Examining Ionic Liquid Effects on Mononuclear Rearrangement of Heterocycles Using QM/MM Simulations

The mononuclear rearrangement of heterocycles (MRH) reaction of the Z-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole into 4-benzoylamino-2,5-diphenyl-1,2,3-triazole derives a sizable rate enhancement in the 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF₄] ionic liquid as compared to the hexafluorophosphate-based [BMIM][PF₆] and conventional organic solvents. However, the origin of the rate difference between [BMIM][BF₄] and [BMIM][PF₆] has proven difficult to rationalize as no experimental trend relates the physical properties of the solvents, e.g., polarity and viscosity, to the rates of reaction. QM/MM calculations in combination with free-energy perturbation theory and Monte Carlo sampling have been carried out for the MRH reaction to elucidate the disparities in rates when using ionic liquids, methanol, and acetonitrile. Activation barriers and solute-solvent interactions have been computed for both an uncatalyzed and a specific base-catalyzed mechanism. Energetic and structural analyses determined that favorable π⁺-π interactions between the BMIM cation, the substrate phenyl rings, and the bicyclic quasi-aromatic 10π oxadiazole/triazole transition state region imposed a preordered geometric arrangement that enhanced the rate of reaction. An ionic liquid clathrate formation enforced a coplanar orientation of the phenyl rings that maximized the electronic effects exerted on the reaction route. In addition, site-specific electrostatic stabilization between the ions and the MRH substrate was more prevalent in [BMIM][BF₄] as compared to [BMIM][PF₆].