Different borohydrides are available depending on the metal cations and ligands such as sodium borohydride, lithium borohydride, zinc borohydride, lithium or potassium trialkylborohydride and sodium cyanoborohydride. All the borohydrides reduce aldehydes and ketones to the corresponding alcohols but due to its counter cations and ligands it shows some special selectivity towards some functional groups.
2.1.5.1 Sodium Borohydride (NaBH4)
The sodium borohydride is an ideal reducing agent for the reduction of aldehydes and ketones to give the corresponding alcohols (Scheme 1). Unconjugated ketones and aldehydes are rapidly reduced, usually in preference to a conjugated carbonyl group that may be present elsewhere in the molecules. The reactions are performed in polar solvents such as THF, DME, diglyme, hydroxyl solvent and water but the ethanol is the solvent of choice.
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Scheme 1 |
In contrast, in the presence of CeCl3, conjugated ketone is reduced much faster compared to the normal ketone (Scheme 2). The combination of NaBH4 and CeCl3 is called Luche reagent and the reaction is known as Luche reduction.
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Scheme 2 |
Proposed Mechanism
The cerium reagent coordinates to the carbonyl, making only a 1,2-addition possible (Scheme 3).
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Scheme 3 |
The esters react slowly and the carboxylic acids in normal condition do not react but are reduced to alcohol in presence of catalytic amount of iodine in reflux condition in THF (Scheme 4). This method provides effective route for the conversions of amino acids to amino alcohols, which serve as ligands for asymmetric catalysis.
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Scheme 4 |