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 or, abbreviated . It is soluble in organic solvents tetrahydrofuran and hexane.
Triethylborane is prepared by the reaction of trimethyl borate with triethylaluminium:
Et3Al + (MeO)3B → Et3B + (MeO)3AlThe molecule is monomeric, unlike H3B and Et3Al, which tend to dimerize. It has a planar BC3 core.
Triethylborane was used to ignite the JP-7 fuel in the Pratt & Whitney J58 turbojet/ramjet engines powering the Lockheed SR-71 Blackbird[1] and its predecessor, the A-12 OXCART. Triethylborane is suitable because it ignites readily upon exposure to oxygen. It was chosen as an ignition method for reliability reasons, and in the case of the Blackbird, because JP-7 fuel has very low volatility and is difficult to ignite. Conventional ignition plugs posed a high risk of malfunction. Triethylborane was used to start each engine and to ignite the afterburners.[2]
Mixed with 10–15% triethylaluminium, it was used before lift-off to ignite the F-1 engines on the Saturn V rocket.[3]
The Merlin engines that power the SpaceX Falcon 9 rocket use a triethylaluminium-triethylborane mixture (TEA-TEB) as a first- and second-stage ignitor.[4]
The Firefly Aerospace Alpha launch vehicle's Reaver engines are also ignited by a triethylaluminium-triethylborane mixture.[5]
Industrially, triethylborane is used as an initiator in radical reactions, where it is effective even at low temperatures.[6] 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.[7] 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, 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.[8] The Et stands for ethyl group .
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.[9]
Triethylborane is the precursor to the reducing agents lithium triethylborohydride ("Superhydride") and sodium triethylborohydride.[10]
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.[11] [12]
Triethylborane is strongly pyrophoric, with an autoignition temperature of,[13] burning with an apple-green flame characteristic for boron compounds. Thus, it is typically handled and stored using air-free techniques. Triethylborane is also acutely toxic if swallowed, with an of 235 mg/kg in rat test subjects.[14]