高等有机化学——反应机理-6.ppt

第四课 周环反应 1、基础知识 A pericyclic reaction is a reaction in which bonds are formed or broken at the termini of one or more conjugated systems. The electrons move around in a circle, all bonds are made and broken simultaneously, and no intermediates intervene. 4.1.1 Classes of Pericyclic Reactions Electrocyclic reactions (ring openings or ring closings), Cycloadditions Sigmatropic rearrangements Ene reactions. Electrocyclic reactions Cycloadditions Cheletropic reactions Sigmatropic rearrangements Sigmatropic rearrangements Sigmatropic rearrangements Ene Reaction 四种周环反应总结 立体专一性 Polyene MOs (加入Me2S 使过氧化物还原为醛) Paterno–Büchi reaction （2）[2+2] Cycloadditions （胸腺嘧啶） （导致皮肤癌） Wittig reaction （3）[4+1] Retro-cycloadditions 4.3.2 Regioselectivity 定位规则：Diels–Alder reactions proceed to put the most electron donating substituent on the diene and the most electron withdrawing substituent on the dienophile either “ortho” or “para” to one another. (1) Normal electron-demand Diels–Alder reactions (2) Reverse electron-demand Diels–Alder reactions [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 4.3.4 Stereoselectivity 练习 4.2.2 Stereospecificity 1,3,5-hexatrienes 偶——热——顺 奇——热——反 偶——光——反 奇——光——顺 The Woodward–Hoffmann rules apply only to concerted, pericyclic reactions. A reaction can be forced to proceed through the higher energy TS if the lower energy one is raised prohibitively high in energy by geometric constraints. 4.2.3 Stereoselectivity 4.3 Cycloadditions 4.3.1 Typical Reactions 4.3.1.1 The Diels–Alder Reaction Most Diels–Alder reactions occur with what is called normal electron-demand, in which an electron-rich (nucleophilic) diene reacts with an electronpoor (electrophilic) dienophile. Normal electron-demand Diels–Alder reactions 用前线轨道理论解释反应活性 炔烃的D-A反应活性低 (Aldrin) Very electron poor dienes can undergo Diels–Alder reactions with electronrich dienophiles in the inverse electron-demand Diels–Alder reaction. The dominant interaction in the TS of inverse electron-demand