Highly Selective and Solvent-Dependent Reduction of Nitrobenzene to N-Phenylhydroxylamine, Azoxybenzene, and Aniline Catalyzed by Phosphino-Modified Polymer Immobilized Ionic Liquid-Stabilized AuNPs

Abstract

Gold nanoparticles stabilized by phosphine-decorated polymer immobilized ionic liquids (AuNP@PPh2-PIILP) is an extremely efficient multiproduct selective catalyst for the sodium borohydride-mediated reduction of nitrobenzene giving N-phenylhydroxylamine, azoxybenzene, or aniline as the sole product under mild conditions and a very low catalyst loading. The use of a single nanoparticle-based catalyst for the partial and complete reduction of nitroarenes to afford three different products with exceptionally high selectivities is unprecedented. Under optimum conditions, thermodynamically unfavorable N-phenylhydroxylamine can be obtained as the sole product in near quantitative yield in water, whereas a change in reaction solvent to ethanol results in a dramatic switch in selectivity to afford azoxybenzene. The key to obtaining such a high selectivity for N-phenylhydroxylamine is the use of a nitrogen atmosphere at room temperature as reactions conducted under an inert atmosphere occur via the direct pathway and are essentially irreversible, while reactions in air afford significant amounts of azoxy-based products by virtue of competing condensation due to reversible formation of N-phenylhydroxylamine. Ultimately, aniline can also be obtained quantitatively and selectively by adjusting the reaction temperature and time accordingly. Introduction of PEG onto the polyionic liquid resulted in a dramatic improvement in catalyst efficiency such that N-phenylhydroxylamine could be obtained with a turnover number (TON) of 100 000 (turnover frequency (TOF) of 73 000 h–1, with >99% selectivity), azoxybenzene with a TON of 55 000 (TOF of 37 000 h–1 with 100% selectivity), and aniline with a TON of 500 000 (TOF of 62 500 h–1, with 100% selectivity). As the combination of ionic liquid and phosphine is required to achieve high activity and selectivity, further studies are currently underway to explore whether interfacial electronic effects influence adsorption and thereby selectivity and whether channeling of the substrate by the electrostatic potential around the AuNPs is responsible for the high activity. This is the first report of a AuNP-based system that can selectively reduce nitroarenes to either of two synthetically important intermediates as well as aniline and, in this regard, is an exciting discovery that will form the basis to develop a continuous flow process enabling facile scale-up.

Highly Active Hydrogenation Catalysts Based on Pd Nanoparticles Dispersed along Hierarchical Porous Silica Covered with Polydopamine as Interfacial Glue

“…The reduction of nitroarenes is one of the most used catalytic processes since the aniline and their derivatives represent a large part of the market in the organic chemical industry. This reaction implies the transformation of compounds of pollutant nature into intermediates in the industrial productions of agrochemicals, polymers, pharmaceuticals, dyes, and pigments [19][20][21][22][23][24][25].…”

Poly(vinylbenzyl chloride-co-divinyl benzene) polyHIPE monolith-supported o-hydroxynaphthaldehyde propylenediamine Schiff base ligand complex of copper(ii) ions as a catalyst for the epoxidation of cyclohexene

“…5,6 However, to improve the activity of catalysts on polymer gel-bead, spacers such as polyethylene glycol are used, but cleaving and poor loading of catalyst remained major drawbacks. 3,[12][13][14] Some efforts were made to increase the loading capacity of supports by controlling their swelling properties and improving the stability of supported catalysts. 4,15 Property modication in polymer gel-bead supports was found to be more useful in continuous ow reactors than batch reactors.…”

“…4,15 Property modication in polymer gel-bead supports was found to be more useful in continuous ow reactors than batch reactors. 14,[16][17][18][19] However, polymer gel-beads suffered from a channeling effect, which made interior active sites to remain unutilized in ow-through processes. 20 Considering these drawbacks of polymer gel-bead supports, the polyHIPE monoliths without interstitial space 5,6 are found to be more useful and ideal for continuous ow reactors.…”

“…[53][54][55] Though complexes of metal ions on inorganic supports are used as catalysts in various reactions, [56][57][58] polymer supports provide a better microenvironment to both reactants and catalysts. 14,37 Studies have indicated that supported Schiff base complexes of transition metal ions were more catalytic in the epoxidation of olens 59,60 than the unsupported ones. 61,62 To utilize the properties of polyHIPE monoliths, some studies were conducted using physically 63 or chemically bound catalysts on porous polyHIPE monoliths.…”

Citrate Stabilized Au‐FexOy Nanocomposites for Variable Exchange Bias, Catalytic Properties and Reversible Interaction with Doxorubicin

“…We have monitored the yield of amine products not only from 4‐NP, but also for other nitroarenes through HPLC for 5Fe , 6Fe , 7Fe and 9Fe as catalysts (Table , Figure S16). The results have been also compared with other catalysts ( C1 ‐ C12 ) reported in literature . In our study, we could isolate up to 98% free amine derivatives from the reactions.…”