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International Journal of Molecular Sciences, Vol.8, No.6, 553-563, 2007
Asymmetric glyoxylate-ene reactions catalyzed by chiral Pd(II) complexes in the ionic liquid [bmim][PF6]
The room temperature ionic liquid [ bmim][PF6] was employed as the reaction medium in the asymmetric glyoxylate-ene reaction of alpha-methyl styrene (4a) with ethyl glyoxylate using chiral palladium(II) complexes as the catalysts. [Pd(S-BINAP)(3,5-CF3PhCN) (2)](SbF6)(2) ( 1b) showed the highest catalytic activity. Under the reaction conditions of 40 degrees C, 0.5 h, and 1b/4a molar ratio of 0.05, ethyl alpha-hydroxy-4-phenyl-4-pentenoate was obtained in excellent chemical yield ( 94%) with high enantioselectivity ( 70 %). Other alpha-hydroxy esters can also be obtained in high chemical yields and enantioselectities through the glyoxylate-ene reactions of alkenes with glyoxylates catalyzed by 1b in [bmim][PF6]. Moreover, the ionic liquid [bmim][PF6] which contained the palladium( II) complex could be recycled and reused several times without significant loss of the catalytic activity.
Keywords:ionic liquid;glyoxylate-ene reaction;palladium;chiral;1-n-butyl-3-methylimidazolium hexafluorophosphate