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Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li[Al H 4] or LiAlH 4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. [4] This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides.
The first step involves deprotonation of the carboxylic acid. The final step in the reduction of carboxylic acids and esters is hydrolysis of the aluminium alcoxide. [8] Esters (and amides) are more easily reduced than the parent carboxylic acids. Their reduction affords alcohols and amines, respectively. [9] The idealized equation for the ...
Lithium aluminium hydride (LAH) modified with chiral alkoxide ligands has been shown to proceed in good yield and high enantioselectivity. Chelating ligands such as BINOL [6] are used to avoid disproportionation and background reduction by LAH. Chiral diamines and amino alcohols have also been used to modify LAH for enantioselective reductions. (4)
Here 42 functional groups (39 hydroxyls, one diol, an amine group, and a carboxylic acid) required protection. These proceeded through 8 different protecting groups (a methyl ester, five acetals, 20 TBDMS esters, nine p‑methoxybenzyl ethers, four benzoates, a methyl hemiacetal, an acetone acetal and an SEM ester). [109] Palytoxin
In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups. [34] In many ways, the reactivity of aluminium hydride is similar to that of lithium aluminium hydride. Aluminium hydride will reduce aldehydes, ketones, carboxylic acids, anhydrides, acid chlorides, esters, and lactones to their corresponding alcohols.
Instead, LiBHEt 3 is reserved for difficult substrates, such as sterically hindered carbonyls, as illustrated by reduction of 2,2,4,4-tetramethyl-3-pentanone. Otherwise, it reduces acid anhydrides to alcohols and the carboxylic acid, not to the diol. Similarly lactones reduce to diols. α,β-Enones undergo 1,4-addition to give lithium enolates.
Lithium borohydride (LiBH 4) is a borohydride and known in organic synthesis as a reducing agent for esters.Although less common than the related sodium borohydride, the lithium salt offers some advantages, being a stronger reducing agent and highly soluble in ethers, whilst remaining safer to handle than lithium aluminium hydride.
The Barton decarboxylation is a radical reaction in which a carboxylic acid is converted to a thiohydroxamate ester (commonly referred to as a Barton ester). The product is then heated in the presence of a radical initiator and a suitable hydrogen donor to afford the decarboxylated product.