Isotope Effects Reveal an Alternative Mechanism for "Iminium-Ion" Catalysis

A novel mechanism for the epoxidation of enals with hydrogen peroxide catalyzed by diarylprolinol silyl ether supported by experimental ¹³C kinetic isotope effects (KIEs) and density functional theory calculations is presented. Normal ¹³C KIEs, measured on both the carbonyl- and β-carbon atoms of the enal, suggest participation of both carbon atoms in the rate-determining step. Calculations show that the widely accepted iminium-ion mechanism does not account for this experimental observation. A syn-S_N2' substitution mechanism, which avoids formation of a discrete iminium-ion intermediate, emerges as the most likely mechanism based on agreement between experimental and predicted KIEs.