Monosaccharides as silicon chelators: pentacoordinate bis(diolato)(phenyl)silicates with the cis-furanose isomers of common pentoses and hexoses

Five-coordinate phenylsilicates are formed from the reaction of trimethoxy(phenyl)silane with monosaccharides in methanol in the presence of a stoichiometric amount of base. Five complexes have been isolated and characterized with two ketoses and three aldopentoses. The silicon central atom in K(18]crown-6)]PhSi(beta-D-Fruf 2,3H-2)2].MeOH (1, Fru=fructose) is part of two chelate rings, with the ligands being beta-D-fructofuranose-O2,O3 dianions. The beta-furanose isomer is best suited for silicon ligation because it exhibits a torsion angle close to 0 degrees for the most acidic diol function, thus assuring a flat chelate ring. The same structural principles are also found in K(18]crown-6)]PhSi(beta-D-Araf1,2H-2)2].2 MeOH (2, Ara=arabinose), K(18]crown-6)]PhSi(alpha-D-Ribf1,2H-2)2] (3, Rib=ribose), K(18]crown-6)]PhSi(alpha-D-Xylf1,2H-2)2]. acetone (4, Xyl=xylose), and K(18]crown-6)]PhSi(alpha-D-Rulf2,3H-2)2] (5, Rul=ribulose).