Effects of beta-cyclodextrin on the keto-enol equilibrium of benzoylacetone and on enol reactivity

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.