Mechanistic insights on the epoxidation of alkenes by high-valent non-heme Fe(iv) and Fe(v) oxidants: a comparative theoretical study

Abstract

In enzymatic and synthetic catalytic oxidation, highly reactive iron–oxo intermediates play an important role as oxidants in the processes of hydroxylation, epoxidation, halogenation reactions, etc. Synthetic iron–oxo species also have the ability to catalyse chemo- or enantioselective reactions similar to enzymatic catalysis. In this context, a report on the [(PDP)Fe^II(CF₃SO₃)₂)]/H₂O₂/AcOH system has gained attention as this activates the CC bond of the alkene and has the catalytic ability to perform chemo- or enantioselective conversion of the alkenes. In this study, we have employed density functional methods for the formation of Fe^IVO as well as Fe^VO from O⋯O bond cleavage and also shown that the formation of Fe^VO dominates over the corresponding Fe^IVO species. In addition to having favourable formation energy, a lower barrier height was computed for the CC bond activation of cis-2-butene in the S = 3/2 state in a concerted manner rather than forming a radical intermediate in a stepwise manner from the Fe^VO unit. The concerted mechanism was found to be responsible for the chemo- and enantioselective product, which was also observed in the experimental findings. Furthermore, NCI plots also support the less steric interaction during the formation of Fe^VO compared to Fe^IVO species. Additionally, during the epoxidation reaction, these steric effects significantly contributed towards the concerted route over the stepwise pathway.