The B(C6F5)3 boron Lewis acid route to arene-annulated pentalenes

Borylative Cyclization of 1,6-Allenynes Driven by BCl3

A metal-free intramolecular borylative cyclization of 1,6-allenynes driven by BCl₃ was developed. This method provides a general and practical strategy to construct valuable pyrrolidines containing all-carbon quaternary centers or 3,5-dihydroazepine derivatives depending on the substituents of the allene, with conjugative and sterically hindered phenyl groups favoring the latter.

1,2-Carbopentafluorophenylation of Alkynes: The Metallomimetic Pull-Push Reactivity of Tris(pentafluorophenyl)borane

We report the novel single-step 1,2-dicarbofunctionalization of an arylacetylene with an allylsilane and tris(pentafluorophenyl)borane [B(C₆ F₅ )₃ ] involving C-C bond formation with C-H bond scission at the β-position to the silicon atom of an allylsilane and B→C migration of a C₆ F₅ group. The 1,2-carbopentafluorophenylation occurs smoothly without the requirement for a catalyst or heating. Mechanistic studies suggest that the metallomimetic "pull-push" reactivity of B(C₆ F₅ )₃ imparts consecutive electrophilic and nucleophilic characteristics to the benzylic carbon of the arylacetylene. Subsequent photochemical 6π-electrocyclization affords tetrafluoronaphthalenes, which are important in the pharmaceutical and materials sciences. Owing to the unique reactivity of B(C₆ F₅ )₃ , the 1,2-carbopentafluorophenylation using 2-substituted furan proceeded with ring opening, and the reaction using silyl enolates formed a C-C bond with C-O bond scission at the silyloxy-substituted carbon.

Recent Progress in High Linearly Fused Polycyclic Conjugated Hydrocarbons (PCHs, n > 6) with Well-Defined Structures

Although polycyclic conjugated hydrocarbons (PCHs) and their analogues have gained great progress in the fields of organic photoelectronic materials, the in-depth study on present PCHs is still limited to hexacene or below because longer PCHs are insoluble, unstable, and tediously synthesized. Very recently, various strategies including on-surface synthesis are developed to address these issues and many higher novel PCHs are constructed. Therefore, it is necessary to review these advances. Here, the recent synthetic approach, basic physicochemical properties, single-crystal packing behaviors, and potential applications of the linearly fused PCHs (higher than hexacene), including acenes or π-extended acenes with fused six-membered benzenoid rings and other four-membered, five-membered or even seven-membered and eight-membered fused compounds, are summarized.

Gold-Catalyzed Regiospecific Annulation of Unsymmetrically Substituted 1,5-Diynes for the Precise Synthesis of Bispentalenes

Precise control of the selectivity in organic synthesis is important to access the desired molecules. We demonstrate a regiospecific annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzene derivatives for a geometry-controlled synthesis of linear bispentalenes, which is one of the promising structures for material science. A gold-catalyzed annulation of unsymmetrically substituted 1,2-di(arylethynyl)benzene could produce two isomeric pentalenes, but both electronic and steric effects on the aromatics at the terminal position of the alkyne prove to be crucial for the selectivity; especially a regiospecific annulation was achieved with sterically blocked substituents; namely, 2,4,6-trimetyl benzene or 2,4-dimethyl benzene. This approach enables the geometrically controlled synthesis of linear bispentalenes from 1,2,4,5-tetraethynylbenzene or 2,3,6,7-tetraethynylnaphthalene. Moreover, the annulation of a series of tetraynes with a different substitution pattern regioselectively provided the bispentalene scaffolds. A computational study revealed that this is the result of a kinetic control induced by the bulky NHC ligands.

A DFT Study of the Modulation of the Antiaromatic and Open-Shell Character of Dibenzo[a,f]pentalene by Employing Three Strategies: Additional Benzoannulation, BN/CC Isosterism, and Substitution

Dibenzo[a,f]pentalene ([a,f]DBP) is a highly antiaromatic molecule having appreciable open-shell singlet character in its ground state. In this work, DFT calculations at the B3LYP/6-311+G(d,p) level of theory were performed to explore the efficiency of three strategies, that is, BN/CC isosterism, substitution, and (di)benzoannulation of [a,f]DBP, in controlling its electronic state and (anti)aromaticity. To evaluate the type and extent of the latter, the harmonic oscillator model of aromaticity (HOMA) and aromatic fluctuation (FLU) indices were used, along with the nucleus-independent chemical shift NICS-XY-scan procedure. The results suggest that all three strategies could be employed to produce either the closed-shell system or open-shell species, which may be in the singlet or triplet ground state. Triplet states have been characterized as aromatic, which is in accordance with Baird's rule. All the singlet states were found to have weaker global paratropicity than [a,f]DBP. Additional (di)benzo fusion adds local aromatic subunit(s) and mainly retains the topology of the paratropic ring currents of the basic molecule. The substitution of two carbon atoms by the isoelectronic BN pair, or the introduction of substituents, results either in the same type and very similar topology of ring currents as in the parent compound, or leads to (anti)aromatic and nonaromatic subunits. The triplet states of all the examined compounds are also discussed.

The effect of two types of dibenzoannulation of pentalene on molecular energies and magnetically induced currents

The effect of two types of dibenzo-fusion of pentalene in the singlet and triplet states on its molecular energies and magnetically induced ring currents was examined via density functional calculations. The isomerization energy decomposition analysis (IEDA) together with the calculated aromaticity indices (NICS(1)zz, HOMA and FLUπ), estimation of resonance energies (RE) and extra cyclic resonance energies (ECRE) via the NBO method, and the NICS-XY-scans revealed that the π-electronic system is the most important factor controlling the molecular energies. The [a,f] topology features greater delocalization, which results in two opposing effects: larger ECRE, but weaker π-bonding. The latter is mainly responsible for the higher energy of [a,f]-dibenzopentalene (DBP) (ΔEiso = 21.7 kcal mol-1), with the other effects being σ-orbital and electrostatic interactions. The reversal of energetic stability in the triplet states (ΔEiso = -10.8 kcal mol-1) mainly comes from the reduced Pauli repulsion in [a,f]-DBP, which stabilizes the unpaired spin density over the central trimethylenemethane subunit vs. the central pentalene subunit in [a,e]-DBP. Although the [a,e] topology only reduces the diatropic and paratropic currents of the elementary subunits, benzene and pentalene, the [a,f] topology also creates strong global paratropicity involving the benzene rings. Both DBP isomers are characterized by global and smaller semi-global and local diatropic currents in the triplet state.

Borylative cyclisation of diynes using BCl3 and borocations

Products from the borylative cyclisation of diynes using BCl₃ is dependent on substituent effects, however, some control of product outcome is achieved using borocations or BCl₃ in the presence of [BCl₄]⁻.

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.

N-Heterocycle-Fused Pentalenes by a Gold-Catalyzed Annulation of Diethynyl-Quinoxalines and -Phenazines

The gold-catalyzed annulation of diethynyl N-heterocycles for the synthesis of quinoxaline-/phenazine-based pentalenes and the study of their optoelectronic properties are described. The inhibition of the gold catalyst by the nitrogen centers in the substrate and the product could be overcome by increasing the reaction temperature to 130 °C, which usually leads to catalyst decomposition in gold catalysis. At 130 °C, 6,7-di(arylethynyl)quinoxalines in chlorobenzene give the corresponding pentalenes. The annulation of 2,3-di(arylethynyl)quinoxalines requires an even higher temperature under microwave irradiation. The quinoxaline-based pentalenes showed lower LUMO levels compared to the corresponding naphthalene-based pentalenes.

Synthesis, Characterization, and Functionalization of 1-Boraphenalenes

1-Boraphenalenes have been synthesized by reaction of BBr3 with 1-(aryl-ethynyl)naphthalenes, 1-ethynylnaphthalene, and 1-(pent-1-yn-1-yl)naphthalene and they can be selectively functionalized at boron or carbon to form bench-stable products. All of these 1-boraphenalenes have LUMOs localized on the planar C12 B core that are closely comparable in character to isoelectronic phenalenyl cations. In contrast to the comparable LUMOs, the aromatic stabilization of the C5 B ring in 1-boraphenalenes is dramatically lower than the C6 rings in phenalenyl cations. This is due to the occupied orbitals of π symmetry being less delocalised in the 1-boraphenalenes.

FLP reactivity of [Ph3C]+ and (o-tolyl)3P and the capture of a Staudinger reaction intermediate

The frustrated Lewis pair (FLP) derived from the trityl cation and (o-tolyl)₃P effects the activation of 1,4-cyclohexadiene and 1-bromo-4-ethynylbenzene and heterolytically cleaves the S-S bond of diphenyl disulfide. The FLP also captures pentafluorophenyl azide as the Staudinger reaction intermediate, a species that reacts with Ph₃SiH to give the silyl analog.

On the Gold-Catalyzed Generation of Vinyl Cations from 1,5-Diynes

Conjugated 1,5-diynes bearing two aromatic units at the alkyne termini were converted in the presence of a gold catalyst. Under mild conditions, aryl-substituted dibenzopentalenes were generated. Calculations predict that aurated vinyl cations are key intermediates of the reaction. A bidirectional approach provided selective access to the angular annulated product in high yield, which was explained by calculations.

Borane-catalyzed indole synthesis through intramolecular hydroamination

Catalytic metal-free intramolecular hydroamination for the synthesis of indoles and tetrahydroisoquinolines was achieved.

The Chemistry of a Non-Interacting Vicinal Frustrated Phosphane/Borane Lewis Pair

The dimesitylphosphinocyclopentene/HB(C₆ F₅ )₂ -derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane-borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO₂ . It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C₆ F₅ )₂ ] to generate a FLP-η² -formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.

Selective synthesis of unsymmetric dibenzo[a,e]pentalenes by a rhodium-catalysed stitching reaction

A rhodium-catalysed stitching reaction between 2-(silylethynyl)arylboronates and 2-(silylethynyl)aryl bromides has been developed for the synthesis of unsymmetric dibenzo[a,e]pentalenes. The introduction of appropriately sized silyl groups on the starting substrates led to a high crossover selectivity without using an excess amount of either substrate. The present stitching reaction could produce a variety of unsymmetric dibenzo[a,e]pentalene derivatives, including those with electronically different substituents on the fused benzene rings as well as heteroarene fused compounds. Desilylative halogenation was also demonstrated to synthesise the corresponding halogenated dibenzo[a,e]pentalenes, which can be used as building blocks for further chemical transformations.

Contrasting Frustrated Lewis Pair Reactivity with Selenium- and Boron-Based Lewis Acids

The activation of π-bonds in diynyl esters has been investigated by using soft and hard Lewis acids. In the case of the soft selenium Lewis acid PhSeCl, sequential activation of the alkyne bonds leads initially to an isocoumarin (1 equiv PhSeCl) and then to a tetracyclic conjugated structure with the isocoumarin subunit fused to a benzoselenopyran (3 equiv PhSeCl). Conversely, the reaction with the hard Lewis acidic borane B(C6 F5 )3 initiates a cascade reaction to yield a complex π-conjugated system containing phthalide and indene subunits.

Benzo- and Naphthopentalenes: Syntheses, Structures, and Properties

Benzo- and naphthopentalene derivatives were synthesized, and the effects of structural variations on their antiaromaticity and optoelectronic and electrochemical properties were examined experimentally and theoretically in detail. The results unveiled that with increasing the bond order of the carbon-carbon bond ([5,6] junction) shared by the pentalene and aromatic moieties, the 8π antiaromatic character of pentalene is enhanced and the HOMO-LUMO gap is decreased, which accompanies both the elevation of the HOMO level and the lowering of the LUMO level. The ethynylene units between the pentalene skeleton and the phenyl groups proved to extend π-conjugation sufficiently.

Frustrated Lewis Pair-Catalyzed Cycloisomerization of 1,5-Enynes via a 5-endo-dig Cyclization/Protodeborylation Sequence

The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh3 was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.

The Impact of Antiaromatic Subunits in [4n+2] π-Systems: Bispentalenes with [4n+2] π-Electron Perimeters and Antiaromatic Character

Three series of stable, neutral, π-extended bispentalene derivatives, with two pentalenes fused to a central benzene or naphthalene moiety, have been prepared through a modified double carbopalladation cascade reaction. While these chromophores feature skeletons with [4n+2] π-electron perimeters, the two 8 π-electron pentalene subunits strongly influence bonding and spectral properties. (1)H NMR spectra showed large upfield shifts of the protons in the pentalene moieties, comparable to antiaromatic monobenzopentalenes. Further investigations on magnetic ring currents through NICS-XY-scans suggest a global paratropic current and a local diatropic current at the central benzene ring in two of the series, while the third series, with a central naphthalene ring, showed more localized ring currents, with stronger paratropic ring currents on the pentalene moieties. X-ray diffraction analyses revealed planar bispentalene cores with large double- and single-bond alternation in the pentalene units, characteristic for antiaromaticity, and small alternation in the central aromatic rings. In agreement with TD-DFT calculations, both optical and electrochemical data showed much smaller HOMO-LUMO energy gaps compared to other neutral, acene-like hydrocarbons with the same number of fused rings. Both experimental and computational results suggest that the molecular properties of the presented bispentalenes are dominated by the antiaromatic pentalene-subunits despite the [4n+2] π-electron perimeter of the skeletons.

Vertically-expanded imidazo[1,2-a]pyridines and imidazo[1,5-a]pyridine via dehydrogenative coupling

The intramolecular dehydrogenative coupling mediated by potassium constitutes the general methodology leading to weakly emitting π-expanded heterocycles.