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 Rule of cycloadditions 6.3.1.2 Electrocyclic Reactions 4n, 4n+2 Ring strain, aromaticity, stabilization of charge by substituent If the total number of electrons involved can be expressed in the form (4n+2), where n is an integer, a thermal pericyclic cycloaddition is symmetry allowed when both components react in a suprafacial manner (or both components react in an antarafacial manner). If the total number of electrons can be expressed in the form (4n), it is allowed if one of the components reacts in a suprafacial manner and the other antarafacial. Stereochemistry Woodward-Hoffman rule (1965) Disrotatory () and Conrotatory Thermal electrocyclic reactions involving a total number of electrons that can be expressed in the form (4n+2) are disrotatory, and thermal electrocyclic reactions in which the total number of electrons can be expressed in the form (4n) are conrotatory Robert Burns Woodward Roald Hoffmann (1917 – 1979) 1937- ) Examples 6.3.1.3 Sigmatropic Rearrangements [1,5] [1,n] H-shift (4n+2) only Sigmatropic rearrangements of hydrogen are suprafacial if the total number of electrons is a (4n+2) number and antarafacial if the total number of electrons is a (4n) number. 2e 4e H => C When the total number of electrons is a (4n+2) number, [1,n] sigmatropic rearrangements of elements other than hydrogen are either suprafacial with retention of configuration in the migrating Ref) group or are antarafacial with inversion of Wagner-Meerwein rearrangement configuration in the migrating group. When the total number of electrons is a (4n) number, [1,n] sigmatropic rearrangements of elements other than hydrogen are either antarafacial with retention of configuration in the migrating group or are suprafacial with inversion of configuration in the migrating group Ref) Ref) Beckmann rearrangement Baeyer-Villiger When the total number of electrons is a (4n+2) Suprafacial [1,3]-shift Suprafacial [1,4]-shift number, [m,n]-sigmatropic rearrangements are allowed if both migrations are suprafacial or both antarafacial. When the total number of electrons is a (4n) number, [m,n]-sigmatropic rearrangements are allowed if one migration is suprafacial and the other antarafacial. (4n+2) Ireland-Claisen rearrangement Cope rearrangement 10 electrons 6.3.1.4 Group Transfer Reaction Stevens rearrangement When the total number of electrons is a (4n+2) number, group transfer reactions are allowed with all-suprafacial stereochemistry. The Generalised Woodward-Hoffmann Rule 6.3.2.1 Cycloadditions (1) which of these components is acting in a suprafacial manner and which in an antarafacial manner? 1969 (1) in which of these components can the number of electrons be expressed in the form (4q+2) and in which in the form (4r), A ground-state pericyclic change is where q and r are integers? symmetry allowed when the total number of (4q+2)s and (4r)a components is odd. 6.3.2.2 Electrocyclic Reactions When both lines are drawn into the same lobe it is suprafacial, and when there is one line dawn into the top lobe and one into the bottom, it is antarafacial. Since this is neither a p nor a orbital, it is given the Greek letter w. 6.3.2.3 Sigmatropic Rearrangement 6.3.2.4 Group Transfer Reactions Report No8 (7/15) 1. Explain the following rearrangement..