Solvent-Dependent Conformational Switching of N-Methyl-N,N'-diarylsquaramide.

N,N'-Diarylsquaramide and N,N'-dialkylsquaramide are conformationally stable linkers with extended (trans, trans) and folded (cis, cis) structures, respectively, independently of external conditions. Here, we show that N-monomethylated N,N'-diarylsquaramides generally take a (trans, cis) structure in the crystal but show a solvent-dependent conformational equilibrium in solution. In particular, the stable conformer of N-methyl-N,N'-bis(1-naphthyl)squaramide (1f) changes depending upon the solvent. Thus, aromatic N-monomethylated squaramides could find application as components of environment-responsive molecular switches.

1T/1H-SnS2 Sheets for Electrochemical CO2 Reduction to Formate.

Understanding the catalytic mechanism of highly active two-dimensional electrocatalysts is crucial to their rational design. Herein, we reveal the element dependence of the reactivity of two-dimensional metal dichalcogenide sheets for electrocatalytic CO2 reduction. We found that tin(IV) disulfide (SnS2) and molybdenum(IV) disulfide (MoS2) sheets exhibited Faradaic efficiencies of 63.3% and ∼0%, respectively, for formic acid. Scanning electrochemical cell microscopy and theoretical calculations were used to identify the catalytically active sites of SnS2 as terraces and edges. Owing to the effective utilization of the entire surface area, SnS2 can effectively accelerate catalytic reactions. This finding provides a direction for material research in two-dimensional electrocatalysts for energy-efficient chemical production from electrochemical CO2 reduction, as well as for other energy devices.

Conformational Properties of Aromatic Amides Bearing Imidazole Ring and Acid-Induced Trans-Cis Amide Switching.

Aromatic amides bearing secondary amide bond exist in trans conformation both in the crystal and in solution, whereas the conformation of the N-methylated derivatives is cis in the crystal and predominantly cis in various solvents. The cis conformational preference of N-alkylated benzanilide provides access to aromatic foldamers such as oligo(N-alkyl-p-benzamide)s, which adopt dynamic helical structures. Here, the conformational properties of imidazole-substituted amide in the crystal and in solution were examined. Imidazole-substituted amides 2a and 4a existed mainly in the cis conformation in solution. The ratio of the cis conformer of N-methyl-N-(1-methyl-1H-imidazol-4-yl)benzamide (4a) was smaller than that of N,1-dimethyl-N-phenyl-1H-imidazole-2-carboxamide (2a) or N-methylbenzanilide, but the introduction of a substituent strongly affected the conformer ratio. Compounds 6a and 7a bearing an electron-withdrawing group on the imidazole ring existed predominantly in trans form. On the other hand, the introduction of an electron-withdrawing group on the phenyl ring or a bulky substituent on the amide nitrogen of 4a increased the ratio of cis conformer. Further, the major conformer of N-alkylated N-imidazolylamides was switched from cis to trans by the addition of acid. These results suggest that imidazole-substituted amides might be applicable as conformational switches in aromatic foldamers to enable environment-dependent structural change.

Accessing Improbable Foldamer Shapes with Strained Macrocycles.

The alkylation of some secondary amide functions with a dimethoxybenzyl (DMB) group in oligomers of 8-amino-2-quinolinecarboxylic acid destabilizes the otherwise favored helical conformations, and allows for cyclization to take place. A cyclic hexamer and a cyclic heptamer were produced in this manner. After DMB removal, X-ray crystallography and NMR show that the macrocycles adopt strained conformations that would be improbable in noncyclic species. The high helix folding propensity of the main chain is partly expressed in these conformations, but it remains frustrated by macrocyclization. Despite being homomeric, the macrocycles possess inequivalent monomer units. Experimental and computational studies highlight specific fluxional pathways within these structures. Extensive simulated annealing molecular dynamics allow for the prediction of the conformations for larger macrocycles with up to sixteen monomers.

Construction of Aromatic Multilayered Structures Based on the Conformational Properties of N,N'-Dimethylated Squaramide.

Squaramide is a highly rigid four-membered ring system bearing two carbonyl and two amino groups, and its derivatives have found applications in many fields. We synthesized several N,N'-dimethylated oligosquaramides linked via phenyl groups, and investigated their structures in the crystalline state and in solution. Compounds 1, 2 (bissquaramides), and 13 (trissquaramide) exist as folded structures in the crystalline state, in which the aromatic rings are located in a face-to-face position. In their multilayered structures, the benzene rings are more nearly parallel and are closer to each other, compared with those in N,N'-dimethylated oligoureas. Individual molecules of meta-connected compounds 2 and 13 show a helical structure with all-R or all-S axis chirality, but afford only racemic crystals. The NMR spectra indicated that these compounds retain well-ordered folded structures in solution. The unique steric and electronic properties of N,N'-dimethylated squaramide can provide access to novel functional aromatic multilayered and helical foldamers.

Oxaphosphacyclophanes Constructed from a Bis(triphenylphosphine oxide) Moiety Linked by Dioxyalkyl Chains: Synthesis and Crystal Structures.

Oxaphosphacyclophanes, a bis(triphenylphosphine oxide) moiety linked by dioxypropane, dioxybutane, dioxypentane, and dioxyhexane, were synthesized from the reaction of bis(4-hydroxyphenyl)phenylphosphine oxide and dibromoalkanes. Single-crystal X-ray diffraction analysis and exploring intermolecular interactions by using the Hirshfeld surface revealed that the butylene-linked oxaphosphacyclophane (2) had a one-dimensional (1D) supramolecular structure via CH/π interactions between the terminal phenyl group of 2 and a phenylene group of the cavities. In the packing diagram of pentylene-linked oxaphosphacyclophane (3) with antiform, one macrocycle recognized two terminal phenyl groups to form a 1D supramolecular chain via π/π interactions. The syn-form of 3 was obtained by the recrystallization from toluene and CHCl3. In this crystal, a toluene molecule avoid construction of a 1D chain. The antiform of hexylene-linked oxaphosphacyclophane 4 was obtained by the recrystallization from cyclohexane and CHCl3. Cyclohexane was captured in the cavity via CH/π interactions. Two molecules of 4 with the syn-form recognized each other to construct a dimeric structure via CH/π interactions.

Synthesis and chiroptical properties of cylindrical macrocycles comprising two calix[3]aramide moieties.

Calix[3]aramide-based cylindrical macrocycles were synthesized by the one-step amide coupling reaction of a monomer containing two meta-alkylaminobenzoic acid units linked by para-phenylene bridges. The major products included a meso-form and an enantiomeric pair, with stereochemistry derived from the direction of the amide bonds and their fixed conformation. Mirror-image ECD, VCD, and CPL spectra were observed in the enantiomeric pair and the absolute structure was determined by comparing measured and calculated ECD and VCD spectra.

Hollow and Solid Spheres Assembled from Functionalized Macrocycles Containing Adamantane.

An adamantane-based macrocycle possessing eight hydroxyl groups (1) was synthesized, in which the macrocyclic framework comprises two disubstituted adamantane molecules bearing phenyl derivatives connected to two biphenylene spacers by oxygen atoms. Furthermore, functionalized macrocycles containing methyl (2) and methoxycarbonylmethyl (3) groups were prepared. From the X-ray crystallographic analysis, the backbone of the macrocycles in all crystals had a nearly hexagonal shape with a cavity and these macrocycles could be arranged into different tubular structures dependent on the substituents. In acetone, macrocycle (1) formed stable hollow spherical aggregates with multilayer membranes. In contrast, macrocycle (3) exhibited no production of self-assembled materials in chloroform. The addition of hexane into the solution caused the generation of solid spheres and their fused network aggregates, which were finally transformed into crystals owing to the solvent effects.

Coordination structure and extraction behavior of a silver ion with N-substituted-9-aza-3,6,12,15-tetrathiaheptadecanes: significant effect of Ph-C-N framework on the extractability.

N-Substituted-9-aza-3,6,12,15-tetrathiaheptadecanes having Ph-C-N frameworks (N-R-ATH; R = benzyl (N-Bn-ATH), 4-nitrobenzyl (N-NO2Bn-ATH), and diphenylmethyl (N-Ph2CH-ATH)) were synthesized, and their Ag(i) complexes were structurally characterized. X-Ray crystal structure analyses of [Ag(N-R-ATH)](BF4) (R = Bn and Ph2CH) revealed monomeric tetra-S-coordinated complex cation structures without the N-coordination, and a benzene ring of the N-R group covered over the amine nitrogen atom. The precise extraction analyses of a Ag(i) ion with ATH derivatives (L = N-R-ATHs and N-H-ATH) associated with the (1)H NMR analyses of the [Ag(L)](+) complexes in polar and non-polar solvents revealed that the introduction of the N-substituent significantly enhanced the extractability of Ag(+), due to the "hydrophobic cover" effect by the Ph-C-N framework in the [Ag(N-R-ATH)](+) complexes.

Control of a Chiral Property of a Calix[3]aramide: The Racemization Suppressed by Intramolecular Cyclic Hydrogen Bonds and DMSO-H2O System-Induced Spontaneous Resolution.

A calix[3]aramide has been synthesized bearing three triphenylphosphinic amide moieties, which formed intramolecular cyclic hydrogen bonds that suppressed its cone/partial cone inversion. The intramolecular cyclic hydrogen bonds were disrupted by DMSO, and the insertion of H2O into the hydrogen bonds triggered the spontaneous resolution of the calix[3]aramide. Within a chiral environment, such as that afforded by the presence of optically active 2-butanol, the calix[3]aramide underwent a symmetry breaking crystallization process.

Multilayered inclusion nanocycles of anionic spiroborates.

Multilayered spiroborate nanocycles were prepared from tris- or tetrakis(dihydroxynaphthalene) and tetrahydroxyanthraquinone as pillar and crossbar units via the reversible formation of a spiroborate linkage. The four-layered spiroborate nanocycle recognized two cationic aromatic guests simultaneously and exhibited the ability to form a supramolecular one-dimensional array by combining with methyl viologen dimer as the ditopic guest.

Nestable Tetrakis(spiroborate) Nanocycles.

Multicomponent construction of the tetrakis(spiroborate) anionic nanocycles was achieved by reacting bis(dihydroxynaphthalene)s with tetrahydroxyanthraquinone in the presence of boric acid in a self-organized manner. These nanocycles exhibited selective molecular recognition behavior toward cationic guests such as methyl viologen derivatives. Formation of a supramolecular ring@ring and a guest@ring@ring structure was observed by combining the anionic nanocycle and the vinylogous analog of cyclobis(paraquat-p-phenylene).

Adamantane-based oxacyclophanes containing pyrazines: synthesis, crystal structure, and self-assembly behavior.

Two adamantane-based oxacyclophanes were synthesized by the SNAr reaction of bis-phenol derivatives based on adamantane with 2,6-dichloropyrazine. Their crystal structures had distorted or twisted 1,3-alternate conformations. The oxacyclophanes indicated no formation of spherical aggregates in acetone. However, addition of water into the solution caused the generation of hollow spherical aggregates with a multilayer membrane. Over time, the hollow spheres were induced into fibrous and network assemblies, which were eventually transformed into single crystals.

[Synthesis and structure of silver(I) coordination polymers with bis(pyridyl) ligands linked by an aromatic sulfonamide].

Aromatic sulfonamides exist in a synclinal conformation with the twisted structure arising from rotation around the S-N bond in both the solid state and in solution. Simple bidentate ligands containing the sulfonamide moiety can be extended to form elongated ligands, and optically active components can be added to form a versatile building block for the construction of coordination polymers with many structures. Mixing the simple ligands 1 and 2 and the elongated ligands 3 and 4 with different Ag(I) salts yielded the corresponding complexes [Ag(1)OTf]n (1a), [Ag(2)]n•nOTf(2a), [Ag(3)OTf]n (3a), [Ag(3)]n•nBF4 (3b), [Ag(4)CH3CN]n•nBF4•nCHCl3 (4b), and [Ag(4)]n•nSbF6•nCH4O (4c). Straight chains and racemic helical polymers were observed in the crystal structure of complexes 1a and 2a, respectively. In the crystal structures of complexes 3a and 4b, infinite 1D straight chains containing a T-shaped coordination geometry about the Ag(I) centers were formed by the reaction of ligands 3 or 4 with Ag(I) salts in CH3CN/CHCl3. A continuous 1D coordination polymer containing a racemic mixture of left- and right-handed helices formed in the crystal structure of complex 3b. Furthermore, a layered coordination polymer consisting of a racemic mixture of left- and right-handed polymers was observed from the crystal structure of complex 4c. The construction of optically pure left- or right-handed 1D helical polymers via the introduction of chiral functional groups on the nitrogen atom of the sulfonamide ligand is currently under investigation in our laboratory.

Acid-mediated ring-expansion reaction of N-aryl-2-vinylazetidines: synthesis and unanticipated reactivity of tetrahydrobenzazocines.

The aza-Clasen rearrangement of N-aryl-2-vinylazetidines has been explored. N-Aryl-2-vinylazetidines were transformed to corresponding tetrahydrobenzazocines in good yields. Unexpectedly, the tetrahydrobenzazocine was unstable and readily isomerized to vinyltetrahydroquinoline in the presence of acid. The mechanism of this ring contraction was studied in detail.

Helicity Induction in Three π-Conjugated Chromophores by Planar Chirality of Calixamide.

Shall we twist? Three-dimensional arrangement of π-conjugated chromophores with triple-stranded helicity was achieved by using the planar chirality of m-calix[3]amide. Based on spectroscopic data and theoretical calculations, the dynamic and preferred helical characters of bithiophene units embedded in the tubular molecule were elucidated, and the absolute configuration was determined.

Trifluoroacetylation-induced Houben-Hoesch-type cyclization of cyanoacetanilides: increased nucleophilicity of CN groups.

Trifluoroacetic anhydride-mediated tandem trifluoroacetylation/cyclization of cyanoacetanilides proceeded efficiently under mild conditions to give 4-hydroxy-3-trifluoroacetylquinolin-2(1H)-ones in good yields. Isolation and direct observation of the reaction intermediates revealed that alpha-trifluoroacetylation resulted in the shortening of C[triple bond]N bonds and that the electron density of CN groups was high. A plausible reaction mechanism based on the results is also described.

Effect of aromatic-aromatic interactions on the conformational stabilities of macrocycle and preorganized structure during macrocyclization.

The crystal structures and dynamic behavior in solution of aromatic amides containing terphenyl groups were revealed by X-ray crystallographic analysis and VT-NMR measurements. Controlling the synthetic yield and molecular shape of the macrocycles with intramolecular aromatic-aromatic interactions was successfully achieved. N,N'-Diethyl-N,N'-diterphenyl-1,3-benzenedicarboxamide (2) exists in the syn conformation and two terphenyl groups are on the same side of the plane of the central benzene ring due to attractive aromatic-aromatic interactions between the two terphenyl moieties. The yield of the macrocyclization reaction of 1,3-benzenedicarboxylic acid with bis(ethylamino)terphenyl (4) was relatively high (55% yield) because an intermediate in the macrocyclization reaction was preorganized in the syn conformation, which is similar to the diamide 2. On the other hand, N,N'-diethyl-N,N'-diterphenyl-1,4-benzenedicarboxamide (3) exists in the anti conformation and two terphenyl groups are positioned on opposite sides of the plane of the central benzene ring. In contrast to the 1,3-derivative, the yield of the macrocyclization reaction of the 1,4-benzenedicarboxylic acid with the diamine 4 was low (19% yield). Although macrocycle 5 exists in a planar conformation in the crystal and in solution, macrocycle 6 exists in a twisted conformation. A deformation of the twist was induced by a tilted T-shaped aromatic-aromatic interaction between the central phenylene rings of the macrocycle.

Chiral spherical molecule constructed from aromatic amides: facile synthesis and highly ordered network structure in the crystal.

A novel chiral spherical molecule composed of aromatic amide was synthesized in short steps. The molecule is constructed from four benzene rings connected by six amide bonds and has multiple functionalizable points and an asymmetric structure. The racemic spherical molecule constructed channel network structures in the crystalline state.

One-pot synthesis of cyclic triamides with a triangular cavity from trans-stilbene and diphenylacetylene monomers.

Base-promoted self-condensation reactions of trans-stilbene and diphenylacetylene monomers bearing 4-alkylamino and 4'-methoxycarbonyl groups were investigated. Reactions of N-propyl monomers under pseudohigh-dilution conditions (a THF solution of monomer was added dropwise to a THF solution of LiHMDS) afforded the corresponding cyclic triamides in good yields. X-ray crystallographic analysis showed that these cyclic triamides possessed an almost equilateral triangle structure with a cavity surrounded by tilted benzene rings.