and the very complicated-looking Jacobsen catalyst (not pictured
reported by katsuki using derivatives of the jacobsen catalyst
Discussions of the mechanism of the oxygen transfer to the double bond have led to controversy. Depending on the substrate and additives, the formation of side products with stereochemistry points to a radical mechanism, whereas alkyl-substituted olefins stereoselectively give only products via a concerted mechanism. The suggested formation of manganaoxetanes receives support from calculations on a theoretical level, and from experiments reported by Katsuki using derivatives of the Jacobsen catalyst.
Best Jacobsen Catalyst: R = tBu
Jacobsen's catalyst,N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)chloride, is a popular reagent for the enantioselective epoxidation ofalkenes. This reagent is successfully prepared in three steps bybeginning organic chemistry students. A mixture of1,2-diaminocyclohexane isomers is purified and resolved bycrystallization (and recrystallization) with L-tartaric acid; a diimineis formed between the resolved trans-1,2-diaminocyclohexane and3,5-di-tert-butyl-2-hydroxybenzaldehyde to produce the Jacobsen ligand;and finally Jacobsen's catalyst is prepared from the ligand by treatmentwith manganese(II) acetate followed by oxidation with air. The studentsthen use their Jacobsen catalyst to enantioselectively epoxidize one ofthe following alkenes: 1,2-dihydronaphthalene, styrene, ora-methylstyrene. After purifying their epoxides by flash chromatography,students determine the enantiopurity by GC using a chiral column. Inthis series of experiments students utilize a wide variety of laboratorytechniques: running a reaction at reflux, aqueous workup with aseparatory funnel, recrystallization, flash chromatography, TLC,polarimetry, IR and NMR spectroscopy, and chiral GC analysis. These labsalso reinforce many important concepts related to chirality,stereochemistry, and optical activity.