Dedicated to Professor Oswald S. Tee on the occasion of his 60 th birthday, and in recognition of his many contributions to chemistry in Canada ( Abstract 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-NO 2 ) and methylthiobenzylidene Meldrum's acids (5-SMe-Z with Z = 4-MeO, 4-Me, H, 4-Br, 4-CF 3 and 3,5-(CF 3 ) 2 ) with OH -, CF 3 CH 2 Oand HOCH 2 CH 2 Sin 50% DMSO-50% water (v/v) at 20 °C is reported. The

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... tions of 5-H-Z lead to reversible attachment of the nucleophile to the substrate; with CF 3 CH 2 Oand HOCH 2 CH 2 Srate (k 1 Nu and k 1 − Nu ) and equilibrium constants (K 1 Nu ) for this process could be determined while for the reactions with OHonly 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 HOCH 2 CH 2 Sthe 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 CF 3 CH 2 Othe kinetic behavior is similar to that with HOCH 2 CH 2 Sbut complications leave some ambiguity about the interpretation of some of the rate data and hence only k 1 Nu is reported. With OHthe 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. Figure 4. Hammett plots for the reactions of 5-H-Z with HOCH 2 CH 2 S -: log K 1 RS (□); log k 1 RS (Ο); log k 1 − RS (∆). The filled symbols refer to Z = 4-MeO. Figure 5. Hammett plots for the reactions of 5-SMe-Z with HOCH 2 CH 2 S -: log K 1 RS (□); log k 1 RS (Ο); log k 1 − RS (∆); log k 2 RS ( ). The filled symbols refer to Z = 4-MeO. importance of 11c. The fact that λ max (334 nm) for 5-SMe-MeO is the same as for the other 5-SMe-Z derivatives (4-Me, H, 4-Br: 334 nm; 4-CF 3 and 3,5-(CF 3 ) 2 : 335 nm) is consistent with our explanation. Conclusions (1) The ρ(K 1 Nu ) values are relatively large, consistent with the notion that the negative charge on the respective intermediates or adducts is not particularly strongly delocalized. (2) The lower ρ(K 1 Nu ) value for HOCH 2 CH 2 Sattachment to 5-SMe-Z compared to 5-H-Z may