Functional chiral hybrid silica gels prepared from (R)- or (S)-binaphthol derivatives
Résumé
We describe the synthesis of new functional bis-trialkoxysilylated binaphthol derivatives I–V containing protected OH groups and cyano-, pyridyl- and carbamate-functionalities starting from optically pure (R)- or (S)-2,2′-dihydroxy-1,1′-binaphthyl (BINOL) and their subsequent transformation to hybrid silica gels via the sol–gel process. (R)-I was obtained by protection of hydroxyl groups of (R)-6,6′-dibromo-BINOL with MEM-chloride, halogen metal exchange with n-butyllithium and coupling of the metalated (R)-BINOL species with chlorotrimethoxysilane. The synthesis of (R)-II, (R)-III, (R)-IV and (R)/(S)-V was realized through silylation of 6,6′-dibromo-BINOL derivatives under Heck-reaction conditions using vinyl-triethoxysilane and palladium acetate/tri-o-tolyl phosphine. The sol–gel hydrolysis and polycondensation of the precursors (R)-I-(R)-V led to hybrid organosilicate gels (R)-A–(R)-E incorporating C2-symmetric chiral binaphthyl moieties. The attached functional groups are preserved after sol–gel transformation. Deprotection of the MEM–BINOL unit of (R)-I was performed following two pathways: in the first case, treatment of the gels (R)-A with a ten-fold excess of zinc bromide, and secondly by hydrolysis polycondensation of (R)-I in the presence of nickel or palladium salts. The solids, consisting of a chiral hybrid network, were characterized by solid state NMR, FTIR, TEM and nitrogen sorption analysis. These materials may open new perspectives for applications in heterogeneous catalysis and molecular recognition.