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The study of the aqueous solutions of the cationic hexameric calixarene 5,11,17,23,29,35-hexakis (N,N-dimethyl-N-hydroxyethylammoniummethylene)-37,38,39,40,41,42-hexametoxycalix[6]arene hexachloride demonstrates their colloidal nature. Within the concentration range from 1 × 10–5 to 0.01 M, this compound forms positively charged aggregates of a much larger size as compared with those of tetrameric cationic calix[4]arene previously studied by us. The aggregation was confirmed both by DLS and TEM experiments; the last method revealed that the 102 to 103 nm sized particles consist of smaller primary aggregates. These aggregates manifest themselves in displaying alteration of absorption spectra and marked shifts of apparent ionization constants of common acid–base indicators. These shifts resemble those registered in micellar solutions of cationic surfactants and cationic polyelectrolyte brushes. The pCl measurements also confirm the formation of calixarene aggregates. The dye associations with the calixarene...

Evidence for interactions between π-electron-rich calixarene cavities and the surface of gold nanoparticles is demonstrated using a comparative synthetic approach involving the capping of the nanoparticles with thiol ligands. In one approach, thiol ligands are systematically titrated onto existing 5.3 nm gold nanoparticles in solution (postsynthetic synthesis approach), whereas in the other a gold solution is reduced in the presence of thiol ligands (direct synthesis approach). The calixarene cavity is placed in close proximity and facing toward the gold nanoparticle surface upon adsorption of ligand 5,11,17,23-tetrakis-mercaptomethyl-25,26,27,28-tetrapropoxylcalix[4]arene (MMC), which comprises four upper-rim thiol groups. The other two ligands used in this investigation contain either four thiol groups on the calixarene lower-rim, 25,26,27,28-tetrakis(4-mercaptobutyloxy)calix[4]arene (MBC), or a model monodentate thiol, 4-methoxybenzenemethanethiol (MBM), as controls. The orientation of the cavity in MM...

Calixarene-Ti complexes were grafted onto SiO2 (0.18-0.24 Ti nm-2) to form isolated and accessible Ti centers persistently coordinated to multidentate calixarene ligands. Grafted Ti-tert-butylcalix[4]arenes gave Ti K-edge absorption spectra with pre-edge features at 4968.6-4968.9 eV, independently of Ti surface density and of their use in epoxidation catalysis. The structure and reactivity of grafted Ti-calix[4]arenes were weakly dependent on thermal treatment below 573 K, and the relative epoxidation rates of trans- and cis-alkenes showed that calixarene ligands did not restrict access to Ti centers more than corresponding calcined Ti-SiO2 materials. For all materials, 13C NMR and UV-visible spectroscopies confirmed the presence of Ti-O-Si connectivity and identical ligand-to-metal transitions. Grafted Ti-homooxacalix[3]arene complexes, however, gave weaker pre-edge features at higher energies ( approximately 4969.5 eV), consistent with greater Ti 3d occupancy and coordination numbers greater than four, and 20-fold lower cyclohexene epoxidation rate constants (per Ti) than on calix[4]arene-based materials. These different rates and near-edge spectra result from aldehyde formation caused by unimolecular cleavage of ether linkages in homooxacalix[3]arene ligands during grafting, leading to higher coordination and electron density at Ti centers. Materials based on tert-butylcalix[4]arene and homooxacalix[3]arenes led to similar epoxidation rates and near-edge spectra after calcination, consistent with the conversion of both materials to isolated Ti centers with identical structure. These materials provide a systematic approach for relating oxidation reactivity to Ti 3d occupancy, a descriptor of Lewis acid strength, and Ti coordination, because they provide Ti centers with varying electron density and coordination, but maintain accessible active centers with uniform structure and unrestricted access to reactants.

[structure: see text] Chiral calix[4]arene alpha-aminophosphonic acids were obtained through diastereoselective Pudovik-type addition of sodium ethyl phosphites to the chiral calixarene imines, removal of chiral auxiliary groups, and mild dealkylation of phosphonate fragments. The diacids obtained show inhibitory activity toward porcine kidney alkaline phosphatase that depends considerably on the absolute configuration of the alpha-carbon atoms.

Calix[4]arenedialkyl ethers 3 react with an excess of acylisocyanates to give selectively monoacylated products 4. Intramolecular hydrogen bonds and steric effects of the acylcarbamate fragments are most likely responsible for the high selectivity of this monoprotection. [reaction: see text]

The regioselective reaction of resorcarene octaols 1 with 4 equiv of arylsulfonyl chlorides and Et3N in MeCN gives in 30−60% yield the C2v symmetrical tetrasulfonates 3. The mild electrophilic substitution (aminomethylation, bromination) of resorcarene tetrasulfonates 3 and tetraphosphates 2 takes place only in the 2-positions of unsubstituted resorcinol rings yielding distally disubstituted resorcarenes 4. The acylation of hydroxy groups in compounds 2 and 3 gives C2v symmetrical octaesters 5 including large resorcarene tetracrown-ethers. In the minimized boat conformation of tetrasulfonates 3 the two unsubstituted resorcinol rings are vertical and the sulfonyl fragments are arranged in a C2 symmetrical manner via intramolecular SO- - -H−O hydrogen bonds. This structure is proved by NMR and IR spectroscopy. In the case of tetrasulfonate 3c the two enantiomeric conformations interconvert in CDCl3 with ΔG* = 11.6 kcal/mol. This is in agreement with the ΔE* value predicted by molecular mechanics in vacuo.

[structure: see text]. A diastereomeric mixture of chiral 25-(1S)-camphorsulfonyloxy-26-isopropoxycalix[4]arene 2a (de 15%) and 25-isopropoxy-26-((1S)-10-camphorsulfonyl)calix[4]arene 2b has been obtained by asymmetrical lower rim (1S)-camphorsulfonylation of the monoisopropoxycalix[4]arene. Pure diastereomer 2a has been obtained by simple crystallization, and its absolute configuration has been determinated by X-ray analysis. Enantiomerically pure inherently chiral 5,11-dibromo-26-isopropoxycalix[4]arene 4 has been synthesized by the upper rim dibromination of the diastereomer 2a followed by hydrolytical removal of the auxiliary camphorsulfonyl group.