Selective Mild Oxidation of Anilines into Nitroarenes by Catalytic Activation of Mesoporous Frameworks Linked with Gold‐Loaded Mn₃O₄ Nanoparticles

Abstract

Dedicated to the memory and achievements of Dr. Ioannis TamiolakisThis work reports the synthesis and catalytic application of mesoporous Au-loaded Mn 3 O 4 nanoparticle assemblies (MNAs) with different Au contents, i. e., 0.2, 0.5 and 1 wt %, towards the selective oxidation of anilines into the corresponding nitroarenes. Among common oxidants, as well as several supported gold nanoparticle platforms, Au/Mn 3 O 4 MNAs containing 0.5 wt % Au with an average particle size of 3-4 nm show the best catalytic performance in the presence of tert-butyl hydroperoxide (TBHP) as a mild oxidant. In all cases, the corresponding nitroarenes were isolated in high to excellent yields (85-97 %) and selectivity (> 98 %) from acetonitrile or greener solvents, such as ethyl acetate, after simple flash chromatography purification. The 0.5 % Au/Mn 3 O 4 catalyst can be isolated and reused four times without a significant loss of its activity and can be applied successfully to a lab-scale reaction of ptoluidine (1 mmol) leading to the p-nitrotulene in 83 % yield. The presence of AuNPs on the Mn 3 O 4 surface enhances the catalytic activity for the formation of the desired nitroarene. A reasonable mechanism was proposed including the plausible formation of two intermediates, the corresponding N-aryl hydroxylamine and the nitrosoarene.

Co(OAc)2/TBHP as a Highly Active Catalytic System for the Selective Oxidation of Aromatic Amines to Nitro Derivatives

“…According to the control experiments (Scheme 2 ) and the literature reports, 22 we propose a possible reaction mechanism as shown in Scheme 3 . With our standard conditions, TBHP oxidizes Co(II) to [Co(II)ox] and releases t -BuOH.…”

“…According to the control experiments (Scheme 2) and the literature reports [22] , we propose a possible reaction mechanism as shown in Scheme In conclusion, we have developed efficient catalyst system for the oxidation of arylamines, which provided nitroarenes selectively in good yields, employing the commercial available Co(OAc)2•4H2O and TBHP as the catalyst and oxidant, respectively. Many functional groups such as methoxy, methyl, ester, cyano, trifluoromethyl and halo groups are tolerated under these reactions conditions.…”

“…According to the control experiments (Scheme 2) and the literature reports, 22 In the whole process, TBHP is used to oxidize Co(II) to [Co(II)ox], so that the catalyst can be reused. The mechanism of this reaction is still under investigation in our laboratory.…”

“…Accordingly, several strategies have been published for this chemical transformation by employing various oxidizing agents such as peracetic acid (PAA), 11 3-chloroperoxybenzoic acid (m-CPBA), 12,13 peroxytriflouro acetic acid, 11 dimethyldioxirane, 14 oxone, 15 and potassium permanganate. 16 In addition to this, a number of metal-catalysed methods have been developed for the oxidation of aryl amines to nitroarenes, using tert-butyl hydroperoxide (TBHP) as oxidant in the presence of various catalysts, including [Rh2(cap)4], 17 CrS, 18 KI, 19 Zr(tBu)4, 20 CuAlPO-5, 21 Au/Mn3O4, 22 and Co-Schiff based complexes. 23 Despite significant progress, however, most of these approaches sitll suffer from moderate yield and poor selectivity, as well as the requirement of expensive ligands.…”

Controllable redox reaction cycle enabled by multifunctional Ru-containing polyoxometalate-based catalysts

“…The exploration of tailored conditions to realize maximum catalytic functionalization has aroused great attention in the catalysis industry; [43][44][45] thus, a series of control experiments were conducted for this purpose. For the methanol-Na 2 HAsO 4 system (Scheme 2a), the intermediate phenylhydroxylamine (Ph-NHOH) could be readily oxidized to NSB (Scheme 1, step 2), which in turn reacted with other Ph-NHOH molecules to generate AOB (Scheme 1, step 4), even without the use of any additive (Table S18, † entry 5).…”