Improved conversion and selectivity of a Diels-Alder cycloaddition by use of emulsions of carbon dioxide and water

Water and liquid carbon dioxide are mixed by power ultrasound to promote the Diels-Alder cycloaddition between cyclopentadiene and methyl vinyl ketone. Performing the reaction in emulsions of water and carbon dioxide leads to the high endo/exo selectivity and conversion that are characteristic of water but specific production rates that are greater than when either solvent is used alone. The maximum selectivity measured in this study was 16: 1, which compares favorably to that obtained for the same reaction in pure water (2 1: 1) or for a similar Diels-Alder reaction (cyclopentadiene + butyl acrylate) in supercritical carbon dioxide in the presence of Sc(OTf)(3) (24:1). To interpret our results, we present a simple mathematical model in which the carbon dioxide phase acts as a carrier for water-insoluble reactants and the reaction occurs primarily in the water phase. The standoff distance between the sonic horn and the carbon dioxide/water interface is a key variable and has an optimum value of about 2.0 cm. Our approach shows potential as an environmentally benign and economically attractive means of exploiting the effects of water on chemical reactivity.