Retro-Diels−Alder Reaction in Aqueous Solution:  Toward a Better Understanding of Organic Reactivity in Water

The retro-Diels-Alder (RDA) reaction of anthracenedione 1a proceeds considerably faster in aqueous solutions than in organic solvents. Addition of organic solvents to water retards the reaction, whereas glucose induces a modest acceleration. SDS micelles induce a considerable retardation, but even at high concentrations of surfactant (complete micelle-substrate binding), the cycloreversion is not fully inhibited. Correlation with data for solvatochromic indicators strongly suggest that the origin of the water-induced acceleration involves primarily enhanced hydrogen bonding of water to the activated complex for the RDA reaction of 1a. Activation parameters support this view. A comparison of the present results with previous kinetic data for bimolecular and intramolecular Diels-Alder reactions provides insights into the contributions of hydrogen-bond and hydrophobic interactions to the aqueous accelerations of the latter two types of reactions.