Stereoselective solvolyses of amino acid <i>p</i>‐nitrophenyl esters by optically active imidazole‐containing polymers

Abstract

Abstract Solvolytic reactions of N ‐methoxycarbonyl‐D‐ and L‐phenylalanine p ‐nitrophenyl ester (D‐ 2 and L‐ 2 ) by poly( N ‐methacryloyl‐L‐histidine) and poly( N ‐methacryloyl‐L‐histidine methyl ester) were carried out. However, no significant difference between the solvolytic rates of the two enantiomeric substrates was observed. To investigate the effect of hydrophobic interaction, N ‐acetyl‐, N ‐pentanoyl‐ and N ‐octanoyl‐L‐histidine were prepared and their effect on the ester solvolyses was studied. The rates were generally enhanced as the alkanoyl group became larger. Furthermore N ‐octanoyl‐L‐histidine showed some degree of stereoselective catalysis. Accordingly, when polymers with increased hydrophobicity were prepared by copolymerizing N ‐methacryloyl‐L‐histidine and its methyl ester with dodecyl methacrylate and were subjected to catalyze the solvolyses of optically active esters, the apparent catalytic rate constant for L‐ 2 was observed to be 1,3 times larger than that for the D‐ 2 . The enantio‐selectivity became more pronounced as the hydrophobic character of polymer catalysts was enhanced.