Non-supported and resin-supported oligo(oxyethylenes) as solid–liquid phase-transfer catalysts. Effect of chain length and head-group

Abstract

A series of oligo(oxyethylenes) containing 3–30 ethylene oxide residues and a number of polystyrene resin-supported analogues have been examined as phase-transfer catalysts in the reaction of solid potassium phenoxide with 1-bromobutane in toluene. With both groups of catalysts the activity increases with the length of the oligoether chain. This effect arises despite a fall in the ability of the oligoethers to solubilize the phenoxide, and must therefore be associated with an increase in the nucleophilicity of the phenoxide anion in the oligoether complexed ion pairs. A group of oligo(oxyethylenes) with three ethylene oxide residues and symmetrically substituted with different head-groups was also examined. One with 8-quinolyl end-groups proved to be as active as dibenzo-18-crown-6, whereas, somewhat surprisingly, a corresponding species with 2-methoxyphenyl substituents was a relatively poor catalyst. In this case the geometry of complexation might allow retention of intimate ion pair character. In the case of oligoethers with head-groups OH,OH; OH,OCH₃; and OCH₃,OCH₃ the presence of an hydroxy-group appears to enhance the ability to complex K⁺, but simultaneously reduces catalytic activity, possibly because of interaction with the phenoxide counterion. As a result, the unsymmetrically substituted derivative is the most active catalyst providing an optimum balance of these factors. Two series of resin-supported analogues, one with three ethylene oxide residues in each oligoether chain and one with four, were also synthesised. In each case the derivative with an 8-quinolyl head-group and one related structure derived from 2-pyridylmethanol were the most active catalysts. Again the two species with a 2-methoxyphenyl head-group were less effective than anticipated. In the case of the oligoether chain with four ethylene oxide residues, the activity was somewhat better and there is some correlation with the reported complexing ability of analogus unbound species.