5-yl-pent-1-ene自由基环合成反应机理的理论研究

Theoretical investigation on the cyclization reaction mechanism of 5-yl-pent-1-ene radical

应用密度泛函理论对5-yl-pent-1-ene自由基环合成反应做了详细的理论研究。结果表明,反应经4-exo-trig过程生成四元环产物是一个吸热反应,而经5-endo-trig过程则是一个放热反应,说明在热力学上5-endo-trig过程比4-exo-trig过程有利。环张力的计算结果说明四元环较大的环张力是形成四元环过程需要吸热的一个主要原因,而五元环较小的环张力是反应放热的一个原因。从动力学势垒来看,5-endo-trig反应过程也比4-exo-trig过程稍微有利,这是违反Baldwin规则的。这是由于自由基环合成所形成的四元环张力要比五元环张力大所引起的,因此环张力是在四元环形成过程中动力学上需要克服的主要障碍之一,也是导致其动力学势垒较高的主因。五元环产物的势垒主要是由于进攻角度变化较大而引起的。因此,Baldwin规则中进攻角度决定环合成反应选择性的理论不适用于5-yl-pent-1-ene自由基环合成过程。

A detailed theoretical investigation on the radical cyclization reaction mechanism of 5-yl-pent-1-ene was performed by use of density functional theory. The computation result indicate that the 4-exo-trig reaction channel is a endothermic reaction while the 5-endo-trig reaction pathway is a exothermic reaction, which demonstrates that the 5-endo-trig reaction pathway is thermodynamically favored relative to the 4-exo-trig reaction channel. The computed results about ring strain show that the larger ring strain energy of four-membered ring should be responsibility for the unfavored thermodynamic reaction while the relatively smaller strain energy of the five-membered ring species leads to the exothermic process. From the thermodynamic and kinetic viewpoints, the 5-endo-trig channel is somewhat more favorable that the 4-exo-trig pathway, which is disagreement with the Baldwin＇ s conclusion. This can be explained by the larger four-membered ring strain energy than five-membered one. Therefore, the attack angel change in the Baldwin＇ s rules can not be singly used to explain the radical cyclization reaction regioseletivity of 5-yl-pent- 1 -ene.