Highly Efficient and Selective Reduction of Nitroarenes to N-Arylhydroxylamines Catalysed by Phosphine Oxide-Decorated Polymer Immobilized Ionic Liquid Stabilized Ruthenium Nanoparticles

RuNPs stabilised by phosphine oxide decorated PEG-modified polymer immobilised ionic liquid, RuNP@O=PPh2-PEGPIILS, (2) is a remarkably efficient and selective catalyst for the hydrazine hydrate-mediated partial reduction of nitroarenes to the corresponding N-arylhydoxylamine. Near quantitative conversion to N-phenylhydroxylamine with >99% selectivity was obtained after only 2 h when the reaction was conducted at 25 °C in ethanol under an inert atmosphere using 0.1 mol% catalyst. Under these conditions, the composition-time profile showed that the reduction occurred via the direct pathway whereas reactions in air gave a mixture of azoxy-based products due to competing condensation resulting from reversible formation of N-phenylhydroxylamine. The initial TOF of 6,100 h-1 obtained after 10 min at 40 °C with 0.1 mol% 2 is among the highest to be reported for the metal nanoparticle catalysed reduction of nitrobenzene to N-phenylhydroxylamine and a significant improvement on 5 wt% Ru/C which gave a modest conversion of 21% (initial TOF = 240 h-1) to a mixture of N-phenylhydroxylamine and aniline. A broad range of substituted N-aryl and N-heteroaryl nitroarenes were reduced to the corresponding N-arylhydroxylamine in high yield and with excellent selectivity by adjusting the reaction times. However, reduction of electron rich amino-substituted nitroarenes was extremely slow and resulted in reduction to the aniline with no evidence for the corresponding hydroxylamine. Complete reduction of amino substituted nitroarene is proposed to be facilitated by amine-assisted elimination of hydroxide from the hydroxylamine to afford a readily reducible quinondiimine-derived iminium intermediate that reacts with a surface hydride to liberate the amine. Under optimum conditions the catalyst could be reused five times for the reduction of nitrobenzene to N-phenylhydroxylamine with no detectable change in activity and only slight decrease in selectivity.