Remarkable Activation of Anionic Nucleophiles toward p-Nitrophenyl Acetate by Aqueous Trioctylmethylammonium Chloride: A New Class of the Hydrophobic Aggregate1

Trioctylmethylammonium chloride (TMAC), a typical phase transfer catalyst, forms aggregates in aqueous solutions at very low concentrations (10-4-10-5 M). The aggregate was inferred to be much smaller than the conventional globular micelles from the data of surface tension and specific conductance. The dissociation of 2,6-dichlorophenolindophenol, which is commonly used for detection of the critical micelle concentration of the cationic micelle, was enhanced in proportion to the TMAC concentration at 10-4-10-5 M. This lack of the critical phenomenon suggests the progressive formation of the tight ion pair between the phenolate anion and the cationic TMAC aggregate. The reactivity of lauryl-substituted hydroxa-mate and imidazole nucleophiles toward p-nitrophenyl acetate was remarkably enhanced in the presence of 7 X 10-5 M TMAC in water at 30°vC, pH 9. The rate enhancements amounted to 500 to 104 times, and were much larger than those produced by the conventional hexadecyltrimethylammonium bromide (CTAB) micelle. Less hydrophobic hydroxamate and imidazole nucleophiles were not activated by addition of TMAC. The dissociation of the hydrophobic nucleophiles was promoted in the presence of the TMAC aggregate and, for example, pKa,2 of the lauryl-substituted imidazole was lowered by 2.5 pK units relative to that of the hydrophilic counterpart. Therefore, the large rate enhancement observed is produced by adsorption of hydrophobic nucleophile onto TMAC aggregates by which highly nucleophilic ion pairs are formed. Finally, the acetylimidazole intermediate is hydrolyzed very rapidly, and the imadazole-TMAC system is an extremely efficient catalyst for the hydrolysis of phenyl esters.