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Among the methods used for hydrogenations in organic chemistry chemistry, the ones that rely on the use of zinc are numerous. Zinc is used as the pure metal in chips or in powder, in association with other metals in the form of a couple: combined zinc1, the couples zinc-copper2, zinc-palladium and zinc-platinum3 are described in the literature. The couple zinc-nickel does not appear to have catched the attention of chemists. The work that we present has for goal to signal his interest. The zinc-nickel couple is active especially in the presence of a base like ammonia or an alkaline hydroxide. This is most often in the presence of ammonia than we have it employed. Nevertheless we were able to reduce the galactose in dulcite, the levulose in mannite, in neutral environment.
Preparation of the Zn/Ni couple
It is easily obtained by treating an ammonia solution of a nickel salt, the sulfate for example, with powdered zinc. The nickel is reduced to a black precipitate while the zinc powder is oxidized to soluble zinc salts, and at the same time the finely divided nickel powder at the same time reduces the organic substrate upon contact. In certain cases it is necessary to eliminate the re-formed ionized nickel, because the organics gives an insoluble complex with it in the presence of ammonia, or it can be prevented by substituting potassium carbonate for the ammonia in those cases.
Below, the hydrogenation of many different organic functional groups with this couple is shown.
Malic, crotonic, oleic, and cinnamic acid have been respectively transformed in succinic, butyric, stearic and phenylpropionic acids.
Formaldehyde, salicylaldehyde and vanillin has been reduced to methanol, salicylalcohol and vanillic alcohol. Salicylaldehyde forms an insoluble complex with nickel in ammoniacal solutions4, its reduction has been furnished in the presence of potassium carbonate. The β-keto-substituted phenylpyruvic and benzoylpropionic acid has been transformed to phenylacetic acid and γ-phenylbutyrolactone respectively.
Ketoximes are reduced to the primary amine. 2-Phenethylamine and Phenylisopropylamine has been obtained from phenylacetaldoxime and phenylacetoxime, respectively. The isonitrosoketones are reduced to amino-alcohols (Pyrazines are not formed as a by-product in this reduction, for example in the reduction of α-isonitrosoacetophenone to β-phenylethanolamine)
Benzyl cyanide is hydrogenated in aqueous alcohol to phenethylamine, and ethyl-2-amino-4-cyano-5-pyrimidine furnishes first the orangish red complex describes by M. Delépine5, then the corresponding diamine.
p-Nitrobenzoic acid is hydrogenated smoothly to p-aminobenzoic acid.
If the usual reaction mixture is mixed with anhydrous alcohol to increase the solubility of the ketone to be reacted, one obtains with a satisfactory yield, in certain cases, the corresponding primary amine. Cyclohexanone, in particular, furnishes in these conditions 60-70% of primary cyclohexylamine. Phenylacetone and β-naphtanone have been transformed equally in amphetamine and β-naphtanamine.
The examples that we have just quoted show the properties of the zinc-nickel couple. The simple preparation and the facile conditions of employment shows that this couple warrants further investigation.