Evidence for Stepwise Oxetane Formation in an FeCl3 Catalyzed Carbonyl-Olefin Metathesis Reaction

Experimental 13 C kinetic isotope effects (KIEs) provide compelling qualitative evidence for a stepwise mechanism for oxetane formation in the intramolecular FeCl3 catalyzed ring-closing carbonyl-olefin metathesis (COM) reaction forming polycyclic aromatic hydrocarbons. This finding is in stark contrast to the current mechanistic understanding of this reaction, where prior studies have proposed an asynchronous concerted [2+2] cycloaddition to be the operational mechanism for oxetane formation. Theoretical studies including AIMD simulations, predicted KIEs from density functional theory (DFT) studies, and quasiclassical direct dynamics trajectory studies provide quantitative validation for the stepwise pathway. Our computed free-energy profile reveals similar barrier heights for formation of the betaine intermediate and the subsequent ring-closure to form the oxetane intermediate resulting in both steps contributing to the observed KIEs. Calculations show that this stepwise mechanism is insensitive to the substitution pattern on the olefin (2-methyl-1-propenyl versus styrenyl) and the nature of the carbonyl (ketone versus aldehyde) for COM reactions forming polycyclic aromatics.