定义Definition
周环反应是经环状过渡态、键的断裂与形成协同进行（一步完成、无中间体）的反应，主要包括环加成、电环化、σ 迁移、螯键（cheletropic）和基团转移反应。其立体化学由轨道对称性严格控制。
Pericyclic reactions proceed in one concerted step through a cyclic transition state, with no intermediates. They include cycloadditions, electrocyclic reactions, sigmatropic shifts, cheletropic reactions and group transfers, and their stereochemistry is strictly governed by orbital symmetry.
伍德沃德–霍夫曼规则Woodward–Hoffmann rules
1965 年伍德沃德与霍夫曼提出轨道对称守恒原理：热反应中参与电子数为 4n+2 时同面–同面（或顺旋/对旋）途径允许；4n 时需光照或采取异面途径。福井谦一的前线轨道理论（HOMO–LUMO 相互作用）给出等价解释。霍夫曼与福井共获 1981 年诺贝尔化学奖。
In 1965 Woodward and Hoffmann proposed conservation of orbital symmetry: thermally, 4n+2-electron processes are allowed suprafacially (or with the matching rotation sense), while 4n-electron ones need light or antarafacial geometry. Fukui’s frontier-orbital theory (HOMO–LUMO) is equivalent; Hoffmann and Fukui shared the 1981 Nobel Prize.
特点Features
受溶剂极性、催化剂影响较小（路易斯酸可加速狄尔斯–阿尔德）；立体专一；光照与加热常得到不同立体化学产物（如维生素 D 的光化学开环与热 σ 迁移）。
Relatively insensitive to solvent polarity (though Lewis acids speed Diels–Alder); stereospecific; light and heat often give different stereochemistry (vitamin D formation involves a photochemical ring opening and a thermal sigmatropic shift).
芳香过渡态Aromatic transition states
德瓦尔–齐默尔曼规则：允许的周环过渡态相当于“芳香”的环状电子体系（休克尔型 4n+2 或默比乌斯型 4n）。
Dewar–Zimmerman rule: allowed pericyclic transition states are ‘aromatic’ cyclic arrays (Hückel 4n+2 or Möbius 4n).
反应Reactions
加热或光照
Heat or light
反应物轨道在环状过渡态中连续重叠。
Reactant orbitals overlap continuously in a cyclic TS.
一步完成成键与断键。
Bonds form and break in one step.
狄尔斯–阿尔德反应。
The Diels–Alder reaction.
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.
六个 π 电子环状协同移动，两根新 σ 键同时形成
Six π electrons move in a ring; both new σ bonds form at once
环己烯
Cyclohexene
年表Timeline
- 1928
狄尔斯–阿尔德反应
Diels–Alder reaction
- 1952
福井谦一提出前线轨道理论
Fukui proposes frontier-orbital theory
- 1965
伍德沃德–霍夫曼规则
Woodward–Hoffmann rules
- 1981
霍夫曼、福井获诺贝尔化学奖
Hoffmann and Fukui share the Nobel Prize