Enantioselective Chemisorption on a Chirally Modified Surface in Ultrahigh Vacuum: Adsorption of Propylene Oxide on 2-Butoxide-Covered Palladium(111)

Abstract

The enantioselective chemisorption of (S)- and (R)-propylene oxide is measured on a Pd(111) surface chirally modified using (S)- and (R)-2-butanol. Reflection−absorption infrared spectroscopic (RAIRS) data suggest that adsorbed 2-butanol forms 2-butoxide species when heated to ∼150 K and converts to a ketone with a concomitant loss in chirality at 200 K. Methyl ethyl ketone, ethylene, methane, CO, and hydrogen are found as products in temperature-programmed desorption (TPD). Propylene oxide adsorbs reversibly on Pd(111) at 80 K without undergoing any thermal decomposition, thus providing an ideal probe of surface chirality. The coverage of (R)-propylene oxide adsorbing on an (R)-2-butoxide-covered surface, ratioed to that on one covered by (S)-2-butoxide, reaches a maximum value of ∼2 at a relative 2-butoxide coverage of ∼25% of saturation and decreases to unity at a coverage of ∼50% of saturation. This implies that the enantioselectivity depends critically on coverage and arises due to chiral “pockets” formed on the surface.