BODIPY photocatalyzed oxidation of thioanisole under visible light

Article abstract of DOI:10.1016/j.catcom.2011.09.007

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes are organic dyes which have excellent thermal and photochemical stability, high fluorescence quantum yield and good solubility. Four BODIPY dyes were made and used to investigate the oxidation of th

Full text of DOI:10.1016/j.catcom.2011.09.007

Catalysis Communications 16 (2011) 94–97
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Catalysis Communications
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Short Communication
BODIPY photocatalyzed oxidation of thioanisole under visible light
a
Wenliang Li a,b, Zhigang Xie a, , Xiabin Jing
⁎
a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2011
Received in revised form 28 August 2011
Accepted 11 September 2011
Available online 17 September 2011
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes are organic dyes which have excellent thermal
and photochemical stability, high fluorescence quantum yield and good solubility. Four BODIPY dyes were
made and used to investigate the oxidation of thioanisole under visible light. BODIPY dyes were shown
to be highly active photocatalysts which are comparable to the metal complex, like Ru(bpy)32+. This
work highlights the potential of using BODIPY as photocatalysts for a number of important organic
transformations.
Keywords:
Photocatalysis
Bodipy
© 2011 Elsevier B.V. All rights reserved.
Thioanisole
Photo-oxidation
1. Introduction
photocatalysts to investigate α-oxyamination reactions between
1,3-dicarbonyl compounds and the radical TEMOP (TEMPO
=
The development of photocatalysts for organic reaction driven by
visible light is gaining increasing interest from organic chemists
because of their mild conditions for substrate activation and the
potential to mediate thermodynamically uphill reactions by harvesting
energy from sunlight in these reactions [1–4]. The simple inability of
most common organic molecules to absorb light in the visible
region limits the wide application of photochemical reactions [5].
Many research groups have reported the use of the widely applicable
and extensively studied organometallic complexes such as [Ru(bpy3)]2+
and [Ir(ppy)2(dtb-bpy)]+ (bpy = bipyridine, ppy = 2-phenylpyridine,
dtb-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine) [6–10]. MacMillan et al.
reported the cooperative combination of photoredox catalysis with
organocatalysis for the enantioselective α-alkylation of aldehydes
[11,12].
2,2,6,6-tetramethylpiperidine-1-oxyl) [17]. We hypothesized that
the BODIPY dyes could also be used in photocatalysis.
BODIPY dyes have been known as one of the most versatile fluor-
ophore and used as drug delivery agents [18], light-harvesters [19]
and sensitizers for solar cells [20,21]. The excellent thermal and
photochemical stabilities, high fluorescence quantum yield, good
solubility, and chemical robustness have added to the general attrac-
tiveness of these materials [22]. In this work, we synthesized four
BODIPY dyes and studied their photocatalytic activity to oxidation
of thioanisole.
2. Experimental
2.1. Synthesis of BODIPY
But the potential toxicity and high cost of the organometallic
complexes as well as their limited availability in the future are disad-
vantages of these metal-based catalysts [13,14]. So looking for the
metal-free photocatalyst to enable the direct utilization of visible
light in organic reaction is important for green synthesis. Wang et
al. reported the aerobic oxidation of amines and alcohols by carbon
nitride photocatalysis [15,16]. Alternatively, organic dyes could be
used in photoredox catalysis. Zeitler et al. reported the successful
application of organic dyes as effective photocatalysts for the
cooperative organocatalytic asymmetric intermolecular α-alkylation
of aldehydes [13]. Tan et al. used the Rose Bengal as visible light
The BODIPY 1, 2 and 3 were synthesized by following the litera-
ture procedure [23,24]. BODIPY 4 was prepared as follows: to a mix-
ture of 4-(diphenylamino)benzaldehyde (546 mg, 2.0 mmol), 2,4-
dimethypyrrole (412 mg, 4.4 mmol) in CH2Cl2 (100 mL) was added
to trifluoroacetic acid (2 drops) under nitrogen. The reaction mixture
was stirred over night at room temperature, then 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ, 454 mg, 2.0 mmol) in CH2Cl2
(10 mL) was added and stirred for 30 min. Et3N (3 mL) and
BF3·Et2O (3 mL) were added under ice-cold conditions, the mixture
was stirred for 30 min before warming up to room temperature, and
was stirred for additional 3 h at room temperature. The reaction mix-
ture was washed with water (3×100 mL), and the organic layers
were combined and dried over anhydrous MgSO4. The solvent was
evaporated in vacuum, and the residue was purified by column
⁎
Corresponding author. Tel./fax: +86 431 85262775.
E-mail address: xiez@ciac.jl.cn (Z. Xie).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.09.007

W. Li et al. / Catalysis Communications 16 (2011) 94–97
95
chromatography (silica gel, petroleum ether: EtOAc=10:1, v/v) to
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
afford red needles (206 mg, 21%). 1H NMR (CDCl3): δ 1.57 (s, 6H);
2.53 (s, 6H); 5.98 (s, 2H); 7.02–7.04 (d, 2H, J=7.2 Hz); 7.06–7.08
(m, 2H); 7.09–7.10 (m, 4H,); 7.14–7.17 (m, 2H); 7.27–7.30 (m, 4H).
HRMS (TOF MS EI+) calculated: 491.23, found: 491.3.
2.2. General procedure for photocatalytic oxidation of thioanisole
To a 10 mL vial equipped with a magnetic stir bar were added
BODIPY catalysts (2.5 μmol, 0.005 equiv), thioanisole (62 μL,
0.5 mmol, 1.0 equiv), and methanol (0.5 mL). The reaction mixture
was stirred at room temperature in air at a distance of ~5 cm from
a 24 W fluorescent lamp with a filter (λ=395 nm). Fluorescent
lamps could emit a small amount of ultraviolet light, so a filter was
used to eliminate UV light. 1H NMR spectra was taken of the reac-
tion mixture, and the ratio of integrated intensity between the 1H
NMR peaks of the substrate and product was used to calculate the
conversion yields (Supplementary data, Fig. S3 and Fig. S4).
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 1. Normalized absorption (solid line) and fluorescence (dashed line, λex =502 nm)
spectra of BODIPY 4 in CHCl3.
thioanisole to methyl phenyl sulfoxide with more than 89% conver-
sion in methanol after 24 h.
3. Results and discussion
The time curve of conversion with all four BODIPY dyes was
shown in Fig. 2. All four BODIPY dyes show the similar profile of
thioanisole oxidation. 34% of conversion was observed after 8 h with
BODIPY 2 as catalysts, and then the rate of reaction increased with
68% conversion after 12 h. The reaction is almost done after 24 h
with BODIPY 2, 3, and 4. BODIPY 1 shows reduced conversion with
respect to the other three BODIPY, the main reason is probably that
The BODIPY dyes can be made by using a highly electrophilic car-
bonyl compound (acid anhydride, acyl chloride or aldehyde) to form
the methane bridge between two pyrrole units. We synthesized the
BODIPY 1, 2 and 3 by following the literature method, and their 1H
NMR spectra were identical to that of references [23,24]. As shown
in Scheme 1, BODIPY 4 was obtained by using 4-(diphenylamino)
benzaldehyde as the bridging unit for pyrrole units. 1H NMR and MS
spectra confirmed the structure (Supplementary data, Figs. S1
and S2). Its absorption and fluorescence spectra were recorded in
chloroform at room temperature. As shown in Fig. 1, the maximum
wavelengths of absorption and emission are 502 and 603 nm, respec-
tively. BODIPY 4 displays near-IR emission, which is similar to the
reported BODIPY [25].
Table 1
BODIPY dyes and their structures.
Compound Structure
λmax
(abs) (em)
(nm) (nm)
λmax
Solvent
Ethanol
References
[22]
In the photocatalytic experiment, a 24 W household fluorescent
light bulb was used as the visible light source, which runs within a
wide spectral window (400 to 700 nm). Because fluorescent lamps
BODIPY 1
495
511
N
B
F
N
still could emit
a small amount of ultraviolet light, a filter
F
(λ=395 nm) was used to eliminate UV light. As shown in Table 1,
BODIPY, which has a strong absorption band in the range of 450–
550 nm, was maybe an active catalyst for the oxidation of thioanisole
to give the product methyl phenyl sulfoxide.
BODIPY 2
497
507
Acetonitrile
[23]
Sulfoxides are important intermediates for synthesis of valuable
compounds such as pharmaceuticals and other chemicals [26,27].
Selective oxidation of sulfides to sulfoxides is one of the most funda-
mental processes from a synthetic point of view [28]. Photocatalytic
oxidation of sulfide to sulfoxide has been reported using Ru(bpy)32+
in acetonitrile in the presence of a lead ruthenate pyrochlore mineral
as an electron shuttle [29]. In the present study, the photocatalyzed
sulfide oxidation was run at room temperature without any additives
in the presence of BODIPY. As shown in Table 2, all four BODIPY dyes
are highly effective photocatalyts for the aerobic oxidation of
N
B
N
F
F
Br
BODIPY 3
494
516
Dichloromethane [24]
N
F
N
B
F
N
B
NH
N
BODIPY 4
502
603
Chloroform
N
B
BF3·Et2O
21% yield
O
H
N
N
N
N
F
F
F
F
Scheme 1. Synthesis of BODIPY 4.

96
W. Li et al. / Catalysis Communications 16 (2011) 94–97
Table 2
which is consistent to the previous reports [32,33]. Four BODIPY dyes
Oxidation of thioanisolea
don't show big difference in reactivity (Table 2, entries 1, 2, 6, 7), pos-
sible reason is that all four BODIPY dyes have similar chemical structure
and absorbance around 500 nm. No sulfone was detected by 1H NMR,
demonstrating a high degree of selectivity of this reaction. When the
methyl p-tolyl sulfide was used as substrates, 89% yield was obtained
after 24 h (Table 2, entry 5, 1H NMR can be found in Supplementary
data, Fig. S4).
Further investigations were conducted using other organic dyes
like Rhodamine B and Nile red (Fig. 3 and Table 2, entries 8 and 9). Al-
though these two dyes could absorb the visible light too, only low
conversions were observed. This means that not all organic dyes
could be used as photocatalysts in oxidation of thioanisole. In order
to compare with metal-based catalysts, the widely used photocatalyst
Ru(bpy)32+ was used at same reaction condition (Table 2, entry 10).
The conversion was 98% after 24 h. It means that the photocatalytic
activities of the BODIPY dyes are comparable to Ru(bpy)32+ for the ox-
idation of thioanisole.
A number of control experiments were also carried out to demon-
strate the photocatalytic nature of the reactions. The reaction under
light without BODIPY or in the dark with BODIPY showed no conver-
sion of the sulfide to sulfoxide (Table 2, entries 11 and 12), and only
7% of sulfide was converted when the reaction was carried out
under N2 protection (Table 2, entry 13). These results showed that
light, BODIPY catalyst and oxygen are essential for this reaction. In
addition, in order to know whether the activity of the BODIPY is
maintained after the catalytic tests applied, a crossover experiment
was run at same condition. First, thioanisole was used in the BODIPY
2 catalyzed reaction, and 99% conversion was obtained after 24 h,
then the methyl p-tolyl sulfide was added to the above mixture.
After the mixture was stirred for another 24 h, 88% conversion was
obtained for the methyl p-tolyl sulfide. This conversion is comparable
to the reaction of methyl p-tolyl sulfide catalyzed by Bodipy 2
(Table 2, entry 5). This result shows that the BODIPY is stable under
this reaction condition.
Based on foregoing result, we proposed a mechanism that it is
highly likely that the oxidation of thioanisole is mediated by the pho-
tochemically generated singlet oxygen [30,31]. As shown in Fig. 4,
photoexcitated by visible light, BODIPY accepted a photon from the
visible light to form BODIPY*; then, the singlet oxygen was generated
by energy transfer from BODIPY*. Finally, the thioanisole was oxi-
dized to form the methyl phenyl sulfoxide by singlet oxygen. Actually,
the singlet oxygen as an intermediate in the photosensitized reaction
has been reported and discussed in detail [34,35]. BODIPY dyes could
generate singlet oxygen from the triplet excited state in terms of the
energy requirements and have been used in photodynamic therapy
[36,37].
O
Bodipy
.
S
S
methanol, in air
b
Entry
Catalyst/reaction condition
Solvent
Conversion (%)
1
2
3
BODIPY 1
BODIPY 2
BODIPY 2
BODIPY 2
BODIPY 2
BODIPY 3
BODIPY 4
Methanol
Methanol
Acetonitrile
Chloroform
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
89
99
23
4
89
99
99
21
23
98
0
4
5c
6
7
8
9
10
11
12
13
Rhodamine B
Nile red
Ru(bpy)32+
No catalyst with light
No light with BODIPY 2
BODIPY 2 under N2 protection
0
7
a
All the reactions were run at room temperature for 24 h with 0.5 mol% BODIPY
catalyst.
b
c
Conversion was determined by 1H NMR.
The substrates is the methyl p-tolyl sulfide.
100
80
60
40
20
0
BODIPY 1
BODIPY 2
BODIPY 3
BODIPY 4
0
5
10
15
20
25
Time (h)
Fig. 2. The conversion of thioanisole with different time.
BODIPY 1 lacks phenyl groups on the 8th-position. A long time for
photocatalyzed reaction can be found elsewhere [13,17]. This kind
of conversion profile was seldom seen in the literature, but it can be
explained. The key step for oxidation of thioanisole is that BODIPY ab-
sorb the light, and then transfer the energy to oxygen. So any factors
that could affect this process is the possible reason to the unusual
conversion profile, such as the low power of fluorescent light source
and low content of oxygen in the air.
4. Conclusion
Among the solvent tested, methanol exhibits the highest activity
(Table 2, entries 2−4). Compared to the aprotic solvent (acetonitrile)
and non-polar solvent (chloroform), the effect of a protic solvent
(methanol) on the reaction is to dramatically favor product formation,
In summary, four BODIPY dyes were synthesized, their structures
were confirmed by 1H NMR and MS spectra, and they were used as
photocatalysts for oxidative reactions of thioanisole. In this way, a
Cl-
N
N
O
+N
N
N
N
N
N
Ru 2+
N
O
CO2H
N
O
Rhodamine B
Nile red
2+
3
Ru(bpy)
Fig. 3. Structures of other dyes used.

W. Li et al. / Catalysis Communications 16 (2011) 94–97
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BODIPY
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*
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