Synthesis and Use of Jacobsen's Catalyst: Enantioselective Epoxidation in the Introductory Organic Laboratory

Abstract

Jacobsen's catalyst, N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride, is a popular reagent for the enantioselective epoxidation of alkenes. This reagent is successfully prepared in three steps by beginning organic chemistry students. A mixture of 1,2-diaminocyclohexane isomers is purified and resolved by crystallization (and recrystallization) with L-tartaric acid; a diimine is formed between the resolved trans-1,2-diaminocyclohexane and 3,5-di-tert-butyl-2-hydroxybenzaldehyde to produce the Jacobsen ligand; and finally Jacobsen's catalyst is prepared from the ligand by treatment with manganese(II) acetate followed by oxidation with air. The students then use their Jacobsen catalyst to enantioselectively epoxidize one of the following alkenes: 1,2-dihydronaphthalene, styrene, or α-methylstyrene. After purifying their epoxides by flash chromatography, students determine the enantiopurity by GC using a chiral column. In this series of experiments students utilize a wide variety of laboratory techniques: running a reaction at reflux, aqueous workup with a separatory funnel, recrystallization, flash chromatography, TLC, polarimetry, IR and NMR spectroscopy, and chiral GC analysis. These labs also reinforce many important concepts related to chirality, stereochemistry, and optical activity.