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Iron complexes with chiral tetradentate ligands based on the pdp scaffold (pdp = N,N′-bis(2-pyridylmethyl)-2,2′-bipyrrolidine) are efficient and versatile catalysts for the highly enantioselective epoxidation of a wide range of olefins. The nature of the species responsible for oxygen atom transfer to the olefin in these reactions is under debate. In order to investigate this question, the enantioselectivity of the epoxidation reaction has been used as a mechanistic probe. The enantioselectivities obtained under different reaction conditions for two iron catalysts (S,S)-[Fe(CF3SO3)2(Me2Npdp)] ((S,S)Me2N1Fe) and (S,S)-[Fe(CF3SO3)2(dMMpdp)] ((S,S)dMM1Fe) have been analyzed. Reactions were performed with a series of peracids, and enantioselectivities of these reactions were compared with those obtained by combining peroxides and carboxylic acids. This analysis provides conclusive experimental evidence that the same oxidant is responsible for the asymmetric epoxidation reaction in both scenarios. The study al...