An easy synthesis of robust polymer-supported chiral 1,1′-bi-(2-naphthol)s (BINOLs): application to the catalysis of the oxidation of prochiral thioethers to chiral sulfoxides

Abstract

Polystyrene beads bearing chiral 1,1′-bi-(2-naphthol) (BINOL) moieties are readily prepared by Suzuki couplings between chiral 6-bromo-1,1′-bi-(2-naphthol)s and crosslinked polystyrene beads containing phenylboronic acid residues. With this method, no protecting groups for the phenolic residues are required; any boronic acid groups that do not take part in the Suzuki coupling are removed from the support by in situ hydrolysis, and the BINOL moieties are bound to the support via strong C–C bonds. To test the performance of the new polymer-supported (PS) BINOLs, they were reacted with titanium tetraisopropoxide to give catalysts for the oxidation of aryl methyl thioethers using t-butyl hydroperoxide in tetrahydrofuran at 0 °C. The expected sulfoxides were obtained in up to 91% ee. The results obtained with the present catalysts are comparable to those obtained with PS catalysts prepared using more elaborate syntheses, and that have the catalyst moieties bound to the support by potentially labile linking groups.