Asymmetric Epoxidation of Chromenes Using Manganese(III) Complexes with Novel Chiral Salen-like Schiff Base Ligands
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Asymmetric Epoxidation of Chromenes Using Manganese(III) Complexes with Novel Chiral Salen-like Schiff Base Ligands Longhai Chen • Jian Wei • Ning Tang Feixiang Cheng
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Received: 20 December 2011 / Accepted: 20 February 2012 / Published online: 6 March 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Three novel chiral salen-like schiff base ligands and their Mn(III) complexes containing different amino acid unit have been synthesized and characterized. Asymmetric epoxidation reactions show these complexes are effective catalysts for the chromenes with buffer NaOCl as terminal oxidant and pyridine N-oxide as co-catalyst in the presence of ionic liquid. Good-to -excellent enantioselectivity and acceptable yields can be obtained under optimum reaction conditions. Catalyst 4c gives the highest ee (95%) for 6-chloro-2,2-dimethylchromene among these catalytic performances. Furthermore, compared the enantioselectivity of catalyst 4c with the other two catalysts 4a and 4b, the positive experimental results suggest that the steric effect of the ligands plays an important role in the asymmetric catalysis. Keywords Manganese(III) complex Schiff base Asymmetric catalysis Epoxidation Ionic liquid Amino acid
1 Introduction Chiral epoxides are extremely useful building blocks in the synthesis of chiral compounds for the pharmaceuticals as well as for fine chemicals [1–4]. The use of metal complexes of chiral salen ligands in asymmetric synthesis has been widespread in recent years [5, 6]. Numerous metal complexes L. Chen J. Wei N. Tang College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China F. Cheng (&) College of Chemistry and Chemical Engineering, Qujing Normal University, Qujing 655011, People’s Republic of China e-mail: [email protected]
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containing salen derivatives have been synthesized and extensively used as catalysts for a range of asymmetric reactions, such as aziridination [7], cyclopropanation [8], Diels–Alder cycloaddition [9], lactide polymerization [10], Michael addition [11], CO2 fixation [12], Sulfide oxidation [13], hydrolytic kinetic resolution [14] and so on. Chiral Mn(III) salen complexes have received much interest due to their scope of applications as homogeneous catalysts in asymmetric epoxidation of un-functionalised alkenes, which have been studied extensively by Jacobsen, Katsuki and so on [15, 16]. Although these metallosalen complexes are excellent catalysts for asymmetric reactions, the separation and recycling of homogeneous catalysts is problematic, making the entire catalytic process economically nonviable for industrial processes. Therefore, many attempts have been made to discover the recoverable metallosalen complexes, such as immobilization of catalyst onto inorganic materials [17, 18] or polymer [19], ionic liquid-functionalized of catalyst [20, 21], oligomer [22, 23] and highly active salen catalysts [24, 25]. Among these methods, ionic liquids have generated much excitement in the field of organic synthes
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