Structurally-selective gas-phase ion-molecule reactions of dibenzo-16-crown-5 compounds
作者:Chien-Chung Liou、John Isbell、Hui-Fen Wu、Jennifer S. Brodbelt、Richard A. Bartsch、Jong Chan Lee、Johnny L. Hallman
DOI:10.1002/jms.1190300408
日期:1995.4
Steric and substituent effects can play large roles in influencing the outcomes of organic reactions. In this work, the use of ion-molecule reactions of dibenzo-16-crown-5 compounds (lariat ethers) by tandem mass spectrometry to probe the influence of the pendant groups on the selectivity of their gas-phase reactions was evaluated. Lariat ethers are macrocyclic ethers with pendant substituents that have been developed as new types of hosts for molecular recognition. Dimethyl ether (DME) was the reactant chosen because of its well characterized reactivity with various organic substrates possessing different functional groups. Only those dibenzo-16-crown-5 compounds with no or at most one substituent at the center carbon of the three-carbon bridge form the diagnostic [M + 13]+ product ion through a methylene substitution process. Dibenzo-16-crown-5 compounds with geminal substituents on the center carbon of the three-carbon bridge form the [M + 45]+ ion, but not the characteristic [M + 13]+ ion. Causative factors may be steric blocking of the reaction pathway by the geminal groups or a requirement for the presence of at least one hydrogen on the center carbon of the three-center bridge for formation of the [M + 13]+ ion. CAD, deuterium labelling, molecular orbital calculations and comparisons with model compounds provide additional information about the reaction pathways.
位阻效应和取代基效应在影响有机反应的结果方面发挥着重要作用。在这项工作中,通过串联质谱法利用二苯并-16-冠-5 化合物(套索醚)的离子-分子反应来探讨侧基对其气相反应选择性的影响。套索醚是带有侧取代基的大环醚,已被开发为分子识别的新型主体。选择二甲醚(DME)作为反应物是因为它与具有不同官能团的各种有机底物具有良好的反应活性。只有那些在三碳桥的中心碳上没有取代基或最多有一个取代基的二苯并-16-冠-5 化合物才能通过亚甲基取代过程形成诊断性[M + 13]+ 产物离子。在三碳桥的中心碳上具有偕取代基的二苯并-16-冠-5化合物形成[M+45]+离子,但不是特征性[M+13]+离子。致病因素可能是孪生基团对反应路径的空间阻碍,或者需要在三中心桥的中心碳上存在至少一个氢以形成[M+13]+离子。 CAD、氘标记、分子轨道计算以及与模型化合物的比较提供了有关反应途径的更多信息。