Synthesis And Application Of Chiral Ligand Enhenced By Chiral Sidearm

As kinds of important organic intermediates,chiral epoxides can be transformed to a series of high value-added functional compounds via various reactions.Therefore,many protocols have been developed to obtain chiral epoxides,among which the asymmetric epoxidation of various olefins is the most efficient one,and the asymmetric epoxidation of unfunctionalized olefins is the most challenging topic.It has been found that Salen-Mn(III)complexes are good and efficient catalysts in the asymmetric epoxidation of unfunctionalized olefins,but are still needed to be improved.It is expected that a introduction of a chiral sidearm into the skeleton of a chiral Salen-Mn(III)complex could enhance its catalytic performances of the chiral Salen-Mn(III)complex based on the sidearm strategy.Based on the above,4(R,R)-3,4-diaminopyrrolidine-based chiral salen ligands with NBoc-L-prolyl,N-Boc-D-prolyl,L-prolyl and D-proline as respective sidearms and their manganese(III)complexes were designed and synthesized.All the ligands and complexes were characterized by NMR,FT-IR,UV-Vis,etc.The four complexes as catalysts were firstly evaluated in the asymmetric epoxidation of cis-β-methylstyrene using NaClO as an oxidant.The complexes showed different catalysis varied with the structures,configurations,and steric hindrance of the sidearms.Among the four complexes,complex Mn2 with N-Boc-D-prolyl as a sidearm showed best catalytic performances compare to those of Jacobsen’s catalyst,and better than those of Mn5 with a benzyl sidearm in view of both epoxide yield and enantioselectivity under same reaction conditions.The big enhancement of the N-Boc-D-proline sidearm on the enantioselectivity of the asymmetric epoxidation might be ascribed to the large steric hindrance,and its appropriate configuration which allows the Boc moiety away from the side-on approach pathway of olefins to active center but near the opposite side one in some degree,thus facilitating the approach of olefins to the active center from one direction.Next,the effects of reaction time,amount of catalyst,reaction temperature,amount of axial ligand on the epoxidation of cis-β-methylstyrene with NaClO and m-CPBA as respective oxidants were screened,and optimal reaction conditions were received.The asymmetric epoxidation of several aryl olefins and substituted chromenes were then investigated under the catalysis of Mn2 and optimal reactions with NaClO and m-CPBA as respective oxidants,and moderate to excellent enantioselectivities and yields were achieved.The results indicated that both the two asymmetric epoxidation systems with Mn2 as a catalyst have broad substrate scopes,and also revealed that the introduction of a sidearm with an appropriate steric hindrance and a configuration into a Salen-Mn(III)complex might be a good strategy to improve the catalysis of the complex in the asymmetric epoxidation of unfunctionalized olefins.