Functionalized Biaryls as Building Blocks for Non-Aggregating Molecular Spoked Wheels

Abstract

Molecular spoked wheels (MSWs) showed to be synthetically accessible in various sizes and functionalization in the past. Due to their construction of not only phenylene- but also acetylene- and bisacetylene-units, some of the molecules turned out to decompose when stored under ambient conditions for too long. To overcome such decomposition, the employed structural motifs were limited to all-phenylene subunits, that were more inert towards ambient conditions, but revealed limits considering their lateral expansion. Within the final synthetic step, the formation of intermolecular bonds was observed instead of the desired intramolecular cyclization reaction. Detailed investigation came to the conclusion, that the open-framed precursors’ tendencies to aggregate caused such intermolecular oligomerization. <br /> Hence, this work deals with synthetic strategies to overcome the aggregation of all-phenylene MSW precursors. To do so, the established synthetic strategy was advanced. It was found to be helpful to introduce alkyl groups into the precursors’ spokes to 1) maintain high solubility during its preparation and 2) to separate the open-framed precursors that is it insufficient for them to aggregate. Two promising building blocks are fluorene and biphenol. <br /> Fluorene comes with the advantage, that it can be halogenated regioselectively in 2- and 7-position, allowing the construction of large, nearly linear aromatic precursor molecules via transition metal catalysis in few steps. Besides that, the mentioned alkyl groups can be introduced into the motif straightforward because of its acidic 9,9’-position. Biphenol can be alkylated with similar ease, since its two hydroxy groups can be utilized as strong nucleophiles as well under mild basic conditions. Due to its electronic properties, a late halogenation of (alkylated) biphenol is not possible, requiring an elaborate synthesis of the unit from two moieties. <br /> The synthetic investigations of the final cyclization were supported by quantum chemical simulations predicting their outcome. Due to the slightly bent geometry of fluorene, the resulting MSWs become bowl-shaped, making it unclear, if a cyclization of the rim fragments can occur successfully in the final step. Thus, each cyclization step of the six-fold closure was investigated individually regarding the distance between both fragments as well as the strain introduced into the molecule after a successful bond formation. In the end, the predictions turned out to be in full accordance with the synthetic evidence. <br /> The accessed MSWs were structurally proven via NMR spectroscopy and mass spectrometry and scanning tunnelling microscopy (STM), providing valuable insights into their geometry, the orientation of the newly introduced spoke unit and their packing on highly oriented pyrolytic graphite (HOPG).