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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.
The relatively weak reducer sodium borohydride is typically used for reducing ketones and aldehydes. It tolerates many functional groups (nitro group, nitrile, ester). [6] In their handling properties, lithium aluminium hydride and sodium borohydride (and their derivatives) strongly differ.
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 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.
At low pH values, it efficiently reduces aldehydes and ketones. [7] As the pH increases, the reduction rate slows and instead, the imine intermediate becomes preferential for reduction. [7] For this reason, NaBH 3 CN is an ideal reducing agent for one-pot direct reductive amination reactions that don't isolate the intermediate imine. [2]
The Narasaka–Prasad reduction, sometimes simply called Narasaka reduction, is a diastereoselective reduction of β-hydroxy ketones to the corresponding syn-dialcohols. The reaction employs a boron chelating agent, such as BBu 2 OMe, and a reducing agent, commonly sodium borohydride. This protocol was first discovered by Narasaka in 1984. [1]
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]
The use of triphenylphosphine, thiourea, zinc dust, or dimethyl sulfide produces aldehydes or ketones. While the use of sodium borohydride produces alcohols. (R group can also be hydrogens) The use of hydrogen peroxide can produce carboxylic acids. Amine N-oxides produce aldehydes directly. [8]