Size controlled synthesis of Pd nanoparticles in water and their catalytic application in C–C coupling reactions Sudeshna Sawoo, Dipankar Srimani, Piyali Dutta, Rima Lahiri, Amitabha Sarkar * Department of Organic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700032, India article info Article history: Received 3 February 2009 Received in revised form 20 March 2009 Accepted 23 March 2009 Available online 27 March 2009 Keywords: Pd nanoparticle Fischer carbene complex PEG Water C–C coupling abstract Catalytically active Pd nanoparticles have been synthesized in water by a novel reduction of Pd(II) with a Fischer carbene complex where polyethylene glycol (PEG) was used as stabilizer. PEG molecules wrap around the nanoparticles to impart stability and prevent agglomeration, yet leave enough surface area available on the nanoparticle for catalytic activity. Our method is superior to others in terms of rapid generation and stabilization of Pd nanoparticles in water with a cheap, readily available PEG stabilizer. The size of the nanoparticles generated can be controlled by the concentration of PEG in water medium. The size decreased with the increase in the PEG: Pd ratio. This aqueous nano-sized Pd is a highly efficient catalyst for Suzuki, Heck, Sonogashira, and Stille reaction. Water is used as the only solvent for the coupling reactions. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Organic synthesis is routinely performed in non-aqueous sol- vents, be it in academic research laboratories or industrial pro- duction units. Lack of solubility of generally lipophilic organic or organometallic compounds in water compelled chemists to employ non-aqueous solvents. In contrast, nature, in its unique way, utilizes water for enzymatic transformations of all organic substrates. The use of water, the most abundant and non-toxic solvent for reactions is reclaiming its importance due to pressing environmental, eco- nomical, and safety concerns. 1 Current emphasis on green chem- istry underscores the need for developing catalytic reactions, especially with involving precious metals as mediators, in aqueous medium. 2 Palladium catalyzed carbon–carbon and carbon–heteroatom coupling reactions are extensively used in the synthesis of complex organic molecules. 3 In these reactions, either preformed Pd com- plexes like (PPh 3 ) 4 Pd are used where ligands are P and/or N donors or active Pd complexes are generated in situ. 4 Many of these ligands are expensive, toxic, and sensitive to air and a major challenge lies in the separation of product from expensive catalyst, much to the dismay of large-scale producers. As an alternative, solid supported Pd catalysts have been developed for several reactions. 5 Ionic liq- uids are said to provide a green alternative to conventional organic solvents, 6 but they are not devoid of toxicity. Efforts have been made, therefore, to develop a Pd catalytic system free of organic solvent for such reactions. Water-soluble, phosphine based ligands, 7 and phase-transfer catalysts 8 have been used in an aqueous–or- ganic biphasic reaction, but they operate under rather harsh conditions. Nanoparticle catalysis in water squarely addresses the issue of carrying out efficient reactions under environmentally benign conditions associated with green chemistry and also facilitates separation of products 9 from Pd colloids dispersed in water. Since catalysis takes place on metal surface, nanoparticles are much more reactive than the particulate metal counterpart due to their high surface to volume ratio. 10 In this paper, we report a combination of both these facets use of water as solvent and use of Pd nano- particles, which are synthesized and stabilized in water 11 as cata- lyst for a variety of coupling reactions where predominantly lipophilic organic reactants provide high yield of products in water as the only solvent. 2. Experimental 2.1. General All chemicals were purchased as reagent grade from commercial suppliers and used without further purification, unless otherwise noted. Reactions were monitored by thin layer chromatography (TLC). All 1 H NMR (300 MHz), 13 C NMR (75 MHz) spectra were recorded on a Bruker-Avance DPX300 for a CDCl 3 solution and * Corresponding author. Tel.: þ913324734971; fax: þ913324732805. E-mail address: ocas@iacs.res.in (A. Sarkar). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2009.03.062 Tetrahedron 65 (2009) 4367–4374 Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet