D. A. Evans. B. Breit. B. Dunn Electrocyclic Reactions Chem 206 ELECTROCYCLIC RING CLOSURE/RING OPENING Butadiene to cyclobutene: A 4-electron(4q) system The stereochemical issues: Ring closure can occur in two distinct ways. This has consequences with The orbital lobes that interact heat a The disposition of substituents on the termini Conrotatory Closure: The termini rotate in the same direction Me\Me onorio Hextriene to cyclohexadiene: A 6-electron(4q+2 system AB ATB 83-8 derotation A Disrotatory Closure: The termini rotate in opposite directions derotation It was also noted that changing the "reagent"from heat to light reverse Empirical Observations this reactivity patten. Under photochemical conditions 4 electron systems undergo disrotatory motion, while 6 electron systems undergo conrotatory motion It was noted that butadienes undergo conrotatory closure under thermal conditions, while hexatrienes undergo disrotatory closure under thermal conditions. The microscopic reverse reactions also occur with the same rotational sense (i.e. cyclobutenes open in a conrotatory sense when heated, and cyclohexadienes open in a disrotatory sense when heated hB A A B A B B A B A A B A B A B A B A B A B A B Me Me Me H H Me Me Me H Me H Me Me Me Me Me D D Me Me Me Me H H Me Me Me H Me H Me Me hn hn Me Me D. A. Evans, B. Breit, T. B. Dunn Electrocyclic Reactions Chem 206 Conrotatory Closure: The termini rotate in the same direction The Stereochemical issues: ELECTROCYCLIC RING CLOSURE/RING OPENING: Ring closure can occur in two distinct ways. This has consequences with regard to: ■ The orbital lobes that interact ■ The disposition of substituents on the termini Disrotatory Closure: The termini rotate in opposite directions Empirical Observations: conrotation conrotation disrotation disrotation Butadiene to cyclobutene: A 4-electron (4q) system Hextriene to cyclohexadiene: A 6-electron (4q+2) system It was also noted that changing the "reagent" from heat to light reversed this reactivity pattern. Under photochemical conditions 4 electron systems undergo disrotatory motion, while 6 electron systems undergo conrotatory motion. disrotation controtation heat heat It was noted that butadienes undergo conrotatory closure under thermal conditions, while hexatrienes undergo disrotatory closure under thermal conditions. The microscopic reverse reactions also occur with the same rotational sense (i.e. cyclobutenes open in a conrotatory sense when heated, and cyclohexadienes open in a disrotatory sense when heated.)