Asymmetric Transfer Hydrogenation with Noyori–Ikariya Ru-TsDPEN Catalysts

June 15, 2026 Methodology, Science asymmetric transfer hydrogenation, Noyori, TsDPEN, ruthenium, ketone reduction, chiral

Asymmetric transfer hydrogenation (ATH) is one of the most operationally simple methods for reducing prochiral ketones and imines to chiral alcohols and amines. Developed by Noyori and Ikariya in the mid-1990s, the protocol uses a Ru(II)-TsDPEN complex and a hydrogen donor such as isopropanol or a sodium formate/water mixture, avoiding the high-pressure hydrogen gas equipment required for direct hydrogenation.

The Catalytic Cycle

The active species, generated in situ from a [(TsDPEN)Ru(p-cymene)Cl₂] precursor, operates through an outer-sphere “metal–ligand bifunctional” mechanism. A hydride is delivered from Ru to the carbonyl carbon while the N–H of the TsDPEN ligand transfers a proton to the oxygen, all without substrate coordination to the metal. This concerted [2+2] addition explains the high enantioselectivities and functional-group tolerance that make ATH a workhorse in API synthesis.

Practical Advantages

  • No pressurized H₂: Ideal for kilo-lab and CDMO environments where high-pressure infrastructure is limited.
  • Aqueous compatibility: The formate/water variant is green and scalable.
  • Broad scope: Aromatic ketones, α-substituted ketones, imines, and heteroaromatic substrates.
  • Tunable stereochemistry: Choosing the (S,S) or (R,R) ligand gives access to either enantiomer.

Selecting the Right Configuration

The enantiomer of the product is dictated by the ligand geometry. The [(S,S)-TsDPEN]Ru complex typically delivers the (R)-alcohol from acetophenone-type substrates, while the [(R,R)-TsDPEN]Ru complex delivers the opposite enantiomer. Process chemists therefore stock both enantiomers to flex around evolving regulatory specifications.

  • [(S,S)-TsDPEN]Ru(p-cymene)Cl₂](/products/ss-tsdpenrup-cymenecl/) — (S,S) precatalyst
  • [(R,R)-TsDPEN]Ru(p-cymene)Cl₂](/products/rr-tsdpenrup-cymenecl/) — (R,R) precatalyst
  • (S,S)-TsDPEN — free ligand for catalyst screening
  • (R,R)-TsDPEN — free ligand for catalyst screening

Recent Developments

Recent work has extended ATH to N-heteroaryl ketones, trifluoromethyl ketones, and continuous-flow reactors where the immobilized Ru-TsDPEN catalyst achieves turnover numbers above 10,000 with sub-ppm Ru leaching. These advances are pushing ATH deeper into commercial manufacturing of chiral alcohols and amines.

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