Hoffmann Marcin, Marciniec Bogdan
Department of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780, Poznan, Poland.
J Mol Model. 2007 Apr;13(4):477-83. doi: 10.1007/s00894-006-0166-3. Epub 2007 Jan 10.
Silylative coupling of olefins differs from olefin metathesis. Although in both these reactions ruthenium catalysts play a crucial role and ethylene product is detected, ruthenium-carbene intermediate is formed only in the course of the metathesis reaction. In this study quantum chemical calculations based on the density functional theory (DFT) have been carried out in order to examine the mechanism of the silylative coupling of olefins leading to ethylene elimination. In the first step of the catalytic cycle, a hydrogen atom from the ruthenium catalytic center is transferred preferentially to the carbon atom bound to Si in a vinylsilane. This H transfer is coupled with the formation of Ru-C bond. Next, the rotation around the newly formed C-C single bond occurs so that silicon atom is placed in the vicinity of the ruthenium center. The following step involves the migration of a silyl moiety, and leads to Ru-Si bond formation, coupled with ethylene elimination. The next reaction, that is the insertion of ethylene (alkene) into Ru-Si bond, has an activation barrier almost as high as the reaction of ethylene elimination. However, the posibility of removing gaseous ethylene from the reactive mixture together with the entropic fators suggests that the insertion of alkene that is larger than C(2)H(4) is the rate limiting step in the silylative coupling of olefins. It also suggests that the substituents attached to the silicon atom or the carbon atoms of an alkene by electronic and steric effects may significantly affect silyl migration and thus the effectiveness of the catalytic reaction.
烯烃的硅氢化偶联与烯烃复分解反应不同。尽管在这两种反应中钌催化剂都起着关键作用，且都能检测到乙烯产物，但仅在复分解反应过程中会形成钌-卡宾中间体。在本研究中，基于密度泛函理论（DFT）进行了量子化学计算，以研究导致乙烯消除的烯烃硅氢化偶联反应的机理。在催化循环的第一步，钌催化中心的一个氢原子优先转移到乙烯基硅烷中与硅相连的碳原子上。这种氢转移与Ru-C键的形成相耦合。接下来，围绕新形成的C-C单键发生旋转，使硅原子位于钌中心附近。随后的步骤涉及硅基部分的迁移，并导致Ru-Si键的形成，同时伴随着乙烯的消除。下一个反应，即乙烯（烯烃）插入Ru-Si键，其活化能垒几乎与乙烯消除反应的活化能垒一样高。然而，从反应混合物中除去气态乙烯的可能性以及熵因素表明，大于C₂H₄的烯烃的插入是烯烃硅氢化偶联反应的速率限制步骤。这也表明，通过电子效应和空间效应连接到硅原子或烯烃碳原子上的取代基可能会显著影响硅基迁移，从而影响催化反应的效率。
