Synthesis And Host-Guest Complexation Of Pillararenes

It has been demonstrated that the arrival of any new generation of macrocycles can accelerate the development of supramolecular chemistry and provide new opportunities for material science. Recently, a new kind of macrocyclic receptors, pillararenes, appeared in the supramolecular world. Due to their rigid symmetrical pillar architectures, pillararenes have displayed lots of fascinating properties in the preparation of supramolecular polymers, microtubes, vesicles, and other interesting supramolecular systems. In this dissertation, we focus on the preparation of pillararenes and their derivatives, and investigation of their host-guest properties.In the first part, we found another method to synthesize pillararenes. It was demonstrated that pillararenes could be obtained by the condensation of2,5-dialkoxybenzyl alcohol or2,5-dialkoxybenzyl bromide with an appropriate Lewis acid as the catalyst at room temperature in a very short time. The mechanism of this cyclization reaction is presumed to be a Friedel-Crafts alkylation. The effect of different Lewis acids on the cyclization reaction was investigated. The results showed that the effects of different Lewis acids on the cyclization reactions of2,5-dialkoxybenzyl alcohol and2,5-dialkoxybenzyl bromide were different. When2,5-dialkoxybenzyl alcohol was used as the reactant, the appropriate catalyst was FeCl3, AlCl3, SnCl4, or BF3·O(C2H5)2, while when2,5-dialkoxybenzyl bromide was used, the appropriate catalyst was FeCl3, AlCl3, ZnCl2or SnCl4. Using this present method, pillar[n]arenes (n=5,6) having different alkoxy substituents were prepared successfully. It is clearly demonstrated that dynamic covalent bond formation is at work during the preparation of pillar[5]arene derivatives under Friedel-Crafts conditions. Thermodynamic control explains the high yielding preparation of pillar[5]arenes.In the second part, we have successfully synthesized a cationic water-soluble pillar[5]arene, which bears trimethylammonium groups at both of the two rims. The presence of ten positive charges makes it possible to act as an anion receptor. Consequently, in aqueous media, this water-soluble pillar[5]arene could bind sodium1-octanesulfonate, forming a [2]pseudorataxane mainly driven by hydrophobic and electrostatic interactions.In the third part, we demonstrated that a cationic water-soluble pillar[5]arene with a well-formed hydrophobic cavity could bind neutral guests, even water-insoluble guests to form1:1host-guest complexes in water. This is the first demonstration that neutral guests can be bound by a water-soluble pillararene host in water. All the host-guest systems are in fast exchange on the NMR time scale. The hydrophobic interactions play a dominant role in the formation of these host-guest complexes. NMR binding studies indicated that the binding ability depends upon the guest hydrophobicity. The higher guest hydrophobicity gives the higher host affinity. These neutral guests were encapsulated by the host through the inclusion of the hydrophobic unit into the host cavity. No binding could be detected for methanol and ethanol because they were so hydrophilic for the host to compete with water. This work might be useful for encapsulation of interesting thermally fragile molecules and obtaining a better understanding of the major processes in nature.In the forth part, we have successfully synthesized the per-hydroxylated pillar[6]arene and obtained its single crystal structure, which was a regular hexagon. The X-ray crystallographic analysis showed that the pillar[6]arene molecules were arranged in an up-to-down manner to form infinite channels in the solid state. The pillar[6]arene could bind bispyridinium salts in acetone to form [2]pseudorotaxanes. The crystal structure of the complex between per-hydroxylated pillar[6]arene and paraquat paraquat showed that the presence of the guest in the cavity of the pillar[6]arene twisted the regular hexagonal structure, but it also destroyed the disorder. The easy availability of the per-hydroxylated pillar[6]arene makes these newly established host-guest recognition motifs applicable in the design and preparation of new functional supramolecular systems, including mechanically interlocked threaded structures.