Theoretical Mechanism of Rh-Containing Species Catalyzing the Hydrogenolysis of Lignin Model Compounds Using −CαO–H and −Cα–H Groups as Superior H‑Sources

The hydrogenolysis of lignin model compounds (MCs) into high-value chemicals has received increasing attention, but their catalytic reaction mechanisms are not yet very clear. Here, we report the reaction mechanisms of the hydrogenolysis of MC into 4-acetylanisole (AAL) and guaiacol (GAL) catalyzed by LRuCl3 (L = 4′-(4-methoxyphenyl)-2,2′:6′,2″-terpyridine) with MC, H2, and 1-phenylethan-1-ol (PEO) as the H-sources in aqueous solution with the Bro̷nsted base (NaOH), at the M06/def2-TZVP, 6-311++G (d,p) theoretical level, namely, RS-Self, RS-H2, and RS-PEO, respectively. After dissociation in NaOH aqueous solution, the LRuCl3 compound can form a stable complex LRh (OH)3 as the initial catalytically active species. Their rate-determining steps are related to the cleavage of the −CαO–H bond in both RS-Self-and RS-PEO, whereas it is associated with the heterolysis of the H–H bond in RS-H2. From NaOH aqueous solution, the OH– ligand of LRh(OH)3 promotes the cleavage of the −CαO–H bond with MC and PEO as H-sources, whereas it advances the heterolysis of the H–H bond with H2 as the H-source, owing to its strong Bro̷nsted basicity. Furthermore, the Rh center of LRh(OH)3 mediates the cleavage of the −Cα–H bond, because of its Lewis acidity. The reaction activity lowers as RS-Self > RS-PEO ≫ RS-H2, which is positively dependent on the protonation degree of the departing H atom. Compounds containing both −CαO–H and −Cα–H groups can act as effective donors for H-sources. In the meantime, the stronger the electron-withdrawing groups they contain, the more pronounced the protonation of the −CαO–H group and the higher the reaction activity in providing H-sources. The present results provide insights into utilizing lignin’s own functional groups as a hydrogen source for high-value conversion.