212068-02-5Relevant articles and documents
Kinetics of the reactions of methoxybenzylidene Meldrum's acid with thiolate ions, alkoxide ions, OH-, and water in aqueous DMSO. Detection and kinetic characterization of the S(N)V intermediate
Bernasconi, Claude F.,Ketner, Rodney J.,Chen, Xin,Rappoport, Zvi
, p. 7461 - 7468 (1998)
The nucleophilic vinylic substitution (S(N)V) reactions of methoxybenzylidene Meldrum's acid (5-OMe) follow the common two-step mechanism involving a tetrahedral intermediate. With thiolate and alkoxide ions, this intermediate is shown to accumulate to detectable levels and a detailed kinetic study allowed the determination of the rate constants of the various elementary steps. With OH- as the nucleophile, the intermediate cannot be observed; it is shown that this is the result of the intermediate rapidly breaking down to products by a pathway not available in the reactions with the thiolate or alkoxide ions. Comparison of structure-reactivity data for the reactions of 5-OMe with those of benzylidene Meldrum's acid (5-H) and β-methoxy-α-nitrostilbene (4-OMe) reveal a complex interplay of steric effects, π-donor and π-acceptor resonance effects, and anomeric effects.
Acid-catalyzed breakdown of alkoxide and thiolate ion adducts of benzylidene Meldrum's acid, methoxybenzylidene Meldrum's acid and thiomethoxybenzylidene Meldrum's acid
Bernasconi, Claude F.,Ketner, Rodney J.,Brown, Shoshana D.,Chen, Xin,Rappoport, Zvi
, p. 8829 - 8839 (2007/10/03)
A kinetic study of the acid-catalyzed loss of alkoxide and thiolate ions from alkoxide and thiolate ion adducts, respectively, of benzylidene Meldrum's acid (1-H), methoxybenzylidene Meldrum's acid (1-OMe), and thiomethoxybenzylidene Meldrum's acid (1-SMe) is reported. The reactions appear to be subject to general acid catalysis, although the catalytic effect of buffers is weak and the bulk of the reported data refers to H+-catalysis. α-Carbon protonation and, in some cases, protonation of one of the carbonyl oxygens to form an enol compete with alkoxide or thiolate ion expulsion. This rendered the kinetic analysis more complex but allowed the determination of pK(a) values and of proton-transfer rate constants at the α-carbon. In conjunction with previously reported data on the nucleophilic addition of alkoxide and thiolate ions to the same Meldrum's acid derivatives, rate constants for nucleophilic addition by the respective neutral alcohols and thiols could also be calculated. Various structure-reactivity relationships are discussed that help define transition-state structures. Comparisons with similar reactions of alkoxide ion adducts of β-alkoxy-α-nitrostilbenes provide additional insights.
