Enantioenriched epoxides are a useful chiral building block in asymmetric transformatons, and they are also an important morif sometimes found in natural products. Catalytic asymmetric epoxidation of olefins is the most direct and effective route to the optically active epoxides. Herein, we present the development of three chiral titanium catalysts, di-μ-oxo titanium(salan 3) complex, for asymmetric epoxidation of olefins with a green oxidant, aqueous hydrogen peroxide (Figure 1). Di-μ-oxo titanium complex 1 showed high caalytic performance in the epoxidation of terminal, cis-disubstituted and trisubstituted conjugate olefins (Scheme 1). In the epoxidation of 1,2-dihydronaphthalene, the desired epoxide was obtained in 92% yield with >99% ee in the presence of only 0.02 mol% of the catalyst. Aliphatic olefins, which are a challenging substrate in asymmetric epoxidation, were also converted to the epoxides with good to high enantioselectivity. High reactivity of titanium(salalen) 1 rendered asymmetric epoxidation of cis-alkenylsilanes possible, where the epoxysilanes were obtaine with complete enantioselectivity (Scheme 2). We proposed a peroxotitanium complex, which is activated by hydrogen bonding with amino proton, as active species (Figure 2). On the basis of the putative mechanism of the epoxidation with complex 1, we developed more-easily available titanium(salan) complexes which were prepared in situ from Ti(OiPr)_4 and the corresponding salan ligands (scheme 3). In particular, salan ligand 2 bearing ortho-methoxyphenyl groups at the C3 and C3' positions displayed high enantioselectivity in the epoxidation of conjugate olefins (scheme 4). The method could be applied to a gram-scale synthesis without eroding the enantioselectivity. For example, 50.0g of indene was transformed to indene oxide in 75% isolate yield with 98% ee (Scheme 5). Optically active styrene oxide derivatives are an important class of epoxides. However, there were no general and highly enantioselective (more than 95% ee) catalysts for asymmetric epoxidation of styrenes. Even with titanium(salalen) 1 which has two chiral axes and two chiral centers, the ee value observed in the epoxidation of styrene was 93% ee. Thus, we designed novel C_1-symmetric salan ligand 3 derived from naturally occuring α-amino acid proline for the epoxidation (Scheme 6). The in-situ prepared titanium(salan 3) catalyst exhibited high enantioselectivity ranging from 96 to 98% ee in the epoxidation of styrenes with electron-donating and -withdrawing groups (Scheme 7).
