ortho C-H activation of haloarenes and anisole by an electron-rich iridium(I) complex:: Mechanism and origin of regio- and chemoselectivity.: An experimental and theoretical study
ortho C-H activation of haloarenes and anisole by an electron-rich iridium(I) complex:: Mechanism and origin of regio- and chemoselectivity.: An experimental and theoretical study
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DOI：
10.1021/om060078
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发表时间：
2006-06-19
期刊：
影响因子：
2.8
通讯作者：
Milstein, David
中科院分区：
文献类型：
--
作者：
Ben-Ari, Eyal;Cohen, Revital;Milstein, David
（PNP）Ir(COE)(+) PF6- (1) (PNP=2,6-二（二叔丁基磷甲基）吡啶的反应COE=环烯烯)与苯反应生成稳定的不饱和方形锥体Ir（III）羟基芳基配合物2，该配合物在50℃与其他芳烃反应时发生芳烃交换。1与卤代芳烃（氯苯和溴苯）和苯甲醚在50℃反应时发生选择性的邻位C- h活化。没有观察到c -卤素键活化，即使在正常反应的溴苯的情况下，尽管有空间位阻的光环取代基。邻位活化配合物（8a、9a和10a）对芳烃交换具有较高的阻隔性；也就是说，在高达60摄氏度的温度下加热时，没有发生交换。这些配合物在热力学和动力学上都比相应的元异构体和对异构体（8b, c, 9b， c和10b，c）更稳定。所观察到的选择性是由于杂原子与金属中心的配位，它在动力学上引导金属到邻氢键上，并在热力学上稳定了所得到的配合物。在相同条件下，配合物1与氟苯反应时，由于氟取代基的配位能力较低，没有观察到这种选择性。竞争实验表明，邻位活化配合物8a、9a和10a具有相似的动力学稳定性，而氯和甲氧基配合物8a和10a在热力学上比溴配合物9a更稳定。使用mPW1K交换相关函数和基于pnp系统的各种基集的计算研究，提供了机制洞察力。苯的整个C-H活化过程的速率决定步骤是COE解离形成反应性14e配合物。随后形成eta(2)（C-C）中间体，该中间体转化为eta(2)（C-H）复合物，两者都是C-H活化过程中的重要中间体。在氯苯、溴苯和苯甲醚的情况下，通过杂原子与14e基团配位，然后形成邻位的eta(2)（C-H）配合物，导致选择性活化。计算结果表明，在氯苯的情况下，未观察到的c -卤化物活化过程涉及与C-H活化过程相同的cl -配位中间体，但它经历了更高的活化屏障。邻位C-H活化产物在热力学上也比C-Cl氧化加成配合物更稳定。
Reaction of (PNP)Ir(COE)(+) PF6- (1) (PNP=2,6-bis(di-tert-butylphosphinomethyl) pyridine; COE=cyclooctene) with benzene yields a stable unsaturated square pyramidal Ir(III) hydrido-aryl complex, 2, which undergoes arene exchange upon reaction with other arenes at 50 degrees C. Upon reaction of 1 with haloarenes (chlorobenzene and bromobenzene) and anisole at 50 degrees C, selective ortho C-H activation takes place. No C-halogen bond activation was observed, even in the case of the normally reactive bromobenzene and despite the steric hindrance imposed by the halo substituent. The ortho-activated complexes (8a, 9a, and 10a) exhibited a higher barrier to arene exchange; that is, no exchange took place when heating at a temperature as high as 60 degrees C. These complexes were more stable, both thermodynamically and kinetically, than the corresponding meta- and para-isomers (8b, c, 9b, c, and 10b,c). The observed selectivity is a result of coordination of the heteroatom to the metal center, which kinetically directs the metal to the ortho C-H bond and stabilizes the resulting complex thermodynamically. Upon reaction of complex 1 with fluorobenzene under the same conditions, no such selectivity was observed, due to low coordination ability of the fluorine substituent. Competition experiments showed that the ortho-activated complexes 8a, 9a, and 10a have similar kinetic stability, while thermodynamically the chloro and methoxy complexes 8a and 10a are more stable than the bromo complex 9a. Computational studies, using the mPW1K exchange-correlation functional and a variety of basis sets for PNP-based systems, provide mechanistic insight. The rate-determining step for the overall C-H activation process of benzene is COE dissociation to form a reactive 14e complex. This is followed by formation of a eta(2)(C-C) intermediate, which is converted into an eta(2)(C-H) complex, both being important intermediates in the C-H activation process. In the case of chlorobenzene, bromobenzene, and anisole, eta(1)-coordination via the heteroatom to the 14e species followed by formation of the ortho eta(2)(C-H) complex leads to selective activation. The unobserved C-halide activation process was shown computationally in the case of chlorobenzene to involve the same Cl-coordinated intermediate as in the C-H activation process, but it experiences a higher activation barrier. The ortho C-H activation product is also thermodynamically more stable than the C-Cl oxidative addition complex.