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-

Despite the central importance of monoketones as carbon acids, no intrinsic barriers or intrinsic rate constants for proton transfer to a standard family of bases such as amines have been reported. This paper presents the results of such determinations for the reaction of 2-acelyl-1-methylpyridinium ion (1) with secondary alicyclic amines. They yield log k(o) = 0.92 in water and log k(o) = 1.70 in 50% DMSO-50% water (v/v) at 20 DC. These intrinsic rate constants are lower than for any sterically unhindered carbon acids except For nitroalkanes, reflecting the strong pi-acceptor character of the CO group as well as the strong solvation of the oxygen in the enolate ion. Rare constants, k(1)(OH), for deprotonation of I by OH- have also been determined in water and 50% DMSO-50% water, with somewhat different results than those reported by Tobin and Frey (J. Am. Chem. Sec. 1996, 118, 12253), The k(1)(OH) value Ln water shows a small positive deviation from a correlation of such rate constants with the corresponding pK(a) of 17 simple monoketones and aldehydes reported by Keeffe and Kresge; this deviation as well as the much larger positive deviation for the deprotonation of 1-methyl-3-pyridinioacetophenone (7) and 1-pyridinioacetophenone (8) can be attributed to electrostatic effects resulting from transition-state imbalance.