N-烯基硝酮分子内环加成反应区域选择性的理论研究 Ⅳ.N-4-己烯基硝酮分子内环加成反应的区域选择性

Theoretical Study on the Regioselectivity of Intramolecular N-Alkenylnitrone Cycloaddition

N-烯基硝酮分子内环加成反应可能得到联桥式和氧桥式两种产物.对于N-4-戊烯基硝酮的分子内环加成反应,碳桥式与氧桥式产物的产率分别为47%和23%,约为2:1.当N-5位上的一个H被甲基取代后(即N-4-已烯基硝酮〕.却主要得到碳桥式产物,产率高达95%以上.可见,N-5-甲基的增加使得区域选择性发生了很大的变化.本文用AM1 MO方法和过渡状态理沦研究N-4-己烯基硝酮分子内环加成反应的机理,计算两个平行反应(a,b)的速率常数的比值,得到与实验吻合的结果.计算结果还表明,由于N-5-甲基的电子效应,使得反应b的活化焓降低,同时,N-5-甲基引起的构型变化,使得反应b的活化熵增高(绝对值变小),这使得k_b/k_a值明显增长,是区域选择性发生较大改变的原因.

The regioselectivity of intramolecular N-4-hexenyl nitrone cycloaddition has been investigated by AMI 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 reaction (kb/ka) is calculated to be 25.46, which is in good consistence with experiments. In comparison of this ratio with that of N-4-pentenyl nitrone cycloaddition, which is 1.21, it is realized that N-5-methyl substituent changes the regioselectivity notably. The main reason is that the electronic effect of N-5-methyl makes the activation enthalpy of reaction b reduced and the configuration change by introducing an N-5-methyl group makes the activation entropy of reaction b increased.