Synthesis of Higher Homologues of Cucurbituril Derivatives and Evaluation of their Supramolecular Properties
The cucurbit[n]uril (Q[n]) family provides real potential for a wide range of applications, including drug delivery, complex self-sorting systems, chemical sensors, environmental protection and supramolecular catalysis. However, compared with other host molecules, such as crown ethers, cyclodextrins and calixarenes, its further development has been limited by the challenge of achieving its functionalisation. The introduction of substituents, especially those carrying reactive functional groups, into Q[n] has met with significant problems, such as their unavailability or the fact that they are restricted to the smaller homologues, where n = 5 6 and, in some cases, only n = 5. In this thesis, we describe the synthesis of a new family of cyclopentanoQ[n] (n = 5 7) which opens the way to obtaining significant proportions of the substituted higher homologues. In describing our achievement, we also identify reasons why certain publications report the synthetic difficulties in trying to obtain other substituted higher homologues. We have shown that alkyl substituents on Q[6,7] modify the host guest binding behaviour with regard to increased binding constants and slower rates of exchange. The reasons for these changes have been discussed with reference to their substitutent effects upon the electronic characters of their portals and cavities. In this context, the shape and size of the guest, as well as its charge, are considered. In particular, the neutral guest dioxane and the ionic guest adamantyl ammonium ion have been studied. The rates of guest exchange for substituted Q[7] have been found to be much slower than those for normal Q[7] when studied in detail using the adamantyl ammonium ion and substituted ferrocene ammonium ion derivatives. Two guest exchange mechanisms are possible: dissociation and replacement; and displacement or shunting . Both have been evaluated in relation to the kinetics of the exchange reaction. Our study of the synthesis of the new family of cyclopentanoQ[n] derived from the cyclopentane substituent provides opportunities for achieving similar substituents which bear reactive functional groups. Some of these are explored, and at least two potentially applicable starting glycolurils, in which the cyclopentane substituent ring carries reactive functional groups as esters or acetate-protected alcohols, were successfully synthesised. Our cyclopentanoglycoluril example, which introduces substitution into the Q[n] family by virtue of the positive effect of a five-membered carbon ring on the Q[n] synthesis, and, in particular, achieves the higher homologues, led us to also examine the strained rings of the norbornane structure. Glycolurils derived from norbornane derivatives could potentially offer greater opportunities to the synthesis of higher homologues of substituted Q[n]. We report our findings in relation to the ease or difficulty of the norbornane derivatives to form glycolurils. We discovered intermediates that would normally not have been isolated, however, they help to explain the general reaction process in the synthesis of aliphatic glycolurils, which are less common than aromatic glycolurils. These intermediates provide insight that may prove useful for future aliphatic glycoluril syntheses.
supramoleular chemistry, cucurbituril derivative, glycoluril
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