The effect of alkali metal ions (Li+, Na+, K+, Cs+) on the reactivity of toluenethiolate anion in the demethylation of 2-methoxy-1,3-xylyl-18-crown-5, 2-methoxy-1,3-xylyl-24-crown-7 and the model compounds anisole and 2,6-dimethylanisole was investigated in dimethylformamide (+3.3 M water) at 60 °C. It was found that the metal ion effects are markedly influenced by the presence of the polyether chain in the reaction system. Whereas reactions of the model compounds are slightly inhibited by the presence of alkali metal ions, the latter strongly enhance rate of demethylation of the crown ether derivatives, the observed catalytic factors ranging over nearly three orders of magnitude. These remarkable rate-enhancing effects are ascribed to a strong transition-state stabilization by metal ions through cooperation of electrostatic binding with the negative charge developing on the oxygen atom of the methoxy group undergoing nucleophilic attack and coordinative interaction with the polyether chain.
ALKALI-METAL ION-ASSISTED CLEAVAGE OF CROWN-ETHER ANISOLES BY TOLUENETHIOLATE ANION
CACCIAPAGLIA R;MANDOLINI L;
1992
Abstract
The effect of alkali metal ions (Li+, Na+, K+, Cs+) on the reactivity of toluenethiolate anion in the demethylation of 2-methoxy-1,3-xylyl-18-crown-5, 2-methoxy-1,3-xylyl-24-crown-7 and the model compounds anisole and 2,6-dimethylanisole was investigated in dimethylformamide (+3.3 M water) at 60 °C. It was found that the metal ion effects are markedly influenced by the presence of the polyether chain in the reaction system. Whereas reactions of the model compounds are slightly inhibited by the presence of alkali metal ions, the latter strongly enhance rate of demethylation of the crown ether derivatives, the observed catalytic factors ranging over nearly three orders of magnitude. These remarkable rate-enhancing effects are ascribed to a strong transition-state stabilization by metal ions through cooperation of electrostatic binding with the negative charge developing on the oxygen atom of the methoxy group undergoing nucleophilic attack and coordinative interaction with the polyether chain.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
