CAS OpenIR
Co/N-O自由基催化体系:制备、机理及应用
SAJID MAHMOOD
Thesis Advisor徐宝华
2018-06
Degree Grantor中国科学院大学
Degree Name博士
Degree Discipline应用化学
Keyword钴催化,Nhpi,碳氢化合物的需氧氧化,氧气扩散,Tempo
Abstract

碳氢化合物的直接氧化官能化是有机化学中最基本和最重要的反应,在工业催化中具有很好的应用前景,涉及许多基础化学品,天然产物,精细化学品和医药产品的合成。近年来,醛/醇和醇直接氧化酯化反应制备有机酯的方法因其突出的原子经济性而备受关注,相关方法学研究取得了一些显著成效,但普遍存在的问题是大部分已报道的催化反应仍采用当量的有机/无机氧化剂,反应需要在惰性气氛下进行。发展新的碳氢化合物直接氧化官能化的均相催化体系,尤其是以氧气为末端氧源的绿色催化体系,在更温和的条件下实现高选择性碳氢化合物的氧化以及交叉偶联反应,具有很高的学术价值和实际意义。本论文围绕非贵金属钴均相催化展开研究工作。首先以tBuOOH 为氧化剂实现了Co(II)催化的醛/醇与醇的直接氧化酯化反应;其次针对Co(II)/氮氧自由基需氧氧化催化体系,研究了Co(II)-TEMPO配合物的结构及其在单电子转移中的反应性能,实现了无溶剂Co(II)/NHPI/IL体系催化的碳氢化合物需氧氧化羧酸化反应。具体如下:(1) 构筑了以tBuOOH为氧化剂,Co(II)/CF3COOH(TFA)为共催化剂的催化氧化体系,实现了醛/醇与醇的氧化交叉偶联成酯。机理研究表明醛与醇氧化酯化反应经历三步:首先是醇和醛亲核加成生成半缩醛HOCH(OR1)R2,其次半缩醛与Co(III)-OH缩合生成CoIII-OCH(OR1)R2中间体,其α-H被原位生成的tBuO自由基夺取氧化成酯。第二步缩合反应为决速步,强酸性的TFA有利于提高决速步的反应速率,同时有利于Co(III)得电子还原过程。因此,共催化剂TFA的添加能够显著提高交叉偶联反应的选择性。醇与醇的交叉氧化酯化反应中,α-H活性较高的有机醇(例如苄醇)首先发生氧化脱氢成醛。(2) 以TEMPO为氮氧自由基,研究了TEMPO与CoX2 (X = Cl, I)的配位结构及其与HX (X = Cl, I)反应中的单电子氧化性能。在弱配位性溶剂CH2Cl2中合成TEMPO-CoX2 (X = Cl(1), I(2))的配合物。晶体结构的解析表明配合物1和2中Co(II)配位构型为类十二面体;SQUID磁性以及ESR顺磁共振分析发现二者均顺磁性,但是不同的卤素配体对于零场分裂参数(D)影响显著。理论分析表明TEMPO与Co(II)配位后夺电子活性中心从O转移到N上,实验中配合物(1和2)与HX(X = Cl, I)反应分别生成 [CoCl4(TEMPOH2)2](3)和[CoI2(TEMPOH2)I2](4)。在强配位溶剂CH3CN中,CoCl2与溶剂分子形成配合物[Co(CH3CN)6]2+ 2[CoCl3(CH3CN)]-2[CH3CN] (5),与TEMPO反应生成高氧化活性的 [TEMPO]2+2[CoCl3]-(6)。(3) 探索了咪唑类离子液体(IL)结构对Co/NHPI催化的碳氢化合物氧化反应效率的影响机制,实现了无溶剂Co(II)/NHPI/IL催化氧化三元体系。研究表明,IL在反应物(有机相)和微量水(极性相:包括催化剂Co(II)盐和NHPI)的界面上能够发生自组装,形成一个反胶束微反应环境。反应的初始速率受反胶束的各组分构成和IL的结构变化的影响。例如:合适的水/IL的比例对于提高反应活性至关重要。此外,由C2-烷基化咪唑类离子液体如[bdmim]SbF6和[C12dmim]SbF6组成的界面,不仅有利于形成较高稳定性的三元体系(碳氢化合物/ IL/ H2O),还能增大O2的扩散速率、提升含氧浓度,从而可以实现碳氢化合物的高效氧化。;Direct oxidation of hydrocarbons functionalization is the most basic and important reaction in organic chemistry. It has good application prospects in industrial catalysis and involves the synthesis of many basic chemicals, natural products, fine chemicals and pharmaceutical products. In recent years, the direct esterification of aldehydes / alcohols and alcohols to prepare organic esters has attracted much attention due to its outstanding atom economy. Some notable achievements have been made in related methodologies, but there are some common problem still remains. For instance, stoichiometric amount of organic / inorganic oxidant are normally required and most reactions need to be conducted under an inert atmosphere. Consequently, development of an efficient homogeneous catalytic system for direct hydrocarbon C-H oxidation, especially a catalytic system with oxygen as the terminal oxidant, enables the desired selective oxidation of hydrocarbons and cross-coupling reactions thereafter under milder conditions, which therefore has a highly academic value and practical significance. This dissertation focuses on the research of homogeneous non-noble metal cobalt catalysis. Firstly, Co(II)-catalyzed oxidative cross-coupling of aldehydes/alcohols and alcohol were explored with tBuOOH as the oxidant. Secondly, Co(II)/nitroxide aerobic oxidation catalytic system was studied. In this section, the coordination behavior of TEMPO to Co(II) and the performance of the complexes thereof in the single electron transfer reaction were investigated. Additionally, a solvent-free ternary system of Co(II)/NHPI/IL was achieved for aerobic catalytic oxidation of hydrocarbons. The details as the following:(1) A catalytic system of Co(II)/CF3COOH (TFA) was developed by using tBuOOH as oxidant, thereby the oxidative cross-coupling of aldehydes/alkanols with alkanols was achieved. Mechanistic studies indicated that the oxidative esterification of aldehydes with alkanols involves three primary steps. The first is the formation of hemiacetal HOCH(OR1)R2 upon nucleophilic addition of alcohol to aldehyde. The second is the condensation of hemiacetal with Co(III)-OH to form Co(III)-OCH)R2 intermediate. Final oxidation of α-H therein by the in-situ formed tBuO radical offers the desired ester. Noteworthily, the second step of condensation reaction is the rate-determining step, of which the rate was significantly accelerated in the presence of a strong acid TFA as a co-catalyst, Besides, the reduction of Co(III) to Co(II) was favored in an strong acidic environment. Consequently, it resulted a high selectivity in the desired cross-coupling reaction in the presence of TFA. In the case of oxidative esterification of alkanols with alkanols, it proceeded oxidative dehydrogenation of alkanols with active α-H bond to the corresponding aldehydes, such as benzyl alkanol, as the first step, followed by the general steps of oxidative esterification of aldehydes.(2) With TEMPO as nitroxide, the coordination behavior of TEMPO towards CoX2 (X = Cl, I) and the performance of complexes thereof in the one-electron oxidation with HX (X = Cl, I) were studied. The complex of TEMPO-CoX2 (X = Cl (3), I (4)) was synthesized in weakly coordinating solvent CH2Cl2. X-ray analysis showed that the coordination configuration of Co (II) in complex 3 and 4 was pseudo tetrahedron. Both SQUID magnetic and ESR paramagnetic resonance analysis indicated that both of them were paramagnetic. Importantly, different halogen ligation has significant impact on zero field splitting parameter (D). The theoretical analysis shows that the reactive electron abstraction center of TEMPO shifts from O to N upon coordination to and Co (II). The complexes (3 and 4) were subsequently reacted with HX (X = Cl, I) to form [CoCl4(TEMPOH2)2] (5) and [CoI2(TEMPOH2)I2] (6), respectively. In sharp contrast, the reaction of TEMPO with CoCl2 in strong coordination solvent CH3CN resulted in the formation of form [TEMPO]2+2[CoCl3]- (2). Independently, the complex of CH3CN and CoCl2, [Co(CH3CN)6]2+ 2[CoCl3(CH3CN)]2-[CH3CN] (1), was isolated and characterized. (3) The effect of imidazolium-based ionic liquid (IL) as an additive on the catalytic efficiency of Co(II)/NHPI-catalyzed hydrocarbons oxidation was explored., thereby a ternary system of Co(II)/NHPI/IL was achieved. Studies have shown that IL can self-assemble at the interface of reactant (organic phase) and traces of water (polar phase: including catalyst Co(II) salt and NHPI) to form micro-reaction environment composed by reversed multilamellar vesicles. The initial oxidation rate is affected by the composition of the reversed vesicles and the structural variation of IL launched as well. For instance, a suitable water/IL ratio is essential to attain an optimized reaction efficiency. In addition, the interface consisting of C2-alkylated imidazolium-based ILs such as [bdmim]SbF6 and [C12dmim]SbF6 not only contributes to the formation of higher stability ternary systems (hydrocarbons/IL/H2O) but, also increases the diffusion rate of O2 and increases the oxygen concentration, thereby enabling oxidation of hydrocarbons more efficient. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/26943
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
SAJID MAHMOOD. Co/N-O自由基催化体系:制备、机理及应用[D]. 中国科学院大学,2018.
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