Chiral Pyrazolinylidene Complexes for Asymmetric Catalysis

The strong σ-donating properties of N-heterocyclic carbenes (NHCs) have established them as key ligands in homogeneous catalysis. However, among this class of ligands, pyrazolinylidenes remain largely unexplored. Herein, we introduce the development of the first chiral pyrazolinylidene transition metal complex for asymmetric catalysis. Specifically, a chiral-at-ruthenium catalyst incorporating bidentate N-(2-pyridyl)-substituted pyrazolin-5-ylidene ligands within a C₂-symmetric octahedral framework is reported. The modular ligand synthesis enables systematic variation at both the pyridine and carbene backbones, providing access to a series of air-stable ruthenium(II) complexes. Spectroscopic analysis indicates enhanced σ-donor strength compared to related imidazolin-2-ylidene and mesoionic carbene congeners, contributing to increased electron density at the metal center and a pronounced transeffect. The resulting complexes were evaluated in the enantioselective intramolecular aziridination of an alkene-tethered N-benzoyloxyurea, affording a bicyclic urea-based aziridine containing a quaternary stereocenter in up to 95% yield and 98% enantiomeric excess. These findings reveal pyrazolin-5-ylidenes as powerful ligands for chiral transition metal catalysis and establish a robust catalyst scaffold to be applied to other asymmetric transformations.