Improving the Configurational Stability of Chiral-at-Iron Catalysts Containing Two N-(2-Pyridyl)-Substituted N-Heterocyclic Carbene Ligands

Recently, we introduced the first example of chiralat-iron catalysts in which two achiral N-(2-pyridyl)-substituted Nheterocyclic carbene (NHC) ligands in addition to two labile acetonitriles are coordinated around a central iron, to generate a stereogenic metal center [Hong, Y. et al. Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis. J. Am. Chem. Soc. 2019, 141, 4569-4572]. A more facile synthesis of such chiral-at-iron catalysts was developed, which omits the use of expensive silver salts and an elaborate electrochemical setup. Configurational robustness was improved by replacing the imidazol-2-ylidene carbene moieties with benzimidazol-2-ylidenes. The pi-acceptor properties of the altered NHCs were investigated by Ganter's Se-77 NMR method. The obtained benzimidazol-2-ylidene chiral-at-iron complex is an excellent catalyst for an asymmetric hetero-Diels-Alder reaction under open-flask conditions.

Automated summary

This study introduces a novel chiral-at-iron catalyst and develops a more convenient synthesis method. In the new catalyst, N-(2-pyridyl)-substituted Nheterocyclic carbene (NHC) ligands coordinate around a central iron ion to generate a stereogenic metal center. The pi-acceptor properties of the modified NHCs were investigated, and the chiral-at-iron complex exhibited excellent asymmetric catalytic activity under open-flask conditions.