Bodipy derivatives as organic triplet photosensitizers for aerobic photoorganocatalytic oxidative coupling of amines and photooxidation of dihydroxylnaphthalenes

Article abstract of DOI:10.1021/jo400769u

We used iodo-Bodipy derivatives that show strong absorption of visible light and long-lived triplet excited states as organic catalysts for photoredox catalytic organic reactions. Conventionally most of the photocatalysts are based on the off-the-shelf compounds, usually showing weak absorption in the visible region and short triplet excited state lifetimes. Herein, the organic catalysts are used for two photocatalyzed reactions mediated by singlet oxygen ( ¹O₂), that is, the aerobic oxidative coupling of amines and the photooxidation of dihydroxylnaphthalenes, which is coupled to the subsequent addition of amines to the naphthoquinones, via C-H functionalization of 1,4-naphthoquinone, to produce N-aryl-2-amino-1,4-naphthoquinones (one-pot reaction), which are anticancer and antibiotic reagents. The photoreactions were substantially accelerated with these new iodo-Bodipy organic photocatalysts compared to that catalyzed with the conventional Ru(II)/Ir(III) complexes, which show weak absorption in the visible region and short-lived triplet excited states. Our results will inspire the design and application of new organic triplet photosensitizers that show strong absorption of visible light and long-lived triplet excited state and the application of these catalysts in photoredox catalytic organic reactions.

Full text of DOI:10.1021/jo400769u

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Article
Bodipy Derivatives as Organic Triplet Photosensitizers
for Aerobic Photoorganocatalytic Oxidative Coupling of
Amines and Photooxidation of Dihydroxylnaphthalenes
Ling Huang, Jianzhang Zhao, Song Guo, Caishun Zhang, and Jie Ma
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo400769u • Publication Date (Web): 13 May 2013
Downloaded from http://pubs.acs.org on May 17, 2013
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Bodipy Derivatives as Organic Triplet
Photosensitizers for Aerobic Photoorganocatalytic
Oxidative Coupling of Amines and Photooxidation
of Dihydroxylnaphthalenes
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Ling Huang, Jianzhang Zhao,* Song Guo, Caishun Zhang and Jie Ma
State Key Laboratory of Fine Chemicals,School of Chemical Engineering, Dalian University of
Technology, Dalian 116024 (China)
Eꢀmail: zhaojzh@dlut.edu.cn;
Group homepage: http://finechem.dlut.edu.cn/photochem
RECEIVED DATE (to be automatically inserted)
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Abstract: We used iodoꢀBodipy derivatives that show strong absorption of visible light and
longꢀlived triplet excited states as organic catalysts for photoredox catalytic organic reactions.
Conventionally most of the photocatalysts are based on the offꢀtheꢀshelf compounds, usually
show weak absorption in visible region and short triplet excited state lifetime. Herein the organic
catalysts are used for two photocatalyzed reactions mediated by singlet oxygen (1O2), i.e. the
aerobic oxidative coupling of amines and the photooxidation of dihydroxylnaphthalenes, which
is coupled to the subsequent addition of amines to the naphthoquinones, via C−H
functionalization of 1,4ꢀnaphthoquinone, to produce Nꢀarylꢀ2ꢀaminoꢀ1,4ꢀnaphthoquinones (oneꢀ
pot reaction), which are antiꢀcancer and antibiotic reagents. The photoreactions were
substantially accelerated with these new iodoꢀBodipy organic photocatalysts compared to that
catalyzed with the conventional Ru(II)/Ir(III) complexes which show weak absorption in the
visible region and shortꢀlived triplet excited states. Our result will inspire the design and
application of new organic triplet photosensitizers which show strong absorption of visible light
and longꢀlived triplet excited state, and for the application of these catalysts in photoredox
catalytic organic reactions.
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Keywords: aerobic oxidative coupling; bodipy; photochemistry; triplet photosensitizer; visible
light; photoredox catalysis
1. INTRODUCTION
Triplet photosensitizers promoted photoredox catalytic organic reactions have attracted much
attention,1ꢀ6 such as the azaꢀHenry reaction,7 the oxidative coupling of amine or the crossꢀ
dehydrogenative coupling reactions,8ꢀ10 and hydrogen (H2) production by photolysis of water.11ꢀ13
The photosensitizers used for these reactions are usually the offꢀtheꢀshelf compounds, such as the
Ru(bpy)3[PF6]2 (bpy = 2,2’ꢀbipyridine) or Ir(ppy)3 complex (ppy = 2ꢀphenylpyridine),14 or
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organic triplet photosensitizers, such as Eosin Y or benzophenone.9 Inorganic metarials, such as
polycrystalline cadmium sulfide, was also used for the photoredox catalytic organic
reactions.1e,1g Recently, Eosin Y was used for intermolecular αꢀalkylation of aldehydes and
direct C−H arylation of heteroarenes with aryl diazonium salts,15 dehydrogenative coupling
reactions of amines with nitroalkanes.16 Rose Bengal (RB) was also used for dehydrogenative
coupling reactions of amines with nitroalkanes.17 However, the πꢀconjugation framework of
these known organic photocatalysts is difficult to be modified, thus the absorption wavelength of
these catalysts cannot be readily shifted to the red end of the spectrum.
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The common feature of these offꢀtheꢀshelf photosensitizers is weak or moderate absorption of
visible light, and the difficulty to modify the molecular structures of the photocatalysts to
optimize the catalytic property. For example, the molar absorption coefficient
ε of Ru(II)
complexes are usually less than 20000 Mꢀ1 cmꢀ1 at ca. 450 nm,1ꢀ13 and the absorption wavelength
of the typical Ru(II) and Ir(III) complexes are below 450 nm. Triplet photosensitizers with
absorption in longer wavelength region for photocatalysis were rarely reported. Furthermore, the
redox properties of these photosensitizers are not readily tunable. Therefore, it is highly desired
to develop new organic photosensitizers which show easily derivatizable molecular structures to
promote the photoreactions. To date very few organic triplet photosensitizers have been used for
photocatalytic reactions, such as oxidative coupling of amines.8b To the best of our knowledge,
no photocatalytic oranic reaction was carried out with organic photocatalyts that show strong
absorption in visible region and readily changeable molecular structures.2,8,18
The organic photosensitizers for photocatalysis should be with the following properties, (1)
strong absorption of visible light, preferably in green and red region of the spectrum; (2) with
triplet excited state be efficiently populated upon photoexcitation; (3) the lifetime of the triplet
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excited state should be long so that the single electron transfer (SET, the most popular
mechanism for photocatalytic organic reactions) will be improved; (4) the molecular structure of
the triplet photosensitizers (photocatalysts) should be readily tunable, so that the photophysical
and electrochemical properties can be optimized.
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In order to address the above challenges, herein we prepared a series of iodoꢀBodipy as triplet
photosensitizers which show the aforementioned desired photophysical properties (B-1 – B-4,
Scheme 1). We proved that these organic triplet photosensitizers can be used to promote different
type of photocatalytic reactions, such as the aerobic oxidative coupling of amines, and
photooxidation of dihydronaphthalenes.19 These photocatalytic reactions give a synthetically
important products, such as Schiff base by C−N bond formation,8 and naphthoquinones,19 and
thereafter the C−H functionalization of 1,4ꢀnaphthoquinone to produce Nꢀarylꢀ2ꢀaminoꢀ1,4ꢀ
naphthoquinones (oneꢀpot reaction), which are antiꢀcancer and antibiotic reagents.20 Previously
the above photoreactions were often carried out with the conventional Ru(II) or Ir(III) complexes
as photocatalysts. With the new iodoꢀBodipy based triplet photosensitizers, for which the
molecular structure is readily changeable, which is different from the known organic
photocatalysts such as Eosin Y, these different reactions are greatly accelerated, indicated the
significance of using organic triplet photosensitizer. Our approach will be very useful for the
development of photocatalytic reactions.
2.1. Design and synthesis of the photocatalysts. The design rationales of the photocatalysts
are (1) strong absorption of visible light; (2) high triplet state quantum yield; (3) longꢀlived
triplet excited states and (4) readily changeable molecular structures. Strong absorption of visible
light and efficient production of longꢀlived triplet excited states will ensure abundent
photocatalysts at the excited state, which are beneficial for the energy transfer or the electron
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transfer between the photocatalyst and the substrate molecules.1,2 To fufill these goals, Bodipy
was selected as the chromophore for preparation of the organic photocatalysts (Scheme 1), owing
to its ideal photophysical properties, such as strong absorption of visible light, good
photostability, and versatile derivatizing chemistry.21ꢀ24 In order to ensure efficient intersystem
crossing (ISC), by which the triplet excited state was produced, heavy atom iodine atoms were
attached on the πꢀcore of Bodipy chromophore, instead of the peripheral of the chromophore
(such as the meso phenyl moiety).25ꢀ27 The absorption wavelength of the Bodipy based triplet
photosensitizer can be readily redꢀshifted by attaching a styryl moiety to the Bodipy
chromophore (B-4). The compounds were obtained in satisfactory yields.
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Scheme 1. Synthesis of B-3 and B-4. The known compounds B-1, B-2, TPP, Ru-1, Ir-
1 and Rose Bengal were presented a
I
I
(b)
(a)
N
F
N
F
I
I
I
N
F
N
F
N
F
N
F
B
B
B
B-1
B-3
B-4
I
I
N
Ru
N
N
Ir
HO
O
O
I
N
N
N
N
N
NH
N
N
I
I
N
N
Cl
Cl
COOH
N
F
N
F
HN
B
Cl
Cl
[Ru(bpy)3]2+
Ru-1
[Ir(ppy)2bpy]+
B-2
TPP
Rose Bengal
a Key: (a) Pd(PPh3)2Cl2, CuI. Yield 35%. (b) piperidine, AcOH, DMF, microwave. Yield: 42%.
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2. RESULTS AND DISCUSSIONS
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2.2. Steady state UV−vis absorption and fluorescence spectra. The UV−vis absorption of
the organic photocatalysts and the reference compounds were studied (Figure 1). For the
conventional photosensitizer Ru(bpy)3[PF6]2, moderate absorption in the visible range were
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observed (ε
=16700 Mꢀ1 cmꢀ1 at 451 nm). For the Bodipyꢀbased organic photocatalysts B-1ꢀB-4,
however, much more intense absorption at redꢀshifted range were observed, the molar absorption
coefficients are up to 89000 Mꢀ1 cmꢀ1 in the range of 511 nm − 630 nm. The lifetimes of the
triplet excited states were determined as 1.8 ꢀs – 84.6 ꢀs (Table 1).The photophysical properties
of the triplet photosensitizers (the photocatalysts) were compiled in Table 1.
1.2
3.5
a
B-1
B-2
1.0
0.8
0.6
0.4
0.2
0.0
B-3
b
2.8
2.1
1.4
0.7
0.0
TPP
B-4
RB
Ru-1
400 500 600 700 800
Wavelength / nm
400
600
800
Wavelength / nm
Figure 1. (a) UV−vis absorption spectra of (a) B-1−B-4 and (b) TPP, Ru-1 and RB. In CH3CN,
(1.0 × 10ꢀ5 M; 22 °C).
Notebly Bꢀ4 shows a short triplet excited state lifetime (1.8 ꢀs), but the singlet oxygen
quantum yield (Φꢁ = 0.69) is comparable to that of B-1 (Table 1). The most probable reason is
that the lifetime of B-4 (1.8 ꢀs) is already long enough to efficiently produce singlet oxygen
(1O2) in fluid solution at room temperature, by the tripletꢀtripletꢀenergyꢀtransfer process.
Therefore, there is no drastic difference for the singlet oxygen quantum yield of the triplet
photosensitizers which are with different triplet state lifetime. Similar results were observed
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previously. For example, Ir(ppy)2(bpy) complex is with shortꢀlived triplet excited state (0.34 ꢀs),
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but the O2 quantum yield is high (Φꢁ = 0.97).19 However, singlet oxygen photosensitizing is
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dependent not only on the singlet oxygen quantum yield, but also on the visible lightꢀabsorbing
property of the photosensitizers.
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We investigated the UVꢀvis absorption spectrum of B-4 upon monochromatic light irradiation,
but found no changes for the UV−vis absorption spectra. Therefore, we propose that there is no
significant cisꢀtrans isomerization for the styryl C=C bonds in B-4 (Supporting Information,
Figure S91).
Table 1. Photophysical parameters of the photocatalysts (triplet photosensitizers) a
[b]
λ
abs / nm
ε
λ
em / nm
ΦF / %[c]
2.7
τ
/ ꢀs [d] Φꢁ / % [e]
529
511
547
630
556
411
451
0.85 548
0.89 535
0.80 585
0.77 654
1.31 575
3.08 655
84.6
80.4
85.2
1.8
0.79
0.79
0.86
0.69
0.80
0.62
0.57
B-1
B-2
B-3
B-4
RB
3.6
14.6
5.1
29.0
10.0
6.1
117.6
70.0
0.45
TPP
0.167 607
Ru-1
a
b
In CH3CN (1.0 × 10ꢀ5 M). Molar extinction coefficient at the absorption maxima.
ε
: 105 Mꢀ1
cmꢀ1. With Bodipy as the standard (Φ = 0.72 in THF). Triplet state lifetimes, measured by
transient absorption spectra, in ꢀs. Singlet oxygen quantum yield. With Rose Bengal (RB) as
c
d
e
the standard (Φꢁ = 0.8 In Methanol). 22 °C.
2.3. Photocatalytic oxidative coupling of amines. Firstly the iodoꢀBodipy triplet
photosensitizers were used for oxidative coupling of amines. Recently inorganic material of
carbon nitride and prophyrin derivatives were used as the photosensitizer for the photocatalytic
oxidative coupling of amines was reported.1f,8 With carbon nitride as the photocatalysts, the
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reaction was performed at 80 °C, and 0.5 MPa O2. However, the ideal reaction condition will be
at room temperature and using air as the oxidant.14 Furthermore, the absorption of the
mesoporous graphite carbon nitride in visible range is not strong, especially in the range beyond
450 nm.8 Recently phenothiazine derivatives were used as photocatalysts for oxidative coupling
of amines, but the absorption of those dyes are in blue region and the reaction was carried out
under neat O2 and long reaction time was mandatory (20 h).8b With porphyrin compounds, good
results were obtained, such as fast photocatalytic reaction.1f However, it is clear that much room
is left to increase the molecular divercity of the photocatalysts.
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Table 2. Transformation of benzylamine catalyzed by different triplet photosensitizers a
NH2
N
Entry
photosensitizer
solvent
T [oC] t [h]
Conv. [%] bc
1
2
3
4
5
Rose Bengal
B-1
acetonitrile
acetonitrile
acetonitrile
acetonitrile
20
20
20
20
1 h
1 h
1 h
1 h
1 h
81
100
100
100
100
B-2
B-3
MeCN/CH2Cl2 25
(3:2, v/v)
B-4
7
8
9
7
acetonitrile
20
20
25
20
1 h
1 h
1 h
1 h
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37
TPP
CH2Cl2
TPP
Ru(bpy)3
Ir(ppy)2bpy
acetonitrile
acetonitrile
a Reaction conditions: benzylamine (0.5 mmol), photosensitizers catalyst (0.005 mmol, 1 mol%),
solvent (5 mL), in air,
> 380 nm （20 mW/cm2）, 1 h, 22 °C. b Yield was determined with 1H
The reaction was monitored by TLC. Only two spots were found (except
λ
c
NMR.
photosensitizer), therefore the selectivity of the photoreaction is good.
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Interestingly, with the organic triplet photosensitizers, we found that the oxidative coupling of
benzyl amine can be carried out at much milder conditions compared to that with the mpgꢀC3N4.8
For example, the reaction can be run in aerated solution (not neat O2 atmosphere) at one
atmosphere pressure (1 atm), 22°C and the reaction was completed within one hour. The
conversion and selectivity are excellent (Table 2). In comparison, with mpgꢀC3N4 as the
photosensitizer, the same reaction has to be run at 5 atm neat O2 atmosphere (not air), elevated
temperature (80 °C) and with prolonged reaction time (2−5 hr).8 Furthermore, we found that the
typical offꢀtheꢀshelf organic triplet photosensitizers, such as TPP, Rose Bengal (RB),
[Ir(ppy)2(bpy)]PF6 and Ru-1 (Scheme 1), give lower conversion (12ꢀ81%). On the contrary, the
Bodipy based triplet photosensitizers give quantitative conversion of the benzyl amine to the
imine (Table 2).
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The reaction show excellent tolerance of functional groups on the amines used as the
substrates for the oxidative coupling reaction (Table 3). Moderate to excellent conversion and
selectivity were observed. For example, with triplet photosensitizer B-1 as the photocatalyst and
4ꢀtertꢀbutylbenzylamine as the substrate, 100 % conversion was observed (entry 4 in Table 3).
Furthermore, hydroxyl amine (entry 6, Table 3) was also used as the substrate. The hydroxyl
group was not oxidized, demonstrated the good functional groupꢀtolerance of the photooxidation
with the Bodipy photosensitizers. Heterocyclic amine, such as pyridyl methylamine also proceed
smoothly (Table 2, entry 3). No reaction was found with amine lacking of αꢀH, such as aniline
(entry 9 in Table 3). Regioselective oxidations were observed with unsymmetrical secondary
amines (entries 5 and 8 in Table 3): the reaction proceeded to yield the conjugated Nꢀbenzylidene
products in remarkably high selectivity rather than those generated from oxidation on the less
activated site (entry 5, 6 and 8, Table 3). This observation is in agreement with previouse
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Table 3. Oxidation of various amines using B-1 a
5
N
6
7
NH2
8
9
R
R
R
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Entry Substrate
Product
t
Yield%b TONd
TOF / minꢀ1 e
[h]
1.0 100
0.5 60
100
60
1.6
1.0
1
2
NH2
N
NH2
N
F
F
F
1.0 91
91
1.5
1.6
3
4
NH2
N
N
N
N
NH2
1.0 100
100
N
1.0
1.0
1.5
1.7
89
89
5
6
NH2
N
O
O
O
NH2
100
100
N
O
O
O
1.0
1.7
NH2
100
100
7
N
Cl
NH2
Cl Cl
2.0
2.0
0.4
0.6
44
66
44
66
8
9
N
F
F
F
NH2
N
F3C
F3C
CF3
2.5 74
2.5 62
74
62
1.2
0.4
10
11
NH
N
OH
OH
N
N
H
12 f
13 f
N
4.0 100
4.0 100
100
100
0.4
0.4
N
H
N
H
N
3.0 − c
−
−
NH2
14
N N
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Reaction conditions: benzylamine (0.5 mmol), photosensitizers catalyst (0.005 mmol, 1
b
mol%), solvent (5 mL), in air,
with H NMR. No reaction. Turnover number, calculated by the ratio of moles of
product/moles of catalyst. Turnover frequency. The photoirradiation power density is 37
mW/cm2.
λ
> 380 nm (20 mW/cm2), 1 h, 22 °C. Yield was determined
1
c
d
e
f
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results.1f We propose that the intermediated at the aryl side (such as radical, due to the hydrogen
absraction by 1O2) is more stable than that at the alkyl chain side. As a result, only the aryl imine
was produced for the asymmetrical amines. We also found that the benzylamine with electroꢀ
donating substituents give higher yields than the benzylamines with electronꢀwithdrawing
substituents (Entry 2, 7ꢀ9, Table 3). The photooxidation with other triplet photosensitizers were
carried out (see Supporting Information for detail). The reaction was also run on 1 g scale, yield
up to 95 % was obtained (Supporting Information).
The experiments indicated that oxygen (O2), photoirradiation and the triplet photosensitizers
are all essential for the photocatalytic oxidative coupling of the amines (entries 1ꢀ4, Table 4).
1
The photocatalytic oxidation was greatly inhibited in the presence of histidine, a O2 scavenger.
In the presence of superoxide dismutase (SOD), however, the photooxidation proceeded as
1
normal. Therefore we propose that presence of O2 is responsible for the photooxidation. O2−• is
not involved in the photocatalytic oxidative coupling of the amines, which is supported by the
fact that the reaction cannot be suppressed by 2,6ꢀdiꢀtertꢀbutylmethylphenol.8a
The reaction mechanism of the photocatalytic oxidative coupling reaction were studied by
nanosecond transient absorption spectra, electron spin resonance (ESR) (Figure 2). 5,5ꢀ
Dimethylꢀ1ꢀpyrrolineꢀNoxide (DMPO) and 2,2,6,6ꢀtetramethylpiperidine (TEMP) were
employed as a probe to react with O2−• and 1O2, respectively.9
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Table 4. Mechanism study of the oxidation amines experiments using B-1. The general
reaction is applicable to entries 1-6 a
7
8
9
NH2
N
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Entry Condition
solvent
t / [h] Yield[%] b
1
2
3
4
5
6
In air
acetonitrile
acetonitrile
acetonitrile
acetonitrile
CH2Cl2
1
3
3
3
3
1
99
c
In N2
−
−
4
−
c
No photoirradiation
No B-1
c
3 eqv TFA
2,6ꢀdiꢀtertꢀbutylmethylphenol
(0.05 mmol)
acetonitrile
70
7
8
9
2 eqv. Histidine
No histidine
THF/H2O=1/2
THF/H2O=1/2
acetonitrile
1
1
1
9
80
100
SOD (2 eqv.) d
a Reaction conditions: various amines (0.5 mmol), photosensitizers catalyst B-1 (0.005 mmol, 1
mol%) were dissolved in different solvents. The mixture was irradiated with 35 W Xenon lamp
(λ > 380 nm 20 mW/cm2). b The yields were determined by 1H NMR spectrum. c No reaction. d
SOD stands for superoxide dismutase.
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20
0
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b
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0
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-100
-200
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-20
3450 3500 3550 3600
Magnetic field / G
3400 3450 3500 3550 3600
Magnetic field / G
20
20
10
0
d
c
10
0
-10
-10
-20
-20
3460 3480 3500 3520 3540
3460 3480 3500 3520 3540
Magnetic field / G
Magnetic field / G
Figure 2. ESR spectrum of the mixtures upon photoirradiation. (a) B-1 (1.0 × 10ꢀ4 M),
benzylamine (1.5×10ꢀ3 M) and TEMP (0.12 M); (b) B-1 (1.0 × 10ꢀ4 M) and TEMP (0.12 M); (c)
B-1 (1.0 × 10ꢀ4 M), DMPO (2.0 × 10ꢀ2 M), benzylamine (1.5 × 10ꢀ3 M); (d) B-1 (1.0 × 10ꢀ4 M),
DMPO (2.0 × 10ꢀ2 M). In airꢀsaturated CH3CN. The samples were photoirradiated with 532 nm
laser for 20 s (141 mW/cm2). At 22 °C.
1
For the mixture of B-1 and TEMP, signal of the O2ꢀTEMP adduct was detected upon
photoexcitation (Figure 2b). In the presence of benzylamine, however, the magnitude of the
signal was greatly attenuated (Figure 2a), indicated that the 1O2 was consumed in the presence of
1
benzylamine, therefore, the oxidative coupling of the benzylamine was mediated by O2. With
−•
DMPO, however, the signal of the DMPOꢀO2 adduct did not change in the presence of
−•
benzylamine, indicated that O2 was not involved in the photocatalytic reaction.
1
The reaction mechanism of the O2ꢀmediated aerobatic oxidative coupling of the amines can
be summarized in Scheme 2.1f 1O2 was produced by the triplet photosensitizers. Then the amine
was oxidized to the imine and hydrogen peroxide (H2O2) was produced. Reaction of the imine
with another molecule of amines produce the Schiff base product. NH3 was produced as side
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product.1f We studied the photocatalytic reaction in CDCl3/CHCl3.1f Previously it was found that
the photocatalytic reaction proceeded with higher velocity in deuterated solvents because the
3
4
5
6
7
8
9
1
lifetime of O2 is longer in deuterated solvent than in their proteated counterparts.1f We also
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observed much higher reaction velocity in CDCl3 than that in CHCl3 (see Supporting
1
Infomration, Table S7). This result indicated that O2 is involved in the photocatalytic reactions.
Moreover, as a proof of the proposed mechanism, H2O2 was detected by using KI/CH3COOH.
Brown color was observed by mixing the photocatalytic reaction mixture and KI/CH3COOH (see
Supporting Information, Figure S90).1f Furthermore, the effect of DABCO (1,4ꢀ
diazabicyclo[2.2.2]octane, a singlet oxygen scanvenger) on the photocatalytic oxidative coupling
of benzylamines was also studied. We found that the photocatalytica reaction can be
1
signicicantly surpressed by the DABCO (Supporting Information, Table S8).1f Therefore, O2 is
involved in the photocatalytic reaction. All these results support the proposed mechanism
(Scheme 2).
Scheme 2. Proposed reaction mechanism for the photocatalytic aerobatic oxidation of
amines with the triplet photosensitizers B-1 – B-4
ISC
3[B-1]*
1O2
NH2
R
1[B-1]*
NH3
NH2
R
O2
N
NH
R
R
B-1
R
H2O2
2.4. Photocatalytic oxidation of dihydroxylnaphthalene. In this section we will demonstrate
that the application of organic triplet photosensitizers is not limited to the photocatalytic
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oxidative coupling of amines. Singlet oxygen (1O2) can be produced by the triplet
3
4
5
6
1
photosensitizers in the presence of O2.28ꢀ35 Therefore the O2ꢀmediated photocatalytic oxidation
7
8
9
of 1,5ꢀdihydroxylnaphthalene (DHN) was studied. The reaction can be followed by monitoring
the UVꢀVis changes (Figure 3).19 The product of the photooxidation of DHN, naphthoquinones,
such as juglone, and thereafter the C−H functionalization of 1,4ꢀnaphthoquinone to give Nꢀarylꢀ
2ꢀaminoꢀ1,4ꢀnaphthoquinones, which are antiꢀcancer and antibiotic reagents.20 Previously
cyclometalated Ir(III) complexes were used as triplet photosensitizers for photooxidation of
DHNs, but the Ir(III) complexes show weak absorption in visible region.19
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Recently we reported some visible lightꢀharvesting transition metal complexes as triplet
photosensitizers for photooxidation of DHNs.36ꢀ38 Iodoꢀaza Bodipy was used as triplet
photosensitizers for photooxidation of DHNs, but the photooxidation was not studied in detail.39
Herein we studied the photooxidation of DHNs with various organic triplet photosensitizers. The
photooxidation was also coupled with the oneꢀpot preparation of amine adducts of the
naphthoquinones, which can be potentially used as antiꢀcancer reagents.20
2.5
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0.0
0 min
2 min
4 min 25 min
...
25 min 4 min
30 min 2 min
0 min
0 min
2 min
4 min
...
25 min
30 min
30 min
25 min
...
4 min
2 min
0 min
30 min
a
b
...
300 400 500 600 700 800
Wavelength / nm
300 400 500 600 700 800
Wavelength / nm
Figure 3. UV−vis absorption spectral changes for the photooxidation of DHN. (a) B-1 as a
sensitizer. (b) B-4 as sensitizer (in CH2Cl2/MeOH, 9:1, v/v). c[sensitizers] = 1.0 × 10ꢀ5 M.
c[DHN] = 1.0 × 10ꢀ4 M. Light intensity: 20 mW/cm2, 20 °C.
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First we investigated a few naphthols as the substrates, such as 1ꢀnaphthol, 1,5ꢀDHN and 1,6ꢀ
DHN. In all cases the yields of the naphthoquinones are satisfactory (Table 5). In order to
explore the application of the photooxidation of DHN for preparation of more functionalized
produces, the photooxidation of DHN was coupled to the adduction of aromatic amines into a
oneꢀpot reaction (Table 6).40
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Table 5． Photooxidation of naphthol with B-1 and B-4 a
OH
O
B-1/B-4
Visible Light
HO
HO
Air rt
O
2
1
Entry
1
Substrate 1
Product 2
Yield b
74/81
TON c
37/40
TOF/ minꢀ1 d
0.6/0.7
OH
O
1a
O
2a
OH O
OH
73/77
80/87
36/38
40/43
0.6/0.6
0.7/0.7
2
3
O
2b
OH
1b
OH
O
O
HO
HO
1c
2c
a
Reaction conditions: substrate 1 (0.10 mmol), photocatalyst B-1/B-4 (2 mol%), in
CH2Cl2/CH3OH(v/v, 4:1, 5.0 mL). The reaction mixture was irradiated with 35 W Xe lamp for
30 min (0.72 M NaNO2 solution was used as filter so that light with wavelength
λ < 385 nm was
blocked. The light intensity at the photoreactor is 20 mW/cm2). At 20 °C. b Yield of the isolated
c
products with B-1 and B-4 as the triplet photosensitizers. Turnover number, calculated by the
molar ratio of the production and the catalysts. d Turnover frequency.
In the first step of the reaction, the naphthol was oxidized to naphthoquinones. Then without
isolation, aromatic amines and Cu2+ saltswere added, the adduction products
aminonaphthoquinones were obtained.20 The oneꢀpot reaction was optimized with TPP as the
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Table 6. Photooxidation of naphthol with B-1 and aniline addition to the naphthoquinones
a
5
6
7
8
O
O
OH
NH2
R
Cupper Acetate
Acetic acid
B-1
R
N
solvent
H
9
O
O
1a
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3
Product
Entry
1
Aniline derivatives
Product
Yield% b
O
NH2
N
75
71
H
O
O
3a
2
NH2
N
H
3b
O
O
Br
3
4
Br
NH2
N
3c
H
48
74
O
H3CO
H3CO
O
NH2
N
H
O
3d
O
5
6
NH2
N
H
79
77
O
3e
O
Cl
Cl
NH2
N
H
3f
O
OCH3
O
7
8
H3CO
Cl
NH2
N
73
70
H
3g
O
Cl
O
NH2
N
H
3h
O
a Step 1: Naphthol (0.5 mmol) and photosensitizer B-1 (5mol%) were mixed in CHCl3/Methanol
(10 mL, 4/1, v/v). The mixture was irradiated with 35W Xe lamp (
>385 nm. 30 mW/cm2) for
0.5 h. Step 2: aniline derivatives (0.6 mmol), acetic acid (10 mL) and Cu(AcO)2 (10 mol%) were
added into the solution and the mixture was heated at 65 °C for 3 h. Overall yield of the
λ
b
isolated product, calculated based on naphthol.
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triplet photosensitizer (Supporting Information, Table S6). The result shows that the optimal
reaction conditions for the oneꢀpot preparation of aminonaphthoquinones are 1.5 h for the first
step, 3 h reaction time for the second step of the oneꢀpot reaction. The reactions were carried out
at 65 °C. The reaction is tolerant to aromatic amines substrates with various functional groups
(see Supporting Information for the substrate tolerance study). However, 4ꢀnitroaniline failed to
produce the aminonaphthoquinone (Figure S5). Furthermore, we found that CuSO4 and CuCl can
catalyze the reaction with high efficiency. Previously only Cu(AcO)2 was used as the catalyst.40
With B-1 as the triplet photosensitizer, we found that the yields of the aminonaphthoquinones
are generally higher than that with TPP as the triplet photosensitizer (Table 6). The yields of the
aminonaphthoquinones are generally higher than 70%. We attribute the more efficient
photooxidation with B-1 to the stronger visible lightꢀharvesting ability of B-1 in visible region.
These results are promising for preparation of antiꢀcancer reagents.20 The reactions catalyzed
with other organic triplet photosensitizers show similar results (see Supporting Information).
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Scheme 3. Proposed one pot reaction mechanism for the photooxidation with the triplet
photosensitizers B-1, B-4 or TPP
O
OH
3Sens*
ISC
1O2
N
1Sens*
H
R
O
HO
HOAc/Cu(AcO)2
NH2
O
O
H2O
3O2
hν
Sens
HO
R
The reaction mechanism for the aerobatic photooxidation of the naphthol with the organic
triplet photosensitizers can be summarized in Scheme 3. Singlet oxygen (1O2) was produced by
photosensitizing of the ground state 3O2 by the organic triplet photosensitizers. Then the naphthol
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was oxidized by the O2. The addition of the aromatic amines to the naphthalquinones leads to
the production of the functionalized aminonaphthoquinones. Our strategy offers a useful
approach to prepare the bioactive aminonaphthoquinones.20
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2.5. Conclusions. In summary, we used iodoꢀBodipy derivatives as organic photocatalysts for
two different photoredox organic reactions. These new organic photocatalysts show strong
absorption of visible light, efficient triplet state production and longꢀlived triplet excited state.
All these properties are crucial for photoredox catalytic organic reactions because strong
absorption of visible light and efficient production of triplet state will make the activated
catalysts more abundant. On the other hand, the longꢀlived triplet excited state of the
photocatalysts will ensure effcient single electron transfer (SET) between the photocatalyst and
the substrate molecules. Herein with the new organic photocatalysts we investigated two
different photocatalytic reactions, i.e. the aerobic photocatalysis oxidative coupling of amines (to
produce imines with formation of C−N bond), and the photocatalytic singlet oxygen (1O2)
mediated photooxidation of dihydroxylnaphthalenes, and thereafter the C−H functionalization of
1,4ꢀnaphthoquinone to produce Nꢀarylꢀ2ꢀaminoꢀ1,4ꢀnaphthoquinones (oneꢀpot reaction), which
are antiꢀcancer and antibiotic reagents. Greatly accelerated photoreactions were found for all the
reactions compared to that catalyzed with the conventional Ru(II)/Ir(III) complexes which show
weak absorption in the visible region and shortꢀlived triplet excited states. Currently most of the
triplet photosensitizers used for the photoredox reactions are the offꢀtheꢀshelf known triplet
photosensitizers, such as Ru(II) polyimine complexes or Eosin Y. Our result will inspire the
designing of new organic triplet photosensitizers which show strong absorption of visible light
and longꢀlived triplet excited states, and for the application of these new organic photocatalysts
in photoredox catalytic organic reactions.
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3. EXPERIMENTAL SECTION
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3.1. General Methods. Compounds B-1 and B-2 were prepared following the reported
methods.26
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3.2. B-3. 1 (57.0 mg, 0.1 mmol) and phenylacetylene (11.0 mg 0.1 mmol) was dissolved in
THF/Triethylamine (20 mL, v/v = 1:1). Ar was bubbled through the solution for 30 min, then
Pd(PPh3)2Cl2 (0.005 mmol, 3.5 mg), CuI (0.01 mmol, 2.0 mg) was added. The mixture was
refluxed for 4 h under an argon atmosphere. After removal of the solvent under reduced pressure,
the mixture was purified by column chromatography (Silica gel. CH2Cl2/hexanes, 1/2, v/v) to
1
give deep red solid. Yield: 20 mg (35%). H NMR (400 MHz, CDCl3)
δ = 7.57−7.55 (m, 3 H),
7.50−7.47 (m, 2 H), 7.36−7.34 (m, 3H), 7.32−7.31 (m, 2 H), 2.75 (s, 3 H), 2.70 (s, 3 H), 1.54 (s,
3H), 1.52 ppm (s, 3H). 13C NMR (100 MHz, CDCl3)
δ
= 158.8, 156.7, 145.0, 142.1, 134.8,
132.0, 131.5, 129.6, 129.5, 128.3, 127.9, 123.5, 96.7, 94.6, 85.6, 81.6, 17.0, 16.1, 13.9, 13.6
ppm. MALDIꢀHRMS calcd [C27H22BF2N2I]+ m/z 550.0889; found m/z 550.0867.
3.3. B-4. B-1 (57.0 mg 0.1 mmol) was dissolved in dry DMF (5 mL). Benzaldehyde (42.0 mg
0.4 mmol) was added, followed by acetic acid (3 drops) and piperidine (3 drops). The mixture
was argon saturated before it was subjected to microwave irradiation (12 min, 150 °C, 1 min preꢀ
stirring). After removal of the solvent under reduced pressure, the mixture was purified by
column chromatography (Silica gel. CH2Cl2) to give deep purple solid. Yield: 30.0 mg (42%). 1H
NMR (400 MHz, CDCl3):
δ = 8.18–8.14 (d, J = 16.0 Hz, 2H), 7.73 (s, 1H), 7.69–7.66 (m, 5H),
7.55–7.52 (m, 3H), 7.44–7.41 (m, 4H), 7.37–7.33 (d, J = 16.0 Hz, 2H), 7.31–7.29 (m, 2H), 1.46
ppm (s, 6H). 13C NMR (100 MHz, CDCl3):
δ
= 157.8, 150.8, 146.3, 139.9, 139.7, 136.8, 133.1,
129.0, 128.9, 128.5, 128.3, 128.0, 127.8, 19.5, 118.9, 29.9 ppm. MALDIꢀHRMS calcd
[C33H25BF2N2I2]+ m/z 752.0168, found m/z 752.0171.
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3.4. Typical procedures for photocatalytic oxidative coupling of amines. To a dry 10 mL flask
were added triplet photosensitizer (0.005 mmol, 1.0 mol%, as the photocatalyst), benzylamine
(52 ꢁL, 0.5 mmol) (or other benzylamine derivatives) and acetonitrile (5 mL). The reaction
mixture was stirred at 22 °C under air atmosphere. The solution was then irradiated using a 35 W
xenon lamp through a cut off filter (0.72 M NaNO2 aqueous solution, which is transparent for
light > 385 nm). Thin layer chromatography (TLC) was used to monitor the progress of the
reaction. After the reaction is completed, the solvent was evaporated under reduced pressure. The
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1
residue was directly used in the H NMR spectral study and the conversion yields were
calculated by integrating the proton peaks. The conversion of the reaction was calculated by
integrating the singlet peak of the featured proton in the products (at about 4.87 ppm for ꢀCH=Nꢀ
CH2-) and that of the corresponding proton in the starting materials (at about 3.98 ppm as singlet
for NH
ꢀCH2-) in the 1H NMR spectrum.
2
3.5. Typical procedures for photooxidation of naphthols to produce naphthoquinone. To a
dry 10 mL flask were added triplet photosensitizer (0.010 mmol 2 mol%), naphthol (0.5 mmol)
(or other naphthol derivatives) and CHCl3/methanol (10 mL, v/v, 4:1). The reaction mixture was
stirred at 22 °C under air atmosphere. The solution was then irradiated using a 35 W xenon lamp
through a cut off filter (0.72 M NaNO2 aqueous solution, which is transparent for light > 385
nm). Thin layer chromatography (TLC) was used to monitor the progress of the reaction. After
the reaction is completed, the solvent was evaporated under reduced pressure. The mixture was
purified by column chromatography (silica gel, CH2Cl2).
1
2a. Yield: 74 % (58.5 mg), H NMR (400 MHz, CDCl3/ CD3OD): δ ppm 8.11–
O
8.08 (m, 2 H), 7.82–7.79 (m, 2H), 7.01 (s, 2H). TOF HRMS EI+: Calcd [C10H6O2]+
O
2a
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m/z 158.0368, found m/z 158.0367.
5
6
7
1
2b. Yield: 73 % (63.5 mg), H NMR (400 MHz, CDCl3/DMSOꢀd6): δ ppm
OH O
8
9
11.86 (s, 1H); 7.70 (t, J = 7.6Hz, 1H); 7.61 (d, J = 8.4 Hz, 1H); 7.56 (d, J = 7.6
Hz, 1H); 7.31 (d, J = 8.4Hz, 1H); 7.17 (t, J = 8.0 Hz, 1H). TOF HRMS EI+:
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11
12
13
14
15
16
17
18
19
20
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58
59
60
2b
O
Calcd [C10H6O3]+ m/z 174.0317, found m/z 174.0314.
1
2c. Yield: 80 % (69.6 mg), H NMR (400 MHz, CDCl3/CD3OD): δ ppm 7.97
O
O
HO
(d, J = 8.0 Hz, 1H ), 7.41(s, 1H), 7.16 (d, J = 8.0 Hz, 2 H), 6.91 ( s, 2H ). TOF
HRMS EI+: Calcd [C10H6O3]+ m/z 174.0317, found m/z 174.0315.
2c
3.6. Typical procedures for one pot synthesis of naphthoquinone derivatives.
O
O
OH
NH2
R
B-1
Cupper Acetate
Acetic acid
R
N
solvent
H
O
O
Product
To a dry 10 mL flask were added B-1 (0.025 mmol 5 mol%), naphthol (0.5 mmol), and
CHCl3/methanol (10 mL, v/v, 4:1) coꢀsolvent. The reaction mixture was stirred at 22 °C under
air atmosphere. The solution was then photoirradiated using a 35 W xenon lamp for 0.5 h
through a cut off filter (0.72 M NaNO2 aqueous solution, which is transparent for light > 385
nm). Thin layer chromatography (TLC) was used to monitor the reaction, after the consumption
of naphthol, the photoirradiation was stopped, acetic acid (10 mL) was added, cupper acetate
(0.050 mmol 10 mol%) and 1.2 eqv. aniline derivatives. The reaction mixture was stirred at 65
°C for 3 h under air atmosphere. After the reaction is completed, the solvent was evaporated
under reduced pressure. The residual was purified by column chromatography (silica gel,
CH2Cl2/CH3OH 50/1, v/v).
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3a. Yield: 70 % (87.2 mg), HNMR (400 MHz, CDCl3): δ ppm 8.11 (t, J =
O
O
N
6.12 Hz, 2H), 7.76 (t, J = 7.2 Hz, 2H), 7.67 (t, J = 8.0 Hz, 2H), 7.57 (s, 1H), 7.42
(t, J = 7.64 Hz, 2H), 7.22 (t, J = 6.14 Hz, 1H), 6.42 (s, 1H). TOF HRMS EI+:
H
3a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Calcd [C16H11NO2]+ m/z 249.0797, found m/z 249.0790.
1
3b. Yield: 71 % (93.4 mg), HNMR (400 MHz, CDCl3): δ ppm 8.12 (d, J =
O
O
7.6 Hz, 2H), 7.75 (t, J = 6.2 Hz, 1H), 7.66 (t, J = 7.6 Hz, 1H), 7.49 (s, 1H),
N
H
3b
7.48–7.42 (m, 2H), 7.21–7.14 (m, 2H), 6.36 (s, 1H), 2.36 (s, 3H). TOF
HRMS EI+: Calcd [C17H13NO2]+ m/z 263.0946, found m/z 263.0956.
1
O
3c. Yield: 48 % (78.2 mg), HNMR (400 MHz, CDCl3/DMSOꢀ
Br
N
d6/Acetoneꢀd6): δ ppm 8.22 (d, J = 7.6 Hz, 1H), 7.98 (s, 1H), 7.78 (d, J =
H
3c
O
7.6 Hz, 1H), 7.46–7.40 (m, 2H), 7.31–7.24 (m, 2H), 7.15 (d, J = 8.8 Hz, 1H), 6.91 (d, J = 7.2 Hz,
1H), 6.65–6.59 (m, 1H). TOF HRMS EI+: Calcd [C16H10BrNO2]+ m/z 326.9895, found m/z
326.9900.
3d. Yield: 74 % (103.2 mg), 1HNMR (400 MHz, CDCl3): δ ppm 8.11 (t,
O
N
OCH3
J = 4.4 Hz, 2H); 7.76 (t, J = 8.0 Hz, 1H); 7.67 (t, J = 7.6 Hz, 1H); 7.55 (s,
1H); 7.32 (t, J = 8.0 Hz, 1H); 6.88 (d, J = 7.2 Hz, 1H); 6.81 (s, 1H); 6.77
H
O
3d
(d, J = 8.4 Hz, 1H); 6.45 (s, 1H); 3.82 (s, 3H). TOF HRMS EI+: Calcd [C17H13NO3] m/z
279.0895, found m/z 279.0898.
1
O
O
3e. Yield: 79 % (118.1 mg), HNMR (400Hz, CDCl3):
δ
ppm 8.18 (d, J =
N
7.6 Hz, 1H), 8.10 (d, J = 7.8 Hz, 1H), 7.92–1.90 (m, 2H), 7.82–7.76 (m, 3H),
H
3e
7.71–7.67 (m, 1H), 7.56–7.53 (m, 3H), 7.50–7.44 (m, 1H), 6.01 (s, 1H). TOF
HRMS EI+: Calcd [C20H13NO2] m/z 299.0946, found m/z 299.0950.
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7
8
9
1
3f. Yield: 77 % (108.9 mg), HNMR (400 MHz, CDCl3): δ ppm 8.12 (t,
O
O
Cl
J = 6.0 Hz, 2H), 7.77 (t, J = 7.2 Hz, 1H), 7.68 (t, J = 7.6 Hz, 1H), 7.55 (s,
1H), 7.35 (t, J = 8.0 Hz, 2H), 7.29 (s, 1H), 7.18 (t, J = 7.6 Hz, 1H), 6.43 (s,
N
H
3f
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57
58
59
60
1H). TOF HRMS EI+: Calcd [C16H10ClNO2]+ m/z 283.0400, Found m/z 283.0401.
1
3g. Yield: 73 % (101.8 mg), HNMR (400 MHz, CDCl3):
δ ppm 8.11
O
OCH3
(t, J = 6.8 Hz, 2H), 7.75 (t, J = 8.0 Hz, 2H), 7.44 (s, 1H), 7.21 (d, J = 8.8
Hz, 2H), 6.96 (d, J = 8.8 Hz, 2H), 6.22 (s, 1H), 3.83 (s, 3H). TOF HRMS
N
H
O
3g
EI+: Calcd [C17H13NO3]+ m/z 279.0895, found m/z 279.0903.
1
3h. Yield: 70 % (99.1 mg), HNMR (400 MHz, CDCl3): δ ppm 8.12 (t, J
Cl
O
O
= 7.2 Hz, 2H), 7.77 (t, J = 8.0 Hz, 1H), 7.68 (t, J = 7.6 Hz, 1H), 7.51 (s,
1H), 7.40 (d, J = 7.6 Hz, 2H), 7.23 (d, J = 8.4 Hz, 2H), 6.36 (s, 1H). TOF
N
H
3h
HRMS EI+: Calcd [C16H10ClNO2] m/z 283.0400, found m/z 283.0407.
3.7. Singlet oxygen (1O2) quantum yields. The 1O2 quantum yields (Φꢁ) of the
photosensitizers were calculated with Rose Bengal (RB) at standard (Φꢁ = 0.80 in CH3OH). The
1
absorbance of the O2 scavenger 1,3ꢀdiphenylisobenzofuran (DPBF) was adjusted around 1.0 in
aerated dichloromethane. Then, the photosensitizer was added to cuvette and the absorbance of
photosensitizer was adjusted around 0.2−0.3. The cuvette was irradiated with monochromatic
light at the peak absorption wavelength of the photosensitizer for 10 seconds. Absorbance was
measured for several times after each irradiation. The slope of absorbance maxima of DPBF at
414 nm versus time graph for each photosensitizer were calculated. Singlet oxygen quantum
yield (Φꢁ) were calculated according to a modified equation (1): 28
k bod
F ref
(
)
)
(
)
)
φ
bod =
φ
ref ×
×
(
)
(
)
(1)
k ref
(
F bod
(
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where ‘bod’ and ‘ref’ designate the photosensitizers and ‘RB’ respectively. k is the slope of the
plot of absorbance of DPBF (414 nm) against the irradiation time, F is the absorption correction
factor, which is given by F = 1−10−OD (OD is the optical density at the irradiation wavelength).
3.8. ESR spectra. Samples were quantitatively injected into quartz capillaries for ESR
analysis in the dark. The illumination was carried out in the sample chamber of the ESR
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11
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52
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59
60
−•
spectrometer. Triplet photosensitizers and superoxide radical anion (O2 ) scavengers (5,5ꢀ
dimethylꢀ1ꢀpyrrolineꢀNꢀoxide, DMPO), or singlet oxygen (1O2) scavengers (2,2,6,6ꢀ
tetramethylpiperidine, TEMP) in airꢀsaturated CH3CN was stirred under darkness. Then the
solution was injected into the quartz capillary. The quartz capillary was irradiated with laser for
20 seconds. Then the electron spin resonance (ESR) spectroscopy was studied. A diode pumped
solid state (DPSS) continous laser (532 nm) was used for the irradiation for B-1, For B-4 the
laser wavelength is 635 nm.
■ ACKNOWLEDGMENT
We thank the NSFC (20972024, 21073028 and 21273028), the Royal Society (UK) and NSFC
(ChinaꢀUK CostꢀShare Science Networks, 21011130154), Ministry of Education (NCETꢀ08ꢀ
0077 and SRFDPꢀ20120041130005) and Dalian University of Technology for financial support.
Supporting information
Molecular structure characterization and additional photocatalysis data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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