Electrochemical Oxidative [4+2] Annulation for the π‐Extension of Unfunctionalized Heterobiaryl Compounds

Considering their unique electronic properties and diverse biological activities, regioselective access to fused aromatic compounds is significantly important in the field of organic materials and pharmaceutical science. Herein, we developed electrochemical oxidative [4+2] annulation reactions of heterobiaryl compounds with alkynes or alkenes, leading to the formation of several polycyclic heteroaromatic compounds. This electrosynthetic methodology serves for the straightforward π‐extension of unfunctionalized heterobiaryl compounds. The requirements of additional oxidants and prefunctionalization of starting materials are obviated. Through the in situ generation of heterobiaryl radical cation intermediates, various fused aromatic compounds were obtained with good yields and excellent regioselectivity.     

Rapid Access to Nanographenes and Fused Heteroaromatics by Palladium‐Catalyzed Annulative π‐Extension Reaction of Unfunctionalized Aromatics with Diiodobiaryls

Efficient and rapid access to nanographenes and π‐extended fused heteroaromatics is important in materials science. Herein, we report a palladium‐catalyzed efficient one‐step annulative π‐extension (APEX) reaction of polycyclic aromatic hydrocarbons (PAHs) and heteroaromatics, producing various π‐extended aromatics. In the presence of a cationic Pd complex, triflic acid, silver pivalate, and diiodobiaryls, diverse unfunctionalized PAHs and heteroaromatics were directly transformed into larger PAHs, nanographenes, and π‐extended fused heteroaromatics in a single step. In the reactions that afford [5]helicene substructures, simultaneous dehydrogenative ring closures occur at the fjord regions to form unprecedented larger nanographenes. This successive APEX reaction is notable as it stiches five aryl–aryl bonds by C−H functionalization in a single operation. Moreover, the unique molecular structures, crystal‐packing structures, photophysical properties, and frontier molecular orbitals of the thus‐formed nanographenes were elucidated.     

A Molecular Nanotube with Three‐Dimensional π‐Conjugation

A π‐conjugated twelve‐porphyrin tube is synthesized in 32 % yield by a template‐directed coupling reaction that joins together six porphyrin dimers, forming twelve new C? C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm³. Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six‐porphyrin ring, but are red‐shifted by approximately 300 cm^?1, reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited‐state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.     

Rhodium‐Catalyzed Cyclization of O,ω‐Unsaturated Alkoxyamines: Formation of Oxygen‐Containing Heterocycles

O,ω‐Unsaturated N‐tosyl alkoxyamines undergo unexpected Rh^III‐catalyzed intramolecular cyclization by oxyamination to produce oxygen‐containing heterocycles. Mechanistic studies show that an aziridine intermediate seems to be responsible for the formation of the heterocycles, possibly via a Rh^V species.     

Metal‐Triflate‐Catalyzed Synthesis of Polycyclic Tertiary Alcohols by Cyclization of γ‐Allenic Ketones

It has been established that bismuth(III) triflate catalyzes the cyclization of γ‐allenic ketones under mild reaction conditions. This reaction allows the selective formation of polycyclic tertiary alcohols from cyclic ketone derivatives. The resulting dienols can engage in stereoselective cycloadditions to efficiently afford complex polycyclic systems.     

Gold(I)‐Catalyzed Tandem Transformation with Diynes: Rapid Access to Linear Cyclopentenone‐Fused Polycyclic Molecules

An efficient and convenient synthesis of useful linear cyclopentenone‐fused polycyclic compounds has been achieved through a novel gold(I)‐catalyzed transformation of diynes. The method demonstrates high product yields and tolerates of a wide variety of important functional groups. Gold‐vinylidene formation, methoxy group migration, and Nazarov‐type cyclization are proposed to be the key steps in the reaction pathway. The synthetic utility of this method is demonstrated by converting the product to eight‐membered‐ring‐fused compound.     

Transition‐Metal‐Free Synthesis of N‐Hydroxy Oxindoles by an Aza‐Nazarov‐Type Reaction Involving Azaoxyallyl Cations

A novel transition‐metal‐free method to construct N‐hydroxy oxindoles by an aza‐Nazarov‐type reaction involving azaoxyallyl cation intermediates is described. A variety of functional groups were tolerated under the weak basic reaction conditions and at room temperature. A one‐pot process was also developed to make the reaction even more practical. This method provides alternative access to oxindoles and their biologically active derivatives.     

Synthesis,Properties, and Packing Structures of Corannulene‐Based π‐Systems Containing Heptagons

Introduction of heptagons into hexagonal carbon lattices can generate negatively curved polycyclic aromatic hydrocarbons, which are of significant interest in the field of exotic molecular nanocarbons. We have successfully synthesized and characterized corannulene‐based π‐systems containing heptagons ( 4 and 5 ) as new negatively curved polycyclic aromatic hydrocarbons as well as possible intermediates in the synthesis of warped nanographene 1 . The formation of 4 and 5 represents the first example for which a heptagon is formed under Scholl reaction conditions before all hexagons are formed. Even more interestingly, we discovered that the mode and degree of solid‐phase intermolecular π–π interaction can be altered significantly by the degree of ring closure.     

Concise Synthesis of 3D π‐Extended Polyphenylene Cylinders

The synthesis of structurally well‐defined, monodisperse carbon nanotube (CNT) sidewall segments poses a challenge in materials science. The synthesis of polyphenylene cylinders that comprise typical benzene connectivity to resemble precursors of [9,9] and [15,15] CNTs is now reported, and the products were characterized by X‐ray crystallography. To investigate the oxidative cyclodehydrogenation of ring‐strained molecules as a final step towards a bottom‐up synthesis of CNT sidewall segments, phenylene‐extended cyclic p‐hexaphenylbenzene trimers ([3]CHPB) were prepared, and NMR studies revealed a strain‐induced 1,2‐phenyl shift. It was further shown that an increase in ring size leads to selectively dehydrogenated macrocycles. Larger homologues are envisioned to give smooth condensation reactions toward graphenic sidewalls and should be used in the future as seeds for CNT formation.     

Rapid Access to 2‐Methylene Tetrahydrofurans and γ‐Lactones: A Tandem Four‐Step Process

A one‐pot tandem process involving hydrolysis, Knoevenagel condensation, Michael addition, and Conia‐ene (HKMC) reactions has been developed for the rapid synthesis of indanone‐fused 2‐methylene tetrahydrofurans from the reaction of enynals and propynols. In this process, two rings and four bonds are generated with 100 % atom‐economy and high step‐efficiency. The resulting tetrahydrofurans were readily oxidized into α‐methylene γ‐lactones, which are one of the most important substructures in natural and bioactive compounds.     

Copper‐Catalyzed Formation of α‐Alkoxycycloalkenones from N‐Tosylhydrazones

The combination of 20 mol % of copper iodide and lithium tert‐butoxide triggers the formation of a broad range of substituted, functionalized α‐alkoxy 2H‐naphthalenones from readily available N‐tosylhydrazones. The data suggests that this transformation occurs through cycloaddition of a copper carbenoid with an ester, followed by a Lewis acid‐catalyzed [1,2] alkyl shift of the in situ generated alkoxyepoxide intermediate.     

Selective Synthesis of Six Products from a Single Indolyl α‐Diazocarbonyl Precursor

Indolyl α‐diazocarbonyls can be selectively cyclized to give six distinct products through the careful choice of catalyst and reaction conditions. A range of catalysts were used, including complexes of Rh^II, Pd^II, and Cu^II, as well as SiO₂, to promote diazo decomposition and subsequent cyclization/rearrangement through a range of mechanistic pathways.     

Solubilization of Polycyclic Aromatics in Water by γ‐Cyclodextrin Derivatives

A series of hydrophilic per‐6‐thio‐6‐deoxy‐γ‐cyclodextrins (CDs) were synthesized from per‐6‐iodo‐6‐deoxy‐γ‐CD. These new hosts are able to solubilize polycyclic aromatic guests in aqueous solution to much higher extents than native CDs. Phase‐solubility diagrams were mostly linear in accordance with both 1:1 and 1:2 CD–guest complexes in aqueous solution. The stoichiometry of the inclusion complexes was further investigated by fluorescence spectroscopy, which revealed very pronounced Stokes shifts typical for 1:2 complexes. This finding was further consolidated by quantum mechanical calculations of dimer formation of the guests and space‐filling considerations by using the cross‐sectional areas of the CDs and guests. The calculated dimerization energies correlated well with the binding free energies measured for the 1:2 complexes, and provided the main contribution to the driving force of complexation in the γ‐CD cavity.     

Transition from π Radicals to σ Radicals: Substituent‐Tuned Cyclization of Hydrazonyl Radicals

Hydrazonyl radicals are known for their π‐electronic structures; however, their σ‐electronic structures have not been reported as yet. Herein, we show that readily accessible β,γ‐ and γ,δ‐unsaturated N‐trichloroacetyl and N‐trifluoroacetyl hydrazones can be conveniently converted into hydrazonyl σ radicals, which subsequently undergo 5‐exo‐trig radical cyclization at the N¹ or N² atom to form pyrazolines and azomethine imines, respectively.     

One‐Step Annulative π‐Extension of Alkynes with Dibenzosiloles or Dibenzogermoles by Palladium/o‐chloranil Catalysis

Reliable and short synthetic routes to polycyclic aromatic hydrocarbons and nanographenes are important in materials science. Herein, we report an efficient one‐step annulative π‐extension reaction of alkynes that provides access to diarylphenanthrenes and related nanographene precursors. In the presence of a cationic palladium/o ‐chloranil catalyst system and dibenzosiloles or dibenzogermoles as π‐extending agents, a variety of diarylacetylenes are transformed successfully into 9,10‐diarylphenanthrenes in a single step with good functional‐group tolerance. Furthermore, double π‐extension reactions of 1,4‐bis(phenylethynyl)benzene and diphenyl‐1,3‐butadiyne are demonstrated, affording oligoarylene products, which show potential for application in the synthesis of larger polycyclic aromatic hydrocarbons and nanographenes.