高等有机化学——反应机理6.pptx

第四课第四课 周环反应周环反应1、基础知识、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 reactionsRegioselectivityStereoselectivityStereospecificity4.1.1 Classes of Pericyclic ReactionsnElectrocyclic reactions(ring openings or ring closings),nCycloadditionsnSigmatropic rearrangementsnEne reactions.Electrocyclic reactionsCycloadditionsCheletropic reactionsCycloadditionsSigmatropic rearrangementsSigmatropic rearrangementsSigmatropic rearrangementsEne Reaction四种周环反应总结四种周环反应总结立体专一性立体专一性Polyene MOsWoodwardHoffmann rulesPolyene MOsPolyene MOsPolyene MOs2、电环化反应、电环化反应2.1、典型反应、典型反应2.1、典型反应、典型反应2.1、典型反应、典型反应2.1、典型反应、典型反应练习练习4.12.1、典型反应、典型反应2.1、Favorskii rearrangement2.1、Favorskii rearrangement练习练习练习练习4.2.2 Stereospecificity1,3,5-hexatrienes偶偶热热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 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-demand 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 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(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