Nickel(II)-Catalyzed Site-Selective C-H Bond Trifluoromethylation of Arylamine in Water through a Coordinating Activation Strategy

Article abstract of DOI:10.1021/acs.orglett.7b02823

The first example of nickel(II)-catalyzed site-selective C-H bond trifluoromethylation of arylamine in water is established. In this transformation, a coordinating activation strategy is performed by the utilization of picolinamide as a directing group, and target products are obtained in moderate to good yields. In addition, the catalyst-in-water system can be reutilized eight times with a slight loss of catalytic activity and applied in the green, concise synthesis of acid red 266. Furthermore, a series of control experiments verify that a single-electron transfer mechanism is responsible for this reaction.

Full text of DOI:10.1021/acs.orglett.7b02823

Letter
pubs.acs.org/OrgLett
Nickel(II)-Catalyzed Site-Selective C−H Bond Trifluoromethylation of
Arylamine in Water through a Coordinating Activation Strategy
Jun Xu, Li Qiao, Jiabin Shen, Kejie Chai, Chao Shen, and Pengfei Zhang*
College of Material Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
*
S Supporting Information
ABSTRACT: The first example of nickel(II)-catalyzed site-selective
C−H bond trifluoromethylation of arylamine in water is established. In
this transformation, a coordinating activation strategy is performed by
the utilization of picolinamide as a directing group, and target products
are obtained in moderate to good yields. In addition, the catalyst-in-
water system can be reutilized eight times with a slight loss of catalytic
activity and applied in the green, concise synthesis of acid red 266. Furthermore, a series of control experiments verify that a
single-electron transfer mechanism is responsible for this reaction.
ransition-metal-catalyzed trifluoromethylation has
emerged as a dramatic tactic for the trifluoromethylation
Scheme 2. C−H Trifluoromethylation of Arylamine
T
1
1a−j
of organic molecules. Previous works
from many groups are
focused on the formation of trifluoromethylated compounds
from arylboronic acids, aryl acid, arylamine, and aryl halides,
while more recent efforts have led to fast development of C−H
1k−t
trifluoromethylation.
Trifluoromethylaniline, as an important kind of organic
intermediate, is extensively utilized in the production o2f
pharmaceuticals, agrochemicals, and dyes (Scheme 1).
Scheme 1. Application of Trifluoromethylated Arylamines
Recently, transition-metal-catalyzed C−H trifluoromethylation
of arylamine derivatives has received considerable attention. The
palladium-catalyzed trifluoromethylation of acetamide was
developed by Shi’s group through the mechanism of oxidative
addition/reductive elimination facilitated by a O atom of amide
because of the substituent effect, the substrates were confined to
electron-deficient 4-substituted acetamides. Furthermore, 5 days
were needed for this transformation. Indeed, these protocols
represent great infusive advances, but some important aspects
still need continuous improvement.
1
s
(
Scheme 2a). Shortly after Shi’s report, Xi et al. demonstrated
CuCl-catalyzed ortho-trifluoromethylation of arylamine. Un-
fortunately, this system showed poor regioselectivity, because of
the involvement of radical intermediates and the lack of a stable
In 2005, Daugulis and co-workers exploited the picolinamide
(
PA) moiety as a bidentate directing group, for use in palladium-
4
1t
catalyzed C−H functionalization reactions. Since then, new PA-
directed C−H functionalization reactions have been successfully
achieved using copper instead of noble metal palladium in
auxiliary (Scheme 2b). For these two transformations above,
expensive catalyst or trifluoromethylating reagents, organic
solvents, relatively high temperatures, as well as an inert
atmosphere were indispensable, making these methods inadapt-
able to large-scale synthesis. In 2014, Cao et al. demonstrated
copper-free direct C−H trifluoromethylation of acetanilides with
5
6a
catalytic reaction. Recently, the groups of Nakamura and
Received: September 9, 2017
3
Langlois’ reagent via radical pathway (Scheme 2c). However,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02823
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
6
b
2
You reported an iron-catalyzed directed C(sp )−H and
Subsequently, an investigation of various kinds of directing
groups was performed (Scheme 3). The trifluoromethylation of
3
C(sp )−H functionalization assisted by the PA-directing
group, respectively. In contrast, nickel-catalyzed PA-directed
C−H functionalization is rarely established.
Owing to the characteristics of abundance, low price, and
unique reactivity, nickel is widely employed as a prevalent
6
a,b
Scheme 3. Influence of Directing Group
7
catalyst in many organic reactions. Recently, nickel-catalyzed
C−H functionalization has also received considerable attention8.
Preeminent advances have been demonstrated by many groups.
However, nickel-catalyzed trifluoromethylation is seldom
reported. In that context, we report a nickel(II)-catalyzed
trifluoromethylation of arylamine derivatives at the ortho
position in water through a coordinating activation strategy
(
Scheme 2d).
We initially studied the trifluoromethylation of N-phenyl-
a
Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), NiSO ·6H O (10
4
2
picolinamide by using cobalt chloride (CoCl ) as catalyst,
2
mol %), K S O (2 equiv), H O (4 mL), stirred at 50 °C, under air, for
2
2
8
2
Langlois’ reagent (CF SO Na) as CF source, potassium
b
3
2
3
1
2 h. Isolated yields.
persulfate (K S O ) as initiator, and water (H O) as solvent at
2
2
8
2
5
0 °C under air atmosphere. The unexpected product was
arylamine derivative 1a did furnish the ortho-trifluoromethylated
product 3a in 78% yield. Others bearing N-containing bidentate
directing groups reacted with a low yield (3b−d), and products
successfully isolated in 29% yield (Table 1, entry 1). Accordingly,
a,b
Table 1. Optimization of Reaction Conditions
3
e and 3f were not gained. These results indicated the crucial role
of both heterocyclic nitrogen and amide, most likely as bidentate
5c
binding site for a metal atom.
After obtaining the reliable direct C−H bond trifluoromethy-
lation protocol, we then surveyed the substrate scope of
arylamines (Scheme 4). Pleasingly, most substrates could react
b
entry
catalyst
CoCl2
Co(OAc) ·4H O
initiator
yield (%)
1
2
3
4
5
6
7
8
9
K S O
29
33
2
2
8
8
8
8
8
8
8
8
8
K S O
2 2
a,b
2
2
Scheme 4. Substrate Scope of Arylamines
CoF2
K S O
41
2
2
Fe(OTf)3
K S O
trace
trace
22
2
2
Cu(OAc)2
MnBr2
K S O
2 2
K S O
2
2
NiCl2
K S O
59
2
2
Ni(OTf)2
Ni(acac)2
NiSO ·6H O
K S O
44
2
2
K S O
53
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
3
4
5
6
7
8
9
K S O
78
4
2
2
2
8
K S O
0
2
2
8
NiSO ·6H O
TBHP
DTBP
DCP
23
4
2
NiSO ·6H O
20
4
2
NiSO ·6H O
trace
trace
trace
65
4
2
NiSO ·6H O
CH CO H
4
2
3
3
NiSO ·6H O
H O
4
2
2
2
NiSO ·6H O
Na S O
2 2 8
4
2
NiSO ·6H O
(NH ) S O
72
4
2
4
2
2
8
NiSO ·6H O
0
4
2
c
0
1
2
NiSO ·6H O
K S O
8
trace
76
4
2
2
2
d
e
NiSO ·6H O
K S O
2 2
4
2
8
8
a
Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), NiSO ·6H O (10
NiSO ·6H O
K S O
2 2
78
4
2
4
2
a
mol %), K
2
S
2
O
8
(2 equiv), H
O (4 mL), stirred at 50 °C, under air, for
2
Reaction conditions: 1a (0.5 mmol), 2 (1.0 mmol), catalyst (10 mol
), initiator (2 equiv), H O (4 mL), stirred at 50 °C, under air, for 12
h. Isolated yields. Stirred at rt. Stirred at 100 °C. Under N .
b
1
2 h. Isolated yields.
%
2
b
c
d
e
2
smoothly. Overall, arylamine derivatives with various alkyl and
halogen groups at para position revealed better activities, and the
corresponding products (3g−m) were isolated in 52−71% yield.
Transformation of meta-substituted arylamines with Langlois’
reagent (CF SO Na) delivered the ortho-trifluoromethylated
products (3o−r) in 45−60% yield. Substrates bearing ortho-
substituted groups transformed into products with a low yield of
33% and 23% (3s, 3t). Interestingly, bulky arylamines such as
naphthylamine and quinolylamine also furnished the corre-
sponding products 3w and 3x in 43% and 21% yields,
respectively. Electron-withdrawing substituent-bearing aryl-
several kinds of metal catalysts including cobalt(II), iron(III),
copper(II), manganese(II), and nickel(II) salts were investigated
(
Table 1, entries 2−10), and a better yield of 78% was obtained
3
2
by using NiSO ·6H O as catalyst. As expected, no desired
4
2
product was detected in the absence of any metal catalysts (Table
, entry 11). This result revealed that metal catalyst played a
1
crucial role in this transformation. Further screening in initiators,
temperature, as well as atmosphere did nothing to enhance the
product yield (Table 1, entries 12−22).
B
DOI: 10.1021/acs.orglett.7b02823
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Organic Letters
Letter
amines and heteroarylamines are strong electron-deficient
substrates, which are not conducive to this reaction, so substrates
NH were very considerable for this reaction (see the Supporting
Information). Furthermore, because substrate 1w could not be
converted into any product, we could also eliminate the
mechanism of aryl electrophilic substitution. In addition,
when the two ortho positions of arylamine were substituted by
methyl (1z), the substrate lost reactivity, suggesting that the
trifluoromethylation only happened at the ortho position of
arylamine. Second, the reaction was completely suppressed when
2 equiv of TEMPO (2,2,6,6-tetramethyl-piperidin-1-oxyl) was
(
3n, 3u, 3v, and 3y) did not undergo the C−H bond
5c
trifluoromethylation.
With the effective method of trifluoromethylation in hand, we
next inspected the applicability of this method to prepare acid red
266 which has been widely used in the dye industry (Scheme 5).
Scheme 5. Application of Trifluoromethylation
added and 42% yield of TEMPO−CF adduct was detected by
3
19
F NMR, implying that a radical reaction pathway might be
involved in the transformation (Scheme 7, eq 1). To explore the
Scheme 7. Control Experiments
First, gram-scale synthesis of compound 3k was successful
proceed under optimum conditions, and the target product was
acquired in 61% yield. Second, the hydrolysis reaction was
carried out in the presence of hydrochloric acid, and 4-chloro-2-
(
trifluoromethyl)aniline was obtained in 88% yield and 2-
C−H activation step, further exploration about kinetic isotope
effects (KIE) was performed through the intermolecular
competition. When equal amounts of 1a and 1a-D5 were treated
with Langlois’ reagent (CF SO Na) together under standard
picolinic acid was recovered in 85% yield. Finally, successive
diazotization, C−N coupling, and salt-forming reactions gave rise
to the desired product 5 in 64% yield, thus providing a feasible
pathway to the green, concise synthesis of acid red 266.
Since it was our intention to design a green environmental
protection method for the trifluoromethylation of arylamine
derivatives, a recycling experiment of the catalyst-in-water was
performed (Scheme 6). According to the difference in the
3
2
conditions for 3 h, the mixed products 3a/3a-D4 were obtained
in 31% yield, giving a low ratio (k = 1.04), indicating that the rate-
determining step does not involve C−H cleavage (Scheme 7, eq
9
2). In addition, an irreversible C−H cleavage also can be
approved for the reaction because no compound 1a-D was
detected in the proton-exchange experiment (see the SI).
We considered that a single-electron transfer (SET)
mechanism could be involved on the grounds of experimental
Scheme 6. Recycling Catalyst-in-Water System
8a−c,10
II
results and previous reports (Scheme 8).
Initially, Ni L
n
Scheme 8. Plausible Mechanism
solubility of nickel(II) salt and organic product in water and ethyl
acetate, catalyst-in-water could be retrieved by an easy phase
separation from the organic layer. The retrieved catalyst-in-water
(
containing sulfates) could be reutilized in the next round. The
catalytic activity of nickel(II) catalyst remained stable after eight
rounds, affording the target product in 69% yield.
was combined with substrate 1a to generate intermediate A. In
the meantime, radical B was provided from Langlois’ reagent
(CF SO Na) by oxidation of potassium persulfate (K S O ).
In order to gain more insights into the mechanism of
nickel(II)-catalyzed site-selective C−H bond trifluoromethyla-
tion, a series of control experiments were conducted, such as
structure effect of substrates, radical inhibition, proton exchange,
and kinetic isotope effect experiments. First, specific substrates
3
2
2
2
8
Subsequently, complex A turned into compound C through
radical B addition. Then, complex E and anion D were generated
via a SET process. Meanwhile, anion D and CF SO Na
3
2
(
1w−y) failed to afford the corresponding product, these results
transformed into trifluoromethyl radical F by oxidation of
showed that the N atom of pyridine and amide group with a free
potassium persulfate (K S O ). Followed by the combination of
2
2
8
C
DOI: 10.1021/acs.orglett.7b02823
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
trifluoromethyl radical F and Ni L , a CF Ni L species (H) was
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ASSOCIATED CONTENT
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(
PDF)
AUTHOR INFORMATION
Corresponding Author
E-mail: chxyzpf@hotmail.com.
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ORCID
Pengfei Zhang: 0000-0001-9859-0237
Notes
The authors declare no competing financial interest.
1
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ACKNOWLEDGMENTS
■
Financial support was provided by PCSIRT (IRT 1231), the
Natural Science Foundation of China (No. 21376058), and the
Major Scientific and Technological Innovation Projects of
Hangzhou City (No. 20162011A036).
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DOI: 10.1021/acs.orglett.7b02823
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