高等有機化學——反應機理第四課周環(huán)反響1、根底知識

Apericyclicreactionisareactioninwhichbondsareformedorbrokenattheterminiofoneormoreconjugatedsystems.Theelectronsmovearoundinacircle,allbondsaremadeandbrokensimultaneously,andnointermediatesintervene.TypicalreactionsRegioselectivityStereoselectivityStereospecificity4.1.1ClassesofPericyclicReactionsElectrocyclicreactions(ringopeningsorringclosings),CycloadditionsSigmatropicrearrangementsEnereactions.ElectrocyclicreactionsCycloadditionsCheletropicreactionsCycloadditionsSigmatropicrearrangementsSigmatropicrearrangementsSigmatropicrearrangementsEneReaction四種周環(huán)反響總結立體專一性PolyeneMOsWoodward–HoffmannrulesPolyeneMOsPolyeneMOsPolyeneMOs2、電環(huán)化反響2.1、典型反響2.1、典型反響2.1、典型反響2.1、典型反響練習4.12.1、典型反響2.1、Favorskiirearrangement2.1、Favorskiirearrangement練習練習4.2.2Stereospecificity1,3,5-hexatrienes偶——熱——順奇——熱——反偶——光——反奇——光——順

TheWoodward–Hoffmannrulesapplyonlytoconcerted,pericyclicreactions.AreactioncanbeforcedtoproceedthroughthehigherenergyTSifthelowerenergyoneisraisedprohibitivelyhighinenergybygeometricconstraints.4.2.3Stereoselectivity4.3Cycloadditions4.3.1TypicalReactions4.3.1.1TheDiels–AlderReactionMostDiels–Alderreactionsoccurwithwhatiscallednormalelectron-demand,inwhichanelectron-rich(nucleophilic)diene

reactswithanelectronpoor(electrophilic)dienophile.Normalelectron-demandDiels–Alderreactions用前線軌道理論解釋反響活性炔烴的D-A反響活性低(Aldrin)VeryelectronpoordienescanundergoDiels–Alderreactionswithelectronrichdienophilesintheinverseelectron-demand

Diels–Alderreaction.ThedominantinteractionintheTSofinverseelectron-demandDiels–AlderreactionsisbetweentheLUMOdieneandtheHOMOdienophile.Inverseelectron-demandDiels–Alderreaction4.3.1.2OtherCycloadditions〔1〕[3+2]Cycloadditions(參加Me2S使過氧化物復原為醛)Paterno–Büchireaction〔2〕[2+2]Cycloadditions〔胸腺嘧啶〕〔導致皮膚癌〕Wittigreaction〔3〕[4+1]Retro-cycloadditions4.3.2Regioselectivity定位規(guī)那么：Diels–Alderreactionsproceedtoputthemostelectrondonatingsubstituentonthedieneandthemostelectronwithdrawingsubstituentonthedienophileeither“ortho〞or“para〞tooneanother.(1)Normalelectron-demandDiels–Alderreactions(2)Reverseelectron-demandDiels–Alderreactions[4+2]環(huán)加成反響的立體定向性[3+2]