Depolymerization of Oxidized Lignin Catalyzed by Formic Acid Exploits an Unconventional Elimination Mechanism Involving 3c-4e Bonding: A DFT Mechanistic Study
Depolymerization of Oxidized Lignin Catalyzed by Formic Acid Exploits an Unconventional Elimination Mechanism Involving 3c-4e Bonding: A DFT Mechanistic Study
复制标题
甲酸催化氧化木质素的解聚利用了涉及 3c-4e 键合的非常规消除机制：DFT 机制研究
DOI：
10.1021/acscatal.5b01095
复制
发表时间：
2015-11-01
期刊：
影响因子：
12.9
通讯作者：
Wang, Zhi-Xiang
中科院分区：
文献类型：
--
作者：
Qu, Shuanglin;Dang, Yanfeng;Wang, Zhi-Xiang
由Stahl等人（Nature 2014, 515, 249-252）开发的氧化木质素模型（1(ox)）在甲酸催化下解聚为单芳烃的DFT研究已经完成。转化依次通过甲酰化、消除和水解进行。有趣的是，这种消除过程利用了一种不同于E2和E1cb等已知机制的非常规机制。新的机制的特点是通过一个中间proton-shared 3 c-4e稳定的债券(HCOO圆中心导中心导中心圆点H-circle +中心导中心导中心圆点O-circle - = c - a)和3 c-4e债券转向3 c-4e HCOO圆中心导中心导中心圆点H-circle +中心导中心导中心圆点(OOCH) -O-circle -债券联合离去基团,是最初正则H-bond (HCOO-H中心导中心导中心点OOCH -)。根据这些特点，以及原HCOO-H中心点中心点OOCH-键的重要作用，我们将其机理称为E1H-3c4e消去。E1H-3c4e消去的根本原因是，与附近的羰基（C-a=O）相比，较差的离去甲酸基在稳定HCOO-碱提取h - β所产生的负电荷方面的竞争力较弱，羰基可以利用负电荷与甲酸分子形成稳定的3c-4e键。此外，该研究还描述了甲酸在整个转化过程中的多种作用，这就解释了为什么HCO2H/NaCO2H介质在1（ox）解聚过程中如此优雅地工作。
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, 249-252). 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-4e bond (HCOO circle minus center dot center dot center dot H-circle plus center dot center dot center dot O-circle minus=C-a) and by shifting the 3c-4e bond to the 3c-4e HCOO circle minus center dot center dot center dot H-circle plus center dot center dot center dot(OOCH)-O-circle minus bond in the joint leaving group that is originally a regular H-bond (HCOO-H center dot center dot center dot OOCH-). According to these characteristics, as well as the important role of the original HCOO-H center dot center dot center dot OOCH- 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-beta abstraction by the HCOO- base than the nearby carbonyl group (C-a=O) that can utilize the negative charge to form a stabilizing 3c-4e 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 1(ox) depolymerizaion.