Arene Ring Research Articles

Induction of Λ-helicity in a zinc complex with an alanine-appended aminopyridine ligand.

The crystal structure of tris[dimethyl 5-({1-[(pyridin-2-yl-κN)carbamoyl-κO]ethyl}carbamoyl)benzene-1,3-dicarboxylate]zinc(II) dinitrate acetonitrile trisolvate, [Zn(C19H19N3O6)3](NO3)2·3CH3CN or [Zn(L)3](NO3)2·3CH3CN, (1), has been determined by single-crystal X-ray diffraction. The neutral ligand L coordinates to the Zn2+ cation in a bidentate fashion via the pyridine N atom and an amide O atom, forming a six-membered chelate ring. The Λ-helical chirality of the Zn2+ coordination sphere is induced by pendant L-alanine residues through stacking interactions between the arene groups of two coordinated ligands, assisted by a hydrogen bond between amide groups bonded to the stacked arene rings. The third ligand is coordinated to the Zn2+ cation by the same six-membered chelate ring, but in the opposite direction with respect to the analogous chelate rings of the first two coordinated ligands. Besides ionic interactions between [ZnL3]2+ complexes and NO3- anions, several types of hydrogen bonds and intermolecular stacking interactions contribute to the stability of the solid-state phase.

Synthesis and Application of Silylene-Stabilized Low-Coordinate Ag(I)–Arene Cationic Complexes

We report the first examples of N-heterocyclic silylene-stabilized monocoordinate Ag(I) cationic complexes weakly bound to the free arene rings (C6H6, C6Me6, and C7H8). Further, the application of these electrophilic Ag(I) complexes as catalysts has been investigated toward A3-coupling reactions, which afforded a series of propargylamines in good to excellent yields with low catalyst loading under a solvent-free condition (19 examples).

Distinct roles of Ag(I) and Cu(II) as cocatalysts in the intramolecular cyclization of N-methyl-N-phenylanthranilic acid: A theoretical investigation

• The selectivity is controlled by the additive employed in this reaction. • The silver oxide promotes C(sp 3 )−H activation and C−O cyclization to provide 1,2-dihydro-(4H)−3,1-benzoxazin-4-one. • The copper acetate contributes to C(sp 2 )−H activation and decarboxylative arylation to afford carbazole. • The origins of the selectivity could be attributed to the cation-π interaction between Ag and the substrate arene ring. The mechanisms of the intramolecular cyclization of N-methyl-N-phenylanthranilic acid catalyzed by Pd(OAc) 2 were investigated by employing the density functional theory (DFT) calculations. The selectivity is controlled by the additive employed in this reaction. Silver oxide promotes C(sp 3 )−H activation and C−O cyclization to provide 1,2-dihydro-(4H)-3,1-benzoxazin-4-one, while copper acetate contributes to C(sp 2 )−H activation and decarboxylative arylation to afford carbazole. The origins of the selectivity could be attributed to the cation-π interaction between Ag and the substrate arene ring, which significantly lowers the activation energy for C(sp 3 )−H deprotonation. The distinct roles of Ag(I) and Cu(II) as cocatalysts were clarified through our calculations.

Back Cover: Bowl‐Shaped Pentagon‐ and Heptagon‐Embedded Nanographene Containing a Central Pyrrolo[3,2‐b]pyrrole Core (Angew. Chem. Int. Ed. 27/2021)

A bowl-shaped nitrogen-doped nanographene composed of a pyrrolo[3,2-b]pyrrole core substituted with six arene rings circularly bonded with one another undergoes bowl-to-bowl inversion with a low barrier. This inversion resembles that of an umbrella flipped by the strong wind. The transition state is not planar, but possesses a rather twisted, S-shaped conformation with Ci symmetry. The bowl-inversion mechanism follows a quadruple-well potential model, as reported by Andrzej L. Sobolewski, Michał K. Cyrański, Daniel T. Gryko, and co-workers in their Research Article on page 14998.

Bowl‐Shaped Pentagon‐ and Heptagon‐Embedded Nanographene Containing a Central Pyrrolo[3,2‐b]pyrrole Core

AbstractA bowl‐shaped nitrogen‐doped nanographene composed of a pyrrolo[3,2‐b]pyrrole core substituted with six arene rings circularly bonded with one another has been prepared via a concise synthetic strategy encompassing the multicomponent tetraarylpyrrolopyrrole (TAPP) synthesis, the Scholl reaction, and intramolecular direct arylation. This synthesis represents the first case of programmed sequential intramolecular direct arylation reactions utilizing the different reactivity of C–Br and C–Cl bonds. The target compound contains two central pentagons confined between two adjacent heptagons—the inverse Stone–Thrower–Wales topology. The presence of both five‐ and seven‐membered rings in the final structure is responsible for interesting properties such as a perpendicularly aligned dipole moment, absorption and fluorescence in the orange‐red region, weak emission originating from the charge‐transfer character of a low‐energy absorption band, and a high lying HOMO. In the solid state slipped convex‐to‐convex π–π stacking dominates.

Bowl-Shaped Pentagon- and Heptagon-Embedded Nanographene Containing a Central Pyrrolo[3,2-b]pyrrole Core.

A bowl-shaped nitrogen-doped nanographene composed of a pyrrolo[3,2-b]pyrrole core substituted with six arene rings circularly bonded with one another has been prepared via a concise synthetic strategy encompassing the multicomponent tetraarylpyrrolopyrrole (TAPP) synthesis, the Scholl reaction, and intramolecular direct arylation. This synthesis represents the first case of programmed sequential intramolecular direct arylation reactions utilizing the different reactivity of C-Br and C-Cl bonds. The target compound contains two central pentagons confined between two adjacent heptagons-the inverse Stone-Thrower-Wales topology. The presence of both five- and seven-membered rings in the final structure is responsible for interesting properties such as a perpendicularly aligned dipole moment, absorption and fluorescence in the orange-red region, weak emission originating from the charge-transfer character of a low-energy absorption band, and a high lying HOMO. In the solid state slipped convex-to-convex π-π stacking dominates.

Highly Branched Pillar[5]arene-Derived Porous Aromatic Frameworks (PAFs) for Removal of Organic Pollutants from Water.

The adsorption process is widely used for the treatment of wastewater containing organic pollutants. We fabricated highly branched pillar[5]arene-based porous aromatic frameworks (PAFs), PAF-P5, for the adsorption and removal of organic pollutants (short-chain alkyl derivatives 1-3 and pesticide molecules 4-6) from water with high removal efficiency (RE). However, PAF-P5 was incapable of adsorbing aromatic organic dyes 7-9. Adsorption kinetic studies indicated that the adsorption is mainly driven by strong host-guest interactions between 1-3 and the pillar[5]arene units in PAF-P5, while 4-6 only weakly interacted with the pillar[5]arene units in PAF-P5. Moreover, chemically breaking down the pillar[5]arene rings in PAF-P5 caused changes in the pore size, the microenvironment inside of the pores, and the frame morphology, and the resultant frameworks, PAF-DeP5, exhibited poor adsorption toward 1-6 but adsorbed 7-9 possibly through physical adsorption as implied by fitting the experimental data into the adsorption kinetic models.

1,4-Dibromo-2,5-bis(phenylalkoxy)benzene Derivatives: C–Br...π(arene) Versus C–H...Br and Br...Br Interactions in the Solid State

We have prepared and characterized 1,4-dibromo-2,5-bis(2-phenylethoxy)benzene (1) and 1,4-dibromo-2,5-bis(3-phenylpropoxy)benzene (2). Their single-crystal structures confirm that, at the molecular level, they are similar with the phenylalkoxy chains in extended conformations. However, there are significant differences in packing interactions. The packing in 1 is dominated by C–Br...π(arene) interactions, with each Br located over one C–C bond of the central arene ring of an adjacent molecule. In contrast, the packing of molecules of 2 involves a combination of C–H...Br hydrogen bonds, Br...Br interactions, and arene–arene π-stacking. The single-crystal structures of both orthorhombic and triclinic polymorphs of 1 have been determined and the packing interactions are shown to be essentially identical.

Formation of 1,2,4-oxadiazoles in the course of photooxidation of aromatic azides in acetonitrile

Photolysis of aryl azides at room temperature in acetonitrile in the presence of oxygen proceeds as arene ring opening and acetonitrile trapping to afford 5-methyl-3-(5-oxopenta-1,3-dien-1-yl)-1,2,4-oxadiazoles. In the case of 4-methoxyphenyl azide, a product depriving of acetonitrile reactant, 1-hydroxyimino-3-methylcyclopentadiene-2-carbaldehyde, is also formed.

Thermally Responsive Poly(ethylene oxide)-Based Polyrotaxanes Bearing Hydrogen-Bonding Pillar[5]arene Rings.

Invited for the cover of this issue are Tomoki Ogoshi and co-workers at Kyoto University, Kanazawa University and Tokyo University of Agriculture and Technology. The image depicts musical notation to represent hydrogen bond networks and poly(ethylene oxide) chains. Read the full text of the article at 10.1002/chem.202005099.

Crystal structure of tamsulosin hydrochloride, C20H29N2O5SCl

The crystal structure of tamsulosin hydrochloride has been solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional techniques. Tamsulosin hydrochloride crystallizes in space groupP21(#4) witha= 7.62988(2),b= 9.27652(2),c= 31.84996(12) Å,β= 93.2221(2)°,V= 2250.734(7) Å3, andZ= 4. In the crystal structure, two arene rings are connected by a carbon chain oriented roughly parallel to thec-axis. The crystal structure is characterized by two slabs of tamsulosin hydrochloride molecules perpendicular to thec-axis. As expected, each of the hydrogens on the protonated nitrogen atoms makes a strong hydrogen bond to one of the chloride anions. The result is to link the cations and anions into columns along theb-axis. One hydrogen atom of each sulfonamide group also makes a hydrogen bond to a chloride anion. The other hydrogen atom of each sulfonamide group forms bifurcated hydrogen bonds to two ether oxygen atoms. The powder pattern is included in the Powder Diffraction File™ as entry 00-065-1415.

Thermally Responsive Poly(ethylene oxide)-Based Polyrotaxanes Bearing Hydrogen-Bonding Pillar[5]arene Rings*.

Poly(ethylene oxide)s (PEOs) are useful polymers with good water solubility, biological compatibility, and commercial availability. PEOs with various end groups were threaded into pillar[5]arene rings in a mixture of water and methanol to afford pseudopolyrotaxanes. Corresponding polyrotaxanes were also constructed by capping COOH-terminated pseudopolyrotaxanes with bulky amines, in which multiple hydrogen bonds involving the pillar[5]arene OH groups were critically important to prevent dethreading. The number of threaded ring components could be rationally controlled in these materials, providing a simple and versatile method to tune the mechanical and thermal properties. Specifically, a polyrotaxane with a high-molecular-weight axle became elastic upon heating above the melting point of PEOs and exhibited temperature-dependent shape memory property because of the topological confinement and crosslinked hydrogen bonds.

Tetrakis-1,2,4,5-(bis(3,5-dimethylphenyl)phosphino)benzene (Me16tpbz): A soluble and spectroscopically simple variant of the 1,2,4,5-tetraphosphinobenzene ligand platform

A preparation of 1,2,4,5-tetrakis(bis(3,5-dimethylphenyl)phosphino)benzene (Me16tpbz) has been devised as a two-step synthesis beginning with bis(3,5-dimethylphenyl)chlorophosphine and 1,2,4,5-tetrafluorobenzene, albeit in low overall yield of 10%. The crystal structure of Me16tpbz reveals slight pushing of the phosphorus atoms above and below the central arene ring, suggesting the effect of slight steric pressure, as compared to the fully planar C6P4 core found in 1,2,4,5-tetrakis(diphenylphosphino)benzene (tpbz). Upon exposure to air, solution samples of Me16tpbz degrade to (3,5-Me2C6H3)2PP(O)(C6H3-3,5-Me2)2, identified by X-ray crystallography, which suggests P–C bond scission is a decomposition pathway. The dinickel compound [(mnt)Ni(Me16tpbz)Ni(mnt)] (mnt(2–) = [(NC)2C2S2]2–) has been structurally characterized by X-ray crystallography and reveals a chair-like conformation defined by a 32.3° angle between the NiP2 planes with respect to the central C6P4 mean plane. Compared to the analogous [(mnt)Ni(tpbz)Ni(mnt)], slightly weaker binding of Me16tpbz to Ni is implied by its modestly longer Ni–P bond lengths. The cyclic voltammogram of [(mnt)Ni(Me16tpbz)Ni(mnt)] in CH2Cl2 shows reversible reductions at ~ −1.01 V and −1.21 V, corresponding to sequential Ni2+ + e– → Ni1+ processes, while in DMF, under conditions where [(mnt)Ni(tpbz)Ni(mnt)] shows multiple, reversible reduction processes, its behavior is irreversible and more complex.

Δ9-Tetrahydrocannabinolic acid A, the precursor to Δ9-tetrahydrocannabinol (THC).

While Δ9-tetrahydrocannabinolic acid A (THCA-A) has been reported to be difficult to crystallize and/or amorphous, we have obtained THCA-A in a pure crystalline form by extraction of marijuana and selective fractionation with liquid CO2. THCA-A (systematic name: 1-hydroxy-6,6,9-trimethyl-3-pentyl-6a,7,8,10a-tetrahydro-6H-benzo[c]isochromene-2-carboxylic acid, C22H30O4) crystallizes in the orthorhombic space group P212121, with Z= 8 and Z'= 2. The two independent molecules are related by a pseudo-twofold axis centered between the two -CO2H groups, but the conformations of the two -C5H11 chains are quite different (tgt and ttg; t is trans and g is gauche). The carboxylate groups form an intermolecular R22(8) hydrogen-bonded ring; the two C2O2 carboxylate planes are twisted out of the planes of the attached arene rings in opposite directions by 13.59 (8) and 18.92 (8)°, respectively, with a resultant interplanar angle of 28.89 (8)°. Each molecule also has an intramolecular S(6) hydrogen-bond motif between the ortho -OH group and the dihydropyran-ring O atom. Other conformational aspects of the two independent molecules are quite similar to those found in the previously determined structure of THCA-B. THCA-A has shown promise in a number of medical applications. Demonstration of the crystallinity and details of the crystal structure are expected to provide a standard point of departure for chemical and medical experiments.

Polypseudorotaxanes constructed from pillar[5]arenes and polyamides by interfacial polymerization.

Polypseudorotaxanes constructed from pillar[5]arene rings and polyamide chains were successfully synthesized by interfacial polymerization between diamines and dicarbonyl chlorides in the presence of pillar[5]arene. The dicarbonyl chloride length and the assocation constants of dicarbonyl chloride-pillar[5]arene complexes were important factors in producing polypseudorotaxanes with high cover ratio of pillar[5]arene rings.

Selective cleavage of unactivated arene ring C-C bonds by iridium: key roles of benzylic C-H activation and metal-metal cooperativity.

The cleavage of aromatic C–C bonds is central for conversion of fossil fuels into industrial chemicals and designing novel arene functionalisations through ring opening, expansion and contraction. However, the current progress is hampered by both the lack of experimental examples of selective oxidative addition of aromatic C–C bonds and limited understanding of the factors that favour insertion into the C–C rather than the C–H bonds. Here, we describe the comprehensive mechanism of the only reported chemo- and regioselective insertion of a transition metal into a range of substituted arene rings in simple iridium(i) complexes. The experimental and computational data reveal that this ring cleavage requires both reversible scission of a benzylic C–H bond and cooperativity of two Ir centres sandwiching the arene in the product-determining intermediate. The mechanism explains the chemoselectivity and scope of this unique C–C activation in industrially important methylarenes and provides a general insight into the role of metal–metal cooperativity in the cleavage of unsaturated C–C bonds.

Optically active Pt-terpyridyl coordination assemblies derived from planar chiral metallothioligands

Optically active halogenated arenes containing stereogenic benzylic alcohol reacts with [CpRu(CH3CN)3][PF6] to give the major diastereomer with 90% de, in which the metal center is preferentially placed on one face of the arene ring. Thus starting with (S)-1-(2-chlorophenyl)ethanol the metal complex (pS, S)-[CpRu(HO(CH3)CH-C6H4-Cl)][PF6] (pS, S)-1 was obtained in good yield, while the (R)-alcohol provided (pR, R)-[CpRu(HO(CH3)CH-C6H4-Cl)][PF6] (pR, R)-1 metal complex. These compounds display planar and central chiralities. For comparison purposes the analogous racemic compound (rac)-[CpRu(HO(CH3)CH-C6H4-Cl)][PF6] (rac)-1 was also prepared in good yield as well. Subsequent treatment of enantiopure and racemic complexes 1 with NaSH in THF allows halogen displacement to occur at room temperature and generates the related enantiopure neutral metallothioligands of the type (pS, S)-[CpRu(HO(CH3)CH-C6H4-S)], (pS, S)-2, (pR, R)-[CpRu(HO(CH3)CH-C6H4-S)] (pR, R)-2, and (rac)-[CpRu(HO(CH3)CH-C6H4-S)] (rac)-2 (S = sulfur). These metallothioligands react rapidly with [(MeCN)Pt(terpy)][OTf]2 building blocks to give the chiral coordination assemblies [Pt(terpy)-(pS,S)-2][OTf]2 (pS,S)-3, [Pt(terpy)-(pR,R)-3][OTf]2 (pR,R)-3 and the related racemic complex (rac)-3 in which the metal-chromophore is now placed in a chiral environment. The molecular structures of (rac)-3 and the enantiopure heterobimetallic complex (pR,R)-3 were determined and show different arrangements at the supramolecular level depending on whether a homochiral or heterochiral assembly is obtained. The induced circular dichroism curves for the right-handed (pR,R)-3 and left-handed (pS,S)-3 bimetallic complexes displayed mirror image Cotton bands, confirming the enantiomeric relationship between both compounds. Our method provides an entry to the preparation of a wide range of optically pure coordination compounds with potentially important properties.

Dibenzoate esters of cis-tetralin-2,3-diol as analogs of (-)-epigallocatechin gallate: synthesis and crystal structure of anticancer drug candidates.

(-)-Epigallocatechin gallate (EGCG), the main component of green tea extract, displays multiple biological activities. However, it cannot be used as a drug due to its low cellular absorption, instability and metabolic degradation. Therefore, there is a need to provide analogs that can overcome the limitations of EGCG. In this work, six synthetic analogs of EGCG sharing a common tetralindiol dibenzoate core were synthesized and fully characterized by 1H NMR, 13C NMR, HRMS and IR spectroscopies, and X-ray crystallography. These are (2R,3S)-1,2,3,4-tetrahydronaphthalene-2,3-diyl bis[3,4,5-tris(benzyloxy)benzoate], C66H56O10, and the analogous esters bis(3,4,5-trimethoxybenzoate), C30H32O10, bis(3,4,5-trifluorobenzoate), C24H14F6O4, bis[4-(benzyloxy)benzoate], C38H32O6, bis(4-methoxybenzoate), C26H24O6, and bis(2,4,6-trifluorobenzoate), C24H14F6O4. Structural analysis revealed that the molecular shapes of these dibenzoate esters of tetralindiol are significantly different from that of previously reported dimandelate esters or monobenzoate esters, as the acid moieties extend far from the bicyclic system without folding back over the tetralin fragment. Compounds with small fluorine substituents take a V-shape, whereas larger methoxy and benzyloxy groups determine the formation of an L-shape or a cavity. Intermolecular interactions are dominated by π-π stacking and C-H...π interactions involving the arene rings in the benzoate fragment and the arene ring in the tetrahydronaphthalene moiety. All six crystal structures are determined in centrosymmetric space groups (either P-1, P21/n, C2/c or I2/a).

A highly selective anthraquinone appended oxacalixarene receptor for fluorescent ICT sensing of F− ions: an experimental and computational study

A new anthraquinone appended oxacalix[4]arene (DAQOC) has been synthesized and characterized by different 1H NMR, IR, ESI-MS and 13C NMR spectroscopic techniques. This compound was explored for its sensing abilities towards various anions. DAQOC showed selective anion sensing behaviour towards F− ions which was supported by absorption as well as emission studies. Among other anions, only in the presence of F− ions, a quenching in the fluorescence emission of over 79% was observed due to changes in the intermolecular charge transfer (ICT) process. DAQOC exhibited high selectivity and good sensitivity toward F− ions in the presence of competing ions and the detection limit was found to be 1.23 µM. 1H NMR titration displays that the peak corresponding to –NH protons (at 12.91 ppm) disappears upon interaction with F−, suggesting that the sensing mechanism follows the deprotonation route. The geometrical features of F− bound oxacalixarene species were modelled by the Density Functional Theory (DFT) and NCIPlot calculations. The findings suggested that the appended substituents including nitro groups and anthraquinone can make the calix[4]arene ring electron deficient and thereby more susceptible for F− ions. Moreover, this present chemosensor has been applied for recognition of F− ions from waste water samples which is of direct practical relevance. A new anthraquinone appended oxacalix[4]arene has been explored for its sensing abilities towards F− by absorption and emission studies. An intermolecular charge transfer mechanism has been attributed due to the deprotonation of N–H groups in DAQOC. DAQOC-F− complex is modelled by the Density Functional Theory and NCIPlot calculations.

A stimuli-responsive pillar[5]arene-based hybrid material with enhanced tunable multicolor luminescence and ion-sensing ability.

Tunable luminescent materials are becoming more and more important owing to their broad application potential in various fields. Here we construct a pillar[5]arene-based hybrid material with stimuli-responsive luminescent properties and ion-sensing abilities from a pyridine-modified conjugated pillar[5]arene and a planar chromophore oligo(phenylenevinylene) upon coordination of Cd (II) metal cores. This new material not only shows an optimized luminescence due to the minimized π–π stacking and efficient charge transfer properties benefitting from the existence of pillar[5]arene rings, but also exhibits tunable multicolor emission induced by different external stimuli including solvent, ions and acid, indicating great application potential as a fluorescent sensory material, especially for Fe3+. With this pillar[5]arene-based dual-ligand hybrid material, valid optimization and regulation on the fluorescence of the original chromophore have been achieved, which demonstrates a plausible strategy for the design of tunable solid-state luminescent materials and also a prototypical model for the effective regulation of fluorescent properties of planar π systems using synthetic macrocycle-based building blocks.