Jacobsen Mn-Salen Catalysts for Asymmetric Epoxidation of Unfunctionalized Olefins

April 8, 2026 Methodology, Science Jacobsen, Mn-salen, epoxidation, asymmetric, olefin, chiral

Eric Jacobsen’s Mn(III)-salen complex, introduced in 1990, solved a long-standing problem: the enantioselective epoxidation of unfunctionalized olefins. Unlike the Sharpless epoxidation, which requires an allylic alcohol to direct the catalyst, the Jacobsen catalyst acts on isolated cis-disubstituted and trisubstituted olefins using sodium hypochlorite (NaOCl) as the terminal oxidant.

Catalyst Structure

The salen ligand is a tetradentate N₂O₂ Schiff base derived from (1R,2R)-diaminocyclohexane and 3,5-di-tert-butyl-2-hydroxybenzaldehyde, with the chiral cyclohexanediamine backbone controlling the facial selectivity of oxygen transfer. The Mn(III) center activates NaOCl to a high-valent Mn(V)=O species that delivers the oxygen atom to the olefin.

Substrate Scope

  • cis-Disubstituted olefins — the classic substrate; typically 80–98% ee.
  • Trisubstituted olefins — high ee, valuable for terpene and natural product synthesis.
  • Chromenes and indenes — excellent ee, relevant to medicinal chemistry.
  • trans-Disubstituted olefins — modest ee; alternative methods (Shi epoxidation) often preferred.

Practical Considerations

  • Oxidant: Aqueous NaOCl (bleach) is inexpensive and atom-economical.
  • Additive: 4-Phenylpyridine N-oxide (PNO) accelerates the reaction and improves ee.
  • Solvent: Dichloromethane with phase-transfer conditions.
  • Temperature: 0–4 °C for optimal selectivity.
  • Catalyst loading: 1–5 mol% typical; lower with optimized ligand substitution.

Industrial and Synthetic Impact

Jacobsen epoxidation has been deployed in the synthesis of the HIV protease inhibitor indinavir, the anticoagulant warfarin derivatives, and numerous terpene natural products. The ready availability of both (R,R) and (S,S) enantiomers of the catalyst makes the method practical for medicinal chemistry programs that need both enantiomeric epoxides for SAR exploration.

Modern Context

While Shi’s fructose-derived ketone and chiral dioxirane methods have extended asymmetric epoxidation to trans-olefins, the Jacobsen Mn-salen system remains the default for cis-disubstituted and trisubstituted substrates. Heterogenized versions on silica and polymer supports enable catalyst recycling in flow reactors, pushing the technology further into process chemistry.

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