Chiral ruthenium-oxo complexes for enantioselective epoxidation of trans-stilbene

Two oxoruthenium(IV) complexes containing C-2 symmetric 1,1'-biisoquinoline (biqn) and (R,R)-3,3'(1,2-dimethylethylenedioxy)-2,2'-bipyrine (diopy*) we re prepared, and both are active oxidants for alkene epoxidations. The oxid ation of styrene and cis- and trans-beta-methylstyrenes by [(Cn)(diopy*)Ru- IV(O)](ClO4)(2) did not proceed enantioselectively, but the same oxidant ca n attain a moderate enantioselectivity of 33%ee for the trans-stilbene oxid ation to trans-stilbene oxide. A head-on approach model, where the C=C is d irected from the top to the O=Ru moiety, is proposed to account for the fac ial differentiation of the trans-stilbene oxidation.