Investigating Solvent Effects of an Ionic Liquid on Pericyclic Reactions through Kinetic Analyses of Simple Rearrangements

• Published in: ChemPlusChem (Weinheim, Germany) Germany), 2017-03, Vol.82 (3), p.449-457
• Main Authors: Keaveney, Sinead T., Haines, Ronald S., Harper, Jason B.
• Format: Article
• Language: English
• Subjects: ionic liquids; pericyclic processes; solvent effects; solvophobic effects
• ISSN: 2192-6506; 2192-6506
• DOI: 10.1002/cplu.201600585

Simple Cope and Claisen rearrangements were investigated in an ionic liquid and a range of molecular solvents through a series of kinetic studies. Analysis of the solvent effects on the Cope rearrangement of 3‐phenyl‐1,5‐hexadiene indicated that a solvophobic effect was responsible for the observed rate enhancement in the ionic liquid, and that this was due to preferential solvation of the transition state. A similar solvophobic effect contributes to the ionic liquid solvent effect on the Claisen rearrangement of allyl vinyl ether, although the ability of the ionic liquid to stabilise the incipient charges in the transition state also likely contributes to the rate increase observed in the ionic liquid solvent. The activation parameter data suggest that in this case the ionic liquid was interacting with species along the reaction coordinate through general coulombic interactions (more acetonitrile‐like) rather than through hydrogen‐bonding interactions (less ethanol‐like). Solvophobic effects can be used to explain rate enhancements of simple pericyclic processes in an ionic liquid, along with interaction of the ionic liquid components with the transition state (see figure).