Chiral bis(benzo[1,2-b:4,3-b′]dithiophene) atropisomers: experimental and theoretical investigations of the stereochemical and chiroptical properties

Abstract

Conformational chirality is a feature that may arise from the presence of a hindered rotation around a single bond that corresponds to a stereogenic axis. Atropisomers that feature such an element show interesting characteristics suitable for developing chiral systems with applicability in asymmetric synthesis, catalysis and materials chemistry. Here, we report the investigation of the stereochemical and chiroptical properties of a series of axially chiral intermediates useful in the synthesis of helicenes. Racemates of the four studied atropisomeric bisbenzodithiophenes were resolved by enantioselective HPLC and the single enantiomers were investigated by HPLC, ECD, VCD and CPL. Configurational stability was successfully established by dynamic HPLC or by monitoring thermal racemization, allowing the measurement of the free activation energy values for the stereomutation process. Moreover, the absolute configuration was assigned, thanks to a combination of TD-DFT and DFT calculations and experimental data derived by ECD and VCD, and the emissive chiral properties were measured by recording the CPL spectrum. Attention was paid to the importance of considering also aliphatic pendant groups in order to obtain a good calculation–experiment correspondence for both the ECD and VCD spectra. Considering the significant interest for axially chiral biaryl derivatives, this novel class of bisbenzodithiophenes are expected to have broad applications, not only in asymmetric synthesis, but also as potential chiral ligands or CPL emitters.