AOR No10 (2016.7.15) Thermal Pericyclic Reaction Pericyclic Reactions
6.1 The Four Classes of Pericyclic Reactions
1. Cycloadditions (bimolecular) Cheletropic reaction
Otto Paul Hermann Diels Kurt Alder (1876 – 1954) (1902 – 1958) “protection of diene”
2. Electrocyclic reactions (Unimolecular) [1,7]-shift
-50 kJ/mol Mislow rearrangement 3. Sigmatropic rearrangement (Unimolecular) [2,3]-shift Claisen rearrangement [3,3]-shift 4. Group transfer reaction (Bimolecular) 6.2 Evidence for the Concertedness of Bond Making and Breaking
“ene reaction” No intermediates
Diels-Alder reactions: exothermic high negative entropy of activation, -150 to -200 J K-1 mol-1, with a low enthalpy of activation
Sauer, Angew. Chem., Int. Ed. Engl. 1980, 19, 779 Metalla-ene reaction Little effect by solvent polarity Stereochemistry Isotope effect Substituent effect
M = Li, Mg, Pd
[4 + 2] 6.3 Symmetry-allowed and Symmetry-forbidden Reaction
6.3.1 The Woodward-Hoffmann Rules Class by Class
6.3.1.1. Cycloadditions
[2 + 2]
Diels-Alder reaction: [4 + 2] cycloaddition “number of electrons” 1,3-Dipolar cycloaddition Rule of cycloadditions X,Y,Z,A,B = (C, N, O, S)
A thermal pericyclic cycloaddition is allowed if the total number of electrons involved can be expressed in the form (4n+2), where n is an integer. If the total number of electrons can be expressed in 10-electrons [8 + 2] the form (4n) it is forbidden.
(4n + 2) electrons
[6 + 4]
Suprafacial-Suprafacial Suprafacial-Antarafacial 4n (allowed) (4n + 2) Antarafacial-Antarafacial
[14 + 2]
trans