Elucidation of rate variations for a Diels-Alder reaction in ionic liquids from QM/MM simulations

The impact of acidic and basic ionic liquid 1-ethyl-3-methylimidazolium chloride (EMIC) melts upon cyclopentadiene and methyl acrylate Diels-Alder reaction rates has been investigated using QM/MM calculations. The ability of the ionic liquid to act as a hydrogen bond donor (cation effect), moderated by its hydrogen bond accepting ability (anion effect), has been proposed previously to explain observed endo/exo ratios. However, the molecular factors that endow ionic liquids with their rate enhancing potential remain unknown. New OPLS-AA force field parameters in conjunction with potentials of mean force (PMF) derived from free energy perturbation calculations in Monte Carlo simulations (MC/FEP) are used to compute activation energies. QM/MM simulations using a periodic box of ions reproduce relative rate enhancements for the EMIC melts compared to water and 1-chlorobutane that reproduce kinetic experiments. Solute-solvent interactions in acidic and basic ionic liquid melts have been analyzed at key stationary points along the reaction coordinate. The reaction rate was found to be greater in the acidic rather than the basic melt due to less-dominant ion-pairing in the acidic melt, enabling the EMI cation to better coordinate to the dienophile at the transition state. The simulations suggest that the hydrogen on C2 of the EMI cation does not contribute to stabilization of the transition state, as previously believed, and the interactions with the more sterically exposed hydrogens on C4 and C5 play a larger role. In addition, the relative stabilization of the transition state through electrostatic interactions with the EMI cation in the acidic melt is also greater than that afforded by the weaker Lewis-acid effect provided by hydrogen bonding with water molecules in aqueous solution.