Direct C2-arylation of quinoline N-oxides by boronic esters; a molecular approach on the efficient metal-free method in C–C cross-coupling reactions

Kinetic and thermodynamic aspects of the cross-coupling reaction of quinoline N-oxides by various boronic esters in the gas and solvent phases were investigated, theoretically. Two types of boronic esters were considered for the reaction. There are various substitutes in types 1 and 2 which bonded to oxygen and boron atoms, respectively. For investigation of the chemical kinetic aspects of both types, different characters of the rate-determining step (RDS) were considered. Electron density of the involved bonds at the TS, natural charge density of the atoms and donor–accepter orbital interactions are the criteria which were used for the RDS justification of type 1. Nucleus-independent chemical shifts index was applied for description of type 2 behavior in cross-coupling reactions. Theoretical evaluation reveals that a large charge density on the oxygen atom of the O–B bond leads to a higher ability of proton abstraction which reduces ∆G≠ values in type 1 boronic esters. Furthermore, a large aromaticity character in the quinoline ring at the TS in type 2 boronic esters causes a decrease in the ∆G≠ value. Finally, based on the obtained results, the ability of proton abstraction of type 1 and the aromaticity character of type 2 boronic esters are the main factors in the energy barrier of the studied reactions.