取代基对有机钼化合物中α-氢转移反应势垒和产物稳定性的影响

Effects of Substituents on the Reaction Barriers and the Product Relative Stabilities of the α-Hydrogen Shift Reactions in Organomolybdenum Complexes

使用B3LYP方法研究了发生在有机钼化合物R3R4Mo(≡CH)(CHR1R2)和R3R4Mo(=CH2)(=CR1R2)之间的α-氢转移反应,探讨了R1,R2,R3和R4位置上不同取代基对α-氢转移反应势垒和产物稳定性的影响.研究发现,金属钼有机化合物中,发生α-氢转移的碳原子在过渡态中采用sp2杂化.R1和R2位置上取代基对α-氢转移反应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.当R1,R2位置是甲基时,由于碳原子的pz 轨道与甲基的一个C-H键轨道间存在强的超共轭效应,从而可以较大程度地降低α-氢转移反应的势垒.研究结果还表明,当R3,R4位置为SiH3时的反应势垒较低.所以当R1和R2位置为Me,R3和R4位置为SiH3时,反应势垒最低.第一个甲基取代R1或R2位置的H时,反应势垒降低很大;第二个甲基继续取代时,反应势垒的降低约为第一个甲基的一半.第一个SiH3取代R3或R4位置的甲基时,反应势垒降低较大;第二个SiH3继续取代时,反应势垒的降低小于第一个SiH3的一半.对反应物和产物的相对稳定性的研究表明,第一个甲基和第二个甲基对产物的相对能量的降低几乎相同; 第一个SiH3降低产物的相对能量,但是第二个SiH3使产物的相对能量升高,从而抵消了第一个SiH3对产物的稳定作用.

The theoretical studies on a series of a-hydrogen shift reactions between the alkylidene and alkylidyne organometallic molybdenum complexes R^3R^4Mo（= CH）（CHR^1R^2） and R^3R^4Mo（= CH2）（= CR^1R^2）were carried out with the density functional theory B3LYP method. The optimal structures of the reactants,transition states, and products were obtained and the reaction barriers and the relative energies of the reactants and products were calculated. Calculated results indicate that in all of the 16 transition states studied in this paper the sp^2 hybridization has been adopted by the central carbon atoms, from which the hydrogen atom has been shifted. The reaction barrier will be lowered by the substituents that delocalize the unpair electron in Pz orbital of the central carbon atom. The most preferable substituents are methyl groups for positions R^1 and/or R^2, due to the existence of a hyperconjugation effect between the Pz orbital of the central carbon atom and one of the C-H bond orbitals in methyl group. For the positons R^3 and R^4, the SiH3 group is a favorite substituent. Calculated results also show that the first methyl group lowers the barriers as twice as the second one. The first silyl group lowers the barriers twice more than the second one. The second methy1 group for positions R^1 and/or R^2 stabilizes the product as much as the first one. For positions R^3 and/or R^4, the first silyl group stabilizes the product while the second one will destabilize it.