木质素及其模型化合物醚氧键选择氧化断裂研究

Recently, the energy crisis brings out new opportunities for biomass conversion. Nature efficiently synthesizes aromatic structures and deposits them as lignin. As a potential alternative feedstock to petroleum resource, lignin can be used as raw material for producing aromatic chemicals. However, the structure of lignin is relatively complex and is highly functionalized, and thus the lignin conversion still remains a challenging task. We are driven by the dehydroxylation reaction of phenols in organic synthesis. In this reaction, the ether bond (C-O-aryl) is cleaved via selective oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (or DDQ). The stoichiometric reaction will convert the protection groups to alcohols, aldehydes or acids, and DDQ to 2,3-dichloro-5,6-dicyano-1,4-hydroquinone (DDHQ). Herein we propose a redox catalytic system for DDQ in the selective oxidative cleavage of ether bonds of lignin and its typical model compounds with molecular oxygen. We will focus on the substituent electron effect on DDQ, and particularly on the mechanism of selective oxidative cleavage of ether bonds. In the study, compounds of NOx or NaNO2 will be employed as co-oxidant in order to build a redox cycle of DDQ and DDHQ. In situ ATR FT-IR under reaction pressure will be employed to probe reaction intermediates, based on which we can investigate the reaction mechanism for bond cleavage and redox cycle of DDQ. Futhermore, we will employ our method in the selective conversin of lignin to monomer aromatic compounds. We believe that this study will greatly contribute to the basic understanding of oxidative conversion of lignin and will provide the reference for designing new catalytic system.

能源危机为生物质转化利用提供了契机。木质素是自然界含量最多的芳香族化合物，是潜在的可再生原料，替代和补充石化资源制芳香族化学品。木质素结构组成复杂且高度官能团化，转化利用充满了挑战。我们从有机合成中羟基脱保护的方法中获得启发。2,3-二氯-5,6-二氰对苯醌（DDQ）能氧化断裂醚氧键，生成醇、醛或酸等产物，反应后被还原为2,3-二氯-5,6-二氰对苯氢醌（DDHQ），为化学计量反应。本课题研究以分子氧为氧源，构建对苯醌类化合物为核心的催化体系，用于醚氧键氧化断裂。重点研究取代基的电子效应和结构效应，探索木质素模型化合物醚氧键氧化断裂机制；研究以氮氧化合物等为共氧化剂，实现对苯醌类化合物在反应体系内的循环，建立氧化还原耦合反应体系；研究高压液相原位红外光谱等表征手段，探索木质素模型化合物醚氧键断裂和对苯氢醌的氧化还原循环的机理，并将该方法应用到真实木质素催化氧化转化的过程中。

能源危机为生物质转化利用提供了契机。木质素是自然界含量最多的芳香族化合物，是潜在的可再生原料，替代和补充石化资源制芳香族化学品。木质素结构组成复杂且高度官能团化，转化利用充满了挑战。本课题研究以分子氧为氧源，构建氧化-氧化两步法策略，用于β-O-4醚氧键氧化断裂。首先，VOSO4/TEMPO为催化剂，β-O-4醇氧化到β-O-4酮；β-O-4酮在铜基催化剂作用下进一步氧化裂解到芳香有机酸和酚类。重点研究β-O-4酮在铜基催化剂上C-C键的氧化裂解。同位素动力学实验证明Cβ-H键的断裂是β-O-4酮中C-C键氧化裂解的决速步。Cu(OAc)2/1,10-Phenanthroline 配合物可以有效的活化氧气分子，形成铜-氧二聚体是催化活性中心，可以摘取Cβ-H键。BF3•OEt2的引入促进了C-C键的裂解。 实验和理论计算证明，BF3•OEt2与羰基相互作用，削弱了Cβ-H键，在无配体情况下，Cu(OAc)2/ BF3•OEt2催化剂可以实现β-O-4酮中C-C键氧化裂解的高效裂解。而且，BF3•OEt2能够稳定酚类产物，提高酚类收率。发展了一种串联的光催化氧化-氢解方法来断裂β-O-4醇中的C-O键。Pd/ZnIn2S4催化剂用于在455 nm光照下有氧氧化β-O-4醇中α-C-OH结构，接着在365 nm光照下通过氢解反应实现了TiO2-NaOAc催化断裂α-C=O键。通过双光束切换的方法（DLWS），这一氧化-氢解串联的方法能够一步断裂β-O-4醇并得到酮和酚（最高90%选择性）。反应过程中TiO2表面原位形成了Ti3+，并且光催化氢解过程是Ti3+向β-O-4酮的电子转移实现的。采用密度泛函计算，阐明了β-O-4 中C-O键在Pd（111）面上断裂机理。