synthesis of helically chiral N,N,O-boron-chelated BODIPY 18 and subsequent
resolution by chiral HPLC (Daicel Chiralpak IA; CHCl 3 /hexane 1:1). The resolved
enantiomers showed mirror image CD spectra (λ max ¼ 655 nm); however no CPL
studies on these compounds have been disclosed to date (Fig. 6.14) [48].
6.3 CPL BODIPYs
6.3.1 Mono(BODIPY)S
6.3.1.1 C 2 -Symmetric Mono(BODIPY)S
The first example of a CPL-SOM based on BODIPY was a chiral C 2 -symmetric
mono(BODIPY) published by the group of Gossauer et al. in their study of the
urobilins, an important family of natural-product chromophores in algal and bacterial
photosynthesis [49]. As part of their work examining the conformation of synthetic
urobilin analogues in solution, a number of urobilin-based difluoroboron chelates
were prepared. In the case of the non-racemisable, difluoroboron-chelated urobilin
derivative 19 (Fig. 6.15), study of the CD spectra suggests that the, albeit peripheral,
asymmetric carbon centres can effectively induce chirality to the chromophore (the
observed long-wavelength CD transition correlating with that of the visible absorption maxima at approximately 535 nm). Interestingly, comparison of the CD spectra
of difluoroboron-chelated urobilin derivative 19 in either CH 2 Cl 2 or DMF shows an
inversion of sign of the long-wavelength CD transition. This suggests an inversion of
conformation of 19 in solution, presumably as a consequence of competition
between intramolecular NHÁÁÁF bonding in CH 2 Cl 2 and intermolecular NHÁÁÁF
bonding in DMF (Fig. 6.15). Difluoroboron-chelated urobilin derivative 19 in
CH 2 Cl 2 solution was also reported to exhibit CPL at 546 nm with a g lum of
+0.94Á10
À3 , following indirect excitation (366 nm) of the pendant 1,5-dihydro-2Hpyrrol-2-one groups and subsequent energy transfer to the BODIPY
chromophore [32].
N
N B
Ph
O
18
HO
Fig. 6.14 Helically chiral N,N,O-boron-chelated BODIPY (one enantiomer shown). CD spectra
(top) and UV-Vis spectra (bottom) of (S)-18 (black) and (R)-18 (orange) [adapted with permission
from [48], copyright 2018 Elsevier]
128
M. J. Hall and S. de la Moya
resolution by chiral HPLC (Daicel Chiralpak IA; CHCl 3 /hexane 1:1). The resolved
enantiomers showed mirror image CD spectra (λ max ¼ 655 nm); however no CPL
studies on these compounds have been disclosed to date (Fig. 6.14) [48].
6.3 CPL BODIPYs
6.3.1 Mono(BODIPY)S
6.3.1.1 C 2 -Symmetric Mono(BODIPY)S
The first example of a CPL-SOM based on BODIPY was a chiral C 2 -symmetric
mono(BODIPY) published by the group of Gossauer et al. in their study of the
urobilins, an important family of natural-product chromophores in algal and bacterial
photosynthesis [49]. As part of their work examining the conformation of synthetic
urobilin analogues in solution, a number of urobilin-based difluoroboron chelates
were prepared. In the case of the non-racemisable, difluoroboron-chelated urobilin
derivative 19 (Fig. 6.15), study of the CD spectra suggests that the, albeit peripheral,
asymmetric carbon centres can effectively induce chirality to the chromophore (the
observed long-wavelength CD transition correlating with that of the visible absorption maxima at approximately 535 nm). Interestingly, comparison of the CD spectra
of difluoroboron-chelated urobilin derivative 19 in either CH 2 Cl 2 or DMF shows an
inversion of sign of the long-wavelength CD transition. This suggests an inversion of
conformation of 19 in solution, presumably as a consequence of competition
between intramolecular NHÁÁÁF bonding in CH 2 Cl 2 and intermolecular NHÁÁÁF
bonding in DMF (Fig. 6.15). Difluoroboron-chelated urobilin derivative 19 in
CH 2 Cl 2 solution was also reported to exhibit CPL at 546 nm with a g lum of
+0.94Á10
À3 , following indirect excitation (366 nm) of the pendant 1,5-dihydro-2Hpyrrol-2-one groups and subsequent energy transfer to the BODIPY
chromophore [32].
N
N B
Ph
O
18
HO
Fig. 6.14 Helically chiral N,N,O-boron-chelated BODIPY (one enantiomer shown). CD spectra
(top) and UV-Vis spectra (bottom) of (S)-18 (black) and (R)-18 (orange) [adapted with permission
from [48], copyright 2018 Elsevier]
128
M. J. Hall and S. de la Moya