REACTIONS OF POLYMER-SUPPORTED OMEGA-BROMOALKYLCARBOXYLATES - FORMATION OF LACTONES VERSUS OLIGOMERIZATION

The reactions of 4-bromobutyric acid, 5-bromopentanoic acid, 6-bromohe xanoic acid, 8-bromo-octanoic acid, 11-bromoundecanoic acid and 12-met hanesulfonyloxydodecanoic acid with the bicarbonate forms of various a nion-exchange resins were investigated. Both macroporous resins and 2% crosslinked gel-type resins were used. The macroporous resins and som e of the gel-type resins had quaternary ammonium sites; other gel-type resins had quarternary phosphonium sites. Various loadings of these s ites were used. Ester formation could take place intramolecularly or i ntermolecularly and the ester products were analysed for lactone, diol ide and oligoesters. 4-Bromobutyric acid and 5-bromopentanoic acid rea cted with the macroporous polymer-supported bicarbonate to give the la ctones in high yield. The remaining acids gave mainly oligomers with t he macroporous resins and with the highly loaded gel-type resins. Ln t he case of 11-bromoundecanoic acid it was shown that the oligomers had average degrees of polymerization of up to 27 and later work has show n that a substantial number of these are cyclic. As the loading of the bicarbonate on gel-type resins fell (<33% of the phenyl residues bear ing 'onium ion sites) site isolation became important and lactones and diolides were formed in significant yields. In all but one case, 8-br omo-octanoic acid, the yields were, however, consistent with the conce ntrations of the bicarbonate in the beads compared with the 'effective molarities' for such reactions. With 8-bromo-octanoic acid the yield of lactone obtained with a lightly loaded gel-type resin suggests cons iderable site isolation was achieved. There is evidence that as the ch ain lengths of the omega-bromo acids increases site isolation decrease s due to 'bridging', i.e. the ability of the bromo end groups to reach over to the carboxylate groups on neighbouring sites. Substantial sit e isolation was achieved when 11-bromoundecanoic acid was reacted with a gel-type resin on which most of the 'onium salt sites were in the c hloride form and only a small fraction were in the bicarbonate form. T he reason why site isolation occurred in this case is unclear at prese nt. Copyright (C) 1996 Elsevier Science Ltd.