Catalytic asymmetric epoxidation of enones using La-BINOL-triphenylarsine oxide complex: Structural determination of the asymmetric catalyst

The catalytic asymmetric epoxidation of enones using the La-BINOL-Ph₃As=O complex generated from La(O-i-Pr)₃, BINOL, and Ph₃As=O 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(Ph₃As=O)₂ (7) was observed as the major complex in the complexes' solution, generated from La(O-i-Pr)₃, BINOL, and Ph₃As=O in a ratio of 1:1:1, the possible active species turned out to be the La-BINOL-Ph₃As=O 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.