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Duality of the nucleophilic displacement mechanism

亲核置换机制的二元性

基本信息

批准号：
05453032
负责人：
FUJIO Mizue
金额：
$ 4.8万
依托单位：
KYUSHU UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1993
资助国家：
日本
起止时间：
1993 至 1994
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-05453032/
关键词：
Nucleophilic Displacement Menschutkin Reaction Duality of S_N1-S_N2 Solvolysis Mechanism Ion-pair Return Ion-pair Intermediate O-18 Scrambling C-13 NMR 0-18スクランブリング S_N1S_N2機構二次性溶媒効果

项目摘要

The ^<18>O-scrambling within starting sulfonate during the Menschutkin reaction of pmethylbenzyl-1-^<13>C tosylate-S^<18>O_2 with N,N-dimethylanilines which is typical S_N2 displacement reaction, was determined in MeCN at 35ﾟC by ^<13>C-NMR spectroscopy utilizing the split ^<13>C-NMR signals of alkoxy carbon induced by ^<18>O/^<16>O isotopic substitution. For the ^<13>C peak of benzyl carbon of starting tosylates, a new ^<13>C peak caused by ^<18>O isotope effect gradually grew up with time. This finding of unambiguous ^<18>O-scrambling in starting tosylate demonstrated the existence of the carbocationic intermedates in the typical S_N2 reaction in MeCN.The rate of product formation (k_t) and the decreasing rate of unreacted tosylate (k_i) of this reaction are determined with various DMA concentrations. The difference between k_t and k_i values is unchanged with DMA conentrations, irrespective of the nucleophilicity of DMA.This k_i-k_t value is in accordance with the rate of ^<18>O-scrambling of p-methylbenzyl tosylate in the absence of DMA.This result gives the important evidence that benzyl tosylate ionizes independently via S_N1 mechanism irrespective of concurrent S_N2 mechansim. The subsequent nucleophilic attack in this unimolecular ionization process is so slow that the return from the ion-pair intermediate can not be removed effectively. It is concluded that the S_N1 and S_N2 processes are quite distinct that these results afford no evidence for a single mechanism intermediate between S_N1 and S_N2 in this reaction.

在对甲基苄基-1-^ 13 C甲苯磺酸盐-S^ 18 O_2与N，N-二甲基苯胺的Menschutkin反应期间起始磺酸盐内的^ 18 O-扰乱是典型的S_N2置换反应，在35℃下在MeCN中通过^ 13 C-NMR光谱法利用分离的^ 13 C-NMR信号测定由^ 18 O/^ 16 O同位素取代诱导的烷氧基碳。对于起始甲苯磺酸盐的苄基碳的^ 13 C峰，由^ 18 O同位素效应引起的新的^ 13 C峰随着时间逐渐增长。起始甲苯磺酸盐中明确的^ 18 O-扰乱的这一发现证明了MeCN中典型S_N2反应中碳阳离子中间体的存在。该反应的产物形成速率(k_t)和未反应的甲苯磺酸盐的减少速率(k_i)用不同的DMA浓度确定。无论 DMA 的亲核性如何，k_t 和 k_i 值之间的差异不会随 DMA 浓度而变化。该 k_i-k_t 值与在不存在 DMA 的情况下甲苯磺酸对甲基苄酯的 ^<18>O-扰乱速率一致。该结果提供了甲苯磺酸苄酯通过 S_N1 机制独立电离的重要证据，而与此无关并发S_N2机制。该单分子电离过程中随后的亲核攻击非常缓慢，以至于无法有效去除离子对中间体的返回。结论是，S_N1 和 S_N2 过程非常不同，这些结果没有提供该反应中介于 S_N1 和 S_N2 之间的单一机制的证据。