Water complex hydrides

Sodium borohydride and potassium borohydride [13762-51 -1] are unique among the complex hydrides because they are stable in alkaline solution. Decomposition by hydrolysis is slow in water, but is accelerated by increasing acidity or temperature. 

Hydrides — True hydrides (i.e., those in which the hydrogen is in its anionic or most reduced form) are salt-like compounds in which the hydrogen is combined with alkali metals, either alone as simple hydrides or in association with other elements as complex hydrides. Hydrides react with water to release hydrogen. 

The presence of water is essential for the success of these reductions. In anhydrous THF, for example, treatment of iV-benzoylimidazole with NaBH4 leads to benzyl benzoate as the main product (73%), along with 19% benzyl alcohol.[33] Other reports, however, describe the conversion of carboxylic acid imidazolides to the corresponding alcohols by complex hydrides in organic solvents. Further alcohols have been synthesized via imidazolides ... 

The reaction of complex hydrides with carbonyl compounds can be exemplified by the reduction of an aldehyde with lithium aluminum hydride. The reduction is assumed to involve a hydride transfer from a nucleophile -tetrahydroaluminate ion onto the carbonyl carbon as a place of the lowest electron density. The alkoxide ion thus generated complexes the remaining aluminum hydride and forms an alkoxytrihydroaluminate ion. This intermediate reacts with a second molecule of the aldehyde and forms a dialkoxy-dihydroaluminate ion which reacts with the third molecule of the aldehyde and forms a trialkoxyhydroaluminate ion. Finally the fourth molecule of the aldehyde converts the aluminate to the ultimate stage of tetraalkoxyaluminate ion that on contact with water liberates four molecules of an alcohol, aluminum hydroxide and lithium hydroxide. Four molecules of water are needed to hydrolyze the tetraalkoxyaluminate. The individual intermediates really exist and can also be prepared by a reaction of lithium aluminum hydride... 

Complex hydrides react with water to give hydrogen, a metal hydroxide and borax [90]. Very high hydrogen densities are reached if the water from the combustion of the hydrogen is reused (Table 5.7). 

The most useful reagents for reducing aldehydes and ketones are the metal hydride reagents. Complex hydrides are the source of hydride ions, and the two most commonly used reagents are NaBlTj and LiAlH4. Lithium aluminium hydride is extremely reactive with water and must be used in an anhydrous solvent, e.g. dry ether. 

It ignites vigorously and often explodes in air [1], or on contact with cold water [2], See other COMPLEX HYDRIDES... 

It is explosively hydrolysed by water. See other COMPLEX HYDRIDES... 

Zinc hydride, a moderately stable sohd slowly decomposed by water, may be prepared by the reaction of LiH, NaH, or LiAlH4 on a zinc halide snch as the bromide or iodide. If organometallic derivatives of zinc of the type Li ZnR +2 are treated with LiAlHi, complex hydrides snch as LiZnHs, Li2ZnH4, and LisZnHs may be prepared. 

Table 2. Values of pK for Representative Metal-Hydride Complexes in Waters ...

Interaction mechanisms of ruthenium hydride complex with water and 2,2,2-trifluoroethanol... 

With 2,4-substituted, bispidones of lower symmetry and analoguous 1,5-dicarboxylic acid esters (e.g., compounds 10-48 in Table 1) the reduction under Wolff-Kishner conditions fails, but reaction with complex hydrides leads to the corresponding bispidoles. The treatment with LiAUTj yields a 1 1 mixture of the epimeric tris alcohols, whereas reduction with NaBELj at ambient temperature in various solvents leads to the epimeric mono-alcohols in different ratios up to epimerically pure compounds (see Scheme 9) (116, 117), which can be further reduced to the tris alcohols with LiAlH4. For example, the reduction of 14 with NaBH4 in dry methanol yields 65% syn- (84b) and 35% anri-product (84a), while the reaction in a mixture of dioxane-water leads exclusively to the anti-configuration of alcohol 84a (117). 

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