Functional chiral hybrid silica gels prepared from ( R)- or ( S)-binaphthol derivatives

Abstract

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 C 2-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.