A hydroquinone based palladium catalyst for room temperature nitro reduction in water

Abstract

A hydroquinone based palladium complex [Pd(H 2 L)(Cl) 2 ] (1), acts as an efficient room temperature catalyst for reduction of nitroarenes in water as solvent. 1 also acts as a tandem catalyst for Suzuki-Miyaura cross coupling in ethanol followed by reduction of nitroarenes in one pot with a loading of 0.25 mol% catalyst. Palladium based catalysts are well known in the literature for their efficiency in a wide spectrum of coupling reactions and reduction of various functionalities. 1 Such catalytic reactions are of importance for their use in industrial applications spanning from pharmaceuticals to polymers or syntheses of natural products, dyes. 2 Aryl amine derivatives are very important intermediates in syntheses of many of the above using nitroarene precursors. 3 Room temperature reduction of nitroarenes is advantageous since there are compounds which are susceptible to degradation when heated during reduction. Several transition metal based catalysts are known to perform reduction of nitroarenes. 4 Most metal catalyzed nitro reduction reactions require higher temperatures, 4f,5 hydrogen gas, 3b,6 or use of expensive hydride source like silanes. 4b,5e,g,7 Among the most recent ones the work of Beller and co-workers used a Fephenanthroline catalyst (1.0 mol%) at 100 C in THF with four molar equivalent hydrazine as the hydrogen source for efficient and very selective reduction of nitroarenes. 5b However, metal complexes that reduces nitroarenes at room temperature are scarce ( Table 1). Most of such catalysts are nano particles (Table 1). Hence performing the nitroarene reduction in a green solvent like water at room temperature with a discrete metal complex is challenging. Very few catalysts perform the reduction of nitroarenes at room temperature with water as the solvent (Table 1) viz. Zn-dust (10 eq.) in surfactant (TPGS-750 M) water mixture 8 and the ionic liquid based PVDF-[C 6 (mpy) 2 ] [NiCl 4 ] 22 (5 mol%) catalyst 9 are needed in less amounts, are less toxic to environment are of high interest for their potential applications. 10 Such catalysts are air stable, efficient, economically viable and hence greener. The hydroquinone ligand used in this work is synthesized in a single step and is stable. The Palladium catalyst is active in lesser mol% without a phosphine or carbene ligand. To the best of our knowledge the use of hydroquinone in nitroarene reduction is not known.The reaction between a pyrazole derivatized hydroquinone ligand H 2 L and Pd II (MeCN) 2 Cl 2 in acetonitrile under reux condition led to formation of complex 1 which was further puried by crystallization from acetonitrile solution (Scheme 1). The single crystal X-ray diffraction showed the complex crystallizes in orthorhombic, space group Cmc2(1). The N atoms from the two pyrazoles chelate to the Pd II forming a seven membered ring and the remaining two coordination sites of the Pd II are satised by two chloride ions. The two Pd-N bond distances are ca. 2.024Å with an angle of ca. 85.5 . 1 H NMR of the complex indicate that the co...

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