Depolymerization of Oxidized Lignin Catalyzed by Formic Acid Exploits an Unconventional Elimination Mechanism Involving 3c鈥?e Bonding: A DFT Mechanistic Study

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• 作者：Shuanglin Qu ; Yanfeng Dang ; Chunyu Song ; Jiandong Guo ; Zhi-Xiang Wang
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：November 6, 2015
• 年：2015
• 卷：5
• 期：11
• 页码：6386-6396
• 全文大小：903K
• ISSN：2155-5435

文摘

A DFT study has been performed to gain insight into the formic-acid-catalyzed depolymerization of the oxidized lignin model (1^ox) to monoaromatics, developed by Stahl et al. (Nature 2014, 515, 249鈥?52). The conversion proceeds sequentially via formylation, elimination, and hydrolysis. Intriguingly, the elimination process exploits an unconventional mechanism different from the known ones such as E2 and E1cb. The new mechanism is characterized by passing through an intermediate stabilized by a proton-shared 3c鈥?e bond (HCOO^{鈯?/sup>路路路H鈯?/sup>路路路鈯?/sup>O鈺怌伪) and by shifting the 3c鈥?e bond to the 3c鈥?e HCOO鈯?/sup>路路路H鈯?/sup>路路路鈯?/sup>OOCH bond in the joint leaving group that is originally a regular H-bond (HCOO鈥揌路路路OOCH鈭?. According to these characteristics, as well as the important role of the original HCOO鈥揌路路路OOCH鈥?bond, we term the mechanism as E1H-3c4e elimination. The root-cause of the E1H-3c4e elimination is that the poor leaving formate group is less competitive in stabilizing the negative charge resulted from H尾 abstraction by the HCOO鈥?/sup> base than the nearby carbonyl group (C伪鈺怬) that can utilize the negative charge to form a stabilizing 3c鈥?e bond with a formic acid molecule. In addition, the study characterizes versatile roles of formic acid in achieving the whole transformation, which accounts for why the HCO₂H/NaCO₂H medium works so elegantly for 1ox depolymerizaion.}