Facile access to N-substituted anilines via dehydrogenative aromatization catalysis over supported gold–palladium bimetallic nanoparticles

Abstract

N-Substituted anilines are widely utilized important compounds, and the development of their diverse synthetic procedures is of great significance. Herein, we have successfully developed a widely applicable powerful catalytic route to N-substituted anilines. In the presence of a gold–palladium alloy nanoparticle catalyst (Au–Pd/Al₂O₃) and styrene, various kinds of structurally diverse N-substituted anilines (twenty three examples) could be synthesized starting from cyclohexanones and amines (including aliphatic primary and secondary amines and anilines). The catalytic performance was strongly influenced by the nature of the catalyst. A supported gold catalyst (Au/Al₂O₃) was completely inactive for the present transformation. Although a supported palladium catalyst (Pd/Al₂O₃) gave the desired N-substituted anilines to some extent, the performance was inferior to that of Au–Pd/Al₂O₃. The catalytic activity of the palladium species in Au–Pd/Al₂O₃ was at least ca. three times higher than that in Pd/Al₂O₃. Moreover, the performance of Au–Pd/Al₂O₃ was superior to that of a physical mixture of Au/Al₂O₃ and Pd/Al₂O₃. Thus, palladium was intrinsically effective for the present transformation (dehydrogenative aromatization) and its performance was improved by alloying with gold. The present transformation proceeds through a sequence of the dehydrative condensation of cyclohexanones and amines to produce enamines (or ketimines), followed by the dehydrogenative aromatization to produce the corresponding N-substituted anilines. In the aromatization step, styrene could act as an effective hydrogen acceptor to selectively produce the desired N-substituted anilines without catalyzing the disproportionation of the enamine intermediates. The observed catalysis using Au–Pd/Al₂O₃ was truly heterogeneous in nature, and Au–Pd/Al₂O₃ could be reused.