Stereoselective Ring-Opening Reaction of α-Fluorinated Oxetanes: A Practical and Theoretical Investigation

The ring-opening reaction of fluorinated oxetanes by halides, including alkylidene oxetanes and spirocyclic oxetanes, was highly stereoselective and directed by the presence of a fluorine atom. This reaction allowed a stereoselective preparation of tetrasubstituted alkenes and substituted pyrrolidines containing all-carbon quaternary centers. Theoretical calculations were performed to shed light on experimentally observed regioselectivity in the opening of oxetane derivatives. Transition state calculations were carried out to compare the energy of transition states responsible for forming different diastereoisomers. These calculations were performed using several DFT functionals and benchmarked to DLPNO-CCSD(T)/def2-TZVP calculations. Intrinsic reaction coordinated (IRC) calculations were run to confirm if the found transition states connect reactants and products. The IRC paths were then decomposed into the electrostatic, steric hyperconjugative contributions to reaction barriers by using the natural bond orbital (NBO) analysis. The destabilizing F^δ-···Br^- electrostatic interaction directs the reaction pathway.