BINAL-H: Noyori's Chiral Hydride Reagent for Enantioselective Reduction

March 8, 2026 Methodology, Science BINAL-H, Noyori, chiral hydride, reduction, BINOL, ketone, chiral

BINAL-H, introduced by Noyori in 1979, is a chiral modifying reagent formed by combining lithium aluminum hydride (LiAlH₄) with equimolar (S)- or (R)-BINOL and ethanol. The resulting lithium aluminum(binaphthoxide)(ethoxide)hydride complex reduces prochiral ketones with high enantioselectivity, particularly propargylic, α,β-acetylenic, and aryl alkyl ketones that are challenging for catalytic methods.

Mechanism and Selectivity

BINAL-H operates via a six-membered cyclic transition state in which the aluminum coordinates to the ketone oxygen, the binaphthoxide controls the facial approach, and the hydride is transferred from aluminum to the carbonyl carbon. The bulky binaphthyl group shields one face, directing hydride delivery to the opposite face.

The sense of induction is predictable:

  • (S)-BINAL-H delivers the (R)-alcohol from aryl methyl ketones.
  • (R)-BINAL-H delivers the (S)-alcohol.

Substrate Scope and Strengths

  • Propargylic ketones — the flagship substrate; typically 90–99% ee.
  • Aryl alkyl ketones — acetophenone-type, 80–95% ee.
  • α,β-Acetylenic ketones — propargylic alcohols for natural product synthesis.
  • Aryl ketones with ortho-substituents — especially high ee.

When to Choose BINAL-H

BINAL-H remains valuable when:

  • The substrate is incompatible with transition-metal catalysis (e.g., sulfur-containing, halogen-rich).
  • Catalytic methods (BINAP-Ru, CBS, Noyori ATH) give unsatisfactory ee.
  • Stoichiometric flexibility is desired (no catalyst optimization needed).
  • Propargylic alcohol targets are required for alkyne click chemistry or natural product synthesis.

Modern Context

While catalytic asymmetric hydrogenation has captured much of the industrial market for ketone reduction, BINAL-H retains a niche for propargylic ketones and for late-stage reductions in discovery chemistry where stoichiometric flexibility outweighs catalyst cost. The reagent is freshly prepared before use; both (S)- and (R)-BINOL precursors are commercially available at scale.

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