Synthesis And Catalytic Properties Of Chiral (Pyrrolidine Salen)Mn(Ⅲ) Complexes For Asymmetric Epoxidation Of Unfunctionalized Alkenes

Chiral epoxides are versatile intermediates that can be readily converted into a widevariety of enantiomerically pure compounds by means of regio- and stereo-selective ringopening reactions. In the last two decades, various methods have been developed for thepreparation of chiral epoxides. The asymmetric epoxidation of unfunctionalized alkenescatalyzed by chiral (salen)Mn(â…¢) complexes has been considered as one of the most effectivemethods discovered thus far. The chiral (salen)Mn(â…¢) complexes, especially Jacobsen'scatalyst, display high enantioselectivity for asymmetric epoxidation of conjugated cis-di-, tri-and some tetrasubstituted prochiral alkenes in the NaClO biphasic system, but the epoxidationreaction generally requires a long reaction time even in the presence of an axial ligand. Inview of practical application, there has been a compelling interest in the development ofcatalytic epoxidation systems that can increase reaction activity in the NaClOaqueous/organic biphasic system. The main goal of the present study is to design andsynthesize novel chiral (salen)Mn(â…¢) complexes with high activities, to improve theepoxidation activity in the aforementioned reaction system.Eleven chiral (pyrrolidine salen)Mn(â…¢) complexes were prepared and successfully usedin the asymmetric epoxidation of styrene, substituted chromenes and trans-alkenes withNaClO/PPNO (4-phenylpyridine N-oxide) as oxidant system. Moderate to high yields wereobtained for all alkenes, but high enantioselectivity was obtained only in the case ofsubstituted chromenes. Complexes bearing tertiary amine unit(s) displayed significantlyhigher activity than analogous complexes featuring amide unit(s) and Jacobsen's catalyst,with comparable yields and ee values. Dimeric complexes showed enhanced activities overtheir corresponding monomeric complexes of double equiv, with slightly higher yields andnearly identical enantioselectivities. Furthermore, the alkenes' access pathway was discussedon the basis of the steric effect of the N_aza-substituent in the pyrrolidine backbone ofcomplexes on enantioselectivity of epoxidation.The catalytic properties of chiral (pyrrolidine salen)Mn(â…¢) complexes with excess CH₃Iand complexes with an internal pyridinium salt were explored to figure out the influence ofthe internal quatemary ammonium and/or pyridinium salt(s) in the catalyst on asymmetricepoxidation of substituted chromenes with NaClO/PPNO as oxidant system in theaqueous/organic biphasic medium. The (pyrrolidine salen)Mn(â…¢) complexes with internal quaternary ammonium and/or pyridinium salt(s), either formed in-situ or isolated, displayedsignificantly higher activities than analogous complexes and Jacobsen's catalyst in theaforementioned reaction. The enhancement of the reaction activity is attributed to the phasetransfer capability of the built-in quaternary ammonium and pyridinium salts of the(salen)Mn(â…¢) catalyst. The effect of the internal quaternary ammonium and/or pyridiniumsalt(s) on the epoxidation of substituted chromenes is similar to that of the externalpyridinium salts and ammonium halides.