Cascade Hydrogenation–Cyclization of Levulinic Acid into γ-Valerolactone Catalyzed by Half-Sandwich Iridium Complexes: A Mechanistic Insight from Density Functional Theory

Abstract

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.

Noninnocent Ligands for Efficient Dehydrogenation of Aqueous and Neat Formic Acid under Base-Free Conditions

“…First, having established that the dpa ligand is not intrinsically basic, we hypothesized that SO 4 2– , albeit very poorly basic, could play the role of a base to deprotonate coordinated formic acid. We made this assumption previously, and interestingly, Li and co-workers confirmed this hypothesis by DFT when studying several iridium catalysts including ours in the hydrogenation of levulinic acid into γ-valerolactone . Second, the importance of the bridging N–H moiety in the dpa ligand was considered to establish H-bonding interactions.…”

“…We made this assumption previously, 15b and interestingly, Li and co-workers confirmed this hypothesis by DFT when studying several iridium catalysts including ours in the hydrogenation of levulinic acid into γ-valerolactone. 33 Second, the importance of the bridging N−H moiety in the dpa ligand was considered to establish H-bonding interactions. An energetic profile of the reaction is presented in Figure 9a (for full details of the DFT calculation, see the Supporting Information).…”

Mechanistic Insights into Formation of All-Carbon Quaternary Centers via Scandium-Catalyzed C–H Alkylation of Imidazoles with 1,1-Disubstituted Alkenes

“…All calculations were performed with Gaussian 09 . The functional M06-L, suitable for applications in transition metal chemistry, has been successfully used in numerous studies on transition metal-catalyzed organic reactions. − It was combined with the dispersion correction D3 in this work to enhance computational accuracy. Structures were optimized and characterized by frequency calculations to be energy minima (zero imaginary frequencies) or transition states (only one imaginary frequency) at the M06-L-D3/6-31G­(d,p) level with the solvent (toluene) effects included using the SMD solvation model.…”

Solvent-Assisted Ruthenium Complex Catalyzes Hydrogenation and the Reductive Amination of Carbon Dioxide

“…Homogeneous Ru- or Ir-complex catalysts are normally well-defined and highly active and selective in hydrogenation. − Although Ir complex catalysts exhibited higher hydrogenation activity than those of Ru ones, the Ru catalysts were much cheaper and available and thus were applied more widely in laboratory and industry . Among them, ruthenium hydride and dihydrogen complexes have been recognized as active precatalysts or intermediates in catalytic hydrogenation. , Noyori and co-workers have proposed that the precatalysts were neutral ruthenium­(II) complexes RuCl 2 (binap)­(sol) 2 (sol = alcohol or acetone) .…”