show that in the crystalline state, the ordered and densely molecular packing facilitates the successive intermolecular photocycloaddition between adjacent cyanostilbene molecules and causes accumulated stresses within the crystals, which further induces variable photo‐induced mechanical motions such as peeling, bending, twisting, and so on. In the monodispersed solution state, however, the larger distances between TCS1‐5 molecules afford a photo‐induced radical‐assisted intramolecular photocyclization process and give rise to multiple domino photochromic and photofluorochromic behaviors. Based on their distinct yet controllable thermostability and radical colors, photochromic films are fabricated from TCS1@PMMA and TCS1/TFA@PMMA for Morse code and rewritable photopatterning applications, showing excellent photochromism. By coating TCS3 on a piece of plastic bag, the macroscopic mechanical motion is achieved by amplifying the collective molecular motion from the nanoscale up to the macroscopic dimension. The molecular state‐based strategy reveals the regularity between photoreactions and molecular packing and further provides an opportunity to rapidly access controllable photoresponsive systems. [ABSTRACT FROM AUTHOR]

A carbon–carbon-linked, ultrathin, two-dimensional (2D) polymer film was prepared at the air/water interface through photochemically triggered [2 + 2] cycloaddition. The preorganization of the monomers on the water surface and the subsequent photo-polymerization led to the successful preparation of the ultrathin 2D polymer film. The obtained film is continuous, free standing, and has a large area (over 50 μm2). Transmission electron microscopy (TEM) and atomic force microscopy (AFM) give clear evidence of the ultrathin film morphology. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) indicate successful photo-induced [2 + 2] polymerization. [ABSTRACT FROM AUTHOR]

Two representative organic photoreactions, namely a bimolecular photocycloaddition and a monomolecular photorearrangement, are presented that are accelerated when the reaction is performed "on‐water", that is, at the water‐substrate interface with no solvation of the reaction components. According to the established models of ground‐state reactions "on‐water", the enhanced efficiency of the photoreactions is explained by hydrophobic effects (Paternó‐Büchi reaction) or specific hydrogen bonding (di‐π‐methane rearrangement) at the water‐substrate interface that decrease the energy of the respective transition state. These results point to the potential of this approach to conduct photoreactions more efficiently in an ecologically favorable medium. [ABSTRACT FROM AUTHOR]

Orthogonal photoreactions provide a unique way to locally and independently control (thermo)mechanical properties and functionality of polymer networks simply by choice of the wavelength. Herein, a library of acrylate functional coumarin monomers is synthesized, which are cured by sequence‐dependent wavelength orthogonality. In the presence of a long wavelength absorbing photoinitiator, the monomers undergo rapid curing by visible light induced radical chain growth polymerization. Subsequent irradiation with light in the UV‐A region selectively initiates the [2+2] photocycloaddition of the coumarin chromophores, which is confirmed by FTIR and UV–vis experiments. Through a well‐targeted design, acrylate‐based and thiol‐acrylate resin formulations are prepared, whose fast curing rate, low viscosity, and prolonged storage stability enable the one‐step fabrication of multi‐material structures by digital light processing (DLP) 3D printing. By using a dual‐wavelength printer, which operates at two different wavelengths (405 and 365 nm), objects comprising soft (ε = 22%, σ = 7.5 MPa) and stiff (ε = 2%, σ = 8.3 MPa) domains are printed with a single resin vat. Along with tensile properties, the wavelength selective change in the network structure features a local control of the glass transition temperature (ΔTg = 17 °C) in the 3D‐printed objects. Soft active devices are fabricated by dual‐wavelength DLP 3D printing, with distinct domains having a higher Tg and the local programming of multi shapes is demonstrated. [ABSTRACT FROM AUTHOR]

Harnessing the power of light for chemical transformation is a long-standing goal in organic synthesis, materials fabrication and engineering. Amongst all photochemical reactions, [2 + 2] photocycloadditions are inarguably the most important and most frequently used. These photoreactions have green characteristics by enabling new bond formation in a single step procedure under light irradiation, without the need for heat or chemical catalysis. More recently, substantial progress has been made in red-shifting the activation wavelength of photocycloadditions in response to research trends moving towards green and sustainable processes, and advanced applications in biological environments. In the past 5 years, our team has further expanded the toolbox of photocycloaddition reactions that can be triggered by visible light. In our exploration of photochemical reactivity, we found that reactivity is often red-shifted compared to the substrate's absorption spectrum. Our efforts have resulted in red-shifted photochemical reactions, providing some of the lowest energy – and catalyst-free – photo-activated [2 + 2] cycloadditions (up to 550 nm). More recently, we introduced an additional level of control over such finely wavelength gated reactions by altering the pH of the reaction environment, thus exploiting halochromic effects to enhance or impede the photoreactivity of red-shifted [2 + 2] photocycloaddition reactions. In this account, we discuss the current state of halochromically regulated photochemical reactions and their potential in soft matter materials on selected examples. The photochemical reaction pathways of heterocylic N-containing stilbene functions under visible light irradiation can be controlled by pH and the proximity of the reacting moieties. [ABSTRACT FROM AUTHOR]

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

Photochemical reactions are vital synthetic means for the synthesis of natural products and highly strained molecules. However, it remains an immense challenge to control the chemo- and regioselectivity in the photoreactions of anthracene derivatives while maintaining high reactivity. Herein, we report the synthesis of two photoactive metallarectangles 1a and 1b by coordination-driven self-assembly of 2,6- and 2,7-bifunctionalized anthracenes with a half-sandwich rhodium template. By taking advantage of the rhodium template, the anthracene groups within the metallarectangles can be preorganized in a parallel fashion and exclusively undergo a photochemical [4 + 4] cycloaddition. As a result, the syn-photodimers were obtained in quantitative yields under irradiation at 365 nm. The photocycloaddition of metallarectangles 1a and 1b was found to be reversible via photo- and thermal cleavage reactions, even after repeating three times. Furthermore, pure organic photodimers 3a and 3b, which are difficult to synthesize by conventional organic methods, can be readily dissociated from the metalloassemblies in high yields. [ABSTRACT FROM AUTHOR]

The catalyst-free [2 + 2] photocycloaddition between benzils and simple olefins is reported. The adoption of visible light proved essential for the transformation, as shorter wavelengths led to uncontrolled decomposition. When cyclic olefins were used, the reaction occurred smoothly to afford the expected oxetanes regio- and stereoselectively after 24 h of irradiation. In contrast, in the case of acyclic olefins, longer reaction times were typically required and small amounts (ca. 20%) of [4 + 2] photocycloadducts and by-products deriving from competitive hydrogen atom abstraction were observed. The selectivity of the transformation could be consistently improved by decreasing the reaction temperature, thus restoring the desired [2 + 2] reactivity. An overall mechanistic picture is also offered based on the chemical and photophysical quenching experiments and the stereochemical output is rationalized based on Griesbeck models. [ABSTRACT FROM AUTHOR]

Control over the stereochemistry of excited-state photoreactions remains a significant challenge in organic synthesis. Recently, it has become recognized that the photophysical properties of simple organic substrates can be altered upon coordination to Lewis acid catalysts, and that these changes can be exploited in the design of highly enantioselective catalytic photoreactions. Chromophore activation strategies, wherein simple organic substrates are activated towards photoexcitation upon binding to a Lewis acid catalyst, rank among the most successful asymmetric photoreactions. Herein, we show that chiral Brønsted acids can also catalyze asymmetric excited-state photoreactions by chromophore activation. This principle is demonstrated in the context of a highly enantio- and diastereoselective [2+2] photocycloaddition catalyzed by a chiral phosphoramide organocatalyst. Notably, the cyclobutane products arising from this method feature a trans-cis stereochemistry that is complementary to other enantioselective catalytic [2+2] photocycloadditions reported to date. Lewis acids have recently been shown to enable stereocontrol in photochemical cycloadditions, a difficult task due to the reactivity of excited-state compounds. Here the authors show that chiral Brønsted acids are competent chromophore activators in [2+2] cycloadditions, forming diastereomers disfavored in similar Lewis acid catalyzed photochemical reactions. [ABSTRACT FROM AUTHOR]

Lewis acids have recently been recognized as catalysts enabling enantioselective photochemical transformations. Mechanistic studies on these systems are however rare, either due to their absorption at wavelengths shorter than 260 nm, or due to the limitations of theoretical dynamic studies for larger complexes. In this work, we overcome these challenges and employ sub‐30‐fs transient absorption in the UV, in combination with a highly accurate theoretical treatment on the XMS‐CASPT2 level. We investigate 2‐cyclohexenone and its complex to boron trifluoride and analyze the observed dynamics based on trajectory calculations including non‐adiabatic coupling and intersystem crossing. This approach explains all ultrafast decay pathways observed in the complex. We show that the Lewis acid remains attached to the substrate in the triplet state, which in turn explains why chiral boron‐based Lewis acids induce a high enantioselectivity in photocycloaddition reactions. [ABSTRACT FROM AUTHOR]

The highly strained cubane system is of great interest as a scaffold and rigid linker in both pharmaceutical and materials chemistry. A straightforward approach is reported for the scale-up of a [2+2] photocycloaddition step using convenient home-made flow photoreactors to access dimethyl 1,4-cubanedicarboxylate on decagram-scale in 33–40% yield over 8 steps. The process is demonstrated on 3.4 g·h–1 input with 30 minutes residence time, enabling to reduce the process time and to avoid the use of batch photoreactors. Completion of the characterisation of the photocycloadduct and its hydrates is reported. [ABSTRACT FROM AUTHOR]

The sole method available for the photocycloaddition of unconjugated aliphatic alkenes is the Cu‐catalyzed Salomon–Kochi reaction. The [Cu(OTf)]2⋅benzene catalyst that has been standard in this reaction for many decades, however, is air‐sensitive, prone to photodecomposition, and poorly reactive towards sterically bulky alkene substrates. Using bench‐stable precursors, an improved catalyst system with superior reactivity and photostability has been designed, and it offers significantly expanded substrate scope. The utility of this new catalyst for the preparation of sterically crowded cyclobutane structures is highlighted through the preparation of the cores of the natural products sulcatine G and perforatol. [ABSTRACT FROM AUTHOR]

Chiral eniminium salts, prepared from α,β‐unsaturated aldehydes and a chiral proline derived secondary amine, underwent, upon irradiation with visible light, a ruthenium‐catalyzed (2.5 mol %) intermolecular [2+2] photocycloaddition to olefins, which after hydrolysis led to chiral cyclobutanecarbaldehydes (17 examples, 49–74 % yield), with high diastereo‐ and enantioselectivities. Ru(bpz)3(PF6)2 was utilized as the ruthenium catalyst and laser flash photolysis studies show that the catalyst operates exclusively by triplet‐energy transfer (sensitization). A catalytic system was devised with a chiral secondary amine co‐catalyst. In the catalytic reactions, Ru(bpy)3(PF6)2 was employed, and laser flash photolysis experiments suggest it undergoes both electron and energy transfer. However, experimental evidence supports the hypothesis that energy transfer is the only productive quenching mechanism. Control experiments using Ir(ppy)3 showed no catalysis for the intermolecular [2+2] photocycloaddition of an eniminium ion. [ABSTRACT FROM AUTHOR]

A chiral phosphoric acid with a 2,2'‐binaphthol core was prepared that displays two thioxanthone moieties at the 3,3'‐position as light‐harvesting antennas. Despite its relatively low triplet energy, the phosphoric acid was found to be an efficient catalyst for the enantioselective intermolecular [2+2] photocycloaddition of β‐carboxyl‐substituted cyclic enones (e.r. up to 93:7). Binding of the carboxylic acid to the sensitizer is suggested by NMR studies and by DFT calculations to occur by means of two hydrogen bonds. The binding event not only enables an enantioface differentiation but also modulates the triplet energy of the substrates. [ABSTRACT FROM AUTHOR]

Hydrogels with on‐demand tunable mechanical properties within sensitive biological environments are of critical importance for examining cellular responses to cell culture platforms. Herein, the first bio‐orthogonal hydrogel that can be formed and subsequently tuned in its mechanical properties by simply switching different wavelengths of visible light (i.e., 455 and 420 nm) is reported. Specifically, both the initial hydrogelation and the tuning of the mechanical properties can be fully decoupled and selectively initiated by different colors of light. Sparing the need for any catalysts, the development of such a dual wavelength selective hydrogel for biological applications spans four levels: First, the development of the until today most redshifted photocycloaddition to allow for the selective initiation of only one photoreaction; second, the investigation of its wavelength‐dependent ligation efficiency; third, translation of the ligation chemistry into a hydrogelator, and fourth, establishing a biocompatible hydrogel platform for applications in biomaterials engineering including detachment of fibroblasts from 2D culture areas or primary 3D culture of human mesenchymal stem cells. The introduced platform technology enables the fabrication of a hydrogel of predefined mechanical properties exclusively with visible light. [ABSTRACT FROM AUTHOR]

Starting from readily available 7‐substituted 1‐indanones, products with a tetracyclo[5.3.1.01,704,11]undec‐2‐ene skeleton were obtained upon irradiation at λ=350 nm (eight examples, 49–67 % yield). The assembly of the structurally complex carbon framework proceeds in a three‐photon process comprising an ortho photocycloaddition, a disrotatory [4π] photocyclization, and a di‐π‐methane rearrangement. The flat aromatic core of the starting material is converted into a functionalized polycyclic hydrocarbon with exit vectors in three dimensions. Ring opening reactions at the central cyclopropane ring were explored, which enable the preparation of tricyclo[5.3.1.04,11]undec‐2‐enes and of tricyclo[6.2.1.01,5]undecanes. [ABSTRACT FROM AUTHOR]

In a single photochemical operation (λ≥350 nm) an easily accessible indanone derivative was converted into a structurally complex precursor of the protoilludane sesquiterpenes. The product (60 % yield) contains all 15 carbon atoms of the skeleton in the required connectivity and was transformed into the natural product atlanticone C (9 steps, 6 % overall yield). In addition, it was shown that other protoilludanes, such as Δ6‐protoilludene and paesslerin A, can be prepared in a concise fashion via the photochemical key intermediate. The photochemical reaction cascade comprises an ortho photocycloaddition, a thermal disrotatory ring opening and a regioselective disrotatory [4π] photocyclization. [ABSTRACT FROM AUTHOR]

The intramolecular [2+2] photocycloaddition of 3‐alkenyl‐2‐cycloalkenones was performed in an enantioselective fashion (nine representative examples, 54–86 % yield, 76–96 % ee) upon irradiation at λ=366 nm in the presence of an AlBr3‐activated oxazaborolidine as the Lewis acid. An extensive screening of proline‐derived oxazaborolidines showed that the enantioface differentiation depends strongly on the nature of the aryl group at the 3‐position of the heterocycle. DFT calculations of the Lewis acid–substrate complex indicate that attractive dispersion forces may be responsible for a change of the binding mode. The catalytic [2+2] photocycloaddition was shown to proceed on the triplet hypersurface with a quantum yield of 0.05. The positive effect of Lewis acids on the outcome of a given intramolecular [2+2] photocycloaddition was illustrated by optimizing the key step in a concise total synthesis of the sesquiterpene (±)‐italicene. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • [2 + 2], [3 + 2], and [4 + 2] Photocycloadditions selectively proceeded. • Intramolecular π-π interaction in exciplexes governed the reaction modes. • Di-π-methane rearrangement of [4 + 2] photocycloadducts gave angular triquinanes. Abstract Photoreactions of 1-cyanonaphthalene derivatives containing 5-arylpent-4-enyl groups at C-2 or C-4 of the naphthalene ring were investigated. Photoreactions of C-2 derivatives in benzene or CH 3 CN generated intramolecular [2 + 2] and [3 + 2] photocycloadducts. Substances bearing electron-rich and electron-poor phenyl groups produced respective [2 + 2] and [3 + 2] photocycloadducts preferentially. In contrast, photoreactions of C-4 linked 1-cyanonaphthalenes produced intramolecular [4 + 2] photocycloadducts as main products along with stereoisomers of tricyclic compounds as minor products. The formation of [4 + 2] photocycloadducts was promoted by using benzophenone or Michler's ketone triplet photosensitization, and quenched by O 2 or ferrocene. Intramolecular singlet exciplexes, which serve as important intermediates in the mechanistic pathways for these reactions, governed the site-selectivities in photoreactions of both of the 2- and 4-(5-arylpent-4-enyl)-1-cyanonaphthalenes. In addition, [4 + 2] photocycloadducts were observed to undergo di-π-methane rearrangement under irradiation conditions to produce angular triquinane derivatives. [ABSTRACT FROM AUTHOR]

We describe a templating/covalent capture strategy that enables photochemical formation of 8 cyclobutanes in one noncovalent assembly. This process was characterized by experiment and quantum mechanical/molecular mechanics (ONIOM) calculations. Thus, KI and 16 units of 5′‐cinnamate guanosine form a G‐quadruplex where C=C π bonds in neighboring G4‐quartets are separated by 3.3 Å, enabling [2+2] photocycloaddition in solution. This reaction is high‐yielding (>90 %), regio‐ and diastereoselective. Since all components are in dynamic equilibrium this photocycloaddition is catalytic in K+. A templating/covalent capture strategy enables photochemical formation of eight cyclobutanes in one noncovalent assembly. KI and 16 units of 5′‐cinnamate guanosine form a G‐quadruplex where C=C bonds in neighboring G4‐quartets are separated by 3.3 Å, enabling [2+2] photocycloaddition in solution. This reaction is high‐yielding (>90 %), regio‐ and diastereoselective. [ABSTRACT FROM AUTHOR]

In this paper we present a convenient and scalable synthesis of 1‐(methylamino)anthracene, which is a potent building block for the construction of photoresponsive materials. With this structural motif we have synthesized an anion receptor bearing two anthracene groups. Interaction of a chloride anion with the anthracene moieties increases the association constant. Photocycloaddition allows to obtain a single macrocyclic receptor, which exhibits higher binding constants due to the macrocyclic effect. Facile synthesis provides a novel anthracene‐based amine, which acts as a versatile building block for the construction of photoswitchable receptors. Incorporation of this structural motif into dipicolinic amide provides a photoresponsive anion receptor. Upon irradiation with 360 nm light, cycloaddition occurs to yield macrocyclic receptors with higher affinities for anions. [ABSTRACT FROM AUTHOR]

A 2‐alkenoylpyridine‐bound N,N′‐dioxide–TbIII complex has been found to absorb visible light to reach the excited state, leading to the direct visible‐light‐excited catalytic enantioselective [2+2] cycloaddition of 2‐alkenoylpyridines to various alkenes in the absence of an additional photosensitizer. Diverse enantioenriched cyclobutanes were successfully obtained (yields up to 70 %, >19:1 d.r. 92 % ee). The new chiral terbium(III) complex features a bathochromic shift, lower excitation energy, and facile intersystem crossing due to paramagnetic and heavy‐atom effects, which enable the antenna 2‐alkenoylpyridines to be excited. For comparison, a chiral N,N′‐dioxide–ScIII complex in combination with [Ru(bpy)3]Cl2 was efficient in the enantioselective photocycloaddition reactions of 2′‐hydroxychalcones with alkenes, thereby revealing that both substrates and metal salts have significant effects on the reaction. Exciting terbium complex: The terbium complex of a novel four‐carbon‐linked L4‐RaPr2 ligand efficiently promotes the visible‐light‐excited [2+2] cycloaddition reactions of 2‐alkenoylpyridines with diverse alkenes to produce diverse enantioenriched cyclobutanes (see scheme). A plausible mechanism for the photocycloaddition reaction is discussed. [ABSTRACT FROM AUTHOR]

Visible‐light irradiation (λ=457 nm) enabled the enantioselective ortho photocycloaddition of olefins to phenanthrene‐9‐carboxaldehydes (15 examples, 46–93 % yield, 82–98 % ee). A chiral oxazaborolidine Lewis acid (20 mol %) was employed as the catalyst. It operates by coordination to the aldehyde inducing a bathochromic absorption shift beyond the nπ* absorption of the uncomplexed aldehyde. At long wavelengths the Lewis acid complex is exclusively excited; within the complex, one enantiotopic face of the aromatic aldehyde is efficiently shielded. Lewis acid coordination also alters the type selectivity and the simple diastereoselectivity of the photocycloaddition. Chromophore activation at its best: Lewis acid catalysis enables an enantioselective photocycloaddition of olefins to the C9/C10 double bond of phenanthrene‐9‐carboxaldehydes in which up to four stereogenic centers are created. The chromophore is significantly modified by Lewis acid coordination. [ABSTRACT FROM AUTHOR]

The first law of photochemistry, as described by Theodor von Grotthuß and John W. Draper, states that only the light absorbed by the irradiated matter can effect photochemical change. Consequently, the photochemical behavior of a molecule can be controlled by bringing its absorbance properties in line with the emission of the light source. A compound with a chromophore that only absorbs light at short wavelengths will not be excited by light of longer wavelengths. If one can reversibly modify the photophysical properties of a compound with a chemical activator, then it is possible to photoexcite only the activated species. For α,β‐unsaturated carbonyl compounds, the use of Lewis acids, Brønsted acids, or the formation of the respective iminium ions can bring about the desired chromophore activation to catalyze a photochemical reaction at a given wavelength. In this Minireview, the concept of chromophore activation will be illustrated, and examples of its implementation in enantioselective catalysis will be discussed. Walking the tightrope above the ground state: The absolute stereochemical outcome of a photochemical reaction can be controlled by activating the starting material with Lewis acids or Brønsted acids or through the formation of iminium ions. [ABSTRACT FROM AUTHOR]

Abstract: We describe a photochemical reaction using two starting materials, a cyclopent‐2‐enone and an alkene, which are transformed in a controlled manner via the initial [2+2]‐photocycloaddition adducts into cyclobutene aldehydes (conveniently trapped as stable acetals) or unprecedented angular tricyclic 4:4:4 oxetane‐containing skeletons. These compounds are formed through tandem or triple cascade photochemical reaction processes, respectively. Small libraries of each compound class were prepared, thus suggesting that this photochemistry approach opens new opportunities for synthesis design and for widening molecular diversity. [ABSTRACT FROM AUTHOR]

Abstract: A novel method for the catalytic asymmetric dearomatization by visible‐light‐activated [2+2] photocycloaddition with benzofurans and one example of a benzothiophene is reported, thereby providing chiral tricyclic structures with up to four stereocenters including quaternary stereocenters. The benzofurans and the benzothiophene are functionalized at the 2‐position with a chelating N‐acylpyrazole moiety which permits the coordination of a visible‐light‐activatable chiral‐at‐rhodium Lewis acid catalyst. Computational molecular modeling revealed the origin of the unusual regioselectivity and identified the heteroatom in the heterocycle to be key for the regiocontrol. [ABSTRACT FROM AUTHOR]

Spectral and photochemical properties of a bichromophoric dyad with two photochromes, 2-[(2-(pyren-1-yl)ethenyl]quinoline (PEQ) bridged by ten methylene groups have been investigated. Stationary spectroscopy does not show substantial interaction between the PEQ groups in the dyad, but the appearance of another, more long-living component in the f luorescence decay kinetics indicates possible excimer formation. The formation of the chemically active excimer is also confirmed by the intramolecular [2+2] photocycloaddition reaction to give tetrasubstituted cyclobutane with two vicinal pyrene substituents. This reaction competes with trans-cis photoisomerization of the dyad. It has been proposed that the π-stacking interaction of two PEQ groups with a long system of π-bonds plays an important role in the excimer formation followed by the photocycloaddition reaction. [ABSTRACT FROM AUTHOR]

Abstract: New photocatalysts from the flavin family were found to mediate the [2+2] photocycloaddition reaction. 3‐Butyl‐10‐methyl‐5‐deazaflavin (3 a) and 1‐butyl‐7,8‐dimethoxy‐3‐methylalloxazine (2 e), if irradiated by visible light, were shown to allow an efficient (Φ≈3–10 %) intramolecular cyclisation of various types of substrates including substituted styrene dienes and bis(aryl enones), considered as electron‐rich and electron‐poor substrates, respectively, without any additional reagent. The versatility of the procedure was demonstrated by the cyclisation of photosensitive cinnamyl (E)‐3‐iodoallyl ether. Structure–activity studies found alloxazine 2 e was more active than 7‐monosubstituted (R=Cl, Br and MeO) alloxazines. The introduction of chlorine and bromine atom on the deazaflavin skeleton did not enhance the catalytic efficiency of 3 a. A detailed electrochemical and spectroscopic study explains the reaction mechanism proceeding through energy transfer from the flavin excited triplet state to the diene followed by its cyclisation. [ABSTRACT FROM AUTHOR]

Conventional photocycloaddition of 1,4-dihydropyridines does not afford novel head-to-head 3,6-diazatetraasteranes. Herein, we describe a highly regioselective method to synthesize 3,6-diazatetraasteranes via an intramolecular photodimerization of 1,4-dihydropyridines. First, the 1,4-dihydropyridines were tethered by phthaloyl to direct a proximate parallel arrangement in head-to-head orientation by the rotation of C C single bonds in solution. An intramolecular [2 + 2] photocycloaddition proceeded subsequently to give desired 3,6-diazatetraasteranes in high yield (92–97%) and excellent regioselectivity. Furthermore, two different 1,4-dihydropyridines can also be regiocontrolled by this strategy and produce polysubstituted 3,6-diazatetraasteranes via a cross-photodimerization in a concise and efficient way. In addition, this approach can provide direct access to other polysubstituted polyhedron scaffolds from 1,4-dihydropyridine analogues. [ABSTRACT FROM AUTHOR]

To study the effects of the position of the hexatrienyl moiety on the oxazole ring, novel substituted cis/trans-2/4/5-(2-vinylstyryl)oxazoles have been synthesized. These novel compounds were prepared by Wittig reaction from the diphosphonium salt of α,α'-o-xylenedibromide, formaldehyde and the corresponding 2-methyl-4-, 4-methyl-2-, 2-pheny-5- and 4-methyl-5-oxazolecarbaldehyde, respectively. Aldehydes were synthesized by using several synthetic approaches. By applying intramolecular photocycloaddition, 2-methyl-4-(2-vinylstyryl)-oxazole afforded, as major product, fused oxazoline-benzobicyclo[3.2.1]octene with small quantities of 4-(1,2-dihydronaphthalen-2-yl)-2-methyloxazole, as electrocyclization product. Upon irradiation of 4-methyl-5-(2-vinylstyryl)oxazole, endo- and exo-benzobicyclo[2.1.1]hexene products were formed by [2+2] cycloaddition; this was the first instance of the 1,4-closure to the bicyclo[2.1.1]hexene skeleton in the 2-, 4-, and 5-oxazole- stilbene derivatives studied so far. Derivatives 2-phenyl-5- and 4-methyl-2-(2-vinylstyryl)oxazole did not react and gave only high-weight molecular products but were crucial as a comparison in the overall mechanistic study. We have found that, depending on the position of the hexatrienyl moiety in the oxazole ring, as well as on the position of the methyl/phenyl substituents, these new vinylstyryl-2/4/5-oxazole derivatives show diverse photochemical behavior. [ABSTRACT FROM AUTHOR]

Abstract: Εniminium ions were prepared from the corresponding α,β‐unsaturated carbonyl compounds (enones and enals), and were found to be promoted to their respective triplet states by energy transfer. The photoexcited intermediates underwent intra‐ or intermolecular [2+2] photocycloaddition in good yields (50–78 %) upon irradiation at λ=433 nm or λ=457 nm. Iridium or ruthenium complexes with a sufficiently high triplet energy were identified as efficient catalysts (2.5 mol % catalyst loading) for the reaction. The intermolecular [2+2] photocycloaddition of an eniminium ion derived from a chiral secondary amine proceeded with high enantioselectivity (88 % ee). [ABSTRACT FROM AUTHOR]

The reactivity of photoexcited molecules has been extensively studied for decades but until today direct bond-forming reactions of such excited states in a catalytic and asymmetric fashion are restricted to the synthesis of cyclobutanes via [2 + 2] photocycloadditions. Herein, we demonstrate a previously elusive visible-light-induced catalytic asymmetric [2 + 3] photocycloaddition of alkenes with vinyl azides. A wide range of complex 1-pyrrolines are obtained as single diastereoisomers and with up to >99% enantiomeric excess using a simple reaction setup and mild reaction conditions. The reaction is proposed to proceed through the photoexcitation of a complex out of chiral rhodium catalyst coordinated to α,β-unsaturated N-acylpyrazole substrates. All reactive intermediates remain bound to the catalysts thereby providing a robust catalytic scheme (no exclusion of air necessary) with excellent stereocontrol. This work expands the scope of stereocontrolled bond-forming reactions of photoexcited intermediates by providing catalytic asymmetric access to a key nitrogen heterocycle in organic chemistry. [ABSTRACT FROM AUTHOR]

The title compounds were found to undergo a [2+2] photocycloaddition with olefins at λ = 419 nm in CH2Cl2 as the solvent. The resulting cyclobutanes were isolated in yields of 32-87% (11 examples) and showed a defined relative configuration at C1/C4 in the major diastereoisomer (nitro and aryl trans). The analysis of side products and triplet sensitization experiments support a mechanistic scenario in which a 1,4-diradical is formed as a key intermediate. [ABSTRACT FROM AUTHOR]

Herein, we report the development of continuous flow photoreactors for large scale ESIPT-mediated [3+2]-photocycloaddition of 2-( p- methoxyphenyl)-3-hydroxyflavone and cinnamate-derived dipolarophiles. These reactors can be efficiently numbered up to increase throughput two orders of magnitude greater than the corresponding batch reactions. [ABSTRACT FROM AUTHOR]

Selective excited-state intramolecular proton-transfer (ESIPT) photocycloaddition of 3-hydroxyflavones with trans, trans-1,4-diphenyl-1,3-butadiene is described. Using this methodology, total syntheses of the natural products (±)-foveoglin A and (±)-perviridisin B were accomplished. Enantioselective ESIPT photocycloaddition using TADDOLs as chiral hydrogen-bonding additives provided access to (+)-foveoglin A. Mechanistic studies have revealed the possibility for a photoinduced electron-transfer (PET) pathway. [ABSTRACT FROM AUTHOR]

Polyladderane, the first polymer to contain the ladderane functional group, was synthesized from a gemini monomer through photoreaction in the solid state. The modular design of the gemini monomers used to create polyladderane allowed specific structural modification, resulting in the formation of two distinct polymer products. Monomers were synthesized by connecting two photoreactive units, either sorbic acids (monomer I) or 2-furanacrylic acids (monomer II), with a 1,4-butanediol linker. Single-crystal X-ray diffraction analysis of the monomers confirmed that they packed in the desired head-to-tail orientation and within a viable distance for photoreaction by electronically complementary interaction. Pre-organized gemini monomers were irradiated with UV light and monitored by FT-IR. Two polyladderanes with cis, anti, cis-[3]-ladderane as a characteristic functional group were constructed stereospecifically in 24-36 hours. [ABSTRACT FROM AUTHOR]

An octafluorinated 2,11-diaza[32]paracyclophane derivative 12 was prepared, and its photochemical reaction was investigated. Upon irradiation at 300 nm, the fluorinated azacyclophane 12 underwent efficient photodimerization of the benzene cores to afford the corresponding photoisomer 13, which possessed cage diene benzene dimer structure (pentacyclo[6.4.0.0.2,70.3,1206,9]dodeca-4,10-diene skeleton). The cage diene structure was established by single-crystal X-ray diffraction analysis. The cage diene 13 thermally isomerized to a syn- o, o′-dibenzene isomer 22. The activation parameters for the thermal isomerization were determined to be Ea = 121 kJ mol−1, ΔH≠ = 118 kJ mol−1, ΔS≠(293 K) = 22 J mol−1 K−1, and ΔG≠(293 K) = 111 kJ mol−1. It was revealed that, by photoirradiation at 300 nm, the syn- o, o′-dibenzene isomer 22 underwent facile intramolecular [π4s + π4s] cycloaddition to reproduce the cage diene isomer 13. [ABSTRACT FROM AUTHOR]

Bilobalide is a tetracyclic sesquiterpene containing three contiguous γ-lactone rings and an unusual tert-butyl group, which is found in the leaves of the ginkgo tree (Ginkgo biloba). Three different photochemical approaches towards bilobalide's unique skeleton are presented. A meta photocycloaddition, a [2 + 2] photocycloaddition, and a Paternò–Büchi-reaction were chosen as the respective key steps. [ABSTRACT FROM AUTHOR]

1,3-Dithiane-protected enones (enone dithianes) were found to undergo an intramolecular [2+2] photocycloaddition under visible-light irradiation ( λ=405 nm) in the presence of a Brønsted acid (7.5-10 mol %). Key to the success of the reaction is presumably the formation of colored thionium ions, which are intermediates of the catalytic cycle. Cyclobutanes were thus obtained in very good yields (78-90 %). It is also shown that the dithiane moiety can be reductively or oxidatively removed without affecting the photochemically constructed ring skeleton. [ABSTRACT FROM AUTHOR]

Intramolecular photocycloaddition reactions of cycloalkene linked naphthalenes lead to formation of pentacyclic compounds in a high yielding and stereoselective manner. Intramolecular [2 + 2] cycloadducts are formed initially in photoreactions of cycloalkene linked 1-cyanonaphthalenes. The initially formed intramolecular [2 + 2] cycloadducts absorb light at wavelengths used to promote the photochemical reactions and, as a result, they undergo efficient photocycloreversion to regenerate the starting substrates. In a less efficient competing process, the cycloalkene linked 1-cyanonaphthalenes react to form [3 + 2] intramolecular photoadducts, which do not absorb incident light under the conditions used. Consequently, these photoadducts become the major products in these processes. The relative configuration of the major diastereomers of the pentacyclic products formed from photoreactions of cyclopentene linked naphthalenes ( cis - cis ) differ from those ( cis - trans ) arising from the cyclooctene linked substrates. The results of theoretical calculations, which show that steric factors govern the preferred facial mode of intramolecular addition, are in good agreement with the stereochemical course of these photoreactions. [ABSTRACT FROM AUTHOR]

The synthesis of novel 1-(2-anthryl)-1-phenylethylene (APE) di-telechelic polyisobutylenes is described. Utilization of a difunctional cationic initiator and thein situaddition of the non-homopolymerizable APE lead to the formation of di-anthryl telechelic polyisobutylenes. Products were characterized by1H NMR spectroscopy and Size Exclusion Chromatography. The polymers were UV irradiated at 365 and 254 nm and the reversible photocycloaddition of anthryl moieties was investigated. The chain extension of di-anthryl telechelic PIBs through photocoupling at 365 nm produced higher molecular weight products from low molecular weight precursors. The effect of precursor polymer concentration on the degree of chain extension was investigated, and intermolecular interactions leading to the formation of tetramers was observed. The photocoupled products were UV irradiated at 254 nm to induce the reversal of photocycloaddition of anthryl groups and to follow the consequent photoscission of polymers. [ABSTRACT FROM PUBLISHER]

The seminal synthesis of the sesquiterpene (±)-α-cedrene reported by Wender in 1981 offers a uniquely short and elegant access to the bridged-tricyclic target compound by exploiting an intramolecular arene-olefin photocycloaddition. However, the synthesis was performed only in the racemic series so far. This synthesis was now re-investigated and the catalytic methods for the enantioselective preparation of the chiral key intermediate were evaluated. It was found that Cu-catalyzed allylic substitution of a cinnamyl chloride with MeMgBr in the presence of a Taddol-derived chiral phosphine-phosphite ligand affords the corresponding (1-methylallyl)arene with high enantioselectivity (94% ee). Hydroboration and subsequent Suzuki coupling gave (R)-curcuphenol methyl ether from which (–)-α-cedrene was prepared along the route paved by Wender. [ABSTRACT FROM AUTHOR]