DFT calculations have been performed to illuminate the mechanism of cascade hydrogenation-cyclization of
levulinic acid (LA) into γ-valerolactone (GVL) catalyzed by half-sandwich
iridium complexes. It is shown that the favorable mechanism involves a heterolytic
hydrogen cleavage for Ir-
OH species to form a monohydride
iridium species, concerted reduction of the C═O unit of LA,
hydrogen migration and
dehydration to produce the
iridium alkoxo complex, and cyclization of the
iridium alkoxo complex to generate GVL. The presence of water and counterions are proposed to be important for the hydrogenation where the former works as a
hydrogen donor and the latter acts as a
hydrogen shuttle. Intriguingly, the cyclization process exploits a
metal- and counterion-assisted concerted
dehydration-cyclization mechanism different from the known ones that feature the intramolecular esterification of
4-hydroxyvaleric acid. The effectiveness of the half-sandwich
iridium complex with the double-methoxy group on the bipyridine
ligand-catalyzed system is attributed to the stronger electron-donating methoxy group, which is beneficial to increase the electron density at the Ir center and hence promote the Ir-H bond cleavage. In addition, the calculated free energy barrier for the cascade hydrogenation-cyclization catalyzed by the
iridium complex with a dipyridylamine
ligand is comparable with that promoted by the
iridium complex with the double-methoxy group on the bipyridine
ligand (24.8 vs 26.8 kcal/mol). The present work rationalizes the experimental findings and provides in-depth insights into the catalysis of the half-sandwich
iridium complexes.