N-烯基硝酮分子内环加成反应区域选择性的理论研究——Ⅰ.N-3-丁烯基硝酮分子内环加成反应的区域专一性

Theoretical Study on the Regiochemistry of Intramsiecular N-Alkenylnitrone Cycloaddition Ⅰ. The Regiospecificity of Intramolecular N-3-Butenyl Nitrone Cycloaddition

N-烯基硝酮的分子内环加成反应可能得到氧桥和碳桥的两种产物,但N-3-丁烯基硝酮却只有单一的氧桥产物,已被用来合成一系列新的区域专一性化合物.弄清此反应机理是很有意义的.本文用过渡状态理论和AM1方法进行研究,结果表明,虽然存在得到两种产物的平行反应,但由于这两个反应的速率常数之比很大,基本上只得到单一的氧桥产物.生成氧桥产物的反应活化焓比较小,活化熵比较大.活化熵对反应速率常数比值的贡献是不可忽略的.

The regiospecificity of intramolecular iV-3-butenyl nitrone cycloaddition has been investigated by semiempirical molecular orbital AM1 method and energy gradient technique. The calculated results show that the cycloaddition consists of two similar parallel reactions in which the conformers (R_a and R_b) of N-3-butenyl nitrone generate the regioisomers P_a and P_b via transition states TS_a and TS_b respectively. The geometries of these reactants, TS and products were optimized on the potential energy surface. The regiospecificity is determined by the ratio of the rate constants of the two reactions. The value is 124.1, which is in good consistence with experiments. The determining factors of the rate ratio come from the joint contribution of the activation enthalpy and activation entropy.