A Highly Selective Palladium-Catalyzed Aerobic Oxidative Aniline-Aniline Cross-Coupling Reaction

Article abstract of DOI:10.1021/acs.orglett.9b02527

The first catalytic oxidative aniline-aniline cross-coupling reaction using oxygen as the terminal oxidant is reported. Anilines possessing a pyrrolidino group can be preferentially oxidized under mild aerobic conditions and reacted with other anilines to afford a variety of nonsymmetrical 2-aminobiphenyls with high selectivities. A heterogeneous palladium catalyst is used for the dehydrogenative cross-coupling of anilines with structurally diverse arenes. This reaction does not require stoichiometric oxidants and is an economical and environmentally friendly method.

Full text of DOI:10.1021/acs.orglett.9b02527

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
A Highly Selective Palladium-Catalyzed Aerobic Oxidative Aniline−
Aniline Cross-Coupling Reaction
Kenji Matsumoto,* Satoshi Takeda, Tsukasa Hirokane, and Masahiro Yoshida
Faculty of Pharmaceutical Sciences, Tokushima Bunri University, 180 Nishihama-Boji, Yamashiro-cho, Tokushima 770-8514, Japan
S
* Supporting Information
ABSTRACT: The first catalytic oxidative aniline−aniline
cross-coupling reaction using oxygen as the terminal oxidant
is reported. Anilines possessing a pyrrolidino group can be
preferentially oxidized under mild aerobic conditions and
reacted with other anilines to afford a variety of nonsymmetrical
2-aminobiphenyls with high selectivities. A heterogeneous
palladium catalyst is used for the dehydrogenative cross-
coupling of anilines with structurally diverse arenes. This
reaction does not require stoichiometric oxidants and is an economical and environmentally friendly method.
Scheme 1. Oxidative Cross-Coupling of Aryl Amines
onsymmetrical biaryls are fundamental scaffolds for
1
Nnatural products, catalysts, and industrial materials. In
particular, 2-aminobinaphthyls (NOBINs) are prominent
functionalized nonsymmetrical biaryls that are used in
asymmetric catalysis and are key sources of chiral ligands.2
Furthermore, 2-aminobiphenyls are widely found in bio-
logically active natural products and pharmaceuticals, such as
anticancer agents and anti-HIV agents,3 and are also employed
for the expeditious preparation of key terpene indole alkaloid
intermediates.4 Consequently, many synthetic approaches for
the efficient construction of the biaryl linkage in nonsym-
metrical 2-aminobiaryls have been developed.5 Among them,
the direct oxidative (dehydrogenative) cross-coupling of aryl
amines is the most promising strategy for the preparation of
nonsymmetrical 2-aminobiaryls, mainly because the approach
circumvents the necessity for substrate prefunctionalization
and delivers high atom and step economies.6 The catalytic
oxidative coupling of naphthols and phenols has been well-
studied, and the selective cross-coupling has also been
reported.7,8 In contrast, the oxidative coupling of aryl amines
has received less attention, and the catalytic methods available
for these coupling reactions are still limited because of the
facile oxidation of aryl amines, which can lead to the formation
of undesired competitive products.9−11 Recently, Kita and co-
workers reported a catalytic oxidative cross-coupling of N-
sulfonyl anilides with electron-rich arenes using hypervalent
iodine reagents.11a However, catalytic cross-couplings of
different aryl amines with similar chemical properties, such
as aniline−aniline or anilide−anilide cross-couplings, remain
highly challenging and suffer from low coupling selectivities
due to the small differences in the oxidation potentials of the
coupling partners. Although Waldvogel and co-workers
developed an interesting electrochemical oxidative anilide−
anilide cross-coupling (Scheme 1A),12 unprotected anilines are
unsuitable for such electrochemical reactions because of their
high tendency for anodic overoxidation.
We previously studied heterogeneous metal-catalyzed
aerobic oxidative couplings13 and developed reactions for the
homocoupling and selective cross-coupling of aryl amines for
the preparation of symmetrical and nonsymmetrical 2-
aminobiaryls in high yields (Scheme 1B).14 Despite these
advances, there have been no reports on the catalytic aniline−
aniline cross-coupling reaction. Continuing our interest in this
area, we sought to develop the catalytic cross-coupling of two
different anilines. Since molecular oxygen is considered the
ideal oxidant, the direct oxidative cross-coupling of anilines
using molecular oxygen as the sole oxidant is highly
desirable.15 Herein we demonstrate the first catalytic
dehydrogenative aniline−aniline cross-coupling reaction with
Received: July 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02527
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a heterogeneous palladium catalyst under mild aerobic
conditions (Scheme 1C).16
Table 1. Optimization of the Catalytic Aniline−Aniline
a
Cross-Coupling Reaction
For the selective aniline−aniline cross-coupling to be
successful, overoxidation and homocoupling need to be
prevented. In our previous report on the naphthylamine−
aniline cross-coupling reaction,14b the lower oxidation
potential of naphthylamine 1 due to its extended conjugation
compared to that of N,N-dimethylaniline (2) allowed its
preferential oxidation in the presence of 2 to facilitate the
formation of cross-coupled product 3 with high selectivity
(Scheme 2). A difference in the oxidation potentials of the two
entry
catalyst
acid
time (h)
yield (%)
55
6
1
2
3
4
5
6
7
8
9
5% Rh/C
5% Rh/C
5% Rh/C
5% Rh/C
5% Rh/Al2O3
5% Pt/C
5% Pt/Al2O3
5% Pd/C
5% Pd/Al2O3
5% Ru/C
5% Cu/C
5% Pd/Al2O3
−
TFA
MsOH
74
53
46
46
45
5
3.5
54
23
112
70
97
23
23
Scheme 2. Working Hypothesis for the Catalytic
Dehydrogenative Aniline−Aniline Cross-Coupling Reaction
CHF2CO2H
CH3CO2H
CHF2CO2H
CHF2CO2H
CHF2CO2H
CHF2CO2H
CHF2CO2H
CHF2CO2H
CHF2CO2H
−
62
no reaction
28
2
31
59
82
no reaction
no reaction
no reaction
no reaction
51
10
11
12
13
CHF2CO2H
CHF2CO2H
b
14
5% Pd/Al2O3
a
4a, 2 (3 equiv), catalyst (5 mol %), acid (10 equiv), C6H5CF3, rt, O2.
b
Isolated yields are presented. 1.5 equiv of 2 was used.
anilines would therefore clearly enable the desired aniline−
aniline cross-coupling to take place. Since the pyrrolidino
group is a highly powerful electron-donating substituent,17 we
hypothesized that N-aryl pyrrolidine 4 would have a lower
oxidation potential than 2, which could promote its selective
oxidation to afford radical cation 5.18 The preferential reaction
of 5 with the less sterically hindered aniline 2 would provide
nonsymmetrical 2-aminobiphenyl 6 instead of the homocou-
pling products.
acid was the most effective catalytic system for this cross-
coupling reaction under mild aerobic conditions.19
With the optimized conditions in hand, we subsequently
investigated the scope of the cross-coupling reaction using
various N-aryl pyrrolidines (Scheme 3). When the p-tert-butyl-
substituted aniline was employed, the desired product 6b was
obtained in 92% yield with no detectable formation of the
homodimer. The reactions of anilines possessing electron-
donating substituents such as methoxy, phenoxy, and 2-
acetoxyethyl groups at the para position also proceeded to
afford 6c−e, respectively, in good to moderate yields with high
cross-coupling selectivities. When the reaction rate was too
low, the rate and yield could generally be improved by
increasing the amount of acid. An aniline with an electron-
withdrawing ester substituent was also tolerated, affording 6f in
74% yield despite the use of TFA as the solvent. Furthermore,
3-fluorine-substituted aniline reacted with 2 to provide 6g in
90% yield. Other 3,4-disubstituted anilines were also efficiently
cross-coupled to afford the desired products (6h−j) in high
regioselectivities.
Having achieved excellent yields with high cross-coupling
selectivities using 4, we investigated the dehydrogenative cross-
coupling reaction using various electron-rich arenes (Scheme
4). Thus, various substituted anilines were reacted with 4a and
4b to afford 7a, 7b, 7c, 7e, and 7f in good yields with high
selectivities. Among them, 3-substituted anilines were suitable
coupling partners, affording 7c, 7e, and 7g in excellent yields.
However, no reaction occurred with N-phenylmorpholine
because of its low nucleophilicity. The presence of an amide
group was also tolerated, and the reaction with acetaminophen
afforded 7h in 66% yield. On the basis of these remarkable
results, we then conducted the cross-coupling of 4b with
various phenols and anisoles. The cross-coupling reaction
using 2,6-dimethylphenol yielded the desired product 7i in
80% yield, while the reactions at the ortho positions of 4-
Thus, N-(4-biphenyl)pyrrolidine (4a) was selected as the
substrate for optimization of the aniline−aniline cross-coupling
reaction with 2 using oxygen as the terminal oxidant (Table 1).
In a previous study,14a,b we found that a heterogeneous Rh/C
catalyst could effectively promote the oxidative coupling of aryl
amines, and the use of a suitable acid was effective for
preventing side reactions such as overoxidation. Building on
these conditions, we attempted the cross-coupling of 4a and 2
with 5 mol % Rh/C in the presence of trifluoroacetic acid
(TFA) and observed the selective formation of the desired
cross-coupled product 6a in 55% yield with no detectable
formation of the product from homocoupling of 4a (entry 1).
We subsequently examined the effect of various acids on this
reaction and found difluoroacetic acid to be the most efficient
under the standard conditions (entries 2−4). The use of
another rhodium catalyst, Rh/Al2O3, delivered 6a in a lower
yield (entry 5). While catalysis with Pt/C and Pt/Al2O3
afforded unsatisfactory results because of competitive reac-
tions, the Pd/Al2O3-catalyzed reaction proceeded efficiently to
afford 6a in 82% yield (entries 6−9). In contrast, Ru/C and
Cu/C were found to be less active (entries 10 and 11). Neither
the catalyst nor difluoroacetic acid alone was able to afford the
desired product (entries 12 and 13). When a smaller amount
of 2 was used, the yield decreased to 51%, although the
reaction proceeded efficiently (entry 14). It was therefore
apparent that the combination of Pd/Al2O3 and difluoroacetic
B
DOI: 10.1021/acs.orglett.9b02527
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Cross-Coupling of Various N-Aryl Pyrrolidines
with N,N-Dimethylaniline
Scheme 4. Cross-Coupling of N-Aryl Pyrrolidines with
a
a
Various Arenes
a
Reaction conditions: 4, 2 (3 equiv), 5% Pd/Al2O3 (5 mol %),
CHF2CO2H (10 equiv), CF3C6H5, rt, O2. Isolated yields are
b
c
d
presented. 30 equiv of CHF2CO2H was used. TFA was used. 6-
isomer:2-isomer = 16:1.
substituted phenols and 2-naphthol afforded 7j−l in excellent
yields with perfect selectivities. When the highly activated
anisole 1,2,4-trimethoxybenzene was used, the yield of 7m
decreased to 30% because of the preferential homocoupling of
the anisole. To our delight, the reaction with 2,5-dimethox-
ytoluene proceeded well to afford 7n in 73% yield. These
results therefore indicate that the reaction has a broad
substrate scope and that N-aryl pyrrolidines can be cross-
coupled with a wide range of electron-rich arenes such as
anilines, phenols, and anisoles in an efficient manner.
a
Reaction conditions: 4, arene (3 equiv), 5% Pd/Al2O3 (5 mol %),
CHF2CO2H (30 equiv), CF3C6H5, rt, O2. Isolated yields are
presented. 10 equiv of CHF2CO2H was used.
b
Scheme 5. Synthetic Utility of the C−H/C−H Cross-
Coupling Reaction
To demonstrate the potential application of the present
cross-coupling reaction, the late-stage modification of a
pharmaceutical agent was examined (Scheme 5). The cross-
coupling of 4b with estrone, which is a known estrogen
receptor agonist, proceeded smoothly under the standard
conditions to give optically active 7o in 78% yield in a highly
selective manner. The structure of 7o was unambiguously
assigned by X-ray analysis. These nonsymmetrical 2-amino-
biphenyls are expected to have broad utilities as catalyst ligands
and organocatalysts and in drug discovery.
To gain insight into the reaction mechanism, several control
experiments were then conducted (Scheme 6). Initially, the
reaction was performed under an air or argon atmosphere;
however, the yields of 6a decreased dramatically in both cases,
suggesting that oxygen is critical for promoting the present
heterogeneous metal-catalyzed cross-coupling reaction. Sub-
sequently, we found that when the radical trapping agent
TEMPO was added under the standard conditions, the
reaction failed to produce the desired product. With the use
of galvinoxyl as an alternative radical trapping reagent, the
cross-coupling reaction was once again completely inhibited.
These results suggested that a radical cation is likely involved
as a reactive species under the reaction conditions employed
herein. Furthermore, the N,N-dimethylamino analogue 8 and
piperidino analogue 9 failed to afford the expected products,
C
DOI: 10.1021/acs.orglett.9b02527
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Organic Letters
Letter
mentally friendly alternative to previously reported methods.
Further studies of the synthetic applications of our method and
additional mechanistic details are underway, and the results
will be presented in due course.
Scheme 6. Control Experiments To Probe the Mechanism
of the Cross-Coupling Reaction
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02527.
Experimental details and 1H NMR and 13C NMR
spectra (PDF)
Accession Codes
CCDC 1936765 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
indicating the significant effect of the amine substituent in
achieving a highly selective and efficient aniline−aniline cross-
coupling. On the basis of these results, a proposed mechanism
is presented in Scheme 7. More specifically, ammonium ion 10
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kmatsumoto@ph.bunri-u.ac.jp
ORCID
Scheme 7. Proposed Mechanism
Kenji Matsumoto: 0000-0002-0294-7092
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Dr. H. Imagawa and Prof. Dr. H. Kaku
(Tokushima Bunri University) for X-ray diffraction analysis
and R. Nakano (Tokushima Bunri University) for assistance
with the synthesis and analysis of the pyrrolidine starting
materials. This work was partly supported by JSPS KAKENHI
Grants 17K15430 and 19K06985 from MEXT, The Futaba
Foundation, and the Cooperative Research Program of the
Network Joint Research Center for Materials and Devices.
undergoes a one-electron transfer reaction with the palladium
catalyst, resulting in the formation of radical cation
intermediate 11 and the reduced palladium species.20,21
Radical cation 11 then couples preferentially with the other
less sterically hindered aniline, and subsequent tautomerization
affords the cross-coupled product 12 with excellent selectivity
since the bulky amino group on the aniline suppresses
homocoupling at the ortho position. In contrast, since the
dimethylamino and piperidino groups are more bulky but less
nucleophilic than pyrrolidine,22 the homo- and cross-coupling
reactions of 8 and 9 do not proceed. The para substituents of
10 were also necessary for the efficient cross-coupling and
required to prevent the homocoupling of radical cation 11 at
the para position. Subsequently, molecular oxygen oxidizes the
reduced catalyst, regenerating the active palladium metal.
In conclusion, we have successfully developed the first
heterogeneously catalyzed aerobic oxidative aniline−aniline
cross-coupling reaction, which provides nonsymmetrical 2-
aminobiphenyls in high yields and selectivities. The presented
method operates under mild conditions, displays a broad
scope, and allows the synthesis of a range of substituted and
optically active 2-aminobiphenyls in good to excellent yields.
The combination of the heterogeneous catalyst with oxygen as
a terminal oxidant provides an economical and environ-
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