Transition Structure and Substituent Effects on Aqueous Acceleration of the Claisen Rearrangement

A combined quantum mechanical and statistical mechanical approach is used to assess the transition structure and substituent effects on the rate acceleration of Claisen rearrangements in aqueous solution. It is demonstrated that the computed aqueous solvent effect depends critically on the nature of the transition structure, and a correlation between transition state stabilization and dipole moment in water is obtained. In addition, transition structures for a series of substituted allyl vinyl ether compounds have been optimized at the RHF/6-31G(d) level. The results indicate that transition structures are less tight for donor substitutions on the allyl fragment and acceptor groups on the oxallyl unit than the parent AVE. This leads to increased aqueous rate acceleration, accompanying enhanced ionic characters at the transition state for the Claisen rearrangement. Substituted R2-cyano and R₆= methoxy allyl vinyl ether is predicted to have a synergetic effect with a solvent-induced rate increase of 6 × 10⁵.