Zirkonium-katalysierte Oxidation von primären aromatischen Aminen zu Nitroverbindungen mittert-Butylhydroperoxid

Synthesis of Nitroarenes by Oxidation of Aryl Amines

Nitro compounds are an important class of organic molecules with broad application in organic synthesis, medicinal chemistry, and materials science. Among the variety of methodologies available for their synthesis, the direct oxidation of primary amines represents an attractive alternative route. Efforts towards the development of oxidative procedures for the synthesis of nitro derivatives have spanned over the past decades, leading to a wide variety of protocols for the selective oxidative conversion of amines to nitro derivatives. Methods for the synthesis of nitroarenes via oxidation of aryl amines, with particular emphasis on recent advances in the field, are summarised in this review.

Selective Mild Oxidation of Anilines into Nitroarenes by Catalytic Activation of Mesoporous Frameworks Linked with Gold-Loaded Mn3 O4 Nanoparticles

This work reports the synthesis and catalytic application of mesoporous Au-loaded Mn3 O4 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/Mn3 O4 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/Mn3 O4 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 p-toluidine (1 mmol) leading to the p-nitrotulene in 83 % yield. The presence of AuNPs on the Mn3 O4 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.

Mutagenicity in Salmonella typhimurium TA98 and TA100 of nitroso and respective hydroxylamine compounds

Five aromatic nitroso compounds were prepared and their mutagenicity in Salmonella typhimurium strains TA98 and TA100 compared with that of the corresponding hydroxylamines and the previously studied nitroarenes. A remarkable correspondence of the dose-response curves was observed between the nitroso and the respective hydroxylamine compounds. This effect could be observed in TA98 and TA100. It was only marginally dependent on the metabolical activation by rat liver S9-mix. Even the presence of a bulky alkyl substituent either near to the functional group, or far away from it, previously shown to considerably influence the mutagenic properties of nitroarenes, does not remarkably affect the properties of the nitroso and hydroxylamine species. The similarity between the latter two is likely to be due to a fast reduction of the nitrosoarenes to the hydroxylamine species under the test conditions. It seems that enzymes are not responsible for that reduction step, because sterical crowding near the functional group does not influence that behaviour. The test results of the aromatic hydroxylamines bearing a bulky substituent show that there are at least two ways to influence the mutagenicity of an aromatic nitro compound by such a group. A substituent near the functional group (ortho-position) disturbs the enzymatic reduction of the nitro group, because 3-tert-butyl-4-hydroxylaminobiphenyl and its corresponding nitroso compound are highly mutagenic, whereas 3-tert-butyl-4-nitrobiphenyl was previously shown to be inactive even after addition of S9-mix. In contrast, 4'-tert-butyl-4-hydroxylaminobiphenyl with the tert-butyl group "far away" from the hydroxylamino functionality clearly shows decreased mutagenic activity suggesting a different influence of a substituent in that position. In addition, the substance shows only little cell toxicity even at higher concentrations. Both effects could be due to a reduced effective dose of the hydroxylamine in the cells compared to the non-alkylated compound, caused by a faster degradation of the hydroxylamine or a hindered interaction between that substance and the cells.

From mutagenic to non-mutagenic nitroarenes: effect of bulky alkyl substituents on the mutagenic activity of 4-nitrobiphenyl in Salmonella typhimurium. Part I. Substituents ortho to the nitro group and in 2'-position

Eleven alkyl substituted derivatives of 4-nitrobiphenyl (4NBp) and two corresponding nitroso compounds were synthesised and tested for mutagenic potency in strains TA98 and TA100 of Salmonella typhimurium. The mutagenicity of compounds substituted ortho to the nitro group (3-methyl-, 3-ethyl-, 3-isopropyl-, 3-tertbutyl-, 3, 5-diethyl-, 3,5-diisopropyl-, and 3,5-ditertbutyl-4NBp) decreased with growing steric demand of the alkyl substituents in both tester strains. The most sterically hindered compounds were non-mutagenic even at highest concentrations. This reduction of mutagenicity is correlated with deviations from the coplanar orientation of the nitro group relative to the aromatic plane. Since a comparable decrease of mutagenicity for the nitroso compounds (4NOBp and 3-tertbutyl-4NOBp) was not observed, the first reduction step of non-planar nitro groups must be inhibited. Alkyl groups in the 2'-position of 4NBp (2'-methyl-, 2'-ethyl-, 2'-isopropyl-, and 2',4', 6'-trimethyl-4NBp) also reduced mutagenic activity with increasing size and removed it completely for the most sterically hindered species (2'-isopropyl-, 2',4',6'-trimethyl-4NBp). In this case, the co-planarity of the nitro group is not affected but the twisting of the two aromatic rings, which is associated with a less effective charge delocalisation of the nitrenium ion. The experimental mutagenicities of all nitro compounds were compared to predicted values, that are based on recently developed empirical equations. While there was reasonable correspondence for the parent compound (4NBp), its ortho methylated derivative (3-methyl-4NBp) and two highly hydrophobic dialkylated species (3,5-diisopropyl- and 3, 5-ditertbutyl-4NBp), predictions for all other alkyl substituted compounds were too high. Thus, steric parameters should be included to improve the general validity of predictions by means of quantitative structure-activity relationships (QSAR).