Structural Effects Controlling the Rate of the Retro-Diels-Alder Reaction in Anthracene Cycloadducts

Article abstract of DOI:10.1021/jo00266a006

We have undertaken a fairly broad study of how the structure of an anthracene cycloadduct affects the rate of its cycloreversion reaction.Based on the rate constants for retro-Diels-Alder (rDA) reactions of a variety of anthracene-type adducts conducted in diphenyl ether, we draw the following conclusions.The rDA reaction of anthracene cycloadducts is influenced by diene substituents in the following ways: (1) electron-donating groups increase the reaction rate, and the accelerating effect is subject to geometric modulation for a conjugating substituent like dimethylamino; (2) electron-withdrawing groups may decrease or increase the reaction rate , although the effect is rarely large; and (3) steric acceleration is relatively small and demonstrates an unprecedented bell-shaped structure-reactivity profile.Peripheral substitution of the adduct with siloxy groups results in a significant acceleration, even though the groups are three bonds removed from the reaction site.The same reaction is influenced by dienophile substituents in the following ways: (1) electron-withdrawing groups increase the rate of the reaction; (2) strongly conjugating substituents make the reaction much faster than predicted by classical electron-withdrawing or -donating ability due to a change to polar mechanism; and (3) there is no observable steric effect.