Synthesis of Chiral BODIPYs through Point-to-Helical Chirality Control

Abstract

Molecules can exhibit helical chirality when their structures show a screw-shaped geometry and therefore can be either left- or right-handed. In the case of helically chiral fluorophores, they have the potential to emit circularly polarised luminescence (CPL). The 4,4-difluoro-4boron-3a,4a-diaza-s-indacene (BODIPY) fluorophores are a widely used class of molecules, which can exhibit helical structures which show CPL. In this thesis, we have focused on the synthesis of helically chiral BODIPY architectures via point-to-helical chirality control. This was performed through the introduction of a point chiral group which induces helical chirality in another region of the molecule. Herein we describe the synthesis of both 3,5-dichloro and -dibromo BODIPYs, as suitable intermediates enroute to N,N,O,C helically chiral BODIPYs. We have also developed a novel approach to 3,5-diiodo BODIPYs, which prove to be excellent substrates for both SNAr and Pd cross-coupling reactions. We also describe our development of novel synthetic routes to helically chiral BODIPYs, through the use of point-to-helical chirality control employing a range of naturally occurring α-amino acids to act similarly to chiral auxiliaries (note, these will be described as chiral directing groups throughout this thesis). Helicity can be introduced to the BODIPYs, with diastereomeric excesses (de) of up to 84%, showing excellent stereochemical control. Assignment of helical stereochemistry was performed via single crystal X-ray crystallography and electronic circular dichroism (ECD) spectra, demonstrating near mirror-image ECD spectra for the pairs of diastereomers. The development of point-to-helical chirality routes to chiral BODIPYs opens up new strategies for the future synthesis of CPL emissive organic fluorophores.