During the course of this PhD Thesis supervised by Professor J. Lacour at the Organic Chemistry Department of the University of Geneva, the synthesis and study of novel cationic [6]helicenes were performed.
In fact, the derivatives previously reported by our group were bearing oxygen or nitrogen atoms as bridging units of the carbon skeleton. They displayed remarkable (chir)optical properties based on the delocalisation of the positive charge through their conjugated π-system, as well as very different chemical and configurational stabilities. However, these features were hard to rationalise so far. Herein, the scope of such derivatives is expanded via the synthesis and study of novel cationic and neutral [6]helicenes bearing sulfur or selenium bridging atoms in Chapter 1.
In a second chapter, the scope of novel derivatives is expanded via the introduction of an isopropyl staple motif, and the synthesis of both the cationic and anionic variants is described.
In a third chapter, this manuscript also presents the efforts towards the preparation of phosphorus-containing cationic [6]helicenes, and several synthetic strategies are described. However, despite all the attempts, the synthesis of such compounds was not successful so far.
Finally, in the last chapter, a comparative study of the properties of the novel derivatives is presented. It includes an original discussion of their structural features via the correlation of the increase of the helical pitch with the distortion induced by the bridging units. Somehow related to these structural features, the aromaticity of some of the novel derivatives is commented via advanced density functional theory (DFT) calculations. A study of the chemical stability of these compounds is also globally presented via the correlation of their pKR+ values with their reduction potentials, indicating that their chemical stability depends only on the electronic effects of the various substituents. Finally, the configurational stability of some of the novel derivatives is studied. The absence of correlation with their structural features led to the reevaluation of the postulated racemization pathway for such compounds. In fact, the racemization barriers globally follow the order of chemical stabilities. An alternative mechanism is thus proposed, involving first the formation of sp3-neutral adducts, favoured for the most electron-poor compounds, and the racemization of these intermediates instead of the cationic variants.
