Stereoselective Epoxidation of Unfunctionalized Aliphatic Alkenes with a Macrocyclic Tetra-NHC Iron(II) Catalyst

Abstract

The development of atom-economical and earth-abundant catalysis for enantiopure epoxides is highly valuable since these strained chiral rings are key intermediates for many synthetic applications. However, one class of alkenes has remained particularly challenging for effective chiral catalysis: unfunctionalized aliphatic alkenes. Furthermore, despite epoxides’ significance, no catalyst for asymmetric epoxidation of unfunctionalized aliphatic alkenes has been developed using an iron complex as the catalyst and H₂O₂ as the oxidant. This combination of catalyst and oxidant is highly attractive due to the high terrestrial abundance of iron and the benign byproduct (water) from the oxidation reaction. This report showcases a C₂-symmetric macrocyclic tetra-NHC iron(II) catalyst that is highly effective for epoxidation of cyclic and acyclic aliphatic alkenes, giving high yields and up to moderate %ee values while employing H₂O₂ as the oxidant. DFT analyses elucidate the free-energy pathway for the formation of the postulated iron-oxo intermediate as well as the key transition states for the induction of stereochemistry during the formation of the epoxide. Both the iron(II)/iron(IV)-oxo and iron(III)/iron(V)-oxo cycles were considered as possible pathways.