定义Definition
瓦格纳-梅尔外因重排是碳正离子中相邻碳上的氢、烷基或芳基带着电子对发生 1,2-迁移，生成更稳定碳正离子的一类重排，常伴随骨架改变。
The Wagner–Meerwein rearrangement is a class of carbocation rearrangements in which a hydrogen, alkyl or aryl group on an adjacent carbon migrates with its electron pair (a 1,2-shift) to give a more stable cation, often changing the carbon skeleton.
原理Principle
驱动力是碳正离子稳定性：伯 < 仲 < 叔、烯丙基、苄基；环张力释放（如四元环扩成五元环）也能驱动迁移。迁移经三中心两电子的桥连过渡态，迁移基团构型保持。
The driving force is cation stability (primary < secondary < tertiary, allylic, benzylic) and relief of ring strain (four- to five-membered expansion). Migration passes through a three-centre two-electron bridged transition state with retention at the migrating group.
由来与非经典碳正离子之争Origin and the non-classical ion debate
叶戈尔·瓦格纳（1899）在萜类（樟脑、冰片）化学中观察到骨架重排，汉斯·梅尔外因（1914–1922）提出碳正离子中间体解释。降冰片基正离子是否为桥连“非经典”离子曾引发温斯坦与 H. C. 布朗长达数十年的争论，奥拉在超强酸中的 NMR 研究和 2013 年的晶体结构最终支持非经典结构。
Yegor Wagner (1899) saw skeletal rearrangements in terpene chemistry (camphor, borneol) and Hans Meerwein (1914–22) explained them with carbocations. Whether the norbornyl cation is a bridged ‘non-classical’ ion fuelled a decades-long Winstein–Brown debate, settled in favour of the non-classical structure by Olah’s superacid NMR and a 2013 crystal structure.
意义Significance
解释了 SN1、E1、傅–克烷基化中的重排副产物；萜类、甾体生物合成中（如羊毛甾醇形成）存在一连串酶控的氢迁移和甲基迁移。
Explains rearranged products in SN1, E1 and Friedel–Crafts alkylation; terpene and steroid biosynthesis (e.g. lanosterol formation) involves enzyme-controlled cascades of hydride and methyl shifts.
反应Reactions
能产生碳正离子的条件：强酸、路易斯酸、SN1/E1 溶剂解
Any cation-generating conditions: strong acids, Lewis acids, SN1/E1 solvolysis
生成碳正离子。
A carbocation forms.
邻位基团经桥连过渡态迁移。
A neighbouring group migrates through a bridged transition state.
新的更稳定碳正离子被亲核试剂捕获或失去质子。
The more stable cation is trapped or loses a proton.
新戊基型重排。
A neopentyl-type rearrangement.
萜类骨架重排。
A terpene skeletal rearrangement.
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.
甲基带着电子对迁移到相邻的正碳上
A methyl moves with its electron pair to the adjacent cationic carbon
仲碳正离子变成更稳定的叔碳正离子
The secondary cation becomes a more stable tertiary one
年表Timeline
- 1899
瓦格纳研究萜类骨架重排
Wagner studies terpene skeletal rearrangements
- 1922
梅尔外因提出碳正离子机理
Meerwein proposes the carbocation mechanism
- 1949
温斯坦提出非经典降冰片基正离子
Winstein proposes the non-classical norbornyl cation
- 2013
降冰片基正离子晶体结构确证桥连结构
Crystal structure confirms the bridged norbornyl cation