Abstract
Chiral MnIII (salen) complex supported on modified ZPS-PVPA (zirconium poly(styrene-phenylvinylphosphonate)) and ZPS-IPPA (zirconium poly(styrene-isopropenyl phosphonate)) were prepared using –CH2Cl as a reactive surface modifier by a covalent grafting method. The supported catalysts showed higher chiral induction (ee: 72%–83%) compared with the corresponding homogeneous catalyst (ee: 54%) for asymmetric epoxidation of α-methylstrene in the presence of 4-phenylpyridine N-oxide (PPNO) as axial base using NaClO as an oxidant. ZPS-PVPA-based catalyst 1, with a larger pore diameter and surface area, was found to be more active than ZPS-IPPA-based catalyst 2. In addition, bulkier alkene-like indene, was efficiently epoxidized with these supported catalysts (ee: 96%–99%), the results were much higher than those for the homogeneous system (ee: 65%). Moreover, the prepared catalysts were relatively stable and can be recycled at least eight times without significant loss of activity and enantioselectivity.
Highlights
The asymmetric epoxidation of alkenes into unique labile three-membered ether rings—which are useful organic building blocks for the synthesis of pharmaceuticals, agrochemicals, and fine chemicals—is one of the fundamental organic transformations [1,2,3]
The racemic epoxides were prepared by epoxidation of the corresponding olefins by 3-chloroperbenzoic acid (m-CPBA) in CH2 Cl2 and confirmed by NMR (BrukerAV-300, Bruker, Germany), and the gas chromatography (GC) was calibrated with the samples of n-nonane, olefins, and corresponding racemic epoxides
The first we propose is that the effect of phenylpyridine N-oxide (PPNO) is due to a setwe of propose is(Scheme that the 5), effect of PPNO
Summary
The asymmetric epoxidation of alkenes into unique labile three-membered ether rings—which are useful organic building blocks for the synthesis of pharmaceuticals, agrochemicals, and fine chemicals—is one of the fundamental organic transformations [1,2,3]. Chiral MnIII (salen) complexes, firstly reported by the Jacobsen [1,2] and Katsuki groups [4], have emerged as extremely efficient systems for the asymmetric epoxidation of unfunctionalized olefins. Homogeneous catalysis is advantageous in terms of product yield and efficiency, it suffers from limitations of catalyst recovery and a problem with residual catalyst in the synthesized molecules. Extension of these methods for large-scale synthesis—and for making pharmaceutically imperative molecules—becomes a matter of environmental and economic concern. The use of supported catalysts offers an attractive solution owing to their easy separation, convenient handing, non-toxic nature, and reusability
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