An efficient method to prepare aryl acetates by the carbonylation of aryl methyl ethers or phenols

Article abstract of DOI:10.1039/d0nj05050d

Synthesis of valuable chemicals from lignin based compounds is critical for the application of biomass. Here, we develop a method of preparing aryl acetates by the carbonylation of aryl methyl ethers or phenols under low CO pressure. Good to excellent yields of aryl acetates were obtained using different substrates, and a possible reaction mechanism was proposed by conducting a series of control experiments. This method may provide a potential way for the utilization of lignin.

Full text of DOI:10.1039/d0nj05050d

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An efficient method to prepare aryl acetates by
the carbonylation of aryl methyl ethers or
phenols†
Cite this: New J. Chem., 2021,
45, 2683
a
Dejin Zhang,ab Guoqiang Yang,a Junping Xiong,a Jia Liu,
Xingbang Hu *a and
Zhibing Zhang*a
Synthesis of valuable chemicals from lignin based compounds is critical for the application of biomass.
Here, we develop a method of preparing aryl acetates by the carbonylation of aryl methyl ethers or
phenols under low CO pressure. Good to excellent yields of aryl acetates were obtained using different
substrates, and a possible reaction mechanism was proposed by conducting a series of control
experiments. This method may provide a potential way for the utilization of lignin.
Received 14th October 2020,
Accepted 16th December 2020
DOI: 10.1039/d0nj05050d
rsc.li/njc
preparation of aryl acetates is of great importance. In tradi-
tional studies, acetic anhydride or acyl halides were used as
Introduction
With the depletion of fossil fuels, renewable resources such as starting materials in the presence of base (such as NaOH). Such
biomass are expected to serve as feedstock in the preparation of methods are less environmentally friendly and corrosive
energy and chemicals.1 Lignin, consisting of methoxylated (Scheme 1a).7 In 1995, the acylation of anisole with carboxylic
phenylpropanoid units, is the only large volume renewable acids was reported, HZSM-5 zeolite was used as a catalyst,
feedstock that comprises aromatics.2 In consideration of the phenyl carboxylic ester and 4-acyl anisole were obtained at
importance of aromatic compounds in many key commercial different temperatures.8 In addition, a new acetylation procedure
chemicals, the transformation of lignin and its derivatives has was developed. Demethylation of aryl methyl ethers with BBr3
attracted much attention.3 However, the utilization of lignin to and acetylation of phenols with acetic acid were combined, and
prepare various valuable chemicals remains challenging this method would be useful in the functional group changing
because of its natural complexity and notorious inactive from OMe to OAc.9 As a cheap and abundant C1 resource, carbon
properties.4 In general, two steps need to be performed before monoxide (CO) has attracted much attention in recent years.10
the application of lignin: depolymerization of lignin and trans- However, to the best of our knowledge, there were only a few
formation of depolymerization products into value-added com- studies reported on preparing aryl acetates with CO as feedstock.
pounds. At present, much attention has been paid to the The carbonylation of aryl methyl ethers to synthesise aryl acetates
depolymerization of lignin. Phenols and aryl methyl ethers was reported by Mei;11 LiI and LiBF4 were employed to promote
were obtained in most of the reported processes.5 Therefore, the cleavage of ether bonds, and good yields of aryl acetates were
the conversion of phenols and aryl methyl ethers into valuable
chemicals is significant to the emergence of a sustainable
industry based on lignin.
Aryl acetates are important intermediates in the pharma-
ceutical industry. Many functional medicines prepared from
aryl acetates are making a significant contribution to the clinic
treatment of hepatitis and cholecystitis, and besides, they are
also effective in the detection of cancer.6 Hence, the efficient
a School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, P. R. China. E-mail: huxb@nju.edu.cn, zbzhang@nju.edu.cn
b School of Chemistry and Chemical Engineering, Suzhou University, Suzhou, Anhui
234000, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Synthesis of aryl acetates.
d0nj05050d
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Table 1 Optimization of the reaction conditionsa
obtained. However, to our surprise, the phenolic hydroxyl group
was not favorable for the transformation into esters in the current
reaction system, although 2 MPa CO was used (Scheme 1b).
Here, we reported a new efficient catalytic system to prepare
aryl acetates with good yields using phenols or aryl methyl as
the starting materials. The reactions performed quite well at
low pressure (5 bar of CO). Compared with the traditional
methods, this method may be more economical and environ-
mentally friendly. Considering that the phenolic hydroxyl
group and the methoxy group are the main functional groups
in the depolymerization products of lignin, the reported
method is a potential way to the utilization of lignin.
Entry
Catalyst
Solvent
Yieldg/(%)
1
2
IrCl3
—
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Benzene
Toluene
DMF
1,4-Dioxane
Heptane
MeCN
93
0
47
82
93
0
0
0
88
86
0
3b
4c
5d
6e
7
IrCl3
IrCl3
IrCl3
IrCl3
CoCl2
NiCl2
IrCl3
IrCl3
IrCl3
IrCl3
IrCl3
[Ir(COD)Cl]2
IrCl3
8
9
10
11
12
13
14
15f
Experimental
Chemicals
0
80
92
17
MeCN
CO (99.99%) was purchased from Nanjing Tianze Co. Ltd, IrCl3
was obtained from Energy Chemical Co. Ltd, and other chemicals
a
Standard conditions: 2 mmol anisole, 2 mol% IrCl3 as a catalyst, 5 mL
were purchased from commercial companies and no further of solvent, 5 mol% PPh3, 3 mmol CH3I, 5 bar CO, 180 1C, 20 h. Yields
b
c
were determined by GC and GC–MS. 150 1C. 1 mol% IrCl3 was used.
10 bar CO. 1 bar CO. 3 mmol HI was used instead of CH3I. GC
purification was done.
d
e
f
g
yield.
Carbonylation of aryl methyl ethers or phenols
The reaction was carried out in a high-pressure autoclave
(50 mL) with a magnetic stirrer. In a typical experiment, 2 mmol
aryl methyl ethers or phenols, 2 mol% IrCl3, 5 mol% PPh3 and
3 mmol CH3I were loaded into a reactor. Subsequently, the
reactor was purged with CO to remove air, and then, CO was
charged into an autoclave to 5 bar. Finally, the reactor was
heated to 180 1C and stirred for 20 hours. After the reaction, the
reactor was cooled to room temperature. Silica gel column
chromatography was used to purify the products with petroleum
investigated. The higher pressure of CO cannot further increase
the yield of phenylacetate. But, there was no phenylacetate
generated when the pressure of CO was as low as 1 bar (entries
5 and 6). CoCl2 and NiCl2 were employed to investigate the
catalytic performances and the results indicated that both
CoCl2 and NiCl2 have no catalytic activity under the standard
conditions (entries 7 and 8). The solvent has a remarkable
influence on the yield of phenylacetate. MeCN gave the best
result among the solvents investigated here (entries 9–13).
Additionally, [Ir(COD)Cl]2 was employed to catalyze the reac-
tion; however, there was no clear difference between IrCl3 and
[Ir(COD)Cl]2 (entry 14). The ligand is of great importance to
change the electron density of the metal. Therefore, the effect
of different ligands on the yields of phenylacetate was explored
(Fig. 1). The results indicated that PPh3 was more suitable for
ether and ethyl acetate mixture as the eluent. GC–MS, 1H and 13
NMR were employed to characterize the products.
C
Analysis methods
GC-2014 (Shimadzu, Japan) equipped with an RTX-5 column
(30 m ꢀ 250 mm) was used to determine the yields of the
products. GCMS-QP 2030 NX (Shimadzu, Japan) and Bruker
Avance II 400 MHz NMR spectrometers were employed to prove
the structure of the reaction products.
Results and discussion
Optimization of the reaction conditions
In a first reaction, the carbonylation of anisole to phenylacetate
was used as a probe reaction to optimize the conditions
(Table 1). Up to 93% yield of phenylacetate can be obtained
in the presence of 2 mol% IrCl3 (entry 1). The control reaction
in the absence of IrCl3 cannot happen at all (entry 2). The
decrease of the reaction temperature was attempted and a
lower yield of 47% was obtained at 150 1C (entry 3). In addition,
the dosage of IrCl3 was also explored. 82% yield of phenylacetate
can be obtained with 1 mol% catalyst (entry 4). The pressure of
CO is of great importance to the carbonylation reaction. Hence,
the effect of the CO pressure on the yield of phenylacetate was
Fig. 1 Yields of phenylacetate catalyzed by IrCl3 with different ligands.
Reaction conditions: 2 mmol anisole, 2 mol% IrCl3, 5 mL MeCN, 5 mol%
ligand, 3 mmol CH3I, 5 bar CO, 180 1C, 20 h. (A) No ligand, (B) 2,4,6-
collidine, (C) 1,3-bis(diphenylphosphino)propane, (D) 2,20-bipyridine, and
(E) PPh3.
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the preparation of phenylacetate even when only 2 mmol% donating or an electron-withdrawing group have no significant
IrCl3 was used.
influence on the yields of the target products.
Scope of substrates
Scope of phenols
Scope of aryl methyl ethers. With the optimized reaction As we all know, the phenolic hydroxyl group is also an impor-
conditions in hand, a variety of aryl methyl ethers containing tant part of lignin. Hence, a variety of phenols with different
different functional groups were employed to investigate the functional groups were adopted to investigate the reaction of
scope of substrates (Table 2). In general, the corresponding aryl the phenolic hydroxyl group (Table 3). To our delight, the
acetates were obtained with good to excellent yields. A variety of corresponding aryl acetates were obtained with excellent yields.
substituents, such as methyl (entries 1, 2, 7 and 8), isopropyl However, compared with using aryl methyl ethers as substrates,
(entry 3), tert-butyl (entry 5), –CN (entry 6) and halogens (entries CH3I was consumed to produce aryl acetate when phenols were
9 and 10), were tolerated. The substrates with an electron- adopted.
Table 3 Scope of phenols
Table 2 Scope of aryl methyl ethers
Entry
1
Substrates
Products
Yields/(%)
95
Entry
1
Substrates
Products
Yields/(%)
90
2
3
95
92
2
79
3
93
91
66
4
78
75
92
65
4
5a
5
6
88
73
6
7a
7b
8
9
83
85
8b
75
9a
76
57
10
11
78
83
10a
Conditions: 2 mmol aryl methyl ethers, 2 mol% IrCl3, 5 mL MeCN, Conditions: 2 mmol phenols, 2 mol% IrCl3, 5 mL MeCN, 5 mol% PPh3,
5 mol% PPh3, 3 mmol CH3I, 5 bar CO, 180 1C, 20 h. Isolated yields were 3 mmol CH3I, 5 bar CO, 180 1C, 20 h. Isolated yields were reported.
a
a
b
reported. Yield was determined by 1H NMR.
Yield was determined by 1H NMR. 1 mmol substrate was used.
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Scheme 2 Reactions with the lignin monomers A, B and C.
Fig. 2 Proposed mechanism.
Reactions with lignin monomers
CO in our experiments, phenylacetate was obtained at room
temperature with phenol as the substrate (Scheme 3B). However,
no phenylacetate was formed at room temperature when anisole
was used. When the same reaction was performed at 180 1C,
phenylacetate can be produced (Schemes 3C and D). The high
dissociation energy of the C–O bond may be responsible for the
difference.13
To demonstrate the ability of the catalytic system to promote
the conversion from lignin based materials to aryl acetate, the
lignin monomers guaiacol (A), 4-methoxyphenol (B) and syringol
(C) were investigated in our experiments (Scheme 2). 89% yield
of 1,2-phenylene diacetate was obtained when guaiacol was used.
Both the phenolic hydroxyl group and the methoxy group can
convert to ester at the same time. Compared with guaiacol,
a similar result was obtained when 4-methoxyphenol was
employed, and 91% yield of the corresponding product was
obtained under standard reaction conditions. For the reaction
of syringol, there products with a ratio of 39 : 30 : 31 can be
obtained. More CH3I are required if we want to increase the
selectivity of the triple-ester product (49% yield of triple-ester
product when 5 mmol CH3I was used).
Based on the control experiments and previous studies,14
a
possible reaction mechanism was proposed (Fig. 2).
[Ir(CO)2Cl2]ꢁ was generated in situ by the reaction between
IrCl3 and CO. The oxidative addition of CH3I with A happens,
which might be the rate-determining step in the carbonylation
process. Then, an acetyl group (C) was generated from B with
the migration of methyl, and coordination of CO occurred
immediately. Subsequently, the complex D underwent reduc-
tive elimination to produce the acetyl iodide and catalytic active
species A. Finally, acetyl iodide can react with aryl methyl ethers
to produce aryl acetate, and CH3I was regenerated in situ.
Reaction mechanism
Several control experiments were performed to gain more
mechanistic insight into the reaction pathway. Under standard
conditions, 3 mmol of CH3I was added. In order to confirm the
cleavage of ether bonds, iodoethane was added. It was interesting
to find that there was still 35% of phenylacetate existing in the
reaction solution, and the other product was phenyl propionate.
Hence, we can conclude that CH3I is not consumed in the
reaction system. The methoxy group in anisole will transform
to CH3I in situ (Scheme 3A). Subsequently, according to the
literature reported by Voronkov,12 acetyl iodide was employed
to explore the reaction pathway. When there were no CH3I and
Conclusions
In conclusion, we have developed a new catalytic system for the
preparation of aryl acetates under low CO pressure. Good to
excellent yields of aryl acetates can be obtained with different
phenols, aryl methyl ethers and lignin based materials. This
method may provide an effective way for the preparation of aryl
acetates and the utilization of lignin.
Conflicts of interest
We have no conflicts of interest to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21776122, 21878141, and 21576129)
and the Jiangsu Province NSF (BE2019095 and BM2018007).
The authors also want to thank Mr Shouyan Shao, Guisheng
Zhu, and Peijun Liu from Jiangsu SOPO (Group) CO., LTD for
their support with the research.
Scheme 3 Controlled experiments.
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