Asymmetric Epoxidation of Olefins Catalyzed by Substituted Aminobenzimidazole Manganese Complexes Derived from <i>L</i>‐Proline
Jing Tian, Jin Lin, Jisheng Zhang, Chungu Xia, Wei Sun
Abstract
Abstract A family of manganese complexes [Mn( R peb)(OTf) 2 ] (peb=1‐(1‐ethyl‐1 H ‐benzo[ d ]imidazol‐2‐yl)‐ N ‐((1‐((1‐ethyl‐1 H ‐benzo[ d ]imidazol‐2‐yl)methyl) pyrrolidin‐2‐yl)methyl)‐ N ‐methylmethanamine)) derived from L ‐proline has been synthesized and characterized, where R refers to the group at the diamine backbone. X‐ray crystallographic analyses indicate that all the manganese complexes [Mn( R peb)(OTf) 2 ] exhibit cis ‐ α topology. These types of complexes are shown to catalyze the asymmetric epoxidation of olefins employing H 2 O 2 as a terminal oxidant with up to 96% ee. Obviously, the R group of the diamine backbone can influence the catalytic activity and enantioselectivity in the asymmetric epoxidation of olefins. In particular, Mn( i‐Pr peb)(OTf) 2 bearing an isopropyl arm, cannot catalyze the epoxidation reaction with H 2 O 2 as the oxidant. However, when PhI(OAc) 2 is used as the oxidant instead, all the manganese complexes including Mn( i‐Pr peb)(OTf) 2 can promote the epoxidation reactions efficiently. Taken together, these results indicate that isopropyl substitution on the R peb ligand inhibits the formation of active Mn(V)‐oxo species in the H 2 O 2 /carboxylic acid system via an acid‐assisted pathway. magnified image