Requirements for the Formation of a Chiral Template

Abstract

The chemisorptive enantioselectivity of propylene oxide is examined on Pd(111) surfaces templated by chiral 2-methylbutanoate and 2-aminobutanoate species. It has been found previously that chiral propylene oxide is chemisorbed enantiospecifically onto Pd(111) surfaces modified by either (R)-or (S)-2-butoxide. The enantiomeric excess (ee) varied with template coverage, reaching a maximum of ∼31%. Templating the surface using 2-methylbutanoate, where the chiral center is identical to that in the 2-butoxide species, but is now anchored to the surface by a carboxylate rather than an alkoxide linkage, shows no enantiospecificity. The enantioselectivity is restored when the methyl group is replaced by an amine group, where a maximum ee value of ∼27% is found. DFT calculations and infrared measurements suggest that the structures of the butyl group on the surface are similar for both 2-butoxide and 2-methylbutanoate species, implying that gross conformational changes are not responsible for differences in chemisorptive enantioselectivity. There is no clear correlation between the location of the chiral center and enantioselectivity, suggesting that differences in the template adsorption site are also not responsible for the lack of enantioselectivity. It is proposed that the 2-butyl group in 2-methylbutanoate species is less rigidly bonded to the surface than that in 2-butoxides, allowing the chiral center to rotate azimuthally. It is postulated that the role of the amino group in 2-aminobutanoate species is to anchor the chiral group to the surface to inhibit azimuthal rotation.

Enantiospecific Chemisorption of Small Molecules on Intrinsically Chiral Cu Surfaces

“…Herein, we consider a chiral Cu surface of the type that has been used in a number of experiments [15][16][17][18] and examine two molecular adsorbates, one of which, propylene oxide, has been used in several experimental studies of surface chirality. [1,16,19] We recently used DFT methods very similar to those used below to determine the structure of chiral adlayers of amino acids on low-index Cu surfaces and found very good agreement between the calculated structures and those observed experimentally. [20,21] Our calculations examined the adsorption of the amino-(fluoro)methoxy (FAM) species and propylene oxide on a Cu(874) surface.…”

“…PO has previously been used experimentally as a probe for enantiospecific binding on both intrinsically chiral metal surfaces [16] and chirally templated surfaces. [1,19] We first examined the adsorption of PO on Cu(111) and found that the molecule favors a configuration in which the terminal methyl group is oriented away from the surface. Test calculations with PO on Cu(874) S showed that PO binds much more strongly to step-edge sites than to terrace sites, so only adsorption configurations with the molecule above the step edge were considered further.…”

Effect of Interaction Energies on the Adsorption of Glycine onto a Cu(110) Surface: A Monte Carlo Simulation

“…One promising route for achieving this is to use chiral modifiers to endow enantioselective properties onto a metallic surface (Baiker and Blaser 1997). This modification of the surface may act selectively towards the adsorption of another chiral probe species in two ways: (a) through a oneto-one mechanism, either by the formation of a complex between an individual modifier molecule and the reactant or by the creation of a single selective site on one side of the modifier (Tungler et al 1989;Baiker 1997;LeBlond et al 1999;Bürgi and Baiker 2000;Kubota and Zaera 2001;Chu et al 2003;Ma and Zaera 2005) and (b) through a template mechanism, in which the chiral modifier adsorbs forming a superstructure, while the template which contains void spaces acts selectively on the adsorption of one of the enantiomers of the probe chiral species through symmetry effects (Raval 2001;Ortega Lorenzo et al 1999;Humblot et al 2004;Stacchiola et al 2002Stacchiola et al , 2005Romá et al 2004;Lee and Zaera 2005).…”

“…In all these studies, chiral propylene oxide was used as the probe molecule to measure the enantioselective characteristics of the template. However, an understanding at a molecular level of the enantioselective process in a template system is far from being attained (Romá et al 2003(Romá et al , 2004Szabelski 2004;Szabelski and Sholl 2007a,b;López et al 2008). Some studies (Szabelski 2004;Szabelski and Sholl 2007a,b) have shown how enantioselectivity of a chiral species can arise on a substrate with a pattern of strong and weak adsorbing sites.…”

“…This mechanism could certainly apply in many cases. However, there are other systems, especially those studied experimentally by Zaera (2005, 2006) and Stacchiola et al (2002Stacchiola et al ( , 2005, where preferential adsorption of one of the enantiomers could not be due to a difference in the adsorption energy of different sites. In fact, in these experiments, chiral probe molecules B were adsorbed onto a surface with a template formed by the pre-adsorption of another chiral molecule A -say its R-enantiomer -in such a way that first B(R) was adsorbed to saturation, then the amount adsorbed was measured via thermal desorption, and finally B(S) was adsorbed to saturation and the amount adsorbed again measured via thermal desorption.…”