Cyclodextrin anion acetylation

Scheme 12.1 Acetylation of a cyclodextrin anion by a bound substrate via a tetrahedral intermediate.

The other point that was discovered was that some reaction rates were accelerated by operating in a mixed solvent rather than in pure water. The one that was examined most carefully was the acetyl transfer from bound ra-f-butylphenyl acetate to /3-cyclodextrin with buffers that in water give a pH of 9.5. It was observed that the reaction was almost 50-fold accelerated in a 60% DMS0-H20 solvent compared with the reaction rate in pure water. Part of this acceleration came from an increase in the apparent basicity of the medium, since relative pK s are solvent dependent part of it was also a solvent effect on the reaction rate of the cyclodextrin anion with the substrate. Thus, in 60% DMSO-H20 the /3-cyclodextrin reaction with this substrate was 13,000-fold faster than was the rate of hydrolysis of the substrate in an aqueous buffer of the same composition. Of this approximately 50-fold acceleration over cyclodextrin in water, about 10-fold was caused by changes in the pK s in the system and about 5-fold was caused by a change in the reaction rate of the cyclodextrin. 

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