Diels-Alder Reaction with Hydrophilic Dienes and Dienophiles

Mechanistic aspects of the dramatic enhancement of rates of Diels-Alder reaction between some neutral substrates when carried out in water as compared to organic solvents are mainly attributed to two effects: hydrophobic effect and hydrogen bonding effect. Herein, a semi-quantitative rates approach using hydrophilic substrates in solvents of varying polarity towards the manifestation of the mechanism of water mediated Diels-Alder reaction is reported. These studies showed a relatively fast reaction in water as compared to water-organic and organic solvents therein cast doubt on the original theory of the hydrophobic effect and supports an effect similar to hydrogen bonding catalysis by water. Moreover, Diels-Alder reaction in the presence of a catalytic amount of water along with the organic solvent is more efficient as compared to the reaction in pure organic solvent and thereby further supports the hydrogen bonding effect.