Organocatalysis — boron-mediated reduction
CBS reduction (Corey-Bakshi-Shibata)
How the CBS reduction (Corey-Bakshi-Shibata) works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: (S)- or (R)-Me-CBS oxazaborolidine (5–10 mol%), BH₃·THF or BH₃·SMe₂. Conditions: THF, –20 to 25 °C, anhydrous. Bonds formed: C–H (hydride); C–O–B; bonds broken: B–H; C=O π. Stereochemistry: Predictable absolute configuration: hydride to the face set by the chiral boron-N framework. Rate and selectivity: Hydride transfer is rate-limiting; catalyst turnover fast.
At a glance
Step 1: Borane binds N, ketone binds B
A chiral oxazaborolidine catalyst's boron coordinates the borane reductant (BH₃), while its nitrogen simultaneously coordinates the ketone substrate's oxygen — bringing both reactive partners together in one well-defined, chiral six-membered assembly.
Step 2: Hydride transfer through a six-membered TS
Hydride transfers directly from boron to the ketone's carbonyl carbon through this same six-membered transition state — the geometry set by the chiral catalyst determines which face of the ketone the hydride approaches from.
Step 3: Alkoxyborane released, catalyst regenerated
The resulting alkoxide displaces the product alcohol and regenerates the active catalyst-borane complex — this remains one of the most reliable, widely used methods for reducing a simple ketone to a single enantiomer of secondary alcohol on demand.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Me
- Methyl. –CH₃.
- Ph
- Phenyl. –C₆H₅.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- BH3
- Borane. Electron-deficient Lewis acid that adds H–B across alkenes.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| CBS reduction (Corey-Bakshi-Shibata) | MacMillan iminium organocatalysis | Proline-catalyzed aldol reaction | |
|---|---|---|---|
| Reagents | (S)- or (R)-Me-CBS oxazaborolidine (5–10 mol%), BH₃·THF or BH₃·SMe₂ | Chiral imidazolidinone·HCl (5–20 mol%), enal, nucleophile or diene | L-Proline (10–30 mol%), acetone (donor), aldehyde (acceptor) |
| Conditions | THF, –20 to 25 °C, anhydrous | MeOH/H₂O or CH₂Cl₂, 25 °C | DMSO or DMF, 25 °C |
| Stereochemistry | Predictable absolute configuration: hydride to the face set by the chiral boron-N framework | High ee: bulky group shields one iminium face | High ee; Zimmerman–Traxler-like TS with COOH H-bond |
| Rate / selectivity | Hydride transfer is rate-limiting; catalyst turnover fast | Nucleophilic addition to the iminium is rate-limiting | C–C bond formation from enamine is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the CBS reduction (Corey-Bakshi-Shibata)?
(S)- or (R)-Me-CBS oxazaborolidine (5–10 mol%), BH₃·THF or BH₃·SMe₂. Conditions: THF, –20 to 25 °C, anhydrous.
Which bonds are formed and which are broken?
Formed: C–H (hydride); C–O–B. Broken: B–H; C=O π.
What is the stereochemical outcome?
Predictable absolute configuration: hydride to the face set by the chiral boron-N framework
What controls the rate or the selectivity?
Hydride transfer is rate-limiting; catalyst turnover fast
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam