Molecular imprinting of polymerised catalytic complexes in asymmetric catalysis

Abstract

The hydride transfer reduction of prochiral ketones using rhodium based catalysts has been studied. In homogenous catalysis, acetophenone was quantitatively converted to ( R) phenyl ethanol 3 in 7 days, (67% ee) using two equivalent of (1 S,2 S)- N, N′-dimethyl-1,2-diphenylethane diamine per Rh. We then prepared polyurea supported complex: rhodium containing catalyst were prepared by complexation of the metal to nitrogen ligands in the backbone. Using this catalyst, acetophenone was quantitatively converted to ( S) phenyl ethanol in 1 day, enantiomeric excess was 60%. We have then polymerised preformed [rhodium(I)-( N, N′-dimethyl-1,2-diphenylethane diamine) 2] with di- and triisocyanates. Using the ( S, S) isomer of the diamine leads to the reduction of propiophenone into ( R) in 6 days with 47% ee In order to use molecular imprinting effect, we have polymerised [rhodium(I)-( N, N′-dimethyl-1,2-diphenylethane diamine) 2] in the presence of a chiral template: pure ( R) or ( S) sodium phenylethanolate. Reduction of various substrates was carried out. We have shown that the imprinting effect is obvious for molecule related in structure to the template (propiophenone, 4′-trifluoromethyl acetophenone) and it is not efficient if the substrate has a structure too different from the template one. The studying of conversion vs. global conversion has shown that the mechanism occurs via two parallel reactions on the same site without any interconversion of the final products. Phenyl ethyl ketone was reduced quantitatively in 2 days to ( R) phenyl ethanol ee 70%.