Bolaform and Classical Cationic Metallomicelles as Catalysts of the Cleavage of p-Nitrophenyl Picolinate

Article abstract of DOI:10.1021/ja00183a034

Bolaform micelles made of ligand surfactants 6-(((12-(trimethylammonio)-n-dodecanoyl)amino)methyl)-2-(hydroxymethyl)pyridine bromide (1) or 6-(((10-(trimethylammonio)-n-decyl)thio)methyl)-2-(hydroxymethyl)pyridine bromide (3) in the presence of Cu(II) or Zn(II) ions are good catalysts of the hydrolysis of p-nitrophenyl picolinate (PNPP).In the presence of 4.6x1E-4 M Cu(II) at pH=6.25 and 25 deg C, 2x1E-3 M 1 and 3 cleave PNPP with kinetic advantages of 7.6x1E5 and 1.6x1E6, respectively, over the uncatalyzed hydrolysis of the substrate.Classical micelles made of surfactant 6-(((2-(n-hexadecyldimethylammonio)ethyl)thio)methyl)-2-(hydroxymethyl)pyridine bromide (5) are less efficient ligands for Cu(II) or Zn(II) ions and poorer catalysts of the cleavage of PNPP than 1 or 3 due to the proximity and hence to the electrostatic repulsion between the ammonium group and the metal ion coordination site.The critical feature of the catalytic process is, quite likely, the formation of a ternary complex (8) involving ligand, metal ion, and substrate.The (deprotonated) hydroxyl bound to the pyridine moiety of the catalyst in the ternary complex acts as a nucleophile leading to a transacylation intermediate that undergoes a rapid, metal ion assisted hydrolysis "turning over" the catalyst.In all cases the micellar systems are better catalysts than the corresponding neutral nonmicellar models.This indicates that concentration effects coupled with a more facile acid dissociation of the hydroxyl and, possibly, with an enhanced electrophilicity of the metal ion in the aggregates largely offset the adverse electrostatic interactions between the metal ions and the positively charged micellar surface.