Jacobsen epoxidation
The Jacobsen Epoxidation, sometimes also referred to as Jacobsen-Katsuki Epoxidation is a chemical reaction which allows enantioselective epoxidation of unfunctionalized alkyl- and aryl- substituted olefins. It is complementary to the Sharpless epoxidation (used to form epoxides from the double bond in allylic alcohols). The Jacobsen epoxidation gains its stereoselectivity from a C2 symmetric manganese(III) salen-like ligand, which is used in catalytic amounts. The manganese atom transfers an oxygen atom from chlorine bleach or similar oxidant. The reaction is named after its inventor, Eric Jacobsen, and sometimes also including Tsutomu Katsuki. Chiral-directing catalysts are useful to organic chemists trying to control the stereochemistry of biologically active compounds and develop enantiopure drugs.
Several improved procedures have been developed.
A general reaction scheme follows:
In the early 1990s, Jacobsen and Katsuki independently released their initial findings about their catalysts for the enantioselective epoxidation of isolated alkenes. In 1991, Jacobsen published work where he attempted to perfect the catalyst. He was able to obtain ee values above 90% for a variety of ligands. Also, the amount of catalyst used was no more than 15% of the amount of alkene used in the reaction.
The degree of enantioselectivity depends on numerous factors, namely the structure of the olefin, the nature of the axial donor ligand on the active oxomanganese species and the reaction temperature. Cyclic and acyclic cis-1,2-disubstituted olefins are epoxidized with almost 100% enantioselectivity whereas trans-1,2-disubstituted olefins are poor substrates for Jacobsen's catalysts but yet give higher enantioselectivities when Katsuki's catalysts are used. Furthermore, the enantioselective epoxidation of conjugated dienes is much higher than that of the nonconjugated dienes.
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