MacMillan Imidazolidinone Organocatalysts: SOMO Catalysis and Asymmetric Diels–Alder

March 30, 2026 Methodology, Science MacMillan, imidazolidinone, organocatalysis, SOMO, Diels-Alder, chiral

David MacMillan’s imidazolidinone organocatalysts, introduced in 2000, established iminium activation as a complementary strategy to enamine catalysis. The catalysts condense with α,β-unsaturated aldehydes to form a chiral iminium ion that lowers the LUMO energy of the enal while shielding one face, enabling highly enantioselective reactions with a wide range of nucleophiles.

Two Activation Modes

Iminium Activation

The catalyst condenses with an enal to generate a chiral iminium ion. The bulky aryl groups of the imidazolidinone shield the β-carbon from one face, directing nucleophilic attack to the opposite face. This mode powers Diels–Alder, conjugate addition, and Friedel–Crafts reactions.

Enamine Activation

The catalyst condenses with a saturated aldehyde to generate a chiral enamine. The SOMO-catalysis variant uses a single-electron oxidant (e.g., CAN) to generate a radical cation that couples with weak nucleophiles (allylsilanes, silyl enol ethers) enantioselectively — a mode inaccessible to conventional two-electron catalysis.

Key Reactions

  1. Diels–Alder — enals with cyclopentadiene, 1,3-butadienes; >90% ee.
  2. Mukaiyama–Michael — silyl ketene acetals to enals; quaternary stereocenters.
  3. Friedel–Crafts alkylation — pyrroles and indoles to enals.
  4. SOMO catalysis — α-alkylation, α-arylation, α-vinylation of aldehydes.

Why Imidazolidinones Matter

  • Metal-free: Avoids residual metal contamination in API synthesis.
  • High catalyst loadings tolerated: 5–20 mol% typical; compatible with complex substrates.
  • Both enantiomers available: Enables rapid access to either antipode.

Continuing Influence

MacMillan organocatalysis (recognized with the 2021 Nobel Prize in Chemistry, shared with List) has expanded into photoredox-organocatalytic dual cycles that construct complex molecules under mild visible-light conditions. The original imidazolidinone catalysts remain foundational references for any new organocatalytic campaign.

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