CO2 as a reagent and a solvent
Dense CO2 (in both its liquid and supercritical states) is the natural green candidate for the replacement of conventional liquid solvents. In this field, our group has focussed the attention on two innovative research areas:
a) The use of CO2 as an "organic" medium for reactions carried out under liquid-supercritical phase-transfer catalysis conditions (LSc-PTC). This work, which was begun in collaboration with the "NSF Center for Environmentally Responsible Solvents" at the University of North Carolina at Chapel Hill, is based on the use of both conventional onium salts and onium salts supported over inorganic matrixes (alumina and silica). Under LSc-PTC conditions, model bimolecular nucleophilic substitutions have been investigated and the effects of the CO2 pressure on the reaction kinetics and on the partitioning of reagents between the liquid and the supercritical phases, have been analysed.
b) The use of supercritical CO2 (scCO2) as a reagent/solvent for the synthesis of alkyl carbamates. The synthesis of alkyl carbamates, particularly of methyl carbamates, has been investigated through the reaction of aliphatic amines with scCO2 and dimethyl carbonate (Scheme)
scCO2 had several effects on the reaction outcome. From the synthetic standpoints, the most notable ones were observed at a definite range of medium-to-high pressures (150-200 bars). Under these conditions, both the reaction selectivity and the chemoselectivity for ambident amines (i.e. hydroxyl-benzylamines, aminobezylamines, etc…) were significantly improved.