Hydrocarbon C-H bond activation by rhodium porphyrins
Hydrocarbon C-H bond activation by rhodium porphyrins
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DOI：
10.1142/s108842460400009x
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发表时间：
2004-01-01
影响因子：
1.5
通讯作者：
Wayland, BB
中科院分区：
文献类型：
--
作者：
Cui, WH;Wayland, BB
铑卟啉提供与芳香族和脂肪族烃的各种C-H键反应，这些反应部分通过卟啉配体的空间要求获得不寻常的选择性。铑（III）卟啉通过亲电取代选择性地与芳香族C-H键反应，几乎不活化脂肪族C-H键。铑（II）卟啉通过金属中心的自由基途径与烷基芳烃和烷烃选择性地在烷基C-H键处反应，完全排除芳香C-H键的活化。铑（II）金属自由基与烷基C-H键的反应具有大的氘同位素效应、小的活化能谱和大的负活化熵，与近线性对称的四中心过渡态（（RhHC...）我... C...* Rh）。这种过渡态的性质和铑卟啉的尺寸提供了排除芳香族C-H键反应的空间限制，并为甲烷活化作为最小的烷烃底物提供了高动力学偏好。铑（II）系栓双卟啉双金属自由基配合物将C-H键反应转化为双分子过程，具有显著增加的反应速率和对甲烷活化的高选择性。版权所有（c）2004年卟啉和酞菁协会。
Rhodium porphyrins provide a variety of C-H bond reactions with both aromatic and aliphatic hydrocarbons that acquire unusual selectivity in part through the steric requirements of the porphyrin ligand. Rhodium(III) porphyrins selectively react with aromatic C-H bonds by electrophilic substitution with the virtual exclusion of aliphatic C-H bond activation. Rhodium(II) porphyrins react by a metal-centered radical pathway with alkyl aromatics and alkanes selectively at the alkyl C-H bond with total exclusion of aromatic C-H bond activation. Reactions of rhodium(II) metalloradicals with alkyl C-H bonds have large deuterium isotope effects, small activation enthalpies and large negative activation entropies consistent with a near linear symmetrical four-centered transition state ((RhHC...)-H-...-C-...*Rh). The nature of this transition state and the dimensions of rhodium porphyrins provide steric constraints that preclude aromatic C-H bond reactions and give high kinetic preference for methane activation as the smallest alkane substrate. Rhodium(II) tethered diporphyrin bimetalloradical complexes convert the C-H bond reactions to bimolecular processes with dramatically increased reaction rates and high selectivity for methane activation. Copyright (c) 2004 Society of Porphyrins & Phthalocyanines.