INTRODUCTIONThe enantioselective hydrogenation of α-ketoesters over Ptbased catalysts and the closely related hydrogenation of β-ketoesters over Ni catalysts are two of the most heavily researched examples of heterogeneous asymmetric catalysis. 1À6 In each case, the hydrogenation reaction results in a racemic mixture of products when performed over bare metal catalysts. The key step in achieving enantioselective behavior is the adsorption of chiral modifiers from solution. In the Ni system, more than 100 modifiers have been investigated, including amino acids, amino acid derivatives, peptides, and hydroxyl acids, with α-hydroxy acids (e.g., tartaric acid) and α-amino acids 1 achieving the highest enantioselectivities.There are many examples of ultrahigh vacuum (UHV)-based studies aimed at providing insight into the role of the chiral modifier in achieving enantioselective behavior. 7À20 It is generally accepted that enantioselectivity is facilitated by establishing control over the adsorption geometry of the pro-chiral reagent, to favor the adsorption of one enantiotopic face as opposed to the other. This may be achieved by a direct docking interaction between a single chiral modifier and a pro-chiral reagent as is proposed to occur in the hydrogenation of α-ketoesters over Pt. 5 Alternatively, we have proposed that the formation of 2-D supramolecular assemblies of modifier and reactant could be important in the Ni-based hydrogenation reaction. 9,21 In addition, a number of groups have proposed that adsorption at step or step-kink defects can favor one enantiotopic face of a pro-chiral molecule. 22,23 Recently, to enhance the relevance of model studies, we have performed a number of investigations of chirally modified Ni surfaces prepared at the liquidÀsolid interface using surface vibrational spectroscopy in the form of reflection absorption infrared spectroscopy (RAIRS). 24À26 In particular, we have shown that the tautomeric form of methylacetoacetate (MAA) is very sensitive to the nature of the chirally modified surface. In particular, for the Ni/(R,R)-tartaric acid, 26 Ni/(S)-glutamic acid, 25 and Ni/(S)-aspartic acid 24 systems, it was determined that, following modification under conditions where enantioselectivity is optimized, MAA preferentially adopts the diketo tautomeric form. The preparation of chirally modified Ni catalysts involves treatment in an aqueous solution of modifier at a well-defined pH followed by washing and filtering. In the case of tartaric acid-the best known modifier for this reaction-this preparation procedure results in approximately 20% of a saturated monolayer of tartaric acid 26,27 or ∼0.05À0.06 ML of tartaric acid (where 1 ML is defined as 1 tartaric acid molecule per surface Ni atom).In the case of (S)-aspartic acid, under modification conditions where the catalyst performs most enantioselectively (modification at pH 5 and 300 K 28 ), we found that the washing procedure removes aspartate to levels below the detection levels of RAIRS 24 and X-ray photoelectron spectroscopy (XP...