Near‐Enantiopure Trimerization of 9‐Ethynylphenanthrene on a Chiral Metal Surface

Abstract

Enantioselectivity in heterogeneous catalysis strongly depends on the chirality transfer between catalyst surface and all reactants, intermediates, and the product along the reaction pathway. Herein we report the first enantioselective on‐surface synthesis of molecular structures from an initial racemic mixture and without the need of enantiopure modifier molecules. The reaction consists of a trimerization via an unidentified bonding motif of prochiral 9‐ethynylphenanthrene (9‐EP) upon annealing to 500 K on the chiral Pd3‐terminated PdGa{111} surfaces into essentially enantiopure, homochiral 9‐EP propellers. The observed behavior strongly contrasts the reaction of 9‐EP on the chiral Pd1‐terminated PdGa{111} surfaces, where 9‐EP monomers that are in nearly enantiopure configuration, dimerize without enantiomeric excess. Our findings demonstrate strong chiral recognition and a significant ensemble effect in the PdGa system, hence highlighting the huge potential of chiral intermetallic compounds for enantioselective synthesis and underlining the importance to control the catalytically active sites at the atomic level.

Asymmetric azide-alkyne Huisgen cycloaddition on chiral metal surfaces

“…2j-k). The adsorption configuration of 9-EP molecules incorporated in these trimer heterostructures with respect to the PdGa substrate is identical with that of R enantiomers incorporated into pure 9-EP trimers 32 . The heterostructure trimers exclusively appear in the enantiomeric form shown in Fig.…”

“…Chirality transfer from the PdGa{111} surfaces onto molecular processes has been demonstrated with acetylene (C 2 H 2 ; achiral) and 9-ethynylphenanthrene (9-EP; prochiral, thus appearing in two distinguishable surface enantiomers R and S when confined to a planar configuration) 31,32 . Specifically, C 2 H 2 has been reported to exhibit directed rotation on the Pd 3 -terminated PdGa{111} surfaces 31 .…”

“…Specifically, C 2 H 2 has been reported to exhibit directed rotation on the Pd 3 -terminated PdGa{111} surfaces 31 . On the other hand, for 9-EP, enantiopure trimerization from an initial racemic mixture of monomers on the Pd 3and enantioselective adsorption of individual monomers on the Pd 1 -terminated PdGa{111} surfaces have been shown 32 .…”