A biphase H2O/CO2 system as a versatile reaction medium for organic synthesis

Abstract

We review activity on the usage of a biphase H₂O/CO₂ system in organic synthesis as a reaction medium of green chemistry. The formation of self-neutralizing carbonic acid in such a system eliminates the problem of salt disposal, typical for acid-catalyzed reactions with the usual mineral and organic acids. A large variety of different reactions to be performed in the biphase H₂O/CO₂ system are discussed in detail, including cyclization/cycloaddition, hydroformylation, hydrogenation, reduction, coupling, rearrangement, substitution, addition, halogenation, hydrolysis, oxidation and others. These reactions cover a significant part of modern organic synthesis. The main physical properties of carbonic acid being formed in the biphase H₂O/CO₂ system and their dependence on the temperature and pressure of saturating CO₂ are analyzed. The problem with the search for the most optimal reaction conditions from the viewpoint of selection of appropriate pressure and temperature regions for the best yields and selectivity achievable is addressed in general. Comparison with formation and utilization of peroxycarbonic acids, alkylcarbonic acids and carbamic acids by means of saturation with pressurized CO₂ of some other biphase systems is discussed in relationship to organic synthesis as well. The influence of a CO₂ admixture on the unique properties of high temperature water, another promising green solvent, is also considered.