定义Definition
伯奇还原是用碱金属（Li、Na）溶于液氨并加入醇作质子源，把芳环部分还原为非共轭的 1,4-环己二烯的反应。
The Birch reduction uses an alkali metal (Li, Na) dissolved in liquid ammonia with an alcohol proton source to partially reduce an aromatic ring to an unconjugated 1,4-cyclohexadiene.
机理Mechanism
碱金属溶于液氨生成溶剂化电子（深蓝色）。芳环接受一个电子成自由基负离子，被醇质子化得自由基，再得一个电子成碳负离子，再次质子化得 1,4-环己二烯。动力学控制使双键不共轭。
The metal dissolves to give solvated electrons (deep blue). The arene takes one electron to form a radical anion, is protonated by the alcohol, takes a second electron and is protonated again, giving the 1,4-diene under kinetic control.
区域选择性Regioselectivity
给电子基（–OMe、烷基）留在未被还原的双键碳上（产物为 1-取代-1,4-二烯）；吸电子基（–COOH、–COR）所在碳被还原（得 1-取代-2,5-二烯）。苯甲醚伯奇还原后酸水解可得环己烯酮，是合成甾体激素的重要步骤。
Donor groups (–OMe, alkyl) end up on a remaining double bond (1-substituted 1,4-dienes); acceptor groups (–COOH, –COR) end up on a reduced carbon (2,5-dienes). Birch reduction of anisoles followed by acid hydrolysis gives cyclohexenones — key in steroid-hormone synthesis.
由来Origin
澳大利亚化学家亚瑟·伯奇于 1944 年在牛津报道，并用于合成 19-去甲甾体（口服避孕药的基础）。
Reported by the Australian chemist Arthur Birch at Oxford in 1944 and used to make 19-norsteroids, the basis of oral contraceptives.
反应Reactions
−33 °C 液氨（或 −78 °C），Li 或 Na，叔丁醇或乙醇
Liquid ammonia at −33 °C (or −78 °C), Li or Na, tert-butanol or ethanol
单电子转移生成自由基负离子。
Single-electron transfer gives a radical anion.
醇质子化得自由基。
Protonation by the alcohol gives a radical.
第二次电子转移得碳负离子。
A second electron gives a carbanion.
再质子化得 1,4-环己二烯。
Protonation gives the 1,4-diene.
合成环己烯酮。
A route to cyclohexenones.
年表Timeline
- 1944
伯奇报道该还原
Birch reports the reduction
- 1950
伯奇合成 19-去甲睾酮，推动口服避孕药研发
Birch synthesises 19-nortestosterone, feeding oral-contraceptive development