Reversible Redox Reaction Between Antiaromatic and Aromatic States of 32π‐Expanded Isophlorins

32π‐antiaromatic expanded isophlorins with a varying number of thiophene and furan rings adopt either planar, ring‐inverted, or twisted conformations depending on the number of furan rings in the macrocycle. However, they exhibit identical reactivity with respect to their oxidation to aromatic 30π‐dicationic species under acidic conditions. These 32π‐antiaromatic macrocycles can also be oxidized with [Et₃O⁺SbCl₆^?]and NOBF₄ to generate dications, thus confirming ring oxidation of macrocycles. Furthermore, they can be reduced back to their parent 32π‐antiaromatic state by triethylamine, Zn, or FeCl₂. Single‐crystal X‐ray diffraction analysis confirmed a figure‐eight conformation for a hexafuran system, which opens to a planar structure upon oxidation.     

Reaction between Furan‐ or Thiophene‐2‐carbonyl Chloride,Isocyanides, and Dialkyl Acetylenedicarboxy­lates: Multicomponent Synthesis of 2,2′‐Bifurans and 2‐(Thiophen‐2‐yl)furans

An efficient multi‐component synthesis of highly functionalized 2,2′‐bifurans and 2‐(thiophen‐2‐yl)furans is described. A mixture of furan‐ or thiophene‐2‐carbonyl chloride, an isocyanide, and a dialkyl acetylenedicarboxylate undergoes a smooth addition reaction in dry CH₂Cl₂ at ambient temperature to produce 2‐amino‐5‐(4‐chlorofuran‐2‐yl)furan‐3,4‐dicarboxylates and 2‐amino‐5‐(4‐chlorothiophen‐2‐yl)furan‐3,4‐dicarboxylates. A single‐crystal X‐ray‐analysis of a derivative conclusively confirms the structure of these 2,2′‐bifurans and 2‐(thiophen‐2‐yl)furans. A novel electrophilic aromatic substitution reaction can justify the formation of the Cl‐substituted furan or thiophene rings.     

Synthesis and electrophilic substitution reactions of 2-aryl-3,4-bis(carbomethoxy)furans

The thermal decomposition of the products of hydrogenation of the adducts obtained from arylfurans and dimethyl acetylenedicarboxylate leads to the formation of 2-aryl-3,4-bis(carbomethoxy)furans. The bromination of these compounds takes place in the 5 position of the furan ring; depending on the concentration of the nitric acid used, nitration leads to the formation of products of nitration in both the furan and the benzene rings.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 453–456, April, 1982.     

Synthesis and comparative study of the heterocyclic rings on liquid crystalline properties of 2,5-aryl-1,3,4-oxa(thia)diazole derivatives containing furan and thiophene units

A series of heterocyclic liquid crystalline compounds containing 1,3,4-oxadiazole/thiadiazole, furan and thiophene units were synthesised and characterised by means of electrospray ionisation-mass spectrometry (ESI-MS), high-resolution mass spectroscopy (HRMS), ¹H nuclear magnetic resonance (NMR) and ¹³C NMR. The thermal behaviours were investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). The effect of the 1,3,4-oxadiazole, 1,3,4-thiadiazole, furan, thiophene and benzene rings on the liquid crystalline properties was discussed briefly in context with the geometrical and electronic factors. The results showed that the tendency to form mesophases follows the sequence: 1,4-disustituted benzene >2,5-disubstituted thiophene >2,5-disustituted furan and 1,3,4-thiadiazole >1,3,4-oxadiazole.     

Tuning the Electronic Properties and Acid‐Response Behavior of N‐Heteroacene‐Based π‐Conjugated Liquids by Changing the Number of π‐Conjugated Substituents

We have designed and synthesized two room‐temperature‐fluorescent π‐conjugated liquids based on the N‐heteroacene framework ( 1 and 2 ). These two π‐conjugated liquids, which contained one and two thiophene rings, respectively, exhibited different electronic properties and rheology behaviors. Single‐crystal X‐ray analysis of dithiophene‐appended compound 4 revealed that two thiophene rings hindered the interactions of the imino N atoms with acids through the formation of interactions between the S atoms of the thiophene rings and the imino N atoms of the pyrazine group. On the other hand, monothiophene‐appended molecules 1 and 3 each contained an unhindered imino N atom on the opposite site to the thiophene ring. Upon dissolving various acids with different pK_a values in compounds 1 and 2 , these slight structural differences gave rise to marked differences in their acid‐response behaviors, thereby resulting in the emission of variously colored fluorescence in the liquid state. Furthermore, when acids with lower pK_a values was dissolved in compounds 1 and 2 , phase transition occurred from an isotropic liquid state to a self‐organized liquid‐crystalline phase.     

Competition Between π–π and CH/π Interactions: A Comparison of the Structural and Electronic Properties of Alkoxy‐Substituted 1,8‐Bis((propyloxyphenyl)ethynyl)naphthalenes

The structural and electronic consequences of π–π and C?H/π interactions in two alkoxy‐substituted 1,8‐bis‐ ((propyloxyphenyl)ethynyl)naphthalenes are explored by using X‐ray crystallography and electronic structure computations. The crystal structure of analogue 4 , bearing an alkoxy side chain in the 4‐position of each of the phenyl rings, adopts a π‐stacked geometry, whereas analogue 8 , bearing alkoxy groups at both the 2‐ and the 5‐positions of each ring, has a geometry in which the rings are splayed away from a π‐stacked arrangement. Symmetry‐adapted perturbation theory analysis was performed on the two analogues to evaluate the interactions between the phenylethynyl arms in each molecule in terms of electrostatic, steric, polarization, and London dispersion components. The computations support the expectation that the π‐stacked geometry of the alkoxyphenyl units in 4 is simply a consequence of maximizing π–π interactions. However, the splayed geometry of 8 results from a more subtle competition between different noncovalent interactions: this geometry provides a favorable anti‐alignment of C?O bond dipoles, and two C?H/π interactions in which hydrogen atoms of the alkyl side chains interact favorably with the π electrons of the other phenyl ring. These favorable interactions overcome competing π–π interactions to give rise to a geometry in which the phenylethynyl substituents are in an offset, unstacked arrangement.     

Synthesis of thiophene/phenylene co‐oligomers. IV. 6‐ to 8‐ring molecules

We report the synthesis of various thiophene/phenylene co‐oligomers with a total number of thiophene and benzene (phenylene) rings of 6 to 8. These compounds include a phenyl‐capped sexithiophene, a thienyl‐capped quaterphenylene, as well as block and alternating co‐oligomers. The synthesis is based on either the Suzuki coupling reaction or the direct dimerization coupling. The latter method produces symmetric molecules with an even total ring number. These reaction schemes enabled us to obtain the target compounds in high quality. Although the resulting materials are difficult to dissolve in organic solvents and therefore difficult to identify by usual ¹H nmr spectroscopy, they have successfully been identified through Fourier‐transform ir spectroscopy. The specific group frequencies of ring‐stretching and out‐of‐plane deformation modes are characteristic of the substitution pattern of the individual thiophene and benzene rings.     

Synthesis and structure of some imines containing furoxan ring derived from isosafrole

A series of 14 imines containing furoxan and benzene rings has been prepared starting from isosafrole. The structure of reported compounds have been confirmed by elemental analysis, EI MS, UV, IR, and NMR spectroscopy. It is shown that, on treatment with Na₂S₂O₄, the nitro group on the benzene ring was reduced to amino group, but the N→O group of the furoxan ring was not. The ¹H‐ and ¹³C NMR signals are assigned based on their spin‐spin splitting patterns, in some cases, NOESY and HMBC spectra are used. The NOESY spectra indicate that for reported imines, the benzene and the furoxan rings could not be co‐planar; the imine group has E‐configuration.     

Synthesis of Oligo(thienylfuran)s with Thiophene Rings at Both Ends and Their Structural,Electronic, and Field‐Effect Properties

Oligo(thienylfuran)s with thiophene rings at both ends ( SOSOSOS , DE‐SOSOS , DH‐SOSOS , DE‐SOSOSOS , and DH‐SOSOSOS ; S and O denote thiophene and furan rings, respectively, DE and DH denote diethyl‐ and dihexyl‐substituted, respectively) were newly synthesized by repetitive Stille coupling reactions. The UV/Vis maximum absorptions of the oligomers, SO , SOSO , SOSOS , SOSOSO , and SOSOSOS , exhibited a clear bathochromic shift with increasing number of heterocycles. The value of the oxidation peak potential (E_pa¹) determined by cyclic voltammetry decreased with an increase in the number of heterocycles by 0.06–0.08 V per heterocycle. The crystal‐packing structures of DE‐SOSOS and DH‐SOSOS determined by X‐ray crystallography have a herringbone motif and are denser than the reported structures of pentacene and α‐sexithiophene. The morphologies of thin films prepared by vacuum deposition and spin coating were investigated by atomic force microscopy and X‐ray diffraction. Among these films, those of DE‐SOSOS and DH‐SOSOS exhibited highly ordered arrangements. The devices based on vacuum‐deposited and spin‐coated films of DE‐SOSOS and DH‐SOSOS displayed the highest FET mobilities of 10^?2–10^?3 cm² V^?1 s^?1 among the oligomers reported in this study.     

Synthesis and mesophase behaviour of rigid rod‐like phenylthiophene‐based amphiphilic diol derivatives

Novel amphiphilic block molecules consisting of a rigid 2‐phenylthiophene or 5‐phenylbithienyl core, with a polar glycerol group attached to the phenyl ring and one or two alkyl chains attached to the thiophene ring on the other side, have been synthesised by using Ni(0) and Pd(0) catalyzed coupling reactions as key steps. The thermotropic and solvent‐induced liquid crystalline behaviour of these compounds was investigated by polarising optical microscopy and X‐ray diffraction. The influence of the length, number and position of the alkyl chains, and the length of the rigid core, on their mesophase behaviour was investigated. Compounds with one alkyl chain in the terminal 5‐position on the thiophene ring form only smectic A phases, compounds with two adjacent alkyl chains attached in the 4‐ and 5‐positions of the thiophene ring exhibit thermotropic columnar mesophases, and those with two long alkyl chains attached to the 3‐ and 5‐positions form columnar LC phases only in the presence of water. Another compound containing the longer 5‐phenylbithienyl core unit and two alkyl chains attached in lateral positions to each of the thiophene rings is not mesogenic.     

Medium‐Sized‐Ring Analogues of Dibenzodiazepines by a Conformationally Induced Smiles Ring Expansion

Analogues of dibenzodiazepines, in which the seven‐membered nitrogen heterocycle is replaced by a 9–12‐membered ring, were made by an unactivated Smiles rearrangement of five‐ to eight‐membered heterocyclic anthranilamides. The conformational preference of the tertiary amide in the starting material leads to intramolecular migration of a range of aryl rings, even those lacking electron‐withdrawing activating groups, and provides a method for n →n +4 ring expansion. The medium‐ring products adopt a chiral ground state with an intramolecular, transannular hydrogen bond. The rate of interconversion of their enantiomeric conformers depends on solvent polarity. Ring size and adjacent steric hindrance modulate this hidden hydrophilicity, thus making this scaffold a good candidate for drug development.     

Photophysics of Push–Pull Distyrylfurans,Thiophenes and Pyridines by Fast and Ultrafast Techniques

Time‐resolved transient absorption and fluorescence spectroscopy with nano‐ and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate constant of the main decay processes (fluorescence, singlet–triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excited singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge‐transfer process, which is operative even in a non‐polar solvent. Both the thiophene and pyridine moieties enhance the S₁→T₁ rate with respect to the furan one. This is due to the heavy‐atom effect (thiophene compounds) and to the ¹(π,π)*→³(n,π)* transition (pyridine compounds), which enhance the spin‐orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push–pull compounds: in fact, a particularly fast ¹LE*→¹CT* process was found for dimethylamino derivatives in the most polar solvents (time constant, τ≤400 fs), while it takes place in tens of picoseconds in non‐polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the dimethylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet‐state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed.     

Researches on condensation products of some 2, 2′-dithieniyl compounds. I

Crotonaldehyde-type condensation of 5-acetyl-2, 2-dithienyl with some aromatic and heterotyclic aldehydes and nitroaldehydes in the presence of alkali is used to synthesize a number of hitherto undescribed unsaturated 2, 2-dithienyl ketones. The visible and UV absorption spectra of these compounds of the ketones synthesized are measured in ethanol and 96% sulfuric acid solutions. In sulfuric acid all the compounds investigated form intensely colored halochromic solutions. Comparison of the spectral characteristics of the compounds prepared shows that replacement of the benzene ring by furan and thiophene rings gives rise to a bathochromic effect which is significantly greater in acid solution than in neutral solution. For the neutral solution, introduction of a single nitro group into the benzene or thiophene ring gives rise to a K-band bathchromic shift, while in acid solution there is a hypsochromic effect.     

16‐Cyano‐13α‐(5‐methyl‐1,3,4‐oxa­diazol‐2‐yl)‐13,16‐seco‐17‐nor­androst‐5‐en‐3β‐yl acetate

In the title compound, C₂₃H₃₁N₃O₃, the outer cyclo­hexane rings have chair conformations, while the central cyclohexene ring adopts a half‐chair conformation. In the solid state, intra‐ and intermolecular C—H⋯N interactions are observed.     

(η5‐Cyclopentadienyl)(η6‐phenoxathiin 10,10‐dioxide)iron(II) hexafluoridophosphate and phenoxathiin 10,10‐dioxide

In the structure of the title complex salt, [Fe(C₅H₅)(C₁₂H₈O₃S)]PF₆, the coordinated cyclopentadienyl (Cp) and benzene ring planes are almost parallel, with a hinge angle between the planes of 0.8 (2)°. The hinge angle between the planes of the peripheral (coordinated and uncoordinated) benzene rings in the coordinated phenoxathiin 10,10‐dioxide molecule is 169.9 (2)°, and the FeCp moiety is located inside the shallow fold of the heterocycle. The hinge angle between the benzene ring planes in the free heterocycle, C₁₂H₈O₃S, is 171.49 (6)°.     

Hydrogen bonding between aromatic H and F groups leading to a stripe structure with R‐ and S‐columns: the crystal structure of (2,7‐dimethoxynaphthalen‐1‐yl)(3‐fluorophenyl)methanone and comparison with its 1‐aroylnaphthalene analogues

In the molecule of (2,7‐dimethoxynaphthalen‐1‐yl)(3‐fluorophenyl)methanone, C₁₉H₁₅FO₃, (I), the dihedral angle between the plane of the naphthalene ring system and that of the benzene ring is 85.90 (5)°. The molecules exhibit axial chirality, with either an R‐ or an S‐stereogenic axis. In the crystal structure, each enantiomer is stacked into a columnar structure and the columns are arranged alternately to form a stripe structure. A pair of (methoxy)C—H...F hydrogen bonds and π–π interactions between the benzene rings of the aroyl groups link an R‐ and an S‐isomer to form a dimeric pair. These dimeric pairs are piled up in a columnar fashion through (benzene)C—H...O=C and (benzene)C—H...OCH₃ hydrogen bonds. The analogous 1‐benzoylated compound, namely (2,7‐dimethoxynaphthalen‐1‐yl)(phenyl)methanone [Kato et al. (2010). Acta Cryst. E 66 , o2659], (II), affords three independent molecules having slightly different dihedral angles between the benzene and naphthalene rings. The three independent molecules form separate columns and the three types of column are connected to each other via two C—H...OCH₃ hydrogen bonds and one C—H...O=C hydrogen bond. Two of the three columns are formed by the same enantiomeric isomer, whereas the remaining column consists of the counterpart isomer. In the case of the fluorinated 1‐benzoylated naphthalene analogue, namely (2,7‐dimethoxynaphthalen‐1‐yl)(4‐fluorophenyl)methanone [Watanabe et al. (2011). Acta Cryst. E 67 , o1466], (III), the molecular packing is similar to that of (I), i.e. it consists of stripes of R‐ and S‐enantiomeric columns. A pair of C—H...F hydrogen bonds between R‐ and S‐isomers, and C—H...O=C hydrogen bonds between R(or S)‐isomers, are also observed. Consequently, the stripe structure is apparently induced by the formation of R...S dimeric pairs stacked in a columnar fashion. The pair of C—H...F hydrogen bonds effectively stabilizes the dimeric pair of R‐ and S‐enantiomers. In addition, the co‐existence of C—H...F and C—H...O=C hydrogen bonds makes possible the formation of a structure with just one independent molecule.     

One-pot synthesis of poly-substituted tetramic acids for the preparation of putative turn mimics

A one-pot synthesis of poly-substituted tetramic acids and of their six-membered ring analogs have been obtained in one step by reaction of N-Boc dipeptides, activated as their O-succinimidyl esters (Boc-AA-AA-OSu), with the sodium anion of dibenzyl malonate. The adducts spontaneously cyclize to form five or six-member rings. To check whether this class of compounds may be used to promote reverse-turn conformations, one adduct was further derivatized. The formation of a hydrogen bond between the NH–Boc and the carbonyl at C2 of the heterocycle is highlighted, upon analysis of the product by IR, ¹H NMR, and MD techniques, thus suggesting that these compounds are good candidates to promote reverse-turn conformations or other secondary structures and may be used for the formation of new foldamers.     

Cyclometallated compounds. XV.

The title complex, tetra‐μ‐acetato‐O:O′‐bis{[μ‐1,4‐bis(2‐­pyridyl­oxy)­phenyl­ene‐N,C²:N′,C⁶]dipalladium(II)} bis­(tri­chloro­methane) dihydrate, [Pd₄(C₁₆H₁₀N₂O₂)₂(C₂H₃O₂)₄]·2CHCl₃·2H₂O, the product of the reaction of 1,4‐bis(2‐pyridyl­oxy)­benzene with palladium acetate, is shown to be a tetranuclear, rather than a polymeric, species. It crystallizes about a centre of inversion and has two doubly cyclo­palladated ligands bridged by four acetate groups. The cyclo­palladated ligand is far from planar in the complex and has the central benzene rings π‐stacked. The chelate rings exist in shallow boat conformations.     

Deconstructive Reorganization: De Novo Synthesis of Hydroxylated Benzofuran

An unprecedented deconstructive reorganization strategy for the de novo synthesis of hydroxylated benzofurans from kojic acid‐ or maltol‐derived alkynes is reported. In this reaction, both the benzene and furan rings were simultaneously constructed, whereas the pyrone moiety of the kojic acid or maltol was deconstructed and then reorganized into the benzene ring as a six‐carbon component. Through this strategy, at least one free hydroxyl group was introduced into the benzene ring in a substitution‐pattern tunable fashion without protection–deprotection and redox adjustment. With this method, a large number of hydroxylated benzofuran derivatives with different substitution‐patterns have been prepared efficiently. This methodology has also been shown as the key step in a collective total synthesis of hydroxylated benzofuran‐containing natural products (11 examples).