geminal
19 F–
19 F and
19 F–
11 B couplings, Fig. 6.7). Resolution of racemic BODIPY
9 into its two enantiomers was achieved through semi-preparative chiral HPLC
(Chiralpak AD-H, heptane/2-propanol). CD spectroscopy of the resolved axially
chiral enantiomers, (R)-9 and (S)-9, did not however reveal any significant Cotton
effect at long wavelengths (corresponding to the S 0 –S 1 transitions of the BODIPY),
only short-wavelength transitions were observed associated with the electronic
transitions of the meso-aryl group (Fig. 6.7). However the CD spectra of each
enantiomer was sufficient to allow absolute stereochemical assignment, through a
comparison of the experimental and calculated CD spectra for the postulated (R)-9
enantiomer (CD spectra were calculated via a Boltzmann-weighted averaging of the
calculated CD spectra of each low energy conformer of (R)-9, obtained at via
TD-DFT (cam-B3LYP/6-311++G(2d,p)).
Akkaya et al. employed a similar design principle in the construction of
atropisomeric bis(BODIPY) 10 and tris(BODIPY) 11 (Fig. 6.8) [38]. In the case
of axially chiral bis(BODIPY) 10, one of the involved BODIPY chromophores is
laterally differentiated through the introduction of a single 2-formyl group to the
non-formylated parent bis(BODIPY) through a Vilsmeier-Haack formylation, whilst
its meso-position is occupied by a second 2-linked BODIPY moiety. Rotation
around the BODIPY-BODIPY bond is thus restricted by a double methylÀmethyl
clash, resulting in atropisomeric bis(BODIPY) 10. Further elaboration, through the
introduction of a third meso-linked BODIPY at the 2-position of the central
N
B
N
OEt
O
F
F
(R)-9
–144.25 –144.50 –144.75 –145.00
20
15
10
5
0
–5
–10
–15
–20
180 200 220 240 260 280 300 320 340
exp
10–(+)
calc
(R)
Wavelength (nm)
CD spectra (calculated vs experimental)
ppm
Fig. 6.7 Axially chiral BODIPYs (R)-9 (one enantiomer shown),
19 F NMR spectrum of BODIPY
9 (showing ABX coupling pattern), and comparison of the experimental and calculated CD spectra
for BODIPYs (R)-9 [adapted with permission from [37], published by the Royal Society of
Chemistry]
N B
N
F F
N
B
N
F
F
H
O
N B
N
F F
N
B
N
F
F
N B
N
F F
10
11
Fig. 6.8 Atropisomeric bis
(BODIPY) 10 and tris
(BODIPY) 11 (one
enantiomer shown for each)
124
M. J. Hall and S. de la Moya
19 F–
19 F and
19 F–
11 B couplings, Fig. 6.7). Resolution of racemic BODIPY
9 into its two enantiomers was achieved through semi-preparative chiral HPLC
(Chiralpak AD-H, heptane/2-propanol). CD spectroscopy of the resolved axially
chiral enantiomers, (R)-9 and (S)-9, did not however reveal any significant Cotton
effect at long wavelengths (corresponding to the S 0 –S 1 transitions of the BODIPY),
only short-wavelength transitions were observed associated with the electronic
transitions of the meso-aryl group (Fig. 6.7). However the CD spectra of each
enantiomer was sufficient to allow absolute stereochemical assignment, through a
comparison of the experimental and calculated CD spectra for the postulated (R)-9
enantiomer (CD spectra were calculated via a Boltzmann-weighted averaging of the
calculated CD spectra of each low energy conformer of (R)-9, obtained at via
TD-DFT (cam-B3LYP/6-311++G(2d,p)).
Akkaya et al. employed a similar design principle in the construction of
atropisomeric bis(BODIPY) 10 and tris(BODIPY) 11 (Fig. 6.8) [38]. In the case
of axially chiral bis(BODIPY) 10, one of the involved BODIPY chromophores is
laterally differentiated through the introduction of a single 2-formyl group to the
non-formylated parent bis(BODIPY) through a Vilsmeier-Haack formylation, whilst
its meso-position is occupied by a second 2-linked BODIPY moiety. Rotation
around the BODIPY-BODIPY bond is thus restricted by a double methylÀmethyl
clash, resulting in atropisomeric bis(BODIPY) 10. Further elaboration, through the
introduction of a third meso-linked BODIPY at the 2-position of the central
N
B
N
OEt
O
F
F
(R)-9
–144.25 –144.50 –144.75 –145.00
20
15
10
5
0
–5
–10
–15
–20
180 200 220 240 260 280 300 320 340
exp
10–(+)
calc
(R)
Wavelength (nm)
CD spectra (calculated vs experimental)
ppm
Fig. 6.7 Axially chiral BODIPYs (R)-9 (one enantiomer shown),
19 F NMR spectrum of BODIPY
9 (showing ABX coupling pattern), and comparison of the experimental and calculated CD spectra
for BODIPYs (R)-9 [adapted with permission from [37], published by the Royal Society of
Chemistry]
N B
N
F F
N
B
N
F
F
H
O
N B
N
F F
N
B
N
F
F
N B
N
F F
10
11
Fig. 6.8 Atropisomeric bis
(BODIPY) 10 and tris
(BODIPY) 11 (one
enantiomer shown for each)
124
M. J. Hall and S. de la Moya