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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]
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 .
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 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]
First, the nickel metal dehydrogenates the alcohol to form a ketone and Ni-H complex. Then, the ketone reacts with ammonia to form an imine. Finally, the imine reacts with Ni-H to regenerate catalyst and form primary amine. An example of a homogeneous catalytic system is the reductive amination of ketones done with an iridium catalyst. [20]
In addition to their reduction to alcohols, aldehydes and ketones can be converted to amines, i.e., reductive amination. [18] Because of its cyano substituent, NaBH 3 CN is a weak reducer at moderate pH (>4), so it preferentially reduces iminium cations that exist in the presence of carbonyls:
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]