A theoretical study of the mechanism for the homogeneous catalytic reversible dehydrogenation—hydrogenation of nitrogen heterocycles

Abstract

A mechanism for the dehydrogenation reaction of 1,2,3,4-tetrahydroquinoline to quinoline derivatives, catalyzed by a Cp*Ir complex containing a 2-pyridonate ligand, is proposed and supported by theoretical calculations at the B3LYP level. The proposed mechanism involves two stages which are all thermodynamically unfavorable (endothermic by 36.3 kcal mol⁻¹ and 18.4 kcal mol⁻¹, respectively). The apparent activation energies of the first and second stages of the reaction are 30.8 kcal mol⁻¹ and 34.0 kcal mol⁻¹, respectively, and are considered overestimates of the entropy change of reaction. Owing to a decrease in the oxidative ability of iridium(III) coordinated to large electronegative nitrogen and chlorine, ligand promoted hydrogen abstraction is crucial at both stages of dehydrogenation, in which the oxidation state of iridium(III) does not change, and the ligand 2-pyridonate is converted to 2-hydroxypyridine. Cp*Ir(C₅NH₄OH)ClH, an important intermediate, releases hydrogen through an energy barrier of 23.5 kcal mol⁻¹.