Aerobic oxidation of biomass-derived furfural to furoic acid was studied with an N-heterocyclic carbene as a homogeneous catalyst. Carbene species generated in situ on 1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride with a strong organic base (1,8-diazabicyclo[5.4.0]undec-7-ene) was highly active and selective for the formation of furoic acid in dimethyl sulfoxide at 40 °C. This reaction initiates the formation of a Breslow intermediate between an N-heterocyclic carbene and a furfural molecule and the subsequent activation of molecular O₂. While the active carbene catalyst promoted furfural dimerization to afford furoin as a side reaction, furoin was decomposed into the Breslow intermediate and furfural through a reverse reaction, which were then converted quantitatively to furoic acid. Kinetic studies revealed that the apparent activation energy for this furfural oxidation was only 20 kJ mol^–1, which is significantly lower than that with a supported Au catalyst (30.4 kJ mol^–1). The N-heterocyclic carbene catalyst can oxidize various furan-based aldehydes with high selectivity; however, the electron-withdrawing group bonded to the furan ring has a negative effect on the reaction rate. Furfural can also be oxidized selectively to furoic acid, even in the presence of byproducts that are formed by acid-catalyzed dehydration of xylose with Amberlyst-70. As a result, a sequential reaction system based on initial dehydration and subsequent aerobic oxidation was developed for the production of furoic acid from xylose, without the need for furfural purification, using Amberlyst-70 (a solid acid) and an N-heterocyclic carbene catalyst.

中文翻译：

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N-杂环碳烯催化剂对糠醛的无金属选择性氧化为糠酸

以N-杂环卡宾为均相催化剂，研究了生物质衍生糠醛的好氧氧化为糠酸。在具有强有机碱（1,8-二氮杂双环[5.4.0] undec-7-ene）的1,3-双（2,4,6-三甲基苯基）咪唑鎓氯化物上原位产生的碳氢化合物具有很高的活性和选择性40°C下在二甲基亚砜中形成糠酸。该反应引发N-杂环卡宾与糠醛分子之间的Breslow中间体的形成以及随后的分子O _2的活化。。尽管活性卡宾催化剂促进糠醛二聚化以提供副反应的糠醛，但糠醛通过逆反应分解为Breslow中间体和糠醛，然后定量转化为糠酸。动力学研究表明，糠醛氧化的表观活化能仅为20 kJ mol ^–1，明显低于负载型Au催化剂（30.4 kJ mol ^–1）的活化能。）。N-杂环卡宾催化剂可以高选择性地氧化各种呋喃基醛。然而，结合在呋喃环上的吸电子基团对反应速率有负面影响。糠醛也可以选择性地氧化为糠酸，即使在副产物的存在下，副产物是由木糖用Amberlyst-70酸催化脱水形成的。结果，开发了一种基于初始脱水和随后的好氧氧化的顺序反应系统，用于使用Amberlyst-70（固体酸）和N-杂环卡宾催化剂从木糖生产糠酸，而无需糠醛纯化。 。