Beta-H transfer from the metallacyclobutane: a key step in the deactivation and byproduct formation for the well-defined silica-supported rhenium alkylidene alkene metathesis catalyst.
Beta-H transfer from the metallacyclobutane: a key step in the deactivation and byproduct formation for the well-defined silica-supported rhenium alkylidene alkene metathesis catalyst.
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金属环丁烷的 Beta-H 转移：明确的二氧化硅负载铼亚烷基烯烃复分解催化剂失活和副产物形成的关键步骤。
DOI：
10.1021/ja800189a
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发表时间：
2008
影响因子：
15
通讯作者：
M. Röper
中科院分区：
文献类型：
--
作者：
A. Leduc;Alain Salameh;D. Soulivong;M. Chabanas;J. Basset;C. Copéret;X. Solans;E. Clot;O. Eisenstein;V. Böhm;M. Röper
表面络合物[([三键]SiO)Re([三键]CtBu)(=CHtBu)(CH2tBu)](1)是一种初始活性高、产率好的高效丙烯复分解催化剂。然而，随着时间的推移，它经历了一个快速的失活过程，在活性位点和乙烯中是一级的。值得注意的是，1-丁烯和戊烯是丙烯复分解过程中意想不到的产物，作为初级产物形成，相对于Re (>>1 equiv/Re)，它们的形成与无活性物质的形成无关。分子模型系统的DFT计算表明，副产物的形成和失活始于β - h转移，转移到具有方基金字塔几何结构的金属环丁烷中间体上的弱sigma给体配体（硅氧基）。这一关键步骤的能垒略高于烯烃分解的能垒。在β - h转移之后，最容易获得的途径是在Re-H键中插入乙烯。得到的五配位三过羧基配合物通过(1)α - h萃取得到意想不到的1-丁烯副产物和催化剂的再生或(2)β - h萃取导致脱接枝进行重排。这些失活和副产物形成途径与实验数据完全一致。
The surface complex [([triple bond]SiO)Re([triple bond]CtBu)(=CHtBu)(CH2tBu)] (1) is a highly efficient propene metathesis catalyst with high initial activities and a good productivity. However, it undergoes a fast deactivation process with time on stream, which is first order in active sites and ethene. Noteworthy, 1-butene and pentenes, unexpected products in the metathesis of propene, are formed as primary products, in large amount relative to Re (>>1 equiv/Re), showing that their formation is not associated with the formation of inactive species. DFT calculations on molecular model systems show that byproduct formation and deactivation start by a beta-H transfer trans to the weak sigma-donor ligand (siloxy) at the metallacyclobutane intermediate having a square-based pyramid geometry. This key step has an energy barrier slightly higher than that calculated for olefin metathesis. After beta-H transfer, the most accessible pathway is the insertion of ethene in the Re-H bond. The resulting pentacoordinated trisperhydrocarbyl complex rearranges via either (1) alpha-H abstraction yielding the unexpected 1-butene byproduct and the regeneration of the catalyst or (2) beta-H abstraction leading to degrafting. These deactivation and byproduct formation pathways are in full agreement with the experimental data.