Υπερμοριακή χημεία: σύνθεση, χαρακτηρισμός, ιδιότητες και εφαρμογές δενδρομερών

Abstract

Supramolecular chemistry is defined as the chemistry beyond the molecule. Contrary to the classic molecular chemistry, which is based on strong covalent bonds, supramolecular chemistry is based on weak covalent intermolecular forces. Dendrimers are among the most attractive macromolecules, consisting of a multi-functional core and successive branched repeating units extending radially outward. They have attracted a great deal of attention producing various functions in many fields. A particular interest in dendrimer synthesis is devoted to the discovery of specific functions and properties that are a direct consequence of the dendritic architecture. The ability of the dendritic shell to encapsulate functional core moieties and to create specific site-isolated nanoenvironments, and thereby affect molecular properties, has developed the field of host–guest properties of dendritic molecules into a special area of supramolecular chemistry. The combination of dendrimers and other macromolecular topologies as building blocks is thus an attractive approach to the study of molecular recognition. The present PhD thesis joins together two parts of research fields of supramolecular chemistry, the crown- ethers and the dendrimers. It was expected that the conjunction of the above compounds will lead firstly to new type of compounds with interesting molecular architecture and mainly in new application uses of these compounds. Obviously, the cation-binding and extraction ability of crown ethers is critically affected by several factors including the symmetry of the crown ether, the size-fit relationship, the type of the donor atoms, multiplicity, and cooperative effects of neighbouring binding sites. The preorganization of a host molecule towards a certain guest compound is a central determinant of binding power and one of the most important strategies to increase complexing ability and selectivity. Among these, the attachment of an oligo(oxyethylene) chain to a crown ether is perhaps the simplest approach. The introduction of dendritic wedges may thus confer a third dimension to the crown macrocycle, related to the macrobicyclic and macrotricyclic cryptands, which display much stronger complexation towards metallic cations than the conventional crowns. In the present work we describe the design, synthesis, characterization, complexation and phace-transfer experiments of a series of new crown ether-functionalized dendrimers. Cation-active crown ether moieties of different sizes, that is, benzo-12-crown-4, benzo-15-crown-5, and benzo-18-crown-6 were functionalized with dendritic branches of Fréchet-type of first, second or third generation, which incorporate oxygen as electron donor atoms. Treatment of the corresponding branched benzyl alcohol of the first, second or third generation with excess NaH in refluxing THF, followed by the addition of the corresponding bis(bromomethyl)-substituted crown ether, afforded the crown ether-functionalized dendrimers. The bis(bromomethyl)-substituted benzo-12-crown-4, benzo-15-crown-5 and benzo-18-crown-6 were prepared in one step from the corresponding commercially available crown ethers, by bromomethylation with paraformaldehyde, NaBr, H₂SO₄, and acetic acid in dichloromethane. All the crown ether-functionalized dendrimers were monodisperse in pure form. They were unambiguously characterized by IR, ¹H and ¹³C NMR spectroscopy as well as by ESI high resolution mass spectrometry. A series of experiments were carried out to define the complexation properties of the novel crown ether-functionalized dendrimers. Extraction experiments of alkali cations (Li⁺, Na⁺, K⁺, Rb⁺ & Cs⁺) from an aqua layer to organic layer, showed, by using UV-spectroscopy, in which dendrimers the complexation ability was increased. Finally, the new crown ether-functionalized dendrimers were tested as phase transfer catalysts in the oxidation reaction of 3,5-bis-t-butyl-catechol to the corresponding 3,5-bis-t-butyl-quinone.