Selectivity in Host-Guest Chemistry
This thesis describes host-guest complexes with the use of a biphenyl crown ether or cyclodextrin as host. Complexes between the biphenyl crown ether and salts form phenylalanine esters were studied. Butylphenylalanine ester forms only 1:1 complexes; although excess butylphenylalanine was used, it was impossible to complex both sites of the crown ether. A negative cooperativity is shown for this complex. Diesters from phenylalanine and .alpha., .omega.-n diols, n=6, 8, 10 and 12, all form 1:1 complexes with both of the sites of the crown ether occupied. These complexes also show an alternating effect: for the guest having 6 and 10 CH2-groups, a C2- symmetric complex dominates. In the complexes with 8 and 12 CH2-groups, a C1-symmetric complex are relatively more stable. Using cyclodextrin as the host molecule, the Z/E-selectivity for Wittig reactions between semistabilized ylids and aromatic aldehydes with the ability to form complexes with cyclodextrin was affected. An increase in the Z-selectivity from 57% to 92% was reached in N,N-dimethyl formamide and an increase in the E-selectivity from 67 to 80% was reached for the same Wittig reaction in ethanol.
.gamma.-cyclodextrin was shown to form a water-soluble complex with C60 upon boiling an aqueous solution of .gamma.-cyclodextrin and fullerene soot. The complexation is very selective for C60; when C70-enriched soot was boiled with an aqueous solution of .gamma.-cyclodextrin only C60 dissolved. The complex was identified and investigated by NMR and UV/VIS techniques. The magenta-colored complex between C60 and .gamma.-cyclodextrin turns yellow on heating. The formation of the yellow solution is accompanied with the disappearence of the 1H NMR signals for the cyclodextrin complex and an increase of the signals for free cyclodextrin. The monomeric C60-complex is reformed when the yellow solution is heated with excess .gamma.-cyclodextrin.