| Popis: |
BAL2 demethylation of 2-(methoxycarbonyl)-1,3-xylylene-18-crown-5 by benzenemethanethiolate anion in dimethylformamide (+ 1.6M water) at 35°C is significantly promoted by alkali-metal counterions, the efficiency order being Na+ > K+ > Cs+ > Li+. The contrasting behaviour of the model compound, methyl 2,6-dimethylbenzoate, whose demethylation reaction is in fact slightly inhibited by metal ions, points to the crucial role played by the polyether bridge of the crown ether substrate. In line with previous work from this laboratory, selective transition-state stabilisation in the metal-ion-promoted cleavage of the crown-ether substrate is ascribed to a favourable combination of electrostatic binding of the metal ions with the negative charge being transferred from the nucleophile to the convergent methoxycarbonyl group and coordinative binding to the neutral donors of the polyether bridge. |
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