Solvophobic Acceleration of Diels-Alder Reactions in Supercritical Carbon Dioxide

摘要

The rate of the Diels-Alder reaction between N-ethylmaleimide and 9-hydroxymethylanthracene in supercritical carbon dioxide (scCO_2) was determined by following the disappearance of 9-hydroxymethylanthracene with in situ UV/vis absorption spectroscopy. The reaction conditions were 45-75 ℃ and 90-190 bar, which correspond to fluid densities (based on pure carbon dioxide) ranging between approximately 340 and 730 kg m~(-3). The measured reaction rate at low scCO2 fluid densities was nearly 25× faster than that reported in acetonitrile at the same temperature (45 ℃). An inverse relationship between reaction rate and fluid density/pressure was observed at all temperatures in scCO_2. The apparent activation volumes were large and positive (350 cm~3 mol~(-1)) and only a weak function of reduced temperature. A solvophobic mechanism analogous to those observed in conventional solvents is postulated to describe (a) the rate acceleration observed for this reaction in scCO_2 relative to that in acetonitrile, (b) the observed relationship between reaction rate and pressure/temperature/density, and (c) the large, positive activation volumes. Solubility measurements in scCO_2, rate measurements in conventional solvents, and an empirical correlation are used to support this theory. Our results advance the general understanding of reactivity in supercritical fluids and provide a rationale for selecting reactions which can be accelerated when conducted in scCO_2.