Intrinsic Rate Constants for Proton Transfer from a Monoketone to Amine Bases and Electrostatic Effects on the Intrinsic Rate Constants for the Deprotonation of Cationic Ketones by OH-
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- 作者：Claude F. Bernasconi ; José ; A. Moreira ; Lisa L. Huang ; and Kevin W. Kittredge
- 刊名：Journal of the American Chemical Society
- 出版年：1999
- 出版时间：March 3, 1999
- 年：1999
- 卷：121
- 期：8
- 页码：1674 - 1680
- 全文大小：171K
- 年卷期：v.121,no.8(March 3, 1999)
- ISSN：1520-5126
文摘
Despite the central importance of monoketones as carbon acids, no intrinsic barriers or intrinsicrate constants for proton transfer to a standard family of bases such as amines have been reported. This paperpresents the results of such determinations for the reaction of 2-acetyl-1-methylpyridinium ion (1) with secondaryalicyclic amines. They yield log ko = 0.92 in water and log ko = 1.70 in 50% DMSO-50% water (v/v) at 20
C. These intrinsic rate constants are lower than for any sterically unhindered carbon acids except fornitroalkanes, reflecting the strong 
-acceptor character of the CO group as well as the strong solvation of theoxygen in the enolate ion. Rate constants, 
, for deprotonation of 1 by OH- have also been determined inwater and 50% DMSO-50% water, with somewhat different results than those reported by Tobin and Frey (J.Am. Chem. Soc. 1996, 118, 12253). The 
value in water shows a small positive deviation from acorrelation of such rate constants with the corresponding pKa of 17 simple monoketones and aldehydes reportedby Keeffe and Kresge; this deviation as well as the much larger positive deviation for the deprotonation of1-methyl-3-pyridinioacetophenone (7) and 1-pyridinioacetophenone (8) can be attributed to electrostatic effectsresulting from transition-state imbalance.
C. These intrinsic rate constants are lower than for any sterically unhindered carbon acids except fornitroalkanes, reflecting the strong -acceptor character of the CO group as well as the strong solvation of theoxygen in the enolate ion. Rate constants, , for deprotonation of 1 by OH- have also been determined inwater and 50% DMSO-50% water, with somewhat different results than those reported by Tobin and Frey (J.Am. Chem. Soc. 1996, 118, 12253). The value in water shows a small positive deviation from acorrelation of such rate constants with the corresponding pKa of 17 simple monoketones and aldehydes reportedby Keeffe and Kresge; this deviation as well as the much larger positive deviation for the deprotonation of1-methyl-3-pyridinioacetophenone (7) and 1-pyridinioacetophenone (8) can be attributed to electrostatic effectsresulting from transition-state imbalance.