Selective recognition of the ditopic trimethylammonium cations by water-soluble aminocalix[4]arene

In this Letter, the role of the depth of the cavity in the recognition processes of the guests by the hosts has been investigated. The hosts 2, 3 interact with both the cationic function and the aromatic moiety in the guests 5, 6 but with a slight preference for the cationic functions. The host 4 selectively recognizes the trimethylammonium functions of the guests 5 and 6. However, the host 1 selectively recognizes the aromatic moiety of the ditopic trimethylammonium guests 5 and 6. The recognition and orientation of the guest in the cavity of the host are directly dependent on the depth of the hydrophobic cavity of the host.     

One-pot synthesis of macrocyclic compounds possessing two cyclobutane rings by sequential inter- and intramolecular [2+2] photocycloaddition reactions

Sensitized photocycloaddition reactions of 6,6′-dimethyl-4,4′-[1,3-bis(methylenoxy)phenylene]-di-2-pyrone (1) with electron-poor α,ω-diolefins such as ethylene diacrylate (2a) and polyoxyethylene dimethacrylates (2b-d) afforded site- and stereoselective macrocyclic dioxatetralactones (3a-d) and (4b) having 18- to 25-membered rings across the C5-C6 and C5′-C6′ double bonds, or C5-C6 and C3′-C4′ double bonds in 1, respectively. Similar photoreactions of 1 with electron-rich α,ω-diolefins such as poly(ethylene glycol)divinyl ether (2e and 2f) afforded crown ether-type macrocyclic compounds (5e and 5f) having 18- and 21-membered rings across the C3-C4 and C3′-C4′ double bonds in 1, respectively. The stereochemical features of 3b, 5e-xx, and 5e-nn were determined by the X-ray crystal analysis. The reaction mechanism was inferred by MO methods.     

Guest-encapsulation behavior in a self-assembled heterodimeric capsule

Tetra(4-pyridyl)-cavitand 1 and tetrakis(4-hydroxyphenyl)-cavitand 2 self-assemble into a heterodimeric capsule 1·2 via four PhOH?pyridyl hydrogen bonds in CDCl₃, wherein one molecule of 1,4-disubstituted-benzene as a guest is encapsulated to form a ternary complex, guest@(1·2). The X-ray crystallographic analysis of (methyl p-ethoxybenzoate)@(1·2) confirmed that the methyl ester and ethoxy groups of the encapsulated guest are oriented to the cavity ends of the 1 and 2 units, respectively. The scope and limitation of guest encapsulation in 1·2, including guest-binding selectivity and orientational isomeric selectivity, are described from the viewpoint of size complementarity and CH-π, CH-halogen, and halogen-π interactions between guest and the cavity of 1·2.     

Thiacalix[4]arene-alkali metal assemblies: crystal structures and guest-binding capabilities of supramolecular architectures supported by metal coordination and cation-π interactions

To investigate alkali metal complexation with sulfur-linked calixarene analogues and their guest-binding properties for gaseous organic guest molecules, we elucidated a crystal structure of a cesium complex with p-H-thiacalix[4]arene (1·4H) ligands and guest-binding properties of the cesium complex (2) and the previously reported rubidium complex (3). In crystals of the complex 2, a ‘sandwich-like’ binuclear complex was formed by inter-molecular coordination of cesium cations to the thiacalixarene molecules and methanol molecules, mutually interacting by aromatic-H?S hydrogen bonding and alkali metal cation-π interactions between the alkali metal cation and thiacalixarene aromatic rings outside of the cavities. On the guest-binding behaviors both complexes 2 and 3 toward organic guest molecules, methanol, ethanol, and 1-propanol as polar molecules, the complex 2 has no methanol adsorption ability, but the complex 3 showed vapor adsorption properties for all guest molecules. In particular, both complexes exhibited a high adsorption capability toward ethanol molecule. As results of gaseous guest adsorption measurements for alcohol molecules, the guest-binding of these complexes are significantly different because the properties depend heavily on structural natures between complexes 2 and 3.     

Decomposition of vinyl ethers by alkalide K, K(15-crown-5)2 via organopotassium intermediates

The structure of vinyl ethers determines the direction of the C-O bond cleavage by alkalide K⁻, K⁺(15-crown-5)₂1. Highly reactive organopotassium compounds are intermediate products formed in the system containing phenyl vinyl ether, butyl vinyl ether, ethylene glycol butyl vinyl ether or triethylene glycol methyl vinyl ether. Vinylpotassium and butylpotassium react with 15-crown-5. The oxacyclic ring of the latter is opened in this case. Organopotassium ethers possessing CH₂CH₂O units eliminate ethylene. It results in various potassium alkoxides. The reaction of 1 with butyl vinyl ether occurs very slow as compared to other vinyl ethers and most of other reagents used till now.     

Change in conformation upon complexation of double-armed terephthalamide hosts: dynamic molecular recognition of ditopic guests with strong CD signaling

Conformation of novel terephthalamide hosts 1 changes from anti to syn upon complexation with a bidentate guest (2 or 3). Chiral sense in the helical syn-form of the double-armed host molecules 1 is biased by the asymmetric centers on the chiral guest [(R,R)/(S,S)-2a], which can be detected by the drastic change in circular dichroism (CD) spectrum thanks to the exciton coupling of two chromophores (‘arms’) linked to the amide nitrogens. Asymmetric centers on the host molecule also exhibit preference for the twisting direction upon change in geometry from the anti- to syn-form. Thus, the achiral guests (2b or 3) can also be detected by modulation of CD spectrum upon complexation with the chiral host [(R,R)-1a].     

Binding studies of tetrathiafulvalene-calix[4]pyrroles with electron-deficient guests

The neutral meso-octamethylporphyrinogen derivative, tetraTTF-calix[4]pyrrole 1 (TTF=tetrathiafulvalene), acts as a multi-faceted receptor in that it interacts with an assortment of different guests in different ways. The conformation of receptor 1 can be reversibly switched between the 1,3-alternate conformation (i.e., 1, Fig. 1) and the cone conformation (i.e., 1·Cl⁻, Fig. 2) by the repetitive addition of chloride and sodium ions. In this paper, the results of detailed and systematic complexation studies involving both 1 and its chloride-bound complex, 1·Cl⁻, with a variety of guests are described. Receptor 1 binds quasi-planar nitroaromatic guests in its 1,3-alternate conformation, while release of these guests takes place upon addition of chloride anions. On the other hand, spherical fullerene guests are strongly bound by 1·Cl⁻. Finally, it was found that a bidentate guest, consisting of a quasi-planar 2,5,7-trinitro-9-dicyanomethylenefluorene moiety tethered to a spherical C₆₀ fullerene, could be recognized by receptor 1 in either its 1,3-alternate or its chloride-bound cone conformation, albeit through very different binding modes.     

Molecular recognition of biotin, barbital and tolbutamide with new synthetic receptors

We have measured, by means of NMR titrations, the binding constants for the complexes between hosts N,N′-bis(6-methylpyridin-2-yl)-1,3-benzenedicarboxamide (7) and 4-chloro-N,N′-bis(6-methylpyridin-2-yl)-2,6-pyridinedicarboxamide (8, hydrated) with biotin methyl ester (1), N,N′-dimethylurea (2), 2-imidazolidone (3), N,N′-trimethylenurea (4), barbital (5) and tolbutamide (6) as guests. Molecular Mechanics calculations (Monte Carlo Conformational Search, AMBER and OPLS force fields, MacroModel v.8.1) on the complexes formed between the foregoing guests and hosts 7 and 8, comparatively with 4-oxo-N,N′-bis(6-methylpyridin-2-yl)-1,4-dihydro-2,6-pyridinedicarboxamide (9a) have been carried out in order to determine the correlation between experimental and theoretical results and to understand the behaviour of the designed new hosts. Finally we have performed single point DFT [B3LYP/6-31G(d,p)] calculations on the optimised Molecular Mechanics geometries for the complexes between hosts 7-9 and water.     

Base-induced cyclizations. Reactions of the 1-halo-6-(2-hydroxyethoxy)-cyclohexenes with potassium t-butoxide

Reactions of 1-bromo-6-(2-hydroxyethoxy)cyclohexene (2) and its chloro analog 3 with potassium t-butoxide in dimethyl sulfoxide at 60–70° gave cyclohex-2-enone ethylene ketal (7) and cis-2,5-dioxabicyclo[4.4.0]dec-7-ene (8) as the major products. Under these conditions, 1-(2-hydroxyethoxy)-1,4-cyclohexadiene (13) is also converted to 7 and elimination products, benzene and ethylene glycol. Conversion of 13 to 7 was shown to be reversible by examination of 7 that had been treated with t-BuOK. in DMSO-d₆. In tetrahydrofuran, 2 and t-BuOK gave benzene as a major product, together with small amounts of 2,5-dioxabicyclo[4.4.0]-dec-6-ene (6), 7, and 8. Mechanisms are proposed for these substitution reactions.     

Water-soluble aminocalix[4]arene receptors with hydrophobic and hydrophilic mouths

We report the new water-soluble aminocalix[4]arene hosts 1 and 2 with deep hydrophobic cavity facilitating hydrophobic mouth and hydrophilic mouth, respectively. The ¹H NMR titrations revealed that host 1 shows high selectivity for neutral guests 9 and 10, with log K of 4.2 and 4.6, respectively. The host 2 shows log K of 4.9 for binding with guest 15. Moreover, the binding ability of the host 2 for guest 14 is stronger by a factor of 1000 than that of the host 1.     

2-(1-Isopropyl-2-benzimidazolyl)-6-(1-aryliminoethyl)pyridyl transition metal (Fe, Co, and Ni) dichlorides: Syntheses, characterizations and their catalytic behaviors toward ethylene reactivity

A series of 2-(1-isopropyl-2-benzimidazolyl)-6-(1-aryliminoethyl)pyridyl metal complexes [iron (II) (1a-6a), cobalt (II) (1b-6b) and nickel (II) (1c-6c)] were synthesized and fully characterized by elemental and spectroscopic analyses. Single-crystal X-ray diffraction analyses of five coordinated complexes 5a, 3b, 5b, 1c and 2c reveal 5a and 5b as distorted trigonal-bipyramidal geometry, and 3b, 1c and 2c as distorted square pyramidal geometry. All complexes performed ethylene reactivity with the assistance of various organoaluminums. The iron complexes displayed good activities in the presence of MAO and MMAO. Upon activated by Et₂AlCl, the cobalt analogues showed moderate ethylene reactivity, while the nickel analogues exhibited relatively higher activities.     

An investigation of the role of the disparate redox states of the tetrathiafulvalene unit in modulating hydrogen bonding interactions in solution

We have investigated the electrochemically controlled hydrogen bonding interactions between tetrathiafulvalene host 3 and guests 4 or 5. Stabilisation of the 3⁺ state is dependent upon the nature of the guest species, whereas both guests prevent precipitation of the electrochemically generated 3²⁺ species at the working electrode via hydrogen bonded molecular recognition processes.     

Manganese(III) acetate based oxidative cyclizations of 3-oxopropanenitriles with conjugated alkenes and synthesis of 4,5-dihydrofuran-3-carbonitriles containing heterocycles

4,5-Dihydrofuran-3-carbonitriles 3a-i were obtained through oxidative cyclizations of 3-oxo-3-phenylpropanenitrile 1a, 3-oxo-3-thien-2-ylpropanenitrile 1b, 3-(2-furyl)-3-oxopropanenitirle 1c, 3-(1-benzofuran-2-yl)-3-oxopropanenitrile 1d, and 4,4-dimethyl-3-oxopropanenitrile 1e mediated manganese(III) acetate with 1,1-diphenyl-1-butene 2a and 1,2-diphenyl-1-pentene 2b. The treatments of these 3-oxopropanenitriles with 2-thienyl substituted alkenes such as 2-[(E)-2-phenylvinyl]thiophene 2c, 2-[(E)-1-methyl-2-phenylvinyl]thiophene 2d, and 2-(1-phenylvinyl)thiophene 2e formed 5-(2-thienyl)-4,5-dihydrofuran-3-carbonitriles 3j-r in good yields (45-93%). As a result, 2-thienyl substituted alkenes formed products in higher yields than phenyl substituted derivatives.     

Electrooxidation of activated α,ω-diols to cyclic tetramethylene acetals of the corresponding dials

Activated α,ω-diols such as ethylene glycol 1a, 1,3-propanediol 1b, but-2-ene-1,4-diol 1c and hexa-2,4-diyne-1,6-diol 1d can be converted into cyclic tetramethylene acetals of the corresponding α,ω-dials (2a-d) in 70% yield by a simple anodic oxidation in dry tetrahydrofuran on a glassy carbon anode. Glycerol 3 when subjected to similar anodic oxidation gave a structure 4 containing three seven-membered 1,3-dioxepane rings.     

Oxymethylcrowned chromene: photoswitchable stoichiometry of metal ion complex and ion-responsive photochromism

Chromene derivatives bearing oxymethyl-12-crown-4 (1), -15-crown-5 (2), -18-crown-6 (3) ether moieties, and non-cyclic analogue (4) were synthesized, and their metal ion binding properties and photochromism were examined. NMR titration with alkali metal ions revealed that 1 formed a 1:2 complex (metal ion: ligand) with Na⁺, while Li⁺ afforded a 1:1 complex of 1. In cases of K⁺ and Rb⁺, the complexes were a mixture of 1:1 and 1:2 complexes, but the formation of 1:1 complex was observed again with Cs⁺. Under UV irradiation, however, the complex stoichiometry of 1 with all alkali metal ions was 1:1. As a comparison of NMR spectra between the Li⁺ and Na⁺ complexes of 1 indicated considerable upfield shift for the chromene moiety of the Na⁺ complex, π-π stacking of the chromene moiety seems to induce formation of the 1:2 complex. These results indicate that the chromene moiety is not only to show photochromism but also to induce aggregation to form the 1:2 complex resulted in switching of the complex stoichiometry by UV irradiation. The formation of 1:2 complex appeared only with 1 because flexibility of the crown moieties for 2 and 3 interfered the formation of 1:2 complex. Studies on photochromism in the presence of a metal ion demonstrated that the chromene derivatives bearing crown ether moieties show ion-responsive photochromism depending on the metal ion binding ability of their crown ether moieties.     

Reaction of azulene with 1,2-bis[4-(dimethylamino)phenyl]-1,2-ethanediol in a mixed solvent of methanol and acetonitrile in the presence of hydrochloric acid: a facile one-pot synthesis and properties of new triarylethylenes possessing an azulen-1-yl group

Reaction of azulene (1) with 1,2-bis[4-(dimethylamino)phenyl]-1,2-ethanediol (2) in a mixed solvent of methanol and acetonitrile in the presence of 36% hydrochloric acid at 60 °C for 3 h gives 2-(azulen-1-yl)-1,1-bis[4-(dimethylamino)phenyl]ethylene (3) (8% yield), 1-(azulen-1-yl)-(E)-1,2-bis[4-(dimethylamino)phenyl]ethylene (4) (28% yield), and 1,3-bis{2,2-bis[4-(dimethylamino)phenyl]ethenyl}azulene (5) (9% yield). Besides the above products, this reaction affords 1,1-di(azulen-1-yl)-2,2-bis[4-(dimethylamino)phenyl]ethane (6) (15% yield), a meso form (1R,2S)-1,2-di(azulen-1-yl)-1,2-bis[4-(dimethylamino)phenyl]ethane (7) (6% yield), and the two enantiomeric forms (1R,2R)- and (1S,2S)-1,2-di(azulen-1-yl)-1,2-bis[4-(dimethylamino)phenyl]ethanes (8) (6% yield). Furthermore, addition reaction of 3 with 1 under the same reaction conditions as the above provides 6, in 46% yield, which upon oxidation with DDQ (=2,3-dichloro-5,6-dicyano-1,4-benzoquinone) in dichloromethane at 25 °C for 24 h yields 1,1-di(azulen-1-yl)-2,2-bis[4-(dimethylamino)phenyl]ethylene (9) in 48% yield. Interestingly, reaction of 1,1-bis[4-(dimethylamino)phenyl]-2-(3-guaiazulenyl)ethylene (11) with 1 in a mixed solvent of methanol and acetonitrile in the presence of 36% hydrochloric acid at 60 °C for 3 h gives guaiazulene (10) and 3, owing to the replacement of a guaiazulen-3-yl group by an azulen-1-yl group, in 91 and 46% yields together with 5 (19% yield) and 6 (13% yield). Similarly, reactions of 2-(3-guaiazulenyl)-1,1-bis(4-methoxyphenyl)ethylene (12) and 1,1-bis{4-[2-(dimethylamino)ethoxy]phenyl}-2-(3-guaiazulenyl)ethylene (13) with 1 under the same reaction conditions as the above provide 10, 2-(azulen-1-yl)-1,1-bis(4-methoxyphenyl)ethylene (16), and 1,3-bis[2,2-bis(4-methoxyphenyl)ethenyl]azulene (17) (93, 34, and 19% yields) from 12 and 10 and 2-(azulen-1-yl)-1,1-bis{4-[2-(dimethylamino)ethoxy]phenyl}ethylene (18) (97 and 58% yields) from 13.     

A close inspection of Ag(I) coordination to molecular baskets. A study of solvation and guest encapsulation in solution and the solid state

Four molecular baskets 1-4, each comprising a modular bowl-shaped platform and a set of distinctive aromatic gates, were synthesized. The gates were made to incorporate a nitrogen heteroatom at different positions, as in 2-4, or without a coordinating nitrogen, as in 1 (Fig. 1). For polydentate 3 and 4, the location of the nitrogen was revealed to have an effect on directing the basket's coordination to Ag(I) cation, and subsequent folding to enclose space. The folded geometries were shown to encompass a helical, Ag(I):3, or C_3v/C_2v symmetrical, Ag(I):4, dynamic arrangement of the gates (from density functional theory). For 2 and 1, however, the presence of Ag(I) caused the sole formation of oligomers and the absence of coordinating interactions, respectively. Variable temperature (VT) ¹H and ¹⁹F NMR measurements of Ag(I):3 did not provide direct evidence for the solvation of its inner space and the encapsulation of the BF_4⁻ counterion. Moreover, CH₃CN or CH₃NC substrates were not found inside of Ag(I):3. The finding is in contrast with the behavior of Cu(I):3, which is known to encapsulate these guests. The intriguing guest selectivity was accounted for by small structural and electronic differences of Ag(I)/Cu(I) folded baskets. The X-ray solid-state structural studies of 2, 3, and 4 revealed the basket's capacity to fill its inner space with small compounds. Thus, 3 was found with an ordered molecule of chloroform, while 4 contained molecules of CH₃OH and H₂O. The basket selectivity for enclosing and positioning guests, in the solid state, was deduced to be guided by their size and weak host-guest and guest-guest noncovalent interactions.     

Synthesis and characterization of iron and cobalt dichloride bearing 2-quinoxalinyl-6-iminopyridines and their catalytic behavior toward ethylene reactivity

The 1-(6-(quinoxalin-2-yl)pyridin-2-yl)ethanone was synthesized in order to prepare a series of N-(1-(6-(quinoxalin-2-yl)pyridine-2-yl)ethylidene)benzenamines (L1-L7), which provided new alternative N^∧N^∧N tridentate ligands coordinating with iron(II) and cobalt(II) dichloride to form complexes of general formula LFeCl₂ (1-7) and LCoCl₂ (8-14). All organic compounds were fully characterized by NMR, IR spectroscopic and elemental analysis along with and magnetic susceptibilities and metal complexes were examined by IR spectroscopic and elemental analysis, while their molecular structures (L1, L4, 1, 4, 10, 13) were confirmed by single crystal X-ray diffraction analysis. Upon activation with methylaluminoxane (MAO), all iron complexes gave good catalytic activities for ethylene reactivity (oligomerization and polymerization), while their cobalt analogues showed moderate activities toward ethylene oligomerization with modified methylaluminoxane (MMAO). Various reaction parameters were investigated for better catalytic activities, the higher activities were observed at elevated ethylene pressure. The iron and cobalt complexes with para-methyl substituents of aryl group linked on imino group showed highest activity.     

Effective receptors for fluoride and acetate ions: synthesis and binding study of pyrrole- and cystine-based cyclopeptido-mimetics

Two conformationally constrained pseudo-cyclopeptides (1, 2) consisting of pyrrole-, pyridine-, and cystine-moieties were designed and synthesized as neutral receptors for anionic guests. The anion recognition abilities of these two receptors were examined photometrically in acetonitrile solution. The UV-vis study revealed that the [1+1] receptor (1) formed 1:1 complexes with anions, whereas the [2+2] receptor (2) led to 1:2 mode binding with anions. Both receptors displayed good affinity and selectivity for fluoride and acetate ions.     

‘Two-point’-bound supramolecular complexes from semi-rigidified dipyridine receptors and zinc porphyrins

Two linear compounds 1 and 2 have been designed and synthesized as new receptors for zinc porphyrins. Both compounds consist of two folded aromatic amide moieties, which are connected with an acetylene linker in 1 or directly in 2. The rigid conformations of their folded moieties are stabilized by intramolecular tri-centered hydrogen bonding, while the whole molecule adopts a ‘S’- or ‘C’-styled conformation depending on the relative orientation of the two rigid moieties. Two pyridine units are introduced at the ends of 1 and 2 for the complexation of zinc porphyrin guests. Although the ¹H NMR investigation indicated that both compounds can bind two zinc porphyrin guests at high concentrations (≥5 mM) in chloroform, the UV-vis studies revealed that, at low concentration of 1 and 2 (4 μM), both compounds complex one zinc porphyrin guest to form structurally unique ‘two-point’-bound 1:1 complexes. The association constants of the 1:1 complexes have been determined with the UV-vis titration experiments.