Three-coordinate copper(I) 2-hydroxy-1,10-phenanthroline dinuclear complex catalyzed homocoupling of arylboronic acids towards biphenyls under air condition

Article abstract of DOI:10.1016/j.tet.2015.11.001

An efficient synthesis of biphenyls via three-coordinate planar dinuclear Cu₂(ophen)₂ complex catalyzed homocoupling of arylboronic acids under air condition and in the condition of alkali-free was demonstrated. Uniquely, this catalyst shows high selectivity towards biphenyls when the reaction was conducted in aqueous solution, which is significantly different from selectivity towards phenols in related Cu-catalysts. Utilization of environmentally friendly water as main solvent, reaction at room temperature and in the absence of base, cheapness, low loading and easy bulk preparation of catalyst make it potential in green synthesis of biphenyls.

Full text of DOI:10.1016/j.tet.2015.11.001

Tetrahedron 71 (2015) 9598e9601
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Three-coordinate copper(I) 2-hydroxy-1,10-phenanthroline
dinuclear complex catalyzed homocoupling of arylboronic acids
towards biphenyls under air condition
*
*
Yan-Hong Wang, Mei-Chen Xu, Jie Liu, Ling-Juan Zhang , Xian-Ming Zhang
School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 August 2015
Received in revised form 28 October 2015
Accepted 3 November 2015
Available online 5 November 2015
An efficient synthesis of biphenyls via three-coordinate planar dinuclear Cu2(ophen)2 complex catalyzed
homocoupling of arylboronic acids under air condition and in the condition of alkali-free was demon-
strated. Uniquely, this catalyst shows high selectivity towards biphenyls when the reaction was con-
ducted in aqueous solution, which is significantly different from selectivity towards phenols in related
Cu-catalysts. Utilization of environmentally friendly water as main solvent, reaction at room tempera-
ture and in the absence of base, cheapness, low loading and easy bulk preparation of catalyst make it
potential in green synthesis of biphenyls.
Keywords:
Dinuclear Cu2(ophen)2
Homocoupling reaction
Alkali-free
Ó 2015 Elsevier Ltd. All rights reserved.
High selectivity
1. Introduction
demonstrated to exhibit good catalytic activity toward the homo-
coupling of arylboronic acids. However, high temperature8,10 and
Symmetrical biaryl compounds are an important class of organic
compounds for both synthetic chemistry and medicinal, including
emulsifiers, optical brighteners, crop protection products and an-
timicrobial, anti-proliferative, anti-diabetic, pharmaceuticals.1
These protocols of Stille coupling, Gomberg-Bachmann reaction,
Ullmann reaction, and Suzuki-Miyaura cross-coupling have been
utilized for the construction of biphenyls. The metal-assisted
homocoupling of aryl halides, aryl Grignard reagents, 1,2-
diarylditellanes, and arenediazonium salts continued to develop
over the past decade. Compared with these methods, synthesis of
symmetrical biaryls through the homocoupling of arylboronic acids
has been paid more attention, because of lower toxicity of aryl-
boronic acids and avoiding use of organometallic reagents. Various
catalyst systems like [Ag2OþCrCl2],2 [RhCl (PPh3)3],3 AuCl,4 Au
supported by Chitosan,5 Au/MAO,6 especially Pd7 catalyzed
homocoupling of arylboronic acids have been extensively reported.
Copper-catalyzed aerobic homocoupling of arylboronic acids are of
increasing interest due to copper being an abundant and low-cost
metal. In the previous studies of copper complexes of active oxy-
complex catalyst preparation10,11 could not be avoided in most of
these approaches. Moreover, it should be noted that the homo-
coupling of arylboronic acids in these Cu-based catalysis systems
are performed in organic solvents, and phenols will be the main
products by using environmentally friendly water as the main
solvent (Scheme 1).9e12 Therefore, the development of an eco-
nomic, effective and environmentally friendly Cu catalyst for the
construction of symmetrical biaryl compounds has important
practical meaning.
gen species, such as Cu(OAc)2,8 CuCl,9 CuII-
b-Cyclodextrin Com-
plex,10 Clay encapsulated Cu(OH)x,11 Cu(BDC)MOF12 etc. have been
Scheme 1. Various catalytic systems for homocoupling of arylboronic acid.
* Corresponding authors. E-mail addresses: zhanglingjuan1985@163.com
(L.-J. Zhang), zhangxm@dns.sxnu.edu.cn (X.-M. Zhang).
http://dx.doi.org/10.1016/j.tet.2015.11.001
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

Y.-H. Wang et al. / Tetrahedron 71 (2015) 9598e9601
9599
Table 1
Efforts in our laboratory are focused on the synthesis of metal
Optimization of reaction conditionsa
organic complexes with novel structures and exploration of their
application on catalytic organic synthesis.13 Ten years ago, we
found that simple hydrothermal ligand reaction of copper salt and
cheap 1,10-phenathroline could bulkily generate interesting planar
dinuclear Cu2(ophen)2(1) complex with hydroxylated N-heterocy-
clic ligands.14 Each Cu(I) atom in 1 with a trigonal geometry is co-
ordinated by an oxygen atom from one ophen ligand and two
nitrogen atoms from another ophen ligand (Fig. 1).
Entry
Cat(mol %)
Sol
Conv.%b
Yield%
Biphenylc
Phenol
1
2
0
DMF
DMF
DMF
DMF
DMSO
Toluene
THF
d
d
d
5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
>99
>99
d
95
88
d
3d
4e
5
12
d
18
11
4
90
73
76
10
72
62
72
10
6
7
8
H2O
d
a
Reaction conditions: phenylboronic acid (1 mmol), catalyst (1.3 mg), solvent
(1 mL), air, rt, 20 h.
b
Determined by GC.
Isolated yield.
60 ꢀC, 10 h.
c
d
e
Nitrogen atmosphere.
Fig. 1. View of planar dinuclear structure of Cu2(ophen)2.
Moreover, we found that this dinuclear 1 is redox active and
could be an intermediate in the formation of tetranuclear mixed-
valence compound 2 [Cu4(ophen)4(tp)] (Fig. S2). According to the
reported references,15 planar unsaturated three-coordinate Cu(I)
center in complex 1 might serve as potent active site to activate
molecular oxygen. We deduced that complex 1 could show catalytic
activity in oxidation reaction of alcohols. Based on above fact and
deduction as well as our continuing interest in catalytic characters
study of metal organic complexes, we envisioned that whether the
aerobic homocoupling reaction could occur with compound 1 as
catalyst. Satisfyingly, we find herein that the homocoupling of
arylboronic acids could be catalytically conducted by 1 under mild
temperature and in the absence of base conditions. More impor-
tantly, the selectivity towards biphenyls is considerable by using
environmentally friendly water as the main solvent, which is hard
to be implemented in reported Cu-based catalyst symtems.9e12
In order to investigate the generality of the method, we explored
various arylboronic acids under optimized conditions. The method
proved to be compatible with a wide range of substrates and the
desired products were obtained in good to excellent yields
(Table 2). In general, both electron-deficient and electron-rich aryl
groups gave the good yields of biphenyls (Table 2, entries 1e3,
5e13). However, substrate 1d, bearing a methyl group at the ortho-
position of phenyl ring, was difficult to form the desired biphenyl
product 2d under identical reaction conditions as above even for
48 h. The result indicated that the method was sensitive to the
steric hindrance of arylboronic acid. We also found that the catalyst
was effective to heterocyclic aryl boronic acid such as 1n (Table 2,
entry 14).
Table 2
[Cu2(ophen)2] catalyzed homocoupling of various arylboronic acidsa
2. Results and discussion
Firstly, we chose phenylboronic acid 1f as the model substrate
under various conditions to optimize the condition and the results
were listed in Table 1. An initial set of experiments was carried out
towards the catalyst in order to determine the activity of Cu2(o-
phen)2. No product was observed in the absence of catalyst in DMF
at room temperature (Table 1, entry 1). Gratifyingly, the phenyl-
boronic acid was quantitatively converted along with a yield 95% of
biphenyl 2f when Cu2(ophen)2 (0.5 mol %) was used (Table 1, entry
2). Therefore, Cu2(ophen)2 complex provides access to an effec-
tively active species for the homocoupling of phenylboronic acid.
The rate of conversion obviously rose upon increasing the tem-
perature from rt to 60 ꢀC (Table 1, entry 3), while the biphenyl
selectivity was greatly decreased. A control experiment conducted
in N2 atmosphere showed that the reaction did not occur (Table 1,
entry 4). Different polar solvents have also been detected. When
the reaction was conducted in polar solvent such as DMSO, the
biphenyl was observed in 72% yield (Table 1, entry 5). Cu2(ophen)2
showed good catalytic activity but poor selectivity in weak polar
solvents. For example, the biphenyl in Toluene and THF were ob-
tained in 62% and 72% yields, respectively, along with small part
phenol 3f (Table 1, entries 6e7). Although only 10% biphenyl was
gained in aqueous solution, the selectivity of biphenyl was very
pleased (Table 1, entry 8). Therefore, the optimal base-free system
under air for this reaction involves Cu2(ophen)2 (0.5 mol %) in DMF
at room temperature (Table 1, entry 2).
Entry
Products
Conv.%b
Yield%c
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2h
2i
4,40-ditBubiphenyl
>99
>99
>99
10
90
90
90
10
96
92
95
93
95
97
92
95
95
98
4,40-dimethylbiphenyl
3,30-dimethylbiphenyl
2,20-dimethylbiphenyl
4,40-dimethoxybiphenyl
3,30-dimethoxybiphenyl
biphenyl
>99
>95
>99
>95
>99
>99
>99
>98
>98
>99
4,40-difluorobiphenyl
4,40-dichlorobiphenyl
4,40-dibromobiphenyl
4,40-dinitrobiphenyl
4,40-dimethoxycarbonaylbiphenyl
4,40-dicyanobiphenyl
3,30-dipyridylbiphenyl
9
10
11
12
13
14
2j
2k
2l
2m
2n
a
Reaction conditions: arylboronic acid (1 mmol), Cu2(ophen)2 (0.5 mol %, 1.3 mg),
DMF (1 mL), rt, 20 h.
b
Determined by GC.
Isolated yield.
c
Generally, when the homocoupling of arylboronic acids were
conducted in Cu-based catalysis systems by using water as the main
solvent, the phenols are observed as main products. The biphenyls
did not exist at all or present as a minor product.9e12 Differing from
general reports, we found that the biphenyls remain the main

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Y.-H. Wang et al. / Tetrahedron 71 (2015) 9598e9601
product rather than the phenols by using Cu2(ophen)2 as catalyst
(Table 1, entry 8). Thus we explored various arylboronic acids to
prove the abnormal phenomenon and the results were listed in
Table 3.
product Ar-Ar could be formed by reductive elimination from B and
the catalyst was regenerated simultaneously.
3. Conclusions
In conclusion, we have described an efficient method for the
synthesis of biphenyls from homocoupling reactions of arylboronic
acids catalyzed by planar dinuclear Cu2(ophen)2. This reaction
features high to excellent yields, mild reaction conditions, high
biphenyls selectivity in most cases, cheapness, low loading and
easy bulk preparation of catalyst, and no need for any base. It is
worth reminding that the catalytic specificity of Cu2(ophen)2 for
high selectivity of biphenyls is reflected when the reaction is con-
ducted in environmentally friendly aqueous solution.
Table 3
[Cu2(ophen)2] catalyzed homocoupling of arylboronic acids in H2O-EtOHa
Entry
R
H2O-EtOH Conv.% Yield%b
Biphenyls Phenols
1
1
2
3
4
6
7
8
1a 4-tBu-
1b 4-CH3-
1c 3-CH3-
1e 4-CH3O-
5:1
5:1
5:1
5:1
5:1
1:0
5:1
5:1
5:1
5:1
5:1
5:1
5:1
>97
>99
>99
>99
>97
10
>99
>99
>99
>99
>95
>99
>99
>99
90
71
88
86
94
10
95
99
87
90
85
70
95
92
7
27
10
13
3
1f
3-CH3O-
1g H-
1g H-
1h 4-F-
d
3
4. Experimental
4.1. General
d
10
6
10
25
d
7
9
1i
1j
4-Cl-
4-Br-
10
11
12
13
14
1k 4-NO2-
1l
4-CH3OCO-
All the solvents were obtained from commercial suppliers and
used without further purification. Thin layer chromatography (TLC)
employed glass 0.25 mm silica gel plates. Flash chromatography
columns were packed with 200e300 mesh silica gel in petroleum
(bp. 60e90 ꢀC). GCeMS spectra were recorded on a Thermo Fisher
GCeMS Poparis Q. GC yields were recorded with Agilent 7820A gas
chromatography instrument with a FID detector. All new com-
pounds were characterized by 1H NMR, 13C NMR and GCeMS. The
known compounds were characterized by 1H NMR. 1H and 13C NMR
datas were recorded with ASCend TM 600 MHz with tetrame-
1m 4-NC-
1n 3-Pyridinylboronic acid 5:1
a
Reaction conditions: arylboronic acid (1 mmol), Cu2(ophen)2 (0.5 mol %, 1.3 mg),
V
H2O:VEtOH¼5:1(1.8 mL), rt.
b
Determined by GC.
During the reaction, the added a small amount of ethanol to
dissolve the substrates. As expected, using 5:1 H2O-EtOH as mix-
ture solvent, the selectivity of homocoupling products was more
efficiently compared with other similar Cu-catalytic systems.
In previous work, we reported that the copper atom of catalyst 1
adopts planar geometry and its exposed axial coordination sites
might serve as potent active site to activate molecular oxygen. In
addition, dimerization of this dinuclear species by coordination
with a terephthalic acid generate a mixed-valence dinuclear cop-
per(I,II) complex 2 (Fig. S2), concomitant with contraction of the
Cu-Cu distance.14 The fact is justified in speculating reaction
mechanism. On the basis of above analysis results and related
reports,8e10,12,16 the possible dimetallic mechanism of the homo-
coupling reaction is proposed as shown in Scheme 2. The overall
process may involve (i) the molecular oxygen is activated by Cu(I) of
catalyst 1; (ii) in the presence of water, oxidation of catalyst 1 by
activated O2 to form Cu(II) intermediate A along with contraction of
the Cu-Cu distance; (iii) transmetallation of the arylboronic acids
with copper(II) complex A to give a dimetallic intermediate B, while
release of byproduct B(OH)3 through attack of the hydroxyl ligand
to the oxophilic boron center; (iv) subsequently, the homocoupled
thylsilane as an internal standard. All chemical shifts (d) were re-
ported in ppm and coupling constants (J) in Hz. All chemical shifts
were reported relative to tetramethylsilane (0 ppm for 1H), and
CDCl3(77.16 ppm for 13C), respectively.
4.2. Synthesis of [Cu2(ophen)2]
[Cu2(ophen)2] was prepared according to the procedure in Ref
14. The mixture of Cu(NO3)2$3H2O (120 mg), 1,10-phenanthroline
(117 mg), and water (10 mL) added into a 23-mL Teflon reactor,
was stirred on magnetic stirrers and adjusted to pH¼8e10 with
NaOH (2 mol/L) solution, then sealed and heated at 160 ꢀC for 120 h.
After cooling, deep dark block crystals were filtered, washed with
distilled water and dried in air. The XRPD pattern of the complex
matched with those of pure [Cu2(ophen)2].
4.3. Catalytic tests: general procedure I for the Cu2(ophen)2
catalyzed homocoupling for Table 2
A solution of the corresponding arylboronic acids (1.0 mmol),
Cu2(ophen)2 (1.3 mg, 0.5 mol %), DMF (1.0 mL) in 5 mL round-
bottomed flask was stirred under air and the reaction was moni-
tored by TLC. After the substrate was consumed, the reaction con-
versions were determined by gas chromatography (GC) analysis (FID
from AGILENT 7820) using a cross-linked (95%)-dimethyl-(5%)-
diphenylpolysil-oxane column (HP-5, 30 mꢁ0.32 mmꢁ0.25
mm),
helium, injector temperature 250 ꢀC, detector temperature 300 ꢀC,
and oven temperature program 45 ꢀC (3 min)e20ꢀC/mine280 ꢀC
(2 min). The resulting mixture was poured into brine (10 mL), and
extracted with diethyl ether (3ꢁ10 mL). The organic layer was
washed with brine, dried over Mg2SO4, the residue was chromato-
graphed via a short column of silica gel (petroleum ether: diethyl
ether¼15:1) and evaporated under reduced pressure. The products
was determined by 1H NMR spectroscopy. All 1H NMR spectra were
measured in CDCl3 with TMS as the internal standard.
Scheme 2. A plausible mechanism of the homocoupling reaction.

Y.-H. Wang et al. / Tetrahedron 71 (2015) 9598e9601
9601
4.4. General procedure II for Table 3
(600 MHz, CDCl3):
(150 MHz, CDCl3)
d
8.39e8.37 (m, 4H), 7.81e7.80 (m, 4H). 13C NMR
147.0, 144.0, 127.3, 123.4.
d
A solution of arylboronic acids (1.0 mmol), Cu2(ophen)2 (1.3 mg,
0.5 mol %) in H2O-EtOH (1.8 mL, VH2O:VEtOH¼n:1) was stirred at
room temperature. After the substrate was consumed, the reaction
conversions were determined by GC analysis.
4.4.12. 4,40-Dimethoxycarbonaylbiphenyl 2l.16a The compound was
prepared according to the procedure I. White solid, mp 108e110 ꢀC.
1H NMR (600 MHz, CDCl3):
13C NMR (150 MHz, CDCl3)
d
8.13 (d, 4H), 7.70 (d, 4H), 3.96 (s, 6H).
165.8, 143.3, 129.2, 128.7, 126.2, 51.2.
d
4.4.1. 4,40-DitBubiphenyl 2a.9 The compound was prepared accord-
ing to the procedure I. White solid, mp 121e123 ꢀC. 1H NMR
4.4.13. 4,40-Dicyanobiphenyl 2m.11 The compound was prepared
according to the procedure I. White solid, mp 238e240 ꢀC. 1H NMR
(600 MHz, CDCl3)
d
7.52 (d, J¼6.0 Hz, 4H), 7.45 (d, J¼6.0 Hz, 4H),1.36 (s,
18H). 13C NMR (150 MHz, CDCl3)
d
149.9,138.2,126.7,125.7, 34.5, 31.4.
(600 MHz, CDCl3):
CDCl3) 143.5, 132.9, 128.0, 118.4, 112.5.
d
7.78 (d, 4H), 7.69 (d, 4H). 13C NMR (150 MHz,
d
4.4.2. 4,40-Dimethylbiphenyl 2b.9 The compound was prepared
according to the procedure I. White solid, mp 120e123 ꢀC. 1H NMR
4.4.14. 3,30-Dipyridylbiphenyl 2n.11 The compound was prepared
according to the procedure I. Yellow oil; 1H NMR (600 MHz, CDCl3):
(600 MHz, CDCl3):
d
7.47 (d, J¼6.0 Hz, 4H), 7.23 (d, J¼6.0 Hz, 4H), 2.37
(s, 6H). 13C NMR (150 MHz, CDCl3)
d
138.3, 136.7, 129.5, 126.9, 21.1.
d
8.88 (d, 2H), 8.69 (m, 2H), 7.93 (m, 2H), 7.46 (m, 2H). 13C NMR
(150 MHz, CDCl3)
d 148.3, 147.2, 133.4, 132.5, 122.8.
4.4.3. 3,30-Dimethylbiphenyl 2c.12 The compound was prepared
according to the procedure I. Colourless oil; 1H NMR (600 MHz,
CDCl3):
d
7.29e7.32 (m, 4H), 7.23e7.25 (m, 2H), 7.06 (d, J¼6.0 Hz,
Acknowledgements
2H), 2.33 (s, 6H). 13C NMR (150 MHz, CDCl3)
127.0, 126.9, 123.3, 20.5.
d 140.4, 137.2, 127.6,
This work was financially supported by the NSFC (Grant
21402112), National 10 000 Talent Plan of China and Key Discipline
Shanxi Province.
4.4.4. 2,20-Dimethylbiphenyl 2d.16a The compound was prepared
according to the procedure I. Colourless oil; 1H NMR (600 MHz,
CDCl3):
(150 MHz, CDCl3)
d
7.15e7.18 (m, 6H), 7.12e7.14 (m, 2H), 1.97 (s, 6H). 13C NMR
d
140.5, 134.7, 128.7, 128.2, 126.1, 124.5, 18.8.
Supplementary data
4.4.5. 4,40-Dimethoxybiphenyl 2e.9 The compound was prepared
according to the procedure I. White solid, mp 173e175 ꢀC. 1H NMR
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2015.11.001.
(600 MHz, CDCl3):
d
7.47 (d, J¼6.0 Hz, 4H), 6.96 (d, J¼6.0 Hz, 4H), 3.84
(s, 6H). 13C NMR (150 MHz, CDCl3)
d
157.7, 132.5, 126.7, 113.1, 54.3.
4.4.6. 3,30-Dimethoxybiphenyl 2f.9 The compound was prepared
according to the procedure I. Colourless oil; 1H NMR (600 MHz,
References and notes
1. (a) Lei, A.; Zhang, X. Tetrahedron Lett. 2002, 43, 2525; (b) Singh, F. V.; Vatsyayan,
R.; Roy, U.; Goel, A. Bioorg. Med. Chem. Lett. 2006, 16, 2734; (c) Meier, H. Angew.
Chem., Int. Ed. 2005, 44, 2482.
CDCl3): d 7.37 (m, 2H), 7.19 (m, 2H), 7.14 (m, 2H), 6.92 (m, 2H), 3.86
(s, 6H). 13C NMR (150 MHz, CDCl3)
105.3, 54.2.
d 158.8, 141.6, 128.7, 118.7, 111.9,
2. Falck, J. R.; Mohapatra, S.; Bondlela, M. S.; Venkataraman, K. Tetrahedron Lett.
2002, 43, 8149.
4.4.7. Biphenyl 2g.9 The compound was prepared according to the
procedure I. White solid, mp 69e70 ꢀC. 1H NMR (600 MHz, CDCl3):
3. Voglera, T.; Studer, A. Adv. Synth. Catal. 2008, 350, 1963.
4. Matsuda, T.; Asai, T.; Shiose, S.; Kato, K. Tetrahedron Lett. 2011, 52, 4779.
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Catal. 2013, 298, 186.
d
7.50 (d, J¼6.0 Hz, 4H), 7.35 (d, J¼6.0 Hz, 4H), 7.26e7.24 (t, J¼6.0 Hz,
2H). 13C NMR (150 MHz, CDCl3)
d
140.2, 127.7, 126.2, 126.1.
7. Pd-catalyst: (a) Wong, M.-S.; Zhang, X.-L. Tetrahedron Lett. 2001, 42, 4087; (b)
Amatore, ;C.; Cammoun, C.; Jutand, A. Eur. J. Org. Chem. 2008, 4567; (c) Mu, B.;
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Guo, S. X.; Gu, X. P.; Qiu, L. L.; Song, H. B.; Fang, J. X. Adv. Synth. Catal. 2009, 351,
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4.4.8. 4,40-Difluorobiphenyl 2h.9 The compound was prepared
according to the procedure I. White solid, mp 86e88 ꢀC. 1H NMR
(600 MHz, CDCl3):
(150 MHz, CDCl3)
d
7.49e7.47 (m, 4H), 7.13e7.10 (m, 4H). 13C NMR
d
162.2, 135.4, 127.6, 114.7.
4.4.9. 4,40-Dichlorobiphenyl 2i.9 The compound was prepared
according to the procedure I. White solid, mp 149e151 ꢀC. 1H NMR
(600 MHz, CDCl3):
(150 MHz, CDCl3)
8. Demir, A. S.; Reis, O.; Emrullahoglu, M. J. Org. Chem. 2003, 68, 10130.
9. Cheng, G.; Luo, M. Eur. J. Org. Chem. 2011, 2519.
d
7.85e7.83 (m, 4H), 7.48e7.47 (m, 4H). 13C NMR
10. Kaboudin, B.; Abedi, Y.; Yokomatsu, T. Eur. J. Org. Chem. 2011, 6656.
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24293.
d
132.4, 129.3, 126.9, 124.8.
4.4.10. 4,40-Dibromobiphenyl 2j.11 The compound was prepared
according to the procedure I. White solid, mp 163e165 ꢀC; 1H NMR
(600 MHz, CDCl3):
d
7.85 (d, J¼6.0 Hz, 4H), 7.48 (d, J¼6.0 Hz, 4H). 13
C
14. Zhang, X.-M.; Tong, M.-L.; Gong, M.-L.; Lee, H.-K.; Luo, L.; Li, K.-F.; Tong, Y.-X.;
NMR (150 MHz, CDCl3)
d 138.9, 132.0, 128.5, 122.0.
Chen, X.-M. Chem.dEur. J. 2002, 8, 3187.
15. Elizabeth, A.; Lewis, B.; Tolman, W. Chem. Rev. 2004, 104, 1047.
16. (a) Kirai, N.; Yamamoto, Y. Eur. J. Org. Chem. 2009, 1864; (b) Kaboudin, B.;
Mostafalu, R.; Yokomatsu, T. Green Chem. 2013, 15, 2266.
4.4.11. 4,40-Dinitrobiphenyl 2k.16a The compound was prepared
according to the procedure I. White solid, mp 241e243 ꢀC; 1H NMR