Redox-neutral depolymerization of lignin-derived aryl ethers catalyzed by Rh(III)-complexes: a mechanistic insight

Density functional theory (DFT) calculations at the TPSSh-D3(BJ)/def2-TZVP (SMD, water) level of theory were performed to understand the mechanism of redox-neutral depolymerization of four types of lignin-derived aryl ether dimers catalyzed by rhodium-terpyridine ([Rh]) and a binuclear Rh complex ([2Rh]). The cleavage of the C_β-O bond in the β-O-4 model compound was initiated by the dehydrogenation of the alcohol moiety into a ketone intermediate, followed by the reductive cleavage of the ether bond, producing phenol and aromatic ketone products. The [Rh]-OH intermediate, generated by the interaction between the Rh-complex and NaOH, facilitated the transformation of the alcohol group to a CO group in the lignin model compound and subsequent H-transfer, selectively forming rhodium-H active species and the ketone intermediate. The [2Rh]-H complex exhibited high reactivity, with energy barriers for a rate-determining C_β-O bond breakage of 35.3 kcal mol^-1. In contrast to 1-phenylethan-1-ol and H₂, lignin itself acted as a good hydrogen source to generate [Rh]-H species. The transformation of β-O-4 model compounds with the γ-OH group occurred via the elimination of the γ-OH group, reduction of the CC bond, and C_β-O bond cleavage steps. However, since lignin itself was unable to supply enough hydrogen to form [Rh]-H species, the aromatic products were obtained in low yields, as observed in the experiment.