Enantioselective cleavage of activated amino acid esters promoted by chiral palladacyclesстатья
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Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.
Аннотация: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 В± 0.4 and 7.6 В± 0.5 dm3 molв€’1 sв€’1 for the S- and R-ester, respectively, in the case of 3a, but are 5.7 В± 0.6 and 26.7 В± 0.5 dm3 molв€’1 sв€’1 for the S- and R-ester, respectively, in the case of 3b (pH 6.23 and 25В°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 α-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.