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$\text{NaBH}_3\text{CN}$ and $\text{Sia}_2\text{BH}$

Selective reagents for Reductive Amination and Hydroboration.

By chemca Team • Updated Jan 2026

While reagents like $LiAlH_4$ and $NaBH_4$ are general reducing agents, Sodium Cyanoborohydride ($\text{NaBH}_3\text{CN}$) and Disiamylborane ($\text{Sia}_2\text{BH}$) offer high selectivity, allowing chemists to target specific functional groups without affecting others.

1. Sodium Cyanoborohydride ($\text{NaBH}_3\text{CN}$)

Selective Reductive Amination

The electron-withdrawing cyano group ($-CN$) makes the boron-hydride bond stronger and the hydride less nucleophilic than in $NaBH_4$.

• It is stable in acidic media ( pH ~ 3) where $NaBH_4$ would decompose.
• It selectively reduces Iminium ions ($C=N^+$) faster than Aldehydes or Ketones ($C=O$).

Major Application: Reductive Amination
Converts Carbonyl compounds + Amine $\to$ Higher Amine.

$$ R-CHO + R'-NH_2 \xrightarrow{pH \approx 6} [R-CH=NR'] \xrightarrow{\text{NaBH}_3\text{CN}} R-CH_2-NH-R' $$

Aldehyde $\to$ Imine $\to$ Amine. The reagent reduces the Imine formed in situ without reducing the unreacted aldehyde.

2. Disiamylborane ($\text{Sia}_2\text{BH}$)

Sterically Hindered Hydroboration

$\text{Sia}_2\text{BH}$ is a bulky borane reagent. Due to steric hindrance, it stops at the mono-addition stage when reacting with alkynes, preventing further reaction to alkanes.

Major Application: Hydration of Terminal Alkynes to Aldehydes
It performs Anti-Markovnikov addition of water to terminal alkynes.

$$ R-C \equiv C-H \xrightarrow{1. \text{Sia}_2\text{BH}, 2. H_2O_2/OH^-} R-CH_2-CHO \text{ (Aldehyde)} $$

Comparison: Acid-catalyzed hydration ($HgSO_4/H_2SO_4$) of terminal alkynes follows Markovnikov's rule and yields Methyl Ketones. $\text{Sia}_2\text{BH}$ is complementary, yielding Aldehydes.

3. Summary of Selectivity

Reagent	Substrate	Product	Key Feature
$\text{NaBH}_3\text{CN}$	Aldehyde + Amine (Imine)	Amine	Reduces $C=N$ in presence of $C=O$ at pH 6.
$\text{Sia}_2\text{BH}$	Terminal Alkyne	Aldehyde	Anti-Markovnikov Hydration. Sterically hindered.
$NaBH_4$	Aldehyde/Ketone	Alcohol	Cannot do reductive amination well (reduces aldehyde too fast).
$Hg^{2+}/H^+$	Terminal Alkyne	Methyl Ketone	Markovnikov Hydration.

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