Res. Chem. Intermed.
, Vol. 31, No. 9, pp. 769–778 (2005)
Also available online - www.vsppub.com
Isomerization of an imine intermediate in a reductive
amination reaction over metal catalysts
Department of Organic Technology, Institute of Chemical Technology, Prague Technická 5,
166 28 Praha 6, Czech Republic
Received 24 January 2004; accepted 4 November 2004
Abstract—The reductive amination reaction of acetone by cyclohexylamine over hydrogenation
metal catalysts was investigated. The study is focused on the formation of side products in the reaction.
It was veriﬁed that the formation of amines having unusual combinations of alkyls is caused by the
metal-catalyzed rearrangement of the double bond around the nitrogen atom in an imine intermediate
and consequent reactions of the isomeric imine. It was found that the isomerization reactions occur
over virtually all of the hydrogenation catalysts studied, while their respective activities for the imine
isomerization decreases in the order Ni = Co > Ru > Pt = Rh > Pd.
Keywords: Imine isomerization; reductive amination; side products; metal catalysts; acetone; cyclo-
Reductive amination is a common method for the preparation of many industrially
important amines. This is a reaction of a carbonyl (aldehyde or ketone) with
ammonia, a primary or secondary amine, and hydrogen over a hydrogenation
catalyst. In fact, this is a system of consecutive reactions, in which an imine
(aldimine or ketimine) is the key intermediate formed in the dehydration of an
aminoalcohol, the product of the condensation of an amine with the carbonyl
compound. In the next step, the imine is hydrogenated to the amine required. The
condensation of the carbonyl group with the amine is an acid-catalyzed reaction
(in the case of reactive aldehydes it occurs at a sufﬁcient rate even if no catalyst is
present); the hydrogenation of the imine intermediate occurs over metal catalysts.
In addition to the sequence of the major reactions, in the system numerous
consecutive and side reactions occur that result in the formation of a wide spectrum
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