Abstract
The chemisorption of two chiral molecules, propylene oxide and glycidol, is studied on tartaric-acid modified Pd(111) surfaces by using temperature-programmed desorption to measure adsorbate coverage. It is found that R-glycidol shows preferential enantioselective chemisorption on (S,S)-tartaric acid modified Pd(111) surfaces, while propylene oxide does not adsorb enantioselectively. The enantioselectivity of glycidol depends on the tartaric acid coverage, and is exhibited for low tartaric acid coverages indicating that the bitartrate phase is responsible for the chiral recognition. The lack of enantioselectivity when using propylene oxide as a chiral probe implies that the enantiospecific interaction between glycidol and bitartate species is due to hydrogen-bonding interactions of the -OH group of glycidol. Scanning tunneling microscopy images were collected for tartaric acid adsorbed on Pd(111) under the same experimental conditions as used for enantioselective experiments. When tartaric acid is dosed at room temperature and immediately cooled to 100 K for imaging, individual bitartrate molecules were found. Density functional theory (DFT) calculations show that bitartrate binds to Pd(111) through its carboxylate groups and the -OH groups are oriented along the long axis of the bitartrate molecule. An enantiospecific interaction is found between glycidol and bitartate species where R-glycidol binds more strongly than S-glycidol to (S,S)-bitartate species by simultaneously forming hydrogen bonds with both the hydroxyl and carboxylate groups, thereby providing three-point bonding.
Highlights
Pharmaceuticals must generally be manufactured in their enantiopure form and are often synthesized using homogeneousphase catalysts, requiring subsequent purification steps.[1]
temperature programmed desorption (TPD) experiments are performed to measure the enantioselectivity of tartaric acid modified-Pd(111) by using PO as a chiral probe
The tartaric acid coverage at which enantioselectivity is measured indicates that bitartrate species provides the chiral modifier, while other tartaric-acid derived structures are not enantioselective
Summary
Pharmaceuticals must generally be manufactured in their enantiopure form and are often synthesized using homogeneousphase catalysts, requiring subsequent purification steps.[1]. The surface chemistry of tartaric acid has been explored on a number of While such studies provide detailed information on the local and extended chiral structures on the surface, with the exception of studies of co-adsorbed tartaric acid and methylacetoacetate on nickel,[15] they provide little information on the interactions that lead to enantioselectivity. Such interactions can be explored using chiral probe molecules (here, R- and S-propylene oxide (PO) or glycidol) on chirally modified surfaces.[35,36] For example, such experiments showed differences in the coverages of R- or S-PO
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