(+)- and (−)-α-Pinene as chiral recognition probes with natural cyclodextrins and their permethylated derivatives. An aqueous NMR study

The binding of (1R)-(+)- and (1S)-(-)-α-pinene to α-, β- and γ-cyclodextrin and to the corresponding permethylated derivatives TMα-, TMβ- and TMγ-cyclodextrin, has been studied in aqueous solution by NMR spectroscopy. The stoichiometries and the association constants have been measured. The signals of both (+)- and (-)-α-pinene were found to follow the slow exchange, whereas those of the cyclodextrins the fast exchange regime, indicating that the type of exchange is independent of the magnitude of binding constants, which vary along the series, and also a result of other factors, in addition to the associated rate constants. The structures of the respective complexes have been derived from 2D ROESY experiments. The cyclodextrins preferentially bind with the (1S)-(-)-α-pinene, but only α-CD exhibits remarkable enantioselectivity. These observations show clearly that formation of a hydrogen bond or the presence of an aromatic ring, previously invoked as some of the requirements for enantioselectivity with CDs, are not necessary. In addition to enantioselectivity, site selectivity was also observed for α- and TMα-CD with the preferred (1S)-(-)-enantiomer, the stoichiometry being 2∶1 in both cases.