第四课 周环反应

1 、基础知识 A pericyclic reaction is a reaction in which bonds are formed or broken at the termini of one or more conjugated systems. The electrons move around in a circle, all bonds are made and broken simultaneously, and no intermediates intervene.  Typical reactions  Regioselectivity  Stereoselectivity  Stereospecificity

4.1.1 Classes of Pericyclic Reactions Electrocyclic reactions (ring openings or ring closings), Cycloadditions Sigmatropic rearrangements Ene reactions.

Electrocyclic reactions

Cycloadditions

Cheletropic reactions Cycloadditions

Sigmatropic rearrangements

Ene Reaction

四种周环反应总结

立体专一性

Polyene MOs Woodward–Hoffmann rules

Polyene MOs

2 、电环化反应 2.1 、典型反应

练习 4.1

2.1 、典型反应

2.1 、 Favorskii rearrangement

练习

4.2.2 Stereospecificity

1,3,5-hexatrienes

偶 —— 热 —— 顺 奇 —— 热 —— 反 偶 —— 光 —— 反 奇 —— 光 —— 顺

 The Woodward–Hoffmann rules apply only to concerted, pericyclic reactions.  A reaction can be forced to proceed through the higher energy TS if the lower energy one is raised prohibitively high in energy by geometric constraints.

4.2.3 Stereoselectivity

4.3 Cycloadditions Typical Reactions The Diels–Alder Reaction

Most Diels–Alder reactions occur with what is called normal electron-demand, in which an electron-rich (nucleophilic) diene reacts with an electronpoor (electrophilic) dienophile. Normal electron-demand Diels–Alder reactions

用前线轨道理论解释反应活性

炔烃的 D-A 反应活性低

(Aldrin) Very electron poor dienes can undergo Diels–Alder reactions with electronrich dienophiles in the inverse electron-demand Diels–Alder reaction. The dominant interaction in the TS of inverse electron-demand Diels–Alder reactions is between the LUMO diene and the HOMO dienophile. Inverse electron-demand Diels–Alder reaction

Other Cycloadditions （ 1 ） [3+2] Cycloadditions

( 加入 Me 2 S 使过氧化物还原为醛 )

Paterno–Büchi reaction （ 2 ） [2+2] Cycloadditions

（胸腺嘧啶） （导致皮肤癌）

Wittig reaction

（ 3 ） [4+1] Retro-cycloadditions

4.3.2 Regioselectivity 定位规则： Diels–Alder reactions proceed to put the most electron donating substituent on the diene and the most electron withdrawing substituent on the dienophile either “ortho” or “para” to one another. (1) Normal electron-demand Diels–Alder reactions

(2) Reverse electron-demand Diels–Alder reactions

[4+2] 环加成反应的立体定向性

[3+2] 环加成反应的立体定向性

[2+2] 环加成反应

4.3.4 Stereoselectivity

4.4.1 Typical Reactions 4.4 Sigmatropic Rearrangements

练习

4.5 Ene Reactions