(BODIPY)s 12 and 13 and bis(azaBODIPY) 14 were synthesized as racemates
through oxidative coupling of the corresponding monomers, with subsequent resolution achieved via chiral HPLC (Chirex, dichloromethane/n-hexane (12 and 13) or
dichloromethane/2-propanol/n-hexane (14)). Interestingly, despite the axial chirality
imbued to these systems, the CD spectra of the resolved enantiomers of 12–14
showed only very low intensity signals between 300 and 800 nm, an interesting
example of cryptochirality (i.e. two enantiomers for which the specific rotation is
non-measurable). This cryptochirality is thought to arise in these systems due to the
existence of two different conformations, for each compound 12–14, around the
biaryl axis which exhibit nearly mirror-image CD spectra, resulting in weak experimental CD spectra dominated by vibronic coupling effects.
An alternative approach to the construction of atropisomeric bis(BODIPY) 15 was
subsequently demonstrated by Ishida et al. in which they constructed a 3,3
0 -linked bis
(BODIPY) unit within the architecture of a π-extended corrorin (Fig. 6.11) [40]. This
atropisomeric bis(BODIPY) 15 could be resolved by chiral HPLC to give the
corresponding P and M isomers, the CD spectra of each enantiomer showing a strong
Cotton couplet (g abs ¼ 0.012 at λ ¼ 501 nm). Interestingly the CD spectra showed a
reversible change in peak shape upon addition of a strong protic acid (trifluoroacetic
acid), arising from a conformational change of the molecule upon protonation of the
inner dipyrrin ring, suggesting possible future applications of these systems in
chiroptical sensing (Fig. 6.11).
6.2.4 BODIPYs Displaying Helical Chirality
During their studies into new red-shifted BODIPYs, Burgess et al. investigated the
synthesis and optical properties of 3,5-diaryl-substituted BODIPY 16, in which the
oxygen atoms of the 3,5-ortho-phenolic groups chelate the central boron atom
Fig. 6.11 Atropisomeric bis(BODIPY) 15, including acid/base induced interconversion (left). CD
spectra of atropisomeric bis(BODIPY) 15 in CH 2 Cl 2 in the absence (solid line) and presence
(broken line) of TFA (right) [adapted with permission from [40], copyright 2018 WILEY-VCH
Verlag GmbH]
126
M. J. Hall and S. de la Moya
through oxidative coupling of the corresponding monomers, with subsequent resolution achieved via chiral HPLC (Chirex, dichloromethane/n-hexane (12 and 13) or
dichloromethane/2-propanol/n-hexane (14)). Interestingly, despite the axial chirality
imbued to these systems, the CD spectra of the resolved enantiomers of 12–14
showed only very low intensity signals between 300 and 800 nm, an interesting
example of cryptochirality (i.e. two enantiomers for which the specific rotation is
non-measurable). This cryptochirality is thought to arise in these systems due to the
existence of two different conformations, for each compound 12–14, around the
biaryl axis which exhibit nearly mirror-image CD spectra, resulting in weak experimental CD spectra dominated by vibronic coupling effects.
An alternative approach to the construction of atropisomeric bis(BODIPY) 15 was
subsequently demonstrated by Ishida et al. in which they constructed a 3,3
0 -linked bis
(BODIPY) unit within the architecture of a π-extended corrorin (Fig. 6.11) [40]. This
atropisomeric bis(BODIPY) 15 could be resolved by chiral HPLC to give the
corresponding P and M isomers, the CD spectra of each enantiomer showing a strong
Cotton couplet (g abs ¼ 0.012 at λ ¼ 501 nm). Interestingly the CD spectra showed a
reversible change in peak shape upon addition of a strong protic acid (trifluoroacetic
acid), arising from a conformational change of the molecule upon protonation of the
inner dipyrrin ring, suggesting possible future applications of these systems in
chiroptical sensing (Fig. 6.11).
6.2.4 BODIPYs Displaying Helical Chirality
During their studies into new red-shifted BODIPYs, Burgess et al. investigated the
synthesis and optical properties of 3,5-diaryl-substituted BODIPY 16, in which the
oxygen atoms of the 3,5-ortho-phenolic groups chelate the central boron atom
Fig. 6.11 Atropisomeric bis(BODIPY) 15, including acid/base induced interconversion (left). CD
spectra of atropisomeric bis(BODIPY) 15 in CH 2 Cl 2 in the absence (solid line) and presence
(broken line) of TFA (right) [adapted with permission from [40], copyright 2018 WILEY-VCH
Verlag GmbH]
126
M. J. Hall and S. de la Moya