Copper-Catalyzed Acetylation of Electron-Rich Phenols and Anilines

Article abstract of DOI:10.1055/s-0037-1611741

An approach has been developed for the copper-catalyzed acetylation of phenols and anilines with potassium thioacetate as an acetylating reagent. Although only electron-rich phenols and anilines are compatible with this protocol, the reaction can provide moderate to high yields under mild conditions. Compared with other acetylating reagents, the current reagent has certain advantages, such as its low cost, easy availability, stability, insensitivity to water or air, and ease of storage.

Full text of DOI:10.1055/s-0037-1611741

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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–E
letter
A
en
J. Zhang et al.
Letter
Synlett
Copper-Catalyzed Acetylation of Electron-Rich Phenols and
Anilines
Jieyu Zhang ◊
Qiumin Ke ◊
Feitao Tian
Bei Jiang
XH
X
O
Cu(OAc)2⋅H2O (0.2 equiv)
R
R
+
O
SK MeCN, 80 °C, air, 8 h
X = NH, O
Yields : 50–93%
17 examples
Chang-An Ji
Lingling Zhang
Jian Yu*
Dayun Huang*
Guobing Yan*
Department of Chemistry, Lishui University, Lishui 323000,
P. R. of China
livelyfish@lsu.edu.cn
dayunhuang@lsu.edu.cn
gbyan@lsu.edu.cn
◊These authors contributed equally.
Received: 21.01.2019
Accepted after revision: 05.02.2019
Published online: 27.02.2019
my, and sensitivity to water or air. To comply with the prin-
ciples of green synthesis and atomic economy, inexpensive
carboxylic acids5, esters,6 and alcohols7 have been devel-
oped as reagents for acetylation reactions. For examples,
Thakur and co-workers reported the synthesis of acetamide
with acetic acid catalyzed by nanoparticulate Al2O3 under
solvent-free conditions.5a Sanz Sharley and Williams
showed that acetic acid is an effective catalyst for the
acetylation of amines with either ethyl acetate or butyl ace-
tate as the acetyl source.6b Zhang and co-workers reported
the aerobic oxidative coupling of amines with alcohols cat-
alyzed by resin-supported Au–Pd nano-alloy catalysts, with
the alcohols acting as acylating agents.7 However, the rela-
tively low reactivity of acids or esters and the reversible na-
ture of the reaction equilibrium resulted in only partial
conversion. Therefore, the continuous removal of water or
alcohols is a critical factor in driving the reaction. To over-
come these problems, the use of enol esters as acylating
agents has recently been reported; in this reaction, the enol
byproduct formed in the reaction was rapidly transformed
into the corresponding aldehyde, which rapidly escaped
from the equilibrium system.8 Sharma and co-workers re-
ported a DABCO-catalyzed esterification of phenols with vi-
nyl acetate as an acetylating reagent under microwave con-
ditions.8b In addition, 1,3-diketone compounds have been
employed in acetylations through cleavage of the C–C single
bond, releasing a ketone molecule.9 Wang and co-workers
developed a metal-free method for the acetylation of ani-
lines with 1,3-diketones as acylating reagents promoted by
H2O2 as a sole oxidant at room temperature in water.9a Fol-
lowing this work, various catalytic systems were explored
for the acetylation of anilines with 1,3-diketones.9c,d In ac-
DOI: 10.1055/s-0037-1611741; Art ID: st-2019-u0036-l
Abstract An approach has been developed for the copper-catalyzed
acetylation of phenols and anilines with potassium thioacetate as an
acetylating reagent. Although only electron-rich phenols and anilines
are compatible with this protocol, the reaction can provide moderate to
high yields under mild conditions. Compared with other acetylating re-
agents, the current reagent has certain advantages, such as its low cost,
easy availability, stability, insensitivity to water or air, and ease of stor-
age.
Key words acetylation, phenols, anilines, copper catalysis, potassium
thioacetate, butyl nitrite
The acetylation of phenols and anilines is a fundamental
transformation in organic synthesis, as this is a common
method for protecting these functional groups protected
during multistep syntheses.1 Aryl acetates and acetamides
are also widely used as agrochemicals, medicines, preserva-
tives, plasticizers, or polymer components.2
Much effort has been devoted to the study of acetylation
reactions, and various acetylating reagents have been ex-
plored for this transformation in recent decades. However,
there is still a considerable demand for the development of
inexpensive, simple, and ecofriendly acylation processes.
Acetic anhydride3 and acetyl chloride4 are the most
commonly used sources of acetyl groups. Transformations
using these reagents are usually performed in the presence
of either basic or acidic catalysts. These reactions, although
still widely used in industry and academia, suffer from such
problems as the use of toxic or corrosive acetylating re-
agents, the formation of waste acids with poor atom econo-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

B
J. Zhang et al.
Letter
Synlett
cord with this strategy of releasing a small molecule,
Sumathi and co-workers reported an efficient method for
the acetylation of amines with N,N-dimethylacetamide
(DMAc) as a source of an acetyl moiety in the presence of
carbonyldiimidazole (CDI); this reaction was accompanied
by the release of dimethylamine gas.10
Table 1 Optimization of the Reaction Conditionsa
OH
O
O
O
Catalyst
+
Solvent, Temperature
SK
1a
2a
3a
Generally, thioacetic acid or potassium thioacetate are
used as sources of thioacetate anions, commonly used as
nucleophilic partners in substitution reactions11a–d or cross-
coupling reaction to give organic thioacetates.11e–g Recently,
these reagents have been used as sources of acetyl groups
for the synthesis of amides through metal-catalyzed cross-
coupling reaction with amines.12 Two activated intermedi-
ates from thioacetic acid or potassium thioacetate can be
present in these reactions: metal complexes of thioacetic
acid12b,c or diacetyl disulfide.12d,e For examples, Gopi and co-
workers reported the copper-catalyzed acetylation of
amines with thioacetic acid as an acetylating reagent.12b,c
Tan and co-workers developed a visible-light-promoted
photoredox-catalyzed method for the synthesis of amides
with potassium thioacetate.12d To the best of our knowl-
edge, the application of these reagents to the acetylation of
phenols and alcohols has not yet been reported. Here, we
report a simple copper-catalyzed acetylation of phenols and
anilines with potassium thioacetate as an acetylating reagent.
We began our investigations by examining the reaction
of 2-naphthol (1a) with potassium thioacetate (2a). First,
various solvents were screened in the absence of a catalyst
(Table1, entries 1–4). To our surprise, the target product 2-
naphthyl acetate was obtained in 68% yield in MeCN at 80
°C under air (entry 1), whereas only traces of the product
were detected in other solvents (entries 2–4). Next, various
copper salts were examined as catalysts for the reaction
and were shown to improve the yield (entries 5–9) with the
highest yield being obtained when Cu(OAc)2·H2O was used
as the catalyst (entry 9). We also found that the amount of
2a had a marked impact on the reaction (entries 9–11), and
when the amount of 2a was increased to 3.0 equivalents, a
93% yield was obtained (entry 11). Finally, we investigated
the effect of the temperature on the reaction (entries 12–
15). The optimal reaction temperature was found to be
80 °C, significantly lower yields being obtained at other
temperatures, especially room temperature, at which the
yield was only 39%.
Entry
Catalyst
Solvent
Temp (°C)
Yieldb (%)
1
2
-
MeCN
DMSO
DMF
80
80
68
-
trace
trace
trace
73
3
-
80
4
-
DCE
80
5
CuI
CuBr
CuCl
CuOAc
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
80
6
80
72
7
80
76
8
80
78
9
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
80
85
10c
11d
12d
13d
14d
15d
80
79
80
93
r.t.
60
39
75
100
120
78
56
a Reaction conditions: 1a (0.3 mmol), 2a (2.0 equiv), solvent (2 mL), cata-
lyst (0.2 equiv), under air, 8 h.
b Isolated yield.
c 2a (1.5 equiv).
d 2a (3.0 equiv).
than bromo-substituted naphthols. In addition, we found
that the reactivity of phenols was lower than that of 1- or 2-
naphthols. The position of the substituent group had a
marked effect on the reaction. For example, the reaction of
4-methoxyphenol gave a 68% yield of the acetate, whereas
the 3-methoxyphenol gave a 56% yield (entries 7 and 8).
Methylated phenols and 4-(dimethylamino)phenol gave only
moderate yields of the corresponding acetates (entries 9–11).
Having successfully investigated the acetylation of
naphthols and phenols, we further expanded the scope of
the substrates to include anilines. Under the standard con-
ditions with the copper catalyst, electron-rich anilines were
compatible with this protocol and gave high yields of the
corresponding amides (Table 3; Method A). In addition,
while screening the conditions for this reaction, we found
that it could be catalyzed by t--BuONO at room tempera-
ture, albeit with slightly lower yields (Table 3; Method B).
The highest yield was obtained from the reaction of 4-me-
thoxyaniline (Table 3, entry 1). Like the acetylation of 4-
and 3-substituted phenols, the reaction of 4-methylaniline
gave a higher yield than that of 3-methylaniline (entries 2
and 3). Aniline gave N-phenylacetamide in yields of 77% and
72% by Methods A and B, respectively (entry 4). Notably, 4-
fluoroaniline also gave a high yield of the corresponding ac-
With the best reaction conditions in hand, we expanded
the scope of the method to various naphthols and phenols.
We found that this reaction gave moderate to high yields
from electron-rich naphthols and phenols (Table 2). A series
of functional groups were tolerated including chloro, bro-
mo, methoxy, methyl, and protected amino groups. The re-
activity of 2-naphthols was higher than that of 1-naphthols
(Table 2, entries 1 and 2). When 1- or 2-naphthols contain-
ing substituent groups were treated with 2a under the opti-
mized conditions, the yields decreased remarkably (entries
3–6), with chloro-substituted naphthols gave higher yields
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

C
J. Zhang et al.
Letter
Synlett
Table 2 (continued)
etamide (entry 5). Interestingly, for 4-aminophenol as a
substrate, the diacetylation product was obtained in mod-
erate yield (entry 6). Other substituted anilines were tried
under the two standard conditions, but the yields were rel-
atively low (not shown). Attempts to apply the process to
the acetylation of aliphatic amines or alcohols proved prob-
lematic for reasons that remain unclear.
Entry
9
ArOH
Product
Yieldb (%)
64
OH
O
O
O
1i
3i
10
11
58
OH
O
Table 2 Scope of Various Naphthols and Phenolsa
N
N
1j
3j
OH
O
O
Cu(OAc)2⋅H2O (0.2 equiv)
+
50
O
OH
O
MeCN, 80 °C, air, 8 h
SK
O
2a
3a–k
1a–k
3k
1k
Entry
1
ArOH
Product
Yieldb (%)
93
a Reaction conditions: 1a–k (0.5 mmol), 2a (3.0 equiv), Cu(OAc)2·H2O (0.2
equiv), MeCN (3 mL), 80 °C, under air, 8 h.
b Isolated yield.
OH
O
O
1a
1b
3a
O
2
3
84
72
To investigate the reaction mechanism, we performed a
radical-trap experiment. When a stoichiometric amount of
the radical scavenger 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO) was added to the reaction mixture under the stan-
dard reaction conditions, only a trace amount of product 3a
was detected. This result suggests that the reaction involves
a free-radical process. On the basis of this results and re-
ports in the literature, we propose a plausible mechanism
for this reaction, as shown in Scheme 1. In the presence of
copper catalyst and air atmosphere, potassium thioacetate
readily forms the acetylsulfanyl radical I, which is then
transformed into the key diacetyl disulfide (II) by a radical
coupling reaction. The acetylation product is then was
formed through nucleophilic attack on the electron-rich an-
iline or phenol.
OH
O
3b
O
OH
O
Cl
1c
Cl
3c
4
63
OH
O
O
In summary, we have described an acetylation of phe-
nols and anilines with potassium thioacetate catalyzed by
copper(II) acetate.13 In addition, the acetylation of anilines
proceeded at room temperature in moderate to high yields
when t-BuONO was used as the catalyst. This simple proce-
dure serves as an important complementary route to previ-
ous acetylation reactions due to certain advantages of this
acetylating reagent, such as its low cost, easy available, sta-
bility, insensitivity to water and air, and ease of storage.
However, this reaction also has some limitations, as it is
only suitable for electron-rich phenols and anilines. Further
studies to expand the substrate scope are currently under-
way in our laboratory.
Br
1d
Br
3d
5
6
65
53
OH
O
O
Br
1e
Br
3e
Br
Br
O
O
O
O
1f
3f
7
8
78
66
MeO
MeO
O
OH
O
1g
3g
O
MeO
3h
O
MeO
1h
OH
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

D
J. Zhang et al.
Letter
Synlett
Table 3 Scope of Various Anilinesa
Funding Information
We thank the National Natural Science Foundation of China (No.
21572094), the Natural Science Foundation of Zhejiang Province (No.
LY18B020005, LQ18B020001), and the China National Students’ Inno-
vation and Entrepreneurship Training Program (No. 201710352005)
Method A
Cu(OAc)2⋅H2O (0.2 equiv)
MeCN, 80 °C, air, 8 h
H
N
NH2
O
+
R
R
O
Method B
t-BuONO (0.2 equiv)
MeCN, 25 °C, air, 4 h
SK
2a
for financial support.
N
ati
o
n
a
lNaturalS
c
i
e
n
c
e
F
o
u
n
d
ati
o
n
of
C
h
i
n
a
(2
1
5
7
2
0
9
4)NaturalS
c
i
e
n
c
e
F
o
u
n
d
ati
o
n
of
Z
h
e
j
i
a
n
g
Pro
v
i
n
c
e
(L
Y
1
8
B
0
2
0
0
0
5)NaturalS
c
i
e
n
c
e
F
o
u
n
d
ati
o
n
of
Z
h
e
j
i
a
n
g
Pro
v
i
n
c
e
(L
Q
1
8
B
0
2
0
0
0
1)
4a–e
5a–e
Entry ArNH2
1
Product
Yieldb (%)
Supporting Information
H
N
NH2
90 (A)
86 (B)
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611741.
S
u
p
p
orit
n
gInformati
o
n
S
u
p
p
orit
n
gInformati
o
n
O
MeO
4a
MeO
5a
References and Notes
2
80 (A)
76 (B)
NH2
H
N
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O
4b
5b
3
73 (A)
64 (B)
NH2
H
N
O
O
4c
5c
4
77 (A)
72 (B)
NH2
H
N
4d
5d
5
85 (A)
78 (B)
H
N
NH2
O
F
4e
F
5e
6
59 (A)c
55 (B)d
NH2
H
N
O
HO
4f
O
O
5f
(4) For selected examples, see: (a) Murashige, R.; Hayashi, Y.;
Ohmori, S.; Torii, A.; Aizu, Y.; Muto, Y.; Murai, Y.; Oda, Y.;
Hashimoto, M. Tetrahedron 2011, 67, 641. (b) Choudhary, V. R.;
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Banerjee, B.; Islam, S. M. RSC Adv. 2014, 4, 42670. (c) Li, N.;
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Zhang, W. Res. Chem. Intermed. 2012, 38, 1961. (b) Sanz Sharley,
D. D.; Williams, J. M. J. Chem. Commun. 2017, 53, 2020.
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(d) Basumatary, G.; Bez, G. Tetrahedron Lett. 2017, 58, 4312.
(e) Yoshida, T.; Kawamura, S.; Nakata, K. Tetrahedron Lett. 2017,
58, 1181.
a Reaction conditions: Method A: 4a–e (0.5 mmol), 2a (3.0 equiv),
Cu(OAc)2·H2O (0.2 equiv), MeCN (3 mL), 80 °C, under air, 8 h; Method B:
4a–e (0.5 mmol), 2a (2.0 equiv), t-BuONO (0.2 equiv), MeCN (3 mL), r.t.,
under air, 4 h.
b Isolated yield.
c 2a (6.0 equiv).
d 2a (4.0 equiv).
O
O
S
O
– 2K+
2
S
2
S
(II)
O
SK
CuII
(I)
ArXH, K+
O
redox
catalysis
CuI
O
O2
+
S
SK
ArX
(7) Zhang, L.; Wang, W.; Wang, A.; Cui, Y.; Yang, X.; Huang, Y.; Liu,
X.; Liu, W.; Son, J.-Y.; Oji, H.; Zhang, T. Green Chem. 2013, 15,
2680.
O2
Scheme 1 Possible mechanism
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H. Synlett 2013, 24, 1870. (b) Kumar, M.; Bagchi, S.; Sharma, A.
New J. Chem. 2015, 39, 8329.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

E
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Letter
Synlett
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(13) Acetylation of Anilines and Phenols with Potassium Thioace-
tate: General Procedure
The appropriate aniline or phenol (0.5 mmol), KSAc (3.0 equiv),
Cu(OAc)2·H2O (0.2 equiv), and MeCN (3 mL) were added to a
screw-capped vial under air, and the vial was placed in a tem-
perature-controlled oil bath at 80 °C. When the reaction was
complete (TLC), the vial was removed from the oil bath and
allowed to cool to r.t. The solution was filtered through a short
column of silica gel that was washed with EtOAc. The filtrate
was concentrated under reduced pressure to give a crude
product that was purified by flash column chromatography
(silica gel, PE–EtOAc).
2-Naphthyl Acetate (3a)
Creamy-white solid powder; yield: 86.5 mg (93%); mp 68–70
°C. 1H NMR (300 MHz, CDCl3): δ = 7.91–7.84 (m, 3 H), 7.61 (s, 1
H), 7.52 (t, J = 3.9 Hz, 2 H), 7.29 (t, J = 7.2 Hz, 1 H), 2.40 (s, 3 H).
13C NMR (75 MHz, CDCl3): δ = 169.8, 148.4, 133.8, 131.6, 129.5,
127.9, 127.7, 126.7, 125.8, 121.2, 118.6, 21.3.
(12) (a) Wu, W.; Zhang, Z.; Liebeskind, L. S. J. Am. Chem. Soc. 2011,
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Commun. 2012, 48, 7085. (c) Mali, S. M.; Bhaisare, R. D.; Gopi, H.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E