Ligand-accelerated catalysis of the Ullmann condensation: Application to hole conducting triarylamines
Ligand-accelerated catalysis of the Ullmann condensation: Application to hole conducting triarylamines
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DOI：
10.1021/jo981804o
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发表时间：
1999-01-22
影响因子：
3.6
通讯作者：
Hu, NX
中科院分区：
文献类型：
--
作者：
Goodbrand, HB;Hu, NX
高纯度三芳基胺可用于静电照相感光体，其中作为聚合物传输层中的浓缩固溶体，它们用作有效的空穴导体。在外加电场的影响下，注入的正电荷通过跳跃机制迁移通过这些层以产生静电潜像。[3]这个过程很容易发生，因为三芳基胺具有容易达到的氧化电位，并且在空穴注入时放弃非键合电子以产生胺阳离子自由基，胺阳离子自由基是负责传输的稳定自旋中心。三芳基胺对许多新兴技术也很重要。传输层对于电致发光器件同样是基本的，在电致发光器件中，基本上与电子照相相反，电子和空穴被分别注入并传输到发射物质，在发射物质中它们的复合产生单线态激子，单线态激子的辐射衰减导致可见光。4它们也是在集成电光开关和调制器的设计中有用的非线性光学发色团的成分。5在这些应用中，都需要达到电子级纯度水平，以获得最佳的器件性能。因此，清洁和有效的合成，可以很容易地扩大到几公斤批量大小是可取的。尽管它们的结构简单，满足这些标准的合成方案是罕见的，三芳基胺的高度重要性导致了广泛的最近的研究关注。构建三芳胺的经典技术是古老的Ullmann缩合。6如通常所实施的，该反应需要二苯胺和未活化的芳基卤在某种形式的铜（金属、合金、铜（I）或铜（II）盐）的催化下在添加的碱的存在下缩合。该反应以其反复无常的性质和对催化剂类型的敏感性而闻名。通常需要涉及高温和延长的反应时间的强侵蚀性条件以确保最好的中等产率。尽管卤化物还原和同偶联通常对产率产生负面影响，但提供了对所有三个环的取代模式的控制。在一个变体中，根据反应物，取代的苯胺与2当量的芳族卤化物的双（芳基化）允许获得其中两个或可能所有三个芳族环被相同取代的产物。在这些情况下，产率往往相当差。人们已尝试缓和反应条件的苛刻性。Fréchet和Gauthier报道了冠醚诱导速率加速并提高某些Ullmann缩合反应的产率。7然而，该方法不需要高温，并且长的反应时间（在最好的情况下为15小时）使其对于大规模工业应用没有吸引力。催化剂效应仍然存在，这在碳酸钾用作碱时是常见的，并且冠的高成本将需要其回收和再利用。
High-purity triarylamines find employment in xerographic photoreceptors where, as concentrated solid solutions in polymeric transport layers, they function as efficient hole conductors. 1, 2 Under the influence of an applied electric field, injected positive charge migrates through such layers by a hopping mechanism to create latent electrostatic images. 3 This process occurs readily since triarylamines possess an easily accessible oxidation potential and on hole injection give up a nonbonding electron to generate amine cation radicals, the stable spin centers responsible for transport. Triarylamines are also important to a number of emerging technologies. Transport layers are equally fundamental to electroluminescent devices where, in essentially the reverse of electrophotography, electrons and holes are separately injected and transported to an emitting species where their recombination produces singlet excitons whose radiative decay results in visible light. 4 They are also constituents of nonlinear optical chromophores useful in the design of integrated electrooptic switches and modulators. 5 In each of these applications, achievement of an electronic grade purity level is required for optimal device performance. Thus, clean and efficient syntheses which can be readily scaled to multi-kilogram lot sizes are desirable. Despite their structural simplicity, synthetic protocols which satisfy these criteria are uncommon and the heightened importance of triarylamines has led to extensive recent research attention. The classic technique for the construction of triarylamines has been the venerable Ullmann condensation. 6 As usually practiced, the reaction entails the condensation of a diphenylamine and an unactivated aryl halide with catalysis by some form of copper (metal, alloy, copper (I) or-(II) salt) in the presence of added base. The reaction is noted for its capricious nature and sensitivity to catalyst type. Strongly aggressive conditions involving high temperature and extended reaction times are generally needed to secure at best moderate yields. Although halide reduction and homocoupling often negatively impact yields, control of the substitution pattern of all three rings is afforded. In a variant, bis (arylation) of a substituted aniline with 2 equiv of an aromatic halide allows access, depending on reactants, to products in which two or perhaps all three of the aromatic ring are identically substituted. Yields tend to be substantially poorer in these cases.Attempts have been made to moderate the harshness of the reaction conditions. Fréchet and Gauthier reported that crown ethers induce rate accelerations and improve yields in certain Ullmann condensations. 7 The procedure, however, does not obviate the requirement for high temperature, and the long reaction times (15 h in the best case) make it unattractive for large scale industrial applications. Catalyst effects still persist, as is common when potassium carbonate is used as base, and the high cost of the crown would necessitate its recovery and reuse.