An In Situ Spectroscopic Study of Prochiral Reactant-Chiral Modifier Interactions on Palladium Catalyst: Case of Alkenoic Acid and Cinchonidine in Various Solvents Shuai Tan and Christopher T. Williams* Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States * S Supporting Information ABSTRACT: In situ attenuated total reflection infrared (ATR-IR) spectroscopy has been used to study the adsorption of a model α,β-unsaturated carboxylic acid (2-methyl-2- pentenoic acid) and chiral modifier (cinchonidine), as well as their intermolecular interactions, on Pd/Al 2 O 3 in typical polar (methanol) and nonpolar (dichloromethane) solvents. It has been found that the solvent has essentially no effect on cinchonidine adsorption. The carboxylic acid tends to adsorb on surface through bridging bidentate carboxylate in MeOH instead of monomer or dimer species, which is prevalent in the case of CH 2 Cl 2 . Moreover, an acid-cinchonidine complex prefers to form in a substrate to modifier ratio of 1:1, regardless of whether it is in the bulk solution or adsorbed on the surface. Thus, direct spectroscopic observation of an important intermediate in this catalytic system has been made. ■ INTRODUCTION Asymmetric hydrogenation has proved to be a promising method for chiral molecules produced in industrial applications, such as for pharmaceuticals and agrochemicals. 1,2 Significant success has been achieved by homogeneous catalysis. 3,4 However, only limited asymmetric synthesis reactions have been approached commercially in the field of heterogeneous catalysis, including CO bond hydrogenation of prochiral ketones and ketoesters over tartaric acid modified Ni 5-7 or cinchona alkaloid modified Pt. 8-10 It has been found that the cinchona alkaloid modified Pd catalyst is more effective than Ni or Pt for CC bond hydrogenation of unsaturated carboxylic acids, including alkenoic acids, 11-15 although the studies of this type of reaction are much less than in the two previous cases. It has been revealed by both theoretical modeling 16,17 and experimental studies 18-21 that cinchona alkaloids exhibit a very rich conformational behavior, which is the key for leading to a preferential formation of an enantiomer. In general, the orientation of quinoclidine N atom (i.e., points away/toward the quinoline ring) is the central role in controlling this behavior. Initially, several UHV-based techniques were applied in the study of adsorption of cinchona alkaloid on metal surface. For instance, the adsorption of 10,11-dihydrocincho- nidine (DHC) on Pt(111) was studied by use of X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). 22,23 Along with mass spectroscopy (MS) and H-D isotope exchange experiments, 24,25 these measure- ments confirmed that the quinoline moiety of the molecule is lying parallel to the metal surface through a π-electron interaction at room temperature, and becomes tilted at 323K, 26 although these UHV-based experiments were far from the real reaction condition. Through the end of the last century, there was no information regarding the surface of chiral-modified metals under conditions approaching those of the actual reaction conditions (i.e., liquid phase, elevated H 2 pressures). The application of in situ vibrational spectroscopic techniques such as infrared reflection-absorption spectroscopy (IRAS) 27,28 and attenuated total reflection infrared (ATR-IR) spectroscopy 29-33 has made it possible to begin to address this situation over the past decade and moving forward. A more important issue in the enantioselective hydro- genation of alkenoic acid other than simple adsorption behavior of the modifier and the substrate is the investigation of intermolecular interactions, which is crucial for catalytic performance in terms of activity and enantioselectivity. A deeper investigation of these aspects needs to be studied in order to further understand and optimize such catalytic systems. Compared to the well-known ketone/ketoester- cinchonidine-Pt system, 34-36 studies of the alkenoic acid- cinchonidine-Pd system are limited under conditions 37 that represent the actual chemical environment encountered during hydrogenation. Therefore, in this paper the in situ ATR-IR technique is applied to examine of adsorption of alkenoic acid and cinchonidine, along with their intermolecular interactions at the Pd surface. Received: April 2, 2013 Revised: July 21, 2013 Article pubs.acs.org/JPCC © XXXX American Chemical Society A dx.doi.org/10.1021/jp403273c | J. Phys. Chem. C XXXX, XXX, XXX-XXX