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

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.

Graphical abstractPS-BINOLs are easily prepared using the above reaction. The PS BINOLs react with titanium isopropoxide to give PS-species that catalyze the oxidation of aryl methyl thioethers by tert-butyl hydroperoxide in THF at 0 °C. These give the sulfoxides in up to 91% ee.Figure optionsDownload full-size imageDownload as PowerPoint slide

(R)-1,1′-Bi(2-naphthol)C20H14O2Ee = 100%[α]D21=+29.5 (c 1.00, THF)Source of chirality: resolution using N-benzylcinchonidinium chloride

(R)-1,1′-Bi(2-naphthol) monopivalateC25H22O3Ee = 100%[α]D23=+58.0 (c 1.00, THF)Source of chirality: esterification of R-1,1′-bi(2-naphthol)

6-Bromo-(S)-1,1′-bi(2-naphthol)C20H13BrO2Ee = 100%[α]D21=+7.6 (c 1.00, THF)Source of chirality: bromination of (S)-1,1′-bi(2-naphthol) monopivalate followed by ester hydrolysis

(R)-Methyl phenyl sulfoxideC7H8OSEe = 84%[α]D20=+133.7 (c 1.00, acetone)Source of chirality: catalyzed oxidation of methyl phenyl ether

(R)-Methyl 4-methylphenyl sulfoxideC8H10OSEe = 90%[α]D25=+128.0 (c 1.00, 95% ethanol)Source of chirality: catalyzed oxidation of methyl 4-methylphenyl ether

(S)-Methyl 4-bromophenyl sulfoxideC7H7BrOSEe = 71%[α]D25=-76.0 (c 0.50, chloroform)Source of chirality: catalyzed oxidation of methyl 4-bromophenyl ether