Investigation of Solvent Effects for the Claisen Rearrangement of Chorismate to Prephenate: Mechanistic Interpretation via Near Attack Conformations

Department of Chemistry, Yale University, 225 Prospect Street, New Haven, CT 06520-8107, USA.
Journal of the American Chemical Society (Impact Factor: 11.44). 07/2003; 125(22):6663-72. DOI: 10.1021/ja021423z
Source: PubMed

ABSTRACT Solvent effects on the rate of the Claisen rearrangement of chorismate to prephenate have been examined in water and methanol. The preequilibrium free-energy differences between diaxial and diequatorial conformers of chorismate, which had previously been implicated as the sole basis for the observed 100-fold rate increase in water over methanol, have been reframed using the near attack conformation (NAC) concept of Bruice and co-workers. Using a combined QM/MM Monte Carlo/free-energy perturbation (MC/FEP) method, 82%, 57%, and 1% of chorismate conformers were found to be NAC structures (NACs) in water, methanol, and the gas phase, respectively. As a consequence, the conversion of non-NACs to NACs provides no free-energy contributions to the overall relative reaction rates in water versus methanol. Free-energy perturbation calculations yielded differences in free energies of activation for the two polar protic solvents and the gas phase. The rate enhancement in water over the gas phase arises from preferential hydration of the transition state (TS) relative to the reactants via increased hydrogen bonding and long-range electrostatic interactions, which accompany bringing the two negatively charged carboxylates into closer proximity. More specifically, there is an increase of 1.3 and 0.6 hydrogen bonds to the carboxylate groups and the ether oxygen, respectively, in going from the reactant to the TS in water. In methanol, the corresponding changes in hydrogen bonding with first shell solvent molecules are small; the rate enhancement arises primarily from the enhanced long-range interactions with solvent molecules. Thus, the reaction occurs faster in water than in methanol due to greater stabilization of the TS in water by specific interactions with first shell solvent molecules.

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    • "Thus, former predictions [14] [15] of solvent effects for the uncatalyzed rearrangement could be in part affected by an inappropriate assignment of the active species. Jorgensen and coworkers realized of the importance of this issue and re-computed their early studies [15] by using Monte Carlo free energy perturbation methods under the NAC framework; in their new results [37], they found a NAC population in water of 82%. This finding contrasts with the value of 10 À4 % provided by Hur and Bruice [23] and with the conclusion reached by Karplus group [20] for the CHAIR conformation (see above; in principle, one would expect a greater abundance in water for a ''true'' GS conformation than for a NAC). "
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