Sodium Binding Effects on Conformational Exchange in a Diquinone Calix[4]arene
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Abstract:
NMR studies show that a 2,4-diethoxycalix[4]arene-1,3-diquinone undergoes an unusually slow conformational change, from a predominantly anti-aryl conformation to that of a syn-aryl conformation, in the presence of Na+ ion. Dramatic changes are observed throughout the entire NMR spectral region. A novel finding is that the timescale of this conversion is unusually long, taking many hours at room temperature to complete. The kinetics of this conformational change over the range of 5−55 °C were measured in a binary solvent system. The rate constants are on the order of 10-5−10-4 s-1 in the presence or absence of Na+. These results suggest a cation effect on the equilibrium position of the anti and syn complexes rather than on the rate of interconversion. The kinetics of interconversion in the presence and absence of the salt are the same. The rate determining step corresponds to rotation of an aryl moiety through the annulus of the calixarene. Additionally, we note that the initially dominant anti-aryl conformer appears to be a kinetic product of the oxidation step of the synthesis with Tl(CF3CO2)3. Variable-temperature NMR studies in the absence of salt show a stable ∼4:1 ratio of anti to syn conformer, while the compound, as initially prepared in solution, exhibits a ratio of >50:1.Keywords:
Conformational isomerism
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Calixarene
Conformational change
Proton NMR
Calixarene
Structural isomer
Sulfonic acid
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Abstract The proton exchange processes in the trifluoroacetates of the title compound were detected by the NMR method: whereas the free energy of activation for the exchange process in the ap rotamer in CDCl3 was 16.4 kcal/mol at 298 K, that in THF-ds was 13.7 kcal/mol. In contrast, the sp rotamer exhibited the free energy of activation for the same process of 14.1 and 13.4 kcal/mol in CDCl3 and THF-d8, respectively. The presence of NH-π interactions in the ap rotamer and the absence of it in the sp are concluded to be responsible to the difference in CDCl3.
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The review consists of two parts.In the first part applications of calixarenes as sensors are described, paying attention to sensors of metal ions.The second part is concerned with other special applications of calixarenes.
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This paper describes a detailed study on the complexation of pyridinium derivatives with calixarenes bound to gold nanoparticles (AuNPs). The studied calixarene derivatives are mixed with alkanethiols to form mixed monolayers on AuNP surfaces. The key findings are: (i) even a small amount (less than 11 mol%) of calixarenes can retain their complexation abilities among a majority of alkanethiols in a mixed monolayer, showing that it is possible to dilute the active calixarene (and possibly other receptors) in gold surfaces, (ii) the chain length of the alkanethiol compared with the calixarene spacer length can be used to fine tune the complexation ability of the calixarene, and there exist calixarene–alkanethiol mixed monolayer compositions in which the particles become unstable due to mismatching ligand spacer lengths, (iii) calixarenes with very short spacers bound to the gold surface can experience an enhancement in the delocalized π-electron density available for cation complexation, likely due to the proximity of the gold-bound sulphur to the calixarene cavity.
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The p-tert-butylcalix[4]arene phosphinoxide derivatives and p-tert-butylcalix[6]-arene phosphinoxide derivatives were synthesized by substitution reaction.IR,1 H NMR and13 C NMR were used to characterize the compounds.The influences of pH,concentration of extractant,extraction time and temperature on the calixarene and its phosphinoxide deriva-tives' extraction of U(Ⅵ)were investigated.The results show that the extraction effect of U(Ⅵ)by p-tert-butylcalix[6]arene is higher than p-tert-butylcalix[4]arene,meanwhile,the extraction efficiencies of U(Ⅵ)by the calixarene phosphinoxide derivatives are higher than the extraction efficiencies of U(Ⅵ)by calixarenes.Compared with calixarenes,the calixarene phosphinoxide derivatives' extraction efficiencies of U(Ⅵ)are increased by 36.49% and37.61%respectively under the same condition.
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Abstract Stereoelectronic hyperconjugative interactions and the relative energies of conformers and transition states of 2‐, 3‐, and 4‐silathiacyclohexane were calculated at the B3LYP/6–311+G(d,p) level of theory. The chair conformer of 2‐silathiacyclohexane is 15.4 and 15.9 kcal mol −1 (1 kcal = 4.184 kJ), respectively, lower in energy than the chair conformers of 3‐ and 4‐silathiacyclohexane. Intrinsic reaction path calculations were used to connect the transition states between the respective chair and twist conformers and different chair–chair conformational interconversion paths were located for 3‐ and 4‐silathiacyclohexane. The energy of the transition state that connects the chair and 2,5‐twist conformers of 3‐silathiacyclohexane is 5.58 kcal mol −1 higher in energy than the chair. The transition state that connects the chair and 2,5‐twist conformers of 4‐silathiacyclohexane is 4.82 kcal mol −1 higher in energy than the chair. The energy differences (Δ E , kcal mol −1 ) between the chair conformer of 2‐silathiacyclohexane and the respective 1,4‐twist (Δ E = 4.16), 2,5‐twist (Δ E = 3.20) and 3,6‐twist (Δ E = 3.87) conformers were calculated. Small relative energy differences were calculated between the chair conformer and the respective 1,4‐twist (Δ E = 3.95), 2,5‐twist (Δ E = 4.07) and 3,6‐twist (Δ E = 3.46) conformers of 3‐silathiacyclohexane. The calculated energy differences (Δ E ) between the chair conformer and the 1,4‐twist and 2,5‐twist conformers of 4‐silathiacyclohexane were 3.50 and 4.04 kcal mol −1 , respectively. The geometric parameters and stereoelectronic hyperconjugative interactions in the silathiacyclohexanes are compared and discussed. Copyright © 2004 John Wiley & Sons, Ltd.
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Calix[4]arenes substituted with amido groups on the upper rim, can be used a starting materials for the synthesis of calixarene ligands. These ligands can be ortho-palladated to form calixarene metalloreceptors capable of coordinating a substrate inside the bowl-shaped cavity of the calixarene. The principles of size and shape selectivity can be demonstrated employing substituted aromatic amines as model substrates.
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