The Diels-Alder Reaction as a Mechanistic Probe for Vibrational Strong Coupling.

Vibrational Strong Coupling (VSC) has recently been reported to alter reaction kinetics. Hypotheses on how it does this have been proposed, but open questions remain regarding the importance of the polarity of the reaction mechanism and of intramolecular vibrational redistribution (IVR), among other factors. We propose the Diels-Alder (DA) reaction as a probe to study chemistry under VSC, owing to the high diversity of its reaction partners. Herein, fixed-width cavities and UV-vis spectroscopy were used to determine the rate constants for the reactions of the diene 1,3-diphenylisobenzofuran (DPIBF) with various dienophiles under different coupling conditions. We investigated the effect of coupling six different solvents and of cooperative coupling of the dienophile through the solvent. Secondly, as the DA reaction can be catalyzed by hydrogen bonding, we investigated how the reaction was influenced by coupling alcohol solvents. Finally, we explored the direct coupling of vibrational modes of the dienophiles, including the stretching mode of the reactive C═C bond. In all cases, no substantial changes to the reaction rate constants were observed among the diverse coupling scenarios explored. This work initiates the use of the DA reaction as a mechanistic platform to understand how VSC changes chemistry and invites further experimental and theoretical studies.