Macrocyclic tetra-NHC ligands are effective auxiliary ligands to support oxidation catalysis, including aziridination and epoxidation, on late transition metals such as iron. The reasons are twofold. First, their strong sigma donor strength promotes high oxidation state intermediates (e.g. iron(IV)) after the initial reaction with an oxidant such as an organic azide or hydrogen peroxide. Second, the NHCs are resistant to oxidation, unlike phosphines. Our recent research on these auxiliary ligands has focused on taking advantage of the underlying symmetry of 18-atom macrocycles. In this case, the ethylene bridge between the NHCs can be functionalized to yield D2-symmetric macrocycles by combining two C2-symmetric diimidazoles. These macrocyclic NHCs can then be ligated to iron either via direct deprotonation with strong bases or through a silver transmetalation, to yield C2-symmetric metal complexes. Single crystal X-ray diffraction of the ligands and associated iron complexes confirms their absolute stereochemistry. Test catalytic reactions for both aziridination and epoxidation with iron complexes have been investigated. Both spectroscopic and structural comparisons will be discussed as we showcase a next generation approach to developing enantioselective versions of these critical oxidation reactions.

D2-symmetric ligand design for chiral catalytic oxidation reactions on iron

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ACS Fall 2022

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