yq有机Chapt-13-1共轭体系.ppt

Chapt 13 Conjugated Unsaturated systems Ø共轭体系共轭体系Ø烯丙基自由基反应烯丙基自由基反应Ø二烯和多烯二烯和多烯Ø共轭体系的加成反应共轭体系的加成反应 ——1,4加成；加成；1,2加成加成ØDiels-Alder反应反应Introduction lConjugated unsaturated systems: systems that have a p orbital on an atom adjacent to a double bond ––– molecules with delocalized p p bonds Øallyl radical (CH2=CHCH2•)have a single electron Øallyl cation (CH2=CHCH2+)  be vacant Øallyl anion (CH2=CHCH2:–)  have a pair of electrons Ø1,3-butadiene (CH2=CH–CH=CH2)  another double bond lConjugation gives these systems special properties: lp- - p p 共轭体系共轭体系ls s- - p p 超共轭体系超共轭体系a a C－－H上的上的σ键键越多，超共越多，超共轭轭效效应应越越强强，，烯烃烯烃越越稳稳定。

定由由 σ电子和电子和 π电子的离域而产生的共轭电子的离域而产生的共轭效应效应.符合马氏规则符合马氏规则Allylic Substitution and the allyl Radical lAllylic substitution: any reaction in which an allylic hydrogen atom is replaced. lAllylic Chlorination (High Temperature)  Mechanism allylic > 3° > 2° > 1° > vinyl or vinylicAllylic Bromination with N-Bromosuccinimide lPropene undergoes allylic bromination when treated with N-bromosuccinimide (NBS) in CCl4 in the presence of peroxides or light. NBS is nearly insoluble in CCl4 and provides a constant but very low concentration of bromine in the reaction mixture. lThe allyl cation (CH2=CHCH2+) is an unusually stable carbocation. The Allyl Cation3°Allyl2°1°Vinyl Substituted allylic反马加成反马加成烯丙式卤代烃（烯丙式卤代烃（CH2=CHCH2Cl）） 烯丙基氯分子中的烯丙基氯分子中的Cl原子与一般卤代烷相比很活泼原子与一般卤代烷相比很活泼(活性与活性与叔卤代烷相当甚至更大叔卤代烷相当甚至更大)，很容易发生亲核取代反应。

很容易发生亲核取代反应SN1、SN2均易进行（一般条件SN1为主）AgNO3/EtOHAgCl快，与叔RX相当甚至更快Mg，乙醚CH2=CHCH2MgCl易CH2=CHCH2ClHBrBrCH2CH2CH2Cl反马氏规则反马氏规则很活很活泼泼 氯原子上的p轨道与双键π轨道相互交盖，形成p-π共轭体系氯原子上的一对未共用电子和π键上的两个π电子发生离域乙烯式卤代烯烃的结构p-π共轭体系比较：比较：CH2=CHCl： C sp2 —— Cl ； CH3CH2Cl： C sp3 — Cl  与与C sp2相连的卤原子相连的卤原子很不活泼，很不活泼，一般不发生亲核取代反应一般不发生亲核取代反应比较：乙烯式卤代烃比较：乙烯式卤代烃(CH2=CHCl)的化学反应的化学反应亲核取代亲核取代格氏试剂格氏试剂Mg，乙醚Mg，CH2=CHMgCl比比RX难难AgNO3/EtOH无AgClHBr亲电加成亲电加成与其它与其它RX相区别相区别比比CH2=CH2难难CH2=CHCl-HClCH≡CH比比RX难难偶极偶极矩矩μμ比比氯氯乙烷小、乙烷小、C — Cl缩短、缩短、C = C键伸长符合马氏规则符合马氏规则很不很不活泼活泼乙烯式卤代烯烃乙烯式卤代烯烃的亲电加成反应的亲电加成反应符合马氏规则符合马氏规则氯原子上的未共用氯原子上的未共用的的p电子离域使碳电子离域使碳正离子正离子较较稳定稳定　　较较不稳定不稳定吸电子基吸电子基Alkadienes and Polyunsaturated Hydrocarbons lAlkadiene and alkatriene  diene and triene;lAlkadiyne and alkenyne  diyne and enyne. 1-Penten-4-yne1,2-Propadiene (allene)(2Z,4E)-2,4-Hexadiene (cis,trans-2,4-hexadiene) lThe multiple bonds of polyunsaturated compounds are classified as being cumulated, conjugated, or isolated. lCumulenes(累积二烯累积二烯)lConjugated diene l isolated diene less stable1,3-Butadiene: Electron Delocalization lBond Length of 1,3-Butadiene 1.34 Å 1.47 Å1.34 ÅC—C1.54 ÅlConformations of 1,3-Butadiene The s-trans conformation of 1,3-butadiene is the predominant one at room temperature. Electrophilic Attack on Conjugated Dienesl1,3-butadiene reacts with one molar equivalent of HCl 1,2-Addition 1,4-Addition lThe mechanism for the addition of HCl: Step 1 l1,3-Butadiene shows 1,4-addition reactions with electrophilic reagents other than HCl. lReactions of this type are quite general with other conjugated dienes.ØConjugated trienes often show 1,6-addition. Step 2 Kinetic Control Versus Thermodynamic ControllThe addition of HBr to 1,3-butadiene When the mixture formed at the lower temperature is brought to higher temperature, the relative amounts of the two products change. 1,4-adduct must be more stable. 速度控制反应速度控制反应（动力学）（动力学）平衡控制反应平衡控制反应（热力学）（热力学）下列化合物与下列化合物与HBrHBr进行亲电加成反应，请指出它们的进行亲电加成反应，请指出它们的相对活性。

相对活性5））>（（4））>（（3））> （（2 ）） > （（1））The Diels-Alder reactionl1, 4-Cycloaddition reaction of dienesl2 new σbonds are formed at the expense of 2 π bondsFactors favoring the Diels-Alder ReactionlDiene: electron-releasing groupslDienophole:electron-withdrawing groupsStereochemistry of the Diels-Alder ReactionlDiels-Alder Reaction is highly stereospecific: Syn Addition and the configuration of the dienophile is retained in the product.The diene reacts in the s-cis conformation rather than s-trans.lDiels-Alder Reaction occurs primarily in an endo (内型内型) rather than an exo (外型外型)fashion when the reaction is kinetically controlled.lAsymmetric Diel-Alder ReactionUsing chiral Lewis acid catalystslIntramolecular Diel-Alder ReactionP616 l13.16 (a)(d)(g)l13.17l13.18 (e)(f)(g)l13.19 (a) (c)(e)l13.24(b)(e)l13.34。