Diels‐Alder Reaction with Hydrophilic Dienes and Dienophiles

Abstract 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.