Acid-Catalyzed Breakdown of Alkoxide and Thiolate Ion Adducts of Benzylidene Meldrum's Acid, Methoxybenzylidene Meldrum's Acid and Thiomethoxybenzylidene Meldrum's Acid

• Published in: Journal of organic chemistry Vol. 64; no. 24; pp. 8829 - 8839
• Main Authors: Bernasconi, Claude F, Ketner, Rodney J, Brown, Shoshana D, Chen, Xin, Rappoport, Zvi
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 26.11.1999
• ISSN: 0022-3263; 1520-6904; 1520-6904
• DOI: 10.1021/jo991041k

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.