Triethylborane

Triethylborane (TEB), also called triethylboron, is an organoborane (a compound with a B-C bond). It is a colorless pyrophoric liquid. Its chemical formula is (C2H5)3B, abbreviated Et3B. It is soluble in organic solvents tetrahydrofuran and hexane.

Not to be confused with triethyl borate.
Triethylborane
Triethylborane
Ball-and-stick model of triethylborane
Names
IUPAC name
Triethylborane
Other names
Triethylborine, triethylboron
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.002.383
EC Number
  • 202-620-9
UNII
Properties
C6H15B
Molar mass 98.00 g/mol
Appearance Colorless to pale yellow liquid
Density 0.677 g/cm3
Melting point −93 °C (−135 °F; 180 K)
Boiling point 95 °C (203 °F; 368 K)
Not applicable; highly reactive
Hazards
Main hazards Spontaneously flammable in air; causes burns
Safety data sheet External SDS
R-phrases (outdated) R11 R14/15 R17 R19 R34 R35 R36/37
S-phrases (outdated) S6 S7/8 S16 S33 S36/37/39 S43A S45 S29
NFPA 704 (fire diamond)
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g. propaneHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 4: Readily capable of detonation or explosive decomposition at normal temperatures and pressures. E.g. nitroglycerinSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
4
3
4
Flash point < −20 °C (−4 °F; 253 K)
−20 °C (−4 °F; 253 K)
Related compounds
Related compounds
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

Preparation and structure

Triethylborane is prepared by the reaction of trimethyl borate with triethylaluminium:[1]

Et3Al + (MeO)3B → Et3B + (MeO)3Al

The molecule is monomeric, unlike H3B and Et3Al, which tend to dimerize. It has a planar BC3 core.[1]

Applications

Turbojet engine

Triethylborane was used to ignite the JP-7 fuel in the Pratt & Whitney J58 turbojet/ramjet engines powering the Lockheed SR-71,[2] and its predecessor A-12 OXCART. Triethylborane is suitable for this because of its pyrophoric properties, especially the fact that it burns with a very high temperature. It was chosen as an ignition method for reliability reasons, and in the case of the Blackbird, because the JP-7 fuel has very low volatility and is difficult to ignite. Conventional ignition plugs posed a high risk of malfunction. It was used to start each engine and to ignite the afterburners.[3]

Rocket

Mixed with 10–15% triethylaluminium, it was used before lift-off to ignite the F-1 engines on the Saturn V rocket.[4]

The SpaceX Falcon 9 rocket also uses a triethylaluminium-triethylborane mixture as a first and second stage ignitor.[5]

Organic chemistry

Industrially, triethylborane is used as an initiator in radical reactions, where it is effective even at low temperatures.[1] As an initiator, it can replace some organotin compounds.

It reacts with metal enolates, yielding enoxytriethylborates that can be alkylated at the α-carbon atom of the ketone more selectively than in its absence. For example, the enolate from treating cyclohexanone with potassium hydride produces 2-allylcyclohexanone in 90% yield when triethylborane is present. Without it, the product mixture contains 43% of the mono-allylated product, 31% di-allylated cyclohexanones, and 28% unreacted starting material.[6] The choice of base and temperature influences whether the more or less stable enolate is produced, allowing control over the position of substituents. Starting from 2-methylcyclohexanone, reacting with potassium hydride and triethylborane in THF at room temperature leads to the more substituted (and more stable) enolate, whilst reaction at −78 °C with potassium hexamethyldisilazide, KN[Si(CH
3)
3]
2 and triethylborane generates the less substituted (and less stable) enolate. After reaction with methyl iodide the former mixture gives 2,2-dimethylcyclohexanone in 90% yield while the latter produces 2,6-dimethylcyclohexanone in 93% yield.[6][7]

It is used in the Barton–McCombie deoxygenation reaction for deoxygenation of alcohols. In combination with lithium tri-tert-butoxyaluminum hydride it cleaves ethers. For example, THF is converted, after hydrolysis, to 1-butanol. It also promotes certain variants of the Reformatskii reaction.[8]

Triethylborane is the precursor to the reducing agents lithium triethylborohydride ("Superhydride") and sodium triethylborohydride.[9]

MH + Et3B → MBHEt3 (M = Li, Na)

Triethylborane reacts with methanol to form diethyl(methoxy)borane, which is used as the chelating agent in the Narasaka–Prasad reduction for the stereoselective generation of syn-1,3-diols from β-hydroxyketones.[10][11]

Safety

Triethylborane is strongly pyrophoric, with an autoignition temperature of −20 °C (−4 °F),[12] burning with an apple-green flame characteristic for boron compounds. Thus, it is typically handled and stored using air-free techniques.

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See also

References
  1. Brotherton, Robert J.; Weber, C. Joseph; Guibert, Clarence R.; Little, John L. (15 June 2000). "Boron Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a04_309.
  2. "Lockheed SR-71 Blackbird". March Field Air Museum. Archived from the original on 2000-03-04. Retrieved 2009-05-05.
  3. "Lockheed SR-71 Blackbird Flight Manual". www.sr-71.org. Retrieved 2011-01-26.
  4. A. Young (2008). The Saturn V F-1 Engine: Powering Apollo Into History. Springer. p. 86. ISBN 0-387-09629-9.
  5. Mission Status Center, June 2, 2010, 1905 GMT, SpaceflightNow, accessed 2010-06-02, Quotation: "The flanges will link the rocket with ground storage tanks containing liquid oxygen, kerosene fuel, helium, gaseous nitrogen and the first stage ignitor source called triethylaluminum-triethylborane, better known as TEA-TEB."
  6. Crich, David, ed. (2008). "Enoxytriethylborates and Enoxydiethylboranes". Reagents for Radical and Radical Ion Chemistry. Handbook of Reagents for Organic Synthesis. 11. John Wiley & Sons. ISBN 9780470065365.
  7. Negishi, Ei-ichi; Chatterjee, Sugata (1983). "Highly regioselective generation of "thermodynamic" enolates and their direct characterization by NMR". Tetrahedron Letters. 24 (13): 1341–1344. doi:10.1016/S0040-4039(00)81651-2.
  8. Yamamoto, Yoshinori; Yoshimitsu, Takehiko; Wood, John L.; Schacherer, Laura Nicole (15 March 2007). "Triethylborane". Encyclopedia of Reagents for Organic Synthesis. Wiley. doi:10.1002/047084289X.rt219.pub3.
  9. Binger, P.; Köster, R. (1974). "Sodium triethylhydroborate, sodium tetraethylborate, and sodium triethyl-1-propynylborate". Inorganic Syntheses. 15: 136–141. doi:10.1002/9780470132463.ch31.
  10. Chen, Kau-Ming; Gunderson, Karl G.; Hardtmann, Goetz E.; Prasad, Kapa; Repic, Oljan; Shapiro, Michael J. (1987). "A Novel Method for the In situ Generation of Alkoxydialkylboranes and Their Use in the Selective Preparation of 1,3-syn Diols". Chemistry Letters. 16 (10): 1923–1926. doi:10.1246/cl.1987.1923.
  11. Yang, Jaemoon (2008). "Diastereoselective Syn-Reduction of β-Hydroxy Ketones". Six-Membered Transition States in Organic Synthesis. John Wiley & Sons. pp. 151–155. ISBN 9780470199046.
  12. Fuels and Chemicals - Autoignition Temperatures
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