Elsevier

Molecular Catalysis

Volume 484, March 2020, 110720
Molecular Catalysis

Isotopic substitution experiments in the hydrogenation of mandelonitrile over a carbon supported Pd catalyst: A nuclear magnetic resonance study

https://doi.org/10.1016/j.mcat.2019.110720Get rights and content
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Highlights

  • Mandelonitrile deuteration reveals a complex mixture of isotopologue products.

  • Tautomeric reactions are invoked to rationalise the product distribution.

  • Three hydrogen requiring processes are necessary to afford phenethylamine.

  • These processes occur in the order: hydrogenation, hydrogenolysis, hydrogenation.

  • Mechanistic insight in to the hydrogenation of mandelonitrile is enhanced.

Abstract

A mechanistic exploration of the liquid phase hydrogenation of the aromatic cyanohydrin mandelonitrile (C6H5CH(OH)CH2CN) over a carbon supported Pd catalyst to produce the primary amine, phenethylamine (C6H5CH2CH2NH2) is conducted. Prior examination showed the reaction to involve the production of the ketone intermediate 2-aminoacetophenone (C6H5C(O)CH2NH2), formed as a consequence of the presence of an acid catalysed tautomeric side reaction. The corresponding deuteration reaction, reported here and analysed by multinuclear NMR spectroscopy and mass spectrometry, is employed to further investigate accessible pathways. Examination of the resultant product distribution of the deuteration, and the location of deuterium incorporation establishes the role of a hydroxy-imine species as a key reaction intermediate. In addition, the acid catalysed tautomerism to the ketone is shown to be a reversible side reaction, but also a contributor to desired product formation. Moreover, an order for the three critical hydrogen consuming steps in phenethylamine formation is established. Hydrogenation of the nitrile functionality to afford the hydroxy-imine precedes hydrogenolysis of the hydroxyl group, with the final step being hydrogenation of the imine to form the target product, phenethylamine.

Keywords

Mandelonitrile hydrogenation
Primary amine synthesis
Carbon-supported palladium catalyst
Mechanism elucidation
Isotopic substitution

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