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Simple L‑proline, by condensation of its secondary amine with a carbonyl, can drive enantioselective aldol reactions with remarkable efficiency. Proline has served as the basis for many organocatalysts, including prolinamides, 1-amino prolines, prolinols, proline tetrazoles and sulfonamides, and advanced proline-urea hybrids. Proline remains so effective a scaffold for organocatalysis, that revisiting previously reported derivatives has uncovered unexpectedly active catalysts, such as in the case of Seebach’s oxazolidinone. Its ability to rapidly generate enamine intermediates makes it a textbook case of how organocatalysis can be both elegant and practical.
Utilising similar principles, David MacMillan developed a series of imidazolidinone catalysts, initially deployed in asymmetric Diels–Alder reactions but quickly finding applications for various transformations of carbonyl compounds. Particularly useful for forming iminium ions with α,β‑unsaturated aldehydes, these secondary amines create highly activated intermediates that steer reactions toward one enantiomer. This has led to cleaner, more predictable outcomes that have become staples in synthesis – and was recognised with a share of the Nobel Prize in Chemistry in 2021.
Beyond these classic examples, secondary amine organocatalysts are now finding synergy with photocatalysis. In dual catalytic systems, the amine catalyst generates a reactive enamine, while a photocatalyst (often an iridium or organic dye complex) drives single‑electron transfer. This combination has opened up radical pathways for C–C bond formation that were previously inaccessible under mild conditions.
From proline to MacMillan’s designer scaffolds, secondary amine organocatalysts continue to prove that small, metal‑free molecules can deliver big advances in selectivity, sustainability and synthetic creativity.