Ruthenium Complexes with PNN Pincer Ligands Based on (Chiral) Pyrrolidines: Synthesis, Structure, and Dynamic Stereochemistry

Abstract

We report the synthesis of lutidine-based PNN type metal pincer complexes, using achiral (pyrrolidine) as well as chiral ((R,R)-2,5-dimethylpyrrolidine and (R)-2-methylpyrrolidine) substituents at the N side arm of the pincer ligand. With the six-coordinate saturated Ru pincers (PNN)Ru(H)(CO)(Cl), which have an aromatic pyridine ligand backbone, as the starting materials, treatment with strong base (KOtBu) generated the corresponding dearomatized pincer complexes (PNN')Ru(H)(CO). Spectroscopic, crystallographic, and computational studies demonstrate that the C-centered chirality from the chiral pyrrolidine group exerts a small but non-negligible influence on the preferred stereochemistry at Ru (and N in the case of (R)-2-methylpyrrolidine) that is reflected in the equilibrium distribution of diastereomers of these Ru complexes in solution. Our data show that the N- and Ru-based stereogenic centers in this class of compounds are stereochemically labile and the mechanisms for epimerization are discussed. Inversion at the Ru center in the dearomatized complexes is proposed to occur via a rearomatized Ru(0) intermediate in which the Ru-bound hydride is transferred to the ligand. Support for this comes from the spectroscopic characterization of a closely related Ru(0) species that is obtained by reaction with CO. Testing these catalysts in enantioselective oxa-Michael addition or transfer hydrogenation led to racemic products, while a low ee (8%) was observed in the hydrogenation of 4-fluoroacetophenone. The lack of appreciable enantioinduction with these catalysts is ascribed to the kinetic lability of the Ru stereocenter, which results in the formation of equilibrium mixtures in which several diastereomers of the catalyst are present. Copyright © 2020 American Chemical Society.