Enantioselective epoxidation with chiral MN(III)(salen) catalysts: kinetic resolution of aryl-substituted allylic alcohols |
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Authors: | Adam W Humpf H U Roschmann K J Saha-Möller C R |
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Affiliation: | Institute of Organic Chemistry and Department of Food Chemistry, University of Würzburg, Am Hubland, 97074 Würzburg, Germany. adam@chemie.uni-wuerzburg.de |
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Abstract: | A set of aryl-substituted allylic alcohols rac-2 has been epoxidized by chiral Mn(salen*) complexes 1 as the catalyst and iodosyl benzene (PhIO) as the oxygen source. Whereas one enantiomer of the allylic alcohol 2 is preferentially epoxidized to give the threo- or cis-epoxy alcohol 3 (up to 80% ee) as the main product (dr up to >95:5), the other enantiomer of 2 is enriched (up to 53% ee). In the case of 1,1-dimethyl-1,2-dihydronaphthalen-2-ol (2c), the CH oxidation to the enone 4c proceeds enantioselectively and competes with the epoxidation. The absolute configurations of the allylic alcohols 2 and their epoxides 3 have been determined by chemical correlation or CD spectroscopy. The observed diastereo- and enantioselectivities in the epoxidation reactions are rationalized in terms of a beneficial interplay between the hydroxy-directing effect and the attack along the Katsuki trajectory. |
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