取代基对有机钨化合物中α-氢转移势垒的影响  

Effect of Substitutes on the Reaction Barriers of α-Hydrogen Shift Reactions in Organotungsten Complexes

作　　者：王长生[1] 刘阳[1] 齐学洁[1] 

机构地区：[1]辽宁师范大学化学系,大连116029

出　　处：《化学学报》2004年第16期1503-1508,共6页Acta Chimica Sinica

基　　金：国家自然科学基金 (No.2 0 1 730 2 5);辽宁省科学技术基金;辽宁省教育厅基金资助项目

摘　　要：使用密度泛函理论的B3LYP方法 ,以有机过渡金属钨化合物中的α 氢转移反应为研究对象 ,探讨不同位置上不同的取代基对α 氢转移反应势垒影响 .确定了反应物、产物和过渡态的几何构型和反应势垒 .研究结果表明 ,过渡金属钨有机化合物中 ,发生α 氢转移的碳原子在过渡态中采用sp2 杂化 .取代基对α 氢转移势垒的影响取决于取代基对过渡态中碳原子的未参与sp2 杂化的pz 轨道上单电子的离域作用 .R1,R2 位置上为氢原子时 ,由于H的s轨道与过渡态中单电子所占领的碳原子的pz轨道对称性不匹配 ,没有有效的成键作用 ,所以此时α 氢转移反应有最大的反应势垒 .当R1,R2 位置是Me基团时 ,由于碳原子的pz 轨道与甲基的一个C—H键轨道对称性匹配 ,存在强的超共轭效应 ,从而可以最大程度地降低α 氢转移过程的反应势垒 .对于R3 ,R4位置 ,相比于本研究中的其他基团 ,SiH3 与金属原子轨道间的有效成键作用最大 ,所以当R3 ,R4位置是SiH3 基团时 ,可以最大程度地降低αThe theoretical studies on a series of α-hydrogen shift reactions of the organometallic tungsten complexes were carried out with the density functional theory B3LYP method. The optimal structures of the reactants, transition states and the products were determined. Calculated results indicate that in all of the 12 transition states studied in this work the sp 2 hybridization has been adopted by the central carbon atoms, from which the hydrogen atom has shifted. Both electron donating group and electron withdrawing group, that delocalize the unpair electron in p z orbital of the carbon atom in some way, will lead to a lower reaction barrier. There exists the highest barrier when the substituent in the position R 1 or R 2 is a hydrogen atom, because the 1s orbital of hydrogen atom cannot lead to an effective match with the p z orbital of the central carbon atom. The most preferable substituents are methyl groups for positions R 1 and R 2, due to the existence of a hyperconjugation effect between the p z orbital of the central carbon atom and one of the C—H bond in methyl group. For the positions R 3 and R 4 the most favorite substituents are silyl groups SiH 3, which stablize the unpair electron of the central carbon atom by the orbital interaction through the metal atom.

关 键 词：取代基 有机钨化合物 α-氢转移反应 反应势垒 密度泛函 超共轭效应 

分 类 号：O621.2[理学—有机化学]