Heterogeneous Enantioselective Hydrogenation in a Continuous-flow Fixed-bed Reactor System: Hydrogenation of Activated K
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Heterogeneous Enantioselective Hydrogenation in a Continuousflow Fixed-bed Reactor System: Hydrogenation of Activated Ketones and Their Binary Mixtures on Pt–Alumina–Cinchona Alkaloid Catalysts Gyo¨rgy Sz} oll} osi • Zsolt Makra • Mo´nika Fekete Ferenc Fu¨lo¨p • Miha´ly Barto´k
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Received: 17 January 2012 / Accepted: 10 May 2012 / Published online: 30 May 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Under the experimental conditions of the Orito reaction the individual hydrogenation and the competitive hydrogenations of three binary mixtures of methyl benzoylformate (MBF), pyruvic aldehyde dimethyl acetal (PA) and 2,2-diethoxyacetophenone (DAP) on platinum–alumina catalysts modified by cinchonidine, cinchonine, quinine and quinidine (Pt–CD, Pt–CN, Pt–QN, Pt–QD) were studied for the first time using continuous-flow fixed-bed reactor system. Conversions of chiral (Cc) and racemic (Cr) hydrogenations of all three compounds and enantioselectivities (ee) were determined under the same experimental conditions (under 4 MPa H2 pressure, at room temperature using toluene/AcOH 9/1 as solvent).The order of the rates of the enantioselective hydrogenations of the three substrates studied is MBF [ PA [ DAP, and the order of their ee values is MBF * PA [ DAP. The hydrogenation rate and the effect of rate on ee depend on the structure of the cinchona used: hydrogenation of MBF and PA may produce ee values over 90 %, however, the ee values were conspicuously low in the presence of Pt–QN and especially of Pt–QD catalysts. In the chiral hydrogenation of DAP
G. Sz}oll}osi (&) F. Fu¨lo¨p M. Barto´k (&) Stereochemistry Research Group, The Hungarian Academy of Sciences, Do´m te´r 8, Szeged 6720, Hungary e-mail: [email protected] M. Barto´k e-mail: [email protected] Z. Makra M. Fekete M. Barto´k Department of Organic Chemistry, University of Szeged, Do´m te´r 8, Szeged 6720, Hungary F. Fu¨lo¨p Institute of Pharmaceutical Chemistry, University of Szeged, Eo¨tvo¨s u 6, Szeged 6720, Hungary
considering racemic hydrogenation rate decrease (Cc/Cr \ 1) takes place instead of rate enhancement over all four catalysts. The new experimental data supported the so far known fundamental rules of the Orito reaction based on batch studies. Keywords Cinchona alkaloid Competitive hydrogenations Enantioselective Methyl benzoylformate Pyruvaldehyde dimethyl acetal 2,2-Diethoxyacetophenone Pt–alumina Continuous-flow reactor
1 Introduction The heterogeneous enantioselective hydrogenation of prochiral ketones is one of the most important methods for the synthesis of chiral secondary alcohols [1–7]. Besides the widespread application of batch reactors the continuousflow fixed-bed reactor system (CFBR) has also been utilized in the chiral hydrogenation of activated ketones [8–12], mainly to study the mechanism of the Orito reaction [13–17] and in lesser extent to the practical realization of the reactions [18, 19]. Competitive hydrogenation of ketone mixtures over metal catalysts is a widely used method in stu
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