Ru-CNP Complex-Catalyzed Hydrogen Transfer/Annulation Reaction of 2-Nitrobenzylalcohol via an Outer-Sphere Mechanism

The development of efficient catalysts plays a central role in advancing chemical reactions. In this study, a ruthenium complex modified with a N-heterocyclic carbene-imine-phosphine ligand (CNP) was employed to enhance the hydrogen transfer/annulation process of 2-nitrobenzyl alcohols with various primary or secondary alcohols. The NMR and HRMS analyses of the reaction solution revealed the in situ formation of [fac-RuH(CO)(PPh₃)(κ³-CN(H)P)]Cl through the transfer hydrogenation of the imine moiety within the CNP ligand under the reaction conditions. This species, a bifunctional Noyori-type ruthenium complex featuring facial coordination with a CN(H)P ligand, served as a key catalytic intermediate. By leveraging the outer-sphere mechanism facilitated by [fac-RuH(CO)(PPh₃)(κ³-CN(H)P)]Cl, the synthesis of 75 quinolines from 2-nitrobenzyl alcohols and a wide range of alcohols has been achieved with yields as high as 95%.