Toward waste-free production of Heck products with a catalytic palladium system under oxygen
Toward waste-free production of Heck products with a catalytic palladium system under oxygen
复制标题
DOI：
10.1002/anie.200351524
复制
发表时间：
2003-01-01
影响因子：
16.6
通讯作者：
Jacobs, PA
中科院分区：
文献类型：
--
作者：
Dams, M;De Vos, DE;Jacobs, PA
芳环与烯烃的 sp2-sp2 偶联是 Pd 催化的关键转化之一。该反应由 Heck 和 Mizoroki 首创，自那时以来已经取得了很大进展，但它仍然存在一个主要缺点，即产生盐废物。 [1]事实上，使用卤代芳香族化合物需要使用化学计量的碱来进行 PdO 再生，这会导致形成大量的卤化物盐副产物，特别是在大规模工艺中。为了解决这些问题，deVries 及其同事 [2] 和 Gooßen 等人 [3] 分别使用芳香族羧酸酐和酯作为 PdCl2 催化的 sp2-sp2 偶联中芳基组分的来源。当使用苯甲酸酐作为芳基化试剂时，作为副产物产生的苯甲酸可以通过脱水回收到酸酐中；[2]当使用活化的对硝基苯酯时，产生的硝基苯酚可以通过与适当的羧酸反应来酯化，得到起始酯。 [3]尽管CO和H2O是唯一的副产物，但这些反应仍然存在缺点，即它们需要高温（1608℃）、两个独立的步骤以及中等催化剂活性所需的最低碱金属卤化物浓度（最大TON（总周转数=每摩尔Pd的偶联产物摩尔数）为400-150）。在此，我们提出了一种 100% 碳效率和无卤化物合成 Heck 偶联产物的替代方案，其中涉及芳烃与烯烃的直接 Pd 催化偶联（方案 1）。 PdO 中间体可以在适当的 O2 压力下重新氧化为活性 PdII 物质。由于芳烃上不需要反应性取代基，水是唯一的副产物。该反应可以在温和的条件下进行，例如在908℃和O2（0.8 MPa）下进行，具有较高的周转频率，并且不需要溶剂或卤化物促进剂。只有少数方法可以通过直接C-H键活化来实现芳烃与烯烃的过渡金属催化反应。 [4]这些包括由 Ru 或 Rh 介导的芳香酮的螯合辅助烷基化，[5–6] 烯烃或炔烃与 Ir 或 Pd 的加氢芳基化，[7–8] 以及烯烃与 Rh、[9] Ru、[10] 或 Pd 的芳基化。[8c, 11] 最后一种类型的催化偶联反应需要氧化剂。[10, 11] 因为分子氧是最便宜的可用方法我们尝试在初始实验中，在 Pd (OAc) 2 (1 mol%) 和 Mn (OAc) 3 (4 mol%) 存在下，在 O2 存在下，在 908°C 纯甲苯中，将甲苯 (1b) 与反式肉桂酸乙酯 (2b) 偶联。令人惊讶的是，以 91% 的收率获得了芳基化产物 3e，以及二芳基化产物 3-苯基-2, 3-二甲苯基-丙烯酸乙酯 (1.7%)。 [12]相比之下，当反应在此类 Pd 催化偶联反应的经典条件下进行时，即在乙酸和乙酸酐的混合物中，仅以 2.5% 的产率获得 3e，以及等摩尔量的由甲苯乙酰氧基化形成的副产物乙酸苄酯。 [13, 14] 因此，在乙酸中仅观察到较低的反应性和 50% 的选择性，而在无溶剂下条件下反应进行到高转化率和高选择性。
The sp2–sp2 coupling of an aromatic ring with an alkene is one of the key transformations catalyzed by Pd. This reaction was pioneered by Heck and Mizoroki and has undergone a lot of progress since that time, but it still suffers from one major drawback, namely the production of salt waste.[1] Indeed, the use of halogenated aromatic compounds requires the use of stoichiometric amounts of base for Pd0 regeneration, which leads to the formation of large quantities of halide salt byproducts, especially in large-scale processes. To circumvent these problems, aromatic carboxylic acid anhydrides and esters have been used by deVries and co-workers [2] and Gooßen et al.,[3] respectively, as sources of the aryl component in the PdCl2-catalyzed sp2–sp2 coupling. When benzoic anhydride is used as the arylating reagent, the benzoic acid produced as a side product can be recycled to the anhydride by dehydration;[2] when an activated p-nitrophenyl ester is used, the nitrophenol produced can be esterified to give the starting ester by reaction with the appropriate carboxylic acid.[3] Although CO and H2O are the only by-products, these reactions still have the disadvantages that they require high temperatures (1608C), two separate steps, and a minimum concentration of alkali-metal halides for moderate catalyst activity (maximum TON (total turnover number= moles of coupling product per mole of Pd) of 400–150). Herein we propose a 100% carbon-efficient and halidefree alternative for the synthesis of Heck-coupling products, which involves the direct Pd-catalyzed coupling of arenes with olefins (Scheme 1). Pd0 intermediates can be reoxidized to active PdII species under an appropriate pressure of O2. As no reactive substituents are required on the arene, water is the only by-product. The reactions can be performed under mild conditions, for example at 908C and under O2 (0.8 MPa), have a high turnover frequency, and do not require a solvent or a halide promoter.Only a few methods exist for the transition-metalcatalyzed reaction of an arene with an olefin through direct CÀH bond activation.[4] These include the chelation-assisted alkylation of aromatic ketones mediated by Ru or Rh,[5–6] the hydroarylation of alkenes or alkynes with Ir or Pd,[7–8] and the arylation of an olefin with Rh,[9] Ru,[10] or Pd.[8c, 11] This last type of catalytic coupling reaction requires an oxidant.[10, 11] As dioxygen is the cheapest available oxidant, we attempted in an initial experiment to couple toluene (1b) with ethyl trans-cinnamate (2b) in the presence of Pd (OAc) 2 (1 mol%) and Mn (OAc) 3 (4 mol%) under O2 at 908C in neat toluene. Surprisingly, the arylation product 3e was obtained in 91% yield, together with the diarylated product ethyl 3-phenyl-2, 3-ditolyl-propenoate (1.7%).[12] By contrast, when the reaction was carried out under the classical conditions for such Pdcatalyzed coupling reactions, namely in a mixture of acetic acid and acetic anhydride, 3e was obtained in only 2.5% yield, along with an equimolar amount of the by-product benzyl acetate, formed by the acetoxylation of toluene.[13, 14] Thus, in acetic acid only low reactivity and a selectivity of 50% were observed, whereas under solvent-free conditions the reaction proceeded to high conversion with high selectivity.