Mechanistic studies on 14-electron ruthenacyclobutanes: Degenerate exchange with free ethylene
Mechanistic studies on 14-electron ruthenacyclobutanes: Degenerate exchange with free ethylene
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DOI：
10.1021/ja0675245
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发表时间：
2007-02-14
影响因子：
15
通讯作者：
Piers, Warren E.
中科院分区：
文献类型：
--
作者：
Romero, Patricio E.;Piers, Warren E.
亚烷基鏻 [（NHC）Cl 2 Ru =CH（PCy 3）]（+）[B（C6 F5）（4）]（-），1，（NHC = N-杂环卡宾，Cy =环己基，C6 H 11）与2.2当量的乙烯在-50 ℃下反应，形成14-电子的亚杂环丁烷（NHC）Cl 2 Ru（CH 2CH 2CH 2），2。NMR光谱数据表明，2具有C-2 v对称结构，具有扁平的风筝状的双烯环丁烷环，与Ru中心具有显著的C-alpha-C-beta agostic相互作用。通过EXSY光谱测量，C-α和C-β的分子内交换是快速的（在223 K下为14（2）s（-1））。C-α和C-β与游离乙烯的亚甲基的分子间交换要慢得多，在[Ru]和[H2 CCH 2]中均为一级交换（4.8（3）x 10（-4）M-1 s（-1））。该过程的活化参数为Δ H = 13.2（5）kcal mol（-1）和Δ S = -15（2）cal mol（-1）K-1，也与作为该交换过程中关键步骤的速率限制缔合取代一致。在此观察的基础上，提出了分子间交换过程的机制，并讨论了Grubbs催化剂介导的催化烯烃复分解中的传播步骤的机制的影响。
The phosphonium alkylidene [(NHC)Cl2Ru=CH(PCy3)](+)[B(C6F5)(4)](-), 1, (NHC = N-heterocyclic carbene, Cy = cyclohexyl, C6H11) reacts with 2.2 equiv of ethylene at -50 degrees C to form the 14-electron ruthenacyclobutane (NHC)Cl2Ru(CH2CH2CH2), 2. NMR spectroscopic data indicates that 2 has a C-2v symmetric structure with a flat, kite shaped ruthenacyclobutane ring with significant C-alpha-C-beta agostic interactions with the Ru center. Intramolecular exchange of C-alpha and C-beta is fast (14(2) s(-1) at 223 K) as measured by EXSY spectroscopy. Intermolecular exchange of C-alpha and C-beta with the methylene groups of free ethylene is much slower and first order in both [Ru] and [H2CCH2] (4.8(3) x 10(-4) M-1 s(-1)). Activation parameters for this process are Delta H = 13.2(5) kcal mol(-1) and Delta S = -15(2) cal mol(-1) K-1, also consistent with a rate limiting associative substitution as the key step in this exchange process. On the basis of this observation, mechanisms for the intermolecular exchange process are proposed and the implications for the mechanism of the propagation steps in catalytic olefin metathesis as mediated by Grubbs catalysts are discussed.