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1第四课第四课 周环反应周环反应21、基、基础知知识 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 reactionsRegioselectivityStereoselectivityStereospecificity34.1.1 Classes of Pericyclic ReactionsnElectrocyclic reactions(ring openings or ring closings),nCycloadditionsnSigmatropic rearrangementsnEne reactions.4Electrocyclic reactions5Cycloadditions6Cheletropic reactionsCycloadditions7Sigmatropic rearrangements8Sigmatropic rearrangements9Sigmatropic rearrangements10Ene Reaction11四种周四种周环反反应总结1213立体立体专一性一性14Polyene MOsWoodwardHoffmann rules15Polyene MOs16Polyene MOs17Polyene MOs182、电环化反化反应2.1、典型反应、典型反应192.1、典型反应、典型反应202.1、典型反应、典型反应212.1、典型反应、典型反应22练习练习4.1232.1、典型反应、典型反应242.1、Favorskii rearrangement252.1、Favorskii rearrangement2627练习练习28293031练习练习3233344.2.2 Stereospecificity35361,3,5-hexatrienes3738偶偶热热顺顺奇奇热热反反偶偶光光反反奇奇光光顺顺39 The WoodwardHoffmann 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.4041424.2.3 Stereoselectivity434445464.3 Cycloadditions4.3.1 Typical Reactions4.3.1.1 The DielsAlder Reaction4748495051Most DielsAlder 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 DielsAlder reactions 52用前线轨道理论解释反应活性用前线轨道理论解释反应活性5354炔烃的炔烃的D-A反应活性低反应活性低55(Aldrin)Very electron poor dienes can undergo DielsAlder reactions with electronrich dienophiles in the inverse electron-demand DielsAlder reaction.The dominant interaction in the TS of inverse electron-demand DielsAlder reactions is between the LUMOdiene and the HOMOdienophile.Inverse electron-demand DielsAlder reaction565758594.3.1.2 Other Cycloadditions（1）3+2 Cycloadditions 606162(加入加入Me2S 使过氧化物还原为醛使过氧化物还原为醛)63PaternoBchi reaction（2）2+2 Cycloadditions 64（胸腺嘧啶）（胸腺嘧啶）（导致皮肤癌）（导致皮肤癌）65Wittig reaction66（3）4+1 Retro-cycloadditions 67686970714.3.2 Regioselectivity定定位位规规则则：DielsAlder 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 DielsAlder reactions 72(2)Reverse electron-demand DielsAlder reactions 734+24+2环加成反应的立体定向性环加成反应的立体定向性 74753+23+2环加成反应的立体定向性环加成反应的立体定向性 76772+22+2环加成反应环加成反应78794.3.4 Stereoselectivity80818283844.4.1 Typical Reactions4.4 Sigmatropic Rearrangements858687练习练习88899091练习练习924.5 Ene Reactions9394
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