Asymmetric azide-alkyne Huisgen cycloaddition on chiral metal surfaces

Achieving fundamental understanding of enantioselective heterogeneous synthesis is marred by the permanent presence of multitudinous arrangements of catalytically active sites in real catalysts. In this study, we address this issue by using structurally comparatively simple, well-defined, and chiral intermetallic PdGa{111} surfaces as catalytic substrates. We demonstrate the impact of chirality transfer and ensemble effect for the thermally activated azide-alkyne Huisgen cycloaddition between 3-(4-azidophenyl)propionic acid and 9-ethynylphenanthrene on these threefold symmetric intermetallic surfaces under ultrahigh vacuum conditions. Specifically, we encounter a dominating ensemble effect for this reaction as on the Pd-3-terminated PdGa{111} surfaces no stable heterocoupled structures are created, while on the Pd-1-terminated PdGa{111} surfaces, the cycloaddition proceeds regioselectively. Moreover, we observe chirality transfer from the substrate to the reaction products, as they are formed enantioselectively on the Pd-1-terminated PdGa{111} surfaces. Our results evidence a determinant ensemble effect and the immense potential of PdGa as asymmetric heterogeneous catalyst. Mechanistic insight into enantioselective reactions at intrinsically chiral surfaces can be challenging to obtain. Here the catalytic activity of Pd-1- and Pd-3-terminated PdGa{111} surfaces is shown to differ substantially, with Pd-1-terminated surfaces promoting on-surface azide- alkyne cycloadditions enantioand regioselectively.

Automated summary

This study investigates the impact of chirality transfer and ensemble effect on thermally activated azide-alkyne Huisgen cycloaddition on well-defined and chiral intermetallic PdGa{111} surfaces. Dominating ensemble effect and chirality transfer from the substrate to reaction products are observed, revealing the potential of PdGa as an asymmetric heterogeneous catalyst. Mechanistic insight into enantioselective reactions at chiral surfaces is challenging but crucial for understanding catalytic activity differences observed on Pd-1- and Pd-3-terminated PdGa{111} surfaces.