Sodium borohydride, a versatile reducing agent
Sodium borohydride is an inorganic compound with white to gray-white microcrystalline powder appearance finding extensive use as a reducing agent across various industries and laboratory settings.
Synthesis and Properties:
Sodium borohydride is typically synthesized through the reaction of sodium hydride with trimethyl borate in an alcohol solvent, followed by purification. It’s soluble in protic solvents such as water and lower alcohols. It also reacts with these protic solvents to produce H2; however, these reactions are fairly slow. Complete decomposition of a methanol solution requires nearly 90 min at 20 °C. It decomposes in neutral or acidic aqueous solutions, but is stable at pH 14.
Synthetic applications:
NaBH4 reduces many organic carbonyls, depending on the conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols, but it can efficiently reduce acyl chlorides, anhydrides, α-hydroxylactones, thioesters, and imines at room temperature or below. It reduces esters slowly and inefficiently with excess reagent and/or elevated temperatures, while carboxylic acids and amides are not reduced at all, although the NaBH4-MeOH system, formed by the addition of methanol to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols.
The reactivity of NaBH4 can be enhanced or augmented by a variety of compounds.
- AlCl3: reduction of ketones to methylene
- BiCl3: converts epoxides to allylic alcohols
- (C6H5Te)2: reduction of nitroarene
- CeCl3: reduction of ketones in the presence of aldehydes
- CoCl2: reduction of azides to amines
- InCl3: hydrogenolysis of alkyl bromides, double reduction of unsaturated ketones
- LiCl: amine oxides to amines
- NiCl2: deoxygenation of sulfoxides, hydrogenolysis of aryl tosylates, desulfurization, reduction of nitriles
- TiCl4: denitrosatation of nitrosamines
- ZnCl2: reduction of aldehydes
- ZrCl4: reduction of disulfides, reduction of azides to amines, cleavage of allyl aryl ethers
- CuCl2: reduction of nitrostyrenes and nitropropenes to amine
- I2: Oxidation with iodine in tetrahydrofuran gives borane–tetrahydrofuran, which can reduce carboxylic acids to alcohols.
Safety Considerations:
While sodium borohydride is generally considered safe to handle when proper precautions are taken, it is essential to be aware of its reactivity with water and acids, which can lead to the release of flammable hydrogen gas. Additionally, it should be handled in a well-ventilated area due to the potential release of toxic boranes during reactions.
