An In Situ Spectroscopic Study of Prochiral Reactant–Chiral Modifier Interactions on Palladium Catalyst: Case of Alkenoic Acid and Cinchonidine in Various Solvents

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.