Gold-Catalyzed Facile Synthesis and Crystal Structures of Benzene-/Naphthalene-Based Bispentalenes as Organic Semiconductors

The gold-catalyzed facile synthesis of U-shaped and S-shaped bispentalenes is described from easily available tetra(arylethynyl)-benzenes and -naphthalenes. The optoelectronic and transistor properties were also investigated. The selectivity between the U-shaped and S-shaped bispentalene isomers can be tuned by the bulkiness of the ligand and the substrates. The S-shaped naphthalene-based bispentalene shows a one-dimensional face-to-face packing pattern in solid state and a good hole mobility, indicating that the S-shaped bispentalene core is highly suitable for transistor applications. The gold-catalyzed annulation of tetraynes provides a useful protocol in the synthesis of bispentalenes for organic semiconductors.     

Synthesis of Multiply Substituted Polycyclic Aromatic Hydrocarbons by Iridium‐Catalyzed Annulation of Ring‐Fused Benzocyclobutenol with Alkyne through C−C Bond Cleavage

The first iridium‐catalyzed intermolecular cyclization between alkynes and ring‐fused benzocyclobutenols (RBCB) through C?C bond cleavage is described. A variety of elusive polycyclic aromatic hydrocarbons (PAHs) with multiple substituents are obtained in good yields under mild conditions. This procedure provides a unique and expeditious tool for the synthesis of PAHs.     

Gold-Catalyzed Annulation of 1,8-Dialkynylnaphthalenes: Synthesis and Photoelectric Properties of Indenophenalene-Based Derivatives

A simple gold-catalyzed annulation of 1,8-dialkynylnaphthalenes utilizing a cationic gold catalyst was developed. Such a peri-position of two alkynyl substituents has not been studied in gold catalysis before. Dependent on the substrate, the reactions either follow a mechanism involving vinyl cation intermediates or involve a dual gold catalysis mechanism which in an initial 6-endo-dig-cyclization generates gold(I) vinylidene intermediates that are able to insert into C−H bonds. Indenophenalene derivatives were obtained in moderate to high yields. In addition, the bidirectional gold-catalyzed annulation of tetraynes provided even larger conjugated π-systems. The optoelectronic properties of the products were also investigated.     

Gold-Catalyzed One-Step Construction of 2,3-Dihydro-1H-Pyrrolizines with an Electron-Withdrawing group in the 5-position: A Formal Synthesis of 7-Methoxymitosene

What a ring formation! Bicyclic dihydropyrrolizines with an electron-withdrawing group (EWG) at the 5-position are formed in one step from linear azidoenynes under gold catalysis. This novel route involves the use of azide as a nitrene precursor, electronically-controlled regioselectivity, and the generation of destabilized 1-azapentadienium ions and their pericyclic reactions. This method was used for a formal synthesis of 7-methoxymitosene.     

Selective Oxidation of Alkyl‐Substituted Polyaromatics Using Ruthenium‐Ion‐Catalyzed Oxidation

Ruthenium‐ion‐catalyzed oxidation of a range of alkylated polyaromatics has been studied. 2‐Ethylnaphthalene was used as a model substrate, and oxidation can be performed in either a conventional biphasic or in a monophasic solvent system. In either case the reaction rates and product selectivity are identical. The reaction products indicate that the aromatic ring system is oxidized in preference to the alkyl chain. This analysis is possible due to the development of a quantitative NMR protocol to determine the relative amounts of aliphatic and aromatic protons. From a systematic set of substrates we show that as the length of the alkyl chain substituent on a polyaromatic increases, the proportion of products in which the chain remains attached to the aromatic system increases. Larger polyaromatic systems, based on pyrene and phenanthrene, show greater reactivity than those with fewer aromatic rings, and the alkyl chains are more stable to oxidation.     

Gold(III) Chloride Catalyzed Synthesis of Chiral Substituted 3‐Formyl Furans from Carbohydrates: Application in the Synthesis of 1,5‐Dicarbonyl Derivatives and Furo[3,2‐c]pyridine

This report describes a gold(III)‐catalyzed efficient general route to densely substituted chiral 3‐formyl furans under extremely mild conditions from suitably protected 5‐(1‐alkynyl)‐2,3‐dihydropyran‐4‐one using H₂O as a nucleophile. The reaction proceeds through the initial formation of an activated alkyne–gold(III) complex intermediate, followed by either a domino nucleophilic attack/anti‐endo‐dig cyclization, or the formation of a cyclic oxonium ion with subsequent attack by H₂O. To confirm the proposed mechanistic pathway, we employed MeOH as a nucleophile instead of H₂O to result in a substituted furo[3,2‐c]pyran derivative, as anticipated. The similar furo[3,2‐c]pyran skeleton with a hybrid carbohydrate–furan derivative has also been achieved through pyridinium dichromate (PDC) oxidation of a substituted chiral 3‐formyl furan. The corresponding protected 5‐(1‐alkynyl)‐2,3‐dihydropyran‐4‐one can be synthesized from the monosaccharides (both hexoses and pentose) following oxidation, iodination, and Sonogashira coupling sequences. Furthermore, to demonstrate the potentiality of chiral 3‐formyl furan derivatives, a TiBr₄‐catalyzed reaction of these derivatives has been shown to offer efficient access to 1,5‐dicarbonyl compounds, which on treatment with NH₄OAc in slightly acidic conditions afforded substituted furo[3,2‐c]pyridine.     

Recent Progress towards Transition‐Metal‐Catalyzed Synthesis of Fluorenes

Fluorenes are a commonly encountered structural motif in materials science, pharmaceutical chemistry, and organic synthesis. Among various strategies towards the synthesis of this unique structure, transition metal‐catalyzed functionalization has emerged as one of the most efficient methods. This Minireview presents an overview of the recent advances in this emerging area by highlighting the reactions’ specificity and applicability and, where possible, provides a mechanistic rationale.     

Gold‐Catalyzed Regiospecific C−H Annulation of o‐Ethynylbiaryls with Anthranils: π‐Extension by Ring‐Expansion En Route to N‐Doped PAHs

We describe a novel, short, and flexible approach to diverse N‐doped polycyclic aromatic hydrocarbons (PAHs) through gold‐catalyzed π‐extension of anthranils with o‐ethynylbiaryls as reagents. This strategy uses easily accessible starting materials, is simple due to high step and atom economy, and shows good functional‐group compatibility as well as scale‐up potential. Mechanistically, the tandem reaction is proposed to involve a nucleophilic addition/ring opening/regiospecific C?H annulation/protodeauration sequence terminated by a Friedel–Crafts‐type cyclization. Photophysical studies of the products indicated violet‐blue fluorescence emission with quantum yields up to 0.45.     

Synthesis and Structures of Polyphenylphenanthrenes

1,2,3,4,5,6,7,8-Octaphenylphenanthrene ( 4 ) and decaphenylphenanthrene ( 5 ) were prepared by very short syntheses (two or three steps) from tetraphenylfuran and polybrominated benzene derivatives. The X-ray structures of compounds 4 and 5 show them to be quite crowded, with the phenanthrene cores twisted by about 40° due to the clash of the C4 and C5 phenyl groups. Compound 4 was resolved by chromatography on a chiral support, and its free energy of activation for racemization was determined to be 24.6 kcal mol⁻¹ at 40 °C. Computational studies indicate that compound 5 has a racemization barrier approximately 6 kcal mol⁻¹ lower than 4 , and thus 5 would not be configurationally stable at room temperature.     

The Precise Synthesis of Phenylene‐Extended Cyclic Hexa‐peri‐hexabenzocoronenes from Polyarylated [n]Cycloparaphenylenes by the Scholl Reaction

The longitudinal extension of cycloparaphenylenes (CPP) towards ultrashort carbon nanotubes (CNTs) is essential for the solution based bottom‐up synthesis of CNTs. Herein, the longitudinal extension of the CPP skeleton by the introduction of hexaphenylbenzene units towards polyarylated [n]CPPs is described. Further, the applicability of the Scholl reaction to selectively form graphenic sidewalls is demonstrated. The ring size and substitution patterns of the polyarylated [n]CPPs were varied to overcome strain‐induced side reactions during the oxidative cyclodehydrogenation and cyclic para‐hexa‐peri‐hexabenzocoronene trimers ([3]CHBCs) were selectively obtained. This concept is envisioned as an access to ultrashort carbon nanotubes subject to the condition that further benzene rings with the right connectivity will be inserted.     

Pd-Catalyzed Annulation of 1-Halo-8-arylnaphthalenes and Alkynes Leading to Heptagon-Embedded Aromatic Systems

A palladium-catalyzed heptagon-forming annulation reaction between 1-halo-8-arylnaphthalene and diarylacetylene is reported. The reaction is promoted using a catalytic system comprised of Pd(OAc)₂, moderately electron-deficient triarylphosphine P(4-ClC₆H₄)₃, and Ag₂CO₃ to afford benzo[4,5]cyclohepta[1,2,3-de]naphthalene derivatives in moderate to good yields, in preference to fluoranthene as a competing byproduct. Twofold annulation can also be achieved to access a novel heptagon-embedded polycyclic aromatic hydrocarbon compound.     

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.     

Atom‐Economic Synthesis of Fully Substituted 2‐Aminopyrroles via Gold‐Catalyzed Formal [3+2] Cycloaddition between Ynamides and Isoxazoles

A concise and flexible synthesis of fully substituted 2‐aminopyrroles via gold‐catalyzed formal [3+2] cycloaddition between ynamides and isoxazoles has been developed. Under mild reaction conditions, various 2‐aminopyrrole derivatives were obtained in good to excellent yields, thus providing an efficient and atom‐economic way for the construction of fully substituted 2‐aminopyrroles.     

溴代稠环芳烃的氧化溴化法合成

以HBr/H2O2为溴化体系,采用氧化溴化法合成了6种稠环芳烃的溴化物。 合成反应无需催化剂,通过控制反应温度和溴化试剂用量,可以在稠环芳烃化合物的活性位选择性地单溴化或双溴化,产率可达51.1%～94.2%。 产品经熔点和1H NMR法确认,合成操作简单安全,环境污染少,有工业应用前景。     

Synthesis of 4-Substituted-1,2-Dihydroquinolines by Means of Gold-Catalyzed Intramolecular Hydroarylation Reaction of N-Ethoxycarbonyl-N-Propargylanilines

An alternative Au(I)-catalyzed synthetic route to functionalized 1,2-dihydroquinolines is reported. This novel approach is based on the use of N-ethoxycarbonyl protected-N-propargylanilines as building blocks that rapidly undergo the IMHA reaction affording the 6-endo cyclization product in good to high yields. In the presence of N-ethoxycarbonyl-N-propargyl-meta-substituted anilines, the regiodivergent cyclization at the ortho-/para-position is achieved by the means of catalyst fine tuning.     

Regiospecific Photochemical Synthesis of Methylchrysenes

Methylated polycyclic aromatic hydrocarbons (PAHs) are suspected to be some of the toxic compounds in crude oil towards marine life and are needed as single compounds for environmental studies. 1-, 3- and 6-methylchrysene (3a,b,c) were prepared as single isomers by photochemical cyclization of the corresponding stilbenoids in the Mallory reaction using stoichiometric amounts of iodine in 82-88% yield. 2-methylchrysene (3d) was prepared by photochemical cyclization where the regioselectivity was controlled by elimination of an ortho-methoxy group under acidic oxygen free conditions in 72% yield. These conditions failed to form 4-methylchrysene from the corresponding stilbenoid. All stilbenoids were made from a common naphthyl Wittig salt and suitably substituted benzaldehydes. We have also demonstrated that methylchrysenes can be oxidized to the corresponding chrysenecarboxylic acids by KMnO₄ in modest yields.     

Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-Catalyzed [2+2+2] Cycloaddition

The enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-based planar chiral cyclophanes was achieved for the first time by the rhodium-catalyzed intramolecular regio- and enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulvenes followed by stepwise oxidative transformations. The thus synthesized planar chiral bent cyclophanes, that possess bent p-terphenyl- and 9-fluorenone-cores, were converted to 9-fluorenol-based ones with excellent ee values of >99 % by diastereoselective 1,2-reduction. These 9-fluorenol-based cyclophanes exhibited high fluorescence quantum yields, which were significantly higher than that of an acyclic reference molecule (78–82 % vs. 48 %). The bending effect on the chiroptical property was also examined, which revealed that the anisotropy factors (g_abs values) for electronic circular dichroism (ECD) of these 9-fluorenol-based planar chiral bent cyclophanes increase as the tether length becomes shorter.