高等有机化学第四章亲电加成反应.ppt

第四章第四章 亲电加成反应亲电加成反应 1 （1 1）亲电加成）亲电加成 ( (electrophilicelectrophilic addition) addition) （2 2）亲核加成）亲核加成 ( (nucleophilicnucleophilic addition) addition) （3 3）自由基加成）自由基加成 (radical addition)(radical addition) （4 4）环加成）环加成 ( (cycloadditioncycloaddition) ) 碳碳重键碳碳重键 (multiple bonds) (multiple bonds) 的加成反应的加成反应 2 一、碳一、碳- -碳双键的亲电加成反应碳双键的亲电加成反应 n n 烯烃活泼的原因：烯烃活泼的原因： 电子受核束缚力小，电子受核束缚力小， 容易极化。容易极化。 3 1. 烯烃的亲电加成反应历程 4 Mechanism （1）involves prior dissociation of the electrophile and implies that a carbocation is generated that is free of the counterion Y- at its formation. （1 1） Prior dissociation of Prior dissociation of electrophileelectrophile and and formation of formation of carbocationcarbocation intermediate intermediate 5 （2 2） Formation of Formation of carbocationcarbocation ion pair from ion pair from alkenealkene and and electrophileelectrophile. . 6 Mechanism（2）also involves a carbocation intermediate, but it is generated in the presence of an anion and exists initially as an ion pair. Depending on the mutual reactivity of the two ions, they might or might not become free of one another before combining to give product. 7 （3 3） Formation a bridged carbonic Formation a bridged carbonic intermediate from intermediate from alkenealkene and and electrophileelectrophile. . 8 Mechanism （3） leads to a bridged intermediate that undergoes addition by a second step in which the ring is opened by a nucleophile. Mechanism （3）implies stereospecific anti addition. Mechanisms （1）, （2）and （3） are all AdE2 reactions: they are bimolecular electrophilic additions. 9 （4 4） Concerted addition of Concerted addition of electrophileelectrophile and and nucleophilenucleophile in a in a termoleculartermolecular reaction reaction 10 Mechanism （4）is a process that has been observed for several electrophilic additions and implies concerted transfer of the electrophilic and nucleophilic components of the reagent from two separate molecules. It is a termolecular electrophilic addition, AdE3, a mechanism that implies formation of a complex between one molecule of the reagent and the reactant and also is expected to result in anti addition. 11 碳正离子历程碳正离子历程 v = k isopropeneHCl （2）双分子历程 12 生成结构稳定的碳正离子中间体生成结构稳定的碳正离子中间体 n碳正离子中间体的AdE2历程的条件 叔碳正离子中间体、苄基型碳正离子叔碳正离子中间体、苄基型碳正离子 13 通常不具有立体选择性通常不具有立体选择性 14 有时有重排产物出现有时有重排产物出现 重排产物的出现可作为经碳正离子重排产物的出现可作为经碳正离子 中间体历程的证据之一中间体历程的证据之一 15 鎓型离子历程鎓型离子历程 立体化学为立体化学为反式加成反式加成 16 “ “Bromine with two bonds and a positive Bromine with two bonds and a positive charge? In a three-charge? In a three-memberedmembered ring? In 1937, ring? In 1937, two Columbia chemists proposed just such a two Columbia chemists proposed just such a species as an intermediate in the reaction of species as an intermediate in the reaction of ethylene with bromine. They were right.”ethylene with bromine. They were right.” 17 n n 溴鎓离子具有八隅体结构，碳正离子为溴鎓离子具有八隅体结构，碳正离子为 六电子体，前者比后者稳定。六电子体，前者比后者稳定。 Br2, I2, IN3, RSCl or ArSCl, Hg(OCOCH3)2 亲电试剂的原子半径要足够大亲电试剂的原子半径要足够大 思考：思考：为什么形成溴鎓离子而不是碳正离子？为什么形成溴鎓离子而不是碳正离子？ 哪些哪些亲电试剂与烯烃可形成鎓型离子中间体？亲电试剂与烯烃可形成鎓型离子中间体？ 18 鎓离子存在的直接证据？鎓离子存在的直接证据？ BiadamantylideneBiadamantylidene bronomiumbronomium 19 （3 3）三分子历程（）三分子历程（AdAd E E 3 3） n 某些非共轭烯烃与HX加成按AdE3历程进行。 n 立体化学通常为反式加成 20 complex 21 22 2. 2. 烯烃亲电加成反应的立体化学烯烃亲电加成反应的立体化学 （1 1）反式加成（）反式加成（AntiadditionAntiaddition） 23 n AdE2：涉及环状鎓离子的形成 反 式 加 成 24 反式加成反式加成 25 反式加成反式加成 26 n AdE3：一分子烯烃和一分子烯烃与HX形 成的络合物反应。 反式加成反式加成 27 （2 2）顺式加成（）顺式加成（synadditionsynaddition） 碳碳双键与苯基共轭的烯烃碳碳双键与苯基共轭的烯烃和和HXHX的加成反的加成反 应主要给出顺式加成产物应主要给出顺式加成产物 28 Ion pair Owing to the greater stability of the Owing to the greater stability of the benzylicbenzylic carbocationscarbocations formed in these reactions, formed in these reactions, concerted attack by halide ion is not required concerted attack by halide ion is not required for for protonationprotonation. . 29 对比立体选择性对比立体选择性 30 顺式加成顺式加成 p 硼烷与烯烃的加成为硼烷与烯烃的加成为顺式加成顺式加成 31 （3）无立体选择性的加成 32 实例1： l l 解释双键碳原子上解释双键碳原子上连有苯基的烯烃与溴连有苯基的烯烃与溴加加 成时，顺式加成产物增多。如：溴与反成时，顺式加成产物增多。如：溴与反-1-1-苯苯 基丙烯反应所得反、顺式加成产物之比为基丙烯反应所得反、顺式加成产物之比为 88:1288:12；当双键碳原子所连的苯基上有给电子；当双键碳原子所连的苯基上有给电子 基团时，顺式加成产物的比例也明显升高。基团时，顺式加成产物的比例也明显升高。 33 反式反式加成加成 产物产物 （88%88%） 顺式顺式加成加成 产物产物 （12%12%） 34 反式反式加成加成 产物产物 （63%63%） 顺式顺式加成加成 产物产物 （37%37%） 35 l l 某些芳烃与卤素加成，主要生成顺式加成某些芳烃与卤素加成，主要生成顺式加成 产物。产物。 （35%35%） （10%10%） 实例2： 36 提示： 37 邻基参与 （注意） 当双键邻位有带未共用电子对的基团存当双键邻位有带未共用电子对的基团存 在时，该基团会对碳正离子中间体进行亲在时，该基团会对碳正离子中间体进行亲 核进攻。核进攻。 38 例如： 39 对比： 40 41 某些某些五、六元环状内酯或环醚五、六元环状内酯或环醚的形成也和的形成也和 邻基参与有关。邻基参与有关。 42 43 3. 3. 烯烃烯烃亲电加成反应的活性亲电加成反应的活性 （1 1）烯烃结构对加成速率的影响）烯烃结构对加成速率的影响 44 结论：结论：Z Z为推电子基，使双键电子云密度增为推电子基，使双键电子云密度增 加；稳定碳正离子中间体，降低活化能加；稳定碳正离子中间体，降低活化能 45 结论：结论：Z Z为吸电子基，使双键电子云密度为吸电子基，使双键电子云密度 降低；使碳正离子中间体更不稳定，增加降低；使碳正离子中间体更不稳定，增加 活化能活化能 46 Z Z为强吸电子基时为强吸电子基时，烯烃不发生亲电加成，烯烃不发生亲电加成 反应，反应，发生亲核加成反应！发生亲核加成反应！ 47 48 （2 2）亲电试剂）亲电试剂对加成速率的影响对加成速率的影响 对于特定烯烃，卤化氢的加成速率与 酸性强弱一致。 HI HBr HCl HF 对于特定烯烃，混合卤素的加成速率 与其异裂难易程度相符 ICl IBr I2 49 4. 烯烃亲电加成反应的定向规律 n 区域选择性影响因素：电子效应，空间 效应。 50 （1 1）电子效应）电子效应 51 n“Markovnikov 规则”？ n“围绕碳正离子的-CC单键旋转，当带正 电荷碳原子的p轨道轴和-CH键的轨道 轴在同一平面时，这两个轨道可发生部分 重叠，使部分正电荷分散到甲基上，起稳 定碳正离子的作用，这种现象称超共轭作 用”。 52 n 对含强吸电子基团的烯烃与不对称亲电试剂 的加成，从表面上看是反马氏规则的。 53 n 也适用于苯乙烯类与卤化氢等不对称试剂的 加成的区域选择性，且当苯环上连有强吸电 子基时，其区域选择性与吸电子基和双键碳 原子直接相连时相似。 54 （2 2）立体效应）立体效应 55 二、炔烃和丙二烯类的亲电加成反应二、炔烃和丙二烯类的亲电加成反应 1. 炔烃的亲电加成 56 pp Since alkynes have Since alkynes have type type orbitalsorbitals, it is not , it is not surprising that there is a good deal of surprising that there is a good deal of similarity similarity to the reactivity of alkenes.to the reactivity of alkenes. pp The fundamental questions about additions The fundamental questions about additions to alkynes include the following:to alkynes include the following: How reactiveHow reactive are alkynes in comparison are alkynes in comparison with alkenes?with alkenes? What is the stereochemistryWhat is the stereochemistry of additions to of additions to alkynes?alkynes? 57 What is the What is the regiochemistryregiochemistry of additions to of additions to alkynes?alkynes? The important role of bridged ions in The important role of bridged ions in addition reactions of alkenes raises the addition reactions of alkenes raises the question of whether similar species are question of whether similar species are involved with alkynes, where the ring involved with alkynes, where the ring includes a double bond and bridged includes a double bond and bridged intermediates and would be expected to be intermediates and would be expected to be substantially more strained.substantially more strained. 58 pp The The basic basic mechansimsmechansims that are considered that are considered to be involved in to be involved in electrophilicelectrophilic additions to additions to alkynes:alkynes: vinyl cation synsyn + anti + anti AdAd E E2 2 59 This mechanismThis mechanism involves a discrete vinyl involves a discrete vinyl cationcation. In general, this reaction will lead to a . In general, this reaction will lead to a mixture of the two mixture of the two stereoisomericstereoisomeric addition addition ducts. 60 Bridged intermediate Bridged intermediateAdE3 AdE2 61 Mechanisms B and CMechanisms B and C depict bridged depict bridged intermediates formed without or with intermediates formed without or with participation of a second participation of a second electrophilicelectrophilic molecule. They should lead to molecule. They should lead to anti anti addition. addition. 62 Mechanism DMechanism D is a is a termoleculartermolecular process process that would be expected to be a that would be expected to be a stereospecificstereospecific anti addition.anti addition. AdE3 63 In general, alkynes are somewhat less In general, alkynes are somewhat less reactive than alkenes toward many reactive than alkenes toward many electrophileselectrophiles. A major reason for this . A major reason for this difference in reactivity is the substantially difference in reactivity is the substantially higerhiger energy of the vinyl energy of the vinyl cationcation intermediate that is formed by an intermediate that is formed by an electrophilicelectrophilic attack on an attack on an alkynealkyne. . pp Reactivity Reactivity 64 For additions that proceed through bridged For additions that proceed through bridged intermediates, the alkynes are intermediates, the alkynes are also less also less reactive because of additional strain in the reactive because of additional strain in the ermediate. pp Reactivity Reactivity 65 pp Example 1. Example 1. HydrohalogenationHydrohalogenation Aryl-substituted acetylenes give mainly the syn addition product. vinyl cation (as ion pair) 66 Alkyl-substituted acetylenes can react with HCl by either the AdE3 or the AdE2 mechanism, depending on the reactant structure and the reaction conditions. The stereochemistry is anti addition. 67 antianti The presence of BrThe presence of Br - - greatly greatly accelerates the reactionaccelerates the reaction 68 pp Example 2. Example 2. HalogenationHalogenation of alkynes of alkynes DisubstitutedDisubstituted (internal) (internal) antianti 69 2. 2. 丙二烯类丙二烯类的亲电加成的亲电加成 （注意）（注意） allene allylic carbocation?vinyl carbocation 70 pp The kinetically favored The kinetically favored protonationprotonation at a at a spsp 2