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Enantioselective ketone reductions convert prochiral ketones into chiral, non-racemic alcohols and are used heavily for the synthesis of stereodefined alcohols. [ 1 ] Carbonyl reduction, the net addition of H 2 across a carbon-oxygen double bond, is an important way to prepare alcohols.
Sodium triacetoxyborohydride is a milder reducing agent than sodium borohydride or even sodium cyanoborohydride. It reduces aldehydes but not most ketones. It is especially suitable for reductive aminations of aldehydes and ketones. [2] [3] [4]
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 reduction of an ester by lithium aluminium hydride is:
The Luche reduction can be conducted chemoselectively toward ketone in the presence of aldehydes or towards α,β-unsaturated ketones in the presence of a non-conjugated ketone. [5] An enone forms an allylic alcohol in a 1,2-addition, and the competing conjugate 1,4-addition is suppressed.
The CBS reduction has since been utilized by organic chemists as a reliable method for the asymmetric reduction of achiral ketones. Notably, it has found prominent use not only in a number of natural product syntheses, but has been utilized on large scale in industry (See Scope Below). Several reviews have been published. [4] [5] [6]
Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, [5] is an inorganic compound with the formula Na B H 4 (sometimes written as Na[BH 4]). It is a white crystalline solid, usually encountered as an aqueous basic solution .
The compound is prepared by the reaction of tert-butylammonium chloride and sodium borohydride: [2] t-BuNH 3 Cl + NaBH 4 → t-BuNH 2 BH 3 + H 2 + NaCl. In organic synthesis, borane tert-butylamine can be used for selective reduction of certain functional groups including aldehydes, ketones, oximes, and imines. [3]
The selectivity of this reagent is illustrated by its reduction of all three methylcyclohexanones to the less stable methylcyclohexanols in >98% yield. Under certain conditions, L-selectride can selectively reduce enones by conjugate addition of hydride, owing to the greater steric hindrance the bulky hydride reagent experiences at the carbonyl ...