Kinetic results are reported on the rates of enolisation of title compounds in water, in acetate buffers, in dilute hydrochloric acid, in dilute sodium hydroxide solutions and in the presence of several metal-ion salts, as measured by their rates of halogenation at 25°C. The results have been compared with those previously obtained from the corresponding reactions of acetophenone and a number of acetylheterocycles. It is shown that a suitable metal cation (e.g., Cu2+) can be hundreds of times more effective than the proton in catalysing the enolisation process of an acetylheterocyclic compound. This can be due either to the ’soft‘ character of the acetyl group, as, for example, that of 2-acetylpyrrole, or to coordination of both the carbonyl oxygen and the heteroatom by the metal cation in the transition state of the reaction, as for 2- and 4-acetylthiazoles.
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