| Abstract | 随着可持续和绿色化学的发展，C-H键的氧化官能化变得越来越重要, 这是由于C-H键大量存在于有机物中, 而且C-H键官能化后仅浪费一个氢原子，符合绿色化学的理念。另一方面，氧气是一种较为理想的氧化剂，它清洁、便宜易得，而且氧化后往往生成副产物水，对环境危害小。因此，利用氧气作为氧化剂进行C-H键的氧化，对绿色化学的发展有着重要的意义。苄基C-H键的需氧氧化反应，主要基于过渡金属催化、非过渡金属催化、光促进、自动需氧氧化等策略来完成。与过渡金属催化方法相比，非金属催化方面的报道较少, 而且使用的催化剂体系往往较为复杂，不利于其应用。针对这一问题，本文以苄基C-H键需氧氧化为研究对象，以“发展简单有效的催化剂体系及阐明反应机理”为主要目标，开展研究工作。主要内容如下：(1) 研究了氮氧物种对苄基C-H键需氧氧化的催化作用，发现HNO3能够催化该类反应。考察了各种反应条件对C-H键氧化的影响，得到了最优反应条件。研究了一系列苄基C-H键向C=O键的转化，结果表明3,4-二氢-1H-2-苯并吡喃、氧杂蒽、芴及它们的衍生物都能顺利地进行需氧氧化，羰基产物的产率在40%-93%之间。利用自由基抑制、控制性实验等方法，对反应机理进行了研究，提出了NO2作用下的机理路径，而以前文献报道氮氧物种通常用作助催化剂。本文的研究结果不仅为酮和酯的合成提供了一种新方法，而且发展了氮氧物种的催化新功能。(2) 基于氮氧物种（TEMPO/HNO3体系）对苄基醚C-H键需氧氧化的催化作用，通过氧化和氰化的串联，实现了苄基醚向芳腈的直接转化。对各种反应条件进行优化，并在最优条件下，考察了多种芳基醚底物的反应。得到的规律为：芳环上吸电子基团的存在对反应不利，多种给电子基团取代的苄基甲基醚能够顺利地进行反应，给出74-78%的目标产物。根据中间体检测、控制性实验等研究结果，提出了TEMPO+/NOx催化下的串联反应机理。该研究在芳腈制备、羟基脱保护等方面，有一定的理论和应用价值。(3) 依靠苄基C-H键的自动需氧氧化，实现了苄基C-H键向C=O官能团的选择性转化。考察了各种因素对反应的影响，优化了反应条件，研究了一系列二苯基甲烷、9,10-二氢蒽、氧杂蒽、芴、3,4-二氢-1H-2-苯并吡喃及它们的衍生物中C-H键的氧化转化。结果表明，它们中的苄基C-H键都能够顺利地进行需氧氧化，得到相应的羰基化合物，产率在21%-94%之间。利用自由基捕捉、分子模拟计算、18O-标记等手段，对反应机理进行了研究；阐明了π键对氧气激活作用下的自由基历程，发现OOH自由基的形成和目标产物的形成都是速决步骤。该研究为羰基化合物的合成提供了一种有效的新方法，加深了对C-H键自动需氧氧化机理的理解。(4) 利用苄基C-H键的自动需氧氧化，实现了苯基丙酮及其衍生物中C-C键的断裂，得到苯甲醛或苯甲酸类化合物。考察了各种条件对反应的影响，得到最佳反应条件。在最优条件下，几个苯丙酮的衍生物有效地转化为目标产物。利用18O-标记、中间体检测等手段，对氧化机理进行了考察，并提出了反应历程。将该方法用于木质素β-O-4类型关键化学键中C-C键的断裂，为木质素自动需氧化裂解为小分子化学品，提供了一些理论支持。 |
| Other Abstract | With the development of sustainable and green chemistry, the oxidative functionalization of C-H bonds is becoming more and more important because the C−H bonds widely exist in organic compounds. Moreover, only one hydrogen atom is wasted in the functionalization of the C-H bonds, which is consistent with the criterion of green chemistry. On the other hand, oxygen is a more ideal oxidant since not only it is inexpensive and readily available, but also the by-product of oxidation is water that has little pollution to the environment. Thus, it is of great significance to the development of green chemistry to use oxygen as the oxidant for the oxidation of the C−H bonds. In the domain of the aerobic oxidation of benzyl C-H bonds, the transition metal-catalyzed, non-transition metal-catalyzed, photo-induced and aerobic auto-oxidated strategies have been utilized to perform the reactions. Compared with the transition metal-catalyzed methods, the non-transition metal-catalyzed methods receive less attention, and the latter often requires an use of sophisticated multi-component catalyst systems, which is disadvantageous to their application. Aiming at this challenge, our research object has been set to develop various simple and effective catalysts, and to clarify the mechanisms regarding with the aerobic oxidation of benzyl C-H bonds.The main research contents and conclusion are as follows: (1) The catalysis of oxynitride species in the aerobic oxidation of benzylic C−H bonds was investigated, and it was herein found that HNO3 had the capacity to catalyze this kind of reaction. After effects of various conditions on the reactions were examined, the optimal conditions were clarified. Subsequently, we evaluated the substrate scope of the conversion of benzylic C−H bonds to C=O bonds, and the obtained results revealed that various isochromans, xanthenes, fluorenes and their derivatives underwent smoothly this transformation to give the carbonylation products in 40-93% yields. In addition, the reaction mechanism was investigated via the radical inhibition and various control experiments, then a NO2-catalyzed mechanism pathway was proposed, while oxynitride species usually played a role of the co-catalyst in many previous literatures. The results in the present chapter not only exhibit a new method for the synthesis of carbonyl compounds, but also show a new catalytic function of nitrogen and oxygen species.(2) Based on the catalysis of oxynitride species in the aerobic oxidation of benzyl C-H bonds, the direct conversion of benzyl ethers into aryl nitriles came true via a combination of oxidation and cyanation. After various reaction conditions were optimized, preliminary studies were conducted with a variety of representative benzyl methyl ethers to explore the scope and generality of this reaction under the optimal conditions. The obtained results suggested that the presence of electron-withdrawing goups bonded to the benzene rings had a negative effect on the reaction. A series of electron-donating group-substituted benzyl methyl ethers were smoothly converted to the targeted products in 74-78% yields. According to the experimental results related to various control experiments and the detection of intermediates, we proposed a mechanism pathway that underwent the stepwise steps under the catalysis of TEMPO+/NOx. The results in the present chapter are of certain academic and practical value in the preparation of aryl nitrile and the deprotection of ether-protected hydroxyl group.(3) By means of the aerobic auto-oxidation, the conversion of benzylic C−H bond to C=O group was actualized. Subsequently, effects of various conditions on the reactions were investigated, and the reaction conditions were optimized. In addition, we evaluated the substrate scope by testing the reactions of diphenylmethanes, 9,10-dihydroanthracene, xanthenes, fluorenes, isochromans and their derivatives, and the obtained results indicated that the benzylic C−H bonds of these compounds underwent smoothly the aerobic oxidation, and the targeted products were obtained in 21%-94% yields. The reaction mechanism was also investigated using radical trapping, DFT calculations and 18O-labelled experiments, and a radical mechanism pathway that involved the activation of oxygen by the π bond was clarified. It was confirmed that the formations of both the OOH radical and the targeted product were the rate-determining steps. The results in the present chapter reveal a new and simple method for the synthesis of carbonyl compounds, and increase the understanding of the mechanism related to the auto-oxidation of the C−H bond.(4) The cleavage of C-C bonds in phenyl acetones was actualized via the aerobic auto-oxidation of benzylic C−H bonds, and the obtained products were benzaldehyde and benzoic acid. Moreover, effects of various conditions on the reactions were examined, and the optimal conditions were clarified. Several phenyl acetones with different substituent groups were smoothly converted to the targeted products under the optimal conditions. The 18O-labelled experiments and the detection of intermediates were also performed to gain preliminary insight into the reaction mechanism, and a rough mechanism pathway was proposed. The present method could been applied into the cleavage of C-C bonds of lignin-related β-O-4 linkage, providing some theoretical information for the aerobic auto-oxidative depolymerization of lignin. |
