A (−)-Menthyl Bonded Silica Phase for Chiral Separations: Synthesis and Solid State NMR Characterization

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Abstract
A new (−)-menthyl bonded silica phase has been prepared by hydrosilation of a hydride silica intermediate. The hydride silica intermediate was synthesized by the reaction of a monoalkoxysilane (CH3)2SiH(OEt) with silica gel, yielding a relatively high surface coverage (4.4 μmol/m2) of SiH groups. This intermediate was then used successfully in the preparation of a monomeric (−)-menthyl bonded silica phase. The bonded phase produced has been used for the chromatographic separation of enantiomers in a reversed phase mode (chiral separations). Solid state 13C and 29Si CP-MAS NMR spectroscopy and DRIFT spectroscopy provides valuable information on the structure of the different surface species formed on silica after modification. The surface coverage of the hydride silica intermediate and of the final bonded silica phase produced are also determined. It is found that this modification procedure can exclusively produce a monomeric coverage of SiH groups on the silica surface and can further produce a final monomeric bonded organic silica phase for the separation of enantiomers.