Synthesis of Trisphenylenes for Supramolecular Applications

Abstract

Benzocyclotrimers are polycyclic molecules, characterized by a single or a double cup shape, and so by the presence of cavities. This feature makes them suitable as hosts in supramolecular chemistry. Benzocyclotrimers has been used as cavitands, nano-capsules, and molecular cages to bind small neutral molecules like acetylene and methane. Recent efforts have been made in order to obtain water-soluble benzocyclotrimers in order to exploit hydrophobic effect for encapsulation and self-aggregation phenomena. While traditional synthesis of Benzocyclotrimers is based on the coupling reaction among three monomers often with chromatographic issues and overall low yields, in this thesis the work has been focused on the research of alternative synthetic routes, involving triphenylenes as alternative and more easily prepared building blocks. The obtained benzocyclotrimers will be used for the synthesis of water-soluble hosts and supramolecular polymers. The starting material hexamethoxytrisphenylene was obtained by iron(III) chloride oxidative coupling of veratrole. The cyclotrimers can be obtained by oxidation of the hemi-O-alkylated hexahydroxytrisphenylene with iodobenzene diacetate (DAIB) in the presence of an alcohol, affording the tris-quinoid intermediate that undergoes Diels-Alder cycloaddition reaction. The crucial step of the whole route is the regioselective formation of the vic-hydroxy-alkoxy moiety in the hemi-O-alkylated hexahydroxytrisphenylene. Indeed, alkylation of the hexahydroxytrisphenylene affords unseparable complex mixtures of randomly substituted products. In order to overcome this problem two possible strategies were undertaken, involving the opening of acetal cycles with Lewis Acid/reductant or with a nucleophile. The research undertaken is a possible alternative to the classic route for the traditional synthesis of benzocyclotrimers based on the coupling of three polycyclic alkenes, which suffers the problems of low yields and reduced possibility of functionalization of the final products. Furthermore, the knowledge acquired during this research regarding the opening of hexahydroxytrisphenylene acetal could be exploited for the synthesis of vic-alcoxy1-alcoxy2 triphenylenes. These substances are difficult to obtain by conventional methods, and their synthesis is a hot topic in the field of liquid crystals preparation.