Highly Stereospecific On‐Surface Dimerization into Bishelicenes: Topochemical Ullmann Coupling of Bromohelicene on Au(111)

The on‐surface dimerization into bis(hexahelicene) on a gold(111) surface has been studied by means of scanning tunneling microscopy and time‐of‐flight secondary mass spectrometry. C−C Ullmann coupling of (rac)‐2‐bromo‐hexahelicene leads to formation of the (M,M)‐ and (P,P)‐diastereomers of 2,2’‐bis(hexahelicene), whilst formation of the (M,P)‐diastereomer is not observed. Upon cooling, the bis(hexahelicene) aggregates into an ordered two‐dimensional lattice with partly randomly distributed enantiomers. The highly specific diastereomeric coupling is explained by the surface alignment of educt in combination with the strong steric overcrowding in a possible surface‐confined (M,P)‐product. Only same handedness please! On‐surface Ullmann C−C coupling of bromohelicene enantiomers leads stereospecifically only to homochiral combinations into bis(helicene).