sodium bicarbonate is added to keep the reaction mixture basic. Examples are hexanal to 1-hexanol (67%), benzaldehyde to benzyl alcohol (84%), ...
Sodium Dithionite
[7775-14~6J · N~04S2 · Sodium Dithionite · (MW 174.12)
(versatile reagent for reduction of aldehydes, ketones, unsaturated conjugated ketones, ' quinones, diunsaturated acids, azo, nitro, and nitroso compounds, ' imines, oximes, tropylium salts, pyridinium salts, pyrazine, and vinyl sulfones; ;~ intramolecular Marschalk cyclizations, , dehalogenation of vic dibromides and a-halo ketones, Claisen rearrangement of allyloxyanthraquinones and for the synthesis of 8-arylaminopurines ' ) Alternate Name: sodium hydrosulfite. Physical Data: mp 52 °C (dec).
.
Solubility: very sol water; sol alcohol. Form Supplied in: white or gray-white crystalline powder. Preparative Methods: by the action of Sulfur Dioxide on Sodium Amalgam in alcoholic solution. Handling, Storage, and Precautions: flammable; moisture sensitive.
Reduction of Aldehydes and Ketones. ~
Sodium dithionite is an alternative and less expensive reducing agent than metal hydrides. The reactions are performed in water for soluble substrates, otherwise a 50:50 mixture of water and dioxane or DMF can be used; sodium bicarbonate is added to keep the reaction mixture basic. Examples are hexanal to 1-hexanol (67%), benzaldehyde to benzyl alcohol (84%), cyclohexanone to cyclohexanol (80%), and acetophenone to 0.· hydroxyethylbenzene (94%). Reduction ofmethylcyclohexanones by N~S204 in benzene-water using adogen . .(commercial mixture of methyl trialkyl Cg-C 10 ammonium chloride) as a phase-transfer agent afforded good yields \ of isomeric mixtures of the corresponding methylcyclohexanols. Reduction using N~S204 proceeds with stereoselectivity similar to that obtained with metal hydrides (Sodium Borohydride) and opposite to dissolving metals reductions, e.g. the reductions of3a.-hydroxy-7-keto-5~-cholanic acid to diols (eq 1).
\ \
.
I I
\
..
H
\ +
(1) H
N~
85:15 94:6 4:96 6:94
100% K,100% Na2S204,100% NaBH4,100%
Regiospecific Reduction of Unsaturated Conjugated Ketones. ' Exclusive reduction of conjugated carbon~carbon double bonds 'is achieved with N~S204 to afford the corresponding saturated carbonyl compound. A two-phase (benzene-water) system using adogen as the phasetransfer catalyst is used. The isolated carbon-carbon double bond remains unaffected. No alcohols are detectable (eqs 2-4).
(2) 91%
65%
.. 63% \
°
(3)
(4)
Quinones to Hydroquinones. Most Quinones are reduced by sodium dithionite to hydroquinones. Naphthacenequinone and higher linear benzologs are exceptional in that they are not reduced by alkaline sodium dithionite. anthraquinone to anthrone.
Sodium dithionite reduces
Conjugated Diunsaturated Acids (and Esters) to Monounsaturated Acids (and Esters). a,~;y,8-Unsaturated acids
are reduced by N~S204 in an alkaline medium (NaOH or NaHC0 3) to a mixture of (Z)-
and (E)-p,y-unsaturated acids (and esters) (40-75% yield) (eq 5).
.. NaOH
74%
2%
Azo to Amine. Azobenzene is reduced to aniline by sodium dithionite. This reaction is used to introduce an amino group into a phenolic compound by first coupling with an aromatic diazonium salt and then reducing the resulting hydroxyazo derivatives with N~S204' e.g. 2- and 4-amino-I-naphthols can be prepared from I-naphthol.
Nitro to Amine. Various aromatic nitro compounds are reduced conveniently to the corresponding aniline
