Journal of the American Chemical Society p. 10297 - 10302 (1992)
Update date:2022-08-11
Topics:
Amyes, Tina L.
Richard, John P.
The exchange for deuterium of the α-protons of ethyl thioacetate and of acetone in 3-quinuclidinone buffers in D2O at 25 °C and pD = 7.7-9.3 was followed by 1H NMR spectroscopy. The exchange reactions lead to the appearance of signals due to the α-CH2D and α-CHD2 species that are cleanly resolved from each other and from the signal due to the α-CH3 species. Observed rate constants for the 3-quinuclidinone-catalyzed exchange were determined during exchange of 30-37% of the first α-proton of each methyl group of ethyl thioacetate or acetone. The rate constants for exchange correspond to those for deprotonation of ethyl thioacetate and acetone by 3-quinuclidinone to give the free enolates, with kB = 2.2 × 10-5 and 5.2 × 10-4 M-1 s-1, respectively. These rate constants were combined with the known pKa of acetone to estimate pKa = 20.4-21.5 for ethyl thioacetate and kBH = 1.7 × 108 to 2 × 109 M-1 s-1 for the reaction of the free thiol ester enolate with the 3-quinuclidinone cation. The lifetime of the buffer acid-enolate intimate ion pair BH+·-CH2COSEt with respect to proton transfer to give B-CH3COSEt is estimated to be from 10-9 to 10-10 s. These results provide evidence against the suggestion that enzyme-catalyzed Claisen condensation and related reactions proceed by concerted mechanisms that are enforced by the insignificant lifetime of the thiol ester enolate in the presence of an acidic amino acid residue at the enzyme.
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