Abstract
1,1'-Bi-2-naphthol(BINOL) is popularly used as an effective chiral ligand for various metal complex catalysts, in which the axial chirality provided by the two naphthyl groups plays an important role. To create an effective cleft-type chiral space at its diol side, we planned to introduce a large aryl substituent at the 3,3'-positions. Thus, two novel BINOL derivatives, (R)-3,3'-di-9-anthryl- and (R)-3,3'-bis(9-anthrylethynyl)-BINOL, and their titanium complexes were synthesized and used for the catalytic asymmetric epoxidation of homoallylic alcohols. These complexes are expected to work as a receptor-type chiral Lewis acid. When cumene hydroperoxide was used as an oxidant, the titanium-anthryl-BINOL complex effectively catalyzed the epoxidation of 3-butenol and 4-substituted homoallylic alcohols in dichloromethane affording the desired 3,4-epoxy alcohols with high ee's (up to 82% ee). Unfortunately, this catalyst was not effective for the reaction of 3-substituted homoallylic alcohols. On the other hand, the use of titanium-anthrylethynyl-BINOL catalyst in THF showed high enantioselectivities (up to 85% ee) rather for this type of substrates. These results indicate that variously substituted chiral 3,4-epoxy alcohols can be obtained in high enantioselectivities by the asymmetric epoxidation of homoallylic alcohols with the two chiral titanium complexes.
