Metal-catalyzed annulation reactions for π-conjugated polycycles

Synthesis of indole-fused heteroacenes by cascade cyclisation involving rhodium(ii)-catalysed intramolecular C–H amination

Indole-fused heteroacenes have been synthesised from cascade cyclisation of 2-[(2-azidophenyl)ethynyl]anilines and -phenols in a catalytic manner under redox-neutral conditions.

Carbocyclization approaches to electron-deficient nanographenes and their analogues

Versatile π-aromatic building blocks and selective coupling transformations enable rapid assembly of complex electron-deficient molecules, useful as n-type organic semiconductors.

Synthesis of distorted nanographenes containing seven- and eight-membered carbocycles

We highlight recent progress in bottom-up synthesis of well-defined distorted polyaromatic hydrocarbons with saddle shapes containing heptagonal and octagonal carbocycles.

Tuning the electronic properties of thiophene-annulated NDIs: the influence of the lateral fusion position

We report a new asymmetric thiophene-annulated naphthalenediimide with high electron mobilities.

Pd-Catalyzed cascade cyclization of o-alkynylanilines via C–H/C–N bond cleavage leading to dibenzo[a,c]carbazoles

A new and efficient Pd-catalyzed cascade cyclization of biaryl-tethered o-alkynylanilines for the formation of dibenzo[a,c]carbazole derivatives has been reported.

Azulene-Fused Linear Polycyclic Aromatic Hydrocarbons with Small Bandgap, High Stability, and Reversible Stimuli Responsiveness

Azulene-fused polycyclic aromatic hydrocarbons (PAHs) were synthesized from commercially available azulene in four steps. The resulting azulene conjugates exhibited significantly narrow HOMO-LUMO bandgaps with high air stability, confirmed by photophysical study. Introduction of azulene also enabled the unique reversible stimuli-responsiveness even with the weak acid and base, which can potentially control the degree of conjugation and optoelectronic properties by simple acid-base and redox processes.

Synthesis of extended polycyclic aromatic hydrocarbons by oxidative tandem spirocyclization and 1,2-aryl migration

The extended polycyclic aromatic hydrocarbons (PAHs) have received significant interdisciplinary attention due to their semiconducting applications in diverse organic electronics as well as intriguing structural interests of well-defined graphene segments. Herein, a highly efficient oxidative spirocyclization and 1,2-aryl migration tandem synthetic method for the construction of extended polyaromatic hydrocarbons (PAHs) has been developed. The CuCl-catalyst/PhCO₃ ^tBu or DDQ oxidation system in the presence of trifluoroacetic acid enables the selective single-electron oxidation to take place preferentially at the more electron-rich alkene moiety of o-biphenylyl-substituted methylenefluorenes, giving rise to the subsequent tandem process. A variety of structurally diverse extended PAHs including functionalized dibenzo[g,p]chrysenes, benzo[f]naphtho[1,2-s]picene, hexabenzo[a,c,fg,j,l,op]tetracene, tetrabenzo[a,c,f,m]phenanthro[9,10-k]tetraphene, tetrabenzo[a,c,f,k]phenanthro[9,10-m]tetraphene, tetrabenzo[a,c,f,o]phenanthro[9,10-m]picene and S-type helicene have been readily synthesized.

Simple synthetic routes to polyaromatic hydrocarbons are important due to their applications in organic electronics. Here the authors report a straightforward synthesis of a diverse range of polyaromatic hydrocarbons, via a one-pot oxidative cyclisation and subsequent 1,2-aryl migration.

Zirconacyclopentadiene-Annulated Polycyclic Aromatic Hydrocarbons

Syntheses of large polycyclic aromatic hydrocarbons (PAHs) and graphene nanostructures demand methods that are capable of selectively and efficiently fusing large numbers of aromatic rings, yet such methods remain scarce. Herein, we report a new approach that is based on the quantitative intramolecular reductive cyclization of an oligo(diyne) with a low-valent zirconocene reagent, which gives a PAH with one or more annulated zirconacyclopentadienes (ZrPAHs). The efficiency of this process is demonstrated by a high-yielding fivefold intramolecular coupling to form a helical ZrPAH with 16 fused rings (from a precursor with no fused rings). Several other PAH topologies are also reported. Protodemetalation of the ZrPAHs allowed full characterization (including by X-ray crystallography) of PAHs containing one or more appended dienes with the ortho-quinodimethane (o-QDM) structure, which are usually too reactive for isolation and are potentially valuable for the fusion of additional rings by Diels-Alder reactions.

Intramolecular Benzoin Reaction Catalyzed by Benzaldehyde Lyase from Pseudomonas Fluorescens Biovar I

Intramolecular benzoin reactions catalyzed by benzaldehyde lyase from Pseudomonas fluorescens biovar I (BAL) are reported. The structure of the substrates envisaged for this reaction consists of two benzaldehyde derivatives linked by an alkyl chain. The structural requirements needed to achieve the intramolecular carbon-carbon bond reaction catalyzed by BAL were established. Thus, a linker consisting of a linear alkyl chain of three carbon atoms connected through ether-type bonds to the 2 and 2' positions of two benzaldehyde moieties, which could be substituted with either Cl, Br, or OCH₃ at either the 3 and 3' or 5 and 5' positions, were suitable substrates for BAL. Reactions with 61-84 % yields of the intramolecular product and ee values between 64 and 98 %, were achieved.

An Annulative Synthetic Strategy for Building Triphenylene Frameworks by Multiple C-H Bond Activations

C-H activation is a versatile tool for appending aryl groups to aromatic systems. However, heavy demands on multiple catalytic cycle operations and site-selectivity have limited its use for graphene segment synthesis. A Pd-catal- yzed one-step synthesis of functionalized triphenylene frameworks is disclosed, which proceeds by 2- or 4-fold C-H arylation of unactivated benzene derivatives. A Pd₂ (dibenzylideneacetone)₃ catalytic system, using cyclic diaryliodonium salts as π-extending agents, leads to site-selective inter- and intramolecular tandem arylation sequences. Moreover, N-substituted triphenylenes are applied to a field-effect transistor sensor for rapid, sensitive, and reversible alcohol vapor detection.

Redox-Neutral Rhodium-Catalyzed [4+1] Annulation through Formal Dehydrogenative Vinylidene Insertion

A synthetic protocol for the expedient construction of 5-methylene-1H-pyrrol-2(5H)-one derivatives through rhodium-catalyzed [4+1] annulation with gem-difluoroacrylate as the C₁ component was reported. By taking advantage of the twofold C-F bond cleavage occurring during the annulation, this reaction not only allows the synthesis of these heterocyclic compounds under overall oxidant-free conditions but also renders the transformation stereospecific. The very mild reaction conditions employed ensure compatibility with a wide variety of synthetically useful functional groups.

Single-Flask Multicomponent Synthesis of Highly Substituted α-Pyrones via a Sequential Enolate Arylation and Alkenylation Strategy

Trisubstituted α-pyrones are obtained by a Pd-catalyzed three-component, single-flask operation via an α-arylation, subsequent α-alkenylation, alkene isomerization, and dienolate lactonization. A variety of coupling components under mild conditions afforded isolated yields of up to 93% of the pyrones with complete control of regioselectivity. Metal dependence was noted for three of the steps of the pathway. Utility of the pyrone products was demonstrated by further transformations providing convenient access to polyaromatic compounds, exhibiting broad molecular diversity.

Pentaleno[1,2-a:4,5']diacenaphthylenes: Uniquely Stabilized Pentalene Derivatives

We demonstrate the preparation of diacenaphthopentalene derivatives via a palladium-catalyzed dimerization of 1-iodo-2-arylethynyl-acenaphthylenes. The resulting 7,14-diarylpentaleno[1,2-a:4,5a']diacenaphthylenes, which contain four linearly fused five-membered rings, are benchtop stable and behave as hole-transporting or ambipolar semiconductors in organic field effect transistors. The X-ray crystal structure shows the important role of the fused naphthalene unit that enforces a formal pentalene subunit at the central five-membered rings and [5]-radialene-like structures at the proximal five-membered rings. Nucleus-independent chemical shift (NICS) calculations show the internal pentalene rings are intermediate in antiaromaticity character between known pentalene and dibenzopentalenes derivatives. The diacenaphthopentalene derivatives give high optical gap materials owing to a forbidden HOMO to LUMO transition, yet have narrow electrochemical gaps and are reduced at small negative potentials giving LUMO energy levels of -3.57 to -3.74 eV.

1,3-Dien-5-ynes: Versatile Building Blocks for the Synthesis of Carbo- and Heterocycles

1,3-Dien-5-ynes have been extensively used as starting materials for the synthesis of a wide number of different carbo- and heterocycles. The aim of this review is to give an overview of their utility in organic synthesis, highlighting the variety of compounds that can be directly accessed from single reactions over these systems. Thus, cycloaromatization processes are initially commented, followed by reactions directed toward the syntheses of five-membered rings, other carbocycles and, finally, heterocycles. The diverse methodologies that have been developed for the synthesis of each of these types of compounds from 1,3-dien-5-ynes are presented, emphasizing the influence of the reaction conditions and the use of additional reagents in the outcome of the transformations.

N-Methyl Transfer Induced Copper-Mediated Oxidative Diamination of Alkynes

A novel intramolecular oxidative diamination of bis(2-aminophenyl)acetylene for the synthesis of the structurally intriguing π-conjugated polyheterocyclic scaffold, 5,10-dihydroindolo[3,2-b]indole (DHII), has been developed under Cu(hfacac)2/O2 oxidation systems. The structure design of bis(2-aminophenyl)acetylene bearing both N,N-dimethylamine and primary amine groups is crucial for constructing the corresponding DHII scaffold. Notably, an intermolecular N-methyl transfer from the nitrogen atom of N,N-dimethylamine to the primary amine takes place, which is a critical step for the successful implementation of the present annulation process.

An Electron-Poor C64 Nanographene by Palladium-Catalyzed Cascade C-C Bond Formation: One-Pot Synthesis and Single-Crystal Structure Analysis

Herein, we report the one‐pot synthesis of an electron‐poor nanographene containing dicarboximide groups at the corners. We efficiently combined palladium‐catalyzed Suzuki–Miyaura cross‐coupling and dehydrohalogenation to synthesize an extended two‐dimensional π‐scaffold of defined size in a single chemical operation starting from N‐(2,6‐diisopropylphenyl)‐4,5‐dibromo‐1,8‐naphthalimide and a tetrasubstituted pyrene boronic acid ester as readily accessible starting materials. The reaction of these precursors under the conditions commonly used for Suzuki–Miyaura cross‐coupling afforded a C₆₄ nanographene through the formation of ten C−C bonds in a one‐pot process. Single‐crystal X‐ray analysis unequivocally confirmed the structure of this unique extended aromatic molecule with a planar geometry. The optical and electrochemical properties of this largest ever synthesized planar electron‐poor nanographene skeleton were also analyzed.

Fused π-Extended Truxenes via a Threefold Borylation as the Key Step

On the basis of a threefold borylated truxene, which is accessible in high yields by iridium-catalyzed borylation under CH-activation, fused π-extended truxenes have been synthesized by a two-step method of first Suzuki-Miyaura cross-coupling reaction and subsequent condensation reaction. The mild condensation method tolerates the presence of a variety of functional groups, such as nitro, fluoro, or carboxyl moieties. Furthermore, by using this approach, N- and S-heteroarene analogues become accessible for the first time, as well as larger structures that represent derivatives of precursors for fullerene C60 or buckybowls. The attached tert-butyl groups make all derivatives sufficiently soluble to allow full spectroscopic and electrochemical investigations. Postfunctionalization of selected derivatives for further synthetic applications of the compounds is also presented.

[N,P]-pyrrole PdCl2complexes catalyzed the formation of dibenzo-α-pyrone and lactam analogues

Catalytic applications on direct intramolecular arylation was developed using a new family of palladium complexes that includes [N,P] ligands based on the pyrrole ring with α-phosphine and phosphole units.

Synthesis of cyclopenta-fused polycyclic aromatic hydrocarbons utilizing aryl-substituted anilines

Cyclopenta-fused polycyclic aromatic hydrocarbons were synthesized from readily available 2-aryl-substituted anilines under extremely mild conditions.

Polycyclic aromatic hydrocarbons with orthogonal tetraimides as n-type semiconductors

We report two novel n-type polycyclic aromatic hydrocarbons bearing orthogonal tetraimides with high electron mobilities.

Competing amination and C–H arylation pathways in Pd/xantphos-catalyzed transformations of binaphthyl triflates: switchable routes to chiral amines and helicene derivatives

A Pd(OAc)₂/xantphos catalyst system can be tuned to promote either amination or C–H arylation of hindered binaphthyl 2-triflates, with xantphos's hemilability playing a key role.