US researchers have demonstrated a polymeric catalyst that is easily removed from solution.

David Bergbreiter's group at Texas A&M University developed the method by attaching the organic catalyst to an inorganic polymer support. This can be extracted from the reaction mixture by adjusting the temperature or adding water to make two liquid phases; the catalyst goes into just one of these, making it easy to separate.

Polymer-bound homogeneous catalysts are usually removed from solution by filtration or precipitation, but these processes can be prohibitively costly. Bergbreiter's earlier success in using relatively inexpensive liquid-liquid separations for recovering organic-polymer-bound catalysts prompted this work using commercially available inorganic polysiloxane supports. 

Polysiloxanes exhibit several attractive properties for this application; they are highly stable, and their hydrophobic nature allows them to be readily extracted into non-polar solvents. The principle was tested by attaching dyes to different polysiloxanes dissolved in mixtures of organic solvents, which were separated into two differently coloured layers by lowering the temperature or by adding water.

Bergbreiter bound an organocatalyst, quinine, to a polysiloxane support in order to catalyse several Michael additions and the catalyst was efficiently recovered in each case. In the future, Bergbreiter hopes to 'explore other polysiloxane supports and other catalysts to establish the scope of this chemistry'.

Paul J O'Sullivan