Study On The Catalytic Action Of TADDOL Metal Complexes On The Reaction Of Nitrone And Electron-Deficient Alkene

The 1, 3-dipolar cycloaddition reaction is a common method for the construction of 5-membered hetereocyclic rings. Due to often high degree of regioselectivity of the 1, 3-dipolar cycloaddition reactions, the attempt to control the stereoselectivity have attracted more attention recently. By using chiral metal complexes as catalysts, the optically active products are directly synthesized from the achiral 1, 3-dipolar reagents and achiral alkenes.In this paper, we study the 1, 3-dipolar cycloaddition reaction of C, N- diphenylnitrone53a and acryloyloxazolidnone66 catalyzed using C₁-symmetrical TADDOLate-TiX₂75 as chiral metal catalysts. The catalyst 75 is synthesized from L-tartaric acid. The influence of catalysts, catalyst amount, solvents, temperature and anion of complex on the region-, diastereo- and enantioselectivity are investigated and the experimental results indicate that the stereoselectivity is decided by the combined action.The regioselectivity of the uncatalyzed reaction of C, N-diphenylnitrone53a and acryloyloxazolidnone 66 is poor, which is an exceptive example in the 1, 3-DC. A significant improvement of the regioselectivity is observed in the presence of TADDOLate-TiX₂ complexs. Enhancing the catalyst amount to 50mol% gives 100% conversion.The diastereoselectivity (endo/exo) of this reaction is reversed compared with the uncatalyzed reaction. A high diastereo- and enantioselectivity is observed by enhancing the catalyst amount and the highest endo/exo ratio and optical purity is 96:4 and 69% ee respectively by using 100mol% 75 as catalysts.The stereoselectivity of this reation has relations with solvents. Enhancing the polarity of solvent leads to a high stereoselectivity in the presence of 10mol% TADDOLate-TiCl2. Toluene and dichloromethane are good solvents for this reaction.The low temperature leads to low stereoselectivity. In dichloromethane, the diastereo- and enantioselectivity reduce to 87:13 and 6%ee respectively in the presence of 100mol%TADDOLate-Ti(OTos)2 at -30℃.The stereoselectivity of this reaction also has relations with anion of catalysts. If the chloride atoms of the catalyst are substituted with the tosylato ligand of the same catalyst amount, slightly changed selectivities are observed.