Enantio-differentiating hydrogenation of methyl acetoacetate over modified Raney nickel catalysts prepared by two-step m
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Catalysis Letters Vol. 112, Nos. 3–4, December 2006 (Ó 2006) DOI: 10.1007/s10562-006-0198-4
Enantio-differentiating hydrogenation of methyl acetoacetate over modified Raney nickel catalysts prepared by two-step modifications Tsutomu Osawa,a,* Kenji Yoshino,b Kosuke Takimoto,b Osamu Takayasu,a and Tadao Haradab a Faculty of Science, University of Toyama, Gofuku, Toyama 930-8555, Japan Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194, Japan
b
Received 10 September 2006; accepted 20 September 2006
Two-step modifications of the Raney nickel were examined for the enantio-differentiating hydrogenation of methyl acetoacetate. Among the two-step modifications attempted in this study, the combination of ‘pre-modification with disodium tartrate and NaBr in water’ and ‘in-situ modification with tartaric acid’ resulted in the highest optical yield (84%). This modification process is a greener process than the conventional one carried out under weakly acidic conditions, because this process generated almost no Ni2+ ions. The pre-modification with the tartrate and NaBr preferentially eliminated Al3+ from the Raney nickel surface. KEY WORDS: tartaric acid-sodium bromide-modified Raney nickel; in-situ modification; pre-modification; two-step modification; enantio-differentiating hydrogenation.
1. Introduction The study of the enantio-differentiating reaction using a homogeneous or heterogeneous catalyst is one of the most active areas in fine chemical synthesis [1,2]. Some homogeneous enantio-differentiating catalysts with the BINAP ligand(s) provide significant results for the enantiomer excess (e.e.) of products [3]. In addition, the investigation of heterogenized catalysts prepared from homogeneous enantio-differentiating catalysts has been attracting increasing attention as advances in clean technology and green chemistry [4]. Generally, heterogeneous enantio-differentiating catalysts possess several advantages over homogeneous ones, such as easy separation, easy regeneration, and long service life. Heterogeneous enantio-differentiating catalysts are expected to play an increasing important role in the development of greener processes and products. Nevertheless, the successful uses of heterogeneous catalysts for the production of enantiomerically pure compounds are rare. Among the enantio-differentiating heterogeneous catalysts, Ni catalysts modified with tartaric acid and sodium bromide (TA–NaBr–MNi) and cinchona-modified Pt or Pd catalysts have already reached the industrial applicable level [1,5,6]. The conventional TA–NaBr–MNi catalysts can be easily prepared by immersion of the activated Ni powder in an aqueous solution of TA and NaBr under weakly acidic and high-temperature conditions (pre-modification method) [6,7]. During the pre-modification process, *To whom correspondence should be addressed. E-mail: [email protected]
the nickel powder is partially oxidized to Ni2+ ions and the ions are then dissolved into the modification solution [7]. The nickel ions possess some toxicity to people or the environment.
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