RESUMEN

The dimerization of tetratolyl- and tetratosyl-urea derivatives 1 and 2, derived from a tetrapentoxy calix[4]arene in the cone conformation and of the corresponding tetra-urea derivatives 3 and 4, in which the cone conformation is rigidified by the two crown-3 tethers, have been studied. All six possible equimolar mixtures were examined by 1H NMR using CDCl3 and CD2Cl2 as solvents. While no heterodimers are found for the combinations 1/3 and 2/4 in either solvent, all remaining combinations lead to the (exclusive) formation of heterodimers in CD2Cl2. In CDCl3 heterodimers are only observed for the combinations of 3 with 2 or 4. These results are discussed in terms of entropic and enthalpic contributions and compared with MD-simulations in a box of chloroform solvent molecules.

Asunto(s)

Calixarenos/química , Urea/análogos & derivados , Urea/química , Simulación por Computador , Dimerización , Entropía , Espectroscopía de Resonancia Magnética , Conformación Molecular , Docilidad , Estereoisomerismo

RESUMEN

Wide rim tetraurea derivatives (2a,b) have been prepared from a calix[4]arene rigidified in the cone conformation by two diethyleneglycol ether bridges between adjacent oxygens. In comparison to the analogous tetraurea derivatives (3a,b) of a tetrapentoxy calix[4]arene, 2a,b show an increased thermodynamic stability in mixtures of CDCl(3) and DMSO-d(6). Their kinetic stability as expressed by the rate of guest exchange (benzene or cyclohexane against the solvent benzene-d(6)) is also strongly increased by factors of 30-38. Noticeable differences for the inclusion of selected guests are found.

Asunto(s)

Calixarenos , Fenoles/química , Urea/análogos & derivados , Urea/síntesis química , Cinética , Espectroscopía de Resonancia Magnética , Conformación Molecular , Estructura Molecular , Termodinámica , Urea/química

RESUMEN

There is more than one way to assemble the two halves of a tethered, urea-substituted calix[4]arene dimer (shown schematically): formation of unimolecular capsules (far left), dimers, or oligomers. By combination of NMR spectroscopy and electrospray mass spectrometry, a hexamethylene spacer was shown to be exactly right to permit the preferential formation of a unimolecular capsule under inclusion of solvent or other guest molecules.

RESUMEN

(E)-Vinylphosphonium salts are conveniently obtained from the reaction of carbonyl compounds with titanium-substituted ylide species (Me(2)N)(3)P=CHTi(OiPr)Cl(2) or (Me(2)N)(3)P=CHTi(OiPr)(2)Cl. While a wide variety of nonenolizable aldehydes are tolerated, the steric bulk surrounding the ylide carbon limits the process to highly activated or unhindered ketones. The vinylphosphonium salts may be converted to allenes by deprotonation and condensation with a second aldehyde, thus accomplishing a two-step double olefination allene synthesis. This methodology, along with a previously reported one-pot reaction, comprises the most convenient route available for the synthesis of 1,3-disubstituted allenes. The metal-substituted ylide reagent may be generated in situ from commercially-available starting materials. Two representative vinylphosphonium salts were shown to undergo reversible isomerization to the (Z)-form upon irradiation.