Mechanism of the mild functionalization of arenes by diboron reagents catalyzed by iridium complexes. Intermediacy and chemistry of bipyridine-ligated iridium trisboryl complexes
Mechanism of the mild functionalization of arenes by diboron reagents catalyzed by iridium complexes. Intermediacy and chemistry of bipyridine-ligated iridium trisboryl complexes
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DOI：
10.1021/ja053433g
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发表时间：
2005-10-19
影响因子：
15
通讯作者：
Hartwig, JF
中科院分区：
文献类型：
--
作者：
Boller, TM;Murphy, JM;Hartwig, JF
本文研究了4，4 '-二叔丁基联吡啶（dtbpy）与烯烃配位的铱卤化物或铱烷氧基配合物催化的二硼试剂B（2）pin（2）（联频哪醇基二硼烷（4））对芳烃的官能化反应机理。本工作确定催化剂的静止状态为[Ir（dtbpy）（COE）（Bpin）（3）]（COE =环辛烯，Bpin = 4，4，5，5-四甲基-1，3，2-二氧硼杂环戊烷基）。以[Ir（COD）（OMe）]（2）、dtbpy、COE和HBpin为原料，通过独立合成得到了[Ir（dtbpy）（COE）（Bpin）（3）]。该络合物由[Ir（COD）（OMe）]（2）、dtbpy、COE和B2 pin 2以低收率形成。动力学研究表明，该配合物与芳烃反应后，可逆解离COE。在COE解离和B（2）pin（2）的还原消除之后，芳烃与Ir（1）络合物[Ir（dtbpy）Bpin]反应的替代机制没有发生到可测量的程度。[Ir（dtbpy）（COE）（Bpin）（3）]与芳烃的反应以及[Ir（COD）（OMe）]（2）和dtbpy催化的B（2）pin（2）与芳烃的反应在贫电子芳烃中比在富电子芳烃中进行得快。然而，化学计量反应和催化反应在富电子杂芳烃噻吩和呋喃中也比在芳烃中发生得更快，这可能是因为η（2）-杂芳烃络合物比η（2）-芳烃络合物更稳定，并且η（2）-杂芳烃或芳烃络合物是氧化加成之前的中间体。催化反应动力学研究表明，[lr（dtbpy）（COE）（Bpin）（3）]通过COE的解离进入催化循环，催化反应和化学计量反应的动力学同位素效应比较表明，反应中间体[lr（dtbpy）（Bpin）（3）]裂解芳烃C-H键。配体交换和C-H活化的障碍允许从计算工作中得出的几个结论的实验评估，最一般的是，我们的结果证实了C-H键断裂是周转限制的结论，但这种键断裂的实验障碍比计算障碍低得多。
This paper describes mechanistic studies on the functionalization of arenes with the diboron reagent B(2)pin(2) (bis-pinacolato diborane(4)) catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and olefin-ligated iridium halide or olefin-ligated iridium alkoxide complexes. This work identifies the catalyst resting state as [Ir(dtbpy)(COE)(Bpin)(3)] (COE = cyclooctene, Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl). [Ir(dtbpy)(COE)(Bpin)(3)] was prepared by independent synthesis in high yield from [Ir(COD)(OMe)](2), dtbpy, COE, and HBpin. This complex is formed in low yield from [lr(COD)(OMe)](2), dtbpy, COE, and B2pin2. Kinetic studies show that this complex reacts with arenas after reversible dissociation of COE. An alternative mechanism in which the arene reacts with the lr(l) complex [Ir(dtbpy)Bpin] after dissociation of COE and reductive elimination of B(2)pin(2) does not occur to a measurable extent. The reaction of [Ir(dtbpy)(COE)(Bpin)(3)] with arenes and the catalytic reaction of B(2)pin(2) with arenes catalyzed by [lr(COD)(OMe)](2) and dtbpy occur faster with electron-poor arenas than with electron-rich arenes. However, both the stoichiometric and catalytic reactions also occur faster with the electron-rich heteroarenes thiophene and furan than with arenes, perhaps because eta(2)-heteroarene complexes are more stable than the eta(2)-arene complexes and the eta(2)-heteroarene or arene complexes are intermediates that precede oxidative addition. Kinetic studies on the catalytic reaction show that [lr(dtbpy)(COE)(Bpin)(3)] enters the catalytic cycle by dissociation of COE, and a comparison of the kinetic isotope effects of the catalytic and stoichiometric reactions shows that the reactive intermediate [lr(dtbpy)(Bpin)(3)] cleaves the arene C-H bond. The barriers for ligand exchange and C-H activation allow an experimental assessment of several conclusions drawn from computational work, Most generally, our results corroborate the conclusion that C-H bond cleavage is turnover-limiting, but the experimental barrier for this bond cleavage is much lower than the calculated barrier.