高等有機化學(xué)-反應(yīng)機理.ppt

高等有機化學(xué)反應(yīng)機理,第四課 周環(huán)反應(yīng),1、基礎(chǔ)知識,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.,Typical reactions Regioselectivity Stereoselectivity Stereospecificity,4.1.1 Classes of Pericyclic Reactions,Electrocyclic reactions (ring openings or ring closings), Cycloadditions Sigmatropic rearrangements Ene reactions.,Electrocyclic reactions,Cycloadditions,Cheletropic reactions,Cycloadditions,Sigmatropic rearrangements,Sigmatropic rearrangements,Sigmatropic rearrangements,Ene Reaction,四種周環(huán)反應(yīng)總結(jié),立體專一性,Polyene MOs,WoodwardHoffmann rules,Polyene MOs,Polyene MOs,Polyene MOs,2、電環(huán)化反應(yīng),2.1、典型反應(yīng),2.1、典型反應(yīng),2.1、典型反應(yīng),2.1、典型反應(yīng),練習(xí)4.1,2.1、典型反應(yīng),2.1、Favorskii rearrangement,2.1、Favorskii rearrangement,練習(xí),練習(xí),4.2.2 Stereospecificity,1,3,5-hexatrienes,偶熱順 奇熱反 偶光反 奇光順,The WoodwardHoffmann 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 DielsAlder Reaction,Most DielsAlder 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 DielsAlder reactions,用前線軌道理論解釋反應(yīng)活性,炔烴的D-A反應(yīng)活性低,(Aldrin),Very electron poor dienes can undergo DielsAlder reactions with electronrich dienophiles in the inverse electron-demand DielsAlder reaction. The dominant interaction in the TS of inverse electron-demand DielsAlder reactions is between the LUMOdiene and the HOMOdienophile.,Inverse electron-demand DielsAlder reaction,4.3.1.2 Other Cycloadditions,（1）3+2 Cycloadditions,(加入Me2S 使過氧化物還原為醛),PaternoBchi reaction,（2）2+2 Cycloadditions,（胸腺嘧啶）,（導(dǎo)致皮膚癌）,Wittig reaction,（3）4+1 Retro-cycloadditions,4.3.2 Regioselectivity,定位規(guī)則：DielsAlder 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 DielsAlder reactions,(2) Reverse electron-demand DielsAlder reactions,4+2環(huán)加成反應(yīng)的立體定向性,3+2環(huán)加成反應(yīng)的立體定向性,2+2環(huán)加成反應(yīng),4.3.4 Stereoselectivity,4.4.1 Typical Reactions,4.4 Sigmatropic Rearrangements,練習(xí),練習(xí),4.5 Ene Reactions,