Enantioselective heterogeneous catalysts are rarities although their inherent technical importance is huge. Under appropriate conditions they afford a high degree of stereochemical control and large rate enhancement effects. Indeed, chiral heterogeneous catalysis is a subject of indisputable importance in current chemical research. Such reactions constitute a relatively unexplored field whose theoretical and practical implications are potentially far reaching. Although the number of known systems is growing, the subject as a whole remains, nevertheless, at a relatively early stage of development as is evident from recent reviews. 1,2 This is especially true in regard to fundamental studies of the surface phenomena involved, even in the case of the most studied reactions, including the one which is the subject of this communications the asymmetric hydrogenation of R-ketoesters on chirally modified Pt surfaces. 3-5 Although considerable effort has been expended in the past decade to gain a detailed insight into the functioning of this system, there are still a number of key issues requiring clarification. Among these is the question of why the behavior of this catalytic system depends on the sequence of introduction of the reactants (methyl pyruvate, hydrogen) and modifier, as demonstrated earlier by transient kinetic measurements. 6 We have employed a combination of complementary methods involving solution phase kinetic measurements on a practical dispersed catalyst and studies on a Pt{111} single-crystal surface by means of STM and NEXAFS. We show that in the absence of the cinchona modifier and under conditions of hydrogen starvation the catalyst deactivates due to blocking of the platinum surface by self-condensation of the methyl pyruvate reactant.Catalytic studies were performed using a 4 mm inner diameter stainless steel tubular fixed-bed reactor system. Details of the reactor, analysis system, and experimental technique are given elsewhere. 7 The catalyst (5 wt % Pt/Al 2 O 3 , Engelhard 4759) was pretreated before use in a separate reactor by flushing with 12 mL‚min -1 N 2 (99.995%) at 673 K for 30 min, followed by a reductive treatment in H 2 (99.999%) for 90 min at the same temperature. After being cooled to room temperature in H 2 , the catalyst was immediately transferred to the reactor and held under nitrogen. Catalyst (500 mg) was applied, resulting in a bed length of 30 mm. Methyl pyruvate (MP, Fluka, 97%) was used without further purification. The reactor was operated at room temperature and a H 2 pressure of 50 bar. STM experiments were carried out using an Omicron UHV STM-1 instrument operating under ultrahigh vacuum conditions (base pressure 5 × 10 -11 mbar). This apparatus incorporated LEED and Auger spectroscopy facilities used for surface characterization 8 prior to adsorption experiments. Images were acquired in constant current mode and control experiments indicated that there were no tip-induced artifacts: neither molecular displacements, nor adsorbate decomposition.The reaction was start...
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