Synthesis of Nitrogen-Containing Polyaromatics by Aza-Annulative π-Extension of Unfunctionalized Aromatics

Palladium-Catalyzed Triple Suzuki-Miyaura Reactions Using Cyclic (Vinyl Triflate)iodonium Salts

By using cyclic (vinyl triflate)iodonium salts, a novel triple Suzuki-Miyaura reaction was accomplished for the synthesis of polyaromatic ethylene derivatives in the presence of palladium catalysts. The reaction exhibits extensive compatibility with a wide range of readily available arylboronic acids, giving triaryl-substituted ethylenes in good yields.

Synthesis of two nitrogen-containing polyaromatic compounds through gold catalysis/DBU-promoted cyclizations

This work reports an efficient synthesis of novel benzo[7,8]indolizino[2,3,4,5-ija]quinazoline derivatives between 2-(2-ethynylaryl)acetonitriles 1 and anthranils 2. The synthetic approach involves the initial formation of 7-formylindole intermediates that can be implemented by DBU to activate a novel indole-nitrile-aldehyde cyclization.

Luminescent Chiral Furanol-PAHs via Straightforward Ni-Catalysed Csp2 -F Functionalization: Mechanistic Insights into the Scholl Reaction

Here we report the stepwise synthesis of new nanographenes (NGs) and polycyclic aromatic hydrocarbons (PAHs) obtained via Scholl ring fusion applied at aromatic homologation compounds, which are obtained through one-step Ni-catalysed Csp2 -F functionalization. The latter are rapidly accessed valid precursors for the Scholl reaction, and screening of experimental conditions allowed us to describe for the first time furanol-bearing PAHs. Mechanistic insights are obtained by DFT to rationalize the formation of the furanol PAHs under moderately acidic conditions. All PAHs and NGs synthesized show moderate/weak fluorescent properties, and all PAHs crystallized show some degree of curvature and are obtained as racemic mixtures. Enantiomeric separation by chiral HPLC of one furanol-bearing PAH allowed the study of their chiroptical CD properties.

Photocatalytic C-N bond construction toward high-value nitrogenous chemicals

The construction of carbon-nitrogen bonds is vital for producing versatile nitrogenous compounds for the chemical and pharmaceutical industries. Among developed synthetic approaches to nitrogenous chemicals, photocatalysis is particularly prominent and has become one of the emerging fields due to its unique advantages of eco-sustainable characteristics, efficient process integration, no need for high-pressure H2, and tunable synthesis methods for developing advanced photocatalytic materials. Here, the review focuses on potential photocatalytic protocols developed for the construction of robust carbon-nitrogen bonds in discrepant activation environments to produce high-value nitrogenous chemicals. The photocatalytic C-N bond construction strategies and involved reaction mechanisms are elucidated.

Gold-Catalyzed 1,2-Aryl Shift and Double Alkyne Benzannulation

The tandem intramolecular hydroarylation of alkynes accompanied by a 1,2-aryl shift is described. Harnessing the unique electron-rich character of 1,4-dihydropyrrolo[3,2-b]pyrrole scaffold, we demonstrate that the hydroarylation of alkynes proceeds at the already occupied positions 2 and 5 leading to a 1,2-aryl shift. Remarkably, the reaction proceeds only in the presence of cationic gold catalyst, and it leads to heretofore unknown π-expanded, centrosymmetric pyrrolo[3,2-b]pyrroles. The utility is verified in the preparation of 13 products that bear six conjugated rings. The observed compatibility with various functional groups allows for increased tunability with regard to the photophysical properties as well as providing sites for further functionalization. Computational studies of the reaction mechanism revealed that the formation of the six-membered rings accompanied with a 1,2-aryl shift is both kinetically and thermodynamically favourable over plausible formation of products containing 7-membered rings. Steady-state UV/Visible spectroscopy reveals that upon photoexcitation, the prepared S-shaped N-doped nanographenes undergo mostly radiative relaxation leading to large fluorescence quantum yields. Their optical properties are rationalized through time-dependent density functional theory calculations. We anticipate that this chemistry will empower the creation of new materials with various functionalities.

Annulative π-Extension by Cp*Co(III)-Catalyzed Ketone-Directed peri-Annulation: An Approach to Access Fused Arenes

A masked-bay-region selective first-row transition-metal Cp*Co(III)-catalyzed annulative π-extension of arene-derived ketones is achieved to afford K-region-functionalized benzo[e]pyrenes, benzotetraphenes, and pyrenes. Comprehensive density functional theory studies buttress the mechanistic pathway comprising key steps like peri-C-H activation, alkyne 1,2-migratory insertion, and nucleophilic attack toward ketone, this attack being the rate-determining step. In addition, π-conjugated 1,1'-bipyrenes, potential photocatalyst pyrene-quinones, and putative n-type semiconductor cyano group-containing dibenzo[de,qr]tetracenes are also accessed.

Time-resolved imaging and analysis of the electron beam-induced formation of an open-cage metallo-azafullerene

The visualization of single-molecule reactions provides crucial insights into chemical processes, and the ability to do so has grown with the advances in high-resolution transmission electron microscopy. There is currently a limited mechanistic understanding of chemical reactions under the electron beam. However, such reactions may enable synthetic methodologies that cannot be accessed by traditional organic chemistry methods. Here we demonstrate the synthetic use of the electron beam, by in-depth single-molecule, atomic-resolution, time-resolved transmission electron microscopy studies, in inducing the formation of a doubly holed fullerene-porphyrin cage structure from a well-defined benzoporphyrin precursor deposited on graphene. Through real-time imaging, we analyse the hybrid's ability to host up to two Pb atoms, and subsequently probe the dynamics of the Pb-Pb binding motif in this exotic metallo-organic cage structure. Through simulation, we conclude that the secondary electrons, which accumulate in the periphery of the irradiated area, can also initiate chemical reactions. Consequently, designing advanced carbon nanostructures by electron-beam lithography will depend on the understanding and limitations of molecular radiation chemistry.

Lithium-Mediated Mechanochemical Cyclodehydrogenation

Cyclodehydrogenation is an essential synthetic method for the preparation of polycyclic aromatic hydrocarbons, polycyclic heteroaromatic compounds, and nanographenes. Among the many examples, anionic cyclodehydrogenation using potassium(0) has attracted synthetic chemists because of its irreplaceable reactivity and utility in obtaining rylene structures from binaphthyl derivatives. However, existing methods are difficult to use in terms of practicality, pyrophoricity, and lack of scalability and applicability. Herein, we report the development of a lithium(0)-mediated mechanochemical anionic cyclodehydrogenation reaction for the first time. This reaction could be easily performed using a conventional and easy-to-handle lithium(0) wire at room temperature, even under air, and the reaction of 1,1'-binaphthyl is complete within 30 min to afford perylene in 94% yield. Using this novel and user-friendly protocol, we investigated substrate scope, reaction mechanism, and gram-scale synthesis. As a result, remarkable applicability and practicality over previous methods, as well as limitations, were comprehensively studied by computational studies and nuclear magnetic resonance analysis. Furthermore, we demonstrated two-, three-, and five-fold cyclodehydrogenations for the synthesis of novel nanographenes. In particular, quinterrylene ([5]rylene or pentarylene), the longest nonsubstituted molecular rylene, was synthesized for the first time.

Electronic Control of the Scholl Reaction: Selective Synthesis of Spiro vs Helical Nanographenes

Scholl oxidation has become an essential reaction in the bottom-up synthesis of molecular nanographenes. Herein, we describe a Scholl reaction controlled by the electronic effects on the starting substrate (1 a, b). Anthracene-based polyphenylenes lead to spironanographenes under Scholl conditions. In contrast, an electron-deficient anthracene substrate affords a helically arranged molecular nanographene formed by two orthogonal dibenzo[fg,ij]phenanthro-[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties linked through an octafluoroanthracene core. Density Functional Theory (DFT) calculations predict that electronic effects control either the first formation of spirocycles and subsequent Scholl reaction to form spironanographene 2, or the expected dehydrogenation reaction leading solely to the helical nanographene 3. The crystal structures of four of the new spiro compounds (syn 2, syn 9, anti 9 and syn 10) were solved by single crystal X-ray diffraction. The photophysical properties of the new molecular nanographene 3 reveal a remarkable dual fluorescent emission.

Rapid access to polycyclic thiopyrylium compounds from unfunctionalized aromatics by thia-APEX reaction

We developed a sulfur-embedding annulative π-extension (thia-APEX) reaction that could construct a sulfur-embedding cationic hexagonal aromatic ring, thiopyrylium, onto unfunctionalized aromatics in one step. The key of thia-APEX is the use of S-imidated ortho-arenoyl arenethiols, and a variety of π-extended thiopyryliums can easily be synthesized. The synthesized thiopyryliums showed diverse absorption and emission properties over the visible light to NIR region, depending on minor structural differences.

One-pot synthesis of 2-arylated and 2-alkylated benzoxazoles and benzimidazoles based on triphenylbismuth dichloride-promoted desulfurization of thioamides

The development of novel and efficient synthesis methods for 2-substituted benzazole derivatives is of interest as they are biologically active substances. Herein, a simple method for the synthesis of 2-aryl- and 2-alkyl-substituted benzazoles is described. The reaction of 2-aminophenols with thioamides at 60 °C in the presence of triphenylbismuth dichloride in 1,2-dichloroethane as a promoter afforded various 2-aryl- and 2-alkylbenzoxazoles in moderate to excellent yields under mild reaction conditions. This method could also be applied to the synthesis of benzimidazoles and benzothiazoles. This study presents the first use of triphenylbismuth dichloride to produce benzimidoyl chloride from thioamides by desulfurization and chlorination, as well as its application to the synthesis of 2-substituted benzazoles.

Two-Fold Intramolecular Phosphacyclization: From Fluorescent Diphosphapyrene Salts to Pentavalent Derivatives

The synthesis of π-extended pyrene-based luminescent compounds containing two six-membered phosphacycles has been realized through a two-step synthesis. It involves a Cu(II)-mediated double cyclization of tertiary diphosphane derivatives to afford dicationic molecules with quaternized phosphorus centers. Subsequent transformation of diphosphonium species into the corresponding P-oxide derivatives has been successfully achieved through Pd(0)-assisted cleavage of the P-Ph bonds, which opens a promising way for the functionalization of polyaromatic P-systems.

Stereoselective Domino Rearrangement peri-Annulation of Cinchona Alkaloid Derivatives with 8-Bromo-1-naphthyl Grignard

The unexpected domino coupling and rearrangement of the Cinchona alkaloid skeleton has been found to occur in the reaction of 9-chloro-9-deoxy-alkaloids with Grignards from peri-dihalogenonaphthalene. The cyclization and migration of the central quinuclidinylmethyl group (C9) from position C-4′ to position C-3′ of quinoline formed a novel chiral ring system of 5-aza-7H-benzo[no]tetraphene, yielding products of unlike configuration. The proposed reaction pathway involves radical intermediates.

1-Substituted Perylene Derivatives by Anionic Cyclodehydrogenation: Analysis of the Reaction Mechanism

Perylene derivatives constitute a promising class of compounds with technological applications mainly due to their optoelectronic properties. One mechanism proposed to synthesize them, starting from binaphthyl derivatives, is anionic cyclodehydrogenation (under reductive conditions). However, the scope of this reaction is limited. In the present study, we report a theoretical and experimental analysis of this particular reaction mechanism for its use in the synthesis of 1-substituted perylenes. Different substituents at position 2 of 1,1'-binaphthalene were evaluated: -OCH₃, -OSi(CH₃)₂C(CH₃)₃, and -N(CH₃)₂. Based on density functional theory (DFT) calculations on the proposed mechanism, we suggest that the cyclization takes place from binaphthyl dianion instead of its radical anion. This dianion has an open-shell diradical nature, and this could be the species that was detected by EPR in previous studies. The O-substituted derivatives could not afford the perylene derivatives since their radical anions fragment and the necessary binaphthyl dianion could not be formed. On the other hand, 49% of N,N-dimethylperylen-1-amine was obtained starting from the N-substituted 2-binapthyl derivative as a substrate, employing a simpler experimental methodology.

Rapid access to polycyclic N-heteroarenes from unactivated, simple azines via a base-promoted Minisci-type annulation

Conventional synthetic methods to yield polycyclic heteroarenes have largely relied on metal-mediated arylation reactions requiring pre-functionalised substrates. However, the functionalisation of unactivated azines has been restricted because of their intrinsic low reactivity. Herein, we report a transition-metal-free, radical relay π-extension approach to produce N-doped polycyclic aromatic compounds directly from simple azines and cyclic iodonium salts. Mechanistic and electron paramagnetic resonance studies provide evidence for the in situ generation of organic electron donors, while chemical trapping and electrochemical experiments implicate an iodanyl radical intermediate serving as a formal biaryl radical equivalent. This intermediate, formed by one-electron reduction of the cyclic iodonium salt, acts as the key intermediate driving the Minisci-type arylation reaction. The synthetic utility of this radical-based annulative π-extension method is highlighted by the preparation of an N-doped heptacyclic nanographene fragment through fourfold C–H arylation.

The functionalisation of unactivated azines has been restricted because of their intrinsic low reactivity. Here the authors show a transition-metal-free, radical relay π-extension approach to produce N-doped polycyclic aromatic compounds directly from simple azines and cyclic iodonium salts.

Computational, Mechanistic, and Experimental Insights into Regioselective Catalytic C-C Bond Activation in Linear 1-Aza-[3]triphenylene

C-C bond activation by transition metal complexes in ring-strained compounds followed by annulation with unsaturated compounds is an efficient approach to generate structurally more complex compounds. However, the site of catalytic C-C bond activation is difficult to predict in unsymmetrically substituted polycyclic systems. Here, we report a study on the (regio)selective catalytic cleavage of selected C-C bonds in 1-aza-[3]triphenylene, followed by annulation with alkynes, forming products with extended π-conjugated frameworks. Based on density functional theory (DFT) calculations, we established the stability of possible transition metal intermediates formed by oxidative addition to the C-C bond and thus identified the likely site of C-C bond activation. The computationally predicted selectivity was confirmed by the following experimental tests for the corresponding Ir-catalyzed C-C cleavage reaction followed by an alkyne insertion that yielded mixtures of two mono-insertion products isolated with yields of 34-36%, due to the close reactivity of two bonds during the first C-C bond activation. Similar results were obtained for twofold Ir- or Rh-catalyzed insertion reactions, with higher yields of 72-77%. In a broader context, by combining DFT calculations, which provided insights into the relative reactivity of individual C-C bonds, with experimental results, our approach allows us to synthesize previously unknown pentacyclic azaaromatic compounds.

Paired electrolysis enabled annulation for the quinolyl-modification of bioactive molecules

A paired electrolysis enabled cascade annulation that enables the efficient synthesis of highly functionalized quinoline-substituted bioactive molecules from readily available starting materials is reported. Using this methodology, two goals, namely, the direct synthesis of quinolines and the introduction of quinoline moieties to bioactive molecules, can be simultaneously achieved in one simple operation. The use of electroreduction for the activation of isatin, together with the further anodic oxidation of KI to catalytically result in a cascade annulation, highlight the unique possibilities associated with electrochemical activation methods. This transformation can tolerate a wide range of functional groups and can also be used as a functionalization tactic in pharmaceutical research as well as other areas.

A paired electrolysis enabled cascade annulation that enables the efficient synthesis of highly functionalized quinoline-substituted bioactive molecules from readily available starting materials is reported.

Synthesis of Distorted Nitrogen-Doped Nanographenes by Partially Oxidative Cyclodehydrogenation Reaction

A series of partially fused N-doped nanographenes are synthesized via the oxidative cyclodehydrogenation of oligoaryl-substituted dibenzo[ e,l ]pyrene ( 1 ), including compounds ( 2 - 4 ) with five, six, and seven new C-C bonds are formed, respectively, implying stepwise C-C bond fusion and extended π-conjugation. Single-crystal X-ray diffraction analysis of compound 4a revealed that the presence of sterically demanding groups hindered the formation of planar and fully fused nanographene in the oxidative cyclodehydrogenation reaction step. Optical study of compounds 2 to 4 showed that extended π-conjugation leads to a regular stepwise bathochromic shift in the absorption and emission spectra. Furthermore, the HOMO-LUMO gaps of these compounds exhibit a decrease as C-C bond formation proceeds. Thus, the optoelectronic properties of nanographenes are highly dependent on the formation of new C-C bonds in the molecular skeleton.

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

This review surveys recent progress in the chemistry of polycyclic heteroaromatic molecules with a focus on structural diversity and synthetic methodology. The article covers literature published during the period of 2016-2020, providing an update to our first review of this topic (Chem. Rev. 2017, 117 (4), 3479-3716).

1,3-Dipolar cycloaddition of azomethine ylides and imidoyl halides for synthesis of π-extended imidazolium salts

A new synthetic approach to π-extended imidazolium salts is developed based on 1,3-dipolar cycloaddition of polycyclic aromatic azomethine ylides with imidoyl chlorides in the presence of cesium fluoride as a key additive.

Infinitene: A Helically Twisted Figure-Eight [12]Circulene Topoisomer

New forms of molecular nanocarbon particularly looped polyarenes adopting various topologies contribute to the fundamental science and practical applications. Here we report the synthesis of an infinity-shaped polyarene, infinitene (1) (cyclo[c.c.c.c.c.c.e.e.e.e.e.e]dodecakisbenzene), comprising consecutively fused 12-benzene rings forming an enclosed loop with a strain energy of 60.2 kcal·mol^-1. Infinitene (1) represents a topoisomer of still-hypothetical [12]circulene, and its scaffold can be formally visualized as the outcome of the "stitching" of two homochiral [6]helicene subunits by both their ends. The synthetic strategy encompasses transformation of a rationally designed dithiacyclophane to cyclophadiene through the Stevens rearrangement and pyrolysis of the corresponding S,S'-bis(oxide) followed by the photocyclization. The structure of 1 is a unique hybrid of helicene and circulene with a molecular formula of C₄₈H₂₄, which can be regarded as an isomer for kekulene, [6,6]carbon nanobelt ([6,6]CNB), and [12]cyclacene. Infinitene (1) is a bench-stable yellow solid with green fluorescence and soluble to common organic solvents. Its figure-eight molecular structure was unambiguously confirmed by X-ray crystallography. The scaffold of 1 is significantly compressed as manifested by a remarkably shortened distance (3.152-3.192 Å) between the centroids of two π-π stacked central benzene rings and the closest C···C distance of 2.920 Å. Fundamental photophysical properties of 1 were thoroughly elucidated by UV-vis absorption and fluorescence spectroscopic studies and density functional theory calculations. Its configurational stability enabled separation of the corresponding enantiomers (P,P) and (M,M) by a chiral HPLC. Circular dichroism (CD) and circularly polarized luminescence (CPL) measurements revealed that 1 has moderate |g_CD| and |g_CPL| values.

Diazapentabenzocorannulenium: A Hydrophilic/Biophilic Cationic Buckybowl

Polycyclic aromatic molecules are promising functional materials for a wide range of applications, especially in organic electronics. However, their largely hydrophobic nature has impeded further applications. As such, imparting high solubility/hydrophilicity to polycyclic aromatic molecules leads to a breakthrough in this research field. Herein, we report the synthesis of diazapentabenzocorannulenium, a cationic nitrogen-embedded buckybowl bearing a central imidazolium core, by a bottom-up strategy from polycyclic aromatic azomethine ylide. X-ray crystallography analyses have revealed a bowl-shaped molecular structure that is capable of forming charge-segregated one-dimensional columns by bowl-in-bowl packing. In addition to its fluorescence capabilities and high dispersibility in water, the molecule was found to selectively localize in the mitochondria of various tumor cells, showing potential as viable mitochondria-selective fluorescent probes.

Novel Synthetic Approach to Heteroatom Doped Polycyclic Aromatic Hydrocarbons: Optimizing the Bottom-Up Approach to Atomically Precise Doped Nanographenes

The success of the rational bottom-up approach to nanostructured carbon materials and the discovery of the importance of their doping with heteroatoms puts under the spotlight all synthetic organic approaches to polycyclic aromatic hydrocarbons. The construction of atomically precise heteroatom doped nanographenes has evidenced the importance of controlling its geometry and the position of the doping heteroatoms, since these parameters influence their chemical-physical properties and their applications. The growing interest towards this research topic is testified by the large number of works published in this area, which have transformed a once "fundamental research" into applied research at the cutting edge of technology. This review analyzes the most recent synthetic approaches to this class of compounds.

Annulative π-Extension (APEX) of Indoles to Pyrido[1,2-a]indoles Using 4-Oxo Peroxides as C4 Units

Annulative π-extension (APEX) of 3-substituted indoles to pyrido[1,2-a]indoles is developed by using 4-oxo peroxides as π-extending reagents, which are employed as versatile C4 building blocks. This transformation is initiated by Brønsted acid-mediated Hock rearrangement of the peroxyl group. Notably, the pyrido[1,2-a]indole products are obtained by elimination of the indole moiety from the corresponding dihydropyrido[1,2-a]indoles, which could be selectively formed at room temperature.

A Highly Luminescent Nitrogen-Doped Nanographene as an Acid- and Metal-Sensitive Fluorophore for Optical Imaging

Dibenzo[hi,st]ovalene (DBOV) has excellent photophysical properties, including strong fluorescence and high ambient stability. Moreover, the optical blinking properties of DBOV have enabled optical super-resolution single-molecule localization microscopy with an imaging resolution beyond the diffraction limit. Various organic and inorganic fluorescent probes have been developed for super-resolution imaging, but those sensitive to pH and/or metal ions have remained elusive. Here, we report a diaza-derivative of DBOV (N-DBOV), synthesized in eight steps with a total yield of 15%. Nitrogen (N)-bearing zigzag edges were formed through oxidative cyclization of amino groups in the last step. UV–vis and fluorescence spectroscopy of N-DBOV revealed its promising optical properties comparable to those of the parent DBOV, while cyclic voltammetry and density functional theory calculations highlighted its lower orbital energy levels and potential n-type semiconductor character. Notably, in contrast to that of the parent DBOV, the strong luminescence of N-DBOV is dependent on pH and the presence of heavy metal ions, indicating the potential of N-DBOV in sensing applications. N-DBOV also exhibited pH-responsive blinking, which enables pH-sensitive super-resolution imaging. Therefore, N-DBOV appears to be a highly promising candidate for fluorescence sensing in biology and environmental analytics.

One-Pot Synthesis of Triazatriphenylene Using the Povarov Reaction

The Povarov reaction combines aromatic amines, aldehydes, and alkynes in a single step and is regarded as an annulative π-extension reaction of aromatic amines. In this study, the Povarov reaction was investigated as an efficient tool for the synthesis of aza-polycyclic aromatic hydrocarbons via multiple π-extensions. The double Povarov reaction of 1,4-diaminobenzene yielded the 4,7-phenanthroline derivative as the major product, regardless of the steric repulsion in the product. The site selectivity mainly depended on the HOMO distribution of the intermediate rather than the steric factor. Based on these insights, a 1,5,9-triazatriphenylene derivative was synthesized via a triple Povarov reaction. The structures of the synthesized compounds were unambiguously determined by single-crystal X-ray diffraction analysis. The triazatriphenylene derivative formed a smooth and stable thin film upon vacuum vapor deposition and served as a hole-blocking material in organic light-emitting diodes.

Synthesis, Structure, and Chiroptical Properties of Indolo- and Pyridopyrrolo-Carbazole-Based C2 -Symmetric Azahelicenes

Treatment of 11,12-bis(1,1'-biphenyl-3-yl or 6-phenylpyridin-2-yl)-substituted 11,12-dihydro-indolo[2,3-a]carbazole with an oxidizing system of Pd(II)/Ag(I) induced effective double dehydrogenative cyclization to afford the corresponding π-extended azahelicenes. The optical resolutions were readily achieved by a preparative chiral HPLC. It was found that the pyridopyrrolo-carbazole-based azahelicene that contains four nitrogen atoms exhibits ca. 6 times larger dissymmetry factors both in circularly dichroism (CD) and circularly polarized luminescence (CPL), |g_CD | and |g_CPL | values being 1.1×10^-2 and 4.4×10^-3 , respectively, as compared with the parent indolocarbazole-based azahelicene. Theoretical calculations at the RI-CC2 level were employed to rationalize the observed enhanced chiroptical responses. The (chir)optical properties of the former helicene was further tuned by a protonation leading to remarkable red-shift with a considerable enhancement of the |g_CPL | value.

Robust Cobalt Catalyst for Nitrile/Alkyne [2+2+2] Cycloaddition: Synthesis of Polyarylpyridines and Their Mechanochemical Cyclodehydrogenation to Nitrogen-Containing Polyaromatics*

The transition-metal-catalyzed [2+2+2] cycloaddition of nitriles and alkynes is an established synthetic approach to pyridines; however, these cycloadditions often rely on the use of tethered diynes or cyanoalkynes as one of the reactants. Thus, examples of efficient, fully intermolecular catalytic [2+2+2] pyridine synthesis, especially those employing unactivated nitriles and internal alkynes leading to pentasubstituted pyridines, remain scarce. Herein, we report on simple and inexpensive catalytic systems based on cobalt(II) iodide, 1,3-bis(diphenylphosphino)propane, and Zn that promote [2+2+2] cycloaddition of various nitriles and diarylacetylenes for the synthesis of a broad range of polyarylated pyridines. DFT studies support a reaction pathway involving oxidative coupling of two alkynes, insertion of the nitrile into a cobaltacyclopentadiene, and C-N reductive elimination. The resulting tetra- and pentaarylpyridines serve as precursors to hitherto unprecedented nitrogen-containing polycyclic aromatic hydrocarbons via mechanochemically assisted multifold reductive cyclodehydrogenation.

Homologation of the Fischer Indolization: A Quinoline Synthesis via Homo-Diaza-Cope Rearrangement

We disclose a new Brønsted acid promoted quinoline synthesis, proceeding via homo-diaza-Cope rearrangement of N-aryl-N'-cyclopropyl hydrazines. Our strategy can be considered a homologation of Fischer's classical indole synthesis and delivers 6-membered N-heterocycles, including previously inaccessible pyridine derivatives. This approach can also be used as a pyridannulation methodology toward constructing polycyclic polyheteroaromatics. A computational analysis has been employed to probe plausible activation modes and to interrogate the role of the catalyst.

Method for the Large-Scale Synthesis of Multifunctional 1,4-Dihydro-pyrrolo[3,2-b]pyrroles

A thorough investigation has enabled the optimization of the synthesis of 1,4-dihydro-pyrrolo[3,2-b]pyrroles. Although salts of such metals as vanadium, niobium, cerium, and manganese were found to facilitate the formation of 1,4-dihydro-pyrrolo[3,2-b]pyrroles from amines, aldehydes, and diacetyl, we confirmed that iron salts are the most efficient catalysts. The conditions identified (first step: toluene/AcOH = 1:1, 1 h, 50 °C; second step: toluene/AcOH = 1:1, Fe(ClO₄)₃·H₂O, 16 h, 50 °C) resulted in the formation of tetraarylpyrrolo[3,2-b]pyrroles in a 6–69% yield. For the first time, very electron-rich substituents (4-Me₂NC₆H₄, 3-(OH)C₆H₄, pyrrol-2-yl) originating from aldehydes and sterically hindered substituents (2-ClC₆H₄, 2-BrC₆H₄, 2-CNC₆H₄, 2-(CO₂Me)C₆H₄, 2-(TMS-C≡C)C₆H₄) present on anilines can be appended to the pyrrolo[3,2-b]pyrrole core. It is now also possible to prepare 1,4-dihydropyrrolo[3,2-b]pyrroles bearing an ordered arrangement of N-substituents and C-substituents ranging from coumarin, quinoline, phthalimide to truxene. These advances in scope enable independent regulations of many desired photophysical properties, including the Stokes shift value and emission color ranging from violet-blue through deep blue, green, yellow to red. Simultaneously, the optimized conditions have finally allowed the synthesis of these extremely promising heterocycles in amounts of more than 10 g per run without a concomitant decrease in yield or product contamination. Empowered with better functional group compatibility, novel derivatization strategies were developed.

Copper-Catalyzed N,N-Diarylation of Amides for the Construction of 9,10-Dihydroacridine Structure and Applications in the Synthesis of Diverse Nitrogen-Embedded Polyacenes

We reported herein CuI/DMEDA catalyzed N,N-diarylation reaction of amides with various di(o-bromoaryl)methanes to produce diverse 9,10-dihydroacridine derivatives. The resulting 9,10-dihydroacridine derivatives were oxidized selectively under mild conditions to afford acridine, acridinone, and acridinium derivatives. The copper-catalyzed N,N-diarylation reaction coupled with oxidative aromatization reaction enabled the facile construction of nitrogen atom-embedded tetracenes and pentacenes of different ortho-fused patterns. The luminescence properties, especially the effect of fusion pattern on fluorescence emission of acquired N-polycenes, were also demonstrated.

How To Make Nitroaromatic Compounds Glow: Next-Generation Large X-Shaped, Centrosymmetric Diketopyrrolopyrroles

Red‐emissive π‐expanded diketopyrrolopyrroles (DPPs) with fluorescence reaching λ=750 nm can be easily synthesized by a three‐step strategy involving the preparation of diketopyrrolopyrrole followed by N‐arylation and subsequent intramolecular palladium‐catalyzed direct arylation. Comprehensive spectroscopic assays combined with first‐principles calculations corroborated that both N‐arylated and fused DPPs reach a locally excited (S₁) state after excitation, followed by internal conversion to states with solvent and structural relaxation, before eventually undergoing intersystem crossing. Only the structurally relaxed state is fluorescent, with lifetimes in the range of several nanoseconds and tens of picoseconds in nonpolar and polar solvents, respectively. The lifetimes correlate with the fluorescence quantum yields, which range from 6 % to 88 % in nonpolar solvents and from 0.4 % and 3.2 % in polar solvents. A very inefficient (T₁) population is responsible for fluorescence quantum yields as high as 88 % for the fully fused DPP in polar solvents.