α-/β-formylated boron-dipyrrin (BODIPY) dyes: Regioselective syntheses and photophysical properties

Article abstract of DOI:10.1002/ejoc.201100736

Formylation has been performed on pyrrole-unsubstituted dipyrromethanes 1 and boron-dipyrrin (BODIPY) dyes 4 based on a Vilsmeier-Haack reaction. It is highly regioselective and complementary and occurs exclusively at the α-and β-position, respectively, f

Full text of DOI:10.1002/ejoc.201100736

FULL PAPER
DOI: 10.1002/ejoc.201100736
α-/β-Formylated Boron–Dipyrrin (BODIPY) Dyes: Regioselective Syntheses
and Photophysical Properties
Changjiang Yu,[a] Lijuan Jiao,*[a] Hao Yin,[b] Jinyuan Zhou,[a] Weidong Pang,[a]
Yangchun Wu,[a] Zhaoyun Wang,[a] Gaosheng Yang,[a] and Erhong Hao[a]
Keywords: BODIPY / Fluorescence / Dyes/pigments / Synthetic methods
Formylation has been performed on pyrrole-unsubstituted
mylation enables the syntheses of a variety of α- and β-sub-
dipyrromethanes 1 and boron–dipyrrin (BODIPY) dyes 4
stituted BODIPY dyes. The installation of formyl groups af-
based on a Vilsmeier–Haack reaction. It is highly regioselec- fects the electronic properties of the BODIPY chromophore,
tive and complementary and occurs exclusively at the α- and
β-position, respectively, for pyrrole-unsubstituted dipyrro-
methanes 1 and BODIPY dyes 4. This regioselective for-
resulting in red- and blueshifts of the absorption and emis-
sion maxima, respectively, for the α- and β-formylated
BODIPYs 3 and 5.
efficient synthesis of a series of β-formylated BODIPYs,[19]
which constitute a good platform for further functionaliza-
tion of the BODIPY core at the β-position.[20] However,
all of these β-functionalization methods face regioselectivity
issues on the BODIPY core, such as the methyl groups in
1,3,5,7-tetramethyl-substituted BODYPYs, because the re-
giochemistry is predetermined by pyrrole-substituted
BODIPYs A and B (Figure 1), which block the other posi-
tions from participating in these reactions.
Introduction
Boron–dipyrrin (BODIPY) dyes have been widely used
as bright fluorescent dyes for cellular imaging due to their
remarkable photophysical properties, such as photostability,
large extinction coefficients and high fluorescent quantum
yields.[1,2] Recent improvements in functionalization meth-
ods for BODIPY has allowed fine-tuning of the properties
of the chromophore and brought renewed research interest
in BODIPYs for diverse fields, such as chemosensors,[3,4]
long-wavelength absorbing/emitting fluorescent dyes,[5–8] la-
ser dyes,[9] photosensitizers,[10] sensitizers for solar cells,[11]
energy-transfer cassettes,[12] light harvesters[13] and fluores-
cent organic devices.[14]
Post-modification methods on some ready-made BOD-
IPY frameworks are convenient for preparing α- and β-
functionalized BODIPYs (Figure 1). Of these compounds,
α-functionalized BODIPYs are often achieved by Knoeven-
agel condensation,[3a,3b,6] Sonogashira/Suzuki coupling[3g,5]
or oxidative formylation.[15] Functionalization at the β-posi-
tion is mainly achieved by sulfonation,[16] nitration,[17] pal-
ladium-catalyzed C–H functionalization,[18] and halogena-
tion reactions.[5e,10] Recently, our group has reported the
[a] Laboratory of Functionalized Molecular Solids, Ministry of
Education, and Anhui Key Laboratory of Molecular Based
Materials, College of Chemistry and Material Science, Anhui
Normal University,
Wuhu 241000, China
Fax: +86-553-3869303
E-mail: jiao421@mail.ahnu.edu.cn
[b] Hefei National Laboratory for Physical Sciences at Microscale,
University of Science and Technology of China,
Anhui 230026, China
Figure 1. IUPAC numbering system for the BODIPY core, chemi-
cal structures for 1,3,5,7-tetramethyl-substituted BODIPYs A and
B, and their β-formylated products C and D.[20]
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100736 or from
the author.
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α-/β-Formylated Boron–Dipyrrin (BODIPY) Dyes
In this case, we envisioned the possibility of regioselective These pioneering works further supported our above ra-
direct β-formylation on these pyrrole-unsubstituted tionale. While we were preparing this manuscript, Ravi-
BODIPYs 4 (Schemes 1 and 2). Formylation is an electro- kanth and co-workers reported the generation of several
philic substitution reaction and would prefer to occur at 3,5-diformyl-substituted BODIPYs through BF3 complex-
the least positively charged positions of the BODIPY core. ation of the corresponding 3,5-diformyldipyrromethenes.[22]
Indeed, the Mulliken charge analysis of pyrrole-unsubsti- Herein, we report the regioselective syntheses of α- and β-
tuted BODIPY 4a (–0.37 for the β-carbon atom) shown in formylated BODIPYs 3, 5 and 6, and their photophysical
Figure 2, and Figure S1 in the Supporting Information, properties.
1
which is consistent with the H NMR spectroscopy results
(δ = 6.55 ppm for the β-proton), clearly shows that the β-
position is the least positively charged site. Thus, by care-
fully controlling the reaction conditions, it would be pos-
Results and Discussion
sible to achieve regioselective β-formylation of pyrrole-un-
substituted BODIPYs.
Syntheses of the BODIPY Dyes
In contrast to their 1,3,5,7-tetramethyl-substituted
BODIPY analogues A and B (Figure 1), pyrrole-unsubsti-
tuted BODIPYs 4 (Scheme 1) have attracted relatively little
research interest until very recently due to synthetic diffi-
culties. Traditionally, this type of BODIPY was prepared
in modest overall yields (ca. 10%)[24b] by using Lindsey’s
method[23,24] of treating a large excess of pyrrole with alde-
hydes under acid-catalyzed conditions. Recent improvement
has been made by using InCl3 as the catalyst to reduce the
amount of pyrrole used to only slightly more than a stoi-
chiometric amount; however, two additional steps, the in-
stallation and removal of an α-thioalkyl group in the initial
and final stages of the synthesis, are required.[24c] Thus, ef-
ficient synthesis of these pyrrole-unsubstituted BODIPYs
has remained a challenge, until a recent report by Dehaen
and co-workers regarding the water-phase synthesis of pyr-
Figure 2. Top view of the calculated structure for meso-phenyl-sub-
stituted BODIPY 4a, Mulliken charges (MC) of the three pyrrole
carbon atoms and the relevant chemical shifts in the 1H NMR
spectrum for the corresponding hydrogen atoms on these carbon
atoms in CDCl3. H atoms are omitted for clarity.
While we designed this project, direct regioselective func- role-unsubstituted dipyrromethanes.[25] This has resulted in
tionalization of pyrrole-unsubstituted BODIPYs, such as increased research interest in this type of mole-
4a, was reported by Osuka and co-workers for iridium-cata- cule.[16,22,23,26] By using this method, a series of meso-substi-
lyzed regioselective α- or β-borylation of BODIPY dyes,[21a] tuted dipyrromethanes, such as 1a,b, shown in Scheme 1,
and by Dehaen and co-workers for regioselective nucleo- were efficiently generated on a multi-gram scale by conden-
philic substitution at the α-position of BODIPY.[21b,21c] sation of aromatic aldehydes with pyrrole (3 equiv.) under
Scheme 1. Regioselective α- and β-formylation of dipyrromethanes 1a,b and BODIPYs 4a,b, respectively to generate α- and β-formylated
BODIPY dyes 3a,b,d and 5a,b. DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
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HCl-catalyzed conditions at room temperature in water. gioselectivity at the β-position. As expected, no α-formyl-
The resulting dipyrromethanes were easily isolated from the ated BODIPYs or other isomers were isolated from this re-
reaction mixture as precipitates in an essentially pure state action under various conditions. This highly regioselective
in high yields. Most of these dipyrromethanes were directly β-formylation is in agreement with the MC analysis results
used for the DDQ oxidation and subsequent BF3·OEt2 described above, indicating that the β-position is the most
complexation reactions to generate the desired BODIPYs reactive site under these reaction conditions. In comparison
4a–i (see Schemes 1 and 2 and Table 1) in 21–62% isolated with the 1,3,5,7-tetramethyl-substituted BODIPY ana-
yields.
logues A and B, the BODIPYs 4a,b showed a reduced reac-
tivity in this formylation reaction. The reaction required a
higher temperature (80 °C), a large excess of the Vilsmeier
reagents and a much longer reaction time (10 h). This dif-
ferent reactivity may be attributed to reduced electron den-
sity on the BODIPY core in BODIPYs 4a,b due to the ab-
sence of electron-donating pyrrole substituents.
Table 1. Syntheses of BODIPYs 5.
1
Mono-α- and -β-formylation was confirmed by H and
13C NMR spectroscopy and X-ray analysis results, which
are shown in Figures 3 and 4. BODIPYs 3a and 5a showed
molecular ion signals at 296.0936 and 296.0926, respec-
tively, in HRMS (calcd. 296.0932) corresponding to the
1
monoformylated BODIPY product. In the H NMR spec-
tra, BODIPYs 3a and 5a showed characteristic signals at δ
= 10.39 and 9.79 ppm, respectively, for the monoformyl
group. Crystals suitable for the X-ray analysis were ob-
tained by the slow concentration of solutions of BODIPYs
3a and 5a in dichloromethane in air. Both α- and β-mono-
formylated BODIPYs 3a and 5a showed a planar BODIPY
framework with dihedral angles of 54 and 56°, respectively,
between the meso-phenyl substituent and the BODIPY
core. The plane defined by the F–B–F atoms for these two
BODIPY molecules is perpendicular to that of BODIPY
core, similar to previously reported results.[23b] The average
B–N distances for BODIPYs 3a and 5a are 1.548(2) and
The reaction of meso-aryldipyrromethanes 1a,b under 1.540(3) Å, respectively, which implies typical delocalization
standard Vilsmeier–Haack conditions with 1 or 2 equiv. of of the positive charge. In 3a, there is also an intramolecular
Vilsmeier reagent (POCl3/DMF) generated the correspond- hydrogen bond between the two fluorine atoms and the for-
ing α-formylated dipyrromethanes 2a,b and α,α-diformyl- myl proton[15b] in the α-position, with an average C–H···F
ated dipyrromethanes[27] 2c,d, respectively, in around 50% distance of 2.74 Å. The formation of α,α-diformylated
isolated yield with perfect regioselectivity at the α-positions. BODIPY 3d was also confirmed by HRMS and NMR
The reaction of the resulting α-formylated dipyrromethanes
2a,b in the DDQ oxidation under ice-cold conditions and
subsequent treatment with Et3N and BF3·Et2O smoothly
generated the desired α-formylated BODIPYs 3a,b in
around 15% isolated yields. Under similar conditions, α,α-
diformylated dipyrromethane 2d was also smoothly con-
verted into the corresponding α,α-diformylated BODIPY
3d[22] in an overall yield of 21%, as shown in Scheme 1. In
contrast, no α,α-diformylated BODIPY was obtained for
α,α-diformylated meso-phenyldipyrromethane 2c under
these conditions. This may be attributed to the instability
of dipyrromethene, which is the DDQ oxidization product
of dipyrromethane 2c, under these reaction conditions in
the presence of a large excess of Et3N. Changing the reac-
tion conditions, such as temperature, solvent and the rea-
gent ratio of BF3·Et2O/Et3N, still failed to convert dipyrro-
methane 2c into the desired α,α-diformylated BODIPY.
On the other hand, treatment of 4a or 4b under modified
Vilsmeier reaction conditions led to the isolation of β-for-
mylated BODIPYs 5a or 5b in yields of 75% with high re- proton is indicated by thin dashed lines for clarity.
Figure 3. Top (a) and side views (b) of the X-ray structure of
BODIPY 3a. Hydrogen bonding between F atoms and the formyl
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α-/β-Formylated Boron–Dipyrrin (BODIPY) Dyes
spectroscopy. A molecular ion signal at 354.0985 in HRMS der Vilsmeier reaction conditions. Instead, these p-substi-
(calcd. 354.0987) corresponded to the diformylated tuted dimethylanilines are usually converted into the di-
BODIPY, and a characteristic signal at δ = 10.48 ppm was benzo[b,f][1,5]diazocines and tetrahydroquinazolinium salts
observed for the two formyl groups.
under these conditions, according to the literature.[29]
Figure 4. Top (a) and side views (b) of the X-ray structure of
BODIPY 5a.
Scheme 2. Syntheses of mono-β-formylated BODIPY 5i and
meso,β-diformylated BODIPY 6.
The F atoms in BODIPY molecules are strongly electro-
negative and are able to form intermolecular hydrogen
bonds (C–H···F) with hydrogen atoms.[28] In the solid state,
each molecule of 5a forms eight such C–H···F intermo-
lecular hydrogen-bonding interactions (with the aldehyde
proton, the hydrogen atom at the 3-position of the
BODIPY core, and the ortho-/meta-hydrogen atoms of the
meso-phenyl substituent); the H···F hydrogen-bond length
is in the range of 2.409(3)–2.845(3) Å, as shown in Fig-
ure S6 in the Supporting Information. In this way, each mo-
lecule of 5a is connected to three other neighbouring mole-
cules of 5a, which eventually leads to the formation of the
crystal-packing structure observed for 5a. In this crystal-
packing structure, all molecules of 5a are almost parallel to
each other in a head-to-tail orientation.
To test the versatility of this regioselective β-formylation
reaction, BODIPYs 4c–h were also used for this optimized
formylation reaction, as shown in Table 1. In comparison
with 4a, BODIPYs 4c–h showed similar reactivities in this
formylation reaction and generated exclusively β-formyl-
ated BODIPYs 5c–h in 54–81% yields. In the 1H NMR
spectra, each of these BODIPYs 5c–h gave a characteristic
formyl proton signal as a singlet in the range δ = 9.74–
9.90 ppm, as summarized in Table 1.
This interesting result promoted us to investigate the op-
timized reaction conditions for the formylation of meso-[p-
(dimethylamino)phenyl]-substituted BODIPY 4i. Because
diformylated BODIPY 6 resulted from further formylation
of BODIPY 5i, less Vilsmeier reagent (30 equiv.) and a
lower reaction temperature (60 °C) were used for the for-
mylation reaction to achieve the β-monoformylated
BODIPY 5i. Under these conditions, the desired β-mono-
formylated BODIPY 5i was isolated as the major product
in 70% yield. To improve the yield of meso,β-diformylated
BODIPY 6, various reaction conditions regarding solvent
and temperature were investigated, but no clear improve-
ment in the yield was obtained. On the other hand, increas-
ing the amount of Vilsmeier reagent from 160 to 250 equiv.
increased the yield to 57%. In contrast, no such meso,β-
diformylated BODIPY was obtained for the other meso-
aryl-substituted BODIPYs 4a–h under various formylation
conditions. Thus, the presence of a strongly electron-donat-
ing substituent at the meso-aryl group facilitates this for-
mylation reaction.
However, the treatment of meso-substituted p-(dimeth-
ylamino)phenyl-substituted BODIPY 4i with 160 equiv. of
Vilsmeier reagent at 80 °C gave mainly the meso,β-diformyl-
Photophysical Characterization of BODIPYs 3
BODIPYs 4, 5 and 6 are colourful to the eye, and most
ated BODIPY 6 instead of the desired β-monoformylated of them are brilliant upon irradiation. The photophysical
BODIPY 5i, as shown in Scheme 2. The formation of properties of these molecules are summarized in Table 2.
BODIPY 6 may be attributed to increased electron density Pyrrole-unsubstituted BODIPYs 4 showed a strong absorp-
at the meso-phenyl carbon atoms adjacent to the dimeth- tion band at around (500Ϯ15) nm, and most of them gave
ylamine substituent, which facilitates further formylation at a weak fluorescence emission at (530Ϯ15) nm with the ex-
the meso-position for β-monoformylated BODIPY 5i. The ception of BODIPY 4g, which gave an emission maximum
second formylation of BODIPY 4i at the p-(dimethyl- at 626 nm. The longest-wavelength emission and the largest
amino)phenyl moiety is surprising, because no formylated Stokes shift observed in BODIPY 4g may be attributed to
product is obtained for p-substituted dimethylanilines un- the presence of the 2-thiophene group at the meso position
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of the BODIPY core, which participates in a twisted intra- cular charger transfer due to the existence of formyl
molecular charge transfer (TICT), as described in the litera- groups.[15b] On the other hand, β-formylation results in a
ture.[26d,26e]
slight blueshift of the spectrum for BODIPY 5b (around
5 nm). Similarly, the installation of a formyl group at the β-
position of the BODIPY core generally gave a blueshift in
both the absorption (3–12 nm) and emission (3–10 nm)
maxima for most of the β-formylated BODIPYs 5, as sum-
marized in Table 2. For the meso-substituted p-(dimeth-
ylamino)phenyl-substituted BODIPY 4i, β-formylation re-
sulted in a 12 nm blueshift of the absorption and a 4 nm
redshift of the emission maxima in BODIPY 5i, whereas
the meso,β-diformylation gave absorption and emission
spectra in BODIPY 6 similar to those of the starting
BODIPY 4i. In comparison with the starting BODIPYs 4,
formylation had little effect on the fluorescence quantum
yields of the resulting formylated BODIPYs 3d, 5 and 6,
except for the α-monoformylated BODIPYs 3a,b, which
gave enhanced fluorescence quantum yields, as summarized
in Table 2.
Table 2. Photophysical properties of BODIPYs 3, 4, 5 and 6 in
dichloromethane at room temperature.
BOD-
IPY
λmax
[nm]
logεmax
λem
[nm]
Stokes shift
[cm–1]
Φ[a]
3a
3b
3d
4a
5a
4b
5b
4c
5c
4d
5d
4e
5e
4f
519
517
544
500
496
502
499
498
493
508
505
502
498
512
508
510
505
501
497
491
479
494
4.11
4.15
4.43
4.52
4.39
4.50
4.13
4.68
4.25
4.13
4.61
4.73
4.46
4.86
4.39
4.47
4.08
4.67
4.49
4.82
4.35
4.21
542
538
590
527
521
529
525
529
519
545
540
528
525
534
532
626
624
521
518
517
521
518
818
755
1430
1025
967
1017
992
1177
1016
1336
1283
981
1033
805
888
3633
3776
766
0.29
0.24
0.08
0.03
0.03
0.02
0.01
0.05
0.03
0.01
0.01
0.02
0.03
0.71
0.71
0.01
0.01
0.84
0.87
0.01
0.01
0.02
5f
4g
5g
4h
5h
4i
816
1024
1683
900
5i
6
[a] The fluorescence quantum yields were calculated by using fluo-
rescein in a 0.1 n aqueous solution of NaOH (Φ = 0.90) as the
standard.
Figure 5. Normalized UV/Vis absorption (a) and normalized
fluorescence emission (b) spectra of BODIPYs 4b (solid line), 3b
(dashed line), 5b (dotted line) and 3d (dash-dotted line) in CH2Cl2.
In comparison with the 1,3,5,7-tetramethyl-substituted
BODIPY analogues, the low fluorescence quantum yields
observed for most of the BODIPYs 4 may be attributed to
free rotation of the meso substituent around a carbon–
Conclusions
We have developed a regioselective formylation of pyr-
carbon single bond,[24b,24c] which increased the non-radia- role-unsubstituted dipyrromethanes and BODIPY dyes at
tive deactivation process in these BODIPYs. The high fluo- the α- and β-positions, as well as an interesting meso,β-di-
rescence emission observed in BODIPYs 4f and 4h was formylation reaction for the preparation of some meso,β-
comparable to that of the 1,3,5,7-tetramethyl-substituted diformylated BODIPYs. The high regioselectivity of this
BODIPY analogues and may be attributed to the presence formylation procedure offers a new way to synthesize dif-
of methyl and chloro substituents at the 2,6-positions of the ferent kinds of α- and β-substituted BODIPY dyes. This
meso-aryl groups. The steric hindrance effect prevents formylation affected the photophysical properties of the
free rotation of the meso group in these two BODIP- BODIPY chromophore, leading to a redshift in both the
Ys.[23b,24b,24c]
absorption and emission spectra in the resulting α-formyl-
The installation of formyl group(s) on the BODIPY core ated and α,α-diformylated BODIPYs 3a,b and 3d, and a
affected the photophysical properties of these molecules, as slight blueshift in the β-formylated BODIPYs 5. Extending
demonstrated in the normalized absorption and emission the high regioselectivity of this formylation reaction to the
spectra of BODIPY dyes 3b, 4b, 5b and 3d shown in Fig- other electrophilic substitution reactions, such as the halo-
ure 5. These formylated BODIPY dyes show good disper- genation reaction, is currently underway in our laboratory.
sion of UV/Vis absorbance and fluorescence emission char-
acteristics for BODIPY dyes. α-Formylation leads to a red-
Experimental Section
shift of absorption (around 15 and 40 nm for BODIPYs 3b
and 3d, respectively) and emission maxima (around 10 and
60 nm for BODIPYs 3b and 3d, respectively), with the
largest spectral shifts for the α,α-diformylated BODIPY 3d.
Several interesting shoulders on the signals of α-formylated
General Procedure for the Preparation of the Mono-α-formylated
BODIPYs 3a–b: DDQ (150 mg, 0.66 mmol) was slowly added to a
round-bottomed flask containing α-formyldipyrromethane 2a,b[27]
(0.6 mmol) in CH2Cl2 (20 mL) cooled on an ice bath. The reaction
BODIPYs 3b and 3d may be attributed to the intramole- mixture was stirred at this temperature for 20 min before triethyl-
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α-/β-Formylated Boron–Dipyrrin (BODIPY) Dyes
amine (0.6 mL) and BF3·Et2O (0.6 mL) were quickly added to the
reaction mixture. The reaction was further stirred at room tempera-
ture for 15 min and extracted with CH2Cl2 (3ϫ 40 mL). The or-
ganic layers were combined, washed with water (2ϫ 100 mL), dried
with anhydrous Na2SO4, and the solvent was removed under re-
duced pressure. The crude product was further purified by column
chromatography on silica gel using dichloromethane as the eluent
to give the desired α-formylated BODIPYs as reddish powders.
BODIPY 4c: The general procedure was applied with 4-bro-
mobenzaldehyde (368 mg, 2.0 mmol) as the aromatic aldehyde. Fi-
nal purification was performed by column chromatography on sil-
ica gel (CH2Cl2/hexane = 1:2, v/v) to give BODIPY 4c (430 mg,
62% yield). 1H NMR (300 MHz, CDCl3): δ = 7.96 (s, 2 H, py),
7.68 (s, 2 H, ph), 7.45 (s, 2 H, ph), 6.91 (s, 2 H, py), 6.56 (s, 2 H,
py) ppm. 13C NMR (75 MHz, CDCl3): δ = 145.8, 144.6, 134.7,
132.6, 131.8, 131.3, 125.5, 118.9 ppm. HRMS (EI): calcd. for
C15H10BBrF2N2 [M]+ 346.0088; found 346.0093.
BODIPY 3a: Compound 2a (150 mg, 0.6 mmol) was used for this
reaction to give BODIPY 3a in 14% yield (25 mg). 1H NMR
(300 MHz, CDCl3): δ = 10.39 (s, 1 H, CHO), 8.24 (s, 1 H, py),
7.64–7.52 (m, 5 H, ph), 7.16 (s, 1 H, py), 7.10 (s, 1 H, py), 6.87 (s,
1 H, py), 6.75 (s, 1 H, py) ppm. 13C NMR (75 MHz, CDCl3): δ =
184.3, 150.1, 149.2, 147.7, 137.0, 136.7, 135.2, 133.2, 131.5, 130.6,
128.7, 121.8, 118.3 ppm. HRMS (EI): calcd. for C16H11BF2N2O
[M]+ 296.0932; found 296.0936.
BODIPY 4e: The general procedure was applied with 4-chloro-
benzaldehyde (280 mg, 2.0 mmol) as the aromatic aldehyde. Final
purification was performed by column chromatography on silica
gel (CH2Cl2/hexane = 1:2, v/v) to give BODIPY 4e as a greenish
powder (364 mg, 60% yield). 1H NMR (300 MHz, CDCl3): δ =
7.96 (s, 2 H, py), 7.53 (s, 4 H, ph), 6.92 (s, 2 H, py), 6.57 (s, 2 H,
py) ppm. 13C NMR (75 MHz, CDCl3): δ = 144.8, 143.5, 136.2,
133.7, 131.1, 130.7, 130.3, 127.9, 117.8 ppm. HRMS (EI): calcd.
for C15H10BClF2N2 [M]+ 302.0594; found 302.0590.
BODIPY 3b: Compound 2b (168 mg, 0.6 mmol) was used for this
reaction to give BODIPY 3b in 16% yield (32 mg). 1H NMR
(300 MHz, CDCl3): δ = 10.39 (s, 1 H, CHO), 8.19 (s, 1 H, py), 7.57
(d, J = 7.8 Hz, 2 H, ph), 7.19 (s, 1 H), 7.11–7.08 (m, 3 H, ph, py),
6.91 (s, 1 H, py), 6.74 (s, 1 H, py), 3.94 (s, 3 H, CH3) ppm. 13C
NMR (75 MHz, CDCl3): δ = 184.4, 162.8, 149.2, 149.0, 147.2,
136.7, 134.9, 132.8, 128.4, 125.8, 121.5, 118.1, 114.4, 55.7 ppm.
HRMS (EI): calcd. for C17H13BF2N2O2 [M]+ 326.1038; found
326.1031.
BODIPY 4f: The general procedure was applied with 2,6-dichlo-
robenzaldehyde (348 mg, 2.0 mmol) as the aromatic aldehyde. Final
purification was performed by column chromatography on silica
gel (CH2Cl2/hexane = 1:2, v/v) to give BODIPY 4f (343 mg, 43%).
1H NMR (300 MHz, CDCl3): δ = 7.95 (s, 2 H, py), 7.53–7.41 (m,
3 H, ph), 6.71 (d, J = 3.9 Hz, 2 H, py), 6.52 (d, J = 3.6 Hz, 2 H,
py) ppm. 13C NMR (75 MHz, CDCl3): δ = 145.4, 140.7, 135.1,
134.8, 131.3, 130.0, 128.3, 119.1 ppm. HRMS (EI): calcd. for
C15H9BCl2F2N2 [M]+ 336.0204; found 336.0203.
Preparation of the α,α-Diformylated BODIPY 3d: The key synthetic
precursor 2d for 3d was prepared according to the literature,[27]
and was directly used for subsequent oxidation and complexation
reactions. DDQ (250 mg, 1.1 mmol) in CH2Cl2 (10 mL) was added
to a round-bottomed flask containing 2d (308 mg, 1.0 mmol) in
CH2Cl2 (60 mL) cooled in an ice bath. The reaction mixture was
stirred at this temperature for 30 min, then triethylamine (2.0 mL)
and BF3·Et2O (3.0 mL) were quickly added. The reaction mixture
was further stirred at room temperature for an additional 30 min,
poured into water (100 mL) and extracted with CH2Cl2 (3ϫ
50 mL). The organic layers were combined, dried with anhydrous
Na2SO4, and the solvent was removed under vacuum. The crude
product was further purified by column chromatography (silica gel,
CH2Cl2/hexane = 3:1, v/v) to give 3d as a green powder (21%,
75 mg). 1H NMR (300 MHz, CDCl3): δ = 10.48 (s, 2 H, CHO),
7.63 (d, J = 8.1 Hz, 2 H, ph), 7.21 (s, 2 H, py), 7.14 (d, J = 5.4 Hz,
4 H, ph), 3.97 (s, 3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ
= 184.3, 163.7, 150.4, 137.6, 133.5, 132.4, 125.9, 119.9, 114.8,
55.8 ppm. HRMS (EI): calcd. for C18H13BF2N2O3 [M]+ 354.0987;
found 354.0985.
BODIPY 4i: The general procedure was applied with p-(dimeth-
ylamino)benzaldehyde (298 mg, 2.0 mmol) as the aromatic alde-
hyde. Final purification was performed by column chromatography
on silica gel (CH2Cl2/hexane = 1:1, v/v) to give BODIPY 4i
(131 mg, 21%). 1H NMR (300 MHz, CDCl3): δ = 7.87 (s, 2 H, py),
7.57 (d, J = 7.8 Hz, 2 H, ph), 7.04 (s, 2 H, py), 6.81 (d, J = 7.8 Hz,
2 H, ph), 6.54 (s, 2 H, py), 3.11 (s, 6 H, CH3) ppm. 13C NMR
(75 MHz, CDCl3): δ = 152.6, 148.5, 141.7, 134.5, 133.1, 130.8,
121.8, 117.6, 111.5, 40.1 ppm. HRMS (EI): calcd. for C17H16BF2N3
[M]+ 311.1405; found 311.1403.
General Procedure for the Preparation of BODIPYs 5: A mixture
of DMF (3 mL, 39 mmol) and POCl3 (3 mL, 32 mmol) was cooled
in an ice bath and stirred under argon for 5 min. After warming to
room temperature, the reaction mixture was further stirred for
30 min. Then, BODIPY 4 (0.2 mmol) in ClCH2CH2Cl (10 mL) was
added to the reaction mixture. After raising the temperature to
80 °C, the reaction mixture was further stirred for 10 h, cooled to
room temperature and slowly poured into a saturated aqueous
solution of K2CO3 (200 mL) cooled in an ice bath. After warming
to room temperature, the reaction mixture was further stirred for
1 h and extracted with CH2Cl2 (3ϫ 100 mL). The organic layers
were combined, washed with water (2ϫ 100 mL), dried with anhy-
drous NaSO4, and the solvent was removed under reduced pressure.
The crude product was further purified by gel column chromatog-
raphy on silica gel using a mixture of dichloromethane (or ethyl
acetate) and hexane as the eluent to give the β-formylated
BODIPYs 5 as reddish-brown powders.
General Procedure for the Preparation of Starting BODIPYs 4:
BODIPYs 4 were prepared in a one-pot, two-step synthetic pro-
cedure from dipyrromethanes prepared according to the litera-
ture.[25] This involved DDQ oxidation of the dipyrromethanes and
BF3 complexation of the resultant dipyrromethenes. DDQ (454 mg,
2.0 mmol) in CH2Cl2 (10 mL) was added to a round-bottomed
flask containing the dipyrromethane (2.0 mmol) in CH2Cl2 (40 mL)
cooled in an ice bath. The reaction mixture was stirred at this tem-
perature for 10 min, then triethylamine (4 mL) and BF3·Et2O
(4 mL) were quickly added to the reaction mixture. The reaction
mixture was further stirred at room temperature for 2 h, washed
with water (50 mL), a 0.2 m aqueous solution of NaOH (100 mL),
and water (50 mL). The organic layer was collected, dried with an-
hydrous NaSO4, and the solvent was removed under vacuum. The
crude product was further purified by column chromatography on
silica gel using a mixture of dichloromethane and hexane as the
eluent to give the desired BODIPYs 4 as powders.
BODIPY 5a: BODIPY 4a (65 mg, 0.24 mmol) was used as the
starting material. Final purification was performed by column
chromatography on silica gel (CH2Cl2/hexane = 1:1, v/v) to give
1
BODIPY 5a (54 mg, 75%). H NMR (300 MHz, CDCl3): δ = 9.79
(s, 1 H, CHO), 8.21 (s, 1 H, py), 8.11 (s, 1 H, py), 7.56–7.51 (m, 5
H, ph), 7.24 (s, 1 H, py), 7.09 (s, 1 H, py), 6.66 (s, 1 H, py) ppm.
13C NMR (75 MHz, CDCl3): δ = 184.9, 149.5, 142.9, 137.0, 135.0,
Eur. J. Org. Chem. 2011, 5460–5468
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5465

L. Jiao et al.
FULL PAPER
134.7, 132.9, 131.8, 131.6, 130.5, 128.8, 128.7, 121.5 ppm. HRMS
(EI): calcd. for C16H11BF2N2O [M]+ 296.0932; found 296.0926.
128.9, 128.6, 121.5 ppm. HRMS (EI): calcd. for C14H9BF2N2OS
[M]+ 302.0497; found 302.0490.
BODIPY 5h: BODIPY 4h (50 mg, 0.16 mmol) was used as the
starting material. Final purification was performed by column
chromatography on silica gel (CH2Cl2/hexane = 1:2, v/v) to give
BODIPY 5h (44 mg, 81%). 1H NMR (300 MHz, CDCl3): δ = 9.74
(s, 1 H, CHO), 8.18 (s, 1 H, py), 8.09 (s, 1 H, py), 6.96–6.85 (m, 4
H, ph and py), 6.59 (s, 1 H, py), 2.30 (s, 3 H, CH3), 2.02 (s, 6 H,
CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 183.8, 148.9, 141.9,
138.6, 136.8, 136.7, 135.1, 132.6, 130.8, 130.7, 127.6, 127.4, 126.2,
120.6, 20.1, 18.9 ppm. HRMS (EI): calcd. for C19H17BF2N2O
[M]+ 338.1402; found 338.1408.
BODIPY 5b: BODIPY 4b (60 mg, 0.20 mmol) was used as the
starting material. Final purification was performed by column
chromatography on silica gel (CH2Cl2/hexane = 3:2, v/v) to give
BODIPY 5b (49 mg, 75%). 1H NMR (300 MHz, CDCl3): δ = 9.88
(s, 1 H, CHO), 8.27 (s, 1 H, py), 8.15 (s, 1 H, py), 7.60 (d, J =
7.8 Hz, 2 H, ph), 7.37 (s, 1 H, py), 7.20 (s, 1 H, py), 7.11 (d, J =
8.1 Hz, 2 H, ph), 6.73 (s, 1 H, py), 3.94 (s, 3 H, CH3) ppm. 13C
NMR (75 MHz, CDCl3): δ = 161.9, 148.6, 147.4, 141.5, 135.7,
133.6, 131.7, 130.7, 129.5, 127.8, 127.3, 124.5, 120.1, 113.5,
54.6 ppm. HRMS (EI): calcd. for C17H13BF2N2O2 [M]+ 326.1038;
found 326.1034.
BODIPY 5i: A mixture of DMF (0.5 mL, 6.5 mmol) and POCl3
(0.45 mL, 4.8 mmol) was cooled in an ice bath and stirred under
argon for 5 min. After warming to room temperature, the reaction
mixture was further stirred for 30 min. BODIPY 4i (50 mg,
0.16 mmol) in ClCH2CH2Cl (10 mL) was added to this reaction
mixture. After the temperature had been raised to 60 °C, the reac-
tion mixture was further stirred for 10 h. Workup was performed
by applying the general procedure described for the preparation of
BODIPYs 5. The crude product was purified by column
chromatography on silica gel (EtOAc/hexane = 1:2, v/v) to give
BODIPY 5i as a reddish-brown powder (38 mg, 70%). 1H NMR
(300 MHz, CDCl3): δ = 9.87 (s, 1 H, CHO), 8.21 (s, 1 H, py), 8.04
(s, 1 H, py), 7.60–7.57 (m, 2 H, ph), 7.41 (s, 1 H, py), 7.23 (s, 1 H,
py), 6.84 (d, J = 8.2 Hz, 2 H, ph), 6.69 (s, 1 H, py), 3.14 (s, 6 H,
CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 185.2, 153.3, 150.2,
145.6, 141.3, 135.8, 134.5, 133.7, 131.4, 127.3, 121.3, 120.0, 111.9,
40.2 ppm. HRMS (EI): calcd. for C18H16BF2N3O [M]+ 339.1354;
found 339.1345.
BODIPY 5c: BODIPY 4c (60 mg, 0.17 mmol) was used as the
starting material. Final purification was performed by column
chromatography on silica gel (CH2Cl2/hexane = 1:1, v/v) to give
BODIPY 5b (46 mg, 71%). 1H NMR (300 MHz, CDCl3): δ = 9.87
(s, 1 H, CHO), 8.30 (s, 1 H, py), 8.20 (s, 1 H, py), 7.75 (d, J =
8.4 Hz, 2 H, ph), 7.49 (d, J = 8.4 Hz, 2 H, ph), 7.29 (s, 1 H, py),
7.15 (d, J = 4.5 Hz, 1 H, py), 6.76 (d, J = 4.2 Hz, 1 H, py) ppm.
13C NMR (75 MHz, CDCl3): δ = 183.8, 148.9, 146.9, 142.2, 135.7,
133.6, 133.2, 131.2, 130.8, 130.6, 127.4, 125.5, 120.8, 113.7 ppm.
HRMS (EI): calcd. for C16H11BBrF2N2O [M + H]+ 374.0038;
found 374.0032.
BODIPY 5d: BODIPY 4d (60 mg, 0.19 mmol) was used as the
starting material. Final purification was performed by column
chromatography on silica gel (CH2Cl2/hexane = 3:2, v/v) to give
1
BODIPY 5d (35 mg, 54%). H NMR (300 MHz, [D6]DMSO): δ =
9.84 (s, 1 H, CHO), 8.63 (s, 2 H, py), 8.44 (s, 2 H, ph), 8.02 (s, 2
H, ph), 7.32 (s, 2 H, py), 6.96 (s, 1 H, py) ppm. 13C NMR (75 MHz,
[D6]DMSO): δ = 186.3, 153.0, 149.5, 146.3, 144.3, 138.7, 137.0,
136.0, 134.0, 132.6, 128.6, 124.3, 123.7, 110.4 ppm. HRMS (EI):
calcd. for C16H10BF2N3O3 [M]+ 341.0783; found 341.0775.
BODIPY 6: The general procedure was applied with the require-
ment for an excess amount of the Vilsmeier reagents: a mixture of
DMF (3 mL, 39 mmol) and POCl3 (3 mL, 32 mmol) was used for
the formylation of BODIPY 4i (40 mg, 0.13 mmol). Final purifica-
tion was performed by column chromatography on silica gel
(EtOAc/hexane = 4:3, v/v) to give BODIPY 6 as a reddish-brown
powder (27 mg, 57% yield). 1H NMR (300 MHz, CDCl3): δ =
10.11 (s, 1 H, CHO), 9.89 (s, 1 H, CHO), 8.27 (s, 1 H, py), 8.14 (s,
1 H, py), 7.99 (d, J = 2.4 Hz, 1 H, py), 7.73 (dd, J = 8.7 Hz, 1 H,
ph), 7.36 (s, 1 H, ph), 7.22 (d, J = 4.5 Hz, 1 H, py), 7.16 (d, J =
8.7 Hz, 1 H, ph), 6.75 (d, J = 3.9 Hz, 1 H, py), 3.16 (s, 6 H, CH3)
ppm. 13C NMR (75 MHz, CDCl3): δ = 189.0, 185.0, 156.2, 148.5,
148.0, 142.4, 136.7, 136.6 (7), 136.4, 134.1, 131.6, 127.6, 124.4,
124.3 (8), 123.0, 121.2, 121.1, 116.9, 44.7 ppm. HRMS (EI): calcd.
for C19H16BN3O2 [M]+ 367.1304; found 367.1309.
BODIPY 5e: BODIPY 4e (60 mg, 0.20 mmol) was used as the
starting material. Final purification was performed by column
chromatography on silica gel (CH2Cl2/hexane = 1:1, v/v) to give
1
BODIPY 5e (45 mg, 69%). H NMR (300 MHz, CDCl3): δ = 9.87
(s, 1 H, CHO), 8.30 (s, 1 H, py), 8.20 (s, 1 H, py), 7.60–7.53 (m, 4
H, ph), 7.29 (s, 1 H, py), 7.15 (d, J = 4.4 Hz, 1 H, py), 6.75 (d, J
= 4.4 Hz, 1 H, py) ppm. 13C NMR (75 MHz, CDCl3): δ = 184.8,
149.9, 147.9, 143.2, 138.2, 136.8, 134.7, 134.3, 131.8, 131.7, 131.2,
129.3, 128.5, 121.9 ppm. HRMS (EI): calcd. for C16H10BClF2N2O
[M]+ 330.0455; found 330.0437.
BODIPY 5f: BODIPY 4f (60 mg, 0.18 mmol) was used as the start-
ing material. Final purification was performed by column
chromatography on silica gel (CH2Cl2/hexane = 3:2, v/v) to give
Supporting Information (see footnote on the first page of this arti-
cle): General methods, molecular modeling, X-ray analysis, copies
of 1H and 13C NMR and HR mass spectra for all new compounds.
1
BODIPY 5f (47 mg, 72%). H NMR (300 MHz, CDCl3): δ = 9.84
(s, 1 H, CHO), 8.29 (s, 1 H, py), 8.23 (s, 1 H, py), 7.53–7.49 (m, 3
H, ph), 7.06 (s, 1 H, py), 6.95 (d, J = 4.5 Hz, 1 H, py), 6.72 (d, J
= 4.5 Hz, 1 H, py) ppm. 13C NMR (75 MHz, CDCl3): δ = 184.7,
151.4, 143.6, 142.9, 137.5, 134.9, 133.9, 133.4, 131.8, 130.2, 128.4,
126.8, 122.3 ppm. HRMS (EI): calcd. for C16H9BCl2F2N2O [M]+
364.0153; found 364.0156.
Acknowledgments
This work was supported by the National Nature Science Founda-
tion of China (grants 20802002, 20902004, 20972001, and
21072005). Funding from Anhui Province (grants 090416221 and
KJ2009A130), the Scientific Research Foundation for Returned
Overseas Chinese Scholars, the State Education Ministry, and the
Doctoral Fund of the Ministry of Education of China (Grant
20093424120001) are also acknowledged.
BODIPY 5g: BODIPY 4g (60 mg, 0.22 mmol) was used as the
starting material. Final purification was performed by column
chromatography on silica gel (CH2Cl2/hexane = 3:2, v/v) to give
BODIPY 5g (45 mg, 68%). 1H NMR (300 MHz, CDCl3): δ = 9.90
(s, 1 H, CHO), 8.28 (s, 1 H, py), 8.16 (s, 1 H, py), 7.84 (d, J =
4.1 Hz, 1 H, th), 7.66 (s, 2 H, py), 7.49 (s, 1 H, th), 7.34 (s, 1 H,
th), 6.77 (s, 1 H, py) ppm. 13C NMR (75 MHz, CDCl3): δ = 185.0,
148.9, 142.7, 141.6, 136.2, 134.8, 134.2, 133.9, 133.7, 133.2, 131.7,
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Received: May 26, 2011
Published Online: August 10, 2011
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