Copper-catalyzed intermolecular amination of acidic aryl C-H bonds with primary aromatic amines

Article abstract of DOI:10.1002/adsc.200900856

A copper-catalyzed intermolecular aerobic oxidative C-H bond amination of various polyfluorobenzenes with an array of primary aromatic amines was achieved for the first time. The reactivity of the polyfluorobenzenes was observed to depend on the acidity of C-H bonds. Under similar reaction conditions, the C-H bond amination of azoles also occurred.

Full text of DOI:10.1002/adsc.200900856

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DOI: 10.1002/adsc.200900856
À
Copper-Catalyzed Intermolecular Amination of Acidic Aryl C H
Bonds with Primary Aromatic Amines
Huaiqing Zhao,a,b Min Wang,a Weiping Su,a,* and Maochun Honga
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy
of Sciences, Fuzhou, Fujian 350002, Peopleꢀs Republic of China
Fax : (+86)-591-8377-1575; phone: (+86)-591-8377-1575; e-mail: wpsu@fjirsm.ac.cn
Graduate University of Chinese Academy of Sciences, Beijing 100039, Peopleꢀs Republic of China
b
Received: December 12, 2009; Revised: March 28, 2010; Published online: May 3, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000856.
Abstract: A copper-catalyzed intermolecular aero-
bic oxidative C H bond amination of various poly-
eties of nitrogen nucleophiles provided a strategic so-
lution to this problem, as illustrated by the elegant
studies of Yu[3b–d] and White.[3h–i,5a,b] By using a Pd(II)/
Pd(IV) catalyst system, Gaunt[3f] achieved the intra-
À
fluorobenzenes with an array of primary aromatic
amines was achieved for the first time. The reactivi-
ty of the polyfluorobenzenes was observed to
À
molecular C H bond amination of secondary amines
À
depend on the acidity of C H bonds. Under similar
under ambient conditions, demonstrating the poten-
tial of the high-valent metal catalyst system to pro-
À
reaction conditions, the C H bond amination of
azoles also occurred.
À
mote the C H bond amination reaction involving
amines as nucleophiles. Stahl and co-workers dis-
À
Keywords: amination; aromatic amines; C N bond
formation; copper catalysis
closed that a Cu
with nitrogen nucleophiles to generate C N bonds.
Notably, copper catalysts have proved to be active
and versatile in oxidative amination of both arylbor-
onic acids[9] and alkynes,[6b] where a broad range of
amines could be used as coupling partners. These
ACHTUNGTRNE(NUNG III)-aryl complex readily reacted
[8]
À
Although many classes of arylamines are now accessi- seminal studies led us to envision that Cu-catalyzed
[10,11,12]
À
ble by metal-catalyzed cross-coupling reactions of aryl intermolecular amination of acidic C H bonds
halides with amines,[1] the prevalence of aromatic C
with amines is possible under oxidative conditions.
N bonds in pharmaceuticals and bioactive natural Herein, we report a Cu-catalyzed method for inter-
products continues to give an impetus to the develop- molecular C H bond amination of electron-deficient
ment of new methods for their construction. In this polyfluoroarenes with an array of primary aromatic
À
À
À
context, the direct amination of C H bonds repre- amines, which occurred under mild conditions using
sents an extremely attractive objective because such a O2 in combination with 2,2,6,6-tetramethyl-1-piperidi-
transformation affords the most efficient synthetic nyloxy (TEMPO) as the oxidant (Scheme 1). The flu-
route to valuable target compounds.[2] Recently, sig- orinated diarylamines generated from this reaction
nificant advances have been made in the development have demonstrated importance in medicinal chemis-
[3,4a–d]
À
of the intramolecular amination of C H bonds,
try.[13] This protocol also enables the intermolecular
À
and the pioneering studies by Yuꢀs group and others C H bond amination of azoles.
have also contributed to the catalytic intermolecular
Our investigation began by examining the reaction
amination of C H bonds using amides as nitrogen nu- of 2,3,5,6-tetrafluoroanisole (1) with 4-nitroaniline (2)
À
cleophiles.[4e–h,5,6a–e] However, to date, the intermolecu-
lar amination of C H bonds with amines remains a
À
challenging goal.[6f,7]
À
An inherent challenge in the amination of C H
bonds arises from the coordination of nitrogen nucle-
ophiles to the metal catalyst center, which reduces the
electrophilicity of metal catalyst and hence interferes
À
with the electrophilic C H bond cleavage. Installation
of the electron-withdrawing groups on the N H moi- bonds with primary aromatic amines.
À
Scheme 1. Copper-catalyzed amination of acidic aryl C H
À
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Huaiqing Zhao et al.
Table 1. Selected screening results for Cu-catalyzed amination of 2,3,5,6-tetrafluoroanisole with 4-nitroaniline.[a]
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Entry
Amine (equiv.)
Base (equiv.)
Additives (equiv.)
Solvent
Yield[b] [%]
1[c]
2
3
4
5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
2.5
3
3
3
3
3
2
2
2
2
2
2
2
2.5
2.5
2.5
2.5
2.5
2.5
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
DMF
DMF
trace
41
59
TEMPO (1)
TEMPO (0.5)
pyCl2NO (1)
60
trace
trace
71
77
88
17
68
66
64
6
7
8
9
NMO (1)
TEMPO (0.5)
TEMPO (0.5)
TEMPO (0.5)
TEMPO (3)
TEMPO (0.5)
TEMPO (0.5), py (0.8)
TEMPO (0.5), TBAA (1)
10[d]
11
12
13
[a]
0.2 mmol 1, DMF (2 mL). Reaction temperature and time were optimized.
Isolated yields.
[b]
[c]
[d]
This reaction was run in the absence of Cu
This reaction was run under nitrogen.
ACHTUNGERTN(NUNG OAc)2.
in the presence of t-BuOK (2–2.5 equiv.) as a base oxidant, the reaction carried out under anaerobic con-
and Cu(OAc)2 (20 mol%) as a catalyst under 1 atm of ditions yielded the desired product in a much lower
ACHTUNGTRENNUNG
O2. Interestingly, the reaction 1 with 2 gave rise to yield (17%) (entry 10, Table 1), presumably because
substitution of one fluoro group by the aniline nucleo- TEMPO would be reduced into the TEMPO anion
phile via an SNAr pathway in the absence of which should ligate the Cu-center to generate an un-
CuACHTUNGTRENNUNG(OAc)2 (entry 1, Table 1) while adding CuAHCTUGNTREN(NUGN OAc)2 reactive Cu species. Further experiments revealed
to the reaction system switched the reaction pathway that the use of coordinating solvents such as DMSO
À
to the C H bond amination, affording the desired and pyridine (py) or the presence of anionic ligands,
product in modest yields (entry 2, Table 1). TEMPO for example, (n-Bu)4NOAc (TBAA), led to a de-
has been demonstrated to be an efficient oxidant for crease in yield (entries 11–13, Table 1). It is unlikely
the Cu-catalyzed amination of arylboronic acids that this reaction involves a radical process since no
under air atmosphere.[9a] Indeed, introducing 1 equiva- product was observed from the trapping of radical in-
lent of TEMPO improved this reaction, affording a termediates by TEMPO.[14]
synthetically useful yield (59%) (entry 3, Table 1),
We explored the substrate scope of this reaction by
and reducing the amount of TEMPO to 0.5 equiva- employing the conditions of entry 9 in Table 1. As
lents provided a comparable result (entry 4, Table 1). shown in Table 2, many tetrafluorobenzenes that con-
However, replacement of TEMPO with its analogues tain an additional substituent reacted in good yields
such as 2,6-dichloropyridine 1-oxide (pyCl2NO) and (3a, 3l–p). Both pentafluorobenzene and pentachloro-
4-methylmorpholine N-oxide (NMO) completely shut benzene gave acceptable yields (3s, 3u). With the
down this reaction (entries 5 and 6, Table 1). Another polyACHTUNTRGNEUNGfluorobenzenes possessing more than one poten-
increase in the yield was obtained by increasing the tial site for reaction, only monoamination occurred
amounts of both amine and base relative to polyfluor- (3j, 3k, 3q and 3r). While 1,2,4-trifluorobenzene can
obenzene (entries 7 and 8, Table 1). When 3 equiva- undergo direct amination (3t), 1,3,5-trifluorobenezene
lents of 2 and 2.5 equivalents of t-BuOK were used, and 1,3-difluorobenezene were both unreactive under
À
an 88% yield was obtained (entry 9, Table 1).
any conditions tried, presumably because the C H
À
The GC analysis showed that 90% of the amount bond between two C F bonds in 1,2,4-trifluoroben-
of TEMPO could be recovered from this reaction zene is more acidic than those in 1,3,5-trifluorobene-
system, indicating that TEMPO participated in the zene and 1,3-difluorobenezene.[15] On the other hand,
catalytic process and was regenerated through a cata- tetrafluoropyridine, which is more acidic than poly-
À
lytic cycle. Using 3 equivalents of TEMPO alone as fluorobenzenes the provided C H bond amination
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Copper-Catalyzed Intermolecular Amination of Acidic Aryl C H Bonds
Table 2. Copper-catalyzed amination of electron-deficient arenes with amines.[a]
[a]
Reaction conditions: 0.2 mmol 4, 0.6 mmol 5, 0.04 mmol CuACTHNURTGNENG(U OAc)2, (0.5–0.8 mmol) t-BuOK,
0.1 mmol TEMPO, 1 atm O2, DMF (2 mL), 408C, 24 h. Isolated yields.
5 mmol scale.
Reaction temperature was 808C.
[b]
[c]
product in a substantially lower yield because substi-
With respect to aromatic amines, an electron-with-
tution of the fluoro group by the nitrogen nucleophile drawing para-substituent on the aromatic ring is nec-
occurred. This reaction can be scaled up as exempli- essary. 4-Nitroaniline and its derivatives bearing a va-
fied by the reaction of 1 with 2 where the yield ob- riety of additional substituents proved to be efficient
tained on a 5-mmol scale was similar to that on a amination reagents for this reaction (3b–f). The ani-
0.2 mmol scale (3a, Table 2).
lines bearing a cyano substituent such as 3,4-dicyano-
AHCTUNGTREGaNNNU niline and 4-cyano-3-trifluoromethylaniline (3g, 3h),
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Huaiqing Zhao et al.
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and the aminopyridines bearing an electron-withdraw- required elevated reaction temperatures (60–808C)
ing group such as methyl 6-aminonicotinate were re- (Table 3).
active, albeit in lower yields (3i). On the basis of the
Combining the reported mechanisms for similar re-
observed difference in reaction outcomes among dif- actions[6,8,9,16] and our experimental observations, we
ferent aromatic amines, we speculate that the reactivi- propose a possible mechanism to explain the Cu-cata-
À
ty increase that the para- substituted aromatic amines lyzed C H amination (Scheme 2). Based on the previ-
gained from electron-withdrawing groups could arise ously reported results that the base-promoted H/D
À
from an enhancement of the N H acidity. Alterna- exchange of polyfluorobenzenes occurred regardless
tively, it is also possible that electron-withdrawing of the presence or absence of a copper source,[12b] we
À
groups may prevent oxidation of aromatic amines, reason that the arenes bearing acidic C H bonds
which was supported by our observation that aniline would be deprotonated by the strong base and subse-
was oxidized into 1,2-diphenyldiazene and other un- quently coordinated to the copper center to form
known compounds under the standard conditions.
aryl-Cu(II) intermediate B. A similar aryl-Cu(II) spe-
The above studies have disclosed that the acidity of cies was also proposed as an intermediate in the Cu-
À
the C H bond was an important factor affecting the mediated oxidative coupling of arylboronic acids with
[9a,b]
À
À
À
C H bond amination of polyfluorobenzenes. In the nucleophiles for C N/C O bond formation,
and
À
light of this, we reasoned that the C H amination of recent EPR studies provided evidence supporting its
azoles would be possible since the acidity of the C H presence.[16] The complex B could further incorporate
À
bond in the 2-position of azoles is similar to that of the deprotonated amine via the replacement of a
polyfluorobenzenes.[15] As expected, the conditions es- ligand to yield the unsymmetrical complex C that
tablished for the direct amination of polyfluoroben- may either undergo oxidation to Cu
zenes also enabled the direct amination of azoles, but to expedite C N bond reductive elimination or direct-
ly deliver the cross-coupling product by reductive
(III) complex D
À
Table 3. Copper-catalyzed amination of azoles with amines.[a]
[a]
Reaction conditions: 0.2 mmol 6, 0.6 mmol 5, 0.04 mmol CuACTHNURTGNENG(U OAc)2, (0.5–0.7 mmol) t-BuOK,
0.1 mmol TEMPO, 1 atm O2, DMF (2 mL), 808C, 24 h. Isolated yields.
Reaction temperature was 608C.
[b]
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Copper-Catalyzed Intermolecular Amination of Acidic Aryl C H Bonds
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Scheme 2. Proposed mechanism for copper-catalyzed amination of acidic aryl C H bonds with primary aromatic amines.
BuOK (0.0561 g, 0.50 mmol) and 4-nitroaniline (0.0828 g,
elimination, followed by re-oxidation of the catalyst.
0.60 mmol). The tube was fitted with a rubber septum and
removed out of the glove box. 2 mL of DMF were added to
the Schlenk tube though the rubber septum using a syringe.
The septum-sealed tube was evacuated and refilled with O2
three times. Under oxygen, 2,3,5,6-tetrafluoroanisole (28 mL,
0.20 mmol) was added. The rubber septum was replaced
with a Teflon screwcap under an oxygen flow. The mixture
was stirred at 408C for 24 h. After cooling down, the reac-
Although coordination of aniline ligands to the Cu
center can enhance the acidity of the N H bond, it is
more reasonable to assume that deprotonation of ani-
lines precedes their coordination to the Cu center be-
cause anilines bearing electron-withdrawing substitu-
ents are very poor coordinating ligands (in fact, elec-
tron-deficient anilines should be deprotonated by t-
À
BuOK even in the absence of copper, based on the tion mixture was diluted with ether (10 mL), filtered
pKa values).[17] The proposed mechanism rationalizes
through a pad of silica gel, followed by washing the pad of
the silica gel with the same solvent (20 mL). The filtrate was
the beneficial effect of using excess anilines: the pres-
washed with water (3ꢂ15 mL) then washed with brine
ence of excess aniline accelerates the formation of un-
(15 mL). The organic phase was dried over Na2SO4, filtered,
symmetrical complex C and suppresses the symmetri-
cal CuACHTUNGTRENNUNG(aryl)2 complex. The latter Cu complex leads to
homocoupling.
It is also possible that the Cu-catalyzed amination
of azoles follows the reaction pathway proposed by
Chang for the Ag-mediated amination of azoles.[7] If
and concentrated under reduced pressure. The residue was
then purified by column chromatography on silica gel using
(5–10% ether/petroleum ether) as the eluent afforded the
product as a yellow solid; yield: 56 mg (88%).
ACTHNUTRGNEUNG
1H NMR(400 MHz), 13C NMR (100 MHz) and 19F NMR
(377 MHz) spectra were recorded respectively using
a
this process is operative, azoles would undergo nucle- Bruker AVANCE 400 spectrometer. Elemental analyses, gas
chromatography (GC) and gas chromatography-mass spec-
tra (GC-MS) were performed in the analysis center of our
institute.
ophilic attack of the deprotonated anilines and subse-
quent oxidation to form the amination product.
In conclusion, we have described a Cu-catalyzed
À
method for the intermolecular amination of C H
bonds of both polyfluorobenzenes and azoles with pri-
mary aromatic amines using O2/TEMPO as an oxida-
tion system. Investigations into the origin of the bene-
ficial effects of TEMPO and the improvement of this
Acknowledgements
Financial support from the 973 Program (2006CB932903,
2009CB939803), NSFC (20821061, 20671089, 20925102),
“The Distinguished Oversea Scholar Project”, “One Hun-
dred Talent Project”, the Knowledge Innovation Program of
CAS and the Key Project from CAS is greatly appreciated.
À
catalyst system for the intermolecular C H bond ami-
nation reaction with a broad range of substrate scope
are underway.
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À
[5] For intermolecular amination of C H bonds with
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A. R. Mazzotti, M. C. White, J. Am. Chem. Soc. 2009,
131, 11701–11706; b) S. A. Read, M. C. White, J. Am.
Chem. Soc. 2008, 130, 3316–3318; c) G. Liu, G. Yin, L.
Wu, Angew. Chem. 2008, 120, 4811–4814; Angew.
Chem. Int. Ed. 2008, 47, 4733–4736.
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Adv. Synth. Catal. 2010, 352, 1301 – 1306