KINETICS AND MECHANISM OF THE INSERTION OF OLEFINS INTO TRANSITION-METAL HYDRIDE BONDS
KINETICS AND MECHANISM OF THE INSERTION OF OLEFINS INTO TRANSITION-METAL HYDRIDE BONDS
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DOI：
10.1021/ja00295a020
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发表时间：
1985-01-01
影响因子：
15
通讯作者：
BERCAW, JE
中科院分区：
文献类型：
--
作者：
DOHERTY, NM;BERCAW, JE
通过Cp* 2NbH 3与相应的烯烃（R= H、Ph、p-Me 2NC 6 H4、/>-MeOC 6 H4、p-MeC 6 H4、p-CF 3C 6 H4）反应以及Cp* 2NbCl 2与相应的烷基格利雅试剂（R= H、Me）反应，制备了全甲基茂（III）烯烃氢化物配合物Cp* 2Nb（CH 2 = CHR）（H）（Cp*= rr-C5 Me 5）。烯烃迁移插入到铑-氢化物键中的产物被一氧化碳或甲基异氰化物捕获，得到全甲基铌茂（III）烷基羰基和甲基异氰化物衍生物Cp* 2Nb（CH 2CH 2 R）（L）。利用磁化转移和聚结技术，通过~ H NMR谱研究了双脱烯烃插入的动力学。通过研究乙烯和烯烃（CH 2 = CH 3，R^ H）与[Cp* 2NbH]竞争结合的平衡，评估了烯烃配位的基态效应。数据表明，在基态，空间效应和电子效应竞争。在一个包含协调环状四中心过渡态的模型下讨论了烯烃插入Nb-H键的机理。电子效应在过渡态中占主导地位，表明氢化物以H~移动，在碳原子上产生部分正电荷，尽管对位取代苯乙烯的插入速率的最佳线性自由能拟合表明p值为-0.64。动力学和热力学同位素效应支持所提出的模型。
Permethylniobocene (III) olefin hydride complexes, Cp* 2Nb (CH2= CHR)(H)(Cp*= rr-C5Me5), havebeen prepared by the reaction of Cp* 2NbH3 with the corresponding olefin (R= H, Ph, p-Me2NC6H4,/>-MeOC6H4, p-MeC6H4, p-CF3C6H4) and by the reaction of Cp* 2NbCl2 with the corresponding alkyl Grignard reagent (R= H, Me). The product of migratory insertion of the olefin into the niobium-hydride bond is trapped by carbon monoxide or methyl isocyanide to give the permethylniobocene (III) alkyl carbonyl and methyl isocyanide derivatives, Cp* 2Nb (CH2CH2R)(L). The kinetics of the hydride-olefin insertion have been studied by* H NMR spectroscopy using magnetization transfer and coalescence techniques. Ground-state effects of olefin coordination were assessed by examining equilibria for competitive binding of ethylene and olefin (CH2= CHR, R^ H) to [Cp* 2NbH], The data indicate that steric and electronic effects compete in the ground state. The mechanism of olefin insertion into the Nb-H bond is discussed in terms of a model involving a concerted cyclic four-center transition state. Electronic effects are found to dominate in the transition state and indicate the development of partial positive charge at the ß carbon with the hydride moving as H~, although the best linear free energy fit of therate of insertion for the para-substituted styrenes indicates a rather modest value of-0.64 for p. Kinetic and thermodynamic isotope effects support the proposed model.