Facile synthesis of dimeric BODIPY and its catalytic activity for sulfide oxidation under visible light

Article abstract of DOI:10.1039/c4ra01501k

An orthogonal dimeric BODIPY was easily prepared via condensation of 2,4-dimethylpyrrole and oxalyl dichloride, and was utilized as a visible-light-driven photocatalyst for the oxidation of sulfides. High catalytic efficiencies, and mild and green conditi

Full text of DOI:10.1039/c4ra01501k

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Facile synthesis of dimeric BODIPY and its catalytic
activity for sulfide oxidation under visible light†
Cite this: RSC Adv., 2014, 4, 14786
ab
Liang Wang,ab Jing Cao,ab Jian-wei Wang,ab Qun Chen,ab Ai-jun Cui
*
ab
*
and Ming-yang He
Received 20th February 2014
Accepted 14th March 2014
DOI: 10.1039/c4ra01501k
www.rsc.org/advances
An orthogonal dimeric BODIPY was easily prepared via condensation such as Ru(bpy)3Cl2 (bpy ¼ 2,20-bipyridine) or Nile Red, BODIPY
of 2,4-dimethylpyrrole and oxalyl dichloride, and was utilized as a derivatives are less-toxic and low cost, but with strong absorp-
visible-light-driven photocatalyst for the oxidation of sulfides. High tion of visible light, long-lived triplet excited state and readily
catalytic efficiencies, and mild and green conditions are the major tunable molecular structures. To our knowledge, the reported
advantages of this protocol. Moreover, meso-carbalkoxylated BODI- BODIPY photocatalysts are focused on iodo-BODIPYs since
PYs could also be prepared using a similar one-pot condensation of uorophores bearing heavy atom generally have a high inter-
2,4-dimethylpyrrole, oxalyl dichloride and a series of alcohols.
system crossing quantum yield (Fisc) and a high singlet oxygen
quantum yield (FO) due to the heavy atom effect.20,21 Recently,
Akkaya et al. designed two kinds of orthogonal dimeric BODI-
PYs with respectable singlet oxygen quantum yields and
increased intersystem crossing but without heavy atoms.11 Their
applications as photocatalysts in organic reactions, however,
were not reported.
Herein, we wish to report a modied and facile route for the
synthesis of orthogonal dimeric BODIPY (1, Fig. 1) and its
application for the oxidation of suldes.
An initial investigation focused on the preparation of
dimeric BODIPY 1 (Scheme 1). The reported methods generally
involved the following steps: (1) condensation of acetoxyacetyl
chloride and 2 equiv. of 2,4-dimethyl pyrrole under reux in
dichloromethane followed by treatment of the reaction mixture
with 4 equiv. of BF3$OEt2 and diisopropylethylamine; (2)
hydrolysis under basic conditions; (3) oxidation to the corre-
sponding meso-formyl BODIPY using standard Dess–Martin
Photoredox catalytic organic reactions driven by visible light
have been gaining increasing interest due to the mild condi-
tions for substrate activation, leading to the construction of
complex organic compounds with a feasible synthetic method.1
However, the potential toxicity, high cost as well as the limited
availability of the current organometallic photocatalysts are the
major drawbacks. Thus, looking for a metal-free, readily avail-
able or easily prepared, and green photocatalysts is still a
challenge in this eld.
Boron–dipyrromethene (BODIPY) compounds have received
much attention because of their unique properties such as high
uorescence quantum yields (Ff), large molar absorption coef-
cients (3), excellent thermal and photochemical stabilities.2–4
Much effort in decoration of the BODIPY scaffold with reactive
functionalities either at meso-position or 8-position have been
realized for tuning their uorescence characteristics,5–11 which
provides them a prominent place as outstanding uorophores
for use in uorescent materials, labels and probes.12–14 Besides,
BODIPY derivatives have shown good photocatalytic activities
including oxidations, cross-dehydrogenative coupling reactions
as reported by Ramaiah group, Jing group as well as Zhao
group.15–19 Comparing with the conventional photocatalysts
aJiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou
University, 213164, P. R. China
bSchool of Petrochemical Engineering, Changzhou University, Changzhou 213164, P. R.
China. E-mail: lwcczu@126.com; hemingyangjpu@yahoo.com; Fax: +86-519-
86330251; Tel: +86-(0)13961477422
† Electronic supplementary information (ESI) available: Experimental section and
characterization data. See DOI: 10.1039/c4ra01501k
Fig. 1 The orthogonal dimeric BODIPY and other photocatalysts
surveyed in this study.
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Scheme 1 Preparation of dimeric BODIPY 1.
Fig. 2 Normalized absorption (solid) and emission (dash) spectra of
BODIPY 1 in dichloromethane.
oxidation conditions; and (4) standard BODIPY synthetic
progress. It was obvious that this route was long and several
purication processes were required, while a 7.2% total yield
was obtained.11,22 In our study, 2,4-dimethylpyrrole and oxalyl
dichloride were selected as starting materials and it was pleased
to obtain the dimeric BODIPY 1 in 10% yield via only one-step
condensation.
and emission at 515 and 588 nm, respectively. Its s and FO were
determined to be 10.9 ns (in reference to rhodamine 6G in
ethanol) and 0.46 (in reference to methylene blue in CH2Cl2),
respectively,11 which is much higher than the reported unha-
logenated BODIPYs and many other organic chromophores and
photosensitizers under comparable conditions.
Further studies showed that a series of carbalkoxylated
BODIPYs could be prepared via the one-pot condensation of 2,4-
dimethylpyrrole, oxalyl dichloride and substituted alcohols,
affording the BODIPYs 6–10 in satisfactory yields. This is due to
the fact that acyl chloride is more active than the aldehyde and
no oxidation process is required during the rst step. To the
best of our knowledge, this is the rst report for the preparation
of carbalkoxylated BODIPYs. Replacing the alcohols with
amines, however, provided no products (Scheme 2).
The photophysical properties of BODIPY 1 and 6–10 were
tested. For BODIPY 1, the maximum wavelengths of absorption
and emission in CH2Cl2 were 515 and 606 nm, respectively
(Fig. 2), which was much higher than carbalkoxylated BODIPYs
6–10. This was attributed to the high conjugation of dimeric
BODIPY 1. The photophysical properties of BODIPY 1 in other
solvents gave obvious differences and the largest stocks shi
(ꢀ102 nm) was found in methanol, while its uorescence
quantum yield was much lower (0.004) than that in hexane
(0.721). For BODIPYs 6–10, generally, relatively lower uores-
cence quantum yields were obtained. This was due to the strong
electron-withdrawing effect of alkoxycarbonyl groups at the
meso-position, leading to decreased uorescence.
The selective oxidation of suldes to the corresponding
sulfoxides is one of the most fundamental organic trans-
formations due to the fact that sulfoxides are important inter-
mediates for various valuable compounds.23 The photocatalytic
activities of BODIPY 1 were then evaluated using thioanisole as
the substrate (Fig. 3). The reaction was carried out in methanol
at room temperature without any additives. A 24 W household
uorescent lamp with a highpass lter (l ¼ 395 nm) was used as
the visible light source (400–700 nm).16 As shown from Fig. 3,
BODIPY 1 was highly effective for the oxidation of thioanisole to
the corresponding (methylsulnyl)benzene. The conversion was
up to 99% within 6 h and no overoxidation product was detected
(Table 1).
Other parameters on the reaction were also evaluated (Table
2). Among the solvents tested, methanol showed the highest
activity. Reactions in non-poplar solvents such as CH2Cl2 and
toluene gave trace yields (Table 2, entries 3 and 4). Other
Akkaya et al. also detected the photophysical properties of
BODIPY 1 in chloroform, which showed a maximum absorption
Fig. 3 Time effect on the conversion of thioanisole. Reaction condi-
tions: thioanisole (0.5 mmol), MeOH (1 mL), BODIPY 1 (1 mol%), 24 W
fluorescent lamp, rt.
Scheme 2 Preparation of carbalkoxylated BODIPY 6–10.
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Table 1 Photophysical properties of BODIPY 1 and 6–10
Table 2 Oxidation of thioanisolea
labs
lem
Stocks shi
(nm)
BODIPY
Solvent
(nm)
(nm)
Ffa
1
Hexane
CH2Cl2
THF
Ethanol
Methanol
Hexane
CH2Cl2
THF
Ethanol
Methanol
Hexane
CH2Cl2
THF
Ethanol
Methanol
Hexane
CH2Cl2
THF
Ethanol
Methanol
Hexane
CH2Cl2
THF
Ethanol
Methanol
Hexane
CH2Cl2
THF
514
515
514
512
512
512
513
512
510
510
506
512
509
510
509
511
512
512
510
510
505
512
507
510
509
511
513
511
510
510
563
606
605
610
614
563
532
531
525
522
535
538
526
531
525
534
533
536
529
523
537
536
534
532
522
540
543
542
542
524
49
91
91
98
102
51
19
19
15
12
29
28
17
21
16
23
21
24
19
13
32
24
27
22
13
29
30
31
32
14
0.721
0.085
0.164
0.019
0.004
0.046
0.007
0.022
0.005
0.006
0.002
0.009
0.024
0.008
0.017
0.007
0.016
0.042
0.013
0.001
0.006
0.017
0.036
0.014
0.003
0.003
0.012
0.030
0.009
0.004
Entry
Catalyst
Solvent
Conversionb (%)
1
2
3
4
5
6
7
8
BODIPY 1
BODIPY 1
BODIPY 1
BODIPY 1
BODIPY 2
BODIPY 3
BODIPY 4
BODIPY 5
BODIPY 6
BODIPY 7
BODIPY 8
BODIPY 9
BODIPY 10
Rhodamine B
Nile red
MeOH
CH3CN
CH2Cl2
Toluene
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
99
18
Trace
Trace
42
65
53
86
33
41
32
37
30
10
14
79
6
7
9
10
11
12
13
14
15
16
8
Ru(bpy)3Cl2
9
a
Reaction conditions: thioanisole (0.5 mmol), MeOH (1 mL), catalyst (1
b
mol%), 24 W uorescent lamp, rt, 6 h. Conversion based on NMR.
10
Ethanol
Methanol
a
Fluorescence quantum yields (F) were calculated based on BODIPY 2
in anhydrous ethanol (F ¼ 0.98, c ¼ 10 mmol Lꢁ1).
BODIPY derivatives 2–10 including either unsubstituted or
halogenated BODIPYs exhibited relative lower activities (30–
86%, entries 5–13). Further investigations clearly show that
other photocatalysts such as rhodamine B and Nile Red were
much less effective for the transformation, and only ca. 10%
yields were observed. It was worth noting that a 79% yield was
obtained aer 6 h when Ru(bpy)3Cl2 was utilized, indicating
that the photocatalytic activity of BODIPY 1 was higher than the
widely used photocatalyst Ru(bpy)3Cl2 in this reaction. Control
experiments also showed that no conversion of thioanisole was
obtained in the absence of any catalyst or visible light. In
addition, trace amount of product was obtained under N2
protection. All these suggested that BODIPY, visible light and
oxygen were essential for this reaction (Scheme 3).
Scheme 3 Control experiments.
Based on the results and the plausible mechanism proposed
by Jing,16,17 the photochemically generated singlet oxygen is the
key. It is highly likely that the reaction proceed via the following
Table 3 Photocatalytic oxidation of other sulfidesa
A series of suldes were tested under the optimized reaction
conditions to evaluate the scope and limitations of the current
procedure (Table 3). In general, all the reactions proceeded
smoothly to give the corresponding products in good yields
(82–99%). Suldes bearing both electron-withdrawing and
electron-donating groups showed good activities. The
hindrance was also examined and ortho-substituted sulde gave
relative lower yield (Table 3, entry 5). Moreover, no sulfone
products were detected in the reactions, demonstrating excel-
lent selectivities of these reactions.
Entry
Ar
R1
Time (h)
Conversionb (%)
1
2
3
4
5
6
Ph
H
H
H
H
H
Me
6
6
6
6
12
6
99
90
88
95
82
94
4-MePh
4-OMePh
4-ClPh
2-ClPh
Ph
a
Reaction conditions: sulde (0.5 mmol), MeOH (1 mL), BODIPY 1 (1
b
mol%), 24 W uorescent lamp, rt. Conversion based on NMR.
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removed by evaporation under vacuum and a dark residue was
obtained which was puried via chromatography on silica gel
column, with the eluting solvent of 1 : 1 hexane–dichloro-
methane, giving a red powder (0.51 g, 10%). 1H NMR (500 MHz,
CDCl3) d: 6.03 (s, 4H, ArH), 2.57 (12H, s, CH3), 1.90 (3H, s, CH3);
13C NMR (125 MHz, CDCl3) d: 157.2, 142.6, 121.6, 14.9, 14.3.
Scheme 4 Proposed mechanism.
Typical procedure for one-pot synthesis of BODIPY 6–11
In N2 bubbled 40 mL dichloromethane, 2,4-dimethylpyrrole
(1 mL, 10 mmol), oxalyl dichloride (0.43 mL, 5 mmol) and an
alcohol (5 mmol) were mixed. The reaction mixture turned red
immediately and was kept stirring for 1 h at room temperature.
Aer completion of the reaction, BF3–Et2O (6 mL) was added to
the above mixture, followed by dropwise addition of triethyl-
amine (4 mL). Aer stirring for 3 h at room temperature, the
solvent was removed by evaporation under vacuum and a dark
residue was obtained which was puried via chromatography
on silica gel column, with the eluting solvent of 1 : 1 hexane–
dichloromethane, giving a red powder. Other BODIPYs 2–5 were
prepared according to the literature.10
pathway: rst, BODIPY 1 accepted a photon from the visible
light to form the excited BODIPY 1*; then the singlet oxygen
(1O2)was generated by energy transfer from BODIPY 1* and O2.
Alternatively, BODIPY 1* maybe underwent intersystem
crossing (ISC) from 1BODIPY 1* to the triplet excited state
3BODIPY 1*, which then reacted with ground state triplet
oxygen (3O2) by an energy transfer process, giving singlet oxygen
1O2. Finally, the sulde was oxidized to form the sulfoxide by
singlet oxygen (Scheme 4).
In summary, a simple one-pot condensation of 2,4-dime-
thylpyrrole and oxalyl dichloride to provide an orthogonal
dimeric BODIPY 1 has been developed. BODIPY 1 was
successfully utilized as a visible-light-driven photocatalyst for
the oxidation of suldes, affording the corresponding sulfox-
ides in excellent yields and selectivities. In addition, meso-car-
balkoxylated BODIPYs, for the rst time, were prepared using
the similar way via one-pot condensation of 2,4-dime-
thylpyrrole, oxalyl dichloride and a series of alcohols, which was
a good complement for the current BODIPY derivatives. Further
investigations on the BODIPY-catalyzed organic reactions are
currently underway in our laboratory.
BODIPY 6. Red solid. M.p.: 293.2–294.5. 1H NMR (500 MHz,
CDCl3) d: 6.06 (s, 2H), 4.44 (q, J ¼ 5 Hz, 2H), 2.53 (s, 6H), 2.14 (s,
6H), 1.44 (t, J ¼ 5 Hz, 3H); 13C NMR (125 MHz, CDCl3) d: 165.3,
157.6, 141.1, 129.2, 128.8, 62.7, 14.8, 13.8, 12.8. HRMS-EI: calcd
for C16H19BF2N2O2 320.1508 [M]+; found 320.1517.
BODIPY 7. Red solid. M.p.: 204.1–205.2. 1H NMR (500 MHz,
CDCl3) d: 6.06 (s, 2H), 5.25 (m, 1H), 2.53 (s, 6H), 2.18 (s, 6H),
2.05–2.09 (m, 2H); 13C NMR (125 MHz, CDCl3) d: 164.9, 157.5,
141.1, 129.8, 128.8, 121.1, 71.5, 21.7, 14.8, 13.1. HRMS-EI: calcd
for C17H21BF2N2O2 334.1644 [M]+; found 334.1652.
BODIPY 8. Red solid. M.p.: 235.8–236.9. 1H NMR (500 MHz,
CDCl3) d: 6.06 (s, 2H), 5.04 (m, 1H), 2.53 (s, 6H), 2.18 (s, 6H),
2.06–2.09 (m, 2H), 1.79–1.83 (m, 2H), 1.52–1.63 (m, 2H), 1.52–
1.61 (m, 2H), 1.29–1.46 (m, 2H); 13C NMR (125 MHz, CDCl3) d:
164.9, 157.4, 141.2, 129.9, 128.8, 121.1, 76.4, 31.5, 25.1, 23.9,
14.8, 13.1. HRMS-EI: calcd for C20H25BF2N2O2 374.1977 [M]+;
found 374.1983.
Experimental
General remarks
2,4-dimethylpyrrole, oxalyl dichloride were obtained from
Aldrich (Shanghai, China). Other commercially available
reagents were used without further purication. 1H- and 13C-
NMR spectra were recorded at 500 MHz in CDCl3 using TMS as
internal standard Chemical shis were reported in ppm (d), and
coupling constants (J), in Hz. High resolution mass spectra were
determined by EI in a Thermosher MAT 95 XP. Absorption
spectra were performed by using a Varian Cary6000i UV-VIS-NIR
absorption spectrophotometer. All the sulfoxides and BODIPYs
2–5 are known compounds and were identied by comparing of
their physical and spectra data with those reported in the
literature.
BODIPY 9. Red solid. M.p.: 232.9–234.0. 1H NMR (500 MHz,
CDCl3) d: 6.06 (s, 2H), 4.34 (t, J ¼ 7 Hz, 2H), 2.53 (s, 6H), 2.13 (s,
6H), 1.73–1.78 (m, 2H), 1.38–1.42 (m, 2H), 1.26–1.30 (m, 16H),
0.87–0.89 (t, J ¼ 2 Hz, 3H); 13C NMR (125 MHz, CDCl3) d: 165.4,
157.5, 141.1, 129.2, 128.8, 121.2, 67.1, 31.9, 29.6, 29.5, 29.4, 29.3,
29.2, 28.1, 26.0, 22.7, 14.8, 14.1, 12.7. HRMS-EI: calcd for
C
26H39BF2N2O2 460.3073 [M]+; found 460.3080.
BODIPY 10. Red solid. M.p.: 212.7–214.0. 1H NMR (500 MHz,
CDCl3) d: 7.22–7.31 (m, 5H), 6.02 (s, 2H), 4.55 (t, J ¼ 5 Hz, 2H),
3.08 (t, J ¼ 5 Hz, 2H), 2.52 (s, 6H), 1.94 (s, 6H); 13C NMR (125
MHz, CDCl3) d: 165.1, 157.6, 141.2, 137.0, 129.0, 128.8, 128.7,
127.0, 121.1, 67.5, 34.6, 14.8, 12.4. HRMS-EI: calcd for
Typical procedure for one-pot synthesis of BODIPY 1
C
22H23BF2N2O2 396.1821 [M]+; found 396.1826.
In N2 bubbled 40 mL dichloromethane, 2,4-dimethylpyrrole
(2.05 mL, 20 mmol) and oxalyl dichloride (0.43 mL, 5 mmol)
were mixed. The reaction mixture turned red immediately and
Typical procedure for the oxidation of sulde
was kept stirring for 2 h at room temperature. Aer completion To a 10 mL vial equipped with a magnetic stir bar were added
of the reaction, BF3–Et2O (6 mL) was added to the above BODIPY catalysts (0.05 mmol, 0.01 equiv.), sulde (0.5 mmol,
mixture, followed by dropwise addition of triethylamine (4 mL). 1.0 equiv.), and methanol (1 mL). The reaction mixture was
Aer stirring for 3 h at room temperature, the solvent was stirred at room temperature in air at a distance of ꢀ5 cm from a
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