Reactions of polymer-supported ω-bromoalkylcarboxylates: formation of lactones versus oligomerization
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文摘
The reactions of 4-bromobutyric acid, 5-bromopentanoic acid, 6-bromohexanoic acid, 8-bromo-octanoic acid, 11-bromoundecanoic acid and 12-methanesulfonyloxydodecanoic acid with the bicarbonate forms of various anion-exchange resins were investigated. Both macroporous resins and 2 % crosslinked gel-type resins were used. The macroporous resins and some of the gel-type resins had quaternary ammonium sites; other gel-type resins had quarternary phosphonium sites. Various loadings of these sites were used. Ester formation could take place intramolecularly or intermolecularly and the ester products were analysed for lactone, diolide and oligoesters. 4-Bromobutyric acid and 5-bromopentanoic acid reacted with the macroporous polymer-supported bicarbonate to give the lactones in high yield. The remaining acids gave mainly oligomers with the macroporous resins and with the highly loaded gel-type resins. In the case of 11-bromoundecanoic acid it was shown that the oligomers had average degrees of polymerization of up to 27 and later work has shown that a substantial number of these are cyclic. As the loading of the bicarbonate on gel-type resins fell (<33 % of the phenyl residues bearing 'onium ion sites) site isolation became important and lactones and diolides were formed in significant yields. In all but one case, 8-bromo-octanoic acid, the yields were, however, consistent with the concentrations 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 considerable site isolation was achieved. There is evidence that as the chain lengths of the ω-bromo acids increases site isolation decreases due to ‘bridging’, i.e. the ability of the bromo end groups to reach over to the carboxylate groups on neighbouring sites. Substantial site 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 chloride form and only a small fraction were in the bicarbonate form. The reason why site isolation occurred in this case is unclear at present.