Olefin Metathesis by Grubbs-Hoveyda Complexes: Computational and Experimental Studies of the Mechanism and Substrate-Dependent Kinetics
Olefin Metathesis by Grubbs-Hoveyda Complexes: Computational and Experimental Studies of the Mechanism and Substrate-Dependent Kinetics
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DOI：
10.1021/cs400164w
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发表时间：
2013-09-01
期刊：
影响因子：
12.9
通讯作者：
Vincent, Mark A.
中科院分区：
文献类型：
--
作者：
Ashworth, Ian W.;Hillier, Ian H.;Vincent, Mark A.
在密度泛函理论（DFT）（M06-L）水平上研究了Grubbs-Hoveyda型预催化剂以乙烯、丙烯、1-己烯和乙烯基乙醚（EVE）为底物的活化势能面.导致14 e费歇尔卡宾和苯乙烯的反应的能量有利的途径开始于起始步骤，其中进入的底物和离开的烯烃配体都清楚地与钌中心相关联。对于这些基板，除乙烯，速率决定步骤被预测为形成的α-环丁烷（MCB）。初始反应物为底物与催化剂前体之间的弱货车范德华络合物。该模型产生良好的协议之间的计算激活参数的父Grubbs-Hoveyda和Grela复杂的EVE基板，和实验值，在这里报道。另一种模型，它采取的初始反应物是两个孤立的分子，需要估计的熵损失形成的初始复杂的解决方案，这是很难评估。我们估计这个数量产生的交换机制，这是接近的值，我们找到的替代机制，其中的速率决定步骤是初始的分解的预催化剂的速率决定步骤的障碍。这两种机制的相对能量学涉及不同的起始步骤，但具有相似的活化屏障，很可能取决于预催化剂和底物，这与Plenio及其同事最近的实验结果一致。
The potential energy surfaces for the activation of Grubbs-Hoveyda-type precatalysts with the substrates ethene, propene, 1-hexene, and ethyl vinyl ether (EVE) have been probed at the density functional theory (DFT) (M06-L) level. The energetically favored pathway of the reaction leading to a 14e Fischer carbene and styrene starts with an initiation step in which the incoming substrate and outgoing alkene ligand are both clearly associated with the ruthenium center. For these substrates, with the exception of ethene, the rate determining step is predicted to be the formation of the metallocyclobutane (MCB). We have taken the initial reactant to be a weak van der Waals complex between substrate and precatalyst. This model yields good agreement between the computed activation parameters for both the parent Grubbs-Hoveyda and Grela complex with EVE substrate, and the experimental values, reported here. The alternative model which takes the initial reactant to be two isolated molecules requires an estimate of the entropy loss on formation of the initial complex in solution which is difficult to evaluate. Our estimate of this quantity yields a barrier for the rate determining step for the interchange mechanism which is close to the value we find for the alternative mechanism in which the rate determining step is the initial dissociation of the precatalyst. The relative energetics of these two mechanisms involving different initiation steps but with similar activation barriers, could well be dependent upon the precatalyst and substrate in line with the recent experimental findings of Plenio and co-workers.