第4章 消除反应

第4章消除反应Chapter4EliminationReactions4.1E1消除4.2E2消除&4.3E1CB消除4.4Ei消除4.5E1和E2之竞争4.6消除和取代之竞争a-消除

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b-消除a-消除:b-消除:ab消除和取代总是在竞争？消除反应的区域选择性——Zaitsev消除和Hofmann消除SaytzeffvsHofmanneliminationSaytzeff烯烃Hofmann烯烃碱体积越大，越有利于Hofmann烯烃的生成！EffectoftheleavinggroupEspeciallyquarternaryammoniumleavinggroupsfavortheHofmannproductThethreeeliminationsE1cB:theprotonisremovedfirst,ananionisformedE1:theleavinggroupdepartsfirst,acationisformedE2:allprocessesoccuratthesametimeE1

机理：决速步4.1E1消除反应动力学：V=k[RX]E1反应与SN1反应竞争——碱性增强，消除产物增多！区域选择性：Zaitsev规则为何生成Zaitsev烯烃？E2机理：4.2E2消除反应动力学：V=k[R-X][B]4.2.1Zaitsev消除KineticIsotopeEffectSubstitutedeuteriumforhydrogenatpositionEffectonrateiskineticisotopeeffect(kH/kD=deuteriumisotopeeffect)RateisreducedinE2reactionHeavierisotopebondisslowertobreakShowsC-Hbondisbrokeninrate-limitingstep底物结构对反应速度的影响：Relativereactivity:R3CX>R2CHX>RCH2Xtrans:cis=6:1——Antiperiplanarallowsorbitaloverlapandminimizesstericinteractions立体化学：反式共平面EliminationFromCyclohexanes4.2.2Hofmann消除：——季铵碱热解成烯烃、叔胺和水的反应区域选择性：Hofmann规则对Hofmann消除的解释：例外：274.3E1cB消除28E1cBReactionWithanacidicproton;E1cBBiologicalElimination:Withapoorleavinggroup;Moreexamples:4.4Ei消除（Intramolecularsynelimination）（1）Cope消除：立体化学：顺式共平面消除逆Cope消除：（2）酯的裂解：（3）b-乙酰氧基酯在碱性条件下消除：J.Org.Chem.2007,72,793-798（4）亚砜和硒氧化物的消除（6）邻二卤代烃在DMSO作用下消除：J.Org.Chem.2007,72,6016-60214.5E1和E2的竞争StrongbaseisneededforE2butnotforE1E2isstereospecifc,E1isnotE1givesZaitsevorientation底物结构的影响4.6消除与取代之竞争StericbulkfavorseliminationWealreadysawthatSN2substitutiononatertiarycarbonisnotpossible,thereforeE2eliminationwillprevail(besideE1elimination)碱性与亲核性的影响ThereisastrongsimilaritybetweentheE2andSN2reactions:strongnucle