Chromogenic azo-coupled calix[4]arenes

Novel 1,3-alternate calix[4]azacrowns having an azo chromophoric pendent group were synthesized, and their 1,3-alternate conformations were confirmed by X-ray crystal structure. In view of the hypsochromical UV band shifting upon cation complexation, azo-coupled calix[4]azacrown-5 (3) showed the most selective shifting with alkali and alkaline metal ions. In addition, 3 revealed K+ ion selectivity not only due to the size comparability between the K+ ion and the azacrown-5 loop but also due to a significant K+-pi interaction between the two aromatic rings and the K+ ion. The UV band shifting is also dependent on the lipophilicity of the species of counteranion used.     

Regioselective ipso-nitration of calix[4]arenes

A novel protection/deprotection method leading to the regioselective ipso-substitution of calix[4]arenes is described. The introduction of nosyl (p-nitrobenzenesulfonyl) groups into the lower rim of partly alkylated tert-butylcalix[4]arenes leads subsequently to the exclusive ipso-nitration of the alkylated phenol rings, while the protecting groups can be easily removed in the next step. This method gives dialkoxy- or trialkoxy-substituted calix[4]arenes with nitro groups on the alkylated rings and tert-butyl groups on the remaining ones. The above substitution pattern is complementary to the isomers so far known in the chemistry of calix[4]arenes and could be used in the design of novel type of calixarene-based receptors.     

Head-to-tail connected double calix[4]arenes

Summary New macrotricyclic compounds consisting of two calix[4]arene substructures connected by aliphatic chains of various length (three to five carbon atoms) between two oppositep-positions and two distal phenolic oxygens have been synthesized. Starting withp-tert-butyl-calix[4]arene, two O-protected phenolic units are attachedvia ether links in 1,3-position by reaction with the corresponding tosylates. After deprotection, the new calix[4]arene is formed by fragment condensation with 2,6-bisbromomethylated 4-alkylphenols. The structure of one example (8c) has been confirmed by single crystal X-ray analysis. Both calixarene parts assume the cone conformation, a molecule of acetonitrile being included in both cavities.

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Doppelte Calix[4]arene in Kopf-Schwanz-Verknüpfung

Zusammenfassung Neue makrotricyclische Verbindungen, in denen zwei Calix[4]aren-Einheiten durch aliphatische Ketten unterschiedlicher Länge (drei bis fünf C-Atome) zwischen zwei gegenüberliegendenp-Positionen und zwei gegenüberliegenden Phenolsauerstoffen verknüpft sind, wurden hergestellt. Ausgehend vonp-tert-Butyl-calix[4]aren werden zunächst zwei O-geschützte Phenolbausteine in 1,3-Stellung durch Umsetzung mit den entsprechenden Tosylaten etherartig gebunden. Nach Abspaltung der Benzylether-Schutzgruppe wird durch Fragmentkondensation mit 2,6-bisbrommethylierten 4-Alkylphenolen das neue Calix[4]aren gebildet. Für8c wurde die Struktur durch Einkristallröntgenstrukturanalyse bestätigt. Beide Calixarenteile nehmen diecone-Konformation ein, wobei in die beiden Hohlräume je ein Molekül Acetonitril eingeschlossen wird.

     

Anion receptors based on ureido-substituted thiacalix[4]arenes and calix[4]arenes

The regioselective nitration of 25,27-dipropoxythiacalix[4]arene was carried out as a key step in the synthesis of thiacalix[4]arene derivative bearing two arylureido functions on the upper rim. The preorganisation of ureido units using the thiacalix[4]arene/calix[4]arene moieties as a molecular scaffold gave novel anion receptors. These compounds, albeit based on hydrogen bonding interactions, show good complexation ability even in highly HB-competitive solvent, such as DMSO. Direct comparison of otherwise identical structures 6a and 7a revealed remarkable dominance of the thiacalix[4]arene derivative over its classical analogue in anion binding.     

Upper rim tetrathiafulvalene-bridged calix[4]arenes

The synthesis of novel upper rim calix[4]arene-tetrathiafulvalene conjugates 1a-d has been performed by bridging the tetrachloromethylated calix[4]arene derivative 4 with the corresponding tetrathiafulvalene-dithiolates. The cyclic voltammetry of 1a-d shows a two-step oxidation behavior, whereas NMR binding titrations showed their binding affinity to pyridinium salts. X-ray structure of 4 features calixarene fixed in the pinched cone conformation; its crystal packing is defined by the network of C-H···Cl weak hydrogen bonds.     

Convenient direct synthesis of bisformylated calix[4]arenes via ipso substitution

[reaction: see text] A facile synthesis of bisformylated calix[4]arenes via ipso substitution of p-tert-butylcalix[4]arenes through treatment with hexamethylenetetramine/trifluoroacetic acid is described. Under identical conditions, p-tert-butylcalix[4]arene tetramethyl ether 4 gives proximally substituted bisformylated derivative 4a in a pinched cone conformation.     

Porous molecular crystals of calix[4]arenes

X-ray diffraction, thermogravimetric, and adsorption measurements were applied to study the thermal decomposition of the complexes of 25,26,27,28-tetrahydroxycalix[4]arene and 4-tert-butylcalix[4]arene with the solvents resulting in the formation of porous phases. Diffusion of the "guest" (solvent, gas) through the molecular crystal of calixarene-host proceeds via the consecutive formation of a series of thermodynamically unstable porous phases of the "host." Owing to the structural reorganization of calixarene, the transport of the "guest" occurs through the "host" matrix followed by complexation and formation of the thermodynamically stable structure.     

Conformational properties of cyanomethoxy calix[4]arenes

O-Alkylation of the dinitro calix[4]arene 2, easily available by selective ipso-nitration of the di-cyanomethyl ether 1, with allylbromide (DMF/Cs2CO3) gave tetraethers 3 and 4 with anti- and syn-orientations of the two allyl ether residues. The two possible stereoisomers of in the partial cone and 1,2-alternate conformation exist as an equilibrium mixture which could be quantitatively analysed by 1H NMR spectroscopy. The temperature dependence of this equilibrium leads to deltaH0 = -7.6 to -9.7 kJ mol(-1) in different solvents (tetrachloroethane, benzene, dimethylsulfoxide). Since 3(1,2-alt) could be obtained in pure form, its isomerisation to the equilibrium mixture with 3(paco) could be followed also kinetically. An activation energy of E(a) = 110.5 kJ mol(-1) was found for this reaction in DMSO-d6. The results were confirmed by similar studies with tetraethers 5 and 6 obtained by O-allylation of 1, although exact thermodynamic and kinetic studies were not possible in this case, since the NMR signals of the respective isomers were strongly overlapping. Single crystal X-ray structures were obtained for 2, 3(1,2-alt), 4(paco) and 5(1,2-alt).     

Dendritic octa-CMPO derivatives of calix[4]arenes

Calix[4]arenes substituted at the narrow or wide rim by eight carbamoylmethyl-phosphine oxide (CMPO) functions in a dendritic manner were synthesised and studied in extraction of Eu³⁺ and Am³⁺ from aqueous nitric acid into o-nitrophenylhexyl ether. ¹H NMR relaxivity titrations for a wide rim octa-CMPO reveal the clear formation of a solvent-free 1:2 ligand/metal complex, while the wide rim tetra-CMPO formed oligomeric complexes under similar conditions.     

Self-sorting dimerization of tetraurea calix[4]arenes

Calix[4]arenes substituted by four urea functions are self-complementary molecules that spontaneously combine in apolar solvents in the presence of an ammonium salt to form dimeric capsules held together by a belt of hydrogen bonds. In the presence of tetraethylammonium salts, the Et4N+ cation is included as a guest. The sorting between dimeric capsules formed in a mixture of calix[4]arenes directly depends on the steric crowding of the substituents grafted on the urea groups whether aromatic derivatives or aliphatic chains linking urea functions in mono-, di-, or tetraloop structures. Simple rules allow one to anticipate which capsules will be exclusively formed when calix[4]arenes are mixed in different proportions. The stabilization of the dimeric structures by hydrogen bonds is thwarted by the overlaps of aliphatic loops and/or by bulky groups that cannot pass through these loops. Despite the structural similarity of the calixarenes, the exclusive formation of dimers of well-defined compositions and clear titration breaks are observed by electrospray mass spectrometry. This technique yields reliable information on stoichiometries and composition despite measurements in the gas phase rather than in solution and it does not suffer from excessive peak overlaps in contrast with NMR.     

The synthesis of metallocene calix[4]arenes

The condensation of 1,1-bis(chlorocarbonyl)metallocenes andp-tert-butylcalix[4]arene in toluene leads to novel metallocene calix[4]arenes in which the metallocene subunit bridges the opposite hydroxy groups of the parent calixarene.Presented at the Fourth International Symposium on Inclusion Phenomena and the Third International Symposium on cyclodextrins, Lancaster, U.K., 20–25 July 1986.     

Long synthetic nanotubes from calix[4]arenes

We report the synthesis and encapsulation properties of long (up to 5 nm) molecular nanotubes 1-4, which are based on calix[4]arenes and can be filled with multiple nitrosonium (NO(+)) ions upon reaction with NO(2)/N(2)O(4) gases. These are among the largest nanoscale molecular containers prepared to date and can entrap up to five guests. The structure and properties of tubular complexes 1(NO(+))(2)-4(NO(+))(5) were studied by UV/Vis, FTIR, and (1)H NMR spectroscopy in solution, and also by molecular modeling. Entrapment of NO(+) in 1(NO(+))(2)-4(NO(+))(5) is reversible, and addition of [18]crown-6 quickly recovers starting tubes 1-4. The FTIR and titration data revealed enhanced binding of NO(+) in longer tubes, which may be due to cooperativity. The described nanotubes may serve as materials for storing and converting NO(x) and also offer a promise to further develop supramolecular chemistry of molecular containers. These findings also open wider perspectives towards applications of synthetic nanotubes as alternatives to carbon nanotubes.     

Cationic calix[4]arenes as anion-selective ionophores

1,3-Alternate cationic calix[4]arene proved highly selective for proton/halogens symport transport and showed antiproliferative activity against murine monocyte/macrophage J774.A1 cancer cells.     

Fourfold [2]rotaxanes based on calix[4]arenes

Tetraurea calix[4]arenes with four loops form exclusively heterodimers with open-chain urea calix[4]arenes when they are dissolved in aprotic solvents. These assemblies can be considered as pseudorotaxanes. If open-chain tetraureas ending with maleic imide functions are used, their Diels-Alder reaction with 1,4,5,8-tetrapentoxyanthracene leads to tetra[2]rotaxanes which cannot be split into the single calixarene parts by hydrogen bond breaking solvents.     

Water sorption on crystalline calix[4]arenes

The water sorption isotherms on seven samples of crystalline calix[4]arenes were obtained by the isopiestic method. The experimental isotherms were described by the Harkins-Jura equation for condensed films. The conclusion was made that water was adsorbed on the surface of calixarene crystals or through pores of a fairly large diameter with the formation of a condensed film. Original Russian Text ? O.V. Surov, M.I. Voronova, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 5, pp. 943–947.     

Azobenzene-bridged calix[8]arenes

An azobenzene bridge was introduced into the lower (or smaller) rim of p-tert-butylcalix[8]arene (1) and 1,5-calix[8]crown-3 (2) to form 1,4-singly bridged (3) and 1,5:3,7-doubly bridged (4) calix[8]arene derivatives, respectively. Trans and cis isomers of conformationally rigid 4 were isolated. The quantum yields of the trans-cis photoisomerisation reactions have been measured.     

Thermodynamic parameters of sublimation of calix[4]arenes

The temperature dependences of the vapor pressure of calix[4]arenes were determined by the Knudsen effusion method. Some calix[4]arenes can form congruently subliming intramolecular compounds with a solvent. The thermodynamic parameters of the sublimation of the compounds were calculated. Molecules of organic solvents incorporated in the cavity of calix[4]arenes stabilize the crystal lattice, increasing the enthalpy of sublimation. The electrostatic interactions presumably make a significant contribution to stabilization of the crystal lattice of stoichiometric complexes of calix[4]arenes with solvents.     

Regioselective upper rim substitution of calix[4]arenes

The electrophilic substitution of 25,27-dipropoxy-26,28-dinosyloxycalix[4]arene leads exclusively to the para-substitution of the alkylated phenol rings, while in the next step, the protecting nosyl group can be easily removed using a basic hydrolysis. The overall process yields dialkoxycalix[4]arenes with the substitution on the alkylated rings—the substitution pattern, which is complementary to the common dialkoxycalix[4]arenes with substituted nonalkylated phenolic units. The usefulness of this protection/deprotection procedure was documented by the synthesis of novel type of calixarene dipropoxy derivatives, and by the preparation of a novel anion receptor based on this substitution pattern.     

New preorganized calix[4]arenes. Part II. Double Schiff-based calix[4]arenes derived from 4,6,10,12,16,18,22,24,25,26,27,28-dodecamethyl-5,11,17,23-tetrahydroxycalix[4]arene

We describe the condensation of an appropriate tetraaldehyde derivative of mesitol calix[4]arene with 1,n-diaminoalkanes to afford double Schiff-base calix[4]arene receptors in the 1,3-alternate conformation.