Selective Synthesis of Primary Anilines from NH₃ and Cyclohexanones by Utilizing Preferential Adsorption of Styrene on the Pd Nanoparticle Surface

Abstract

Dehydrogenative aromatization is one of the attractive alternative methods for directly synthesizing primary anilines from NH 3 and cyclohexanones.However,the selective synthesis of primary anilines is quite difficult because the desired primary aniline products and the cyclohexanone substrates readily undergo condensation affording the corresponding imines (i.e., N-cyclohexylidene-anilines), followed by hydrogenation to produce N-cyclohexylanilines as the major products.Inthis study,primary anilines were selectively synthesized in the presence of supported Pd nanoparticle catalysts (e.g.,P d/HAP,H AP = hydroxyapatite,C a 10 (PO 4 ) 6 -(OH) 2 )b yu tilizing competitive adsorption unique to heterogeneous catalysis;i no ther words,w hen styrene was used as ah ydrogen acceptor,w hichp referentially adsorbs on the Pd nanoparticle surface in the presence of N-cyclohexylideneanilines,v arious structurally diverse primary anilines were selectively synthesized from readily accessible NH 3 and cyclohexanones.T he Pd/HAP catalyst was reused several times though its catalytic performance gradually declined.

Continuous Synthesis of Aryl Amines from Phenols Utilizing Integrated Packed‐Bed Flow Systems

“…This result indicated that the side product 5 aa was generated through hydrogen transfer, that is, the imine intermediate was reduced by the Pd−H species formed through the dehydrogenation of another imine . It was hypothesized that this problem could be mitigated through the use of oxidants or hydrogen scavengers, and the corresponding screening (Figure b, entries 6–8) revealed that styrene (2 equiv) acted as a good hydrogen scavenger to achieve excellent product selectivity. Under optimal reaction conditions [column reactor diameter=10 mm, length=10 cm; catalyst=mixture of Pd(OH) 2 /C and Celite ® (1:10 w/w, ≈4.9 g)], the desired diaryl amine 4 aa could be continuously produced as the sole product in excellent yield for at least 24 hours (Figure b, entry 8).…”

“…Thus, consecutive condensation and dehydrogenation would proceed smoothly to afford the desired aryl amines without any by‐products because of the absence of hydrogen. Furthermore, division of the flow reactors would enable the use of a hydrogen scavenger in the second stage to more reliably control product selectivity. This segmentation and integration strategy could significantly improve the amine scope and product selectivity, while some features of flow reactors loaded with heterogeneous catalysts could not only increase reaction efficiency, but also allow reaction conditions milder than those used in the corresponding batch reactions to realize the continuous synthesis of various aryl amines …”