Styrene derivatives can be successfully epoxidized using 23a-b with good enantioselectivity (Scheme 14). The reaction is effective using the combination of N -morpholine oxide and m -chloroperbenzoic acid.

Examples:

Mechanism

The epoxidation may proceed via a concerted (A) or radical-mediated (B) stepwise manner that depends on the electronic and oxidation state of the oxo species (Scheme 15) .

Scheme 15

To account the degree and sense of the enantioselectivity, side-on perpendicular approach of the alkene to the high valent metal-oxo intermediate has been invoked (Scheme 16).

Scheme 16