A theoretical study of steric and electronic effects in the rhodium-catalyzed carbonylation reactions
A theoretical study of steric and electronic effects in the rhodium-catalyzed carbonylation reactions
复制标题
DOI：
10.1021/ja016468z
复制
发表时间：
2001-12-12
影响因子：
15
通讯作者：
Solà, M
中科院分区：
文献类型：
--
作者：
Cavallo, L;Solà, M
我们用QM和QM/MM方法研究了Rh催化的羰化反应主要步骤中的空间效应和电子效应。所有考虑的体系在CH3I氧化加成之前都采用正方形平面几何结构。对于CH3I氧化加成后的八面体配合物，各种模型之间的比较表明，CH3I氧化加成的能量增加是由于取代基在配体上的空间压力降低的。用QM/MM和QM模型得到的基本相似的结果表明，电子效应在决定氧化加成的能量方面并不是特别相关。关于P，P-Ph八面体络合物。CO基团转化为添加的CH3基团或转化为P原子之一的几何构型具有相似的能量。各种模型的比较表明，络合配体上取代基的存在降低了CO插入反应的能垒。这种效应与系统从六配位的八面体几何结构向五配位的正方形金字塔几何图形移动时对络合物的空间压力的缓解有关。P，S-Ph体系的能垒计算值与实验值吻合较好，而P，P-Ph体系的能垒被低估。在连续体模型中考虑了溶剂效应，可以得到更好的一致性。热力学产物采用四方锥体几何构型，COCH3基团位于顶端。
We present a QM and QM/MM study of steric and electronic effects in the main steps of Rh-catalyzed carbonylation reactions. All the considered systems adopt a square-planar geometry prior to CH3I oxidative addition. As regards the octahedral complexes after CH3I oxidative addition, a comparison between the various models indicates that the energy gain due to the CH3I oxidative addition is reduced by the steric pressure of the substituents on the ligand. The substantially similar results obtained with the QM/MM and QM models indicate that electronic effects are not particularly relevant in determining the energetic of oxidative addition. As regards the P,P-Ph octahedral complex. the geometries in which the CO group is trans to the added CH3 group, or trans to one of the P atoms, are of similar energy. A comparison between the various models indicates that the energy barrier of the CO insertion reaction is lowered by the presence of substituents on the chelating ligands. This effect is related to a relief of the steric pressure on the complex as the systems move from a six-coordinated octahedral geometry toward a five-coordinated square-pyramidal geometry. The energy barrier calculated for the P,S-Ph system is in rather good agreement with the experimental value, whereas that of the P,P-Ph system is somewhat underestimated. Inclusion of solvent effects with a continuum model leads to a slightly better agreement. The thermodynamic products adopt a square-pyramidal geometry with the COCH3 group in the apical position.