E. Iglesias et al., Effects of beta-cyclodextrin on the keto-enol equilibrium of benzoylacetone and on enol reactivity, J ORG CHEM, 64(11), 1999, pp. 3954-3963
Both beta-cyclodextrin and sodium dodecyl sulfate micelles shift the benzoy
lacetone keto-enol equilibrium to the enol tautomer by preferentially bindi
ng the enol form. The UV-vis spectroscopic method was used to quantify the
temperature and solvent effects on the keto-enol equilibrium of benzoylacet
one in aqueous acid medium. The comparison between the thermodynamic parame
ters resulting from the binding of the benzoylacetone enol to sodium dodecy
l sulfate micelles and from the inclusion of both keto and enol tautomers i
nto the beta-cyclodextrin cavity allows us to draw a picture of the possibl
e complex formed in each case. H-1 NMR results suggest that benzoylacetone-
enol protrudes deeper inside the beta-cyclodextrin cavity, whereas the keto
tautomer could only have the phenyl ring enclosed in the beta-cyclodextrin
cavity interior. Nitrosation in acid medium of benzoylacetone in the prese
nce of beta-cyclodextrin is reduced below that of free benzoylacetone, indi
cating that the cyclodextrin complex protects the benzoylacetone enol tauto
mer, which is in perfect accordance with our picture of the enol beta-cyclo
dextrin complex.