Water in oil microemulsions as reaction media for a Diels-Alder reaction between N-ethylmaleimide and cyclopentadiene

The Diels-Alder reaction between N-ethylmaleimide and cyclopentadiene in water/AOT/isooctane microemulsions, where AOT denotes sodium bis(2-ethylhexyl)sulfosuccinate, was studied. The rate of the reaction was found to be higher than that obtained in pure isooctane, irrespective of the particular microemulsion composition used. The efficiency of this catalytic action ranged from a factor of 3 at low water contents (viz., W = H2O]/AOT] = 2) to 15 at W = 35. On the basis of these results, the reaction takes place simultaneously in the continuous medium and at the microemulsion interface. The favorable arrangement of the reactants at the interface results in more than 95% of the reaction occurring in this microenvironment. The kinetic analysis revealed the rate constant at the microemulsion interface to change with the water content. For small W values a bimolecular rate constant at the interface close to that observed in hexane was obtained. This value increases with W and for W > 20, a value close to that obtained in ethanol was found. This can be ascribed to the absence of hydrogen bonding at the microemulsion interface as well as the accelerating effects due to enforced hydrophobic interactions.