Chemistry:Sodium cyanoborohydride

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Sodium cyanoborohydride
Line-bond structure of sodium cyanoborohydride
Names
IUPAC name
Sodium cyanoboranuide
Other names
Sodium cyanotrihydridoborate
Identifiers
3D model (JSmol)
EC Number
  • 247-317-2
UNII
Properties
Na[BH
3(CN)]
Molar mass 62.84 g·mol−1
Appearance white powder, hygroscopic
Density 1.083 g/cm (25°C)3
Melting point 242 °C (468 °F; 515 K) decomposes
212 g/(100 mL) (29 °C)
Solubility soluble in water, ethanol, diglyme, tetrahydrofuran, methanol
slightly soluble in methanol
insoluble in diethyl ether
Structure
4 at boron atom
Tetrahedral at boron atom
Hazards
Main hazards Flammable solid, fatal if swallowed, in contact with skin or if inhaled
Contact with acids liberates very toxic gas
Contact with water liberates highly flammable gas
Safety data sheet Sigma Aldrich[1]
GHS pictograms GHS02: FlammableGHS05: CorrosiveGHS06: Toxic
GHS Signal word Danger
H228, H300, H310, H330, H314, H410
P210, P260, P264, P273, P280, P284
NFPA 704 (fire diamond)
Flammability code 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g. gasolineHealth code 4: Very short exposure could cause death or major residual injury. E.g. VX gasReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazards (white): no codeNFPA 704 four-colored diamond
3
4
2
5 mg/m3 (TWA)
Related compounds
Other anions
 Sodium borohydride
Related compounds
 Lithium aluminium hydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Sodium cyanoborohydride is a chemical compound with the formula Na[[[Boron|B]]H
3(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.[2]

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.[2] 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]−.[2]

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.

Imine to amine reduction using sodium cyanoborohydride.

Reductive amination (Borch reaction)

Reductive amination, sometimes called the Borch reaction, is the conversion of a carbonyl into an amine through an intermediate imine.[3] 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.[4]

Reductive amination using sodium cyanoborohydride.

Reductive deoxygenation of ketones

Aromatic ketones and aldehydes can be reductively deoxygenated using sodium cyanoborohydride.[5] 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.[5]

Reductive deoxygenation of a ketone using sodium cyanoborohydride.

Preparation

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

From sodium cyanide and diborane

Sodium cyanoborohydride can be synthesized from sodium cyanide and diborane.[6]

This method of preparation can be used for other compounds of the formula RBH3CN where R is an alkali metal, a quaternary ammonium radical, or a phosphonium radical.[6] The final products are useful as hydrolysis stable reductants and as synthetic intermediates.[6]

Preparation sodium cyanoborohydride from sodium cyanide and diborane.

Selectivity

Since sodium cyanoborohydride is a mild reducing agent, it gives good chemoselectivity for reaction with certain functional groups in the presence of others. For example, sodium cyanoborohydride is generally incapable of reducing amides, ethers, esters and lactones, nitriles, or epoxides.[7] Therefore, it can selectively reduce some functionalities in the presence of others.

Some examples of selective reduction include:

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 was the first to synthesize a cyanoborohydride by treating lithium borohydride with hydrogen cyanide in 1951.[7] The corresponding compound, sodium cyanoborohydride, was synthesized following a similar rationale by reacting sodium borohydride with hydrogen cyanide.[9] The synthesis was later refined to use sodium cyanide and borane in THF making the process safer.[9]

See also

References

  1. ↑ Sigma-Aldrich Co., Sodium cyanoborohydride. Retrieved on 2014-11-09.
  2. ↑ 2.0 2.1 2.2 Baxter, Ellen W.; Reitz, Allen B. (9 January 2002). "Reductive Aminations of Carbonyl Compounds with Borohydride and Borane Reducing Agents". Organic Reactions: 1–714. doi:10.1002/0471264180.or059.01. ISBN 0-471-26418-0. 
  3. ↑ Richard F. Borch (1988). "Reductive Amination with Sodium Cyanoborohydride: N,N-Dimethylcyclohexylamine". Organic Syntheses. http://www.orgsyn.org/demo.aspx?prep=CV6P0499. ; Collective Volume, 6, pp. 499 
  4. ↑ Richard F. Borch and Mark D. Bernstein and H. Dupont Durst (1971). "Cyanohydridoborate Anion as a Selective Reducing Agent". J. Am. Chem. Soc. 93 (12): 2897–2904. doi:10.1021/ja00741a013. 
  5. ↑ 5.0 5.1 Box, Vernon G. S.; Meleties, Panayiotis C. (1998-09-24). "Reductive, selective deoxygenation of acylbenzo[bfurans, aromatic aldehydes and ketones with NaBH3CN-TMSCl"]. Tetrahedron Letters 39 (39): 7059–7062. doi:10.1016/S0040-4039(98)01519-6. ISSN 0040-4039. https://www.sciencedirect.com/science/article/pii/S0040403998015196. 
  6. ↑ 6.0 6.1 6.2 Hui, Benjamin C. (October 1980). "Synthesis and properties of borohydride derivatives" (in en). Inorganic Chemistry 19 (10): 3185–3186. doi:10.1021/ic50212a075. ISSN 0020-1669. https://pubs.acs.org/doi/abs/10.1021/ic50212a075. 
  7. ↑ 7.0 7.1 7.2 7.3 LANE, Clinton F. (1975). "Sodium Cyanoborohydride - A Highly Selective Reducing Agent for Organic Functional Groups". Synthesis 1975 (3): 135–146. doi:10.1055/s-1975-23685. ISSN 0039-7881. http://dx.doi.org/10.1055/s-1975-23685. 
  8. ↑ Paul, Avishek; Shipman, Michael A.; Onabule, Dolapo Y.; Sproules, Stephen; Symes, Mark D. (2021-04-15). "Selective aldehyde reductions in neutral water catalysed by encapsulation in a supramolecular cage" (in en). Chemical Science 12 (14): 5082–5090. doi:10.1039/D1SC00896J. ISSN 2041-6539. PMID 34163748. 
  9. ↑ 9.0 9.1 Abdel-Magid, Ahmed F., ed (1996-08-13) (in en). Reductions in Organic Synthesis: Recent Advances and Practical Applications. ACS Symposium Series. 641. Washington, DC: American Chemical Society. doi:10.1021/bk-1996-0641.ch001. ISBN 978-0-8412-3381-2. https://pubs.acs.org/doi/book/10.1021/bk-1996-0641. 



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