What Causes Aqueous Acceleration of the Claisen Rearrangement?

We report the results of applying a new self-consistent-field solvation model to the Claisen rearrangement of allyl vinyl ether, all possible methoxy-substituted derivatives, two alkylated derivatives, and one carboxymethylated derivative in order to understand the effects of aqueous solvation on the reaction rates. We have employed the AM1-SM2 version of the model to calculate the changes in free energies of solvation in passing from the lowest-energy conformations of the starting materials to both chair and boat transition states. The hydrophobic effect is always accelerative but always small and not very structure sensitive. Other first-hydration-shell effects attributable to hydrophilic parts of the reagents are more sensitive to the substitution pattern. The polarization contributions to the activation energies are usually larger. A favorable polarization contribution is found to be associated with efficient sequestration of charges of opposite sign into separated regions of space. We conclude that aqueous acceleration of the Claisen rearrangement is caused by electric polarization and first-hydration-shell hydrophilic effects, with the relative magnitudes and even the signs of these effects being quite sensitive to substitution pattern.