第四级铵碱Hofmann消除反应的研究Ⅱ.——关于Hofmann消除反应的构象规则

Studies of the Thermal Elimination of Quaternary Ammonium Hydroxides Ⅱ On the Conformational Rule for Hofmann Elimination

本文制备了七个各含有一个异己基的第四级铵碱,并使之发生Hofmann消除反应。目的是为了给异己基在Hofmann消除反应的构象规则消除易度序列中确定其位置,并探索异己基在该消除反应中的机理倾向。气相色谱分析的结果表明异己基比异戊基和异丁基容易消除,属于“含侧链较远的第一级烷基”,并且其反应过程倾向于反式E_2机理。

Seven quaternary ammonium hydroxides iso-Hex-NMe2-R OH-, in which the R group equals to Et, n-Pr, iso-Pr, n-Bu, iso-Bu, sec-Bu and iso-Am respectively, were prepared and subjected to Hofmann Elimination. It can be seen from Table Ⅱ that:(1) Et, n-Pr, iso-Pr, sec-Bu and n-Bu are the alkyl groups eliminated easier than iso-Hex group, while iso-Bu and iso-Am are the alkyl groups eliminated more difficult than iso-Hex group. This result means that iso-Hex group can be put in the Reactivity Sequence for Alkyl Groups proposed for the Conformational Rule and can be considered as 'a primary alkyl group with a remote side alkyl group'.(2) The fact that iso-Hex group eliminated easier than iso-Am group implicates that the elimination of iso-Am and iso-Hex groups tends to the trans E2 mechanism rather than to the cis one, since it has been shown by calculation that via the trans, mechanism the energy of the needed conformation for iso-Hex group is lower than that for iso-Am group, while via the cis mechanism the situation is just opposite.