Theoretical Investigations of Substrate Behavior in FeCl₃-Catalyzed Carbonyl-Olefin Metathesis

FeCl₃-catalyzed ring-closing carbonyl-olefin metathesis is a powerful method for the formation of cyclic olefins. Multiple substrate classes are known to display this reactivity; however, two substrates have been reported to form an oxetane, and do not undergo retro-[2 + 2] fragmentation into the cyclic olefin and a byproduct carbonyl. Specifically, phenanthrene producing polycyclic aromatic hydrocarbons yield an oxetane when electrophilic fluorine is introduced α to the substrate carbonyl. Herein, we report the application of quantum chemical modeling of enthalpies and NBO charges to investigate this divergent reactivity. In particular, the replacement of C-H bonds with C-F bonds eliminates hyperconjugative stabilization of the retro-[2 + 2] transition state. Taken together, this model suggests that charge stabilization at the reactive carbonyl carbon dictates the ability of the oxetane to fragment into the metathesis product. However, we also observe that electron-deficient carbonyls have a significantly lower barrier to Fe(III)-mediated oxetane formation. Balancing the factors implicated by our model, we predict the structures of possible metathesis-active molecules as well as oxetane-forming molecules.