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A Pochini

Publications and source records attributed to A Pochini.

8 recordsLinked to original sources
PubMed2001

Anion allosteric effect in the recognition of tetramethylammonium salts by calix[4]arene cone conformers.

Rigid calix[4]arene cone conformers, which are efficient receptors for quaternary ammonium salts, are usually obtained through the functionalization of their lower rim with suitable groups. Using flexible cone conformer of calix[4]arene, bearing four 4-hydroxybenzyl groups as cooperative and rigidifying structural elements at the upper rim of the calix, which act as anion binding groups, a new heteroditopic cavitand, 7, was synthesized. Whereas the tetramethoxy derivative 8 does not show any complexing ability, its tetrahydroxy analogue 7 recognizes tetramethylammonium salts with high efficiency. The binding abilities of this new receptor toward a series of tetramethylammonium salts (tosylate, chloride, acetate, trifluoroacetate, and picrate) have been investigated in CDCl(3) solution and compared to the monotopic and rigidified, through the lower rim, cone biscrown-3-calix[4]arene 9. The results obtained confirmed that in CDCl(3) ion pairing strongly affects binding. In particular, the rigid monotopic receptor 9 experiences good efficiency toward tetramethylammonium salts having anions with low ion-pairing ability such as trifluoroacetate or picrate. On the contrary, for the new heteroditopic cavitand 7, a reverse order of efficiency was found. In the latter case a different complexation mode was hypothesized in which the tetramethylammonium cation is deeply entrapped into the host cavity and its counteranion participates to the recognition process by coordination via hydrogen bonding by the four OH groups. To further support the role of the anion in the recognition process, a "dual host" approach, employing 7 or 9 in the presence of a specific receptor for chloride anion (10), was utilized. Molecular modeling studies confirmed that in the complexes formed by 7 and TMA salts the counteranion is involved in hydrogen bonding with the host OH groups and that the guests are bound as ligand-separated ion pairs.

Allosteric Regulation↗
PubMed2000

Recognition of amides by new rigid Calix

The synthesis of new hosts specifically designed for the recognition of amides, characterized by two binding regions: a rigid calix[4]arene cavity and a sidearm, inserted at its rim, able to form strong hydrogen bonds, is described. The binding abilities of the new receptors toward amides of general structure R(1)CONR(2)R(3) have been investigated in CDCl(3) solution by (1)H NMR spectroscopy. When the additional binding site is the N-phenylureido group spaced by a methylene unit from the apolar cavity, binding constants up to 756 M(-)(1) were measured. Neither the two separate potential binding sites, nor the model host, where the calix[4]arene skeleton is flexible show detectable binding ability toward the series of guests examined. The rigidity of the calix[4]arene apolar cavity is the key control element in determining the efficiency of these molecular recognition processes. The presence of NH groups in the guest controls the efficiency and selectivity of binding.

Journal Article↗
PubMed2000

Calix

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Journal Article↗
PubMed2000

Rigidified calixarenes bearing four carbamoylmethylphosphineoxide or carbamoylmethylphosphoryl functions at the wide rim

Conformationally rigidified tetraCMPO derivatives have been prepared from calix[4]arene bis(crown ether) 4a in which adjacent oxygens are bridged at the narrow rim by two diethylene glycol links. Acylation of the tetraamine 4c with the CMPO-active ester 5b gave the tetraphosphine oxide 6a, while the tetraphosphinate 6b and the tetraphosphonate 6c were obtained by Arbuzov reaction of tetrabromoacetamido derivative 7 with PhP(OEt)2 or P(OEt)3. The extraction ability of these CMPO derivatives was checked for selected lanthanides and actinides and compared with the analogous compounds 1b, 10b and 10d derived from calix[4]arene tetrapentyl ether. All rigidified bis(crown ether) ligands are more effective extractants than their pentyl ether counterparts and require only 1/10 of the concentration (cL= 10 4M) to obtain the same distribution coefficients, while with CMPO itself a 2,000-fold concentration is necessary. This could be a consequence of a better preorganisation of the ligating functions owing to the rigidity which on the other hand did not change the observed selectivity for americium (DAm/DEu=9-19) and for light lanthanides over heavy ones. NMR relaxivity titration curves show that the complex of Gd3+ with ligand 6a is highly oligomerised in anhydrous acetonitrile over a large range of ligand:metal concentration ratios. Nuclear magnetic relaxation dispersion (NMRD) profiles also showed that large oligomers were formed, and their mean tumbling times were deduced from the Solomon-Bloembergen-Morgan equations. The NMR spectra of dia- and paramagnetic lanthanide complexes with 6a agreed with the presence of two conformers with an elongated calix[4]arene skeleton in which the distances between opposite methylene groups are different. Contrary to what was observed with ligand 2a, the addition of nitrate ions does not labilize the metal complexes, presumably because of the rigidification effect of the ether bridges. Single-crystal X-ray structures were obtained for the active ester 5b and for diphenylphosphorylacetic acid 5a.

Journal Article↗
PubMed2000

Dimeric capsules by the self-assembly of triureidocalix

A number of calix[6]arenes bearing ureas at the upper rim positions of alternate rings 1, 3 and 5 were prepared and studied in detail by NMR spectroscopy and gel permeation chromatography. N-Unsubstituted ureas were shown to dimerize through a cyclic array of hydrogen bonds to give cylindrical cavities capable of encapsulating small molecules such as dichloromethane, benzene and fluorobenzene. Slow equilibria between dimer and monomer were observed in [D6]DMSO-CDCl3 mixtures. By contrast, N-substituted ureas are monomeric. All urea monomers with bulky O-substituents display a solvent-dependent, slow equilibrium between C3v and Cs cone conformations.

Journal Article↗
PubMed1997

Recognition of cationic species with synthetic receptors.

In the past year progress in the study of cationic species has been made, particularly in our understanding of the factors which control the selective recognition of biologically important cations such as ammonium, alkali and alkaline earth metal ions, and of metal ions used in biomedicine such as lanthanides and iron(III). Based on this knowledge, several new hosts with improved transport, photophysical and biological properties have been designed.

Binding Sites↗