定义Definition
σ 迁移反应是一个 σ 键沿共轭 π 体系协同迁移到新位置、同时 π 键重新分布的周环反应，用 [i,j] 表示迁移前后 σ 键两端原子位置的变化，如 [1,5]-氢迁移、[3,3]-迁移。
A sigmatropic reaction is a concerted pericyclic migration of a σ bond along a conjugated π system with reorganisation of the π bonds, labelled [i,j] by how far each end of the σ bond moves — e.g. [1,5]-H shifts and [3,3] shifts.
规则Rules
[1,j]-氢迁移：[1,5]-H 同面迁移热允许（环戊二烯取代基快速“走动”）；[1,3]-H 同面热禁阻、[1,7]-H 热允许需异面（维生素 D 中前维生素 D → 维生素 D）。[3,3]-迁移（克莱森、科普）经六电子芳香过渡态热允许。[2,3]-迁移（维蒂希、米斯洛–埃文斯）亦常见。
[1,j]-H shifts: suprafacial [1,5]-H is thermally allowed (substituents ‘walk’ around cyclopentadiene); suprafacial [1,3]-H is forbidden; thermal [1,7]-H is antarafacial (previtamin D → vitamin D). [3,3] shifts (Claisen, Cope) proceed through six-electron aromatic TSs; [2,3] shifts (Wittig, Mislow–Evans) are also common.
立体化学Stereochemistry
[3,3]-迁移优先经椅式过渡态，可精确预测新双键几何与新立体中心，因此能高效传递手性。
[3,3] shifts prefer chair transition states, predicting alkene geometry and new stereocentres precisely and transferring chirality efficiently.
实例Examples
芳香克莱森重排、费歇尔吲哚合成中的 [3,3]、分支酸变位酶催化的生物 [3,3] 重排、5-甲基环戊二烯的 [1,5]-H 异构化。
The aromatic Claisen rearrangement, the [3,3] step of the Fischer indole synthesis, the enzymatic chorismate-mutase [3,3] shift and [1,5]-H isomerisation of 5-methylcyclopentadiene.
反应Reactions
加热（部分可光照）
Heat (some photochemically)
σ 键与 π 体系在环状过渡态中重叠。
The σ bond and π system overlap in a cyclic TS.
σ 键在一端断裂、另一端形成，π 键迁移。
The σ bond breaks at one end and forms at the other as π bonds shift.
[3,3]-σ 迁移。
A [3,3]-sigmatropic shift.
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.
椅式六元环过渡态，[3,3] σ 迁移
Chair-like six-membered transition state, a [3,3] sigmatropic shift
γ,δ-不饱和醛（4-戊烯醛）
γ,δ-Unsaturated aldehyde (pent-4-enal)
年表Timeline
- 1912
克莱森重排
Claisen rearrangement
- 1940
科普重排
Cope rearrangement
- 1965
伍德沃德–霍夫曼规则覆盖 σ 迁移
Woodward–Hoffmann rules cover sigmatropic shifts