The stereochemistry of selected supramolecular complexes of cucurbiturils and the effect on homogeneous equilibria and electron transfer of the guest

From a practical perspective, host-guest complexes are vehicles for understanding and using supramolecular interactions for purposeful function in sensors, molecular machines and switches, while from a fundamental perspective they may lead to novel supramolecular protection schemes in basic organic reactions.;This dissertation examines the interactions of cucurbiturils as barrel-shaped hosts, exhibiting a hydrophobic cavity, with substituted benzoylpyrydinium, phenypyrylium, and diazaanthraquinonediium cations as guests.;In water, N-methyl-4-(p-substituted benzoyl)pyridinium cations exist in equilibrium with their hydrated forms (gem-diols), whose concentrations depend on the para substituent. In the presence of cucurbit[7]uril (CB[7]), the benzoyl group shows a preference for the CB[7] cavity, and the ketone to gem-diol equilibrium is shifted toward the keto form, meaning that the stabilization through hydrophobic interactions of the benzoyl group in the CB[7] cavity exceeds the hydrogen bonding stabilization of the gem-diols in the aqueous environment. In an aprotic polar solvent such as dimethylsulfoxide, 4-benzoylpyridinium cations undergo heterogeneous electron transfer simultaneously from both their free state as well as their complexes with CB[7].;In the same line of work, N,N'-dimethyl-2,6-diaza-9,10-anthraquinonediium dication in water not only exists in equilibrium with its gem-diol but also forms aggregates which cause line-broadening in 1H NMR. At low pH (<1), the aggregates break up and the equilibrium is shifted exclusively toward the quinone form. In the presence of CB[7], the quinone form undergoes inclusion with CB[7] by slow exchange in both water and aqueous acid. Both free and CB[7]-intercalated quinone forms are observed by 1H NMR.;To gain more insight on the intercalation of monocationic guests in the cucurbituril cavity not only as a function of their hydrophobic properties but also in terms of their shape, size and the size of the cavity, 4-phenylpyrylium cation (Pylm) was chosen as a guest and both CB[7] and CB[8] as hosts. The size and shape of the guest was modified by 2,6-substitution (Me, iPr, Ph, t-Bu). In water, 2,6-disubsituted-Pylm form dimers, but they enter as such only in CB[8]. All guests insert their 4-phenyl groups in either cavity, except (i Pr-Pylm)2 CB[8] where i Pr-groups are inserted. Stereochemistry is interpreted by an interplay of size and hydrophobicity of pyrylium substituents, solvation effects, and size and flexibility of the hosts.