Catalytic Asymmetric Epoxidation of Enones Using La−BINOL−Triphenylarsine Oxide Complex: Structural Determination of the Asymmetric Catalyst
- 28 February 2001
- journal article
- research article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 123 (12) , 2725-2732
- https://doi.org/10.1021/ja004201e
Abstract
The catalytic asymmetric epoxidation of enones using the La−BINOL−Ph3AsO complex generated from La(O-i-Pr)3, BINOL, and Ph3AsO in a ratio of 1:1:1 is described herein. Using 1−5 mol % of the asymmetric catalyst, a variety of enones, including a dienone and a cis-enone, were found to be epoxidized in a reasonable reaction time, providing the corresponding epoxy ketones in up to 99% yield and with more than 99% ee. The possible structure of the actual asymmetric catalyst has been clarified by various methods, including X-ray crystal structure analysis. This is the first X-ray analysis of an alkali-metal free lanthanoid−BINOL complex. Although La(binaphthoxide)2(Ph3AsO)2 (7) was observed as the major complex in the complexes' solution, generated from La(O-i-Pr)3, BINOL, and Ph3AsO in a ratio of 1:1:1, the possible active species turned out to be the La−BINOL−Ph3AsO complex in a ratio of 1:1:1. A probable reaction mechanism of the catalytic asymmetric epoxidation of enones is also proposed, suggesting that preferential formation of a heterochiral complex is the reason for asymmetric amplification. Moreover, the interesting role of La(O-i-Pr)3 for accelerating the epoxidations while maintaining high ee's is discussed.Keywords
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