双语化学辞典Chem Lexicon
机理 · 有机化学 · 周环反应 Organic chemistry · Pericyclic reactions

电环化反应Electrocyclic reaction

拼音Pinyin diàn huán huà fǎn yìng　别名Also 电环合 / 电开环；electrocyclisation； electrocyclic ring opening

定义Definition

电环化反应是共轭多烯两端碳原子之间形成一个 σ 键而闭环（或其逆过程开环）的周环反应，π 键数减少一个。末端取代基的立体化学由顺旋或对旋方式决定。

An electrocyclic reaction forms a σ bond between the termini of a conjugated polyene to close a ring (or the reverse opening), consuming one π bond; the stereochemistry of terminal substituents follows conrotatory or disrotatory motion.

选择规则Selection rules

4n 个 π 电子（如丁二烯 ⇌ 环丁烯）：加热顺旋、光照对旋；4n+2 个 π 电子（如己三烯 ⇌ 环己二烯）：加热对旋、光照顺旋。可由 HOMO 末端系数的相位判断：两端同相重叠所需的转动方式即允许方式。

4n π electrons (butadiene ⇌ cyclobutene): thermal conrotatory, photochemical disrotatory; 4n+2 (hexatriene ⇌ cyclohexadiene): thermal disrotatory, photochemical conrotatory. The HOMO’s terminal lobes show which rotation brings like phases together.

实例Examples

顺-3,4-二甲基环丁烯加热开环只得 (2E,4Z)-己二烯；(2E,4Z,6E)-辛三烯加热闭环只得顺-5,6-二甲基-1,3-环己二烯。维生素 D 生物合成中 7-脱氢胆固醇在紫外光下顺旋开环生成前维生素 D。纳扎罗夫环化是 4π 阳离子电环化。

cis-3,4-Dimethylcyclobutene opens thermally only to (2E,4Z)-hexadiene; (2E,4Z,6E)-octatriene closes thermally only to cis-5,6-dimethylcyclohexa-1,3-diene. In vitamin D biosynthesis UV light opens 7-dehydrocholesterol conrotatorily to previtamin D. The Nazarov cyclisation is a cationic 4π electrocyclisation.

历史History

伍德沃德在维生素 B₁₂ 全合成中注意到一个电环化的立体化学“异常”，与霍夫曼于 1965 年提出轨道对称守恒规则，首先就是用电环化反应阐述的。

Woodward noticed a puzzling electrocyclic stereochemistry during the vitamin B₁₂ synthesis; the 1965 Woodward–Hoffmann rules were first formulated for electrocyclic reactions.

反应Reactions

1,3-丁二烯 ⇌ 环丁烯4π1,3,5-己三烯 ⇌ 1,3-环己二烯6π1,3-Butadiene ⇌ (4π); 1,3,5- ⇌ 1,3-(6π)

加热或紫外光照

Heat or UV light

  1. 多烯两端 p 轨道转动（顺旋或对旋）。

    Terminal p orbitals rotate (con- or disrotatory).

  2. 同相重叠形成 σ 键，π 体系重组。

    In-phase overlap forms the σ bond and the π system reorganises.

顺-3,4-二甲基环丁烯 △, 顺旋⟶ (2E,4Z)-2,4-己二烯cis--3,4- △,⟶ (2E,4Z)-2,4-

立体专一开环。

Stereospecific ring opening.

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

6π 电环化：两端成键；加热对旋，光照顺旋

6π electrocyclisation; thermal = disrotatory, photochemical = conrotatory

△ 对旋△ disrotatory
2

1,3-环己二烯

Cyclohexa-1,3-diene

年表Timeline

  • 1965

    伍德沃德–霍夫曼提出电环化选择规则

    Woodward and Hoffmann publish electrocyclic selection rules

  • 1972

    维生素 B₁₂ 全合成完成

    Vitamin B₁₂ total synthesis completed