Aerobic Oxidative coupling of aniline catalyzed by one-dimensional manganese hydroxide nanomaterials

Article abstract of DOI:10.1055/s-0037-1612108

The aerobic oxidative coupling of aniline is an effective process for producing aromatic azo compounds, which are widely used in the organic chemical industry. The development of heterogeneous catalysts for this reaction would be advantageous because of their recyclability and convenience in posttreatment. In this work, one-dimensional Mn(OH) ₂ nanostructure with various shapes were synthesized through the adjustment of various surfactants. The as-synthesized Mn(OH) ₂ nanobelts and nanowires showed superior catalytic activity in the activation of oxygen and aniline. Aromatic azo compounds with a variety of substituents were produced through the coupling of the corresponding anilines without additives under ambient conditions.

Full text of DOI:10.1055/s-0037-1612108

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
©
Georg Thieme Verlag Stuttgart · New York
2019, 30, A–E
letter
A
en
Synlett
H. Miao et al.
Letter
Aerobic Oxidative Coupling of Aniline Catalyzed by One-Dimen-
sional Manganese Hydroxide Nanomaterials
Hui Miao*
Kelong Ma
Shiwei Hu
Ruiqian Li
Lin Sun
0
0
0
0-0
0
0
1-7
4
5
1-
1
3
1
1
R
NH2
Mn(OH)2 nanobelts
Air
N
N
R
R
1
0 examples
7–99% conversion
87–99% selectivity
R = 2-Me, 3-Me, 4-Me, 4-OMe, 4-OEt
-Cl, 4-Br, 4-F, 4-OCF3
8
4
Yumin Cui
School of Chemistry and Materials Engineering, Fuyang Normal
College, Anhui Provincial Key Laboratory for Degradation and
Monitoring of Pollution of the Environment, Fuyang, 236037, P.
R. of China
huimiao@mail.ahnu.edu.com
Received: 06.12.2018
Accepted after revision: 11.01.2019
Published online: 18.02.2019
are disadvantageous in industrial applications. Therefore,
the development of efficient nonnoble-metal catalysts that
do not require additives would be highly desirable, but is
also challenging.
DOI: 10.1055/s-0037-1612108; Art ID: st-2018-u0792-l
Abstract The aerobic oxidative coupling of aniline is an effective pro-
cess for producing aromatic azo compounds, which are widely used in
the organic chemical industry. The development of heterogeneous cat-
alysts for this reaction would be advantageous because of their recy-
clability and convenience in posttreatment. In this work, one-dimen-
sional Mn(OH)2 nanostructure with various shapes were synthesized
through the adjustment of various surfactants. The as-synthesized
Manganese-based oxide or hydroxide nanomaterials
have shown significant value in the fields of energy and ca-
talysis.1
2–14
Rapid developments in techniques for the
preparation of nanomaterials permit the synthesis of
Mn(OH) nanomaterials with uniform morphologies, sizes,
2
15–17
and surface structures.
Differences in structure give
Mn(OH) nanobelts and nanowires showed superior catalytic activity in
2
rise to different properties; therefore, the development of
materials-synthesis methods is worthy of further research
the activation of oxygen and aniline. Aromatic azo compounds with a
variety of substituents were produced through the coupling of the cor-
responding anilines without additives under ambient conditions.
to meet the increasing demand for performance improve-
ment.1
8–27
One-dimensional nanomaterials, such as nanow-
Key words anilines, azo compounds, heterogeneous catalysis, man-
ganese hydroxide, nanobelts, nanowires
ires, nanorods, or nanotubes, are among the most popular
nanomaterials.2
8,29
These nanomaterials have unique ther-
mal stabilities and mechanical, electronic-transmission,
and photon-transport properties, giving them a broad
range of potential applications.3
Here, we report the synthesis of one-dimensional
Mn(OH)2 nanostructures with various shapes. By using
oleylamine and oleic acid or dodecanol in various ratios,
Aromatic azo compounds and their derivatives have a
wide range of applications in the fine-chemicals industry,
for example, in the production of dyes, pigments, food addi-
0–33
1–3
tives, and pharmaceuticals. The conventional production
methods for these compounds involves the use of toxic oxi-
dants such as nitrite salts or Mn, Pb, or Hg salts in stoichio-
metric amounts, which complicates purification of the
Mn(OH) nanobelts or nanowires with various aspect ratios
2
were obtained. The as-synthesized Mn(OH) nanobelts and
2
4–6
products and is harmful to the environment.
A process
nanowires showed superior catalytic activity in the activa-
tion of oxygen and aniline, permitting the synthesis of aro-
matic azo compounds with a variety of substituents
through oxidative coupling of corresponding anilines in the
absence of additives under ambient conditions.
involving the coupling of aniline by using molecular oxygen
as the oxidant would not only meets the requirements of
green chemistry, but might also achieve better catalytic
performance. For this purpose, Corma and co-workers de-
veloped a Au/TiO catalyst for the synthesis of aromatic azo
compounds. Other catalysts such as Ag, CuBr, CuCl, and Pt
In our syntheses of Mn(OH) , Mn(acac) (acac = acetyl-
2
2
3
7
acetonate) was used as a precursor and oleylamine (OAm)
was employed as solvent, stabilizing surfactant, and reduc-
ing agent. Dodecanol and oleic acid were also used as sur-
factants. The morphologies of the as-obtained samples
were characterized by transition electron microscopy
(TEM). Figure 1(a) shows the typical morphology of
have also been reported to catalyze the oxidative coupling
8–11
of aniline.
However, most of the reported catalytic
methods require stoichiometric amounts of additives such
as NaOH or pyridine, or use precious metals as catalysts. On
the other hand, complex catalyst-preparation procedures
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

B
Synlett
H. Miao et al.
Letter
Mn(OH) nanobelts obtained by using pure oleylamine as
narrow ones were still present [SI; Figure S2(c)]. Many wid-
er nanobelts with structures similar to those of nanosheets
were obtained when 7 mL of dodecanol were added [SI; Fig-
ure S2(d)].
2
solvent. Most of the Mn(OH) nanobelts were ~10 nm in di-
2
ameter and ~80 nm in length. The X-ray diffraction (XRD)
patterns of an as-synthesized sample is shown in Figure
1(b). The main diffraction peaks are consistent with the
We started our initial evaluation of the Mn(OH) nano-
2
standard values for bulk Mn(OH) (JCPDS Card File No. 18-
belts by using aniline 1a as a model substrate for the oxida-
tive coupling reaction (Table 1). The reaction was first test-
ed with toluene as solvent at 80 °C; a 24% conversion was
achieved with 99% selectivity toward azobenzene after 12
hours (Table 1, entry 1). With p-xylene as solvent, a better
conversion (40%) and 99% selectivity were achieved (entry
2). Other solvents (DMF, DMSO, or 1,4-dioxane) gave mod-
erate activities and selectivities (entries 3–5), and byprod-
ucts were formed through the coupling of aniline to the sol-
vent. The conversion increased at elevated temperatures
2
0787). High-resolution TEM (HRTEM) showed that the
growth of Mn(OH) nanobelts occurred along the (001) di-
2
rection [Figure 1(c)]. The clear lattice fringe spacings of
0.243 and 0.286 nm correspond well to the separations be-
tween the (101) and the (100) lattice planes, respectively, of
Mn(OH) [Figure 1(d)].
2
(entries 6–8), and 90% conversion and 98% selectivity were
achieved in p-xylene at 140 °C (entry 8). On increasing the
reaction time (entries 9 and 10), the conversion of aniline
reached 99% with 98% selectivity after 16 hours (entry 10).
Table 1 Aniline Oxidative Coupling Reaction Using Mn(OH) Nano-
2
belts as Catalyst under Various Conditionsa
NH2
N
Mn(OH)2 nanobelt
T, t, air
N
1a
1b
Entry
Solvent
Temp
°C)
Time
(h)
Conversionb Selectivityb
(%) (%)
(
Figure 1 (a) TEM image of the as-prepared Mn(OH) nanobelts synthe-
sized in pure oleylamine; (b) XRD pattern of a typical sample compared
1
2
3
4
5
6
7
8
9
Toluene
80
12
24
99
2
p-xylene
DMF
80
80
12
12
12
12
12
12
12
14
16
40
11
11
2
99
with standard Mn(OH) (JCPDS Card File No. 18-0787); (c) and (d) HR-
2
TEM images of the Mn(OH) nanobelts.
76
2
DMSO
80
79
1,4-dioxane
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
80
99
We then examined the effects of oleic acid on the mor-
phology of Mn(OH) . When the ratio of oleylamine to oleic
100
120
140
140
140
64
80
90
95
99
99
2
acid was 7:1, we obtained nanobelts with a diameter of 5
nm and a length of 100 nm [see Supplementary Informa-
tion (SI); Figure S1(a)]. The diameter decreased on increas-
ing the amount of oleic acid [SI; Figure S1(b)]. Obvious
nanowires [SI; Figure S1(c)] formed on further increasing
the oleic acid concentration. These nanowires were about 2
μm in length, and might represent a transition state to
nanowires. Nanowires were found to assemble on adding
more oleic acid [SI; Figures S1(d) and S1(e)]. Very fine
nanowires of diameter 1 nm and length 1.5 nm [SI; Figure
S1(f)] were formed when the ratio of oleylamine to oleic
acid was 2:6. Thin nanobelts were obtained by using 1–3
99
98
98
98
10
a
Reaction conditions: aniline (1a; 0.5 mmol), Mn(OH)
solvent (50 mL), under air.
Determined by GC and based on the concentration of aniline. Byproducts
were formed by the reaction of aniline with the solvent.
2
nanobelts (50 mg),
b
Next, we compared the activity of Mn(OH) nanobelts
with that of Mn(OH) nanowires and other commercial Mn-
based catalysts. Under the optimized reaction conditions,
the nanowires showed a similar, but slightly poorer perfor-
mance, compared with the nanobelts (Figure 2). Only 12%
conversion and 86% selectivity were achieved when the
precursor Mn(acac) was used as a catalyst. A reaction with
commercially available γ-MnO2 produced the target azo-
benzene in 53% conversion and 94% selectivity, and similar
2
2
mL of dodecanol, but the diameters of these Mn(OH) nano-
2
belts were larger than those of nanobelts obtained by using
pure oleylamine [SI; Figure S2(a)]. On further increasing the
amount of dodecanol, aggregation of the nanobelts oc-
curred [SI; Figure S2(b)]. Wider nanobelts were obtained on
increasing the amount of dodecanol, although there some
3
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

C
Synlett
H. Miao et al.
Letter
performance was obtained by using commercial Mn O as
substituents (Scheme 1, 2b–5b) or electron-deficient
groups (6b–9b) were smoothly converted into the corre-
sponding aromatic azo compounds in excellent yields and
with excellent selectivities. It is noteworthy that halo-sub-
stituted anilines survived well, leading to halo-substituted
aromatic azo compounds, which could be used as substrate
for further transformations, such as cross-coupling reac-
tions.3
3
4
catalyst. When we used activated MnO (Aldrich) with a
2
high surface area as a catalyst for the aniline coupling reac-
tion with a the reaction time of six hours, the activated
MnO gave a 63% conversion of aniline, whereas Mn(OH)2
2
nanobelts gave 42% conversion (SI; Table S1); however, the
selectivity with the activated MnO catalyst was only 32%,
2
4,35
and byproducts such as nitrosobenzene were formed. On
increasing the reaction time to 16 hours, the Mn(OH) na-
2
nobelts gave almost complete conversion, whereas com-
R
N
mercial MnO gave 71% conversion with 29% selectivity. On
NH2 Mn(OH)2 nanobelts
N
2
R
R
the basis of these results, we speculate that the commercial
140 °C, 16 h, air
activated MnO participates in the reaction as a stoichio-
metric oxidant rather than as a catalyst.
2
2–10a
2–10b
N
N
N
N
2b: 99/99
3b: 98/90
OMe
N
N
N
N
MeO
4b: 99/91
5b: 99/99
OEt
Cl
N
N
N
N
EtO
Cl
Figure 2 Comparison of various Mn-based catalysts for the oxidative
coupling of aniline. Reaction conditions: aniline (0.5mmol), p-xylene (5
mL), 140 °C, 16 h, under air.
6b: 96/97
7b: 92/92
Br
F
N
N
N
N
The specific surface area of one-dimensional Mn(OH)2
nanomaterials and of bulk manganese oxide were deter-
mined (for detailed data, see SI; Table S2). The specific sur-
face area of the bulk materials was very small, whereas the
one-dimensional Mn(OH)2 nanomaterial of various mor-
phologies had similarly large specific surface areas. There-
fore, we believe that the difference in catalytic performance
is due to the huge difference in specific surface area be-
tween the bulk material and the one-dimensional nanoma-
terials. More catalytically active sites are exposed in one-di-
Br
F
8b: 92/88
9b: 88/89
OCF3
N
N
F3CO
1
0b: 89/87
Scheme 1 Scope of the oxidative coupling reaction for aromatic azo
compounds catalyzed by Mn(OH) nanobelts under the optimized con-
ditions. Reaction conditions: aniline 2a–10a (0.5 mmol), Mn(OH) nano-
2
2
mensional Mn(OH) nanomaterials with high specific sur-
belts (50 mg), p-xylene (5 mL), under air, 140 °C, 16 h. The percentage
conversion and selectivity are shown below each compound (e.g.,
2
face areas, which is conducive to the adsorption, activation,
and desorption of substrate molecules. On the other hand,
the facet effect caused by morphological differences might
also have an important effect. Because the catalytic reaction
involves the activation of oxygen, oxygen vacancies on the
surface of materials, which can activate oxygen molecules,
play an important role. Compared with bulk materials, one-
dimensional materials expose more oxygen vacancies,
thereby promoting the catalytic reaction.
99/99); these were determined by GC and are based on the concentra-
tion of the aniline.
The Mn(OH)2 nanobelts catalyst showed outstanding
stability and recyclability, with no obvious decline in activi-
ty and selectivity after five consecutive runs [Figure 3(a)]. A
TEM image [Figure 3(b)] and XRD characterization [Figure
3(c)] revealed that after five cycles, Mn(OH) retained its
2
With the optimized reaction conditions in hand, we
tested anilines with various substituents in the reaction.
Aniline derivatives with electron-rich para-, meta-, or ortho-
nanobelt structure and the used catalyst had the same crys-
talline structure as the fresh catalyst. The recovery yield of
the catalyst was about 98%. The loss was partly caused by
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

D
Synlett
H. Miao et al.
Letter
the centrifugal separation process. The amount of manga-
nese species leached into in the filtrate was determined by
inductively coupled plasma atomic-emission spectroscopy,
which showed that 0.57 μg Mn was present upper solvent,
so 0.002% of Mn was lost. The detected Mn might be due to
traces of catalyst residues in the liquid.
Next, the reaction mixture was divided into two parts
after six hours of reaction. At this point, GC showed that a
4
5% conversion had occurred. The Mn(OH) catalyst was re-
2
moved from one sample by centrifugation, while the other
was left untouched. After stirring the two samples for addi-
tional ten hours, no further conversion of aniline to azoben-
zene was detected in the sample from which the Mn(OH)2
catalyst had been removed, whereas in the untouched sam-
ple, aniline was fully converted into azobenzene. These re-
sults strongly suggest a heterogeneous nature for our
Mn(OH)2 catalyst. To probe the reaction mechanism, we
carried out several control experiments (see SI; Table S3).
Only a trace amount of product was obtained when the re-
Figure 3 (a) Recycling of Mn(OH) nanobelts in the oxidative coupling of
2
aniline. (b) TEM image of Mn(OH) nanobelts after five catalytic cycles.
2
(c) XRD patterns of fresh and spent Mn(OH) nanobelts. (d) Proposed
2
mechanism for the Mn(OH) nanobelt-catalyzed oxidative coupling of
2
aniline.
action was performed under N , and a higher conversion
2
was observed for reactions conducted under oxygen than
under air, confirming the participation of oxygen in the re-
action. These results indicate the importance of molecular
oxygen in the reaction. To further investigate the mecha-
Funding Information
nism of the Mn(OH) -catalyzed oxidative coupling of ani-
2
This work was supported by the National Natural Science Foundation
of China (21402029), the Natural Science Foundation of Higher Edu-
cation Institutions in Anhui Province (KJ2018A0332, KJ2018A0340),
the Natural Science Foundation of Anhui Province (1608085MB34),
the FYNC and Government Cooperation Foundation (XDHX2016013),
and the Students Research Training Program (201810371053,
line, the reaction was initiated in the presence of butylated
hydroxytoluene (BHT), a radical scavenger, especially for
•–
the superoxide radical anion (O2 ). No azobenzene was
formed, suggest that that the reaction progresses via radical
species. We proposed that, under catalytic conditions, the
201710371024, 201710371088).
N
ati
o
n
a
l Natural S
c
i
e
n
c
e
F
o
u
n
d
ati
o
n
of
C
h
i
n
a
(2
1
4
0
2
0
2
9)Natural S
c
i
e
n
c
e
F
o
u
n
d
ati
o
n
of
A
n
h
u
iPro
v
i
n
c
e
(K
J
2
0
1
8
A
0
3
3
2)Natural S
c
i
e
n
c
e
F
o
u
n
d
ati
o
n
of
A
n
h
u
iPro
v
i
n
c
e
(1
6
0
8
0
8
5
M
B
3
4)
Mn(OH) nanobelts can activate molecular oxygen to form a
2
superoxide ion radical. On the basis of the results described
above, we propose the mechanism for Mn(OH) nanobelt-
2
Supporting Information
catalyzed oxidative coupling of aniline that is shown in Fig-
1
1,36
ure 3(c).
We assume that under catalytic conditions, the
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1612108.
•–
S
u
p
p
orti
n
g Inform ati
o
n
S
u
p
p
orti
n
g Inform ati
o
n
Mn(OH) nanobelts can reduce adsorbed O to O by a sin-
2
2
2
gle-electron transfer from the lone pair of electrons on ani-
line, giving rise to an aniline radical cation A. Subsequent
coupling between A and an aniline molecule probably leads
to the formation of intermediate B. Subsequent oxidation of
intermediate B gives the hydrazine intermediate C, which is
rapidly oxidized to the azo product 1b.
References and Notes
(1) Anderson, R. G.; Nickless, G. Analyst 1967, 92, 207.
(
2) Ma, H.; Li, W.; Wang, J.; Xiao, G.; Gong, Y.; Qi, C.; Feng, Y.; Li, X.;
Bao, Z.; Cao, W.; Sun, Q.; Veaceslav, C.; Wang, F.; Lei, Z. Tetrahe-
dron 2012, 68, 8358.
In summary, novel one-dimensional Mn(OH)2 nano-
structures, including nanobelts and nanowires, were syn-
(3) Merino, E. Chem. Soc. Rev. 2011, 40, 3835.
(4) Orito, K.; Hatakeyama, T.; Takeo, M.; Uchiito, S.; Tokuda, M.;
Suginome, H. Tetrahedron 1998, 54, 8403.
37
thesized and used in the oxidative coupling of anilines
with high activity, high selectivity, and good catalyst stabil-
(5) Baer, E.; Tosoni, A. L. J. Am. Chem. Soc. 1956, 78, 2857.
(6) Pratt, E. F.; McGovern, T. P. J. Org. Chem. 1964, 29, 1540.
38
ity. The enhanced activity was attributed to the effective
activation of molecular oxygen to the superoxide ion radi-
cal, which reacts rapidly with anilines. This study provides
an efficient, economic, and environmentally friendly cata-
lytic system for synthesizing aromatic azo compounds.
(7) Grirrane, A.; Corma, A.; García, H. Science 2008, 322, 1661.
(8) Zhang, C.; Jiao, N. Angew. Chem. Int. Ed. 2010, 49, 6174.
(9) Lu, W.; Xi, C. Tetrahedron Lett. 2008, 49, 4011.
(
(
(
10) Hu, L.; Cao, X.; Chen, L.; Zheng, J.; Lu, J.; Sun, X.; Gu, H. Chem.
Commun. 2012, 48, 3445.
11) Cai, S.; Rong, H.; Yu, X.; Liu, X.; Wang, D.; He, W.; Li, Y. ACS Catal.
2013, 3, 478.
12) Zhang, K.; Han, X.; Hu, Z.; Zhang, X.; Tao, Z.; Chen, J. Chem. Soc.
Rev. 2015, 44, 699.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

E
Synlett
H. Miao et al.
Letter
(
(
13) Wang, L.; Wang, D.-L. Electrochim. Acta 2011, 56, 5010.
14) Wang, M.; Ma, J.; Yu, M.; Zhang, Z.; Wang, F. Catal. Sci. Technol.
016, 6, 1940.
(30) Rivière, P.; Nypelö, T. E.; Obersriebnig, M.; Bock, H.; Müller, M.;
Mundigler, N.; Wimmer, R. J. Thermoplast. Compos. Mater. 2017,
30, 1615.
2
(
15) Tang, Q.; Gong, X.; Wu, C.; Chen, Y.; Borgna, A.; Yang, Y. Catal.
Commun. 2009, 10, 1122.
(31) Hochbaum, A. I.; Chen, R.; Diaz Delgado, R.; Liang, W.; Garnett,
E. C.; Najarian, M.; Majumdar, A.; Yang, P. Nature 2008, 451, 163.
(32) Xiao, F.-X.; Miao, J.; Tao, H. B.; Hung, S.-F.; Wang, H.-Y.; Yang, H.
B.; Chen, J.; Chen, R.; Liu, B. Small 2015, 11, 2115.
(16) Yan, D.; Li, Y.; Liu, Y.; Zhuo, R.; Wu, Z.; Geng, B.; Wang, J.; Ren, P.;
Yan, P.; Geng, Z. Mater. Lett. 2014, 136, 7.
(
(
17) Sun, W.; Hsu, A.; Chen, R. J. Power Sources 2011, 196, 627.
18) Fang, H.; Zhang, S.; Wu, X.; Liu, W.; Wen, B.; Du, Z.; Jiang, T.
J. Power Sources 2013, 235, 95.
(33) Ge, M.; Cao, C.; Huang, J.; Li, S.; Chen, Z.; Zhang, K.-Q.; Al-Deyab,
S. S.; Lai, Y. J. Mater. Chem. A 2016, 4, 6772.
(34) Chen, Z.; Wang, S.; Lian, C.; Liu, Y.; Wang, D.; Chen, C.; Peng, Q.;
Li, Y. Chem. Asian J. 2016, 11, 351.
(
(
(
(
(
(
(
(
(
(
(
19) Lei, S.; Tang, K.; Fang, Z.; Zheng, H. Cryst. Growth Des. 2006, 6,
1
757.
(35) Han, F.-S. Chem. Soc. Rev. 2013, 42, 5270.
(36) Dutta, B.; Biswas, S.; Sharma, V.; Savage, N. O.; Alpay, S. P.; Suib,
S. L. Angew. Chem. Int. Ed. 2016, 55, 2171.
20) Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T. Angew.
Chem. Int. Ed. 2004, 43, 1115.
21) Shen, Y. F.; Zerger, R. P.; DeGuzman, R. N.; Suib, S. L.; McCurdy,
L.; Potter, D. I.; O’Young, C. L. Science 1993, 260, 511.
22) Yang, Z.; Gong, J.; Tang, C.; Zhu, W.; Cheng, Z.; Jiang, J.; Ma, A.;
Ding, Q. J. Mater. Sci.: Mater. Electron. 2017, 28, 17533.
23) Zhu, J.; Tang, S.; Xie, H.; Dai, Y.; Meng, X. ACS Appl. Mater. Inter-
faces 2014, 6, 17637.
(37) Mn(OH)2 Nanobelts; Typical Procedure
Mn(acac)3 powder (100 mg) was added to a 10 mL autoclave
containing oleylamine (9 mL) with agitation. The clear solution
was sealed into the autoclave and hydrothermally treated at
180 °C for 18 h, then cooled to r.t. The product was washed with
EtOH (3 × 25 mL) then dispersed in cyclohexane (10 mL).
(38) Azobenzene (1b); Typical Procedure
24) Yan, J.; Wang, Q.; Wei, T.; Fan, Z. Adv. Energy Mater. 2014, 4,
1
300816.
A glass reactor was charged with aniline (1a; 0.5 mmol),
Mn(OH ) nanobelts (50 mg), and p-xylene (50 mL) under O (1
25) Jiao, F.; Harrison, A.; Hill, A. H.; Bruce, P. G. Adv. Mater. (Wein-
heim, Ger.) 2007, 19, 4063.
26) Zhang, X.; Meng, X.; Gong, S.; Li, P.; Jin, L. e.; Cao, Q. Mater. Lett.
2
2
bar), and the mixture was vigorously stirred at 140 °C for 16 h.
The reaction system was then cooled to r.t., and the catalyst was
separated from the mixture by centrifugation. The conversion
was determined by GC. The supernatant was purified by flash
chromatography on a short silica gel (eluent: petroluem ester)
2
016, 179, 73.
27) Yin, B.; Zhang, S.; Jiang, H.; Qu, F.; Wu, X. J. Mater. Chem. A 2015,
, 5722.
3
28) Langley, D.; Giusti, G.; Mayousse, C.; Celle, C.; Bellet, D.;
Simonato, J.-P. Nanotechnology 2013, 24, 452001.
29) Liu, S.; Tang, Z.-R.; Sun, Y.; Colmenares, J. C.; Xu, Y.-J. Chem. Soc.
Rev. 2015, 44, 5053.
to give an orange solid; yield: 88%, 40 mg.
1
H NMR (400 MHz, CDCl , TMS): δ = 7.93–7.92 (d, 4 H), 7.54–
3
13
7.46 (m, 6 H). C NMR (100 MHz, CDCl ): δ = 152.66, 130.99,
3
129.09, 122.84.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E