- Detection and kinetic characterization of SNV intermediates. Reactions of thiomethoxybenzylidene Meldrum's acid with thiolate ions, alkoxide ions, OH-, and water in aqueous DMSO
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The reaction of thiomethoxybenzylidene Meldrum's acid (5-SMe) with thiolate and alkoxide ion nucleophiles is shown to proceed by the two-step addition-elimination SNV mechanism in which the tetrahedral intermediate accumulates to detectable levels. For the reactions with thiolate ions, rate constants for nucleophilic addition (k1RX), its reverse (k-1RX), and for conversion of the intermediate to products (k2RX) were determined. For the reactions with alkoxide ions, only k1RX and k-1RX could be obtained; the intermediate in these reactions did not yield the expected substitution products, and hence no k2RX values could be determined. The reaction with OH- and water are believed to follow the same mechanism, but the respective intermediates remain at steady-state levels, and only k1OH and k1H2O for nucleophilic attack on 5-SMe were measurable. New insights regarding structure-reactivity behavior in SNV reactions are gained from comparisons of rate and equilibrium constants in the reactions of 5-SMe with the corresponding parameters in the reactions of methoxybenzylidene Meldrum's acid (5-OMe) and benzylidene Meldrum's acid (5-H). In particular, the relative importance of steric and π-donor effects of the MeS vs. MeO group in 5-SMe and 5-OMe, respectively, and their role in affecting the intrinsic rate constants for nucleophilic addition, has been clarified by these comparisons. Our results also add support to a previous suggestion that soft-soft type interactions tend to increase intrinsic rate constants for thiolate ion addition to vinylic substrates, especially 5-SMe with the soft MeS group.
- Bernasconi, Claude F.,Ketner, Rodney J.,Chen, Xin,Rappoport, Zvi
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- Acid-catalyzed breakdown of alkoxide and thiolate ion adducts of benzylidene Meldrum's acid, methoxybenzylidene Meldrum's acid and thiomethoxybenzylidene Meldrum's acid
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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.
- Bernasconi, Claude F.,Ketner, Rodney J.,Brown, Shoshana D.,Chen, Xin,Rappoport, Zvi
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p. 8829 - 8839
(2007/10/03)
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- Synthesis of 3-hydroxythiophenes and thiophen-3(2H)-ones by pyrolysis of alkylsulfanylmethylene Meldrum's acid derivatives
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3-Hydroxythiophene 1 and a range of its 2-substituted, 2,2-disubstituted and 5-substituted derivatives have been made by flash vacuum pyrolysis (FVP) of an appropriate alkylsulfanylmethylene derivative of Meldrum's acid 3 or 4.These compounds are readily obtained, either by reaction of methoxymethylene Meldrum's acid with alkylthiols in refluxing acetonitrile, or via the bis(methylsulfanyl) compound 18 by known procedures.The pyrolysis proceeds by a hydrogen-transfer-cyclisation mechanism in which thereis extensive loss of configuration of a chiral centre at the reaction site.The NMR and mass spectra of the Meldrum's acid precursors and the mass spectra of the 3-hydroxythiophenes are briefly discussed.
- Hunter, Gordon A.,McNab, Hamish
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p. 1209 - 1214
(2007/10/02)
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- Synthesis of Methylthio- and Aminoalkylidene Derivatives of Meldrum's Acid
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Reaction of Grignard reagents with isopropylidene bis(methylthio)methylenemalonate (1) resulted in conjugate addition to give an intermediate 2, which could either be hydrolyzed to an isopropylidene (1-methylthioalkylidene)malonate 3 or reacted with ammon
- Huang, Xiang,Chen, Bang-chi
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p. 480 - 481
(2007/10/02)
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