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Sodium Borohydride's Reactivity

        NaBH4 will reduce many organic carbonyls, depending on the precise conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. It will reduce acyl chlorides, thiol esters and imines. However, unlike the powerful reducing agent lithium aluminium hydride, NaBH4 typically will not reduce esters, amides, or carboxylic acids. At room temperature, the only acid derivatives it reduces are acyl chlorides, which have exceptionally high reactivity. With high temperature and concomitant high pressure, sodium borohydride can be forced to react with esters, for example.
        Many other hydride reagents are more strongly reducing. These usually involve replacing hydride with alkyl groups, such as lithium triethylborohydride and L-Selectride (lithium tri-sec-butylborohydride), or replacing B with Al. Variations in the counterion also affect the reactivity of the borohydride.
        Ball-and-stick model of Zr(BH4)4Oxidation of NaBH4 with iodine in tetrahydrofuran gives the BH3-THF complex, which can reduce carboxylic acids. Likewise, the NaBH4-MeOH system, formed by the addition of methanol to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols, for instance, benzyl benzoate to benzyl alcohol. Mixing water or an alcohol with the borohydride converts some of it into unstable hydride ester, which is more efficient at reduction, but the reductant will eventually decompose spontaneously to give hydrogen gas and borates. The same reaction can run also intramolecularly: an α-ketoester converts into a diol, since the alcohol produced will attack the borohydride to produce an ester of the borohydride, which then reduces the neighboring ester.
        Aqueous solutions of sodium borohydride are decomposed by catalytic amounts of cobalt(II) ions to yield sodium borate and hydrogen gas. Pellets of cobalt-doped sodium borohydride are commercially available for use in hydrogen generators, for applications where cylinders of hydrogen would be inconvenient.
        BH4− is an excellent ligand for metal ions. Such borohydride complexes are often prepared by the action of NaBH4 (or the LiBH4) on the corresponding metal halide. One example is the titanocene derivative:

2 (C5H5)2TiCl2 + 4 NaBH4 → 2 (C5H5)2TiBH4 + 4 NaCl + B2H6 + H2