A kinetic study of the reaction of benzylidene Meldrum's acid, PhCH=C(COO)(2)C(CH(3))(2) (5-H), with a series of thiolate and alkoxide ions in 50% DMSO-50% water (v/v) at 20 degrees C is reported. The reactions with RX(-) (X = S or O) lead to adducts of the type PhCH(XR)C(COO)(2)C(CH(3))(2)(-) ((5-H,XR)(-)()), which can be viewed as a model for the intermediate of a nucleophilic vinylic substitution on substrates such as PhC(LG)=C(COO)(2)C(CH(3))(2) (LG = leaving group). Our measurements allowed a determination of rate and equilibrium constants for these processes with RS(-) = n-BuS(-), HOCH(2)CH(2)S(-), MeO(2)CCH(2)CH(2)S(-), and MeO(2)CCH(2)S(-) and RO(-) = OH(-), MeO(-) (only rate constant of breakdown of adduct), HC&tbd1;CCH(2)O(-), and CF(3)CH(2)O(-). Our results show that there are major differences between the alkoxide and thiolate ions with respect to their thermodynamic and kinetic affinities to 5-H. They arise mainly from differences in the polarizability and solvation between the sulfur and the oxygen bases. Similarities and differences between the reactions of thiolate ions with 5-H and alpha-nitrostilbenes (4-H) are also discussed.
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