From Structural Beauty to Chemical Fragility: Diacetylene Bridged “Geländer” Oligomers

The conceptual basis of this thesis is based on Fritz Vögtle’s “Geländer” oligomers, presented first in 1998. In contrast to helicenes and related structures, these helical polyaromatic systems are axially chiral, the rings being arranged along the propagation axis. However, due to their high tendency to racemize and to form the optically inactive meso form, they fell into oblivion. This changed in 2010 when our group rethought the concept of “Geländer” oligomers and corroborated the validity of the concept by introducing newly designed compounds. In these molecules, a longer oligomer is interlinked with a shorter one by being wrapped helically around it like a banister, imparting chirality from one end to the other. The lack of electron mobility in the banister led to a limited display of electronic and optical properties in these compounds. Thus, the following questions presented themselves:
- Is there a chemical design allowing for a “Geländer” oligomer with a fully conjugated banister?
- What would be a good anchor group to investigate these properties in an experimental set-up?
The elucidation of the second question culminated in a novel polar sulfur protection group. The preparation of the protection group was published in Synthesis, also demonstrating its synthetic usefulness and stability.
To answer the first question, we developed a test system by combining a biphenyl with a fully conjugated oligo-para-phenylene-di-ethynylene banister, as published in the European Journal of Organic Chemistry. Having found a suitable banister, the terphenylic “Geländer” oligomer was synthesized and investigated for its dynamic behavior and its chiroptical properties. The synthesis and the results of these investigations were published in Chemistry - A European Journal. The supplementary information for every chapter is given at the end of this thesis.
The three manuscripts are preceded by the review “Thioaryl Protection Groups”, prepared for publication. Aromatic sulfur protection groups are grouped by their chemical nature and discussed systematically. The review also adds additional context to the polar sulfur protection group presented in this thesis and integrates it into the wider world of thioaryl protection groups.