Interaction between chiral molecules are of paramount importance for many applications such as enantioselective catalysis, design of non-linear optics, spin filters and optical resolution of enantiomers by crystallization. Surfaces provide an up–down asymmetry, excluding certain symmetry elements, such as center of inversion or glide plane parallel to the surface, and might induce stereochemical effects during chemical reactions on chiral molecules. Ullmann coupling has been very successful for the carbon–carbon coupling of organic molecules on surfaces. We present a scanning tunneling microscopy (STM) study of the Ullmann coupling of helicenes, a type of helical aromatic hydrocarbons, on Au and Cu. The Ullmann coupling of chiral 2-bromo[4]helicene on Cu(100) gives rise to the exclusive formation of homochiral products, i.e. (M,M) and (P,P)-bis[4]H (Fig. 1), which self-assemble into homochiral mirror domains. The (P,M)-bis[4]H diastereomer is not observed, imposing diastereoselectivity to the Ullmann reaction, which is most likely based on the low inversion barrier of the tetrahelicene subunits. Using 2,3-dibromo[4]helicene as Ullmann reaction precursor leads to trishelicenes (e.g. trisphenanthro[3,4;3’,4’,3”,4”-b,h,n]triphenylene), by forming a new centered C6 ring. These self-assemble into homochiral domains with the bromine atoms embedded between the trishelicenes. The diastereoselectivity in Ullman coupling of chiral 9-bromo[7]helicenes, which have substantially higher inversion barriers, will also be discussed. On Au(111) the coupling of two Br-[7]H produces both heterochiral and homochiral dimers (bis[7]H) arranging in two different long-range ordered structures. On Cu(111), the formation of an intermediate organometallic compound is observed. The self-assembly of bis[7]H surface reaction product will be compared to the self-assembly of bis[7]H obtained by C-C coupling in solution prior to adsorption.

Ullmann coupling of helical polycyclic aromatic hydrocarbons on metal surfaces

255th National Meeting (2018)

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