Mechanism
The mechanism of the Jacobsen-Katsuki epoxidation is not fully understood, but most likely a manganese(V)-species is the reactive intermediate which is formed upon the oxidation of the Mn(III)-salen complex. There are three major pathways. The concerted pathway, the metalla oxetane pathway and the radical pathway. The most accepted mechanism is the concerted pathway mechanism. After the formation of the Mn(V) complex, the catalyst is activated and therefore can form epoxides with alkenes. The alkene comes in from the "top-on" approach (above the plane of the catayst) and the Oxygen atom now is bonded to the two Carbon atoms (previously C=C bond) and is still bonded to the Manganese metal. Then, the Mn--O bond breaks and the epoxide is formed. The Mn(III)-salen complex is regenerated, which can then be oxidized again to form the Mn(V) complex.
The radical intermediate accounts for the formation of mixed epoxides when conjugated olefins are used as substrates.
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