the planes of the two pyrrole rings of 7.3
. The CD spectra of the (R,R)- and (S,S)enantiomers of 31 showed opposing Cotton effects with maxima in the red region of
the visible spectra corresponding to the S 0 –S 1 transition of the BODIPY fluorophore.
The absolute stereochemistry of the (R,R)-enantiomer of 31, which showed a
negative Cotton effect in CD, was assigned by comparison of the experimental
with the corresponding TD-DFT calculated CD spectra. The CPL spectra for the
resolved enantiomers of 31 also showed mirror image signals with |g lum | ¼ 6Á10
À4
(Fig. 6.20).
More recently, Hall et al. described the serendipitous discovery of a new class of
helically chiral mono(BODIPY) [57]. Whilst exploring improved synthetic procedures for the synthesis of N,N,O,O-boron-chelated dipyrromethenes, the unexpected
formation of red-shifted unsymmetrical helically chiral mono(BODIPY) 32 was
observed, thought to arise from a boron metathesis, nucleophilic aromatic substitution (S N Ar), Suzuki coupling and boron-chelation reaction sequence (Fig. 6.21).
Resolution by semi-preparative chiral HPLC (Chiralpak IB, ethyl acetate/n-hexane 15:85) gave both the dextro- and levorotatory enantiomers of the unsymmetrical
helically chiral mono(BODIPY) 32, which in turn gave mirror-image CD spectra
with good correlation with the corresponding absorption spectra (Fig. 6.22). Again
absolute stereochemistry was assigned through comparison of the calculated
(TD-DFT at the cam-B3LYP/6-311++G(3df,2pd) level) and the experimental
N B
N
F F
Mes
O
OMe
O
MeO
lg lum l = 6·10 -4
f = 0.73
31
Fig. 6.20 Helically chiral ring-fused skewed mono(BODIPY) 31 and visible CPL signatures upon
visible irradiation in solution (one enantiomer shown)
N
N
Ar
O
B
O
32
N
N
Ar
Br
B
F F
Br
Fig. 6.21 Synthesis of helically chiral mono(BODIPY) 32. Reaction conditions:
(2-hydroxyphenyl)boronic acid, Na 2 CO 3 , Pd(PPh 3 ) (5 mol%), toluene/1,4-dioxane, 90
C,
80 min. Chemical yield ca. 35%. Ar: p-C 6 H 4 CO 2 CH 3
134
M. J. Hall and S. de la Moya
. The CD spectra of the (R,R)- and (S,S)enantiomers of 31 showed opposing Cotton effects with maxima in the red region of
the visible spectra corresponding to the S 0 –S 1 transition of the BODIPY fluorophore.
The absolute stereochemistry of the (R,R)-enantiomer of 31, which showed a
negative Cotton effect in CD, was assigned by comparison of the experimental
with the corresponding TD-DFT calculated CD spectra. The CPL spectra for the
resolved enantiomers of 31 also showed mirror image signals with |g lum | ¼ 6Á10
À4
(Fig. 6.20).
More recently, Hall et al. described the serendipitous discovery of a new class of
helically chiral mono(BODIPY) [57]. Whilst exploring improved synthetic procedures for the synthesis of N,N,O,O-boron-chelated dipyrromethenes, the unexpected
formation of red-shifted unsymmetrical helically chiral mono(BODIPY) 32 was
observed, thought to arise from a boron metathesis, nucleophilic aromatic substitution (S N Ar), Suzuki coupling and boron-chelation reaction sequence (Fig. 6.21).
Resolution by semi-preparative chiral HPLC (Chiralpak IB, ethyl acetate/n-hexane 15:85) gave both the dextro- and levorotatory enantiomers of the unsymmetrical
helically chiral mono(BODIPY) 32, which in turn gave mirror-image CD spectra
with good correlation with the corresponding absorption spectra (Fig. 6.22). Again
absolute stereochemistry was assigned through comparison of the calculated
(TD-DFT at the cam-B3LYP/6-311++G(3df,2pd) level) and the experimental
N B
N
F F
Mes
O
OMe
O
MeO
lg lum l = 6·10 -4
f = 0.73
31
Fig. 6.20 Helically chiral ring-fused skewed mono(BODIPY) 31 and visible CPL signatures upon
visible irradiation in solution (one enantiomer shown)
N
N
Ar
O
B
O
32
N
N
Ar
Br
B
F F
Br
Fig. 6.21 Synthesis of helically chiral mono(BODIPY) 32. Reaction conditions:
(2-hydroxyphenyl)boronic acid, Na 2 CO 3 , Pd(PPh 3 ) (5 mol%), toluene/1,4-dioxane, 90
C,
80 min. Chemical yield ca. 35%. Ar: p-C 6 H 4 CO 2 CH 3
134
M. J. Hall and S. de la Moya