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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 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 .
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 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 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 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 ...
The imine is then reduced to an amine by sodium cyanoborohydride. This reaction works on both aldehydes and ketones. The carbonyl can be treated with ammonia, a primary amine, or a secondary amine to produce, respectively, 1°, 2°, and 3° amines. [5] Aromatic ketones and aldehydes can be reductively deoxygenated using sodium cyanoborohydride. [6]
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]