The molecular orientation, spatial distribution, and thermal behavior of the powerful chiral catalyst modifier precursor (S)-naphthylethylamine adsorbed on Pt[111] have been studied by NEXAFS, XPS, STM, and temperature programmed reaction. At 300 K, both in the presence and in the absence of coadsorbed hydrogen, the strongly tilted molecules do not form ordered arrays. These results constitute the first direct evidence against the template model and are at least consistent with the 1:1 interaction model of chiral induction in the enantioselective hydrogenation of alkyl pyruvates. Raising the temperature beyond 320 K (the temperature of enantioselectivity collapse) leads either to irreversible dimerization with hydrogen elimination or to dissociation of the ethylamine moiety, depending on whether coadsorbed H(a) is present. Either way, the stereogenic center is destroyed. These findings provide the first direct clue as to the possible origin of enantioselectivity collapse, by a mechanism not previously considered. When NEA and methyl pyruvate are coadsorbed in the presence of H(a), STM reveals entities that could correspond to a 1:1 docking complex between the prochiral reactant and the chiral modifier.

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...

STM, NEXAFS, XPS, and TPR have been used to characterize the adsorption and reactivity of methyl pyruvate on Pt{111}. In the absence of coadsorbed hydrogen, methyl pyruvate polymerizes at room temperature yielding polymer chains, partly dendritic. The STM and XPS data provide independent estimates of the average length, found to be ∼9 monomer units. NEXAFS shows that this polymer contains CdO bonds and no CdC bonds; the CdO bonds are inclined at 64°( 5°with respect to the metal surface. It is proposed that polymerization occurs by hydrogen elimination from the monomer, followed by an aldol condensation that involves elimination of methanol. This mechanism is in excellent accord with the intramolecular bonding, shape, and reactivity of the polymer deduced from the NEXAFS, STM, and TPR results. Coadsorbed hydrogen completely suppresses polymerization. These findings suggest that irreversible deactivation during start-up or steady-state operation of Pt catalysts during enantioselective hydrogenation of alkyl pyruvates can be due to hydrogen starvation which results in polymerization of the prochiral reactant.

The adsorption geometry of quinoid species at platinum and palladium surfaces is relevant to an understanding of enantioselective catalytic hydrogenation. Accordingly, quinoline multilayers and monolayers on Pd{111} have been characterized by XPS and the adsorption geometry within the monolayer determined by NEXAFS at 295 K and 360 K. The molecule lies approximately flat (α < 18° ± 5°), bound to the Pd{111} surface predominantly via the aromatic π-system. This adsorption geometry remains unaffected upon heating to 360 K, and the adsorbed layer does not exhibit long range order under any conditions. Comparison with the corresponding results for Pt{111} indicates that the different intrinsic rates and the reversal in the sense of enantioselectivity observed for pyruvate hydrogenation on palladium catalysts relative to platinum is unlikely to originate in either significantly different modifier adsorption geometries or spatial distributions in the two cases. In addition, it now seems less likely that the collapse in enantioselectivity observed beyond ∼320 K on quinoid-modified palladium catalysts originates in a change in the adsorption geometry of the anchoring moiety of the chiral modifier.