Abstract
Second-order rate constants and isobaric activation parameters for the intermolecular Diels-Alder reactions of cyclopentadiene with alkyl vinyl ketones (2a,b) and 5-substituted-1,4-naphthoquinones (3a-c) as well as for the intramolecular Diels-Alder reaction of N-furfuryl-N-methylmaleamic acid (4) were determined in water, monohydric alcohols, and highly aqueous solutions containing monohydric alcohols and other organic cosolvents. The intermolecular as well as the intramolecular Diels-Alder reaction are characterized by large rate accelerations upon going from an organic solvent to water as the reaction medium. A model for solvation effects on rate constants for Diels-Alder reactions in aqueous solutions is developed. The typical, huge rate enhancements for Diels-Alder reactions in water and in highly aqueous binary mixtures are significantly sensitive to substituent effects and are discussed in terms of enforced pairwise hydrophobic interactions between diene and dienophile.
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