Abstract
Recently, N-substituted anilines have been the object of increasing research interest in the field of organic chemistry due to their role as key intermediates for the synthesis of important compounds such as polymers, dyes, drugs, agrochemicals and pharmaceutical products. Among the various methods reported in literature for the formation of C–N bonds to access secondary anilines, the one-pot reductive amination of aldehydes with nitroarenes is the most interesting procedure, because it allows to obtain diverse N-substituted aryl amines by simple reduction of nitro compounds followed by condensation with aldehydes and subsequent reduction of the imine intermediates. These kinds of tandem reactions are generally catalyzed by transition metal-based catalysts, mainly potentially reusable metal nanoparticles. The rapid growth in the last years in the field of metal-based heterogeneous catalysts for the one-pot reductive amination of aldehydes with nitroarenes demands for a review on the state of the art with a special emphasis on the different kinds of metals used as catalysts and their recyclability features.
Highlights
RA is referred to as direct reductive amination (DRA), when the carbonyl compound, generally an aldehyde or a ketone, and the primary amine are mixed in the presence of the proper reducing agent without prior formation of the intermediate imine
If the primary amines used in RA are anilines, nitroarenes can alternatively be employed as their precursors for accessing secondary amines, because anilines are achieved by the catalytic hydrogenation of nitroarenes [16,17,18]
The high chemoselectivity shown by Au/Al2O3 paved the way to a follow-up article [83] in which the same group reported that Au/Al2O3 could catalyze in a continuous-flow the DRA of aliphatic unsaturated aldehydes with nitroarenes (Scheme 31) obtaining unsaturated secondary amines in moderate to excellent yields (46−99%)
Summary
Secondary amines occupy a paramount role as intermediates and building blocks for the production of numerous agrochemicals, detergents, drugs, dyes, herbicides, pharmaceuticals, pigments, and other fine chemicals [1,2]. The very interesting aspect of this catalytic system stands in both the use of water as the solvent (which notoriously inhibits the imine formation via condensation of aniline and carbonyl group) and the employing of NaBH4 as the reductant, that normally quickly reduces the C=O polar double bond of the aldehyde, even in the absence of metal catalyst, subtracting aldehyde for the condensation step reaction. In 2014, following their study on Pd nanoparticles supported onto magnetite seeds, Li and Ma and co-workers synthesized with the reverse microemulsion method uncommon nano-catalysts constituted by Fe3O4@SiO2 core-shell nanospheres of 35 nm in size, which were functionalized with ethylenediamine groups in turn reacted with succinyl chloride to support Pd NPs of 5 nm in diameter, homogeneously distributed onto the external matrix surface (XL-Pd(0)–Fe3O4@SiO2) [63] These nano-catalysts resulted active and selective in the DRA at room temperature in ethanol under 1 atm H2 starting from nitrobenzene with various benzaldehydes (Scheme 15).
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