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

This paper details the enantioselective performance of styrene/divinylbenze ne-supported Mn- and Cr-based salen complexes for the epoxidation of olefin s and the ring-opening of epoxides to azido-silyl ethers. The Mn catalyst p roduced the epoxides of 1,2-dihydronaphthalene, styrene, and cis-beta-methy lstyrene with enantiomeric excesses (ee's) of 46, 9, and 79%, respectively. For the Cr catalyst, the enantioselective ring-opening of epoxyhexane, pro pylene oxide, and cyclohexene oxide with trimethylsilyl azide proceeded wit h 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 c onditions for epoxidation that resulted in oxidation and decomposition of t he ligand. This process also affects the homogeneous catalyst, thereby limi ting the recyclability of both the homogeneous Mn catalyst and its heteroge nized version for this reaction. The Cr-catalyzed reaction to ring-open epo xides employs milder conditions and allowed reuse of the heterogeneous cata lyst without loss of activity or enantioselectivity through three runs with epoxyhexane. During reaction, the leaching of Cr from the heterogeneous ca talyst is less than 0.1%, suggesting possible reuse of the catalyst over hu ndreds of cycles before reloading the polymer-supported salen ligand with m etal would be necessary. (C) 1999 John Wiley & Sons, Inc.