Nucleophilic Displacements in Supercritical Carbon Dioxide under Phase-Transfer Catalysis Conditions. 2. Effect of Pressure and Kinetics
The nucleophilic displacement on n-octylmesylate (n-C8H17OSO2CH3, 1) with four different anions (I-, Br-, N3-, and SCN-) is investigated under liquid-supercritical phase-transfer catalysis (LSc-PTC) conditions, i.e. in a biphase system of supercritical carbon dioxide (scCO2) and water, in the presence of both silica supported and conventional onium salts. The CO2 pressure greatly affects the concentration of 1 in the sc-phase and plays a major role on its conversion. For example, at 50 °C and with a supported PT-catalyst, the conversion of 1 into n-octyl iodide drops by a factor of 5 as the CO2 pressure is increased from 80 to 150 bar, while in the same pressure range, the solubility of n-octylmesylate in scCO2 shows a 6-fold increase, indicating that the reagent is desorbed from the catalyst. Under LSc-PTC conditions, pseudo-first-order kinetic rate constants, evaluated for the investigated reactions, show that the performance of scCO2 as a PTC solvent and the relative nucleophilicity order of the anions (N3- > I- ≥ Br- > SCN-) are comparable to those of toluene and n-heptane. The behavior of conventional phosphonium salts in the scCO2/H2O biphase system suggests that the reaction may take place either within small droplets of PT-catalyst containing water or in a separate third liquid phase of the PT-catalyst itself.
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