Pinacol Boronates by Direct Arene Borylation with Borenium Cations
Pinacol Boronates by Direct Arene Borylation with Borenium Cations
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DOI：
10.1002/anie.201006196
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发表时间：
2011-01-01
影响因子：
16.6
通讯作者：
Ingleson, Michael J.
中科院分区：
文献类型：
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作者：
Del Grosso, Alessandro;Singleton, Paul J.;Ingleson, Michael J.
芳基硼酸酯是重要的合成结构单元，它们的普遍存在是由于易于处理以及在C13 X（X= C，N，O）键形成反应中的高效性。[1]芳基硼酸酯通常由芳烃通过涉及卤代芳烃中间体和化学计量的硬有机金属试剂或金属催化的交叉偶联（例如，Cu、Ni、Co或Pd）的多步方法合成。[1-7]最近避免卤代芳烃的替代方法包括：[4+ 2]环加成，[8]芳基-CH去质子化，[9，10]重氮离子，[11]和用空间位阻铱催化剂直接芳烃硼基化。[12-14]后者代表了相当大的进步，是高度通用的，并且从母体芳基CH一步进行。然而，它仍然受到许多限制：1）硼基化受空间因素支配;[4，15] 2）在低空间环境中，区域选择性差，例如，苯甲醚与74% Meta和25%帕拉硼基化反应;[4] 3）需要铱催化剂，使得大规模合成不经济。通过亲电芳族取代的通用直接芳烃硼基化路线的开发是高度期望的，其具有提供与铱催化互补的选择性（与空间控制相比的电子）和提高的反应效率（与传统的多步硼基化相比）的潜力。与Friedel-Crafts化学相反，通过亲电取代的直接分子间芳烃硼基化是罕见的，[16-19]需要定向基团来预配位硼（使该过程在分子内）和强制条件。[20-23]我们最近报道了使用硼定位的分子间芳烃硼基化[16]，其中硼基化区域选择性由芳烃电子效应控制。然而，活性硼亲电体由于其高反应性而定义不好。一个相关的分子内芳烃硼基化反应被证明是通过一个三配位的硼定位中间体进行的，[24]被称为borenium阳离子。[25 Borenium阳离子是强亲电体，[27]这是亲电芳香取代的关键先决条件。[17]硼鎓阳离子在分子间硼化反应中是活性亲电体，这是合理的，尽管迄今为止还没有明确的例子。[28]在此，我们报告了新的儿茶酚连接的Borenium阳离子，
Aryl boronate esters are essential synthetic building blocks, their ubiquity arising from ease of handling combined with high efficacy in CÀX (X= C, N, O) bond-forming reactions.[1] Aryl boronate esters are commonly synthesized from arenes by a multistep process involving haloarene intermediates and either stoichiometric hard organometallic reagents or metalcatalyzed cross-coupling (eg, Cu, Ni, Co, or Pd).[1–7] Recent alternative approaches avoiding haloarenes include:[4+ 2] cycloadditions,[8] aryl-CH deprotonation,[9, 10] diazonium ions,[11] and direct arene borylation with sterically hindered iridium catalysts.[12–14] The latter represents a considerable advance, being highly generic and proceeding in one step from the parent aryl CH. It is however still subject to a number of restrictions: 1) borylation is dominated by steric factors;[4, 15] 2) in low steric environments regioselectivities are poor, eg, anisole reacts with 74% meta and 25% para borylation;[4] 3) iridium catalysts are required making larger scale syntheses uneconomical.The development of a generic direct arene borylation route by electrophilic aromatic substitution is highly desirable, with the potential to provide complementary selectivities to iridium catalysis (electronic compared to steric control) and enhanced reaction efficiency (to traditional multistep borylation). In contrast to Friedel–Crafts chemistry, direct intermolecular arene borylation by electrophilic substitution is rare,[16–19] requiring directing groups to precoordinate boron (making the process intramolecular) and forcing conditions.[20–23] We recently reported intermolecular arene borylation using borocations,[16] with borylation regioselectivity controlled by arene electronic effects. However, the active boron electrophile was poorly defined due to its high reactivity. A related intramolecular arene borylation was shown to proceed through a three-coordinate borocation intermediate,[24] termed a borenium cation.[25, 26] Borenium cations are well documented to be strong electrophiles,[27] a key prerequisite for electrophilic aromatic substitution.[17] It is plausible that borenium cations will be active electrophiles in intermolecular borylation, though there have been no definitive examples published to date.[28] Herein we report new catecholato-ligated borenium cations that are sufficiently