Activity and Selectivity Control in Reductive Amination of Butyraldehyde over Noble Metal Catalysts
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Catalysis Letters Vol. 104, Nos. 1–2, October 2005 (Ó 2005) DOI: 10.1007/s10562-005-7431-4
Activity and selectivity control in reductive amination of butyraldehyde over noble metal catalysts J. Bo´disa,b, L. Leffertsd, T.E. Mu¨llera,*, R. Pestmanc, and J.A. Lerchera a
Department Chemie, Lehrstuhl II fu¨r Technische Chemie, Technische Universita¨t Mu¨nchen, Lichtenbergstrasse 4, D-85747Garching, Germany b Department of Organic Chemistry, Babes-Bolyai University, Arany Ja´nos str. No. 11, 3400, Cluj, Romania c DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands d Department of Chemical Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Received 1 June 2005; accepted 20 June 2005
Approaches to control selectivity and activity in the catalytic reductive amination of butyraldehyde with ammonia over carbon supported noble metal catalysts (Ru, Rh, Pd, and Pt) were explored. Detailed analysis of the reaction network shows that the Schiff base N-[butylidene]butan-1-amine is the most prominent initial product and, only after nearly all butyraldehyde had been converted to N-[butylidene]butan-1-amine, amines are detected in the product mixture. From this intermediate, good hydrogenolysis catalysts (Ru, Rh) produce mostly butylamine, while catalysts less active in hydrogenolysis (Pd, Pt) lead to the hydrogenation of N-[butylidene]butan-1-amine to mostly dibutylamine. KEY WORDS: reductive amination; butyraldehyde; amines; noble metal catalysts; carbon supports.
1. Introduction The reductive amination of aldehydes and ketones is one of the most important synthetic routes to amines. The reaction is conventionally claimed to proceed through an intermediately formed imine, which is reduced with hydrogen to the primary amine (scheme 1). Secondary and tertiary amines are assumed to be formed by sequential reactions with the aldehyde or ketone present in the reaction mixture. However, there is evidence that precursors to secondary and tertiary amines could be present at concentrations much higher than the amount of secondary and tertiary amines obtained [1–3]. Thus, the reaction paths, which lead to the final product, and determine the selectivity of a particular catalyst, are still ambiguous. Homogeneous and heterogeneous catalysts are known for the reductive amination of carbonyl compounds. Homogeneous catalysts are based mostly on cobalt and rhodium complexes, [2,4–6] which, with chiral ligands, are also suitable for the stereoselective reductive amination of prochiral ketones [7–11]. With respect to solid catalysts numerous reports exist on supported and bulk transition metal catalysts [3,12–20] most of the information being, however, in the patent literature (see, e.g., nickel [21–26] cobalt [25,27,28] and noble metal based [25,29–32] catalysts). Metal sulfides of Fe, Co, Ni, Rh, W, Re, and Pt were also used in the reductive amination of ketones [33–35]. The relative inactivity of these catalysts for the hydrogenation of aromatic rings,
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