Claisen rearrangements: insight into solvent effects and "on water" reactivity from QM/MM simulations

An "on water" environment, defined by the absence of water solubility of the reactants, has been reported to provide increased rate accelerations, yields, and specificity for several types of organic reaction classes compared to organic solvents. The aromatic Claisen rearrangements of allyl p-R-phenyl ethers (R = CH(3), Br, and OCH(3)) and allyl naphthyl ether have been investigated to determine the origin of the on water effect using QM/MM Monte Carlo calculations and free-energy perturbation theory. The simulations indicate that on water rate enhancements for the rearrangements are derived from the ability of the interfacial waters to stabilize the polar transition state via enhanced hydrogen bonding at the oil/water interface. The position and orientation of the aromatic ethers at the interface are crucial factors affecting solvent accessibility during the reaction pathway; computed solute-solvent energy pair distributions and radial distribution functions showed that hydrophobic substituents on the solute provided a more polar solvent environment than hydrophilic substituents by tilting the reacting oxygen toward the water surface. Calculations in 16 different solvents accurately reproduced the experimental trend of increased rates correlated to increasing solvent polarity. Hydrophobic effects did not provide a substantial contribution to the lowering of the free energy activation barrier (<0.5 kcal/mol), and solvent polarizability via a polarizable force field was also found to be negligible in the observed rate accelerations. New insight into solvent effects for the Claisen rearrangement is presented herein, and a QM/MM approach for computing reactions on a liquid surface is highlighted.