Relative Abundances of Surface Diastereomeric Complexes Formed by Two Chiral Modifiers That Differ by a Methyl Group

Abstract

Enantioselectivity in heterogeneous catalysis can be induced through the adsorption of optically active compounds known as chiral modifiers. The modifiers typically stereodirect prochiral reactants on the metal surface through the formation of diastereomeric complexes. Surface science measurements on model systems show several examples where such complexes display multiple abundant binding configurations. Insights into the factors determining the relative populations of complexation states can be potentially gained from comparing slightly variant chiral modifiers. Here, we compare scanning tunneling microscopy (STM) data for chirality transfer complexes formed by (R)-1-(1-naphthyl)­ethylamine and (R)-1-(8-methyl-1-naphthyl)­ethylamine coadsorbed with the prochiral substrate 2,2,2-trifluoroacetophenone in order to probe for changes induced by the methyl substituent. The comparison shows changes in relative abundances beyond those predicted by only direct steric interaction. Density functional theory (DFT) methods are used to calculate complexation energies. A number of rationalizations are proposed for the experimentally determined differences in relative abundances and also for differences between DFT-predicted and STM-measured populations.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

“…439 Results from recent STM experiments such as those shown in Figure 34c, aided by DFT calculations, have provided direct visualization of how heterogeneous asymmetric induction with single-site resolution is sufficient to distinguish stereochemical conformations. 423,440,441 Notice in particular the different relative positions and orientations of (R)-NEA when interacting with pro-(R) (top-left case in Figure 34c) versus pro-(S) (Figure 34c, bottom-right) methyltrifluoropyruvate, and also the subtle differences in energies involved (-0.52 eV vs −0.59 eV, for a ΔE ≈ 7 kJ/mol). 423 The UHV work discussed in the previous paragraph shows how modern surface-science studies have been able to provide a molecular-level picture of catalytic sites on the surface of heterogeneous catalysts for subtle enantioselective processes.…”

“…Accordingly, most models start from the assumption that the reactant and modifier form weak 1:1 adducts or complexes on the surface. − The initial proposal, put forward by Baiker and coworkers, was based on a one-point hydrogen bond between the amine group of the quinuclidine ring of cinchona alkaloids and the oxygen atom of the keto group of the reactant (Figure a), , but that scheme was later refined by McBreen and coworkers to include additional interactions, in a 3-point model (Figure b) akin to what has been previously reported for enzymes . Results from recent STM experiments such as those shown in Figure c, aided by DFT calculations, have provided direct visualization of how heterogeneous asymmetric induction with single-site resolution is sufficient to distinguish stereochemical conformations. ,, Notice in particular the different relative positions and orientations of (R)-NEA when interacting with pro-(R) (top-left case in Figure c) versus pro-(S) (Figure c, bottom-right) methyl­trifluoropyruvate, and also the subtle differences in energies involved (-0.52 eV vs −0.59 eV, for a Δ E ≈ 7 kJ/mol) …”