Visible-light-induced Pd-catalyzed: Ortho -trifluoromethylation of acetanilides with CF3SO2Na under ambient conditions in the absence of an external photocatalyst

Article abstract of DOI:10.1039/c9cc01014a

A visible-light-induced Pd-catalyzed ortho-trifluoromethylation of acetanilides with CF₃SO₂Na was developed. The reaction proceeded smoothly at room temperature in air without any external photocatalyst or additive, providing the desired products in moderate to good yields with good functional group tolerance and regioselectivity.

Full text of DOI:10.1039/c9cc01014a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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DOI: 10.1039/C9CC01014A
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Visible-light-induced Pd-catalyzed ortho-trifluoromethylation of
acetanilides with CF3SO2Na under ambient conditions in the
absence of an external photocatalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Long Zou,a Pinhua Li,*,a Bin Wang,a and Lei Wang*,a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
A visible-light-induced Pd-catalyzed ortho-trifluoromethylation of because the CF3 moiety possesses wide and specific bioactivities that
acetanilides with CF3SO2Na was developed. The reaction proceeded dramatically change the physical and biological properties of
The previous reports:
smoothly at room temperature in air without any external
Pd(OAc)2 (10 mol%)
photocatalyst or additive, providing the desired products in
moderate to good yields with good functional group tolerance and
regioselectivity.
.
Ru(bpy)3Cl2 6H2O (2.5 mol%)
Ph
N
2 BF4
(a)
+
N
N
MeOH (0.1 M in substrate)
26 W lightbulb, 25 oC, 4 h
R
R
Ph
CF3
H
Pd(OAc)2 (10 mol%)
H
N
H
N
R2
R2
The development of efficient catalytic systems for direct C(sp2)–H
activation and functionalization has received much attention
because these systems can provide a concise route and simple
procedure for the formation of carbon-carbon and carbon-
heteroatom bonds.[1] Among the catalysts capable of C–H activation,
Pd-based catalysts are the most intensively investigated and widely
used in organic transformations with high reactivity.[2] Since
MacMillan developed the first example of visible-light-induced
asymmetric alkylation of aldehydes by merging organo- and
photoredox catalysis in 2008,[3] visible light, a safe and abundant
energy source, photoredox catalysis has become one of the most
active research fields under remarkably mild conditions, and a
number of organic syntheses have been accomplished.[4,5] Recently,
merging of visible light photoredox catalysis with Pd-catalyzed C–H
activation and functionalization has emerged as a powerful strategy
for achieving high-yielding transformations and avoiding the use of
excess oxidants.[6] In 2011, Sanford reported a C–H arylation of 2-
arylpyridines with aryldiazonium salts by merging Pd-catalysis with
Ru-based visible light photoredox catalysis (Scheme 1a).[7] After that,
this dual catalysis system has been widely used in C(sp2)–H acylation,
C(sp2)–H alkenation, C(sp2)–H amination, C(sp2)–H carbonylation,
allylation, dehydrogenation and oxidation.[8]
Cu(OAc)2 (2.2 equiv)
-
(b)
R1
R1
+
S+
CF3
PivOH (5.0 equiv)
O
O
BF4
O
DCE, N2, 110 oC, 24 h
This work:
H
CF3
Pd(OAc)2 (5.0 mol%)
external photocatalyst
DMSO, air, 25 oC, 20 h
blue LED (410 415 nm)
H
N
H
R2
N
R2
R1
R1
(c)
+
CF3SO2Na
O
Scheme 1 Pd-catalyzed regioselective C-H bond functionalization.
parent molecules, including catabolic stability, binding selectivity,
lipophilicity and solubility.[9] There are several methods for the C–H
trifluoromethylation of arylamines. For example, Yu developed a Pd-
catalyzed ortho-trifluoromethylation of 2-phenylpyridines,[10] and Shi
reported a Pd-catalyzed trifluoromethylation of acetamide (Scheme
1b).[11] Recently, Zhang demonstrated Ni-catalyzed ortho-
trifluoromethylation of anilines.[12] Very recently, Xia reported an
iron-catalyzed ortho-trifluoromethylation of anilines under ultra-
violet irradiation (λ = 254 nm). [13] On the other hand, a number of
CF3 precursors, such as CF3I,[14] CF3SO2Cl,[15] CF3SO2Na,[16] the Togni
reagent,[17] and the Umemoto reagent,[18] have been used, and
CF3SO2Na is an ideal CF3 source because of its stability and safety, low
toxicity and low expense. Herein, we report a visible-light-induced
Pd-catalyzed ortho-trifluoromethylation of acetanilides with
CF3SO2Na at room temperature in air in the absence of any external
photocatalyst or additive (Scheme 1c).
In the past few decades, trifluoromethylation has gained much
attention in pharmaceuticals, agrochemicals, and organic materials
To realize this idea, a model reaction of acetanilide (1a) with
CF3SO2Na (2a) was selected. When the reaction mixture of 1a (0.50
mmol), 2a (1.0 mmol), Pd(OAc)2 (5.0 mol%) and Ru(bpy)3Cl2 (3.0
mol%) in DMSO (3.0 mL) was stirred at room temperature under blue
LED (450−455 nm, 6 W) irradiation in air for 20 h, 9% yield of ortho-
trifluoromethylation product 3a was observed (Table 1, entry 1).
Other photocatalysts, fac-Ir(ppy)3, eosin Y, fluorescein and Mes-Acr-
a Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000, P. R.
China, E-mail: leiwang@chnu.edu.cn; pphuali@126.com
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
-
Me+ClO4 , exhibited more reactivity, giving 3a in 41–74% yields
This journal is © The Royal Society of Chemistry 20xx
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(entries 2–5). To our delight, the model reaction was performed in obtained when the reaction was performed under a nitrogen
View Article Online
DOI: 10.1039/C9CC01014A
the absence of an external photocatalyst under irradiation with a
atmosphere (Table S1, SI), indicating that oxygen, as an essential
oxidant, plays an important role in the reaction. No expected product
was detected, as the reaction was carried out in the absence of light
or Pd-catalyst (Table S1, SI).
Table 1 Optimization of the reaction conditionsa
H
CF3
H
H
N
Pd-catalyst (5.0 mol%)
Photocatalyst (3.0 mol%)
N
With the abovementioned optimized conditions, a series of N-
substituted anilides, including N-phenylacetamide (1a), N-
phenylpivalamide (1b), N-phenylbenzamide (1c), N-phenyl-4-
methoxybenzamide (1d), tert-butyl phenylcarbamate (1e), N-
phenylpropionamide (1f), N-phenylbutyramide (1g), and N-
phenylcyclohexanecarboxamide (1h), were examined, and the
results are shown in Scheme 2. As shown in Scheme 2, most of the
N-substituents on the anilides were tolerated, generating the
corresponding products (3a–g) in good yields. Only 37% yield of the
product 3h was obtained when 1h was used. So, 1a (acetanilide) and
its derivatives were chosen as the starting materials because of their
high reactivity and easy availability.
CF -
+
source
3
O
O
DMSO, 6 W LED, rt, 20 h
2
1a
3a
Entry
2
Pd-catalyst
Pd(OAc)2
Photocatalyst
Yield (%)b
1
2
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
Ru(bpy)2Cl2
9
46
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(CH3CN)2Cl2
PdCl2
fac-Ir(ppy)3
3
Eosin Y
74
4
Fluorescein
Mes-Acr+
53
5
41
6
－
8
7
－
77c
69c
67c
47c
23c
n.r.c
n.r.c
tracec
35c
8
－
CF3
H
H
N
H
R
R
9
－
N
Pd(OAc)2 (5.0 mol%)
+
CF3SO2Na
DMSO, rt
O
O
10
11
12
13
14
15
Pd(TFA)2
－
410 415 nm, 20 h
1
2a
3
Pd(PPh3)2Cl2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
－
OMe
CF3
CF3
CF3
H
CF3
－
H
N
H
H
N
N
N
－
O
O
O
O
－
3c
3d
3a
, 79%
, 80%
,77%
3b
, 78%
Pd(OAc)2
－
CF3
CF3
CF3
CF3
H
H
H
H
N
O
N
N
N
O
2a
CF3SO2Na (
)
O
I
O
2b
O
O
CF3SO2Cl (
2c
)
)
O
-
S+
CF3
BF4
TMSCF3
(
3g
, 73%
3h
, 37%
3e
, 53%
3f
, 67%
CF3 2d
(
)
2e
(
)
Scheme 2 The scope of N-substituted anilides [reaction conditions: 1 (0.50
mmol), 2a (1.0 mmol), Pd(OAc)2 (5.0 mol %), and DMSO (3.0 mL) at room
temperature (25 °C) under LED irradiation (410−415 nm, 6 W) in air for 20
h; isolated yield of the product].
a Reaction conditions: 1a (0.50 mmol), 2 (1.0 mmol), Pd-catalyst (5.0 mol%),
photocatalyst (3.0 mol%), DMSO (3.0 mL) at room temperature (25 °C) under
blue LED (450−455 nm, 6 W) irradiation in air for 20 h. b Isolated yield. c blue
LED (410−415 nm).
Next, a variety of acetanilides were examined, and the results are
presented in Scheme 3. The results showed that various functional
groups, including both electron-rich and electron-poor substituents
on the benzene rings in acetanilides, were suitably compatible, and
the desired products were obtained in good yields. For example,
acetanilides with an electron-donating group, such as Me, tBu, Ph, Et
and the weakly electron donating CF3O, at the para-position of the
phenyl rings reacted with CF3SO2Na to afford the corresponding
products (3i–l and 3r) in 64–83% yields. Moreover, acetanilides
bearing halogens (F, Cl and Br) at the para-position of the aromatic
rings generated the ortho-trifluoromethylated products (3m–o) in
63–79% yields. It is important to note that strong electron-
withdrawing groups, such as CN, and CF3, located at the para-position
of the benzene rings of acetanilides were tolerated, affording the
desired products (3p and 3q) in 88% and 81% yields, respectively. To
further assess the scope of this reaction, the introduction of COMe,
CO2Me and CO2Et into the aromatic rings of acetanilides also
proceeded well, providing the anticipated products (3s–u) in 57–82%
yields. In addition, the reactions of meta-substituted acetanilides,
including those containing Me, F, Cl, Br, CN, and OMe, also generated
the expected products (3v–aa) in 49–71% yields owing to a slight
steric effect. The use of ortho-substituted acetanilides, such as N-(2-
6 W blue LED (450−455 nm) in DMSO, providing the desired product
3a in 8% yield (entry 6). A survey of LED sources with different
wavelengths illustrated that 410−415 nm was the best of choice, and
afforded 3a in 77% yield (entry 7). Only a 47% yield of 3a was
observed when the reaction was irradiated with an LED (380−385
nm) (Table S1, SI). An LED (420−425 nm) also promoted the reaction
efficiently and afforded 3a in 72% yield (Table S1, SI). When LEDs with
wavelengths of 510–515 and 520–525 nm were used in the model
reaction, no desired product 3a was detected (Table S1, SI). To
improve the efficiency of the reaction, other Pd sources, including
Pd(CH3CN)2Cl2, PdCl2, Pd(TFA)2 and Pd(PPh3)2Cl2, were used instead
of Pd(OAc)2, and inferior yields (23–69%) of 3a were obtained
(entries 8–11). The solvent plays a very important role in the
reaction. Among the tested solvents, DMSO and CH3CN resulted in
the highest efficiency, and DMF, acetone, THF and DCM exhibited no
reactivity (Table S1, SI). For other CF3 reagents, CF3SO2Cl (2b),
TMSCF3 (2c), the Togni reagent (2d) and the Umemoto reagent (2e)
were used as trifluoromethylation reagents in the reaction, and no
improved yields of 3a were observed (entries 12–15). In addition, the
loading of Pd(OAc)2, the molar ratio of 1a/2a, and the reaction time
were optimized, which are also outlined in Table S1, SI. It was also
found that there was no difference when the reaction atmosphere
was changed from air to oxygen. However, no desired product was
chlorophenyl)acetamide
and
N-(2-bromophenyl)acetamide,
provided the products 3ab and 3ac in 44% and 45% yields,
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CF3
H
respectively. However, other ortho-substituted acetanilides,
including N-(2-methylphenyl)-, N-(2-fluorophenyl)-, N-(2-
(trifluoromethyl)phenyl)- and N-(2-methoxyphenyl)-acetamides
H
H
View Article Online
N
N
Pd(OAc)2 (5.0 mol%)
DOI: 10.1039/C9CC01014A
+
CF3SO2Na
DMSO, air, 25 oC, 20 h
Blue LED (410 415 nm)
O
O
2a
1a
, 10 mmol
3a
, 1.36 g, 67%
failed. In addition to mono-substituted acetanilides, di-substituted
acetanilides,
such
as
N-(3,4-dimethylphenyl)-,
N-(3,5-
SOCl2, MeOH
75 oC, 3 h
CF3
Br
dimethylphenyl)- and N-(3,4-dichlorophenyl)acetamides with
CF3SO2Na, reacted to give the corresponding products (3ad–af) in
39–58% yields. Moreover, the reactions of N-(naphthalen-2-
yl)acetamide and N-(pyridin-4-yl)acetamide with 2a afforded the
expected products 3ag and 3ah in 78% and 67% yields, respectively.
However, N-methyl-N-phenylacetamide was not a suitable substrate
in the reaction, and no product (3ai) was detected with the recovery
of starting materials in quantitative yields.
1) NaNO2, HBr (aq.), 0 o
2) CuBr, HBr (aq.), reflux
C
CF3
5a
, 79%
NH2
CF3
1) NaNO2, H2SO4, 0 o
2) H2O, H2SO4, 100 o
C
OH
C
4a
, 94%
6a
, 82%
Scheme
4 Gram-scale synthesis of 3a and its chemical
transformations.
CF3
H
H
N
H
N
Pd(OAc)2 (5.0 mol%)
To gain insight into the reaction mechanism, control experiments
R1
R1
+
CF3SO2Na
2a
DMSO, rt
410 415 nm, 20 h
O
O
1
were
conducted
(Scheme
S2,
SI).
When
2,2,6,6-
3
CF3
CF3
tetramethylpiperidine N-oxide (TEMPO), a radical scavenger, was
added to the model reaction, the formation of 3a was completely
prevented. To capture the CF3 free radical intermediate formed
during the reaction, 1,1-diphenylethylene was added to the reaction,
and a cross-coupling CF3-radical product was isolated in 43% yield
(Scheme S2, SI), indicating that a CF3 radical was involved in the
reaction.
CF3
H
CF3
H
H
H
N
N
N
N
O
O
O
O
tBu
Et
Ph
Me
3l
3j
3k
, 67%
, 83%
, 64%
3i
, 71%
CF3
CF3
CF3
CF3
H
H
N
H
N
H
N
N
O
O
O
O
Cl
F
Br
NC
3p
, 88%
3n
3m
, 79%
, 63%
3o
, 76%
To further illustrate the important role of molecular oxygen in the
CF3
CF3
CF3
CF3
H
N
H
H
H
N
N
N
reaction, TEMP (2,2,6,6-tetramethylpiperidine) and DMPO (5,5-
•−
O
Me
O
O
MeO
O
dimethylpyrroline N-oxide) were employed to capture 1O2 and O2
,
F3CO
F3C
O
O
respectively, and the resulting EPR spectra were recorded, as
presented in the Supporting Information (Figures S1 and S2). The
results indicated that more highly reactive 1O2 and O2•− were formed
in the reaction system and that molecular oxygen plays an important
role in the reaction as an effective oxidant instead of an additional
oxidant. Moreover, the UV-visible absorption spectra of substrate 1
and product 3 showed that both can absorb light at 410−415 nm
(Figures S3−9 of SI) and act as photocatalysts, thus avoiding the use
of an external catalyst. It should be noted that the formed products
absorb light of longer wavelengths to activate molecular oxygen
more efficiently, and the entire reaction proceeds in an autocatalytic
manner (Figure S10).
3q
, 81%
3t
3s
, 60%
3r
, 82%
, 65%
CF3
CF3
CF3
CF3
H
H
N
H
N
H
N
N
EtO
O
O
O
O
O
Me
Br
F
3x
, 49%
3v
3w
, 50%
, 71%
3u
, 57%
CF3
CF3
CF3
CF3
H
H
H
H
N
N
N
N
O
O
O
O
Cl
Cl
CN
OMe
CF3
3aa
, 65%
3ab
, 44%
3y
, 65%
3z
, 69%
CF3
CF3
CF3
H
H
N
H
H
N
Me
N
Cl
N
O
O
O
O
Me
Br
Me
Me
Cl
3af
, 56%
3ae
, 58%
3ac
3ad
, 45%
, 39%
CF3
On the basis of experimental results, a possible mechanism for the
photoreaction of acetanilide (1a) with CF3SO2Na (2a) is proposed in
Scheme 5. First, substrate 1 was excited by a blue LED (410−415 nm)
to generate the excited species 1*, which acts as a photocatalyst. The
formed 1* then underwent energy transfer with ground-state
oxygen (3O2) to afford more highly reactive excited- state oxygen
(1O2) along with the regeneration of 1 in the ground state. The
CF3
H
CF3
H
N
N
N
O
N
O
O
3ai
, 0%
3ah
3ag
, 67%
, 78%
Scheme 3 The scope of acetanilides [reaction conditions: 1 (0.50 mmol), 2a
(1.0 mmol), Pd(OAc)2 (5.0 mol%), DMSO (3.0 mL) at room temperature (25 °C)
under LED irradiation (410−415 nm, 6 W) in air for 20 h; isolated yield of the
product].
1
generated O2 reacted with 2a via single electron transfer to afford
•−
•
the superoxide radical anion O2 (I) and a CF3SO2 radical, which
Furthermore, the gram-scale synthesis of 3a was performed and
resulted in 67% isolated yield, as shown in Scheme 4. In addition, the
transformation of 3a into its derivatives was also investigated. The
hydrolysis of 3a generated the corresponding product of 2-
(trifluoromethyl)aniline in 94% yield, which could be transformed
into 1-bromo-2-(trifluoromethyl)benzene and 2-(trifluoromethyl)-
phenol in 79% and 82% yields, respectively, according to the
reported procedure.[19]
•
generated a CF3 radical (II) and SO2. On the other hand, the Pd-
catalytic cycle could be initiated by C–H activation to afford
palladacycle intermediate III, which underwent a reaction with the
formed II to afford PdIII-intermediate IV, followed by oxidation of I to
generate PdIV-intermediate V along with the formation of H2O2
2−
through an O2 intermediate. Finally, reductive elimination of V
occurred to afford the product 3a and to regenerate PdII-catalyst to
finish the catalytic cycle. Of course, the obtained 3a also acts as a
photocatalyst, similar to 1a.
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Journal Name
CF3
8
For selected examples, see: (a) C. Zhou, P. Li, X. Zhu anVdieLw. AWrtaicnlegO, Onlringe.
H
N
H
N
Lett., 2015, 17, 6198; (b) J. Zoller, D. C. Fabry, M. A. Ronge and M. Rueping,
DOI: 10.1039/C9CC01014A
O
O
Angew. Chem., Int. Ed., 2014, 53, 13264; (c) S. Choi, T. Chatterjee, W. J.
Choi, Y. You and E. J. Cho, ACS Catal., 2015, 5, 4796; (d) K. Liu, M. Zou and
A.-W. Lei, J. Org. Chem., 2016, 81, 7088; (e) S. B. Lang, K. M. O’Nele and
J. A. Tunge, J. Am. Chem. Soc., 2014, 136, 13606; (f) S. Kato, Y. Saga, M.
Kojima, H. Fuse, S. Matsunaga, A. Fukatsu, M. Kondo, S. Masaoka and M.
Kanai, J. Am. Chem. Soc., 2017, 139, 2204; (g) M. D. Kärkäs, I. Bosque, B.
S. Matsuura and C. R. J. Stephenson, Org. Lett., 2016, 18, 5166.
(a) T. Hiyama, Organofluorine Compounds: Chemistry and Applications,
Springer: Berlin, 2000; (b) R. E. Banks and C. J. Tatlow, Organofluorine
Chemistry, Principles and Commercial Applications, Springer: New York,
1994.
1a
3a
Pd(OAc)2
H
N
O
HOAc
Pd
AcO
AcO
CF3
V
H
2-
H+
O2
N
9
O
H
N
Pd
H2O2
OAc III
O
Pd
10 X. Wang, L. Truesdale and J.-Q. Yu, J. Am. Chem. Soc., 2010, 132, 3648.
11 L.-S. Zhang, K. Chen, G. Chen, B.-J. Li, S. Luo, Q.-Y. Guo, J.-B. Wei and Z.-
J. Shi, Org. Lett., 2013, 15, 10.
12 J. Xu, L. Qiao, J. Shen, K. Chai, C. Shen and P. Zhang, Org. Lett., 2017, 19,
5661.
hv
AcO
CF3
O2
IV
I
CF3
II
1
Substrate (
Product (
)
1
Substrate* ( )*
CF3SO2
3
)
3
Product* ( )*
13 C. Xia, K. Wang, G. Wang and G. Duan, Org. Biomol. Chem., 2018, 16, 2214.
14 For selected papers, see: (a) P. V. Pham, D. A. Nagib and D. W. C.
MacMillan, Angew. Chem., Int. Ed., 2011, 50, 6119; (b) S. N. Iqbal, E. Choi
and E. J. Kim, J. Org. Chem., 2012, 77, 11383; (c) N. Iqbal, J. Jung, S. Park
and E. J. Cho, Angew. Chem., Int. Ed., 2014, 53, 539.
SO2
1O2
3O2
2a
CF3SO2Na (
)
Scheme 5 Plausible mechanism.
15 For selected papers, see: (a) D. A. Nagib and D. W. C. MacMillan, Nature,
2011, 480, 224; (b) S. W. Oh, Y. R. Malpani, N. Ha, Y.-S. Jung and S. B. Han,
Org. Lett., 2014, 16, 1310; (c) D. B. Bagal, G. Kachkovskyi, M. Knorn T.
Rawner, B. M. Bhanage and O. Reiser, Angew. Chem., Int. Ed., 2015, 54,
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Acknowledgements
We gratefully acknowledge the National Natural Science Foundation
of China (21772062, 21572078), the Young Talent Key Project of
Anhui Province (170808J02) and the Project of Anhui (KJ2015TD002)
for financial support.
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