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Borane–tetrahydrofuran (BH3–THF) is a charge-transfer complex that is a useful surrogate for diborane1 in organic synthesis. It can be used to reduce carboxylic acids to alcohols or nitriles to primary amines. It reacts with olefins to add the BH2 functional group. Alkyl- or arylboranes formed in this way can further react with unsaturated compounds such as olefins, imines, ketones, and alkynes (the hydroboration reaction) to make useful boron-containing intermediates.
Charge-transfer complexes such as BH3–THF form when a Lewis base (THF in this case) “donates” its unshared electron pair to an electron-deficient Lewis acid such as borane. The electrostatic attraction between the components is not as strong as a covalent bond; but in many cases it is sufficient to make the complex behave as a covalent compound. As an example, BH3–THF can be distilled without decomposing.
BH3–THF’s relative stability makes it a convenient way to use extremely toxic and flammable diborane. But it is not without its own risks. In particular, it reacts violently with water to produce copious amounts of hydrogen. As a result, BH3–THF is normally sold as a 1 M (≈10 wt%) solution in THF. It is available from numerous chemical supply houses worldwide.
1. Under standard conditions, borane dimerizes to produce the more electronically stable diborane.
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