Proposed Mechanism
A 1:1:1 mixture of La(OiPr)3 , BINOL and Ph3As=O may afford the active complex a in the reaction medium (Scheme 6). Activation of the enone b by coordination to lanthanum metal followed by 1,4-addition of lanthanum peroxide may lead to the formation of enolate c that could provide the epoxide and intermediate d . The latter with TBHP can provide the active complex a to regenerate the catalytic cycle.
Scheme 6
Replacement of La(OiPr)3 by Sm(Oi-Pr)3 , (R)-BINOL 5 by (R)-H8-BINOL 6, Ph3As=O by Ph3P=O and TBHP by CHMP greatly enhances the yield and enantiomeric purity under similar condition for alkenes bearing amides (Scheme 7).
Scheme 7
The catalyst derived from 7 and Y(OiPr)3 catalyzes the epoxidation of α,β -unsaturated esters with excellent enantioselectivity (Scheme 8). The system is compatible with alkenes bearing heteroaromatic rings.
Scheme 8