Selective Reduction of Nitroarenes to Arylamines by the Cooperative Action of Methylhydrazine and a Tris(N-heterocyclic thioamidate) Cobalt(III) Complex

Abstract

We report an efficient catalytic protocol that chemoselectively reduces nitroarenes to arylamines, by using methylhydrazine as a reducing agent in combination with the easily synthesized and robust catalyst tris­(N-heterocyclic thioamidate) Co­(III) complex [Co­(κS,N-tfmp2S)3], tfmp2S = 4-(trifluoromethyl)-pyrimidine-2-thiolate. A series of arylamines and heterocyclic amines were formed in excellent yields and chemoselectivity. High conversion yields of nitroarenes into the corresponding amines were observed by using polar protic solvents, such as MeOH and i PrOH. Among several hydrogen donors that were examined, methylhydrazine demonstrated the best performance. Preliminary mechanistic investigations, supported by UV–vis and NMR spectroscopy, cyclic voltammetry, and high-resolution mass spectrometry, suggest a cooperative action of methylhydrazine and [Co­(κS,N-tfmp2S)3] via a coordination activation pathway that leads to the formation of a reduced cobalt species, responsible for the catalytic transformation. In general, the corresponding N-arylhydroxylamines were identified as the sole intermediates. Nevertheless, the corresponding nitrosoarenes can also be formed as intermediates, which, however, are rapidly transformed into the desired arylamines in the presence of methylhydrazine through a noncatalytic path. On the basis of the observed high chemoselectivity and yields, and the fast and clean reaction processes, the present catalytic system [Co­(κS,N-tfmp2S)3]/MeNHNH2 shows promise for the efficient synthesis of aromatic amines that could find various industrial applications.

Microwave‐assisted reduction of aromatic nitro compounds with novel oxo‐rhenium complexes

“…As we can see from the selected examples reported, both 2 and 5 perform as good as the systems that employ more expensive metals (for instance, PtIPr, [ 49 ] Pd (AcO) 2 [ 50 ] or RhCI (PPh 3 ) 3 [ 54 ] ). When we compare 2 and 5 with other catalysts that performed catalysis in presence of microwaves, we can see that the results were slightly better when compared with Co (κS,N‐tfmp2S) 3 [ 58 ] or Cu (OAc) 2, [ 60 ] but absolutely comparable in terms of yields with all the others. [ 59,62,63 ] Comparing our results with those of the groups that reduced nitrobenzene with methods that do not include a metal as a catalyst or as a reducing agent, we can see that the yields are similar but still the excess of the reducing agent that they employed would be still problematic for large scale processes.…”

“…Maleczka [50] Fe (III) aminebisphenolate 0.012 HSi (OC₂H₅)₃(4.0) 0.6 80 4 84 Zhu et al [51] FeBr 2 /PPh 3 0.1 PhSiH 3 (2.5) 1.0 110 16 85 Junge et al [52] NiCl 2 (aNHC) 2 0.05 PhSiH 3 (2.0) 1.0 60 1 98 Vijaykumar and Mandal [53] RhCI (PPh 3 ) 3 0.2 HSi (OC₂H₅₃(5.0) 10 110 2 86 Brinkman et al [54] Pd/C 0.03 B 2 (OH) 4 /H 2 O(3.3/10.0) 0.6 50 24 95 Zhou et al [55] Pd NPs 0.03 DMAB(3.0) 0.5 RT 0.17 99 Patil et al [56] Imm-Ru-IL 0.013 DMAB(3.0) 0.5 RT 2 99 Patil et al [57] Co (κS,N-tfmp2S) 3 0.05 MeNHNH 2 (6.0) 1.0 70 8 79 Ioannou et al [58] Co (κS,N-tfmp2S) 3 0.05 MeNHNH 2 (6.0) 1.0 100(MW) 0.5 85 Ioannou et al [58] NiO 0.67 EtOH/KOH 5.0 N.A.…”

Recent Developments in Manganese, Iron and Cobalt Homogeneous Catalyzed Synthesis of Primary Amines via Reduction of Nitroarenes, Nitriles and Carboxamides

“…Nitroarenes bearing halogens and reducible groups such as ester, lactone, ketone, cyano and sulfonamide were able to undergo this reduction reaction, but no reaction took place with heteroaromatic nitro compounds like 2-nitropyridine, and nitroalkane substrates under such conditions. [26] To the best of our knowledge, there is no report described homogeneous cobalt catalyzed transfer hydrogenation to generate aniline derivatives starting from nitroarenes using alcohols or formic acid as hydrogen donor.…”

Cobalt catalyzed chemoselective reduction of nitroarenes: hydrosilylation under thermal and photochemical reaction conditions

“…Hydrazines were also used as a hydrogen source and several catalysts have been reported. 5 Formic acid has also been used for the catalytic transfer hydrogenation of nitroarenes. 6 Catalytic hydrogenation can be considered as the most effective and green protocol as compared to other reduction methods with stoichiometric reducing reagents.…”