Reactions in and on Water

With the necessity to develop a greener chemistry, organic reactions in or on water are now well studied and used for a number of industrial applications. ... 

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

Biphasic systems were found to have a unique effect on the selectivity of the addition of arylboronic acids to alkynes. It was found that the use of [Rh(COD)OH]2 associated with the water-soluble ligand, m-TPPTC, was highly effective for such a reaction in the biphasic water/toluene system (Eq. 4.51).91 The reaction was completely stereo-and regioselective. In addition, the catalyst did not lose any activity... 

The carbonylation of MeOH catalysed by Rh and Mel and promoted with iodide salts can be operated at lower reactor ]H20] and higher ]MeOAc] than were originally used in commercial plant. The iodide salt overcomes stability issues and higher reaction rates and lower water gas shift rates are obtained. Some formation of reduced C2 species still takes place both as EtCOOH but also acetaldehyde (AcH). The addition of the iodide salt alone extends the region where the overall rate depends only on ]Rh] and ]MeI] to lower ]MeOAc] and ]H20]. 

As discussed in Chapter 8, enhanced reactions of S02 at the interface have also been observed (Jayne et al., 1990). Surface second harmonic generation (SHG) experiments (Donaldson et al., 1995) subsequently identified a unique adsorbed S02 species at the air-water interface that may be involved in this enhanced reaction. Such SHG work on the uptake and reaction of N02 on water would clearly also be of value in understanding the kinetic anomalies. In addition, the use of sum frequency generation (SFG) spectroscopy, which in effect allows one to obtain the infrared spectrum of species present at interfaces, may shed some light on such reactions. 

There are many pharmaceutical applications for the modification of one enantiomer over another, and to this end, many have studied these selective reactions in carbon dioxide. Glowacz et al. (1996) studied the enzymatic hydrolysis of triolein and its partial glycerides and found that stereoselectivity depends on reaction time and enzyme water content. They suggest that the water content varies the local environment of the enzyme in carbon dioxide and changes the local pH value. Rantakyla et al. (1996) also found that the hydrolysis of one stereoisomer over another was water-dependent. They studied the hydrolysis of 3-(4-methoxyphenyl)glycidic acid methylester and found that the 2S,3R enantiomer hydrolyzed more than fivefold faster than the 2R3S form. 

---