Mechanistic DFT insights into the formation of high valent non-heme iron-oxo oxidants: Identifying the active oxidant for the epoxidation of alkene.

Iron-oxo intermediates play an important role in both enzymatic and synthetic oxidation catalysis. Synthetic non heme iron-oxo species have also the capability to promote chemo- and enantioselective oxidations that closely resemble enzymatic processes. In this context, the [(PDP)Fe(II)(CF3SO3)2]/H2O2/CH3COOH catalytic system has emerged as an efficient system for oxidation of alkene and also capable of activating C=C bonds with higher selectivity. In the present study, density functional theory calculations were employed to explore the formation and reactivity of high-valent iron-oxo intermediates generated through O-O bond cleavage. Our computed results reveal that formation of Fe(V)=O oxidant is strongly favoured over the corresponding Fe(IV)=O oxidant. Subsequent mechanistic analysis of epoxidation of cis-2-butene shows that the oxygen atom transfer towards C=C bond proceeds preferentially via a low-barrier concerted pathway rather than through a stepwise mechanism involving radical intermediates by Fe(V)=O oxidant. This concerted oxygen atom transfer pathway accounts for the experimentally observed chemo- and enantioselectivity of the reaction. Noncovalent interaction (NCI) and steric analyses further support reduced steric repulsion and enhanced stabilization along Fe(V)=O mediated concerted pathway compared to Fe(IV)=O route. Overall, this work identifies Fe(V)=O as a key active oxidant and provides a comprehensive mechanistic framework that rationalizes the selectivity and efficiency of epoxidation of the alkene by the non-heme iron catalysis.