Sol–gel derived organic–inorganic composites recognizing molecular asymmetry

A new type of optically active organic-inorganic composite was prepared by a sol–gel method in which tetraethoxysilane (TEOS) is hydrolyzed in the presence of an optically active organic compound (d-lactose, d-glucose, d-sorbitol, d-fructose, l-tartaric acid, l-malic acid, or l-mandelic acid). Optical resolution of tris(pentane-2,4-dionato)metal complexes was performed by using the new sol–gel derived composites, composites prepared by conventional techniques (kneading with l-lactose, l-fructose or l-tartaric acid, and impregnation with an l-lactose, l-fructose or l-tartaric acid solution) and the optically active organic compounds themselves. The sol–gel derived composites showed much higher optical resolution abilities than the composites prepared by conventional techniques. In addition, the optically active organic compounds could not resolve the racemate into the enantiomers under similar conditions. X-ray diffraction and NMR results disclosed that an optically active organic compound in the sol–gel derived composite is highly dispersed, most likely, because it bonds to silicon atoms. Thus, it was deduced that optically active molecules dispersed at a molecular level recognize the chirality of the metal chelate compound. The high-resolution ability of the sol–gel derived composites arises from the combined effect of the silica support (adsorbing power) and the highly dispersed molecules (chiral recognition power).