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

Issue in Honor of Prof. O. S. Tee ARKIVOC 2001 (xii) 161-178 Keywords: Arylidene Meldrum’s acid, kinetic study, transition state imbalances, oxygen and sulfur nucleophiles Introduction In a series of recent papers we have reported kinetic studies of nucleophilic vinylic substitution (SNV) reactions that proceed by the attachment–detachment mechanism (eq 1) typical for substrates activated by electron withdrawing groups (X, Y).1–13 We have focused mainly on reactions where the intermediate (2) accumulates to detectable levels1,2,5–9,11–13 because this allows the determination of all individual rate constants NuNu Nu (k 1, k -1 and k 2 ). Some representative systems studied to date are the reactions of 4-LG, 5-LG, 6-LG and 7-LG with alkoxide and thiolate ions as well as amines in 50% DMSO–50% water at 20 °C. A major reason why 2 accumulates to detectable levels in these reactions is that the p– acceptor groups (X, Y) provide the necessary stabilization of the intermediate by delocalizing the negative charge. This implies that the formation of 2 should show some of the characteristic features of reactions that lead to resonance stabilized anions in general. One such feature is the relatively high intrinsic barrier or low intrinsic rate constant,14 especially for substrates with strong p–acceptors.15–17 Another is imbalanced transition states in the sense that charge delocalization into X,Y lags behind bond formation between carbon and the nucleophiles.15–17 What complicates the reactivity patterns is that other factors such as the p–donor ability of ISSN 1424-6376 Page 162 ©ARKAT USA, Inc