Fixing the Conformation in Helicenes: Enantioselective Synthesis of [5]Helicenes and Synthesis of In-Fjord Substituted Expanded Helicenes

Fixing the Conformation in Helicenes: Enantioselective Synthesis of [5]Helicenes and Synthesis of In-Fjord Substituted Expanded Helicenes

by Pablo Redero García
Doctoral thesis
Date of Examination:2022-11-25
Date of issue:2022-12-16
Advisor:Prof. Dr. Manuel Alcarazo
Referee:Prof. Johannes Jun.-Walker
Referee:Prof. Dr. Inke Siewert
Referee:Prof. Dr. Ricardo Mata
Referee:Dr. Alessandro Bismuto
Referee:Dr. Daniel Janßen-Müller
Persistent Address: http://dx.doi.org/10.53846/goediss-9604

 

 

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Abstract

English

By combining the 3,3’-disubstituted BINOL-derived backbone and different cationic rests, a new family of α-cationic chiral phosphonites has been synthesized. Coordination to Au(I) afforded the corresponding precatalysts. These novel compounds were applied as ancillary ligands in Au(I)-catalyzed synthesis of 1-arylbenzo[5]helicenes. High regio- and enantioselectivities were accomplished through the intramolecular hydroarylation of alkynyl-functionalized [4]helicenes. Hence, a family of 1-arylbenzo[5]helicenes containing a series of substituents at a diverse external positions of the helicene rim as well as by the modification of the hanging phenyl moiety has been prepared. The connectivities in these structures and their absolute configurations were confirmed by X-ray diffraction analysis. Also, the chiroptical properties of the materials prepared were studied by UV-Vis absorption, fluorescence emission and circular dichroism. In accordance with previous results, a number of thia[5]helicenes containing an embedded thiophene ring were obtained by the same methodology with BINOL- and TADDOL-based Au(I) precatalysts. The high versatility of these compounds enabled the obtention of a widely tuned series of thia[5]helicenes exhibiting exceptional regio- and enantioselectivities. This task was achieved in collaboration with Dr. Valentina Pelliccioli. In collaboration with Dr. Samuel Suárez Pantiga, a group of expanded helicenes of different sizes and shapes with phenyl- and biphenyl-substituents at inner fjord region have finally been synthesized via sequential Au(I)-catalyzed hydroarylation of appropriately designed substrates. Along with Dr. Christopher Golz, single crystals of these compounds suitable for X-ray analyses were grown in order to measure and quantify several structural features. Additionally and in cooperation with Prof. Dr. Ricardo Mata and M. Sc. Xaiza Aniban, the conformational dynamics and racemization barriers of the obtained compounds were calculated via electronic structure methods. The estimated energetic profiles were compared to those of unsubstituted core architectures, observing noticeable differences in the relative energy values as well as in the shape of the profile, i. e. number of transition states and intermediates. Also photophysical studies of these expanded helicenes were performed in order to study the effect of the substitution pattern and the extension of the π-system on their chiroptical properties.
Keywords: Organic chemistry, polyaromatic hydrocarbons, helicenes, gold, asymmetric catalysis,
 
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