Polymer-supported phase-transfer catalysts. High catalytic activity of ammonium and phosphonium quaternary salts bonded to a polystyrene matrix

Phase-transfer (PT) catalysts, namely, phosphonium salts, have been immobilized on silica gel; good organofunctionalization has been obtained ranging from 1 .O to 0.7 mequiv [PRd]+/g. The synthesis of phosphonium salt functionalized silica gels 1-4, with different spacer chain lengths between the polymeric matrix and the catalytic center (from 3 to 27 atoms), is reported. Ketone adsorption constants (K&) on nonfunctionalized and functionalized silica gel, and their comparison with pseudo-first-order rate constants (kobsd) in the aqueous sodium borohydride reduction of ketones under PTC conditions, show that adsorplion phenomena are the main factors determining the reaction rate; moreover, C,O-alkylation experiments show that the catalyzed reaction takes place in a very polar environment formed by the functionalized silica gel and the water adsorbed on it. These immobilized catalysts can be reused several times with a little loss of activity, but they are hydrolyzed at high pH values.