Polystyrene-supported quaternary phosphonium

Polystyrene-supported quaternary phosphonium ions with spacer chains between the active site and the aromatic ring (5, 4, 17-19% RS)... 

Full details are now available of a study of polystyrene-supported quaternary ammonium and phosphonium catalysts (80) for triphase catalysis (TC) in a liquid-solid-liquid mode. Long spacer chains between the quaternary centre and the polymer backbone were found to be unnecessary for TC activity, and phosphonium catalysts were more effective than ammonium thus (80 n = 3, A = P) compared favourably with soluble octadecyltributylphosphonium bromide as a PT catalyst for reactions such as halide to halide interconversions. A test reaction, that of potassium acetate with 1-bromo-octane, has been used to examine the TC activity of silica in the solid-solid-liquid mode. Silica impregnated with a cationic surfactant appears to be as effective as alumina (4,163), whereas silica bearing covalently bound quaternary ammonium groups exhibits increased effectiveness. 

Polymer-supported multi-site phase-transfer catalysis seems to require the use of less material in order to provide activity comparable to others253 (Table 27). Quaternary phosphonium ions on polystyrene latices, the particles of which are two orders of magnitude smaller than usual, were shown to be capable of higher activity coagulation of the catalysts under reaction conditions was minimized by specific treatment904. The spacers may also contain ether linkages. 

The use of phase-transfer catalysts bound to polymeric supports has been reported. The catalytic functional groups anchored to the polymer were (i) quaternary ammonium salts (Fig. 13-la,b,c), (ii) phosphonium salts (Fig. 13-ld), (iii) Crown ethers (Fig. 13-le), and (iv) cryptands (Fig. 13-If). Chloromethylated, 2-4% cross-linked polystyrene and silica gel were used as the support polymers, and the catalyst groups were anchored either by the reaction with the corresponding amine or phosphine or by absorption. Spacer-arms were used for linking the crown ether and cryptand (Cinouini et al., 1976 Cinquini et al., 1975 Molinari et al., 1977 Tundo, 1977, 1978). 

Quaternary ammonium and phosphonium ions bound to insoluble polystyrene present an even more complicated mechanistic problem. Polystyrene beads lacking onium ions (or crown ethers, cryptands, or other polar functional groups) have no catalytic activity. The onium ions are distributed throughout the polymer matrix in most catalysts. The reactive anion must be transferred from the aqueous phase to the polymer, where it exists as the counter ion in an anion exchange resin, and the organic reactant must be transferred from the external organic phase into the polymer to meet the anion. In principle, catalysis could occur only at the surface of the polymer beads, but kinetic evidence supports catalysis within the beads for most nucleophilic displacement reactions and for alkylation of phenylacetonitrile. 

Quaternary ammonium (3) and phosphonium ions (61), crown ethers such as (62), cryptands such as (63) and poly(ethylene glycol) ethers (64) bound to PS are catalysts for reactions of water insoluble organic compounds with organic insoluble inorganic salts. " Silica gel, alumina, polystyrene-polypropylene composite fibers, nylon capsule membranes, and polyethylene (Mn 1000-3000) have also been used as supports. The reactions are called phase-transfer-catalyzed because one or both of the reactants are transported from the normal liquid or solid phase into a polymer phase, where the reaction proceeds. 

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