Phenylene-Based Molecular Spoked Wheels: Synthetic Access and Hybride Structures
Final Report Abstract
We could describe all-phenylene molecular spoked wheels (MSWs) with reduced symmetry that are composed of a central hub, six spokes and a rim, which means that not all six corners of the compounds contain the same substituents. Their access is based on the Co-catalyzed trimerization of non-symmetrical acetylenes that gives the respective isomeric 1,3,5- and the 1,2,4-substituted wheel precursors. Using substituents with different polarity, these isomers can be separated by column chromatography because the compounds have a different net dipol moment. The final rim closure by Yamamoto coupling gave MSWs with a rim composed of 18 phenylene units (18 Ph-MSWs). The characterization includes in most cases also investigations by scanning tunneling microscopy – often with submolecular resolution – that unambiguously show the structure of the compounds. While 24 Ph-MSWs could be also obtained, albeit in lower yields, 30 Ph-MSWs were not available by this strategy. Careful and detailed consideration of all experimental results lead to the proposal that aggregation during the final Yamamoto cyclization lead to the formation of oligomers. With that working hypothesis in mind, we could overcome the synthetic difficulties by attaching alkyl chains to the spokes of the compounds. With fluorene spokes we additionally induce a curvature into the final wheel which further reduces their tendency of aggregation. In this context we could also establish a new theoretical method to predict if the final Yamamoto coupling to the wheel structure will be successful: Based on the structure of calculated intermediates of the Yamamoto coupling and our successful formation of highly strained bicyclophanes in an earlier project, we generated a geometrical and an energetical criterion for a prediction of the outcome of the synthesis. This goes far beyond earlier predictive modellings using ball-and-stick models of the desired final structures. This new strategy allowed us to prepare spoked wheels containing fluorene spokes with 18 and 30 phenylene units in the rim in good yields.
Publications
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Expanded all-phenylene molecular spoked wheels: cutouts of graphenylene-3. Organic Chemistry Frontiers, 8(18), 4980-4985.
Sterzenbach, Christopher; Keller, Tristan J.; Kraus, Daniel; Lupton, John M.; Jester, Stefan-S. & Höger, Sigurd
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Nanopatterns of molecular spoked wheels as giant homologues of benzene tricarboxylic acids. Chemical Science, 12(27), 9352-9358.
Keller, Tristan J.; Sterzenbach, Christopher; Bahr, Joshua; Schneiders, Taria L.; Bursch, Markus; Kohn, Julia; Eder, Theresa; Lupton, John M.; Grimme, Stefan; Höger, Sigurd & Jester, Stefan-S.
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From π-Conjugated Rods to Shape-Persistent Rings, Wheels, and Ladders: The Question of Rigidity. Accounts of Chemical Research, 57(17), 2561-2571.
Höger, Sigurd & Lupton, John M.
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Size‐Increased All‐Phenylene Molecular Spoked Wheels – A Combined Theoretical and Experimental Approach. Angewandte Chemie International Edition, 63(46).
Krämer, Philipp; Kohn, Julia; Hofmeister, David Ari; Kersten, Maximilian; Sterzenbach, Christopher; Gres, Antonia; Hansen, Andreas; Jester, Stefan‐Sven; Grimme, Stefan & Höger, Sigurd
