GENERATION AND STABILITY OF A SIMPLE THIOL ESTER ENOLATE IN AQUEOUS-SOLUTION

The exchange for deuterium of the alpha-protons of ethyl thioacetate and of acetone in 3-quinuclidinone buffers in D2O at 25-degrees-C and pD = 7.7-9.3 was followed by H-1 NMR spectroscopy. The exchange reactions lead to the appearance of signals due to the alpha-CH2D and alpha-CHD2 species that are cleanly resolved from each,other and from the signal due to the alpha-CH3 species. Observed rate constants for the 3-quinuclidinone-catalyzed exchange were determined during exchange of 30-37% of the first alpha-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 k(B) = 2.2 X 10(-5) and 5.2 x 10(-4) M-1 s-1, respectively. These rate constants were combined with the known pK(a) of acetone to estimate pK(a) = 20.4-21.5 for ethyl thioacetate and k(BH) = 1.7 x 10(8) to 2 x 10(9) 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.