N-烯基硝酮分子内环加成反应区域选择性的理论研究 3:N-4-甲基-4-戊烯基硝酮分子内环加成反应的区域选择性

N-4-甲基-4-戊烯基硝酮分子内环加成反应可以生成氧桥型(Pa)和碳桥型(Pb)两种产物, 其实验产率比约为8:1。而在N-4位未取代的情况下, Pa与Pb的实验产率比约为1:2。可见N-4-甲基使得区域选择性发生了很大的变化。本文用AM1 MO方法和过渡状态理论研究了N-4-甲基-4-戊烯基硝酮分子内环加成反应的机理。计算了两个平行反应(a, b)的速率常数的比值, 得到与实验吻合的结果。计算表明, N-4-甲基取代后, a, b两反应的活化熵的相对变化是区域选择性改变的主要因素, 活化焓的相对变化只是一个次要因素。

Theoretical study on the regioselectivity of intramolecular N-alkenylnitrone cycloaddition. pt. 3: regioselectivity of intramolecular N-4-methyl-4-pentenyl nitrone cycloaddition

The regioselectivity of intramol. N-4-methyl-4-pentenylnitrone cycloaddn. has been investigated by AM1 MO method and transition state theory. Two isomeric products and corresponding transition states were located by energy gradient technique. The ratio of the rate constants of these two parallel reactions is calculated to be 5.04 (at 383.75 K), which is in good agreement with experiments In comparison of this ratio with that of N-4-pentenylnitrone cycloaddn., which is 0.83, it is realized that N-4-Me substituent changes the regioselectivity notably. The main reason is that the difference between the activation entropy of the two regioisomers is enhanced by introducing an N-4-Me group.