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Polymer-supported salen complexes for heterogeneous asymmetric synthesis: Stability and selectivity

✍ Scribed by Mark D. Angelino; Paul E. Laibinis


Publisher
John Wiley and Sons
Year
1999
Tongue
English
Weight
170 KB
Volume
37
Category
Article
ISSN
0887-624X

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✦ Synopsis


This paper details the enantioselective performance of styrene/divinylbenzene-supported Mn-and Cr-based salen complexes for the epoxidation of olefins and the ring-opening of epoxides to azido-silyl ethers. The Mn catalyst produced the epoxides of 1,2-dihydronaphthalene, styrene, and cis-␤-methylstyrene with enantiomeric excesses (ee's) of 46, 9, and 79%, respectively. For the Cr catalyst, the enantioselective ringopening of epoxyhexane, propylene oxide, and cyclohexene oxide with trimethylsilyl azide proceeded with ee's of 34, 36, and 6%, respectively. Upon recycle of these heterogeneous catalysts, a degradation process was noted for the Mn-catalyst under the conditions for epoxidation that resulted in oxidation and decomposition of the ligand. This process also affects the homogeneous catalyst, thereby limiting the recyclability of both the homogeneous Mn catalyst and its heterogenized version for this reaction. The Cr-catalyzed reaction to ring-open epoxides employs milder conditions and allowed reuse of the heterogeneous catalyst without loss of activity or enantioselectivity through three runs with epoxyhexane. During reaction, the leaching of Cr from the heterogeneous catalyst is less than 0.1%, suggesting possible reuse of the catalyst over hundreds of cycles before reloading the polymer-supported salen ligand with metal would be necessary.


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