Experimental Evidence for pKa-Driven Asynchronicity in C-H Activation by a Terminal Co(III)-Oxo Complex
Experimental Evidence for pKa-Driven Asynchronicity in C-H Activation by a Terminal Co(III)-Oxo Complex
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DOI：
10.1021/jacs.8b13490
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发表时间：
2019-03-06
影响因子：
15
通讯作者：
Anderson, John S.
中科院分区：
文献类型：
--
作者：
Goetz, McKenna K.;Anderson, John S.
过渡金属氧配合物对C-H的活化是生物体系和合成体系氧化化学中的基本反应。这种中心性促使人们努力了解这种反应的模式和机制。因此，我们全面考察了最近合成和表征的后过渡金属氧配合物PhB((T)Buim)(3)(COO)-O-III的C-H活化反应活性。实验测定了该络合物共轭化合物的pK(A)和BDFE(O-H)的精确值，并对观察到的反应活性提供了深入的了解。测定了该络合物与9，10-二氢菲反应的活化参数，并与文献中的例子进行了比较。对PhB((T)Buim)(3)(CoO)-O-III与各种氢原子供体的反应速率的评价表明，该络合物的反应活性取决于目标底物的pK(A)而不是BDEC-H。这一观察结果与许多其他过渡金属氧合化合物的常见反应性范式背道而驰。对PhB((T)Buim)(3)(CoO)-O-III的C-H活化反应的实验和计算分析表明，这些过程的过渡态具有明显的质子转移特征。然而，更多的实验有力地表明，该反应不是通过一个逐步的过程发生的，从而得出结论，这种Co-III-oxo配合物的C-H活化是通过PK(A)驱动的“异步”协同机制进行的。这一结果支持了一种新的反应性模式，该模式可能适用于其他体系，并可能导致过渡金属氧配合物介导的C-H活化反应的选择性。
C-H activation by transition metal oxo complexes is a fundamental reaction in oxidative chemistry carried out by both biological and synthetic systems. This centrality has motivated efforts to understand the patterns and mechanisms of such reactivity. We have therefore thoroughly examined the C-H activation reactivity of the recently synthesized and characterized late transition metal oxo complex PhB ((t)BuIm)(3)(CoO)-O-III. Precise values for the pK(a) and BDFE(O-H )of the conjugates of this complex have been experimentally determined and provide insight into the observed reactivity. The activation parameters for the reaction between this complex and 9,10-dihydroanthracene have also been measured and compared to previous literature examples. Evaluation of the rates of reaction of PhB((t)BuIm)(3)(CoO)-O-III with a variety of hydrogen atom donors demonstrates that the reactivity of this complex is dependent on the pK(a) of the substrate of interest rather than the BDEC-H. This observation runs counter to the commonly cited reactivity paradigm for many other transition metal oxo complexes. Experimental and computational analysis of C-H activation reactions by PhB ((t)BuIm)(3)(CoO)-O-III reveals that the transition state for these processes contains significant proton transfer character. Nevertheless, additional experiments strongly suggest that the reaction does not occur via a stepwise process, leading to the conclusion that C-H activation by this Co-III-oxo complex proceeds by a pK(a)-driven "asynchronous" concerted mechanism. This result supports a new pattern of reactivity that may be applicable to other systems and could result in alternative selectivity for C-H activation reactions mediated by transition metal oxo complexes.