Anthracenedione retro-Diels—Alder

The retro-Diels-Alder (RDA) reaction of anthracenedione (Eq 12.5) proceeds considerably faster in aqueous solution than in organic solvents.32 The addition of organic solvents to water retards the reaction, whereas glucose induces a modest acceleration. The results suggest that the origin of rate acceleration involves mainly enhanced hydrogen bonding of water to the activation complex for the RDA reaction. 

Retro-Diels-Alder reactions of anthracenedione (51a) have been shown to proceed faster in aqueous solution than in organic solvents, apparently as a consequence of enhanced hydrogen bonding of water to the activated complex,30 since hydrophobic interactions with (51a) are of negligible importance. The results have been compared with previous kinetic data for bimolecular and intramolecular Diels-Alder reactions and the corresponding hydrogen bond and hydrophobic interactions have been discussed. 

In the retro-Diels-Alder reaction of anthracenedione [42], the volume of activation is small. Acceleration in water cannot come from a change in the hydration shell of the molecule. Hydrophobic interactions are negligible and aqueous acceleration is caused by the hydrogen-bond donating ability of water, which stabilizes the polarized activated complex. The Gibbs energy of activation displays a fair linear correlation with the Ej parameter. Hexafluoroisopropanol with an Ej value of 65.3 is even more efficient as a solvent than water Ej= 63.1) which appears to be less polar [41]. 

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