ABSTRACT
The synthesis and spectroscopic characterization of self-assembling calix[4]arene based capsules 1a.1a and 1b.1b are described. These compounds feature four urea substituents at the upper rims and four secondary amide fragments at the lower rims that can participate in inter- and intramolecular hydrogen bonding in apolar solution. Communication between the calixarene rims in 1a, b influences the self-assembled cavity's size and shape. Specifically. dimerization results in a perfect cone conformation of the calixarene skeleton in 1a, b and stabilizes a seam of intramolecular amide C=O...H-N hydrogen bonds at the lower rim. This seam is cycloenantiomeric, with either clockwise or counterclockwise arrangements of the head-to-tail amides. Complexation of Na+-cation breaks hydrogen bonds at the lower rim but maintains the capsular assembly. Encapsulation properties of 1a.1a and 1b.1b were studied in nonpolar solvents and their binary mixtures as well as through heterodimerization experiments. The presence of amide groups at the lower rim causes notable differences in the capsule's binding affinities when compared to the corresponding tetraester capsules 1c.1c and 1d.1d. In the monomeric state calixarenes 1a, b are in a pinched cone conformation. The solid state X-ray crystallographic studies with monomeric 1a reveal only two intramolecular C=O...H-N hydrogen bonds between the adjacent amides at the lower rim, and an extensive network of intermolecular hydrogen bonds between urea groups at the upper rim.
ABSTRACT
Synthesis of novel water-soluble cavitands 1 and 2 and their complexes--the caviplexes--is described. The solubility in water derives from four primary ammonium groups on the lower rim and eight secondary amide groups on the upper rim. Cavitands 1 and 2 exist as D2d velcraplex dimers in aqueous solution but the addition of lipophilic guests 15-24 induces conformational changes to the vase-like structures. The internal cavity dimensions are 8 x 10 A, and the exchange rates of guests in the caviplexes are slow on the NMR time-scale (room temperature and 600 MHz). The direct observation of bound species and the stoichiometry of the complexes is reported. The association constants (Ka) between 0.4 x 10(-1) (-deltaG295= 0.7 kcalmol(-1)) and 1.4 x 10(2)M(-1) (-deltaG295=2.9 kcalmol(-1)) in D2O and 1.4 x 10(1)(-deltaG295= 1.7 kcalmol(-1)) and 2.8 x 10(4)M(-1)(-deltaG295=6.0 kcalmol(-1)) in [D4]methanol for aliphatic guests 16-24 were determined. Guest exchange rates of the new hosts 1 and 2 are considerably slower than rates observed for typical open-ended cavities in aqueous solution.
ABSTRACT
The construction of a molecular cavity for recognition and catalysis requires either covalent synthesis or intermolecular self-assembly of complementary units. Intramolecular hydrogen bonding is another tool to control the cavity-forming process. When properly positioned within the same molecular structure, hydrogen bonding sites are responsible for the formation, preorganization, and binding ability of the host. The most typical examples from the supramolecular chemistry of calixarenes, the key cavity-containing building blocks, and derived from them receptor molecules are discussed.
ABSTRACT
A novel class of self-folding velcrands was prepared that dimerize through intermolecular forces. Solvophobic interactions on extended pi surfaces stabilize the dimer similar to velcrands, while eight hydrogen bonds act like snaps to hold the molecules together. The self-complementary array of hydrogen bonding sites were incorporated on the upper rim of a resorcinarene-based cavitand. A dramatic reorganization of shape and size of the internal cavity was manifested through changes in solvent polarity. Specifically, the equilibrium between the extended surface (D2d symmetry) and a deep cavity (C4v symmetry) could be manipulated in mixtures of aromatic solvents (or CDCl3) and [D6]DMSO. The switching of conformations and the dimerization motif are well-suited for the assembly of noncovalent polymeric materials.
ABSTRACT
Stereoisomeric complexes of an open-ended cavitand and its guest molecules (shown schematically) were detected, and complexes within caviplexes were introduced. Some control can be exerted on the stereoisomerism through methylation of the guest. The symbols "0" and "1" represent different ends of the guest molecule.
