高等有机化学-反应机理.ppt

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1、第四课第四课 周环反应周环反应1、基、基础础知知识识 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 reactionsRegioselectivityStere

2、oselectivityStereospecificity4.1.1 Classes of Pericyclic ReactionsnElectrocyclic reactions(ring openings or ring closings),nCycloadditionsnSigmatropic rearrangementsnEne reactions.Electrocyclic reactionsCycloadditionsCheletropic reactionsCycloadditionsSigmatropic rearrangementsSigmatropic rearrangem

3、entsSigmatropic rearrangementsEne Reaction四种周四种周环环反反应总结应总结立体立体专专一性一性Polyene MOsWoodwardHoffmann rulesPolyene MOsPolyene MOsPolyene MOs2、电环电环化反化反应应、典型反应、典型反应、典型反应、典型反应、典型反应、典型反应、典型反应、典型反应练习练习、典型反应、典型反应、Favorskii rearrangement、Favorskii rearrangement练习练习练习练习4.2.2 Stereospecificity1,3,5-hexatrienes偶偶热热

4、顺顺奇奇热热反反偶偶光光反反奇奇光光顺顺 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.4.2.3 Stereoselectivity4.3 Cycloadditions4.3.1 Typical

5、 Reactions4.3.1.1 The DielsAlder ReactionMost 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 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炔烃的炔烃的D-A反应活性低反应

6、活性低(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-d

7、emand DielsAlder reaction4.3.1.2 Other Cycloadditions（1）3+2 Cycloadditions(加入加入Me2S 使过氧化物还原为醛使过氧化物还原为醛)PaternoBchi reaction（2）2+2 Cycloadditions（胸腺嘧啶）（胸腺嘧啶）（导致皮肤癌）（导致皮肤癌）Wittig reaction（3）4+1 Retro-cycloadditions 4.3.2 Regioselectivity定定位位规规则则：DielsAlder reactions proceed to put the most electron do

8、nating 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(2)Reverse electron-demand DielsAlder reactions 4+24+2环加成反应的立体定向性环加成反应的立体定向性 3+23+2环加成反应的立体定向性环加成反应的立体定向性 2+22+2环加成反应环加成反应4.3.4 Stereoselectivity4.4.1 Typical Reactions4.4 Sigmatropic Rearrangements练习练习练习练习4.5 Ene Reactions