Enantioselectivity of Ester Aminolysis by Optically Active Cationic Surfactants in Apolar Solvents

Abstract

In order to clarify the factors affecting enantioselectivity in nonaqueous surfactant solutions, pseudo-first order rate constants ( k_obs)h ave been measured for the aminolysis of p-nitrophenyl esters of L and D-mandelic acids, N-benzyloxycarbonyl L and D-amino acids and acetic acid by reversed micelle-forming surfactants, (+) and (-)-C6HH(CH₃)NH₃^+-OOC(CH₂)10CH₃, CH₃ (CH₂)11NH₂⁺ _L-^-OOCCH (CH₂C6H₅)-NHCO(CH₂)10 CH₃ and C6H₅CH₂NH₃^+-OOC(CH₂)10 CH₃ in hexane and benzene (Fig.2, 4 and 5). By analyzing the k_obs vs. [Surfactant] profile obtained (Fig.3 ), the rate constants and the enantiomer rate ratio (LID) were obtained as a measure of enantioselectivity for surfactant monomers and aggregates (Table 2-4). Higher rate constants and enantioselectivity were observed for the aggregates as compared to the monomers in any reaction system. In the aggregate systems the selectivity was found to be caused by the difference in the free energy of enantiomer-aggregates complex formation, indi eating different binding constants for the complex (Table 2 and 3). This was supported from the facts that neither selectivity nor different binding constant was observed in optically inactive surfactant solutions (Table 2). The effective enantioselectivity was observed for the surfactant having asymmetric center near the cationic part of the catalytic function group, - NH₃^+-OOC-, of the surfactant (Table 2-, 4).