Aminocarbene mechanism of the formation of a tertiary amine in nitrile hydrogenation on a palladium catalyst

Abstract

The hydrogenation of an aliphatic nitrile, molecules of which do not bear hydrogen atoms on the α-carbon with respect to the nitrile group, has been studied to gain a deeper insight into the mechanism of the heterogeneously catalyzed hydrogenation of nitriles. Hydrogenations were performed in the liquid phase on a Pd/C catalyst at a reaction temperature of 110 °C and a hydrogen pressure of 5 MPa. In the hydrogenation of trimethylacetonitrile in the presence of a lower aliphatic N,N-dialkylamine (e.g., diethylamine), the formation of a mixed tertiary amine, dialkylneopentylamine, was observed. Experiments have shown that the disproportionation reactions of the amines present are not responsible for its formation. Since the corresponding enamine incorporating a double bond in the neopentyl chain cannot be derived from the dialkylneopentylamine, the formation of the dialkylneopentylamine cannot be explained in terms of the enamine theory of the formation of a tertiary amine during the hydrogenation of the nitrile. It has been proved experimentally that the entire process of tertiary amine formation by nitrile hydrogenation is generally reversible: butyronitrile results from a reverse process from tributylamine and ammonia on the surface of the metal catalyst. The combination of experimental facts thus obtained gives an indirect proof of the validity of the hypothesis that the aminocarbene mechanism is involved in the formation of secondary and tertiary amines by nitrile hydrogenation.