Conditioning of Palladium-on-carbon (Pd/C) for Uniform Cinchonidine Modification to Perform Highly Enantioselective Hydr
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Conditioning of Palladium-on-carbon (Pd/C) for Uniform Cinchonidine Modification to Perform Highly Enantioselective Hydrogenation Tae Yeon Kim Æ Mai Yokota Æ Takayuki Uchida Æ Takashi Sugimura
Received: 21 May 2009 / Accepted: 4 June 2009 / Published online: 23 June 2009 Ó Springer Science+Business Media, LLC 2009
Abstract Cinchonidine (CD)-modified palladium catalyst gave the high product ee when a commercial wet Pd/C was used after the co-evaporation pretreatment. The improvement in the ee was attributed to removal of water from a specific Pd/C interior. The use of a water-soluble CDHCl salt resulted in the high ee without pretreatment. Keywords Enantioselectivity Hydrogenation Pd/C Pretreatment Carbon texture
1 Introduction Cinchonidine (CD)-modified palladium catalysts have enantio-differentiating ability in the hydrogenation of prochiral olefins [1–9], and a superior catalyst to give products in high enantiomer excess (ee) has been prepared from a specific palladium catalyst by mixing with a solution of CD under the hydrogenation conditions. The catalyst properties to affect the product ee have been keenly studied [10–16], and the best catalyst for the modification so far found is a homemade 40% Pd/TiO2 that provides up to 91% ee in the hydrogenation of phenylcinnamic acid (PCA) under the optimized conditions (atmospheric pressure, 273–300 K, 0.6–1.0 eq. of benzylamine, and a water-containing solvent) (Scheme 1) [10, 11]. Another palladium catalyst good for the CD modification was found several years ago from commercial Pd/C catalysts [17, 18]. With this catalyst, a pretreatment is indispensable to obtain the high ee, and a certain Pd/C pretreated at 353 K (80 °C) in the T. Y. Kim M. Yokota T. Uchida T. Sugimura (&) Graduate School of Material Science, University of Hyogo, 3-2-1 Kohto, Kamigori, Ako-gun, Hyogo 678-1297, Japan e-mail: [email protected]
reaction solvent under hydrogen gives 82% ee after the CD-modification for the hydrogenation of PCA. The CDmodified Pd/C is advantageous in a practical use because the catalyst can be easily prepared from the readymade Pd/C, and more importantly, it has much higher catalytic activity than the CD-modified Pd/TiO2. The properties of the Pd/C suitable for the CD-modification are completely different from the well-optimized and characterized Pd/TiO2 catalysts (e.g., metal dispersion of the Pd/TiO2 is very low at \0.2, while Pd/C is very high at 0.7 or higher), but the reason for the high ee with Pd/C is not clarified yet. Especially, the role of the pretreatment is difficult to be presumed because the pretreatment conditions are so mild that no changes on the metal particle or carbon support are detected. During further study on the enantioselective hydrogenation with Pd/C, we found another pretreatment method that based on evacuation of the catalyst. In this report, we will present the results of the new pretreatment method for Pd/C and analysis of the improvement mechanism in comparison with the 80 °C pretreatment.
2 Experimental 2.1 Mate
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