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

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

本文合成了１２种含环戊基或环己基的第四级铵碱，使之发生Ｈｏｆｍａｎｎ消除反应。利用气相色谱法分析其产物，以检验构象规则对含脂环基的第四级铵碱消除反应的适应性，并确定了环戊基和环己基在Ｈｏｆｍａｎｎ消除反应的构象规则中消除易度序列的位置。研究结果表明：含环戊基和环己基的第四级铵碱所发生的Ｈｏｆｍａｎｎ消除反应能很好地符合构象规则。环戊基和环己基在该消除易度序列中属于＂其它第二级烷基＂一类；在大多数的例子中，环戊基比环己基容易消除。

In the previous work,a conformational rule was proposed for Hofmann elimination of quaternary ammonium hydroxides.Thus,the different types of alkyl group were put in a sequence with decreasing reactivity of the alkyl groups eliminat-ed to olefins,but in this sequence no position was assigned for the eyclo-alkyl group such as cyclo-pentyl and cyclo-hexyl groups.For the purpose of remedy this shortage,twelve quaternary ammonium hydLox-ides,six cyclo-Hex-NMe_2-R OH￣-and six cyclo-Pent-NMe_2-R OH￣-,in which the two R groups both equal to iso-pr,tert-Bu,Sec-Bu,n-Bu,iso-Bu,and iso-Am groups respectively,were prepared and subjected to the thermal elimination. It could be seen from the data listed in Table Ⅱ that:(1) All the"ethylic groups"(iso-pr,tert-Bu,sec-Bu)and the n-alkyl group (n-Bu)were eliminated easier than cyclo-Hex and cyclo-Pent groups,while iso-Bu and iso-Am groups were eliminated more difficuIt than these cyclo-alkvl groups.Therefore,cyclo-Hex and cyclo-Pent groups could be acceptably put in thereactivity sequence as the"other socondary alkyl groups".( 2)From the alike pairs of the cyclo-Hex-NMe_2-R OH￣- and cyclo-Pent-NMe_2-R OH￣-(1 and 7,2and8,3and9,4 and10,5 and11,6and12),it could be seen thatall the pairs,with one exception(5 and 11),showed that in which cyclo-Pent groupwas eliminated easier than or comparable to cyclo-Hex group. Therefore,in mostcases cyclo-Pent group should occupy a position prior to cyclo-Hex group in the re-actlvity sequence.