Jacobsen epoxidation

The Jacobsen epoxidation ( Jacobsen - Katsuki epoxidation ) is found by Eric N. Jacobsen enantioselective alkene epoxidation and belongs to the group of catalytic asymmetric reactions. In contrast to the Sharpless epoxidation is not limited to allylic alcohols, but generally in multi-substituted, non-activated alkenes applicable. For alkenes with oxidants such as sodium hypochlorite " bleach" under catalysis by chiral manganese ( III) salen complexes are converted ( salen = bis ( salicylidene) ethylendiaminato ); served as a model in nature cytochrome P450.

Reaction conditions

Jacobsen epoxidation is typically carried out in dichloromethane or acetonitrile at room temperature, with sodium hypochlorite or meta -chloroperbenzoic acid ( mCPBA ) as the oxidizing agent, and additives, of N- oxides. As substrates, disubstituted or trisubstituted alkenes also can be used. As a pre-catalyst ( Salen ) Mn (III ) Cl complex added.

Mechanism

The catalyst is first converted by the oxidant into the ( salen ) Mn ( V) O-shape. Further mechanistic details are still controversial. The following course is regarded as probable: the reactive ( salen ) Mn ( V) O-shape formed reacts with the alkene to a radical Mn ( IV ) species with a Mn -O bond, which form the second CO single bond to the Mn (III) - complex reacts oxirane.

The chiral information is in this reaction in cyclohexane backbone of the salen ligand. Note the tert-butyl groups at the Salicylaldehyduntereinheiten Without these groups would oxidize phenols to quinones by oxidants such as sodium hypochlorite, but the tBu groups prevent this oxidation and thus destruction of the ligand under the reaction conditions.