Visible Light Photoredox Catalyzed Biaryl Synthesis Using Nitrogen Heterocycles as Promoter

Article abstract of DOI:10.1021/cs5011037

Although transition-metal-catalyzed direct arylation of aromatic C-H bonds is one of the most efficient ways for the construction of biaryl targets, it is also expected for other alternative methods that can use inexpensive catalysts and abundant solar energy to drive chemical reactions. Herein, we describe a new activation mode for biaryl synthesis by using a photosensitive complex of potassium tert-butoxide (KOt-Bu) and nitrogenous heterocyclic ligands via visible light excitation. Under low-energy visible light irradiation, the single-electron transfer from electron-donor KOt-Bu to electron-deficient nitrogenous heterocycle occurred in the inner part of the complex by using potassium as a bridge atom. The ligand accepted the as-photoexcited single electron and transformed into stable radical anions which played a dominant role in the coupling reactions of benzene with aryl halides at ambient temperature. This reaction paradigm features the use of inexpensive catalyst, abundant visible light energy, and more accessible bromobenzene for the construction of biaryl compounds under rather mild conditions. (Figure Presented).

Full text of DOI:10.1021/cs5011037

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Article
Visible Light Photoredox Catalyzed Biaryl
Synthesis Using Nitrogen Heterocycles as Promoter
Zhen Xu, Li Gao, Lele Wang, Meiwei Gong, Wenfeng Wang, and Rusheng Yuan
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs5011037 • Publication Date (Web): 10 Nov 2014
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Visible Light Photoredox Catalyzed Biaryl
Synthesis Using Nitrogen Heterocycles as
Promoter
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Zhen Xu, Li Gao, Lele Wang, Meiwei Gong, Wenfeng Wang and Rusheng Yuan*
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and
Environment, Fuzhou University, Fuzhou, 350002, P. R. China
KEYWORDS: Visible light photoredox; Biaryl synthesis; Catalysis; Phenanthroline radical
anion; Singleꢀelectronꢀtransfer
ABSTRACT: Although transitionꢀmetalꢀcatalyzed direct arylation of aromatic CꢀH bonds is one
of the most efficient ways to the construction of biaryl targets, it is also expected for other
alternative methods that can use inexpensive catalysts and abundant solar energy to drive
chemical reactions. Herein, we describe a new activation mode for biaryl synthesis by using
photosensitive complex of potassium tertꢀbutoxide (KOtꢀBu) and nitrogenous heterocyclic
ligands via visible light excitation. Under lowꢀenergy visible light irradiation, the single electron
transfer from electronꢀdonor KOtꢀBu to electronꢀdeficient nitrogenous heterocycle occurred in
the inner of the complex by using potassium as a bridge atom. The ligand accepted the asꢀ
photoexcited single electron and transformed into stable radical anions which played a dominant
role in the coupling reactions of benzene with aryl halides at ambient temperature. This reaction
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paradigm features the use of inexpensive catalyst, abundant visible light energy and more
accessible bromobenzene for the construction of biaryl compounds under rather mild conditions.
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1. INTRODUCTION
The pursuit of new protocols to construct biaryl compounds is a key issue in modern organic
synthesis since these substances are fundamental framework and starting stuff in a wide variety
of fine chemical reactions. It is unarguable that transitionꢀmetalꢀcatalyzed direct arylation of
aromatic CꢀH bonds is one of the most efficient ways to the synthesis of biaryl targets.1ꢀ6
However, these processes also suffered significant problems such as high economic cost and the
presence of transitionꢀmetal impurities in the final products. Therefore, the transitionꢀmetalꢀfree
process has been emerged as an alternative strategy to perform CꢀC crossꢀcoupling between
unactivated arenes and aryl halides with the promotion of metal tertꢀbutoxide as a base.7ꢀ12 It is
worthwhile to note that some classes of ligands containing nitrogen heterocycle were usually
employed in these baseꢀpromoted systems, such as quinolineꢀ1ꢀaminoꢀ2ꢀcarboxylic acid,8 an
aminoꢀlinked nitrogen heterocyclic carbene (aminoꢀNHC),9 N,N'ꢀdimethylethylenediamine
(DMEDA),10 1,10ꢀphenanthroline,11 and its derivatives.12 The results of previous studies showed
that the nitrogenous bidentate chelating centers have the capability of coordinating with a metal
ion to form fiveꢀmembered ring structures,8,12,13 and thus prompt the singleꢀelectronꢀtransfer
(SET) from metal tertꢀbutoxides to ligands under conventional heating. There has been growing
interest into the utilization of abundant and renewable sunlight in synthetic applications.
Recently, Rossi group14 reported the application of powerful light (two HPIꢀT 400W lamps) for
direct arylation of benzene using potassium tertꢀbutoxide (KOtꢀBu) and dimethyl sulfoxide
(DMSO) since DMSO solvates the KOtꢀBu and affords the solventꢀseparated ion pair to act as an
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electron donor. Li group15 employed visible light for direct arylation by introducing Ir(ppy)3 (an
effective photoredox catalyst) into a reaction system similar to Rossi group’s. However, the
arylation of benzene and more accessible aryl bromide required the combined use of visible light
and heat (70 oC).
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Encouraged by these results, we envisaged that it might be possible to drive the SET process
in baseꢀpromoted systems using nitrogen heterocycle under visibleꢀlight irradiation since the
proposed configuration of K ion to nitrogen heterocycle is analogous to that of polypyridyl
complexes of ruthenium, a wellꢀknown visible light photoredox catalyst which undergoes a
metal to ligand charge transfer (MLCT) under irradiation and generates photoexcited species to
initiate reactions.16 Herein, we established a new activation mode where the photosensitive
complex of potassium tertꢀbutoxide (KOtꢀBu) and nitrogenous bidentate chelating ligands can be
excited by visible light (λ≥420 nm) that are abundant in the solar spectrum and then initiate
coupling reactions via SET process. This reaction paradigm features the use of inexpensive
catalyst and abundant visible light energy for the construction of biaryl compounds under rather
mild conditions.
2. RESULTS AND DISCUSSION
Generally, the observed color of a substance is said to be the complementary to the color of the
absorbed visible light. When KOtꢀBu and 1,10ꢀphenanthroline in benzene was mixed with a
stoichiometric ratio, a yellow solution was observed although either of them in benzene are
colorless (Photograph, Figure 1). This indicates that a complex between KOtꢀBu and 1,10ꢀ
phenanthroline may be formed via coordination and can absorb light in the visible region. When
the above mixed solution was analyzed by electron paramagnetic resonance (EPR) spectroscopy
under visible light irradiation (420 nm＜λ＜780 nm), a strong EPR signal with a sharp splitting
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Figure 1. EPR spectra arising from the benzene solution of 1,10ꢀphenanthroline and KOtꢀBu
under visible light excitation and their following decay after turning off the light (The
photographs showing that the color changes of the mixed solution before and after light
irradiation and the structure of the asꢀformed complex).
at g=2.0036 appeared, and its intensity increased with a prolonged irradiation time (A, Figure 1).
Similar resonance characteristics for KOtꢀBu and 1,10ꢀphenanthroline have also been reported in
studying the generation of radical intermediates in thermocatalysis.10ꢀ13,17 On the contrary, no
signal was obtained when the solution was kept in the dark or in the absence of KOtꢀBu or 1,10ꢀ
phenanthroline. Although the EPR spectrum shows little resolution and broad bands, the
isotropic g factor of about 2.0036 and the hyperfine coupling constant of about 2.9 G (Figure 2a)
was still determined for the equivalent nitrogen atoms’ splitting in a phenanthroline radical anion
which has a singly occupied b1(φ) orbital. This assignment is consistent with the previous
reports for phenanthroline radical anion with a coordinating counterion such as K+ in
tetrahydrofuran.18ꢀ20 The above results were also supported by theoretical calculations (Figures
S2ꢀ4 and Tables S1ꢀ2). The natural bond orbital (NBO) and molecular orbital analyses based on
the optimized structures of the formed complex in neutral and triplet states further show that the
preferential pathway for electron transfer is from the KOtꢀBu part to the 1,10ꢀphenanthroline part
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Table 1. Direct arylation of benzene with 4ꢀbromotoluene.a
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7
8
9
ligand
KOt-Bu
Br
H
+
Vis
1a
2
3a
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Entry
1
Ligand
Condition
Yield (%)
73g
o
Vis, 18 C
L1
N
N L1
Heat, 100oC
2b
71
3
o
ꢀ
Vis, 18 C
3
o
4c
5
0
L1
L1
L1
L1
Vis, 18 C
o
Dark, 18 C
0
Vis, 6 oC
6d
7e
8f
70
78
70
o
UV, 18 C
o
L1
Vis, 18 C
H
N
o
N
Vis, 18 C
9
6
0
L2
H
o
Vis, 18 C
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L3
N
N
N
N
o
Vis, 18 C
6
L4
o
Vis, 18 C
16
4
L5
L6
N N
o
Vis, 18 C
N
N
o
Vis, 18 C
4
N
N
L7
L8
o
Vis, 18 C
5
N
o
Vis, 18 C
<1
81g
L9
N
N
N
Ph
Ph
o
Vis, 18 C
L10
N
a
General reaction conditions: substrate (0.1 mmol), benzene (1 mL), base (4 equiv), ligand (50
o
mol%), temperature (18 C, cycle water cooling), N2 atmosphere, visible light irradiation (420
nm＜λ＜780 nm, 400 mW cmꢀ2, 24 h). b With a heating condition at 100 C in the dark. In the
o
c
d
o
e
absence of base. With a reaction temperature of 6 C. With an UV irradiation (320 nm＜λ＜
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780 nm) for 6 h. f 4ꢀbromotoluene(0.5 mmol) and benzene (5 mL) were used. g Trace amounts of
the homoꢀcoupling products biphenyl and 4,4’ꢀdimethylbiphenyl of the substrates were identified
only from mass spectrum.
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by using potassium as a bridge atom. Therefore, we supposed that the unpaired electron residing
on the ligand resulted in the formation of phenanthroline radical anion. As shown in Fig. 1B, the
EPR signal for the asꢀobtained radical anions decayed gradually and lasted more than ten
minutes after turning off the light. This suggests that the radical anion intermediates are stable
and have a long lifetime to engage in the subsequent electron transfer process. When the
electronꢀdeficient 4ꢀbromotoluene was rapidly injected to this excited system in the dark, a
coupling product 4ꢀmethylbiphenyl was obtained surprisingly and the signal at g=2.0036
disappeared accordingly, demonstrating that the radical anion intermediate was responsible for
the generation of biaryl product (C, Figure 1).
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In view of the above findings, a series of arylation experiments were launched under
continuous light excitation. The arylation of benzene with 4ꢀbromotoluene proceeded under
visible light irradiation using 50 mol% 1,10ꢀphenanthroline in the presence of 4 equiv KOtꢀBu as
a base at 18 oC. The desired crossꢀcoupling product 3a was obtained in 73% yield for 24 h (entry
1 of Table 1), slightly higher than that obtained under thermocatalytic condition at 100 oC (entry
2). Only 3% yield was obtained in the absence of 1,10ꢀphenanthroline (entry 3) and no coupling
product was obtained in the absence of KOtꢀBu or light irradiation (entries 4 and 5), which
indicates that the presence of KOtꢀBu, 1,10ꢀphenanthroline and light were essential to the
o
reaction. Even at 6 C, the yield of 3a was almost not affected (entry 6), which suggests that
thermal process was not involved in the coupling reaction. Upon irradiation with highꢀintensity
visible light, the increased EPR signals were observed, and the peak areas of EPR signals have a
linear dependence with the corresponding yields of the biaryl product, which suggests that the
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(b)
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(c)
(d)
Figure 2. (a) EPR spectra recorded at different wavelengths of incident light. (b) EPR spectra
recorded at different irradiation intensities. (c) The influence of irradiation intensity (12 h) and
wavelength (5 h) of incident light on the yield of biaryl product. (d) EPR spectra for different
nitrogen heterocycles.
amount of radical anion intermediate notably influenced the yield of the crossꢀcoupling product
(Figure 2aꢀc and Figure S6). This also indicates that the coupling process over the asꢀformed
complex is a typical photoredox catalytic reaction. When the reaction was scaled up by a factor
of 5, the coupling product 3a was still obtained in a satisfying yield under the same photon flux
per unit area (entry 8). Even under irradiation of household CFL lamp (16 W) for 60 h, the
arylation of benzene with 4ꢀiodotoluene or bromobenzene still gave the biaryl products in 79%
and 67% yield, respectively (Table S7). This further confirms the availability and effectiveness
of the asꢀestablished complex for biaryl synthesis.
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The configuration stability of the radical anions would significantly influence the ability to
supply electron for initiating subsequent coupling reactions. Therefore, the various ligands
containing nitrogen heterocycles were tested. When 1,10ꢀphenanthroline was replaced by other
ligands (L2ꢀL6, Table 1), much lower efficiency or no efficiency was observed for the coupling
of benzene with 4ꢀbromotoluene (entries 9ꢀ13). Phenanthrolines with nitrogen atoms at the 1,7ꢀ
or 4,7ꢀpositions gave only 4% and 5% yields of 3a, respectively (entries 14, 15). These results
indicated that the nitrogen atom position of the ligands significantly influenced electron
transferring process due to ineffective coordination with KOtꢀBu. The introduction of electronꢀ
donating methyl groups at the 4ꢀ and 7ꢀpositions of 1,10ꢀphenanthroline resulted in little
coupling product (entry 16), whereas the substitution of 1,10ꢀphenanthroline with two phenyl
groups at the 4ꢀ and 7ꢀpositions greatly increased the yield of 3a up to 81% (entry 17). This
might arise from the differences of the electronic effect of methyl groups and the conjugative
effect of phenyl groups, consistent with the previous report using phenanthroline derivates under
elevated temperature.12 The conjugated groups is more favorable for the ligand to accept single
electron from KOtꢀBu in the inner of the complex, which resulted in the formation of more stable
radical anion intermediates for transferring electron in a further step onto 4ꢀbromotoluene. This
could be further confirmed by the evidence that the EPR signal intensity of 4,7ꢀdiphenylꢀ1,10ꢀ
phenanthroline was rather higher than that of 1,10ꢀphenanthroline under the same conditions
(Figure 2d).
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The scope of this crossꢀcoupling reaction was extended to a variety of aryl halides (Table 2). In
most cases, the introduction of both electronꢀdonating (entries 3ꢀ6) and electronꢀwithdrawing
groups (entries 7ꢀ9) into pꢀsubstituted aryl halides were beneficial for this crossꢀcoupling
reaction and afforded good yields for the desired products. However, carbon trifluoride group
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Table 2. Direct arylation of benzene or pyridine with different aryl halides.a
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4
5
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7
8
9
1,10-phenanthroline
Ar
KOt-Bu
Or
Or
+
N
Ar
Ar X
1
N
Vis
5
2
3
4
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Entry
Ar-X
Products
Yield (%)
1
1e
3c
70
72
73
74
89
90
77
91
87
39
Br
I
2
1f
3c
3a
3a
3b
3b
3h
3h
3i
3
Br 1a
Me
Me
Me
4
1b
I
Me
5
Br 1c
MeO
MeO
NC
MeO
6
1d
I
MeO
NC
7b
8b
9
Br 1k
1l
NC
I
NC
F3C
F3C
1m
F3C
I
10
1n
3i
F3C
Br
Me
Br
Me
1g
3d
11
12
13
15
52
67
OMe
Br
OMe
1h
1i
3e
3f
Me
Me
Br
MeO
MeO
1j
3g
3a
5a
14
15
76
20
Br
Cl
1o
Me
Me
Me
Me
16c
95
1a
Br
N
N
Me
17c,d
100
1b
5a
Me
I
a
General reaction conditions: substrate (0.1 mmol), benzene (1 mL), KOtꢀBu (4 equiv), 1,10ꢀ
phenanthroline (50 mol%), temperature (18 oC, cycle water cooling), N2 atmosphere, visible light
irradiation (420 nm＜λ＜780 nm, 400 mW cmꢀ2, 24 h). b Irradiation time of 1 h. c Substrate (0.5
mmol) and pyridine (5 mL) were used. GC yield based on 1a for 48 h and isomer ratio:
o/m/p=1.0/1.2/3.9 determined by NMR. d GC yield based on 1b for 8 h and isomer ratio: o/m/p=
1.0/1.3/3.8 determined by NMR.
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substituted bromobenzene (1n) gave the biphenyl product only in 39% yield, much lower than
that using bromobenzene (entry 10). Surprisingly, the pꢀsubstituted aryl halides with electronꢀ
withdrawing –CN group (1k, entry 7, 1l, entry 8) exhibited extremely high reactivity. The
coupling yields of 77% and 91% were obtained for 1 h respectively, comparable to those of other
aryl halides for 24 h. This is obviously different from the thermocatalytic process with a poor
reactivity for pꢀhalogenated benzonitrile.11 The transformation was sensitive to steric effects, and
the yield of the coupling products for oꢀ, mꢀ and pꢀsubstituted aryl halides increased
progressively (entries 3, 5, 11ꢀ14). The reactivity of aryl halides decreased in the order of iodides,
bromides and chlorides (entries 3, 4, 15). In addition, this reaction can be extended to the
coupling of electronꢀdeficient pyridine and aryl halides, giving high yields of the coupling
products (entries 16,17). Other arenes with different substituted groups were also tested to
demonstrate the applicability of the reaction (Table S9). The arylation of 4ꢀbromoanisole with
substituted arenes afforded the corresponding products as a mixture of regioisomers, and the
orthoꢀarylated product predominated. This result indicated that the substitution of benzenes will
accelerate the coupling to the orthoꢀposition with good regioselectivity, and the reaction went
through a radical pathway.
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To further verify SET mechanism, the coupling reaction was completely terminated by the
addition of a typical electron scavenger (1,4ꢀbenzoquinone, BQ) as a result of the consumption
of single electron by BQ, which was confirmed by the formation of benzoꢀsemiquinone anion
radical (Table 3 and Figure S11). A dominant pathway involving radicals in the cross coupling
reaction was supported by the addition of 2,2,6,6ꢀtetramethylpiperidine Nꢀoxyl (TEMPO), a
typical radical scavenger, to the reaction system. The yield of the coupling product decreased as
the amount of TEMPO was increased, and when TEMPO was added at a concentration of 12
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Table 3. Radical/electron trapping experiments.a
3
4
5
6
7
8
9
1,10-phenanthroline (50 mol%)
KOt-Bu (4 equiv)
+ H
Br
Vis 12 h
2
1a
3a
1 mL
0.1 mmol
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Entry
Additive
Yield (%)
1
2
3
4
5
none
37
0
BQ (2 equiv)
TEMPO (6 mol%)
TEMPO (12 mol%)
7
0
TBA+PF6 (2 equiv)
2
-
a
General reaction conditions: substrate (0.1 mmol), benzene (1 mL), KOtꢀBu (0.4 mmol, 4
equiv), 1,10ꢀphenanthroline (0.05 mmol, 50 mol%), temperature (18 oC, cycle water cooling), N2
atmosphere, visible light irradiation (420 nm<λ<780 nm, 400 mW cmꢀ2, 12 h). TBA+PF6 ＝tetraꢀ
ꢀ
nꢀbutylammonium hexafluorophosphate.
Table 4. Competition reaction between two aryl bromides.a
C6H6 (1 mL)
MeO
F3C
F3C
MeO
1,10-phenanthroline (50 mol%)
KOt-Bu (4 equiv)
+
+
Vis 8 h
Br
Ph
Ph
Br
1n
3b
1c
3i
Entry 1n (mmol)
1c (mmol)
3i (%) 3b (%)
1
2
3
0.05
0.1
0
0.05
0
28
8
1
0
0.1
0
52
a
General reaction conditions: substrate (0.1 mmol), benzene (1 mL), KOtꢀBu (0.4 mmol, 4
equiv), 1,10ꢀphenanthroline (0.05 mmol, 50 mol%), temperature (18 oC, cycle water cooling), N2
atmosphere, visible light irradiation (420 nm<λ<780 nm, 400 mW cmꢀ2, 8 h).
mol%, the transformation was completely inhibited (Table 3, entries 3, 4). The introduction of
TBA+PF6 also decreased the yield of the biaryl product greatly (entry 5), demonstrating the
ꢀ
presence of ion radicals in the reaction.21,22 A further competitive experiment with equimolar
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amounts of 1c and 1n in the presence of benzene was also carried out (Table 4). Although the
presence of 1c or 1n alone gave 52% or 8% of the corresponding coupling product, respectively,
a more electronꢀdeficient 1n with benzene have higher reactivity than 1c in the coexistence of
two components, consistent with the SET mechanism.12,23 The kinetic isotopic effect of benzene
showed a low KH/KD value of 1.04 for the coupling reaction, indicating that the cleavage of CꢀH
bonds in benzene is not the rateꢀdetermining step (Scheme 1). Therefore, we concluded that the
production of radicals via SET process was the dominant step. Additionally, the controlled
reaction using bromobenzene as the sole starting material under the same condition also gave the
coupling products of bromobiphenyl and biphenyl (Table S10 and Scheme S2), which well
supports that the aryl radicals from the reduction of aryl halide are key intermediates for
initiating subsequent reactions with benzene.
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D
1,10-phenanthroline
D
D
D
KOt-Bu
D
D
D
or
+
+
Vis-400mW,28h
D
D
D
Br
D
3a-d5
2-d6
1a
2
3a
(2.5 mL) (2.5 mL)
(0.5 mmol)
72% yield, KH/KD=1.04
Scheme 1. KIE experiment of the model reaction.
On the basis of the above experimental evidence and previous reports, a plausible radical
involving mechanism initiated by a light responsive SET process is proposed in Scheme 2.10ꢀ13,17
When KOtꢀBu and 1,10ꢀphenanthroline in benzene was mixed stoichiometrically, a complex
with visible light absorption formed via a fiveꢀmembered ring structure between K ion and
phenanthroline. Under visible light irradiation, the SET from electronꢀdonor KOtꢀBu to
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Scheme 2. Proposed plausible mechanism.
electronꢀdeficient phenanthroline occurred in the inner of the complex by using potassium as a
bridge atom. Thus, phenanthroline accepted an electron and transformed into radical anion
corresponding to the EPR signal at g=2.0036. Then the electron can be captured by aryl halide to
give an [ArX]·ꢀ radical anion and complex radical cation intermediate C, resulting in the
disappearance of the EPR signal at g=2.0036. The [ArX]·ꢀ radical anion subsequently dissociated
into aryl radical which can combine with benzene to give cyclohexadienyl radical G. DFT
calculations (Table S2) suggest that the radical cation C is unstable and more apt to oxidize G,
which was also reported in previous literatures.9,12 The oxidation of G by radical cation C gave
cation H, which was deprotonated to give biaryl 3a and tꢀBuOH. The regenerated phenanthroline
continued to combine with fresh KOtꢀBu for initiating a new reaction cycle. In view of this, the
phenanthroline can be supposed as the crucial promoter or catalyst of this crossꢀcoupling
reaction. The coordinated complex of phenanthroline with KOtꢀBu can be excited by visible light,
and the corroborative evidence for the formation of phenanthroline radical anion that plays a
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dominant role in the aromatic crossꢀcoupling reactions were uncovered for the first time.
Moreover, we obtained a good yield of biaryl only under visibleꢀlight irradiation (λ≥420 nm) for
more accessible bromobenzene, comparable to that obtained under thermocatalytic condition at
100 oC.
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3. CONCLUSIONS
We have developed a visible light photoredox catalyzed coupling reaction of aryl halides with
benzene through using nitrogen heterocycles as promoter. The conclusive evidence in the EPR
spectra supported that the formed phenanthroline radical anion via SET process played a
dominant role in the formation of biaryl products. This system can also be applied to other aryl
halides and analogous nitrogen heterocycles, and offers a green and sustainable route to the
construction of the biaryl products.
4. EXPERIMENTAL DETAILS
General procedure for the direct arylation of benzene or pyridine with aryl halides. All
reagents were commercially available and used without further treatment. Benzene was
anhydrous, and all glassware used in the reactions was completely dried to exclude the
introduction of water. Prior to the reaction, all manipulations were performed using Schlenk
tubes and in a nitrogenꢀfilled glovebox at room temperature. Typically, a 10 mL Schlenk tube
was charged with 1,10ꢀphenanthroline (9 mg, 0.05 mmol) and KOtꢀBu (45 mg, 0.4 mmol).
Benzene or pyridine (1 mL) and aryl halides (0.1 mmol) were then added. After that, the tube
was removed from the glovebox, and the resulting mixture was stirred upon visible light
irradiation with a xenon lamp (PLSꢀSXE300C, Beijing Perfect Light Co) equipped with an IRꢀ
cutoff filter (λ<780 nm) for providing the visible light (λ>420 nm) or UV light (λ>320 nm)
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through the combination of different filters. The tube reactor was immersed in a transparent
vessel with jacket connected to a circulating cooling water source controlled by a thermostatic
apparatus. Some different UV sources were also provided by 4ꢀW UV lamps with certain
wavelengths centered at 254 nm or 365 nm (Philips, TUV 4W/G4 T5 or TL 4W/05). The
irradiance spectrum and energy of the light incident on the suspension were measured with a
spectroradiometer (International Light Technologies Model ILT950), about 400mW cmꢀ2
(420~780 nm wavelength range) for visible light case, 300 mW cmꢀ2 (320~780 nm wavelength
range) for UV case and 60 mW cmꢀ2 (certain wavelengths centered at 254 nm or 365 nm),
respectively. The illuminated area was estimated to be about 1 cm2 corresponding to the cross
section of the columned tube. After reaction, nꢀoctane (0.05 mmol) was added into the reaction
mixture and then the mixture was centrifugalized. The asꢀobtained supernatant liquid was
injected into the GC equipped with FID detector (Agilent Technologies, GC6890N) for analysis.
A HPꢀ5 5% phenyl methyl siloxane column (30 m × 0.32 mm × 0.5 ꢁm) was used. The chemical
structures of the products were confirmed by comparison with standard chemicals and GCꢀMS
(Agilent Technologies, GC6890N, MS 5975). The yield of the crossꢀcoupling product was
described as the ratio of the moles of the generated biaryl compound in the reaction to the initial
moles of the corresponding aryl halide.
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Procedure of cross-coupling reaction under thermal condition. A 10 mL Schlenk tube was
charged with 1,10ꢀphenanthroline (9 mg, 0.05 mmol), KOtꢀBu (45 mg, 0.4 mmol), benzene (1
mL) and 4ꢀbromotoluene (0.1 mmol) in the glovebox. The sealed tube was then removed from
the glovebox and the containing mixture was stirred at 100°C under dark for 24 h. The final
products were allowed to cool to room temperature and nꢀoctane (0.05 mmol) was added into the
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mixture. After centrifugalizing, the asꢀobtained supernatant liquid was injected into the GC and
GCꢀMS (a solvent delay time of about 5 min was set before the acquisition) for further analysis.
Procedure of cross-coupling reaction for separating products. A 25 mL Schlenk tube was
charged with 1,10ꢀphenanthroline (45 mg, 0.25 mmol), KOtꢀBu (225 mg, 2.0 mmol), benzene or
pyridine (5 mL) and aryl halides (0.5 mmol) in glovebox. Then the tube was removed from the
glovebox and the resulting mixture was stirred upon visible light irradiation (providing 400 mW
cmꢀ2 visible light) for 24 h. The reaction was quenched with water in excess and the residue was
extracted with copious ethyl acetate. The combined organic phase was concentrated under
vacuum. The crude product was purified by semipreparative HPLC (60% MeCN, 0.1% TFA) to
give desired product. Semipreparative HPLC was performed using an ODS column (ODSꢀA,
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1
10×250 mm, 5 µm) at 5 mL/min. H NMR and 13C NMR were recorded on a BRUKER
BIOSPIN AVANCE III spectrometer using TMS as the internal standard.
Procedure of KIE experiment (scheme 1). A 25 mL Schlenk tube was charged with 1,10ꢀ
phenanthroline (45 mg, 0.25 mmol), KOtꢀBu (225 mg, 2.0 mmol), benzene (2.5 mL), benzeneꢀd6
(2.5 mL) and 4ꢀbromotoluene (0.5 mmol) in glovebox. Then the tube was removed from the
glovebox and the resulting mixture was stirred upon visible light irradiation (providing 400 mW
cmꢀ2 visible light) for 24 h. The reaction was quenched with water in excess and the residue was
extracted with copious ethyl acetate. The combined organic phase was concentrated under
vacuum. The crude product was purified by semipreparative HPLC (60% MeCN, 0.1% TFA) to
give the desired product.
Detection of radical anion. The electron paramagnetic resonance (EPR) measurements were
conducted according to the following procedures. A solution containing KOtꢀBu (0.2 mmol),
nitrogen heterocyclic compound (0.025 mmol) and benzene (0.2 mL) was placed into a quartz
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cell, and then the quartz cell was sealed. All these procedures were operated in glovebox under
N2 atmosphere. The quartz cell was then irradiated with different light sources of xenon lamp at
the range of 320 nm~780 nm, and the spectra were recorded in situ with a Bruker A300
instrument operating in the Xꢀband at room temperature.
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ASSOCIATED CONTENT
Supporting Information.
Experimental section, computational details, extra crossꢀcoupling reactions, the copies of NMR
spectra and mass spectra for all the crossꢀcoupling products. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: yuanrs@fzu.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was financially supported by the National Basic Research Program of China
2014CB239303, the National Nature Science Foundation of China (Nos. 21077023, 81001403),
the Natural Science Foundation of Fujian Province of China (Nos. 2010J01035, JA10008 and
JK2011001).
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