Ethyl acetoacetate, proton transfer from

Another reaction in which the cleavage of a carbon-hydrogen bond is important is the bromination of ketones. In the bromination of ethyl acetoacetate and 2-carboethoxycyclopentanone, it was shown that multivalent cations are catalysts. In the latter reaction, cupric, nickelous, lanthanum, zinc, plumbous, manganous, cadmium, magnesium, and calcium ions were effective (45). One can interpret the effect of the metal ion in terms of its catalysis of the proton transfer from the ester to a base, whether the reaction is carried out in dilute hydrochloric acid solution (acid-catalyzed bromination) or in acetate buffer (base-catalyzed bromination). 

Quinone oximes and nitrosoarenols are related as tautomers, i.e. by the transfer of a proton from an oxygen at one end of the molecule to that at the other (equation 37). While both members of a given pair of so-related isomers can be discussed separately (see, e.g., our earlier reviews on nitroso compounds and phenols ) there are no calorimetric measurements on the two forms separately and so discussions have admittedly been inclusive—or very often sometimes, evasive—as to the proper description of these compounds. Indeed, while quantitative discussions of tautomer stabilities have been conducted for condensed phase and gaseous acetylacetone and ethyl acetoacetate, there are no definitive studies for any pair of quinone oximes and nitrosoarenols. In any case. Table 4 summarizes the enthalpy of formation data for these pairs of species. 

The final step drives the reaction to completion. Ethyl acetoacetate is more acidic than any of the other species present, and it is converted to its conjugate base in the final step. A full equivalent of base is needed to bring the reaction to completion. The /i-kctocstcr product is obtained after neutralization and workup. As a practical matter, the alkoxide used as the base must be the same as the alcohol portion of the ester to prevent product mixtures resulting from ester interchange. Because the final proton transfer cannot occur when a-substituted esters are used, such compounds do not condense under the normal reaction conditions. This limitation can be overcome by use of a very strong base that converts the reactant ester completely to its enolate. Entry 2 of Scheme 2.13 illustrates the use of triphenylmethylsodium for this purpose. 

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