Enantiodivergent Photochemical Rearrangements Due to Different Coordination Modes at an Oxazaborolidine Lewis Acid Catalyst

A strong enantiodivergence ranging from +92% ee to −45% ee was observed in the oxadi-π-methane rearrangement of 2,4-cyclohexadienones. Oxazaborolidine-based Lewis acid catalysts of the same absolute configuration were applied in all cases, and the stereochemical outcome is solely a function of the oxazaborolidine substituents. Based on the results of an extended catalyst library screening (27 examples) and by interrogating plausible catalyst-substrate complexes in the ground state with density functional theory (DFT) methods, we could link the switch in enantioselectivity to a change in substrate binding. If the typical substrate binding at the convex catalyst side is inhibited by bulky substituents, our results indicate that substrates instead bind to the concave side, and enantiomeric products result. Studies by TDDFT in the S₁ excited state further clarified the mechanistic picture by connecting efficient product formation with trajectories that reach a conical intersection with more excess energy. Our analysis was validated by the stereochemical outcome achieved with five structurally different catalysts.