Issue 36, 2024

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

Abstract

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 Cβ–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[double bond, length as m-dash]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 Cβ–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[double bond, length as m-dash]C bond, and Cβ–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.

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

Supplementary files

Article information

Article type
Paper
Submitted
04 Jul 2024
Accepted
20 Aug 2024
First published
20 Aug 2024

Phys. Chem. Chem. Phys., 2024,26, 23710-23721

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

Y. Zhang, Y. Luo, C. Hu, D. Tang and Z. Su, Phys. Chem. Chem. Phys., 2024, 26, 23710 DOI: 10.1039/D4CP02660H

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