高等有机化学——反应机理.6

第四课周环反应1、基础知识Apericyclicreactionisareactioninwhichbondsareformedorbrokenattheterminiofoneormoreconjugatedsystems.Theelectronsmovearoundinacircle,allbondsaremadeandbrokensimultaneously,andnointermediatesintervene.TypicalreactionsRegioselectivityStereoselectivityStereospecificity4.1.1ClassesofPericyclicReactionsElectrocyclicreactions(ringopeningsorringclosings),CycloadditionsSigmatropicrearrangementsEnereactions.ElectrocyclicreactionsCycloadditionsCheletropicreactionsCycloadditionsSigmatropicrearrangementsSigmatropicrearrangementsSigmatropicrearrangementsEneReaction四种周环反应总结立体专一性PolyeneMOsWoodward–HoffmannrulesPolyeneMOsPolyeneMOsPolyeneMOs2、电环化反应2.1、典型反应2.1、典型反应2.1、典型反应2.1、典型反应练习4.12.1、典型反应2.1、Favorskiirearrangement2.1、Favorskiirearrangement练习练习4.2.2Stereospecificity1,3,5-hexatrienes偶——热——顺奇——热——反偶——光——反奇——光——顺TheWoodward–Hoffmannrulesapplyonlytoconcerted,pericyclicreactions.AreactioncanbeforcedtoproceedthroughthehigherenergyTSifthelowerenergyoneisraisedprohibitivelyhighinenergybygeometricconstraints.4.2.3Stereoselectivity4.3Cycloadditions4.3.1TypicalReactions4.3.1.1TheDiels–AlderReactionMostDiels–Alderreactionsoccurwithwhatiscallednormalelectron-demand,inwhichanelectron-rich(nucleophilic)dienereactswithanelectronpoor(electrophilic)dienophile.Normalelectron-demandDiels–Alderreactions用前线轨道理论解释反应活性炔烃的D-A反应活性低(Aldrin)VeryelectronpoordienescanundergoDiels–Alderreactionswithelectronrichdienophilesintheinverseelectron-demandDiels–Alderreaction.ThedominantinteractionintheTSofinverseelectron-demandDiels–AlderreactionsisbetweentheLUMOdieneandtheHOMOdienophile.Inverseelectron-demandDiels–Alderreaction4.3.1.2OtherCycloadditions（1）[3+2]Cycloadditions(加入Me2S使过氧化物还原为醛)Paterno–Büchireaction（2）[2+2]Cycloadditions（胸腺嘧啶）（导致皮肤癌）Wittigreaction（3）[4+1]Retro-cycloadditions4.3.2Regioselectivity定位规则：Diels–Alderreactionsproceedtoputthemostelectrondonatingsubstituentonthedieneandthemostelectronwithdrawingsubstituentonthedienophileeither“ortho”or“para”tooneanother.(1)Normalelectron-demandDiels–Alderreactions(2)Reverseelectron-demandDiels–Alderreactions[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4.3.4Stereoselectivity4.4.1TypicalReactions4.4SigmatropicRearrangements练习练习4.5EneReactions