D. A. Evans Diels-Alder Reaction: The Transition Structure Chem 206 Transition State Modelling is Coming of Age a Lewis Acid Catalysis of the reaction is possible: Yates Eaton JAcs1960,82,4436 LUMO, LUMOz The Critical Energy a The lengths of the forming C-c bonds are Ca. 1.5 times the normal bond E(LUMO1)-E(HOMO2 distance. This factor comes out of the ab initio work of Jorgensen& Houk + leading references Jorgensen, JACS 1993, 115, 2936-2942 Houk, Jorgensen, JACS 1989, 111, 9172 4#a E(LUMO2 )-E(HOMO1 Transition Structures of Hydrocarbon Pericyclic Reactions Houk Angew. chem. Int. Ed. 1992, 31, 682-708 a Bond formation is not synchronus with substituted dienophiles (Jorgensen) i AS AE decreases for the relevant ground state FMOS, ran rates increase 2193A 2193A 2.091A 2325A UMO LUMO △s+=-321eu s+=383 rel rate 1 rel rate= 10*5 44 a Diene Reactivity as measured against Maleic anhydride HOMO AE(LUMO3-HOMO1)AE(LUMO2-HOMO1)= Rate Acceleration log k=4.96 log k= 2.36 log k=2.19 log k=2.12 log k=1.83 Lewis acid catalysis not only dramatically increases rates by ca 10* Sauer,Angew.Chem. Int Ed,1980,19,779-807/it also improves reaction regiochemistry endo diastereoselectivityMe Me HOMO3 HOMO2 HOMO1 O H H O HOMO2 HOMO1 Lewis acid catalysis not only dramatically increases rates by ca 10+6 it also improves reaction regiochemistry & endo diastereoselectivity Ethylene & Butadiene Vs Butadiene & Acrolein DE (LUMO3 -HOMO1 ) < DE (LUMO2 -HOMO1 ) Rate Acceleration + LUMO1 + E LUMO3 LUMO2 Dienophile E(LUMO1 ) - E(HOMO2 ) or E(LUMO2) - E(HOMO1) ■ The closer the two orbitals are in energy, the better they interact ■ As DE decreases for the relevant ground state FMOs, rxn rates increase LUMO1 LUMO2 Diene The Critical Energy Difference: energy D. A. Evans Diels-Alder Reaction: The Transition Structure Chem 206 rel rate = 10+5 rel rate = 1 exptl DH ‡ = 14.0 kcal/mol DS ‡ DS = -38.3 eu ‡ = -32.1 eu DH ‡ = 22.5 kcal/mol ■ Bond formation is not synchronus with substituted dienophiles (Jorgensen) 2.091 Å 2.325 Å 2.193 Å 2.193 Å Houk, Jorgensen, JACS 1989, 111, 9172 Jorgensen, JACS 1993, 115, 2936-2942 leading references: ■ The lengths of the forming C–C bonds are Ca. 1.5 times the normal bond distance. This factor comes out of the ab initio work of Jorgensen & Houk Transition State Modelling is Coming of Age Transition Structures of Hydrocarbon Pericyclic Reactions Houk Angew. chem. Int. Ed. 1992, 31, 682-708 ‡ + ■ Diene Reactivity as measured against Maleic anhydride Sauer, Angew. Chem. Int. Ed., 1980, 19, 779-807 log k = 4.96 log k = 2.36 log k = 2.19 log k = 2.12 log k = 1.83 ■ Lewis Acid Catalysis of the reaction is possible: Yates & Eaton, JACS 1960, 82, 4436