Nanofiltration for Homogeneous Catalysis Separation: Soluble Polymer-Supported Palladium Catalysts for Heck, Sonogashira, and Suzuki Coupling of Aryl Halides

Abstract

The anionic polymerization of 4-methylstyrene results in a linear poly(4-methylstyrene), whose substoichiometric bromination with Br2 converts a portion of 5−18% of the −CH3 units into −CH2Br groups, which were then reacted with (1-Ad)2PH to synthesize a polymer loaded with sterically demanding and electron-rich −CH2P(1-Ad)2 groups as the respective air-stable phosphonium salt. Palladium complexes with the phosphinated polymer are efficient catalysts for carbon−carbon coupling reactions of the Sonogashira, Suzuki, and Heck type with aryl chlorides and bromides. The molecular weight of the respective phosphines in the range of 5−35 kDa does not have a significant effect on the catalytic performance. Such polymer-enlarged catalysts are suitable for separation over a solvent-resistant nanofiltration membrane composed of a dense poly(dimethylsiloxane) (PDMS) layer cast on a porous sublayer of poly(acrylonitrile) (PAN). The membrane displays virtually quantitative retention of the polymeric catalyst; furthermore palladium could not be found in the permeate; TXRF and spectrophotometric determinations are indicative of >99.95% retention of the catalyst. The activity of the catalyst retained in the retentate is almost unchanged, as shown by the high (typically >90%) and almost constant yields of the coupling reactions as well as by the constant turnover frequencies of the catalysts following the membrane separation experiments during up to nine cycles of Sonogashira and Suzuki reactions. For the Heck coupling the used polar aprotic solvents best suited for the catalytic reaction lead to rapid deterioration of the membranes tested.