The comparison of the self-assembly 9,9'-bisheptahelicene on the Au(111) surface, studied with scanning tunneling microscopy, with the self-assembly of the same species obtained by on-surface synthesis via Ullmann coupling from 9-bromoheptahelicene reveals a diastereomeric excess for the ( M, P)- meso-form of 50%. The stereoselectivity is explained by a topochemical effect, in which the surface-alignment of the starting material and the organometallic intermediate sterically favor the ( M, P)-transition state over the homochiral transition states.

Department of Chemistry University of Hamburg 20146 Hamburg Germany

Department of Chemistry University of Zurich 8057 Zurich Switzerland

Empa Swiss Federal Laboratories for Materials Science and Technology 8600 Dübendorf Switzerland

Institut für Anorganische und Analytische Chemie Goethe Universität Frankfurt Max von Laue Straße 7 60438 Frankfurt Germany

Institute of Physics of the Czech Academy of Sciences Cukrovarnická 10 18221 Prague 6 Czech Republic

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Stereospecific on-Surface Cyclodehydrogenation of Bishelicenes: Preservation of Handedness from Helical to Planar Chirality