26-3 Stereochemistry of Thermal Electrocyclic Reactions According to frontier orbital theory,the stereochemistry of an electrocyclic reaction is determined by the symmetry of the polyene HOMO Electrons in the HOMO are the highest-energy and are the most reactive For thermal reactions,the ground-state electron configuration is used to identify the HOMO For photochemical reactions,the excited-state electron configuration is used to determine the HOMO
According to frontier orbital theory, the stereochemistry of an electrocyclic reaction is determined by the symmetry of the polyene HOMO • Electrons in the HOMO are the highest-energy and are the most reactive • For thermal reactions, the ground-state electron configuration is used to identify the HOMO • For photochemical reactions, the excited-state electron configuration is used to determine the HOMO 26-3 Stereochemistry of Thermal Electrocyclic Reactions
Ring Closure of Conjugated Trienes Lobes of like sign on the same side of the molecule yields disrotatory ring closure Heat (Disrotatory) CH CH3 CH3 (2E,4Z,6E)-Octa-2,4,6-triene cis-5,6-Dimethylcyclohexa-1,3-diene Heat (Disrotatory) (2E,4Z,6Z)-Octa-2,4,6-triene trans-5,6-Dimethylcyclohexa-1,3-diene
Lobes of like sign on the same side of the molecule yields disrotatory ring closure Ring Closure of Conjugated Trienes
Electrocyclic Opening of a Diene For conjugated dienes the equilibrium favors the ring- opened isomer. 3,4-dimethylcyclobutene rings open in a conrotatory sense Heat (Conrotatory) CH3 CH3 CH3 cis-3,4-Dimethylcyclobutene (2E,4Z)-Hexa-2,4-diene Heat (Conrotatory) H3C H3 trans-3,4-Dimethylcyclobutene (2E,4E)-Hexa-2,4-diene
For conjugated dienes the equilibrium favors the ringopened isomer. 3,4-dimethylcyclobutene rings open in a conrotatory sense Electrocyclic Opening of a Diene
Contrast:Reactions of Dienes and Trienes Conjugated dienes and conjugated trienes react with opposite stereochemistry 。 Polyenes with even number of electron pairs undergo thermal electrocyclic reactions in a conrotatory sense 。 Polyenes with odd number of electron pairs undergo thermal electrocyclic reaction in a disrotatory sense Different symmetries of the diene and triene HOMOs Dienes open and close by a conrotatory path Trienes open and close by a disrotatory path Opposite signs Same sign Diene HOMO Triene HOMO Cengage Leaming:All Rights Peserved
Conjugated dienes and conjugated trienes react with opposite stereochemistry • Polyenes with even number of electron pairs undergo thermal electrocyclic reactions in a conrotatory sense • Polyenes with odd number of electron pairs undergo thermal electrocyclic reaction in a disrotatory sense • Different symmetries of the diene and triene HOMOs • Dienes open and close by a conrotatory path • Trienes open and close by a disrotatory path Contrast: Reactions of Dienes and Trienes