J.P. Richard
Formation and stability of enolates of acetamide and acetate anion: an eigen plot for proton transfer at α-carbonyl carbon
Richard, J.P.; Williams, G.; O'Donoghue, A.C.; Amyes, T.L.
Abstract
Second-order rate constants were determined in D2O for deprotonation of acetamide, N,N-dimethylacetamide, and acetate anion by deuterioxide ion and for deprotonation of acetamide by quinuclidine. The values of kB = 4.8 × 10-8 M-1 s-1 for deprotonation of acetamide by quinuclidine (pKBH = 11.5) and kBH = 2-5 × 109 M-1 s-1 for the encounter-limited reverse protonation of the enolate by protonated quinuclidine give pKaC = 28.4 for ionization of acetamide as a carbon acid. The limiting value of kHOH = 1 × 1011 s-1 for protonation of the enolate of acetate anion by solvent water and kHO = 3.5 × 10-9 M-1 s-1 for deprotonation of acetate anion by HO- give pKaC 33.5 for acetate anion. The change in the rate-limiting step from chemical proton transfer to solvent reorganization results in a downward break in the slope of the plot of log kHO against carbon acid pKa for deprotonation of a wide range of neutral -carbonyl carbon acids by hydroxide ion, from -0.40 to -1.0. Good estimates are reported for the stabilization of the carbonyl group relative to the enol tautomer by electron donation from -SEt, -OMe, -NH2, and -O- substituents. The -NH2 and -OMe groups show similar stabilizing interactions with the carbonyl group, while the interaction of -O- is only 3.4 kcal/mol more stabilizing than for -OH. We propose that destabilization of the enolate intermediates of enzymatic reactions results in an increasing recruitment of metal ions by the enzyme to provide electrophilic catalysis of enolate formation.
Citation
Richard, J., Williams, G., O'Donoghue, A., & Amyes, T. (2002). Formation and stability of enolates of acetamide and acetate anion: an eigen plot for proton transfer at α-carbonyl carbon. Journal of the American Chemical Society, 124(12), 2957-2968. https://doi.org/10.1021/ja0125321
Journal Article Type | Article |
---|---|
Publication Date | Mar 1, 2002 |
Deposit Date | Sep 21, 2007 |
Journal | Journal of the American Chemical Society |
Print ISSN | 0002-7863 |
Electronic ISSN | 1520-5126 |
Publisher | American Chemical Society |
Peer Reviewed | Peer Reviewed |
Volume | 124 |
Issue | 12 |
Pages | 2957-2968 |
DOI | https://doi.org/10.1021/ja0125321 |
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