One-step synthesis of inherently chiral p-tert-butylcalix[4]azacrown

A one-step procedure is developed to synthesize inherently chiral p-tert-butylcalix[4]azacrown 1 through etherification between p-tert-butylcalix[4]arene and compound 3, which can be amplified to efficiently prepare more inherently chiral calix[4]arenes in ABHH substitution pattern.     

First synthesis of lower-rim-substituted aryl ethers of p-tert-butylcalix[4]arene

With the use of S(N)Ar or Ullmann reactions, the synthesis of the first lower-rim aryl ether derivatives of tert-butylcalix[4]arene is reported, as are some of their conformational properties, (1)H NMR spectra, and X-ray crystal structures. The lower-rim aryl pendants reported herein provide for new scaffolds upon which a host of other new molecular architectures can be constructed, thus extending the capability of the versatile calix[4]arenes even further.     

Phosphorus-based p-tert-butylcalix[5]arene ligands

New calix[5]arene trivalent phosphorus derivatives have been synthesized which should be excellent ligands with which to study and control the interaction of a ligand atom with a metal. The larger cavity of the calix[5]arene (compared to calix[4]arene) provides a good balance between constraint and flexibility. Treatment of p-tert-butylcalix[5]arene with 2 equiv of either tris(dimethylamino)phosphine or dichlorophenylphosphine inserts two RP moieties into the calix[5]arene framework to give calix[5](PR)2(OH) (1, R = Me2N; 2, R = Ph). Further treatment of 1 with 4 equiv of HCl gives calix[5](PCl)2(OH) (3). Heating a solution of the monophosphorus compound calix[5](PNMe2)(OH)3 (4) releases dimethylamine to yield both monomeric calix[5](P)(OH)2 (6) and dimeric [calix[5](P)(OH)2](2) (7), the latter having a tubelike geometry. X-ray crystallographic studies confirm the structures and show that 1 and 2 have approximate cone conformations while 3 has an approximate 1,2-alternate conformation. The orientations of the phosphorus lone pairs and oxygen atoms in all derivatives provide a framework for both soft and hard ligand interactions within the calix[5]arene.     

Polymorphism of pure p-tert-butylcalix[4]arene: subtle thermally-induced modifications

A low-density polymorph of the well-known host compound p-tert-butylcalix[4]arene undergoes subtle structural changes when heated and cooled.     

Molecular dynamics simulations of p-tert-butylcalix[4]arene with small guest molecules

Classical molecular dynamics simulations were used to study p-tert-butylcalix[4]arene inclusion compounds with xenon, nitrogen, hydrogen, methane, and sulfur dioxide guest molecules. The calixarene units were taken to be rigid and the intermolecular molecular interactions were modeled as a sum of the van der Waals interactions with parameters from the AMBER force field and electrostatic interactions. Simulations of the high-density alpha phase and low-density beta0 phase of p-tert-butylcalix[4]arene were used to test the force field. The predicted densities of the two phases were found to agree with experimental measurements at 173 K to within 5 %. Simulations were performed with guests placed inside the calixarene cages of the beta0 phase. Guest-host ratios of 1:1 to 1:4 were considered. Changes in the unit-cell volume and density of the phases with the addition of guest molecules and the inclusion energies for the guests were determined. Finally, the dynamics of the guest motion inside the cages were characterized by determining the root-mean-square displacements and velocity autocorrelation functions of the xenon and nitrogen guests.     

Regioselective O-substitution of p-tert-butylcalix[7]arene

[structure: see text]. The first examples of selectively functionalized calix[7]arenes have been obtained by weak-base promoted O-alkylation or O-benzoylation of p-tert-butylcalix[7]arene. Mono-, 1,3- and 1,4-disubstituted calix[7]arenes have been obtained in workable yields, while the 1,2,4,6-tetrasubstitution was achieved with surprisingly high selectivity (50-88% yield) by using K2CO3 as base. A rationale for this finding is proposed.     

Characterization of precursors of p-tert-butylcalix[6]arene synthesis. Mechanism of formation of p-tert-butylcalix[6]arene

Abstract

Gutsche has proposed the existence of “hemicalixarenes” and “pseudocalixarenes” to explain the formation of calixarenes. The characterization and the quantitative determination of molecules present during the reaction, described in “Organic Synthesis”, have permitted us to determine the immediate precursors of p-tert-butylcalix[6]arene. The calix[6]arene could be due to cyclization of linear species named “pseudocalixarenes” and not due to duplication of molecules named “hemicalixarenes”.     

Transition metal complexes of bidentate p-tert-butylcalix[4]arene S-alkyldithiocarbazate Schiff bases

The calix[4]arene-based podand, which incorporates two Schiff-base functions derived from S-alkyldithiocarbazate in 1,3-alternate positions on the lower rim has been prepared and reacted with transition metal ions. X-ray single crystal diffraction of nickel and copper complexes shows that two iminothiolate domains are monoanionic bidentate planar chelators to coordinate one metal in distorted tetrahedral geometry.     

Regioselective intramolecular bridging of p-tert-butylcalix[10]arene

The synthesis and characterization of a series of regioselective intramolecular bridging of calix[10]arene are described for the first time. Reacting p-tert-butylcalix[10]arene with tri-ethylene glycol ditosylate using K₂CO₃ as a base in toluene, 1,2-calix[10]crown-4 2a, 1,4-calix[10]crown-4 2b and 1,6-calix[10]crown-4 2c were obtained in yields of 9%, 14% and 7%, respectively. While using Cs₂CO₃/acetone instead of K₂CO₃/toluene, the 1,4-calix[10]crown-4 2b was obtained selectively in good yield up to 50%.     

Synthesis and complexing properties of four imidazolyl acetamido p-tert-butylcalix[4]arenes

Four imidazolyl acetamido p-tert-butylcalix[4]arenes 5–8 have been prepared by reacting the corresponding methyl esters derivatives 1–4 with histamine in 1:1 mixture of methanol:toluene. The yields ranged from 56 to 68%. 5–8 have been shown to be in cone conformation. The complexation behaviour of 5–8 towards monovalent metal picrates M⁺Pic⁻ with M⁺ = Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺ and divalent metal picrates M²⁺(Pic⁻)₂ with M²⁺ = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Zn²⁺ and Co²⁺ are given. Tentative localisation of the metal cations in the receptors is given. The binding properties towards these cations have been determined along with stoichiometries of the complexes.     

Solvent extraction of europium trifluoromethanesulfonate into nitrobenzene in the presence of p-tert-butylcalix[6]arene and p-tert-butylcalix[8]arene

From extraction experiments and ??-activity measurements, the extraction constants corresponding to the general equilibrium Eu³⁺(aq) + 3A^?(aq) + L(nb) ? EuL³⁺(nb) + 3A^?(nb) taking part in the two-phase water?Cnitrobenzene system (A^? = CF₃SO₃ ^?; L = p-tert-butylcalix[6]arene, p-tert-butylcalix[8]arene; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. Further, the stability constants of the EuL³⁺ complexes in nitrobenzene saturated with water were calculated for a temperature of 25 °C as log ?? _nb(EuL³⁺) = 6.4 ± 0.1 (L = p-tert-butylcalix[6]arene) and log ?? _nb(EuL³⁺) = 11.3 ± 0.1 (L = p-tert-butylcalix[8]arene).     

Spirodienone route for the stereoselective methylene functionalization of p-tert-butylcalix[4]arene.

A new method for the regio- and stereoselective functionalization of two distal methylene groups of p-tert-butylcalixarene (1a) is described. Reaction of the meso bis(spirodienone) calixarene derivative 2a with bromine afforded the tetrabrominated product 3a derived from exo 1,4-additions of bromine to the diene subunits. A phase-transfer-catalyzed reaction of 3a with aqueous NaOH/CH2Cl2 yielded the exo bis(epoxide) calixarene derivative 4. Heating 3a in a vacuum eliminated two molecules of HBr and afforded a product (5b) that retained the C(i) symmetry of the starting material. X-ray analysis indicated that the calixarene derivative 5b possesses two exocyclic double bonds of E configuration. Calixarene 5b undergoes reaction with nucleophiles at the exocyclic double bonds, with concomitant bond shifts and expulsion of the bromine atoms. Selective trans monodeuteration of two methylene groups of 1 was achieved by reaction of 5b with NaBD4 followed by aromatization of the labeled spirodienol derivative. Reaction of 5b with RONa/ROH (R = Me, Et) afforded the methylene-substituted bis(spirodienone) derivatives 9a and 9b possessing two trans alkoxy groups. X-ray crystallography of 9b indicated that the two trans substituents are located at pseudoequatorial positions of the methylene groups. LiAlH4 reduction of the substituted bis(spirodienone) derivatives afforded calix[4]arenes incorporating trans alkoxy groups at two distal methylene positions.     

Simulations of p-tert-butylcalix[4]arene with multiple occupancies of small guest molecules

Classical molecular dynamics simulations were used to study low-density beta(0)-phase p-tert-butylcalix[4]arene inclusion compounds with multiple calix occupancies of xenon, carbon dioxide, methane, and hydrogen guest molecules with guest-host ratios ranging from 1:4 to 4:1. Custom parameterized force fields were used for the guests and the AMBER force field for the calixarene units was validated in our previous work (Chem. Eur. J. 2006, 12, 5231). The inclusion energy and unit cell volume of the calixarene inclusion compound were determined for various guest occupancies and for occupancies greater than 1:1, strong guest-guest interaction effects are observed. The structure and energetics of the 2:1 CO(2)/beta(0)-phase inclusion compound were compared to those of the low-temperature 2:1 CO(2)/calixarene in which the guest molecules occupy both cage and interstitial sites.     

Specific features of the reduction of disubstituted amide derivatives of p-tert-butylcalix[4]arene

Regardless of the reducing agent, the reduction at 25°C and higher of the amide groups in a large number of p-tert-butylcalix[4]arene derivatives containing amide fragments with different substituents on the nitrogen atoms was accompanied by hydrogenolysis of the C-N bond with formation of the corresponding O-(2-hydroxyethyl) calixarenes and by partial cleavage of the ether bond between the calixarene framework and the substituent to give compounds with a lower degree of substitution.     

Extending the chemistry of p-tert-butylcalix[4]arene with H-bonding and secondary coordination

Through hydrogen bonding and secondary coordination, aliphatic amines, together with water and metals, form novel compounds with p-tert-butylcalix[4]arene containing complex clusters and layers.     

Pseudopolymorphism in the p-tert-butylcalix[4]arene--n-butylamine system: directing the structural motifs

p-tert-Butylcalix[4]arene forms three compounds with n-butylamine, two weakly interacting guest-host compounds of 1:1 and 1:2 stoichiometry, as well as a hydrogen-bonded 3:1 compound containing some n-butylamine molecules not coordinated to the host.     

Synthesis of p-tert-butylcalix[4]arene derivatives with trans-alkyl substituents on opposite methylene bridges

Reaction of the bis(spirodiene) calixarene derivative 9 possessing exocyclic double bonds with tetraethylammonium fluoride or chloride afforded bis(spirodienone) calixarene derivatives substituted by the corresponding halogen atoms (11 and 12). Reaction of 9 with alkyl cuprates yielded in one step p-tert-butylcalix[4]arene derivatives with opposite methylene groups substituted in a trans fashion by identical alkyl substituents (methyl, ethyl, or isopropyl). The isopropyl derivative 16 displayed the largest cone-to-cone inversion barrier of the series.     

What we have learned from the study of solid p-tert-butylcalix[4]arene compounds

The study of solid p-tert-butylcalix[4]arene and its compounds with a variety of techniques has provided a good understanding of the versatility of this host molecule, how to induce a number of distinct host-guest motifs, its molecular recognition properties, the complex phase relationships and unique properties such as gas adsorption without having obvious channels.