The norcaradiene–cycloheptatriene equilibrium

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.

Diastereomeric Configuration Drives an On-Surface Specific Rearrangement into Low Bandgap Non-Benzenoid Graphene Nanoribbons

Stereochemistry, usually associated with the three-dimensional arrangement of atoms in molecules, is crucial in processes like life functions, drug action, or molecular reactions. This three-dimensionality typically originates from sp3 hybridization in organic molecules, but it is also present in out-of-plane sp2-based molecules as a consequence of helical structures, twisting processes, and/or the presence of nonbenzenoid rings, the latter significantly influencing their global stereochemistry and leading to the emergence of new exotic properties. In this sense, on-surface synthesis methodologies provide the perfect framework for the precise synthesis and characterization of organic systems at the atomic scale, allowing for the accurate assessment of the associated stereochemical effects. In this work, we demonstrate the importance of the initial diastereomeric configuration in the surface-induced skeletal rearrangement of a substituted cyclooctatetraene (COT) moiety-a historical landmark in the understanding of aromaticity-into a cyclopenta[c,d]azulene (CPA) one in a chevron-like graphene nanoribbon (GNR). These findings are evidenced by combining bond-resolved scanning tunneling microscopy with theoretical ab initio calculations. Interestingly, the major well-defined product, a CPA chevron-like GNR, exhibits the lowest bandgap reported to date for an all-carbon chevron-like GNR, as evidenced by scanning tunneling spectroscopy measurements. This work paves the way for the rational application of stereochemistry in the on-surface synthesis of novel graphene-based nanostructures.

Arene Ring Expansion by Ruthenium η6-Arene Complexes

Transition metal π-arene complexes enable the dearomatization of benzene rings to access diversified unsaturated carbocycles through multistep synthetic procedures involving sequential addition of nucleophiles and electrophiles. This work details a single-step dearomatization process by reaction of Ru(η6-arene) complexes with enolates derived from α-halo or α-(tosyloxy)esters to directly transform π-coordinated arenes to ring-expanded cycloheptatrienes.

Catalytic Dearomative [1,5]-Sigmatropic Carbon Shift of Heterole-Fused Norcaradienes Enabled Concise Helicenation

In contrast to the locked fluxionality of norcaradienes fused by benzene, unexplored less aromatic heterole-fused norcaradienes, creatively generated by intramolecular hydroarylation of heteroaryl alkynylcyclopropanes, reserve a balancing fluxionality that permits a dearomative [1,5]-sigmatropic carbon shift of norcaradienes akin to the reduced aromaticity of heterole. This "walk" shift was confirmed by the isolation of a cycloheptatriene species derived from ring-expansion of a dearomatized alkynylated heterole-fused norcaradiene. A following ester-directed ring-opening rearomatization of these dearomatized heterole-fused norcaradienes gives the products featuring migratory acylmethyls that are competent for helicenation with the neighboring (hetero) arenes via (formal) dehydrative alkenylation. Such balancing reactivity of heterole-fused norcaradienes will open up the opportunity for the development of controllable reactions of fused norcaradienes.

A review of the synthetic strategies toward the antiviral drug tecovirimat

This review provides a comprehensive analysis of synthetic routes for tecovirimat, an antiviral drug used to treat orthopoxvirus infections, including monkeypox and smallpox. We focus on the scale-up synthesis of key intermediates, including cycloheptatriene, as documented in the published literature and patent records. The review highlights the efficiency, yield, and purity of these approaches, as well as the minimization of genotoxic and in-process impurities. Furthermore, we critically evaluate the recently reported optimized industrial-scale synthesis process, highlighting its advantages and limitations, and identifying avenues for further improvement. By obtaining insights from the published literature and patent records, this review elucidates the current state of knowledge regarding key synthesis parameters influencing tecovirimat production and emphasizes the critical importance of optimizing synthesis techniques to achieve remarkable improvements in safety and environmental impact. This review serves as a valuable resource for researchers and industry professionals in the field of R&D and production of APIs, particularly in expediting the safe and efficient industrial production of tecovirimat.

Access to cyclohexadiene and benzofuran derivatives via catalytic arene cyclopropanation of α-cyanodiazocarbonyl compounds

The arene cyclopropanation between diazo compounds and benzene is well known to produce a tautomeric mixture of norcaradiene and cycloheptatriene in favour of the latter species. Nevertheless, previous studies have suggested that the initially formed norcaradiene can be stabilized by a C-7 cyano group with prevention of its 6π-electrocyclic ring opening. According to this feature, a synthetic route to functionalized cyclohexadienes has been designed using α-cyanodiazoacetates and α-diazo-β-ketonitriles as the starting materials, respectively. The Rh2(esp)2-catalyzed arene cyclopropanation of α-cyanodiazoacetates in benzene afforded the expected 7-alkoxycarbonyl-7-cyanonorcaradienes as isolable compounds, which then served as templates for the second cyclopropanation with ethyl diazoacetate or α-cyanodiazocarbonyls to enable the formation of bis(cyclopropanated) adducts. Their subsequent treatment with SmI2 triggered a double ring-opening process, allowing for the generation of 1,4- and/or 1,3-cyclohexadienes as either regio- or diastereomeric mixtures. On the other hand, the norcaradienes generated from phenyl- or methyl-substituted α-diazo-β-ketonitriles were found to undergo an in situ rearrangement to yield dihydrobenzofurans that could be converted to benzofuran derivatives by DDQ oxidation.

Norcaradiene-Cycloheptatriene Equilibrium: A Heavy-Atom Quantum Tunneling Case

The equilibrium between norcaradiene and cycloheptatriene, which has captivated chemists for more than half a century, is revisited by state-of-the-art quantum chemical calculations. Our theoretical data significantly deviate from the experimental results (J. Am. Chem. Soc., 1981, 26, 7791-7792), especially at low temperatures, where isomerization is dominated by heavy-atom tunneling. This effect results in an extremely short half-life for norcaradiene, rendering it undetectable. This work sheds light on this equilibrium, updating the kinetic and thermodynamic data while also expanding the repertoire of organic reactions controlled by this exotic quantum effect.

Impurity study of tecovirimat

This article delineates the systematic identification, synthesis, and impurity control methods used during the manufacturing process development of tecovirimat, an antiviral drug that treats monkeypox. Critical impurities were synthesized, and their chemical structure was confirmed through NMR analysis, GC, and HPLC mass spectrometry. The results established a thorough approach to identify, address, and control impurities to produce high-quality tecovirimat drug substance in accordance with International Conference on Harmonization (ICH)-compliant standards. This study is the first of its kind to evaluate both process and genotoxic impurities in tecovirimat, demonstrating effective control measures during commercial sample investigations and scaling up to a 60-kg batch size. The findings highlight the importance of critical impurity characterization and control in pharmaceutical development and production to ensure the safety and efficacy of the final product.

Asymmetric Synthesis of Hydrodibenzofurans from Norcaradienes: Kinetic Resolution via [3 + 2] Cycloaddition with Quinones

The catalytic asymmetric [3 + 2] cycloaddition of racemic norcaradienes with quinones to construct multicyclic hydrodibenzofurans was achieved by the use of chiral N,N'-dioxide/metal complex catalysts. Kinetic resolution of norcaradienes accompanied by partial racemization occurred, and one enantiomer in prior acted as the C2 synthon to participate in diastereoselective cycloaddition. An enantiodivergent synthesis via a switch of metal ions was observed when naphthoquinone was used as the partner. DFT calculations revealed the profiles of the cycloaddition processes.

Asymmetric [4 + 2], [6 + 2], and [6 + 4] Cycloadditions of Isomeric Formyl Cycloheptatrienes Catalyzed by a Chiral Diamine Catalyst

Novel asymmetric aminocatalytic cycloadditions are described between formyl cycloheptatrienes and 6,6-dimethylfulvene that lead to [4 + 2], [6 + 2], and [4 + 6] cycloadducts. The unprecedented reaction course is dependent on the position of the formyl functionality in the cycloheptatriene core, and each formyl cycloheptatriene isomer displays a distinct reactivity pattern. The formyl cycloheptatriene isomers are activated by a chiral primary diamine catalyst, and the activation mode is dependent on the position of the formyl functionality relative to the cycloheptatriene core. The [4 + 2] and [6 + 2] cycloadducts are formed via rare iminocatalytic inverse electron-demand cycloadditions, while the [4 + 6] cycloadduct is formed by a normal electron-demand cycloaddition. The reactivity displayed by the different formyl cycloheptatrienes was investigated by DFT calculations. These computational studies account for the different reaction paths for the three isomeric formyl cycloheptatrienes. The aminocatalytic [4 + 2], [6 + 2], and [4 + 6] cycloadditions proceed by stepwise processes, and the interplay between conjugation, substrate distortion, and dispersive interactions between the fulvene and aminocatalyst mainly defines the outcome of each cycloaddition.

Open shell versus closed shell bonding interaction in cyclopropane derivatives: EDA-NOCV analyses

Cyclopropane ring is a very common motif in organic/bio-organic compounds. The chemical bonding of this strained ring is taught to all chemistry students. This three-membered cyclic, C3 ring is quite reactive which has attracted both, synthetic and theoretical chemists to rationalize/correlate its stability and bonding with its reactivity and physical properties over a century. There are a few bonding models (mainly the Bent-Bond model and Walsh model) of this C3 ring that are debated to date. Herein, we have carried out energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) to study the two most reactive bonds of cyclopropane rings of 49 different organic compounds containing different functional groups to obtain a much deeper bonding insight toward a more general bonding model of this class of compounds. The EDA-NOCV analyses of fragment orbitals and susequent bond formation revealed that the nature of the CC bond of the cyclopropane (splitting two bonds at a time out of three CC bonds) ring is preferred to form two dative covalent CC bonds (between a singlet olefin-fragment and an excited singlet carbene-fragment with a vacant sp2 orbital and a filled p-orbital) for the majority (37/49) of compounds over two covalent electron sharing bonds in some (7/49) compounds (between an excited triplet olefin and triplet carbene), while a few (5/49) compounds show flexibility to adopt either the electron sharing or dative covalent bond as both are equally possible. The effects of functional groups on the nature of chemical bond in cyclopropane rings have been studied in detail. Our bonding analyses are in line with the QTAIM analyses which produce small negative values of the Laplacian, significantly positive values of bond ellipticity, and accumulation of electron densities around the ring critical point of C3 -rings. These corresponding QTAIM parameters of C3 -rings are quite different for CC single bonds of normal hydrocarbons as expected. The chemical bonding in the majority of cyclopropane rings can be very similar to those of metal-olefin systems.

Recent advances in transition-metal-catalyzed Büchner reaction of alkynes

Medium-sized ring-containing organic molecules, especially seven-membered rings, are significant structural motifs. However, such frameworks are considered difficult structures to access owing to entropic effects and transannular interactions. Compared to the construction of five and six-membered rings, the synthesis of seven-membered rings can be more challenging through traditional cyclization pathways. Büchner reactions are particularly attractive and efficient synthetic strategies to construct functionalized seven-membered ring products from the benzenoid double bond with carbene. In recent years, the field of transition-metal-catalyzed Büchner ring expansion reactions of alkynes has experienced a speedy development and a diverse array of efficient synthetic procedures have been disclosed under mild experimental conditions, as the synthesis of synthetically challenging seven-membered rings is easily achieved. In this review, we will focus on the recent progress in the transition-metal-catalyzed Büchner reaction of alkynes and the mechanistic rationale is depicted where possible, with the reactions being sorted according to the type of catalyst.

Asymmetric synthesis of complex tricyclo[3.2.2.0]nonenes from racemic norcaradienes: kinetic resolution via Diels-Alder reaction

Herein, the enantioselective synthesis of complex tricyclo[3.2.2.0]nonenes through the Diels-Alder reaction is reported. Utilizing racemic norcaradienes prepared from the visible-light-mediated dearomative cyclopropanation of m-xylene as dienes and enone derivatives as dienophiles, the overall process represents a kinetic asymmetric transformation in the presence of a chiral cobalt(ii) complex of chiral N,N'-dioxide. High diastereo- and enantioselectivity could be obtained in most cycloaddition processes and part racemization of norcaradiene is observed. The topographic steric maps of the catalysts were collected to rationalize the relationship between reactivity and enantioselectivity with the catalysts.

Asymmetric dearomative cyclopropanation of naphthalenes to construct polycyclic compounds

Catalytic asymmetric dearomatization (CADA) reactions is an important synthetic method for constructing enantioenriched complex cyclic systems from simple aromatic feedstocks. However, the CADA reactions of nonactivated arenes, such as naphthalenes and benzenes, have been far less explored than those of electronically activated arenes, such as phenols, naphthols and indoles. Herein, we disclose an asymmetric dearomative cyclopropanation of naphthalenes for the rapid construction of polycyclic compounds. With chiral dirhodium carboxylate as a catalyst, the dearomative cyclopropanation proceeded smoothly under mild conditions and afforded benzonorcaradiene-containing tetracycles in good yield and high enantioselectivity (up to 99% ee). Three stereogenic centers, including two all-carbon quaternary centers, were created in the dearomatization reaction. Moreover, a variety of functional groups are well-tolerated in the reaction. The products could be readily converted into other complex polycycles while maintaining the high ee value.

Dearomative Ring Expansion of Polycyclic Arenes

Benzocycloheptenes constitute a common structural motif embedded in many natural products and biologically active compounds. Herein, we report their concise preparation from non‐activated polycyclic arenes using a two‐step sequence involving dearomative [4+2]‐cycloaddition with arenophile in combination with palladium‐catalyzed cyclopropanation, followed by cycloreversion‐initiated ring expansion. The described strategy provides a working alternative to the Buchner reaction, which is limited to monocyclic arenes. Overall, this methylene‐insertion molecular editing approach enables rapid and direct conversion of simple (hetero)arenes into a range of substituted (aza)benzocycloheptatrienes, which can undergo a myriad of downstream functionalizations.

On-Surface Thermal Stability of a Graphenic Structure Incorporating a Tropone Moiety

On-surface synthesis, complementary to wet chemistry, has been demonstrated to be a valid approach for the synthesis of tailored graphenic nanostructures with atomic precision. Among the different existing strategies used to tune the optoelectronic and magnetic properties of these nanostructures, the introduction of non-hexagonal rings inducing out-of-plane distortions is a promising pathway that has been scarcely explored on surfaces. Here, we demonstrate that non-hexagonal rings, in the form of tropone (cycloheptatrienone) moieties, are thermally transformed into phenyl or cyclopentadienone moieties upon an unprecedented surface-mediated retro–Buchner-type reaction involving a decarbonylation or an intramolecular rearrangement of the CO unit, respectively.

Highly enantioselective synthesis of norcaradiene derivatives from naphthyl diazoacetamides using a Ru(ii)-Pheox complex

The highly regio- and enantioselective intramolecular cyclopropanation reactions of naphthyl diazoacetamides have been reported herein.

Exploiting Continuous Processing for Challenging Diazo Transfer and Telescoped Copper-Catalyzed Asymmetric Transformations

Generation and use of triflyl azide in flow enables efficient synthesis of a range of α-diazocarbonyl compounds, including α-diazoketones, α-diazoamides, and an α-diazosulfonyl ester, via both Regitz-type diazo transfer and deacylative/debenzoylative diazo-transfer processes with excellent yields and offers versatility in the solvent employed, in addition to addressing the hazards associated with handling of this highly reactive sulfonyl azide. Telescoping the generation of triflyl azide and diazo-transfer process with highly enantioselective copper-mediated intramolecular aromatic addition and C–H insertion processes demonstrates that the reaction stream containing the α-diazocarbonyl compound can be obtained in sufficient purity to pass directly over the immobilized copper bis(oxazoline) catalyst without detrimentally impacting the catalyst enantioselectivity.

Discovery of a photochemical cascade process by flow-based interception of isomerising alkenes

Herein we report the discovery of a new photochemical cascade process through a flow-based strategy for intercepting diradicals generated from simple alkenes. This continuous process delivers a series of unprecedented polycyclic reaction products. Exploring the scope of this novel process revealed that this approach is general and affords a variety of structurally complex reaction products in high yields (up to 81%), short reaction times (7 min) and high throughputs (up to 5.5 mmol h^-1). A mechanistic rationale is presented that is supported by computations as well as isolation of key intermediates whose identity is confirmed by X-ray crystallography. The presented photochemical cascade process demonstrates the discovery of new chemical reactivity and complex chemical scaffolds by continuously generating and intercepting high-energy intermediates in a highly practical manner.

Visible-Light-Mediated Cyclopropanation Reactions of 3-Diazooxindoles with Arenes

The cyclopropanation reaction of 3-diazooxindoles with arenes was first accomplished using visible-light irradiation. A series of spiro[norcaradiene-7,3'-indolin]-2'-ones were synthesized for the first time in high yields and with excellent diastereoselectivities. The synthetic usefulness of this catalyst-free photochemical methodology is illustrated by the further controllable rearrangement and epoxidation reactions.

Buchner Reaction/Azirine Modification Approach Toward Cycloheptatriene Containing Nitrogen Heterocyclic Scaffolds

The reaction conditions for the Buchner reaction of 2-(diazoacetyl)-2H-azirines with benzene leading to 2-(cyclohepta-2,4,6-trien-1-yl)-2H-azirines have been found. The azirine fragment of these compounds was subsequently used to prepare 5-(cyclohepta-2,4,6-trien-1-yl)isoxazoles and α-(cyclohepta-2,4,6-triene-1-ylcarbonyl)-1H-pyrroles. NMR and DFT calculations showed that the cycloheptatriene-norcaradiene valence isomerization in the corresponding moieties of synthesized azirines, isoxazoles, and pyrroles is very fast at room temperature. Cycloheptatriene valence isomer predominates in azirines and pyrroles, while 5-(cycloheptatrienyl)isoxazoles exist in a pure cycloheptatriene form at 25 °C in chloroform solution. In accordance with the X-ray analysis, two of the synthesized pyrroles crystallized as norcaradiene isomers from a solution of equilibrium mixture of valence isomers. Reductive isoxazole ring opening using the Mo(CO)₆/H₂O reductive system afforded (Z)-3-amino-3-aryl-1-(cyclohepta-2,4,6-trien-1-yl)prop-2-en-1-ones, which are pure cycloheptatriene isomers. α-(Cycloheptatrieneylcarbonyl)-1H-pyrroles were transformed into the α-(benzylcarbonyl)-1H-pyrroles under cooperative TFA/Rh₂(TFA)₄ catalysis.

Asymmetric Intramolecular Buchner Reaction: From High Stereoselectivity to Coexistence of Norcaradiene, Cycloheptatriene, and an Intermediate Form in the Solid State

Bicyclic compounds bearing a quaternary stereogenic center have been obtained using asymmetric intramolecular Buchner reaction with excellent yields and level of enantioselectivity. X-ray crystallography determination of the absolute configuration of one product has led to the serendipitous observation of an unusual behavior within the crystal structure, with equilibrating norcaradiene and cycloheptatriene valence isomers at the solid state, as well as an even more unexpected intermediate form. DFT calculations were performed to support these observations.

Tandem isonitrile insertion/azacyclopropylidene-annulated cyclohexenone–tropone rearrangement of p-QMs and TosMIC: de novo synthesis of pyrrolotropones with anti-cancer activity

TosMIC causes ring expansion of p-QMs: highly substituted pyrrolotropones are rapidly assembled in a domino process mediated by DBU. The utility of the method was highlighted by follow-up transformation and biological activity identification.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Rolf Huisgen's Classic Studies of Cyclic Triene Diels-Alder Reactions Elaborated by Modern Computational Analysis

Rolf Huisgen explored the Diels-Alder reactions of 1,3,5-cycloheptatriene (CHT) and cyclooctatetraene (COT) with the dienophiles maleic anhydride and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) to determine the kinetics and mechanisms of various electrocyclizations and Diels-Alder reactions. These reactions have been examined with density functional theory. Modern computational chemistry has provided information not previously available by experiment. Transition states for all the reactions have been identified, and their Gibbs energies are used to explain the experimental reactivities. Zwitterionic intermediates were not found in the [4+2] cycloadditions of both CHT or COT with PTAD and are thus not involved in these reactions. [2+2+2] cycloadditions, as an alternative path to the Diels-Alder products, are highly disfavored. Rapid double nitrogen inversion was found for the cycloaddition products with PTAD.

Catalytic, Asymmetric Dearomative Synthesis of Complex Cyclohexanes via a Highly Regio- and Stereoselective Arene Cyclopropanation Using α-Cyanodiazoacetates

Arene cyclopropanation offers a direct route to higher-order, non-aromatic carbocycles; however, the inherent issue of dictating site selectivity has cumbered the development of novel intermolecular reactions that directly engage the arene pool. This paper describes a highly regio- and stereoselective, Rh₂[(S)-PTTL]₄-catalyzed arene cyclopropanation using α-cyanodiazoacetates to afford stable norcaradienes bearing three stereogenic centers, one of which is an all-carbon quaternary center. The enantioenriched norcaradienes served as tunable templates for further transformation into stereochemically dense, fused and bicyclic carbocycles containing transmutable functionality.

The Thermal Rearrangement of an NHC-Ligated 3-Benzoborepin to an NHC-Boranorcaradiene

An N-heterocyclic-carbene-ligated 3-benzoborepin with a bridged structure has been synthesized by double radical trans-hydroboration of benzo[3,4]cycloundec-3-ene-1,5-diyne with an N-heterocyclic carbene borane. The thermal reaction of the NHC-ligated borepin at 150 °C gives an isolable NHC-boranorcaradiene. Experiments and density functional theory calculations support a mechanism whereby the borepin initially rearranges to a boranorcaradiene by a thermal 6π-electrocyclic reaction. This is followed by 1,5-boron shift to give a rearranged boranorcaradiene. This shift occurs with stereoinversion at boron through a transition state with open-shell diradical character. This is the first example of the isolation of a boranorcaradiene from a thermal reaction of a borepin.

Cyclopropane-alkene metathesis by gold(i)-catalyzed decarbenation of persistent cyclopropanes

A gold(i)-catalyzed cyclopropane-alkene metathesis has been demonstrated with two new families of cyclopropane derivatives of naphthalene and phenanthrene (benzo-fused norcaradienes). In this process, metal carbene units are transferred from a persistent cyclopropane to an alkene, upon release of naphthalene or phenanthrene, allowing the diastereoselective synthesis of a wide range of aryl and vinyl cyclopropanes.

Corey-Chaykovsky Cyclopropanation of Nitronaphthalenes: Access to Benzonorcaradienes and Related Systems

Nitronaphthalene derivatives react as Michael acceptors in the Corey-Chaykovsky reaction with alkyl phenyl selenones and alkyl diphenyl sulfonium salts. Mechanistic studies reveal that sterically demanding substituents at the carbanionic center favor formation of cyclopropanes and suppress competitive β-elimination to the alkylated products. The transformation, demonstrated also on heterocyclic nitroquinoline and nitroindazolines, is an example of transition metal-free dearomatization method.

Norcaradiene Synthesis via Visible-Light-Mediated Cyclopropanation Reactions of Arenes

Cyclopropanation reactions of carbenes with arenes provide a straightforward pathway to norcaradienes or cycloheptatrienes. This reaction normally requires harsh reaction conditions or transition-metal catalysts. In this report, we describe the metal-free visible-light photolysis of aryl diazoacetates in aromatic solvents, which provides access to the norcaradiene ring system in a highly regio- and stereoselective manner. The mild reaction conditions of this approach also allow chemoselective cyclopropanation of substituted arenes without competing C-H functionalization reactions.

Synthesis of 3-Substituted 3-Bromo-1-phenylallenes from Alkynylcycloheptatrienes

A new method has been developed for the preparation of 3-bromo-1-phenylallenes from 7-alkynylcycloheptatrienes and N-bromosuccinimide. Trisubstituted bromoallenes were obtained at room temperature in moderate to excellent yields. Functionalization of the carbon-bromine bond via Pd- or Cu-catalyzed cross-coupling reactions easily provided substituted allenes.

A Room-Temperature-Stable Phosphanorcaradiene

A room-temperature-stable crystalline phosphanorcaradiene 2 has been synthesized via a C-C bond forming strategy induced by the demetalation of a phosphepine-Au complex 1 using a Lewis base (Ph₃P, IDipp or CyNC). Compound 2 undergoes photolysis to afford a 2 H-phosphirene 4. Ru(PPh₃)₃Cl₂ is shown to catalyze the equilibration of 2 and 4 in solution. In addition, the transformation of 2 to 4 can be achieved by a two-step strategy, namely cyclopropenylidene-induced ring opening to give cyclopropenylidene-stabilized vinylphosphinidene 7 followed by elimination of cyclopropenylidene by an electrophile including TMSOTf and Me₂SBH₃.