D. A. Evans Cycloaddition Reactions-1 Chem 206 Why does maleic anhydride react easily with 1, 3-butadiene, but not with ■ Consider2+2] cycloaddition: Photochemical activation[π2s+π2s] ethylene? So what are the rules"? dThe related reaction of 2 ethylenes is nonconcerted: [2+ 2] cycloaddition HOMO 区一叹 a We also know that the photochemical variant is concerted The frontier orbitals of the reacting species must have the proper symmetries [2+2] Cycloaddition-Examples Nomenclature uprafacial Dy Quadricyclane Dauben, Tet. 1961, 15, 197 Using this nomenclature the Diels-Alder reaction is a T4s+ I2S ■ Consider2+2 I cycloaddition: Thermal activation【π2s+π2S] Schafer, AC 1967. 79. 54 Dewarbenzene-Derivative Prismane-Der akan抛 bonding t2 bonding [π2S+π2a" allowed π2s+π2]" forbidden must be antarafical for indicated stereochem L1967.4357.4723X C C C C C C C C C C C C C C C C C C C C C C C C C C C C O O O O O O X Y Y X C C H Me Me Me Me Me Me Me Me Me Me Me Me HOMO X Y O O O D. A. Evans Cycloaddition Reactions-1 Chem 206 Why does maleic anhydride react easily with 1,3-butadiene, but not with ethylene? So what are the "rules"? [4+2] [2+2] p* p concerted ✻ bonding bonding + energy ✻ + light ✻ p p* new HOMO light ■ Consider [2 + 2] cycloaddition: Photochemical activation [ p2s + p2s] bonding ■ Consider [2 + 2] cycloaddition: Thermal activation [ p2s + p2s] The frontier orbitals of the reacting species must have the proper symmetries heat ■ The related reaction of 2 ethylenes is nonconcerted: [2 + 2] cycloaddition LUMO · · ■ We also know that the photochemical variant is concerted ■ Nomenclature p2s p2a Using this nomenclature, the Diels-Alder reaction is a p4s + p2s cycloaddition p2s p2s antibonding [ p2s + p2s] "forbidden" p2s p2a [ p2s + p2a] "allowed" bonding bonding suprafacial antarafacial [p 2 s + p 2 a] must be antarafical for indicated stereochem TL 1967, 4357, 4723. [2+2] Cycloaddition - Examples hn [p 2 s + p 2 s ] hn [p 2 s + p 2 s ] Quadricyclane Prismane-Der. Dauben, Tet. 1961, 15, 197. Schäfer, AC 1967, 79, 54. Dewarbenzene-Derivative