Institution: | a Department of Chemistry, M.V. Lomonosov Moscow State University, 119899, Moscow, Russia b Division of Chemistry, G.V. Plekhanov Russian Economic Academy, Stremyanny per. 28, 113054, Moscow, Russia c D.I. Mendeleev Moscow Academy of Chemical Technology, Miusskaya sq. 9, 125820, Moscow, Russia |
Abstract: | The ester cleavage of R- and S-isomers N-CBZ-leucine p-nitrophenyl ester intermolecularly catalyzed by R- (a) and S-stereoisomers (b) of the Pd(II) metallacycle Pd(C6H4C*HMeNMe2)Cl(py)] (3) follows the rate expression kobs = ko + kcat 3], where the rate constants kcat equal 25.8 ± 0.4 and 7.6 ± 0.5 dm3 mol?1 s?1 for the S- and R-ester, respectively, in the case of 3a, but are 5.7 ± 0.6 and 26.7 ± 0.5 dm3 mol?1 s?1 for the S- and R-ester, respectively, in the case of 3b (pH 6.23 and 25°C). Thus, the best catalysis occurs when the asymmetric carbons of the leucine ester and Pd(II) complex are R and S, or S and R configured, respectively. Molecular modeling suggests that the stereoselection results from the spatial interaction between the CH2CHMe2 radical of the ester and the -methyl group of 3. A hydrophobic/stacking contact between the leaving 4-nitrophenolate and the coordinated pyridine also seems to play a role. Less efficient intramolecular enantioselection was observed for the hydrolysis of N-t-BOC-S-metthionine p-nitrophenyl ester with R- and S-enantiomers of Pd(C6H4C*HMeNMe2)Cl] coordinated to sulfur. |