Copper-catalyzed synthesis of benzanilides from lignin model substrates 2-phenoxyacetophenones under an air atmosphere

Article abstract of DOI:10.1039/c7nj02589k

The synthesis of chemicals from biomass-derived compounds is an interesting and challenging topic. In this work, using the lignin-derived 2-phenoxyacetophenones as the feedstock we present a novel approach for the synthesis of benzanilides via the reaction of 2-phenoxyacetophenones with anilines catalyzed by CuCl₂ in DMSO at 120 °C under an air atmosphere. This approach has wide scope for 2-phenoxyacetophenones and anilines, and various benzanilides accompanied by the corresponding phenols could be obtained in high yields via changing the 2-phenoxyacetophenones and anilines. The reaction mechanism study indicated that the oxidative cleavage of the C-C bond in 2-phenoxyacetophenones and the formation of a C-N bond occurred simultaneously in the reaction process, resulting in the formation of benzanilides together with phenols.

Full text of DOI:10.1039/c7nj02589k

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Copper-catalyzed synthesis of benzanilides from
lignin model substrates 2-phenoxyacetophenones
under an air atmosphere
Cite this:DOI: 10.1039/c7nj02589k
Xinwei Liu,ab Hongye Zhang,a Cailing Wu,ab Zhenghui Liu,ab Yu Chen,ab Bo Yua and
ab
Zhimin Liu
*
The synthesis of chemicals from biomass-derived compounds is an interesting and challenging topic. In this
work, using the lignin-derived 2-phenoxyacetophenones as the feedstock we present a novel approach for the
synthesis of benzanilides via the reaction of 2-phenoxyacetophenones with anilines catalyzed by CuCl2 in
DMSO at 120 1C under an air atmosphere. This approach has wide scope for 2-phenoxyacetophenones and
anilines, and various benzanilides accompanied by the corresponding phenols could be obtained in high yields
via changing the 2-phenoxyacetophenones and anilines. The reaction mechanism study indicated that the
oxidative cleavage of the C–C bond in 2-phenoxyacetophenones and the formation of a C–N bond occurred
simultaneously in the reaction process, resulting in the formation of benzanilides together with phenols.
Received 16th July 2017,
Accepted 5th December 2017
DOI: 10.1039/c7nj02589k
rsc.li/njc
cleavage of b-O-4 and related conversion have been widely investi-
gated. 2-Phenoxyacetophenones are considered as a kind of lignin-
Introduction
Biomass provides renewable carbon resources, and the utilization derived compound with b-O-4 linkage, which have been applied
of biomass is of significance for sustainable development. The as feedstocks for the production of various aromatics including
catalytic transformation of biomass into chemicals and fuels is a 1-phenylpropane-1,2-dione, benzoic acid, and methyl benzoate.18–23
promising way to green and sustainable development, which has The formation of new chemical bonds, for instance, a C–N bond,
been paid much attention all over the world. Lignin is a kind of using 2-phenoxyacetophenones can widen the applications of
biopolymer in nature with complicated structures that can be lignin in the synthesis of value-added chemicals.
depolymerized mainly into cinnamyl alcohols, p-hydroxycinnamyl
Benzanilides are valuable N-containing compounds with
alcohol, sinapyl alcohol and coniferyl alcohol.1 Therefore, lignin various bioactivities, such as anti-inflammatory,24 anti-tumor25,26
provides a rich resource for the production of aromatics. The and potassium channel activation,27 which are generally produced
production of aromatics from lignin or lignin-derived compounds via petroleum-based routes. In a recent work, 1-benzoylpiperidine
has been widely investigated in the past decades.2,3 Most research was detected as a byproduct in a minor amount from lignin-
studies focus on the direct cleavage of the C–C/C–O bonds in derived compounds under copper/O2 catalysis.28 Herein, we
lignin or lignin-derived compounds via reduction,4–6 oxidation,7–9 present a novel and simple route to synthesize benzanilides
hydrolysis,10,11 thermal cracking12,13 and enzymolysis14,15 to produce via the reaction of 2-phenoxyacetophenones with anilines catalyzed
O-containing chemicals such as aryl ethers, phenols, etc. The by CuCl2 under an oxygen or air atmosphere, as illustrated in
oxidative depolymerization of lignin with O2 is an efficient and clean Scheme 1. In this approach, CuCl2 as the catalyst was very effective
way to cleave C–C bonds, in which transition-metal catalysts for the reaction, and various benzanilides accompanied by phenols
including ruthenium complexes16 and vanadium complexes17 could be obtained in high yields via changing the 2-phenoxy-
are usually used.
acetophenones and anilines. The reaction mechanism study
Due to the complicated structures, lignin is seldom directly indicated that the oxidative cleavage of the C–C bond and the
used as a feedstock for the production of aromatics, while lignin- formation of the C–N bond occurred simultaneously, resulting
derived model compounds or platforms are instead used. The b-O-4 in the formation of benzanilides together with phenols.
structure accounts for ca. 50% linkage in lignin, and thus the
a Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid,
Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China. E-mail: liuzm@iccas.ac.cn
b University of Chinese Academy of Sciences, Beijing 100049, China
Scheme 1 Synthesis of benzanilides via the reaction of 2-phenoxyaceto-
phenones with anilines catalyzed by [Cu] under an O2 atmosphere.
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Experimental section
Materials and methods
CDCl3 or DMSO using an internal reference (the residue proton
peaks of CHCl3 at 77.02 ppm and DMSO at 40.03 ppm) on a
Bruker 400 spectrometer at room temperature.
All reagents and solvents were purchased from Acros, TCI, Alfa-
Aesar, Sigma-Aldrich and Beijing Chemical Company, Ltd. Unless
otherwise noted, materials obtained from commercial suppliers NMR data of the as-synthesized 2-phenoxyacetophenones and
were used without further purification. 2-Phenoxyacetophenones benzanilides
were synthesized as follows.
2-Phenoxyacetophenone. Spectral data are consistent with
those reported in the literature.30 1H NMR (400 MHz, CDCl3) d
Synthesis of 2-phenoxyacetophenones
7.87 (d, J = 7.3 Hz, 1H), 7.65 (d, J = 7.9 Hz, 1H), 7.55 (t, J = 7.3 Hz,
The synthetic reaction was carried out in a 150 mL round-bottle
glass flask equipped with a condenser. 2-Phenoxyacetophenones
1H), 7.48 (t, J = 7.4 Hz, 1H), 7.37 (t, J = 7.9 Hz, 1H), 7.16 (t, J = 7.4
Hz, 1H). 13C NMR (101 MHz, CDCl3) d 194.55 (s), 158.03 (s),
were synthesized via the reaction of the corresponding phenols
134.65 (s), 133.82 (s), 129.56 (s), 128.81 (s), 128.15 (s), 121.65 (s),
114.83 (s), 70.84 (s).
and 2-bromoacetophenones based on the reported procedures.29
Typically, 2-bromoacetophenone (5 mmol) was dissolved in a
solution of K2CO3 (7.5 mmol, 1.036 g) and phenol (5 mmol) in
acetone (50 mL) and loaded in the reactor. The reaction mixture
was then stirred under refluxing temperature for 5 h, and was
filtered and concentrated under vacuum. The residue was purified
using column chromatography with hexane : ethyl acetate (3 : 1),
giving rise to the desired phenoxyacetophenones based on the
corresponding phenols and 2-bromoacetophenones.
2-Phenoxy-1-p-tolylethanone. Spectral data are consistent
with those reported in the literature.31 1H NMR (400 MHz,
CDCl3) d 7.87 (d, J = 7.1 Hz, 1H), 7.80–7.65 (m, 1H), 7.61–7.40
(m, 3H), 7.18 (d, J = 8.2 Hz, 1H), 5.30 (s, 1H), 2.35 (s, 2H).
2-(4-Methoxyphenoxy)-1-(4-methoxyphenyl)ethanone. Spectral
data are consistent with those reported in the literature.32
1H NMR (400 MHz, CDCl3) d 7.99 (d, J = 8.9 Hz, 2H), 6.95 (d,
J = 8.9 Hz, 3H), 6.81 (d, J = 9.1 Hz, 2H), 5.15 (s, 2H), 3.87 (s, 3H),
3.75 (s, 3H). 13C NMR (101 MHz, CDCl3) d 193.41 (s), 163.97 (s),
Synthesis of benzanilides
154.40 (s), 152.31 (s), 130.51 (s), 127.71 (s), 115.92 (s), 114.67 (s),
The synthetic reaction was performed in a 5 mL round-bottle
glass flask equipped with a condenser open to air or connected
to an O2 balloon. Taking the synthesis of benzanilide as an example,
the reaction procedures are as follows. 2-Phenoxyacetophenone
(0.5 mmol), aniline (0.5 mmol), DMSO (1.5 mL) and the catalyst
(0.05 mmol) were loaded into the reactor. After being equipped with
a condenser, the reactor was moved into an oil bath at a designated
temperature (e.g., 120 1C) and kept at that temperature for the
desired reaction time. Subsequently, the reaction solution was
treated to separate the unreacted reactants and products. First, an
aqueous solution of HCl (10 mL, 1.0 M) was added to the reaction
mixture, and then 3 Â 10 mL ethyl acetate was added to extract
organic compounds. Then, the organic phase was washed with
distilled water (3 Â 10 mL), and analyzed by gas chromatography
using n-dodecane as an internal standard to determine the
composition of the reaction solution. The organic phase was
further purified by column chromatography over silica gel to obtain
the desired product (eluent: petroleum ether/ethyl acetate = 10/1),
thus getting the isolated yields of the products. The isolated
113.95 (s), 77.26 (d, J = 11.4 Hz), 77.00 (s), 76.68 (s), 71.67 (s),
55.57 (d, J = 18.1 Hz).
2-(4-Methoxyphenoxy)-1-(3-methoxyphenyl)ethanone. Spectral
data are consistent with those reported in the literature.33
1H NMR (400 MHz, CDCl3) d 7.98 (d, J = 8.8 Hz, 2H), 7.16 (t,
J = 8.4 Hz, 1H), 6.95 (d, J = 8.8 Hz, 2H), 5.18 (s, 2H), 3.86 (s, 3H),
3.76 (s, 3H). 13C NMR (101 MHz, CDCl3) d 192.80 (s), 163.95 (s),
160.80 (s), 159.27 (s), 130.42 (s), 130.15 (s), 129.87 (s), 127.57 (s),
113.92 (s), 107.17 (s), 106.57 (s), 101.43 (s), 77.26 (d, J = 11.7 Hz),
77.00 (s), 76.68 (s), 70.60 (s), 55.41 (s), 55.18 (s).
1-(4-Bromophenyl)-2-phenoxyethanone. Spectral data are
consistent with those reported in the literature.34 1H NMR
(400 MHz, CDCl3) d 7.89 (d, J = 7.5 Hz, 1H), 7.54 (t, J = 7.4
Hz, 1H), 7.42 (t, J = 7.7 Hz, 1H), 7.28 (d, J = 8.7 Hz, 1H), 6.73 (d,
J = 8.8 Hz, 1H), 5.17 (s, 1H). 13C NMR (101 MHz, CDCl3) d 193.96
(s), 157.13 (s), 134.37 (s), 133.97 (s), 132.36 (s), 128.85 (s), 128.03
(s), 116.60 (s), 113.87 (s), 70.82 (s).
Phenylbenzamide. Spectral data are consistent with those
reported in the literature.35 1H NMR (400 MHz, CDCl3) d 7.87
(d, J = 7.3 Hz, 3H), 7.65 (d, J = 7.9 Hz, 2H), 7.52 (dt, J = 27.5, 7.2
Hz, 3H), 7.37 (t, J = 7.9 Hz, 2H), 7.16 (t, J = 7.4 Hz, 1H). 13C NMR
(101 MHz, CDCl3) d 137.92 (s), 135.03 (s), 131.83 (s), 129.10 (s),
1
products were identified by H and 13C NMR analysis.
Characterization
GC analysis was performed on an Agilent 7890B GC system with 128.79 (s), 127.00 (s), 124.57 (s), 120.19 (s).
a HP-INNOWAX capillary column (30 m  0.25 mm  0.25 mm)
4-Methoxyphenol. Spectral data are consistent with those
and an FID detector. The following GC temperature program reported in the literature.36 1H NMR (400 MHz, CDCl3) d
was used: 50 1C hold for 2 min, ramp 20 1C minÀ1 to a final 6.86–6.71 (m, 4H), 5.30 (d, J = 5.5 Hz, 1H), 3.77 (s, 3H). 13C NMR
temperature of 260 1C, and hold for 8 min. Nitrogen was used (101 MHz, CDCl3) d 153.66 (s), 149.53 (s), 116.11 (s), 114.98 (s),
as a carrier gas. The injector temperature was held at 250 1C. 55.87 (s).
Liquid 1H NMR spectra were recorded in CDCl3 or DMSO using
N-p-Tolylbenzamide. Spectral data are consistent with those
an internal reference (the residue proton peaks of CHCl3 at reported in the literature.37 1H NMR (400 MHz, CDCl3) d 7.91–
7.26 ppm and DMSO at 2.5 ppm) on a Bruker 400 spectrometer 7.57 (m, 12H), 7.57–7.41 (m, 18H), 7.18 (d, J = 8.2 Hz, 7H), 4.22
at room temperature. Liquid 13C NMR spectra were recorded in (t, J = 5.9 Hz, 3H), 2.35 (s, 10H). 13C NMR (101 MHz, CDCl3)
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Table 1 Optimization of reaction conditionsa
d 131.73 (s), 129.60 (s), 128.77 (s), 126.97 (s), 120.25 (s),
77.32 (s), 77.00 (s), 76.68 (s), 29.69 (s), 1.00 (s).
4-Methyl-N-phenylbenzamide. Spectral data are consistent
with those reported in the literature.38 1H NMR (400 MHz,
CDCl3) d 7.86 (d, J = 7.3 Hz, 2H), 7.80 (s, 1H), 7.51 (tt, J = 14.8,
7.3 Hz, 5H), 7.17 (d, J = 8.2 Hz, 2H), 2.34 (s, 3H).1H NMR
(400 MHz, CDCl3) d 7.86 (d, J = 7.3 Hz, 2H), 7.80 (s, 1H), 7.51 (tt,
J = 14.8, 7.3 Hz, 5H), 7.17 (d, J = 8.2 Hz, 2H), 2.34 (s, 3H).
4-Methoxy-N-phenylbenzamide. Spectral data are consistent
with those reported in the literature.39 1H NMR (400 MHz,
CDCl3) d 6.86–6.71 (m, 4H), 5.30 (d, J = 5.5 Hz, 1H), 3.77 (s, 3H).
13C NMR (101 MHz, CDCl3) d 153.66 (s), 149.53 (s), 116.11 (s), 115.87
(s), 115.19 (s), 114.98 (s), 77.32 (s), 77.00 (s), 76.68 (s), 55.87 (s).
4-Bromo-N-phenylbenzamide. Spectral data are consistent
with those reported in the literature.40 1H NMR (400 MHz,
DMSO) d 7.99 (d, J = 8.5 Hz, 4H), 7.89–7.80 (m, 8H), 7.44 (t,
J = 7.9 Hz, 4H), 7.19 (t, J = 7.4 Hz, 2H), 3.41 (s, 9H). 13C NMR
(101 MHz, DMSO) d 164.51 (s), 138.93 (s), 134.00 (s), 131.50 (s),
129.61 (s), 128.59 (s), 125.28 (s), 120.53 (s).
Yieldb (%)
Entry
Cat.
Cat. (wt%)
Solvent
T (1C)
3a
4a
1
None
CuCl2
CuCl2
AlCl3
FeCl3
CuBr2
CuCl
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
0
10
10
10
10
10
10
5
15
10
10
10
10
10
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
120
120
120
120
120
120
120
120
120
100
140
120
120
120
0
0
0
0
86
15
9
75
56
68
84
75
87
38
57
24
2c
3
87
21
25
72
64
72
86
77
88
42
44
21
4
5
6
7
8
9
10
11
12
13
14
Toluene
Ethyl acetate
a
3-Methoxy-N-phenylbenzamide. Spectral data are consistent
with those reported in the literature.41 1H NMR (400 MHz,
DMSO) d 10.14 (s, 1H), 8.00–7.93 (m, 2H), 7.70 (d, J = 9.0 Hz,
2H), 7.60–7.47 (m, 3H), 6.97–6.90 (m, 2H), 3.75 (s, 3H). 13C NMR
Conditions: 1a (0.5 mmol), 2a (0.5 mmol), catalyst (10 mol%), DMSO
(1 mL). Isolated yield. The reaction was performed under an argon
atmosphere (1 atm).
b
c
(101 MHz, DMSO) d 165.12 (s), 155.56 (s), 135.08 (s), 132.25 (s), The amount of CuCl2 had a little impact on the reaction
128.44 (s), 128.22 (s), 127.68 (s), 127.41 (s), 122.00 (d, J = 23.1 Hz), (Table 1, entries 3, 8 and 9), and a loading amount of 10 mol%
121.87–121.73 (m), 114.20 (s), 113.28 (s), 55.40 (s), 54.92 (s).
CuCl2 was necessary for a good product yield. The influence
2-Hydroxy-N-phenylbenzamide. Spectral data are consistent of the reaction temperature was investigated in the range
with those reported in the literature.42 1H NMR (400 MHz, of 100–140 1C, and it was demonstrated that 120 1C was
DMSO) d 10.38 (s, 3H), 7.97 (dd, J = 7.9, 1.5 Hz, 3H), 7.70 (d, an appropriate temperature (Table 1, entries 3, 10 and 11).
J = 7.6 Hz, 6H), 7.41 (ddd, J = 25.3, 12.2, 5.0 Hz, 9H), 7.14 (t, The screening of the solvents indicated that DMSO was a
J = 7.4 Hz, 3H), 7.02–6.92 (m, 6H), 3.34 (s, 4H). 13C NMR suitable solvent (Table 1, entries 3 and 12–14), probably
(101 MHz, DMSO) d 166.65 (s), 158.58 (s), 138.16 (s), 129.17 (s), because it has good solvent strength to dissolve the reactants
128.91 (s), 128.63 (s), 121.14 (s), 120.90 (s), 117.44 (s).
and products.
Fig. 1 shows the dependence of the 1a conversion and
product yields on the reaction time. It was indicated that the
reaction initially occurred rapidly, and almost completed within
12 h under the experimental conditions. Two products were
Results and discussion
2-Phenoxyacetophenone (1a) as a model substrate was first simultaneously detected in the reaction process, and their yields
examined to react with aniline (2a) to explore the appropriate increased with the reaction time, reaching almost identical
reaction conditions, and the results are listed in Table 1. It was values as the reaction proceeded for 12 h. Notably, the yield of
indicated that in the absence of a catalyst or O2 no reaction 4a was lower than that of 3a at the shorter reaction time (e.g.,
occurred (Table 1, entries 1 and 2). To our delight, CuCl2 could 4 h), while it was almost identical to that of 4a as reaction time
effectively catalyze the reaction of 1a and 2a in DMSO at 120 1C prolonged (e.g., 8 and 12 h). This implies that the formation of
under an air atmosphere, selectively affording 3a and 4a in 87% 3a occurred more rapidly than that of 4a.
and 86% yields, respectively (Table 1, entry 3). These preliminary
Having the optimized reaction conditions in hand, we
results indicate that the CuCl2 catalyst and O2 were indispensable explored the scopes of the 2-phenoxyacetophenones and anilines
for the reaction. The requirement of O2 indicates that the for the synthesis of various benzanilides, and the results are listed
oxidative cleavage of the C–C bond in the substrate occurred in Table 2. It was indicated that all the tested 2-phenoxyaceto-
during the reaction process. AlCl3, FeCl3and CuBr2 were also phenones could react with anilines under the experimental
effective for the reaction, however, exhibiting much lower activities conditions, producing the corresponding benzanilides and
compared to CuCl2 (Table 1, entries 4–6). For comparison, CuCl phenols in good to excellent yields. These results demonstrated
was also examined, and it was indicated that CuCl showed that various benzanilides could be obtained in satisfactory
lower catalytic activity than CuCl2 (Table 1, entry 7). As for CuCl, yields via changing the 2-phenoxyacetophenones and anilines.
we inferred that Cu(I) species were less oxidative than the Cu(II) The approach presented in this work provides a simple and
species, and under the experimental conditions Cu(I) might be clean way to produce benzanilides, which may have promising
oxidized into Cu(II), which further catalysed the transformation. applications.
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To gain insight into the reaction mechanism, several control
experiments were performed. The reaction of 1a with 2a was
carried out without any catalyst under Ar conditions for 4 h,
and the reaction solution was detected by means of 1H NMR
analysis. As illustrated in Fig. 2, the peak at 5.7 ppm in the
1H NMR spectrum of 1a disappeared after the reaction, implying
the formation of an intermediate enamine.43 In addition, it was
found that formyloxybenzene could transform to phenol and CO2
catalyzed by CuCl2 in DMSO in the presence of O2. Based on the
above results, a possible reaction pathway was proposed as
illustrated in Scheme 2. Firstly, the substrate A reacts with aniline
B, forming enamine C after dehydration. Subsequently, the
single-electron-oxidation of intermediate C with Cu(II)–oxygen
species occurs,44–46 followed by trapping the resultant cation-
radical of C, producing copper peroxide D, which further
cyclizes to afford aminodioxetane E. The concurrent cleavage
of the O–O and C–C bonds in aminodioxetane E results in the
formation of the product benzanilide F and formyloxybenzene
G.47 G further transforms to the product H(phenol) and CO2
under the experimental conditions.
Fig. 1 Dependence of the 1a conversion and the product yields on the
reaction time. Conditions: 1a, 0.5 mmol; 2a, 0.5 mmol; catalyst, 10 mol%,
DMSO, 1 mL.
Table 2 Scopes of 2-phenoxyacetophenones and anilines for the synth-
esis of various benzanilidesa
Substrate
Yieldb (%)
Entry 2-Phenoxyacetophenone Aniline
Benzanilide
Phenol
1
2
3
4
5
6
7
8
Fig. 2 1H NMR spectra of 1a (top) and the solution of 1a reacting with 2a
(bottom).
9
10
a
Reaction conditions: substrate, 0.5 mmol; amine, 0.5 mmol; CuCl2,
b
10 mol%; DMSO, 1 mL; 120 1C, 12 h. Isolated yield.
Scheme 2 Possible reaction pathways.
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17 J. M. W. Chan, S. Bauer, H. Sorek, S. Sreekumar, K. Wang
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B. Yang, Green Chem., 2014, 16, 897.
Conclusions
In summary, we for the first time developed a novel and simple
approach to synthesize benzanilides from the lignin model
substrates 2-phenoxyacetophenones by reacting with anilines
catalysed by CuCl2 in DMSO at 120 1C under an air atmosphere.
This method has wide scope for 2-phenoxyacetophenones and
anilines, and various benzanilides could be obtained in high
yields via changing the 2-phenoxyacetophenones and anilines.
This work opens a new way to acquire aromatics from lignin-
derived feedstocks, which may have promising applications.
23 Z. Strassberger, A. H. Alberts, M. J. Louwerse, S. Tanase and
G. Rothenberg, Green Chem., 2013, 15, 768.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (Grants No. 21373242, and 21125314) and the
Chinese Academy of Sciences (QYZDY-SSW-SLH013-2).
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