A combined NMR, computational and HPLC study of the inclusion of aromatic and fluoroaromatic compounds in cyclodextrins as a model for studying carbohydrate-aromatic interactions

A combined NMR, computational, and HPLC study of the inclusion of aromatic and fluoroaromatic compounds in cyclodextrins has been carried out with the aim of studying this process as a model for carbohydrate⋯aromatic interactions. NMR experiments showed that although β-CD formed an inclusion complex with benzene, no stable inclusion adduct was formed with hexafluorobenzene. MM3* calculations confirmed these data and, when extended to naphthalene, anthracene, phenanthrene, and their partially and fully fluorinated analogues, showed that β-CD formed inclusion adducts only with the hydrocarbons, whereas the partially fluorinated derivatives were shown to enter the cavity only with their non-fluorinated part, and the perfluoro compounds were not included. HPLC experiments, carried out by eluting these compounds through CD-modified HPLC columns, also confirmed these results, at least with the mono- and bicyclic arenes. Indeed, the elution of these derivatives showed that the efficiency of the inclusion, as determined by comparing HPLC retention times, decreased on passing from hydrocarbons to partially fluorinated compounds, and reached a minimum with perfluorinated derivatives. In contrast with this general trend, 1,2,3,4-tetrafluoroanthracene had longer retentions times on β- and γ-CD-modified HPLC columns than anthracene, possibly for solubility reasons. As a whole, these results were interpreted in terms of carbohydrate⋯aromatic interactions between the C5-H5 bonds on the hydrophobic faces of the sugars and the π electrons of the aromatic partner. The interactions seem to have an important dispersive component.