Lithium borohydride stoichiometry

In aprotic solvents, such as dichloromethane and THF, potassium boro-hydride [10], lithium borohydride [8b], lithium triethylborohydride [8a, 11], and diisobutylaluminum hydride (DIBALH) [11] have been used instead of NaBH4. NaBH4 also reduces elemental selenium in alcohol or water to sodium hydrogen selenide or sodium diselenide (Na2Se2) depending on the stoichiometry of the reactants (Scheme 8) [12]. These anions are useful for the synthesis of selenides or diselenides, respectively, via nucleophilic substitution with various electrophiles. 

Gold nanoparticles (2.5-7.5 nm diameter) were obtained by Martino and others [402] by reduction of AUCI4 via lithium borohydride in tetrahydrofuran in the reverse micelle system DDAB (didodecyldimethylammonium bromide)/toluene. A purple-colored gold colloid was observable. The particles were encapsulated by silica gels formed in the micellar system by hydrolysis-condensation of TEOS or a prehydrolyzed TEOS, already added in the system. Gelation of silica was achieved by the addition of tetrabutylammonium hydroxide (quicker in case of pre-hydrolyzed TEOS). The particle size was found to be independent of reaction stoichiometry, gel precursor type (TEOS/pre-hydrolyzed TEOS) and the washing step after synthesis. 

A similar mechanism and stoichiometry underlie reactions of organic compounds with lithium and sodium borohydrides. With modified complex hydrides the stoichiometry depends on the number of hydrogen atoms present in the molecule. 

Unlike sodium borohydride, lithium aluminum hydride reacts violently with water, alcohols, and other protic solvents to liberate hydrogen gas and form metal hydroxides and alkoxides. Therefore, reductions of aldehydes and ketones using this reagent must be carried out in aprotic solvents, most commonly diethyl ether or tetrahydrofuran. The stoichiometry for lithium aluminum hydride reductions is the same as that for sodium borohydride reductions one mole of lithium aluminum hydride per four moles of aldehyde or ketone. Because of the formation of gelatinous aluminum salfs, aqueous acid or base workup is usually used to dissolve them. 

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