双语化学辞典Chem Lexicon
概念 · 有机化学 · 亲核芳香取代 Organic chemistry · Nucleophilic aromatic substitution

迈森海默络合物Meisenheimer complex

拼音Pinyin mài sēn hǎi mò luò hé wù　别名Also 迈森海默配合物；σ-络合物（亲核芳香取代）；Jackson–Meisenheimer complex；anionic σ- complex

定义Definition

迈森海默络合物是亲核试剂加成到缺电子芳环（通常带硝基等强吸电子基）上形成的阴离子 σ 加合物，环上一个碳变为 sp³ 杂化，负电荷离域到吸电子基上。它是加成-消除机理 SNAr 反应的中间体。

A Meisenheimer complex is the anionic σ-adduct formed when a nucleophile adds to an electron-poor arene, typically one bearing nitro groups. One ring carbon becomes sp³ and the negative charge is delocalised onto the electron-withdrawing groups. It is the intermediate of addition–elimination SNAr reactions.

结构与稳定性Structure and stability

负电荷通过共振分布在加成位点的邻、对位上；邻、对位的硝基可把负电荷接收到氧原子上，大大稳定中间体。2,4,6-三硝基苯甲醚与甲醇钠形成的络合物足够稳定，可分离为有色盐。

The negative charge is spread by resonance to the positions ortho and para to the site of attack; nitro groups there take the charge onto oxygen and strongly stabilise the adduct. The complex from 2,4,6-trinitroanisole and sodium methoxide is stable enough to isolate as a coloured salt.

由来Origin

1900 年美国化学家查尔斯·杰克逊和 1902 年德国化学家雅各布·迈森海默分别提出这类加合物的结构；迈森海默证明由苦基乙醚 + 甲醇钠与苦基甲醚 + 乙醇钠得到同一络合物。

Charles Loring Jackson (1900) and Jakob Meisenheimer (1902) proposed the structure; Meisenheimer showed that picryl ethyl ether + methoxide and picryl methyl ether + ethoxide give the same complex.

在 SNAr 中的作用Role in SNAr

SNAr 反应中亲核加成通常是决速步，所以离去基团活性顺序为 F > Cl ≈ Br > I（电负性大的氟使碳更缺电子），与 SN2 相反。近来研究发现部分 SNAr 反应是协同的，并不经过稳定的迈森海默中间体。

Nucleophilic addition is usually rate-determining in SNAr, so leaving-group reactivity is F > Cl ≈ Br > I — the reverse of SN2, because fluorine makes the carbon more electrophilic. Recent work shows some SNAr reactions are concerted with no stable Meisenheimer intermediate.

反应Reactions

Ar–X缺电子+ Nu− ⇌ [Ar(X)(Nu)]−迈森海默络合物 ⟶ Ar–Nu + X−Ar–X(Electron)+ Nu− ⇌ [Ar(X)(Nu)]−(Meisenheimer complex) ⟶ Ar–Nu + X−

强亲核试剂，极性非质子溶剂（DMSO、DMF）有利

Strong nucleophiles; polar aprotic solvents (DMSO, DMF) help

  1. 亲核试剂加成到连有离去基团的碳上。

    The nucleophile adds to the carbon bearing the leaving group.

  2. 形成负电荷离域的 σ 络合物。

    A delocalised anionic σ-complex forms.

  3. 离去基团离去，恢复芳香性。

    The leaving group departs and aromaticity returns.

2,4-二硝基氟苯 + H2N–肽 ⟶ DNP-肽 + HF2,4- + H2N– ⟶ DNP- + HF

桑格用此反应标记蛋白质 N 端。

Sanger's labelling of protein N-termini.

反应机理图Mechanism

红色弯箭头表示一对电子的移动方向：从电子来源（孤对电子或化学键）指向电子去向。Red curly arrows show where an electron pair moves: from its source (lone pair or bond) to its destination.

1

CH₃O⁻ 加到连 F 的碳上，负电荷推到硝基所连的碳

CH₃O⁻ adds to the C–F carbon; the negative charge moves to the carbon bearing NO₂

慢slow
2

迈森海默络合物：负电荷由硝基稳定；F⁻ 离去恢复芳香性

Meisenheimer complex, stabilised by NO₂; loss of F⁻ restores aromaticity

快fast
3

对硝基苯甲醚；F 虽是差的离去基团，却因加成是决速步而最快

4-Nitroanisole; F reacts fastest because addition, not loss of F⁻, is rate-determining

年表Timeline

  • 1900

    杰克逊提出该类加合物

    Jackson proposes such adducts

  • 1902

    迈森海默确证其结构

    Meisenheimer confirms the structure

  • 2018

    实验与计算表明许多 SNAr 为协同过程

    Experiments and computation show many SNAr reactions are concerted