Synthesis of an Aza Chiral Crown Ether Grafted to Nanofibrous Silica Support and Application in Asymmetric Michael Addition

We report on the synthesis of a new, supported phase transfer catalyst and its asymmetric induction. An inorganic-polymer-supported chiral crown ether was prepared by the reaction between an optically active macrocycle deriving from methyl-α-d-glucopyranoside and silica nanofibres. The inorganic carrier of the catalyst has high specific surface area due to its nanofibrous structure, which is favourable for heterogeneous catalytic reactions. SiO₂ fibres were electrospun from silica sol which was prepared via sol–gel reactions from tetraethylorthosilicate. The asymmetric Michael reaction of diethyl acetamidomalonate and β-nitrostyrene was selected for the comparison of the performance of various glucopyranoside-based macrocycles. The asymmetric inductions of macrocycles provided 20–99 % enantiomeric excess. A triethoxysilyl derivative was prepared from the highly enantioselective macrocycle in order to immobilize it on the surface of the silica nanofibres. The supported glucose based monoaza-15-crown-5 type macrocycle was characterized by X-ray photoelectron spectroscopy and compared with non-supported native crown ethers in the asymmetric Michael addition. The immobilized phase-transfer catalyst generated high enantiomeric excess (82 %) in spite of the fact that it was used in a three-phase reaction.