亚甲基环丙烷卡宾的开环与1,2-氢迁移的从头算研究

用从头计算-解析梯度法研究了亚甲基环丙烷卡宾的重排反应。开环重排经Hückel型过渡态,在过渡态前后分别属2电子和4电子反应,取对旋和顺旋方式,遵从4n+2和4n规则。1,2氢重排经迁移氢进攻卡宾碳的空p轨道发生。用RHF/6-31-G^**∥STO-3G方法算得开环势垒为24kJ/mol。1,2氢重排不是开环的竞争反应,其势垒为118kJ/mol。与环丙烷卡宾比较,亚甲基取代未改变其开环反应的类型与机理,但增大开环反应活性。

Ab initio Study of Ring Opening and 1,2-H Shift of Methylcnecyclopropylidenc

Isomerizations of methylenecyclopropylidene were studied using ab initio gradient method. The ring opening occurs via a Huckel aromatic transition state. It is a 2 e reaction before the transition state and a 4 e one thereafter, obeying Huckel 4n+2 and 4n rules, respectively. The 1,2 H migration occurs by the attack of the carbene p orbital on the shift hydrogen. The ring opening barriers is 24 kJ/ mol, and the hydrogen shift barrier is 118 kJ/mol, at the RHF/6-31G**//STO-3G level. Relative to the ring opening of cyclopropylidene, the substitution results in a larger reactivity but does not change the type and the mechanism of the reaction.