Donor Ligand Effect in Asymmetric Epoxidation of Unfunctionalized Olefins with Chiral Salen Complexes

Chiral Metallosalen Complexes: Structures and Catalyst Tuning for Asymmetric Epoxidation and Cyclopropanation

“…[9] A dimeric homochiral Co(salen) and Mn(salen) connected by a covalent bond serves as the efficient catalyst for enantioselective ring-opening reactions of epoxides [4b] and epoxidation with urea ¥ hydrogen peroxide as the oxidant. [74] Furthermore, nature of an apical ligand [38,44,75] and the valency of manganese ion of oxo Mn(salen) [75] have an influence on the catalytic performance. Although M(salen)s are already one of the most widely used chiral catalysts in asymmetric synthesis, these findings will promote the construction of so far undeveloped new M(salen)s and further expand the scope of M(salen) chemistry.…”

“…Indeed, Mn(salen) 10 bearing 5-and 5'-t-butyl groups is a better catalyst than 7 (Scheme 4). [43] The coordination of a donor ligand to oxo-Mn(salen) causes not only conformational change of the salen ligand (vide infra) and change in geometry of oxoMn(salen) (vide supra), [1c,44a] but also reduction of reactivity of oxo-Mn(salen), and the addition of a donor ligand to the reaction mixture generally improves the enantioselectivity of the epoxidation [44] and chemical yield of epoxide. [1c] Thus, chiral Mn(salen)-catalyzed epoxidation has been generally performed in the presence of a donor ligand such as 4-phenylpyridine N-oxide, 4-dimethylaminopyridine N-oxide and 4-tbutylpyridine N-oxide.…”

“…[1c] Thus, chiral Mn(salen)-catalyzed epoxidation has been generally performed in the presence of a donor ligand such as 4-phenylpyridine N-oxide, 4-dimethylaminopyridine N-oxide and 4-tbutylpyridine N-oxide. [33b, 44,45] Despite the above description, a diaxial conformer (diax-11) may participate in epoxidation, when the substituents at the ethylenediamine unit are coordinating groups (R C ): an axially oriented substituent coordinates to the manganese ion with the inversion of the ligand conformation (Scheme 5). Accordingly, olefins approach the Mn O bond from the opposite side (cf.…”

“…In these reactions, olefins are considered to approach the intermediary carbenoid species [(R,R]-43 and (R,S)- 44] along the Co-N bond axis from the downwardly bent naphthalene side and rotate counter-clockwise and clockwise, respectively, to avoid the repulsion between the salen ligand and the olefinic substituent, giving enantiomeric cis-cyclopropane adducts in a highly enantioselective manner ( Figure 10). [12b] However, the olefin approaching toward (R,S)-44 suffers steric repulsion with the 2''-phenyl group and the reaction with 42 is slower than that with 41.…”

Synthesis of enantioenriched sulfoxides

“…109 Similar results were obtained when carrying out manganese salen catalyzed epoxidations in the presence of 4-PPNO, where it acts as a donor ligand in the medium. 110 Recently, Ryu et al used a similar manganese complex 116 to synthesize esomeprazole in 59% yield and 70% ee (Scheme 44). …”