Hydrophobic interactions in the enantioselective solvolyses of optically active activated esters by imidazole-containing polymeric surfactants

Enantioselective solvolyses of optically active activated esters in the aggregate system of optically active polymeric surfactants containing imidazole and benzene moieties were performed. The catalyst polymers copoly(MHis-DEVAB)] employed were copolymers of N-methacryloyl-L -histidine methyl ester (MHis) with N,N-dimethyl-N-hexadecyl-N-10-(p-vinylcarboxanilido) decyl]ammonium bromide(DEVAB). In the solvolyses of N-carbobenzoxy-D - and L -phenylalanine p-nitrophenyl esters (D -NBP and L -NBP) by polymeric catalysts, copoly(MHis-DEVAB) exhibited not only increased catalytic activity but also enhanced enantioselectivity as the mole percent of surfactant monomers in the copolymers increased. The polymeric catalysts showed noticeable enantioselective solvolyses toward D - and L -NBP of the substrates employed. As the reaction temperature was lowered for the solvolyses of D - and L -NBP with the catalyst polymer containing 4.8 mol% of MHis, an increased reaction rate and enhanced enantioselectivity were observed. The coaggregate systems of the polymer and monomeric surfactants were also investigated. The case of a coaggregate system consisting of 70 mol% of cetyldimethylethylammonium bromide with polymeric catalyst showed maximum enantioselective catalysis, viz., k_cat (L )/k_cat(D ) = 6.68. The catalyst polymers in the sonicated solvolytic solutions were confirmed to form large ordered aggregate structure by electron microscopic observation. From these results, it is concluded that hydrophobic interaction in ordered aggregate structure plays an important role in enantioselective catalysis of optically active imidazole-containing polymers.