Secondary Products from the Catalytic Hydrogenation of Nitriles^1,2

Nitrile Hydrogenation on Solid Catalysts – New Insights into the Reaction Mechanism

“…Winans and Adkins [6] examined the hydrogenation of hydrobenzamide and their results were in agreement with von Braun's mechanism. Juday and Adkins [7] investigated the hydrogenation of both aliphatic and aromatic nitriles and found that, for the aliphatic nitriles, their results agreed with von Braun's mechanism whereas, for the aromatic nitriles, the Schiff base is formed by hydrogenolysis of the dimer of aldimine. In 1933, Kindler and Hesse [8] suggested that the tertiary amines are formed by the addition of a secondary amine to aldimine and subsequent hydrogenolysis of 1,1-bis(alkylamino)alkane-1-amine (6) (denoted, for the sake of brevity, as 1-aminotrialkylamine).…”

“…Theoretically, condensation reactions, as represented by the reaction scheme for the formation of secondary and tertiary amines via the hydrogenation of nitriles, can proceed in the bulk fluid phase (liquid or gas), on the surface of the metal, on the surface of the acid support of the catalyst or simultaneously on multiple sites. As early as 1955, during a study of nitrile hydrogenation in the presence of primary amines bearing different alkyl groups, Juday and Adkins [7] observed that, in addition to the asymmetric secondary amine, a symmetric secondary amine was formed in substantial amounts. However, according to the mechanism of Braun, who assumed that the condensation reaction leading to the secondary imine proceeds in the liquid phase, the asymmetric secondary amine should be formed preferentially.…”

The application of a supported palladium catalyst for the hydrogenation of aromatic nitriles

“…Historically, it has proven rather difficult to identify intermediate species in the hydrogenation of nitriles, which is attributed to their inherently high reactivity [5,6]. The presence of imines and enamines as intermediates has since been repeatedly established [7,8] and this mechanism is now almost universally accepted [7,9].…”

“…The addition of primary amine to intermediate aldimine, followed by subsequent hydrogenolysis leads to the formation of secondary amines. However, an equally valid pathway could be the elimination of ammonia to give a Schiff base, which is then hydrogenated to the final secondary amine [9,12]. The exact mechanisms followed are still under debate, but it has been shown that these further steps can take place on the surface of the catalyst/support system [13,14].…”

Impact of solvent on Co/SiO2 activity and selectivity for the synthesis of n-butylamine from butyronitrile hydrogenation

“…The general mechanism of nitrile hydrogenation and coupling to higher amines used by many authors is based on the pioneer work of Braun et al [3], Sabatier and Senderens [4], and others [5,6]. This mechanism is shown in Scheme 1 for butyronitrile (BN) hydrogenation to nbutylamine (BA) and coupling reactions leading to dibutylamine (DBA) and tributylamine (TBA) [7,8].…”