A process for preparing amines of the formula I, by reacting a carbonyl compound of the formula II with an amine of the formula III where R1 and R2 are different and are each an organic radical which has from 1 to 20 carbon atoms and may optionally also comprise heteroatoms, R3 is a C1-C6-alkyl group and R4 is an aryl group which may be partly or fully halogenated and/or may bear from one to three radicals from the group of cyano, nitro, C1-C6-alkyl, C1-C6-haloalkyl, hydroxyl, C1-C6-hydroxyalkyl, C1-C6-alkoxy, C1-C6-haloalkoxy, hydroxycarbonyl, C1-C6-alkoxycarbonyl, C1-C6-alkylamino, di(C1-C6-alkyl)amino, aryl and aryl(C1-C6-alkyl); and * represents the S or R configuration, and ** represents the S and/or R configuration; which comprises performing the reaction in the presence of a heterogeneous imination catalyst, of a hydrogenation catalyst and of hydrogen.