Mechanism of Enantioselective C−C Bond Formation with Bifunctional Chiral Ru Catalysts: NMR and DFT Study

The mechanism of Michael addition reactions of 1,3-dicarbonyl compounds to cyclic enones catalyzed by bifunctional Ru catalysts bearing N-sulfonylated (R,R)-DPEN ligands (DPEN = (R,R)-1,2-diphenylethylenediamine) was studied by NMR and DFT computational analyses. NMR investigation of the stoichiometric reactions of chiral amido Ru complexes, Ru(N-sulfonylated dpen)(η6-arene) 1a−c, with dimethyl malonate 2 and β-keto ester 3 revealed that at decreased temperatures deprotonation proceeds in a stereoselective manner to provide amine complexes. The reaction with malonic ester 2 provided exclusively C-bound amino Ru complexes 6a,c, while the reaction of β-keto ester 3 gave an equilibrium mixture of rapidly interconverting C- and O-bound complexes. The structures of C-bound Ru complex 6c and O-bound Ru complex 9c were determined by single crystal X-ray analysis. A computational study showed that the enatioselective C−C bond formation proceeds through intermediate formation of chelating ion pairs that coordinate a molecule of enone via the Ru metal center producing a highly organized environment for the C−C bond formation, yielding selectively only one enantiomer of the product. Systematic study of a series of the catalyst−substrate combinations revealed that the experimentally observed sense of enantioselection was consistently explained by computational analysis. The tendency of increasing ee with the bulk of the coordinated arene in Ru complex is reproduced computationally by changes in the difference of either ZPPE-corrected energies or Gibbs free energies for S- and R-pathways.