Borohydride
Borohydride refers to the anion BH4− and its salts. Borohydride is also the term used for compounds containing BH4−nXn−, for example cyanoborohydride (B(CN)H3−) and triethylborohydride (B(C2H5)3H−). Such compounds find wide use as reducing agents in organic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, but other salts are well known (see Table). Tetrahydroborates are also of academic and industrial interest in inorganic chemistry.
Alkali metal borohydrides were described in 1940 by Hermann Irving Schlesinger and Herbert C. Brown. They synthesized lithium borohydride (LiBH4) from diborane (B2H6):
Current methods involve reduction of trimethyl borate with sodium hydride.
The borohydride anion has a tetrahedral structure with boron in the center and the hydrogens located at the four vertices of the tetrahedron. In metal complexes, the borohydride ion is bound to the metal by means of bridging hydrogen atoms, usually in one of three different ways: monodentate (one hydrogen bridge), bidentate (two hydrogen bridges), and tridentate (three hydrogen bridges) (η1, η2, or η3). The preferred coordination mode is strongly affected by the nature of the metal and its oxidation state.
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Sodium borohydride is the borohydride that is produced on the largest scale industrially, estimated at 5000 tons/y in 2002. The main use is for the reduction of sulfur dioxide to give sodium dithionite:
Dithionite is used to bleach wood pulp. Sodium borohydride is also used to reduce aldehydes and ketones in the production of pharmaceuticals including chloramphenicol, thiophenicol, vitamin A, atropine, and scopolamine, as well as many flavorings and aromas.
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