Inner‐Sphere C─N Bond Formation at Metal‐Bound Aldehyde in Ru‐Catalyzed N‐Alkylation of Anilines and Quinoline Synthesis

ABSTRACT

N‐alkylation of anilines and synthesis of quinolenes with primary alcohols catalyzed by our recently reported Ru─CNC pincer complexes ( 1a – 1d ), capable of new metal–ligand cooperativity via π–π interactions, is reported. This catalytic system displays a broad substrate scope with low catalyst and base loadings. DFT investigations revealed that the N‐alkylation does not proceed via an outer‐sphere pathway. In the outer‐sphere pathway, no stabilization due to π–π interactions between the extended aromatic ring system of the benzimidazolylidene‐based CNC ligand framework and the aromatic rings of the substrates was observed. An alternative inner‐sphere mechanism was found for this reaction in which C─N bond formation occurs through base‐assisted nucleophilic attack of the aniline on the metal‐bound aldehyde intermediate. The O‐bound intermediate thus formed undergoes an O‐to‐N coordination mode flip followed by C─O bond cleavage to generate the metal‐bound imine intermediate. The subsequent hydride transfer to the imine is followed by alcoholysis to release the final product. Further, DFT calculations reveal the possible explanation of the effect of alkali metal ion in the base on the selectivity of the reaction, as replacing K ⁺ with Na ⁺ increases the activation barrier for the Ru─O to Ru─N coordination flip (24.1 vs. 16 kcal).