Hydride LiAlH

Lithium aluminium hydride, LiAlH, is a very active reducing agent, and is used particularly for the ready reduction of carboxylic acids (or their esters) to primary alcohols R-COOH -> R CH,OH. 

Lithium aluminium hydride LiAlH is a useful and conveuient reagent for the selective reduction of the carbonyl group and of various other polar functional groups. It is obtained by treatment of finely powdered lithium hydride with an ethereal solution of anhydrous aluminium chloride ... 

The final step is to convert the carboxylic acid into a primary alcohol by heating it with lithium aluminium hydride (LiAlH ) dissolved in ether (ethoxyethane). This is a reduction reaction and delivers the target molecule, propan-l-ol. 

Dimethoxyamphetamine. A solution of the above nitropropene (17.0 g) in 500 ml of dry ether is added slowly to a stirred solution of lithium aluminum hydride (LiAlH or LAH) 12 g suspended in 150 ml of dry ether. After completing the addition, the mixture is refluxed for 20 hours, then cooled. The excess LAH is decomposed by careful addition of water. The resulting suspension is filtered and the solid removed is washed with ether. The combined ether solutions are dried with MgS04, and then saturated with dry hydrogen chloride. This precipitates the title compound which is filtered and recrystallized from ethanol. Yield 13 g, mp 111.5-112.5°. 

The reduction of carboxylic acids or esters requires very powerful reducing agents such as lithium aluminum hydride (LiAlH,) or sodium (Na) metal. Aldehydes and ketones are easier to reduce, so they can use sodium borohy-dride (NaBH,j). Examples of these reductions are shown in Figure 3-13. 

The azide ion is a better nucleophile than amines, but it has to be reduced to the cimine cifter nucleophilic substitution. Lithium aluminum hydride (LiAlH ) in ether followed by treatment with water reduces the azide ion to the amine. Figures 13-11 and 13-12 illustrate two examples of this reaction. 

A number of organic species, including amides, oximes, and nitriles, undergo reductive amination, a variety of reduction reactions that produce cimines. In general, these processes involve imines, R=N-R, or related species. Reduction processes include hydrogenation using Raney nickel as the catalyst (for nitriles), the reaction with sodium/EtOH (for oximes), and the use of lithium aluminum hydride, LiAlH (for amides or nitriles). Figure 13-16 illustrates the preparation of amphetamine by reductive amination. 

Pyridine is difficult to reduce (as is benzene ), but pyridinium salts, e.g. alkylpyridinium halides, are partly reduced by hydride transfer reagents such as lithium aluminium hydride (LiAlH ) and sodium borohydride (NaBH4). LiAlH, which must be used in anhydrous conditions, only gives the 1,2-dihydro derivative, but the less vigorous reductant NaBH in aqueous ethanol yields the 1,2,5,6-tetrahydro derivative (Scheme 2.30)1... 

Lithium aluminum hydride, LiAlH, in anhydrous ether can also be used. 

Acid chlorides, R(Ar)COCl, are reduced to R(Ar)CHO by Hj/Pd(S), a moderate catalyst that does not reduce RCHO to RCHjOH (Rosenmund reduction). Acid chlorides, esters (R(Ar)COOR), and nitriles (RC N) are reduced with lithium tri-t-butoxyaluminum hydride, LiAlH[OC(CH3)3]j, at very low temperatures, followed by HjO. The net reaction is a displacement of X by H",... 

Aldehydes are prepared by the hydroboration-oxidation of alkynes (see Section 5.3.1) or selective oxidation of primary alcohols (see Section 5.7.9), and partial reduction of acid chlorides (see Section 5.7.21) and esters (see Section 5.7.22) or nitriles (see Section 5.7.23) with lithium tri-terr-butox-yaluminium hydride [LiAlH(0- Bu)3] and diisobutylaluminium hydride (DIBAH), respectively. 

Sterically bulky reducing agents, e.g. lithium tri-t-butoxyaluminium hydride, can selectively reduce acid chlorides to aldehydes at low temperatures (—78 °C). Lithium tri-t-butoxyaluminium hydride, LiAlH(0-t-Bu)3, has three electronegative oxygen atoms bonded to aluminium, which makes this reagent less nucleophilic than LiAlH4. 

In these formulas, the R or R group may be either an aliphatic or aromatic group. In a ketone, the R and R groups may represent the same group or different groups. These types of compounds are best reduced by complex metal hydrides, such as lithium aluminum hydride (LiAlH) or sodium borohydride (NaBU). 

Carboxylic acids, acid chlorides, acid anhydrides and esters get reduced to primary alcohols when treated with lithium aluminium hydride (LiAlH) (Fig.M). The reaction involves nucleophilic substitution by a hydride ion to give an intermediate aldehyde. This cannot be isolated since the aldehyde immediately undergoes a nucleophilic addition reaction with another hydride ion (Fig.N). The detailed mechanism is as shown in fig.O. 

The synthetic equivalents of the synthon H are the hydride donors sodium borohydride NaBHj, and lithium aluminium hydride LiAlH. Row might you make TM21 using this disconnection ... 

Lithium. Lithium aluminum hydride, LiAlH, is the only lithium compound of interest for Li analysis in IH work. Little work has been reported, except for standard ionization interferences (3) corrected by addition of 1000 ppm Cs to the final solution. 

Reduction to Aldehydes Reduction of carboxylic acids to aldehydes is difficult because aldehydes are more reactive than carboxylic acids toward most reducing agents. Almost any reagent that reduces acids to aldehydes also reduces aldehydes to primary alcohols. In Section 18-10, we saw that lithium tri-ferf-butoxyaluminum hydride, LiAlH(0-f-Bu)3 is a weaker reducing agent than lithium aluminum hydride. It reduces acid chlorides to aldehydes because acid chlorides are strongly activated toward nucleophilic addition of a hydride ion. Under these conditions, the aldehyde reduces more slowly and can be isolated. Therefore, reduction of an acid to an aldehyde is a two-step process Convert the acid to the acid chloride, then reduce using lithium tri-ferf-butoxyaluminum hydride. 

Lithium tri-tert-butoxyaluminum hydride, LiAlH[OC(CH3)3]3, has three electronegative oxygen atoms bonded to aluminum, which make this reagent less nucleophilic than LiAlH4. 

Lithium aluminum hydride, LiAlH, is another reducing agent often used for reduction of ketones and aldehydes. A grayish powder soluble in ether and tetrahydrofuran, LiAlH4 is much more reactive than NaBH4 but also more dangerous. It reacts violently with water and decomposes explosively when heated above 120°C. 

Lithium tri-t-butoxyaluminum hydride, LiAlH[OC(CHj)3]3. Mol. wt. 254.30, sublimes at 280° under vacuum, solubility at 25° in g./lOO g. diglyme, 41 tetrahydrofurane 36, ether, 2 1,2-dimethoxyethane, 4 t-BuOH, 0. Supplier Ventron Corp. The reagent currently costs 1.5 as much as LiAlH and the active hydrogen per gram is only 3.7% that for LiAlHa. 

Complex metal hydrides LiAlH(OR>3 MH bond is more Additions. KH act as... 

Reduction of a variety of compounds that contain the carbonyl group provides s)mthetic methods to produce primary and secondary alcohols. A common, very powerful reducing agent is lithium aluminum hydride, LiAlH other reducing agents include sodium in alcohol and sodium borohydride, NaBH. ... 

This group of reagents is commercially available in large quantities some of its members-notably lithium aluminium hydride (LiAlH ), calcium hydride (CaH2), sodium borohydride (NaBH ) and potassium borohydride (KBH4)-have found widespread use in the purification of chemicals. 

Earlier studies with complex metal hydrides (LiAlH [16], and NaBH4 [17]) indicated the formation of a mixture of cis- and frarw-verbenoT. In our experiments over amorphous Ni alloys, the hydrogenation of verbenone resulted in the formation of verbanone and verbanol. This means that the same consecutive two-step reduction occurred as observed in the case of 2. Under appropriate reaction conditions (373 K), selective saturation of the carbon-carbon double bonds yielded verbanone. At higher temperature (398 K), selective formation of the saturated alcohol (verbanol) was observed in most cases. 

There are two commonly used reducing agents, namely first—sodium horohydride (NaBH ) which reduces exclusively aldehydes, ketones and acyl halides and secondly—lithium aluminium hydride (LiAlH ) which reduces the above compounds as well as compormds belonging to the carboxylic acid family. 

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