Borohydride, lithium

Lithium Borohydride. Lithium borohydride [16949-15-8] LlBH, is made by metathesis between sodium borohydride and lithium chloride (20) ia isopropylamine. 

After the amine is removed, a single extraction with ethyl ether is sufficient to provide a 98% pure product ia ca 75% yield. Lithium borohydride is a hygroscopic, white powder that decomposes slowly at its melting poiat, evolving hydrogen. The heat of formation is —190 kJ/mol (—45.4 kcal/mol). 

Unlike many other borohydrides, lithium borohydride is highly soluble ia ethers including aUphatic ethers, THF, an d polyglycol ethers. It is also very soluble ia amines and ammonia. Dissolution ia water and lower aUphatic alcohols leads to extensive decomposition and hydrogen evolution. 

Lithium borohydride contains 18.5% hydrogen by weight and, on complete hydrolysis, Hberates 4.1 L (STP) hydrogen per gram. 

Lithium borohydride is a more powerful reducing agent than sodium borohydride, but not as powerful as lithium aluminum hydride (Table 6). In contrast to sodium borohydride, the lithium salt, ia general, reduces esters to the corresponding primary alcohol ia refluxing ethers. An equimolar mixture of sodium or potassium borohydride and a lithium haUde can also be used for this purpose (21,22). 

Phenylstibine [58266-50-5] C H Sb, has been obtained by the reduction of phenyldiio do stihine [68972-61-2] CgH3l2Sb, (73) or phenyldichlorostibine [5035-52-9] 031130.2, (74) with lithium borohydride. It has also been prepared by the hydrolysis or methanolysis of phenylbis(trimethylsilyl)stibine [82363-95-9] C22H23Si2Sb (75). Diphenylstibine [5865-81-6] C22H22Sb, can be prepared by the interaction of diphenylchlorostibine [2629-47-2] C22H2QClSb, with either Hthium borohydride (76) or lithium aluminum hydride (77). It is also formed by hydrolysis or methanolysis of diphenyl (trimethylsilyl)stibine [69561-88-2] C H SbSi (75). Dimesitylstibine [121810-02-4] h.3.s been obtained by the protonation of lithium dimesityl stibide with trimethyl ammonium chloride (78). The x-ray crystal stmcture of this secondary stibine has also been reported. 

Good yields of phenylarsine [822-65-17, C H As, have been obtained by the reaction of phenylarsenic tetrachloride [29181-03-17, C H AsCl, or phenyldichloroarsine [696-28-6], C3H3ASCI25 with lithium aluminum hydride or lithium borohydride (41). Electrolytic reduction has also been used to convert arsonic acids to primary arsines (42). Another method for preparing primary arsines involves the reaction of arsine with calcium and subsequent addition of an alkyl haUde. Thus methylarsine [593-52-2], CH As, is obtained in 80% yield (43) ... 

Enamines of A" -3-ketones (45) are stable to lithium aluminum hydride, but lithium borohydride reduces the 3,4-double bond of the enamine system." In the presence of acetic acid the enamine (45) is reduced by sodium borohydride to the A -3-amine (47) via the iminium cation (46). ... 

Semicarbazones are used as protecting groups as a consequence of their stability to reducing agents such as potassium borohydride, sodium boro-hydride and lithium borohydride. Semicarbazones are cleaved by strong acids and by heating in acetic anhydride-pyridine. " ... 

The success of the halo ketone route depends on the stereo- and regio-selectivity in the halo ketone synthesis, as well as on the stereochemistry of reduction of the bromo ketone. Lithium aluminum hydride or sodium borohydride are commonly used to reduce halo ketones to the /mm-halohydrins. However, carefully controlled reaction conditions or alternate reducing reagents, e.g., lithium borohydride, are often required to avoid reductive elimination of the halogen. 

If the lithium borohydride sticks to the spatula or powder funnel, additional methanol can be used to wash the material into the reaction mixture without affecting the yield. On this reaction scale, an additional 25-50 mL of methanol could be used. Using 2.0M LiBH4 in THF instead of solid LiBH4 complicates the following step resulting in lower yields. 

Lithium hydride added diborane to form lithium borohydride (7). 

Davis, W. D., L. S. Mason, and G. Siegeman The Heats of Formation of Sodium Borohydride, Lithium Borohydride and Lithium Aluminium Hydride. J. Amer. chem. Soc. 71, 2775 (1949). 

Olsen, L., and R. T. Sanderson The Reaction of Aluminum Chloride with Lithium Borohydride. J. Inorg. Nucl. Chem. 7, 228 (1958). 

Schaeffer, G. W., J. S. Roscoe, and A. G. Stewart The Reduction of Iron(III) Chloride with Lithium Aluminohydride and Lithium Borohydride Iron(II) Borohydride. J. Amer. chem. Soc. 78, 729 (1956). 

Stewart, A. C., and G. W. Schaeffer The Reaction of Cobalt(II) Bromide with Lithium Borohydride and Lithium Alumohydride. J. Inorg. Nucl. Chem. 3, 194 (1956). 

A suspension of 414 in aquoeus dioxane was treated with proline 433a and iV-phenyl isatin 432a and the reaction mixture was heated at 80-90 °C overnight. The resin-containing cycloaddition product was reduced in aqueous tetrahydrofuran with lithium borohydride for 12 h at room temperature to afford a mixture of mainly 437 and a trace of 438 unlike in solution phase where in equal amount of 437 and 438 were produced (Scheme 99). 

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