Triethylborane is prepared by the reaction of trimethyl borate with triethylaluminium: The molecule is monomeric, unlike H3B and Et3Al, which tend to dimerize. It has a planar BC3 core.
Applications
Turbojet engine
Triethylborane was used to ignite the JP-7fuel in the Pratt & Whitney J58 turbojet/ramjet engines powering the Lockheed SR-71, 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.
Industrially, triethylborane is used as an initiator in radical reactions, where it is effective even at low temperatures. 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. 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 enolate, whilst reaction at −78 °C with potassium hexamethyldisilazide, and triethylborane generates the less substituted 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. 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. Triethylborane is the precursor to the reducing agentslithium triethylborohydride and sodium triethylborohydride. Triethylborane reacts with methanol to form diethylborane, which is used as the chelating agent in the Narasaka-Prasad reduction for the stereoselective generation of syn-1,3-diols from β-hydroxyketones.
