This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 2539–2548 2539 Cite this: Catal. Sci. Technol., 2012, 2, 2539–2548 Controlling selectivity in the reaction network of aldoxime hydrogenation to primary aminesw Ewa Gebauer-Henke, a Walter Leitner, bc Angelina Prokofieva, a Henning Vogt a and Thomas E. Mu¨ller* a Received 27th May 2012, Accepted 5th August 2012 DOI: 10.1039/c2cy20356a During the hydrogenation of oximes, the temporary protection of the nitrogen moiety by a hydroxyl group promises to provide access to a highly selective synthetic route to primary amines. We will show that the reaction, however, proceeds via the more general network of imine and Schiff base chemistry. By choosing the right catalyst and conditions, specific pathways can be selected enabling us to steer the route from a pool of intermediate compounds to either primary or secondary amines. Thus, nickel catalysts provide high selectivity towards primary amines albeit at a moderate activity, while noble metal catalysts show good selectivity towards secondary amines, as well as high activity. Detailed analysis of the reaction sequence over Ni/SiO 2 provides insight into the pathways and provides understanding for the tools to obtain outstanding selectivities in the hydrogenation of oximes and other amine precursors. 1 Introduction Amines are widely used bulk chemicals. A large number of solvents, surfactants and polymers originate from chemical syntheses where amines occur as intermediates. 1 Moreover, amines are important fine chemicals used, e.g., in the manu- facture of pharmaceuticals, agrochemicals, emulsifiers and plasticizing agents. 2,3 Primary amines, in particular, are required in large quantities, and numerous methods for their synthesis have been described, 4–7 including the reductive amination of alcohols 8 and carbonyl compounds, 9 the reduction of nitriles, 1,10–13 azides and amides, 14 and the hydro- amination of olefins. 15–17 Although most of these methods are rather versatile and afford amines in good yields, the primary amines are obtained usually as a mixture with secondary and tertiary amines, and the workup is characterised by energy- consuming purification steps. 18 Thus, up to now, selectivity control in the synthesis of primary amines remains a key issue in technical implementations where heterogeneous catalysts are preferentially employed. Closer inspection of the underlying reaction network shows that several important routes to amine synthesis are closely related. Thus, Schiff bases have been identified as reaction intermediates in the reductive amination of aldehydes, 19–22 as well as during nitrile hydrogenation. 19,23–25 Nitriles, in turn, have been detected during the hydrogenation of amides 26 and Schiff bases as well as the reductive amination of aldehydes. 19 It appears likely that in all of these cases the same reaction network is entered from different access points (Scheme 1). Herein, we explore the reduction of oximes 27,28 as an alternative rarely used way to transform carbonyl compounds into the corresponding amines. Note that oximes are obtained readily by reaction of the corresponding aldehydes with hydroxylamines. 29 Sabatier introduced nickel catalysts in oxime hydrogena- tion, 10 but little is known about the mechanistic pathways of the reaction, and only a few comparative studies with present day catalysts have been conducted. To explain the formation of Schiff bases and secondary amines as intermediates and products, which involve formal condensation of two Scheme 1 Common reaction network in amine syntheses via hydro- genation of unsaturated nitrogen precursors. a CAT Catalytic Center, ITMC, RWTH Aachen University, 52074 Aachen, Germany. E-mail: Thomas.mueller@catalyticcenter.rwth-aachen.de; Fax: +49 241 8022593; Tel: +49 241 8028594 b Institut fu ¨r Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany c Max-Planck-Institut fu ¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu ¨lheim an der Ruhr, Germany w Electronic supplementary information (ESI) available: Further experimental details, detailed characterisation of aldoximes, ketoximes, nitriles and Schiff bases; additional kinetic data; schematic representa- tions of model complexes. See DOI: 10.1039/c2cy20356a Catalysis Science & Technology Dynamic Article Links www.rsc.org/catalysis PAPER