Sodium triacetoxyborohydride (STAB) is a mild, selective borohydride reducing agent widely used in organic synthesis, particularly for the reductive amination of aldehydes and ketones. It is a white, free-flowing powder that is freely soluble in aprotic solvents such as benzene, tetrahydrofuran, acetonitrile and 1,2-dichloroethane, but it reacts rapidly with water and protic solvents. STAB enables the conversion of carbonyl compounds and amines into secondary and tertiary amines under mild conditions, with good functional-group tolerance, and it is a practical alternative to sodium cyanoborohydride because it avoids the toxic by-products of cyanide-based reagents.
What Is Sodium Triacetoxyborohydride (STAB)?
STAB, with the CAS number 56553-60-7 and the molecular formula C6H10BNaO6, is prepared from sodium borohydride by replacing the hydride hydrogens with acetoxy groups. It is a hydride-delivery reagent in which the three acetoxy groups moderate the reducing power, making it far milder than sodium borohydride itself. In reductive amination, STAB acts as a stoichiometric reducing agent rather than a catalyst: it selectively reduces the iminium or imine intermediate formed between the carbonyl compound and the amine, without reducing the carbonyl group itself under typical conditions.
Solubility and Stability
Solubility: freely soluble in benzene, tetrahydrofuran, acetonitrile and 1,2-dichloroethane; sparingly soluble in many other solvents.
Moisture sensitivity: STAB reacts with water and alcohols, decomposing to release hydrogen and boric acid derivatives, so it must be handled under anhydrous conditions.
Storage: keep in a tightly sealed container under an inert atmosphere in a cool, dry place, away from moisture and oxidizing agents.
Key Applications in Organic Synthesis
Reductive amination: converts aldehydes and ketones with primary or secondary amines into secondary and tertiary amines under mild conditions, typically in a solvent such as THF or 1,2-dichloroethane with a mild acid additive.
Lactamization: used in reductive amination of carbonyl compounds and amines that cyclize to lactams.
Aromatic aldehyde amination: effective for the reductive amination of aromatic aldehydes, including hindered substrates.
Selective carbonyl reduction: reduces activated carbonyls in complex molecules while tolerating many other reducible groups, which is valuable in multi-step synthesis and medicinal chemistry.
Compared with sodium cyanoborohydride, STAB offers comparable selectivity with a much more favorable safety profile, and compared with sodium borohydride it avoids over-reduction of carbonyls in reductive amination, making it a first-choice reagent for this transformation.
Preparation of Sodium Triacetoxyborohydride
A typical laboratory preparation suspends sodium borohydride in an aprotic solvent such as benzene or tetrahydrofuran, adds glacial acetic acid, and then slowly drops in acetic anhydride. The mixture is stirred until the reaction is complete, and the resulting solid is collected by filtration, washed with tetrahydrofuran and dried under vacuum. When working at larger scale, tetrahydrofuran or another less hazardous solvent is preferred over benzene, which is a regulated carcinogen in many countries.
Safety and Handling Considerations
STAB is irritating to the skin, eyes and respiratory tract, and contact should be avoided by wearing protective gloves and goggles. Because the reagent is moisture-sensitive and evolves hydrogen on contact with water, work should be carried out in a fume hood away from ignition sources, and waste should be quenched carefully by slow addition to a large volume of cooled water or dilute acid in a ventilated area. Store the reagent under inert gas in an airtight container and keep it separate from water, alcohols and strong oxidizing agents.
Frequently Asked Questions
What is sodium triacetoxyborohydride used for?
STAB is used primarily for the reductive amination of aldehydes and ketones, and for the selective reduction of activated carbonyl compounds in organic synthesis.
Is STAB a catalyst?
No. STAB is a stoichiometric reducing agent: it is consumed in the reaction as it donates hydride, so it is not a catalyst.
Is STAB soluble in water?
No. STAB reacts with water, decomposing rather than dissolving, so it must be used in anhydrous aprotic solvents.
How does STAB differ from sodium borohydride?
STAB is much milder than sodium borohydride because the three acetoxy groups reduce its hydride-donating power, allowing selective reduction of iminium intermediates while leaving most carbonyl groups untouched.
Why use STAB instead of sodium cyanoborohydride?
STAB gives similar reductive-amination selectivity but avoids the generation of toxic cyanide by-products and the associated waste-disposal burden, which is why it is often preferred in modern practice.
How should STAB waste be handled?
Quench waste by slow addition to excess cooled water or dilute acid in a ventilated area, allow the hydrogen to dissipate, and dispose of the aqueous residue according to local regulations.