Enantioselective xatalusis by imidazole-containing polymeric surfactants

Abstract

To explore the role of hydrophobic interaction in the stereoselective solvolyses of optically active activated esters by polymeric imidazoles, N-methacryloyl-L-histidine methyl ester(1) and N-methacryloyl-L-phenylalanyl-L-histidine methyl ester(2) were synthesized and copolymerized with N,N-dimethyl-N-hexadecyl-N-(11-methacryloyloxyundecanyl)ammonium bromide (I), N,N-dimethyl-N-hexadecyl-N-[10-(p-vinylcarboxanilido) decyl] ammonium bromide (II), and N,N-dimethyl-N-hexadecyl-N-[10-(p-methacryloyloxyphenoxycarbonyl) decyl] ammonium bromide (III) to obtain imidazole-containing polymeric surfactants. In the presence of each of these copolymers the stereoselective solvolytic reactions of N-carboethyoxy-D- and L-phenylalanine p-nitrophenyl esters (D-NEP and L-NEP), N-carbobenzoxy-D- and L-phenylalanine p-nitrophenyl esters (D-NBP and L-NBP), N-dodecanoyl-D- and L-phenylalanine p-nitrophenyl esters (D-NDP and L-NDP), and D- and L- $\alpha$-methoxyphenylacetic acid p-nitrophenyl esters (D-Man-PNP and L-Man-PNP) were carried out. In the solvolyses of D-NBP and L-NBP by copoly (I-1), copoly (II-1), copoly (III-1), and copoly(I-2), polymeric catalysts exhibited not only increased catalytic activity but also enhanced enantioselectivity as the mole % of surfactant monomers in all the copolymers increased, indicating that hydrophobic interaction can be one of the most limportant contributions to the high catalytic activity and enantioselectivity. In the solvolyses of activated esters of different alkyl sizes, the polymeric catalysts showed enhanced enantioselective solvolyses toward the more hydrophobic substrates, D-NBP and L-NBP, while in the case of solvolyses of D- and L-NEP and Man-PNP, there was no significant differences between the catalytic rate constants for two enantiomeric substrates. The hydrophobic interaction was temperature dependent. As the reaction temperature was lowered for the solvolyses of D- and L-NBP with catalyst polymer, copoly (II-1), the reacti...