User:Berkeker.22/Sodium cyanoborohydride

Sodium cyanoborohydride is a chemical compound with the formula Na[ BH3(CN)]. It is a colourless salt used in organic synthesis for chemical reduction including that of imines and carbonyls. Sodium cyanoborohydride is a milder reductant than other conventional reducing agents making it more selective.

Structure
Sodium cyanoborohydride is a salt. The cationic sodium ion, [Na]+, interacts with the anionic cyanoborohydride ion, [BH3(CN)]−. The anionic component of the salt is tetrahedral at the boron atom.

The electron-withdrawing cyanide substituent draws electron density away from the negatively charged boron; thus, reducing the electrophilic capabilities of the anionic component. This electronic phenomenon causes sodium cyanoborohydride to have more mild reducing qualities than other reducing agents. For example, Na[BH3(CN)] is less reducing than its counterpart sodium borohydride, containing [BH4]−.

Uses
Sodium cyanoborohydride is a mild reducing agent. It is often used for the reduction of imines and carbonyls.

Reduction of Imines
Imines can be reduced to amines using sodium cyanoborohydride. The reaction will produce two enantiomers in a racemic mixture.

Reductive Amination (Borch Reaction)
Reductive amination is the conversion of a carbonyl into an amine through an intermediate imine. The carbonyl is first treated with ammonia to promote imine formation by nucleophilic attack. The imine is then reduced to an amine by sodium cyanoborohydride. This reaction works on both aldehydes and ketones. The carbonyl can be treated with ammonia, a primary amine, or a secondary amine to produce, respectively, 1°, 2°, and 3° amines. The Borch reaction will produce two enantiomers in a racemic mixture.

Reductive Deoxygenation of Ketones
Aromatic ketones and aldehydes can be reductively deoxygenated using sodium cyanoborohydride. This means that the carbonyl oxygen is being removed completely from the molecule. Deoxygenation using sodium cyanoborohydride is often done in the presence of trimethylsilyl chloride, or TMSCl.

Preparation
Sodium cyanoborohydride can be purchase from most chemical suppliers. It is most commonly synthesized by the following methods:

Preparation from Sodium Cyanide and Diborane
Sodium cyanoborohydride can be synthesized from sodium cyanide and diborane.

This method of preparation can be used for other compounds of the formula RBH3 CN where R is an alkali metal, a quaternary ammonium radical, or a phosphonium radical. The final products are useful as hydrolysis stable reductants and as synthetic intermediates.

Selectivity
Since sodium cyanoborohydride is a mild reducing agent, many functional groups remain inert in its presence. For example, sodium cyanoborohydride is incapable of reducing amides, ethers, lactones, nitriles, and epoxides. Therefore, it can selectively reduce some functionalities in the presence of others.

Some examples of sodium cyanoborohydride include:


 * Reduction of iminium ions in the presence of carbonyls
 * Reduction of aldehydes in the presence of ketones and esters.
 * Reduction of aldehydes in the presence of thiol ester groups

The selectivity of this reducing agent makes it an important tool in organic synthesis. It allows for specific modifications to be made to complex organic molecules.

History
Georg Wittig, the inventor of the Wittig reagent, was the first to synthesize a cyanoborohydride by treating lithium borohydride with hydrogen cyanide in 1951. The corresponding compound, sodium cyanoborohydride, was synthesized following a similar rationale by reacting sodium borohydride with hydrogen cyanide. The synthesis was later refined to use sodium cyanide and borane in THF making the process safer.

Sodium borohydride

 * Chemical structure Na[BH4]
 * Stronger reducing ability

Sodium triacetoxyborohydride

 * Chemical structure Na[BH(OAc)3]
 * Mild reducing ability
 * Reductive amination of aldehydes and ketones
 * Less toxic side products than Na[BH3(CN)] (more safe)

Diisobutylaluminium hydride

 * Often abbreviated as DIBAL-H
 * Mild reducing ability
 * Reduces esters/carboxylic acids to aldehydes

Lithium aluminium hydride

 * Often abbreviated as LAH
 * Strong reducing ability
 * Reduces esters/carboxylic acids to alcohols