Borohydride tetraalkylammonium

For many years, prior to the development of current phase-transfer catalytic techniques, tetraalkylammonium borohydrides have been used in non-hydroxylic solvents [see, e.g. I, 2], Originally, the quaternary ammonium borohydrides were obtained by metathesis in water or an alcohol [3, 4], However, with greater knowledge of the phase-transfer phenomenon, an improved procedure has been developed in which the ammonium salt is transferred into, and subsequently isolated from, dichloromethane [5, 6], In principle, it should be possible to transfer the quaternary ammonium borohydride for use in any non-miscible organic solvent. It should be noted, however, that quaternary ammonium cations are susceptible to hydrogeno-lysis by sodium borohydride in dipolar aprotic solvents to yield tertiary amines [4]. 

Stoichiometric reduction of carbonyl compounds with tetraalkylammonium borohydrides... 

Tetraalkylammonium borohydrides react with elemental selenium and tellurium to form the symmetrical dialkyl selenides and tellurides [44]. Diphenyl selenide yields the alkyl phenyl selenides. 

Diastereocontrolled reduction of amino ketones represents an attractive route to amino alcohols, many of which are pharmacologically important, and has been exhaustively reviewed. Even in the case of a-amino ketones, examples of high stereoselectivity were rare with conventional metal hydride reagents, and mixtures were common as the amino group became more distant. In contrast, a-triazolyl ketones (64) were reduced with high stereoselectivity by tetraalkylammonium borohydrides to the syn-alcohols (63) in dichloromethane or to the anti isomers (65) when titanium tetrachloride was added (Scheme 10). ... 

Preparation. Tetraalkylammonium borohydrides can be prepared by addition of a slight excess of sodium borohydride to a solution or suspension of a tetraalkylammonium hydrogen sulfate in an aqueous solution of NaOH. The resulting tetraalkylammonium borohydride is extracted with methylene chloride. The solid salt can be obtained by evaporation of the methylene chloride and crystallization from ethyl acetate. These salts are mild reducing agents. They are converted into diborane and a tetraalkylammonium halide by treatment in methylene chloride with an alkyl halide (methyl iodide, ethyl bromide). The advantage of generation of diborane in this way is that anhydrous methylene chloride is easily obtained. 

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