Application of carbon dioxide in reaction work-up and purification procedures

The production of fine chemicals, including active pharmaceutical ingredients, can lead to large amounts of waste, including by-products and contaminated solvents. In many cases, it is actually the work-up of the reaction and purification, rather than the reaction itself, which is responsible. CO2-based procedures have been developed,
which can potentially alleviate some of these problems. They can be summarised as
follows:

I. The distribution of certain organic bases/acids between organic and aqueous phases could be tuned by exposing the two phases to CO2, exploiting a pH change due the acidic character of CO2. The induced change could be reversible
by physical decarboxylation. Numerous amines and other organic bases were tested for CO2 aided aqueous
extraction. Connection between their pKa and logP was found, and an empiric formula was proposed to predict the possibility of extraction. Concentration dependence of the CO2 aided aqueous extraction and rate of the
chemical free neutralisation were also investigated for benzyl amines and drug molecules with amine functions.

II. CO2 can enhance the ability to precipitate of certain amines, exploiting a carbamate formation reaction. This would dramatically reduce the amount of acid and base required for preparation of amine salt derivatives and
neutralisation. Tendencies of several secondary amines for CO2 adduct formation were tested, and detailed analysis of these often labile compounds was discussed. Possibility
of carbamate formation based separation was demonstrated.

III. CO2 could be used to induce precipitation of polar compounds from their solution at elevated pressures when utilised as an antisolvent, exploiting its nonpolar character and its solubility in organic solvents. Numerous solute-solvent systems were exposed to pressurised CO2, either above or under its critical temperature. High pressures can often be a limitations for industrial procedures. Operation at lower temperatures allowed
the application of lower pressures, because of the decreased vapour pressure of liquid CO2. Possibility to separate solute mixtures was also demonstrated.