Leaving group effects in gas-phase substitutions and eliminations
Leaving group effects in gas-phase substitutions and eliminations
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DOI：
10.1021/ja047002u
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发表时间：
2004-10-13
影响因子：
15
通讯作者：
Pratt, LM
中科院分区：
文献类型：
--
作者：
Gronert, S;Fagin, AE;Pratt, LM
利用最近发展起来的研究气相S（N）2和E2反应产物分布的方法，研究了离去基团对反应速率和支化比的影响。使用二价阴离子作为亲核试剂，与一系列的烷基溴，碘，和三氟乙酸酯的反应进行了研究。研究中的烷基是乙基、正丙基、正丁基、异丁基、异丙基、仲丁基和叔丁基。数据表明，离去基团的能力直接关系到在气相和冷凝相的反应过程的重复性。气相数据给出了S（N）2和E2反应的反应性顺序为碘离子>三氟乙酸根>溴离子。先前的凝聚相数据表明，取代反应的反应性顺序为碘化物>溴化物>三氟乙酸盐;然而，溴化物和三氟乙酸盐的碱度在凝聚相中是相反的，因此这种反应性模式确实反映了相对反应的碱度。除了这种变化，气相数据平行的凝聚相数据表明，取代基的影响是植根于烷基基板的性质，而不是在溶剂化的差异。实验数据得到MP2/ 6-311+G（d，p）MP2/6-31+（d）水平计算的支持。
Using a methodology recently developed for studying the product distributions of gas-phase S(N)2 and E2 reactions, the effect of the leaving group on the reaction rate and branching ratio was investigated. Using a dianion as the nucleophile, reactions with a series of alkyl bromides, iodides, and trifluoroacetates were examined. The alkyl groups in the study are ethyl, n-propyl, n-butyl, isobutyl, isopropyl, sec-butyl, and tert-butyl. The data indicate that leaving group abilities are directly related to the exothermicities of the reaction processes in both the gas phase and the condensed phase. Gas-phase data give a reactivity order of iodide > trifluoroacetate > bromide for S(N)2 and E2 reactions. Previous condensed phase data indicate a reactivity order of iodide > bromide > trifluoroacetate for substitution reactions; however, the basicities of bromide and trifluoroacetate are reversed in the condensed phase so this reactivity pattern does reflect the relative reaction exothermicities. Aside from this variation, the gas-phase data parallel condensed phase data indicating that the substituent effects are rooted in the nature of the alkyl substrate rather than in differences in solvation. The experimental data are supported by calculations at the MP2/ 6-311+G(d,p)MP2/6-31+(d) level.