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

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.