(+)- and (-)-α-Pinene as chiral recognition probes withnatural cyclodextrins and their permethylated derivatives. An aqueousNMR study
1997
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.
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