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A kinetic study of the reactions of phenyl substituted benzylidene Meldrum’s acids (5-H-Z with Z = 4-MeO, 4-Me, H, 4-Br, 3-Cl and 4-NO2) and methylthiobenzylidene Meldrum’s acids (5SMe-Z with Z = 4-MeO, 4-Me, H, 4-Br, 4-CF3 and 3,5-(CF3)2) with OH, CF3CH2O and HOCH2CH2S in 50% DMSO–50% water (v/v) at 20 °C is reported. The reactions of 5-H-Z lead to reversible attachment of the nucleophile to the substrate; with CF3CH2O and HOCH2CH2S rate (k 1 Nu and k 1 − Nu ) and equilibrium constants (K 1 Nu ) for this process could be determined while for the reactions with OH only k 1 Nu was obtained. The reactions of 5-SMe-Z lead to substitution of the MeS group by the nucleophile via a two–step mechanism. With HOCH2CH2S the first step represents rapid reversible nucleophilic attachment to the substrate which is followed by slow leaving group departure (k 2 Nu ). Rate (k 1 Nu , k 1 − Nu , k 2 Nu ) and equilibrium constants (K 1 Nu ) could be determined. With CF3CH2O the kinetic behavior is similar to that with HOCH2CH2S but complications leave some ambiguity about the interpretation of some of the rate data and hence only k 1 Nu is reported. With OH the nucleophilic attachment step is rate limiting and hence only k 1 Nu could be obtained. Substituent effects on the various rate and equilibrium constants were analyzed by the Hammett and Brønsted equations and provided insights into the π–donor effects of the MeS (leaving group) and the 4-MeO group (substituent) as well as into their mutual interaction, the presence of transition state imbalances, and the effect of soft acid–soft base interactions. Issue in Honor of Prof. O. S. Tee ARKIVOC 2001 (xii) 161-178 ISSN 1424-6376 Page 162 ARKAT USA, Inc

arkivoc

Reactions of substituted benzylidene Meldrum’s acids and methylthiobenzylidene Meldrum’s acids with OH–, CF3CH2O– And HOCH2CH2S– in 50% DMSO-50% water. π-Donor effects, soft acid-base interactions and transition state imbalances