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.