Aminolactone Chiral Modifiers for Heterogeneous Asymmetric Hydrogenation: Corrected Structure of Pantoyl-Naphthylethylamine, In-Situ Hydrogenolysis, and Scanning Tunneling Microscopy Observation of Supramolecular Aminolactone/Substrate Assemblies on Pt(111)

As established by Baiker and co-workers,
pantoyl-naphthylethylamine (PNEA) is an efficient synthetic chiral
modifier for the asymmetric hydrogenation of ketopantolactone (KPL)
to pantolactone on supported Pt catalysts. We report a scanning tunneling
microscopy (STM) study of PNEA and PNEA-derived aminolactone species
on Pt(111) and a reassignment of the relative stereochemistry of the
modifier. Robust organic chemistry methods were used to establish
that the structure of PNEA is R,S rather than R,R. The dissociative
chemisorption of a fraction of PNEA adsorbed on Pt(111) yields two
fragments that we attribute to a process involving C–N bond
scission. We show that C–N bond scission occurs under hydrogenation
conditions on PNEA-modified Pt/Al2O3 catalysts,
forming the aminolactone amino-4,4-dimethyldihydrofuran-2-one (AF).
STM measurements on (S)-AF and 2,2,2-trifluoroacetophenone
coadsorbed on Pt(111) show the formation of isolated 1:1 complexes.
In contrast, measurements on coadsorbed (S)-AF and
KPL show fluxional supramolecular AF/KPL assemblies. The possibility
that such assemblies contribute to the overall enantioselectivity
observed for PNEA-modified Pt catalysts is discussed.