Synthesis and esterolytic chemistry of some (dialkylamino)pyridine-functionalized micellar aggregates. Evidence of catalytic turnover

Abstract

Four new (dialkylamino)pyridine-functionalized surfactants have been synthesized. Micelles were generated either from the surfactant alone in aqueous buffer (pH 8.5 or 9.0) or by comicellization in 1 x 10^-3-1 x 10^-4 M aqueous micellar cetyltrimethylammonium bromide (CTABr) solution at pH 8.5 or 9.0. Such aggregates were used to cleave p-nitrophenyl alkanoates or p-nitrophenyl diphenylphosphate. The nucleophilic reagents and the second-order catalytic rate constants toward esterolysis of the substrate p-nitrophenyl octanoate (at 25 °C, pH 9.0) were [cat.] = 1 x 10^-4 M, [CTABr] = 1 x 10^-3 M, and k_cat. = 440.13 M^-1 s^-1 for 1b, [cat.] = 5 x 10^-4 M, [CTABr] = 5 x 10^-4 M, and k_cat. = 30.8 M^-1 s^-1 for 1c, [cat.] = 5 x 10^-4 M, [CTABr] = 5 x 10^-3 M, and k_cat. = 183.64 M^-1 s^-1 for 2a, and [cat.] = 3 x 10^-4 M and k_cat. = 54.1M^-1 s^-1 for 2b. The catalytic systems, especially 1b/CTABr and 2a/CTABr, also conferred significantly greater reactivity toward the esters derived from alkanoic acids of moderate chain length (C₆-C₁₀) during hydrolytic cleavages relative to their shorter and longer counterparts. Importantly, the catalytic systems comprising the coaggregates of either neutral 1b and CTABr (1 :10) or anionic 2a and CTABr (1 :10) conformed to the Michaelis-Menten kinetic scheme and demonstrated turnover behavior in the presence of excess substrate.