Synthesis and Application of Polystyrene Nanospheres Supported Platinum Catalysts in Enantioselective Hydrogenations
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Synthesis and Application of Polystyrene Nanospheres Supported Platinum Catalysts in Enantioselective Hydrogenations Be´la To¨ro¨k • Aditya Kulkarni • Ryan DeSousa Kalyani Satuluri • Marianna To¨ro¨k • G. K. Surya Prakash
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Received: 3 May 2011 / Accepted: 22 July 2011 / Published online: 13 August 2011 Ó Springer Science+Business Media, LLC 2011
Abstract Nearly monodisperse polystyrene nanospheres coated with platinum have been synthesized and applied successfully in the cinchona alkaloid-modified enantioselective hydrogenation of ethyl pyruvate. During the catalyst preparation broadly varied experimental conditions were used resulting in several catalysts with different properties. The effect of hydrogen pressure, solvent, modifier and polystyrene nanoparticle size on the enantioselectivity was also studied. Based on the results the effect of support on the mechanism of the enantiodifferentiation has been discussed. Keywords Platinum Polystyrene nanospheres Cinchona alkaloids Ethyl pyruvate Hydrogenation Enantioselective Heterogeneous
1 Introduction One of the most intensely studied test reactions in the heterogeneous catalytic asymmetric hydrogenation of a-keto esters is the hydrogenation of ethyl pyruvate to ethyl lactate on cinchona alkaloids modified Pt catalyst (Orito’s reaction) [1, 2]. Since it was first reported by Orito et al., B. To¨ro¨k (&) A. Kulkarni M. To¨ro¨k Department of Chemistry, University of Massachusetts Boston, Boston, MA 02125, USA e-mail: [email protected] B. To¨ro¨k K. Satuluri M. To¨ro¨k Department of Chemistry, Michigan Technological University, Houghton, MI 49931, USA R. DeSousa G. K. S. Prakash Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, CA 90089-1661, USA
extensive efforts were focused on investigating the mechanism of the reaction including the role of modifier, catalyst, catalyst support, solvent and hydrogen pressure [3–8]. An important characteristic feature of this heterogeneous catalytic system is the simultaneous enhancement observed both in reaction rate and enantiomeric excess (ee) [9, 10]. Cinchona alkaloids such as cinchonidine (CD) or methoxycinchonidine (MeOCD) and cinchonine (CN) remained the modifiers of choice over three decades of developments. The use of other modifiers, however, revealed invaluable information regarding important details of the reaction mechanism [11–13]. Several enantiodifferentiation models have been proposed to explain the mode of action of cinchona-modified Pt catalysts or analogues thereof [14–18]. The information gained from these studies such as kinetic and spectroscopic investigations were rationalized to coherent mechanistic proposals and the new developments have been summarized in several recent reviews [19–23]. By now, the modified catalyst model [19–23] enjoys broad acceptance in this field. There is also an agreement in proposing a strong interaction between adsorbed ethyl pyruvate (or a half-hydrogenated intermediate) and the adsorbed mod
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