Homogeneous catalysts based on water-soluble phosphines

Aryl- and alkyl-substituted tertiary phosphines are among the most common ligands used in homogeneous catalysts, but they are hydrophobic, as in general are the products of the catalysis. Therefore, there are two basic problems in homogeneous catalysis, namely the separation and subsequent recycling of the catalyst. These problems can be elegantly solved by using two-phase catalysis where, for example, the catalyst can be in a hydrophilic phase in which the organic products are insoluble. In order to transform homogeneous catalysts based on tertiary phosphines into water-soluble moieties, one approach is to make the phosphines water-soluble. Thus, when the catalytic reaction is complete, the products are in an organic phase and the catalyst is in the aqueous phase. Then, a simple phase separation enables the continuous re-use of the catalyst. However, if we compare biphasic reactions with their monophasic equivalents, we find that rates are lower in two-phase systems. This is mainly due to the fact that when the catalyst is in one phase and the substrates are in another, the interaction between the catalyst and the substrates is lower than in a monophasic system, thus reducing the rate of the reactions. A way to remove disadvantages of both monophasic and biphasic catalysis is to introduce ligands that confer a thermoregulated phase-transfer function to the catalyst. The strategy of the thermoregulated phase-transfer catalysis (TRPTC) is that before the reaction, the catalyst resides in the aqueous phase and the substrates in the organic phase as in regular biphasic catalysis, but at higher temperature the catalyst can transfer into the organic phase to catalyse the reaction, as in a homogeneous system, and then return to the aqueous phase to be separated from the product at lower temperature. (C) 2002 Published by Elsevier Science B.V.