The Suzuki-Miyaura reaction is the most practiced palladium-catalyzed, cross-coupling reaction because of its broad applicability, low toxicity of the metal (B), and the wide variety of commercially available boron substrates. A wide variety of boronic acids and esters, each with different properties, have been developed for this process. Despite the popularity of the Suzuki-Miyaura reaction, the precise manner in which the organic fragment is transferred from boron to palladium has remained elusive for these reagents. Herein, we report the observation and characterization of pretransmetalation intermediates generated from a variety of commonly employed boronic esters. The ability to confirm the intermediacy of pretransmetalation intermediates provided the opportunity to clarify mechanistic aspects of the transfer of the organic moiety from boron to palladium in the key transmetalation step. A series of structural, kinetic, and computational investigations revealed that boronic esters can transmetalate directly without prior hydrolysis. Furthermore, depending on the boronic ester employed, significant rate enhancements for the transfer of the B-aryl groups were observed. Overall, two critical features were identified that enable the transfer of the organic fragment from boron to palladium: (1) the ability to create an empty coordination site on the palladium atom and (2) the nucleophilic character of the ipso carbon bound to boron. Both of these features ultimately relate to the electron density of the oxygen atoms in the boronic ester.

中文翻译：

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阐明硼酸酯在铃木-宫浦反应中的作用：结构、动力学和计算研究

Suzuki-Miyaura 反应是最常用的钯催化的交叉偶联反应，因为它具有广泛的适用性、金属 (B) 的低毒性以及种类繁多的市售硼底物。已经为此过程开发了多种硼酸和酯，每种都具有不同的特性。尽管 Suzuki-Miyaura 反应很受欢迎，但有机片段从硼转移到钯的精确方式对于这些试剂仍然难以捉摸。在此，我们报告了由各种常用硼酸酯生成的预金属化中间体的观察和表征。确认金属转移前中间体的能力为阐明在关键的金属转移步骤中有机部分从硼转移到钯的机制方面提供了机会。一系列结构、动力学和计算研究表明，硼酸酯可以直接进行金属转移而无需预先水解。此外，根据所使用的硼酸酯，观察到 B-芳基转移的显着速率增强。总的来说，确定了两个关键特征，使有机碎片从硼转移到钯：（1）在钯原子上产生空配位的能力和（2）与硼结合的 ipso 碳的亲核特性。