Reactivity in the nucleophilic aromatic substitution reactions of pyridinium ions.
Reactivity in the nucleophilic aromatic substitution reactions of pyridinium ions.
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DOI：
10.1039/c4ob00946k
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发表时间：
2014-08-28
影响因子：
3.2
通讯作者：
Wu W
中科院分区：
文献类型：
--
作者：
Bowler JT;Wong FM;Gronert S;Keeffe JR;Wu W
芳香亲核取代反应（SNAr）中的“元素效应”是由活化芳基底物的离去基团顺序L = F > NO2 > Cl → Br > I表征的。环上取代的N-甲基吡啶鎓化合物与哌啶在甲醇中的取代反应中观察到不同的离去基团顺序：2-CN ≥ 4-CN > 2-F ~ 2-Cl ~ 2-Br ~ 2-I。反应是二级的[哌啶]，机制涉及速率决定哌啶和底物哌啶加成中间体之间的氢键形成，然后由该中间体的去质子化。计算结果表明，氢键复合物的去质子化可能是无障碍的，并伴随着同时损失的离去基团（E2）的L = Cl，Br，和I，但随后，快速损失的离去基团（E1 cB样）较差的离去基团，CN和F。大约50倍的2-和4-氰基底物的反应性归因于吸电子氰基基团在去质子化步骤中的影响。这些结果提供了另一个β-消除反应的例子，该反应在E2-E1 cB机理边界附近。
The “element effect” in nucleophilic aromatic substitution reactions (SNAr) is characterized by the leaving group order, L = F > NO2 > Cl ≈ Br > I, in activated aryl substrates. A different leaving group order is observed in the substitution reactions of ring-substituted N-methylpyridinium compounds with piperidine in methanol: 2-CN ≥ 4-CN > 2-F ~ 2-Cl ~ 2-Br ~ 2-I. The reactions are second-order in [piperidine], the mechanism involving rate determining hydrogen-bond formation between piperidine and the substrate-piperidine addition intermediate followed by deprotonation of this intermediate. Computational results indicate that deprotonation of the H-bonded complex is probably barrier free, and is accompanied by simultaneous loss of the leaving group (E2) for L = Cl, Br, and I, but with subsequent, rapid loss of the leaving group (E1cB-like) for the poorer leaving groups, CN and F. The approximately 50-fold greater reactivity of the 2- and 4-cyano substrates is attributed to the influence of the electron withdrawing cyano group in the deprotonation step. The results provide another example of β-elimination reactions poised near the E2-E1cB mechanistic borderline.
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影响因子：
15
作者：
BUNNETT, JF;GARBISCH, EW;PRUITT, KM
通讯作者：
PRUITT, KM
影响因子：
15
作者：
JENCKS, WP;SALVESEN, K
通讯作者：
SALVESEN, K
影响因子：
15
作者：
Kim, Yongho;Mohrig, Jerry R.;Truhlar, Donald G.
通讯作者：
Truhlar, Donald G.
影响因子：
3.6
作者：
MUSCIO, OJ;RUTHERFORD, DR
通讯作者：
RUTHERFORD, DR
DOI：
10.1039/jr9590000551
发表时间：
1959-01-01
期刊：
JOURNAL OF THE CHEMICAL SOCIETY
影响因子：
--
作者：
NICHOLLS, B;WHITING, MC
通讯作者：
WHITING, MC