Short CommunicationOne pot catalytic NO2 reduction, ring hydrogenation, and N-alkylation from nitroarenes to generate alicyclic amines using Ru/C-NaNO2
Introduction
The efficient synthetic pathway for producing amines has been extensively investigated due to the fact that they are widely used as significant intermediate as well as final product in various chemical industries [1], [2]. Especially, the cyclohexylamine (CHA) finds various applications in pharmacology or as agrochemicals in terms of its being raw material for carbamates and isocyanates. Among these, alicyclic diamines also more substantially used feedstock for the synthesis of non-yellowing polyurethanes.
Conventionally, the CHA has been produced from the following two step; i) reduction of nitrobenzene (NB) to aniline [3], [4], [5], [6], [7], [8], [9], ii) ring hydrogenation of aniline to CHA [1], [2], [10], [11], [12], [13], which makes the process complicated. Therefore, development of a facile one step synthetic pathway to alicyclic amines from nitroarenes is of great importance with respect to process efficacy.
As to the industrial processes to manufacture aniline through NB hydrogenation, the process generally employs above 240 °C with copper-catalysts in two-stage bed reactor [14]. Recently, Langer et al. reported a low-pressure process for the hydrogenation of aniline to produce CHA with high selectivity using rhodium catalysts [11]. The literature indicates that ruthenium is the most used catalyst for ring-hydrogenation of aromatic amines due to its high activity, selectivity, and reproducibility [10], [11], [12], [13]. The N-alkylation of amines using non-halide reagent is also an important process because conventional halogenated substances may generate harmful by-products such as HCl [15], [16], [17]. To overcome disadvantages, alcohols can be employed as direct alkylating agent for the N-alkylation of amines [18], [19], [20], [21]. Since there has been no report on one-pot synthesis of CHA or N-alkylated amines from the reaction of NB with H2 and alcohols, this approach might provide much greener pathway from the view point of process efficiency as well as cost saving.
In this context, a direct synthesis of alicyclic amines and their subsequent production of N-alkylated amines from the reaction of nitroarenes with H2 and alcohols in the presence of a catalyst system comprising carbon supported Ru and NaNO2 are presented.
Section snippets
Experimental
All the chemicals and catalyst were purchased from Aldrich (South Korea) and used without further purification.
Effect of polar aprotic and protic solvents
Catalytic reduction of –NO2 group and ring hydrogenation reaction of NB were carried out over Ru/C catalyst in the presence of various polar aprotic and alcoholic solvents at 170 °C. This temperature was used to figure out all the possible products from these harsh reaction conditions. The results summarized in Fig. 2 show that NB conversion was observed as 100% (data not shown) regardless of solvent used probably due to the high temperature. Among polar aprotic solvents, only NMP showed 100%
Conclusions
Ru/C-NaNO2 catalyst system is found to catalyze various nitroarenes to produce corresponding alicyclic or aromatic amines in high yield. The hydrogenation reaction undergoes following steps; ‒NO2 reduction, ring hydrogenation, and continuing to N-alkylation depending on the reaction conditions used. The more severe reaction conditions, the further the reaction proceeded thus any compound out of the three, e.g., aniline, CHA, N-alkylated CHA, could be produced almost quantitatively by simply
Acknowledgment
This work was supported by the “Fusion Research Program for Green Technologies (NRF-2012M3C1A1054497)” funded by the Ministry of Education, Science and Technology, Korea.
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These authors contributed equally to this work.