Heterogeneous Rhodium-Catalyzed Aerobic Oxidative Dehydrogenative Cross-Coupling: Nonsymmetrical Biaryl Amines

Article abstract of DOI:10.1002/anie.201600400

The first heterogeneously catalyzed oxidative dehydrogenative cross-coupling of aryl amines is reported herein. 2-Naphthylamine analogues were reacted with various electron-rich arenes using a heterogeneous Rh/C catalyst under mild aerobic conditions, thus affording nonsymmetrical biaryl amines in excellent yields with high selectivities. This reaction provides a mild, operationally simple, and efficient approach for the synthesis of biaryls which are important to pharmaceutical and materials chemistry. A jab-cross move: A heterogeneously catalyzed oxidative dehydrogenative cross-coupling of aryl amines is reported. Aryl amines were treated with various arenes using a heterogeneous Rh/C catalyst under mild aerobic conditions to selectively afford cross-coupled products, and provides an efficient synthetic method for the preparation of nonsymmetrical biaryl amines by oxidative C-H activation.

Full text of DOI:10.1002/anie.201600400

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201600400
German Edition: DOI: 10.1002/ange.201600400
Cross-Coupling
Heterogeneous Rhodium-Catalyzed Aerobic Oxidative
Dehydrogenative Cross-Coupling: Nonsymmetrical Biaryl Amines
Kenji Matsumoto, Masahiro Yoshida, and Mitsuru Shindo*
Abstract: The first heterogeneously catalyzed oxidative dehy-
drogenative cross-coupling of aryl amines is reported herein. 2-
Naphthylamine analogues were reacted with various electron-
rich arenes using a heterogeneous Rh/C catalyst under mild
aerobic conditions, thus affording nonsymmetrical biaryl
amines in excellent yields with high selectivities. This reaction
provides a mild, operationally simple, and efficient approach
for the synthesis of biaryls which are important to pharma-
ceutical and materials chemistry.
H
eterogeneous metal catalysts are critical to the synthesis
of fine chemicals and functional materials owing to their
advantages such as high efficiency, robustness, and facile
recyclability and reusability.[1,2] Biaryls are privileged struc-
tures found in many natural products, pharmaceuticals, and
À
Scheme 1. Aryl–aryl bond formation by cross-coupling of aryl amines.
BDD=boron-doped diamond, HFIP=1,1,1,3,3,3-hexafluoro-2-propa-
nol, mCPBA=m-chloroperbenzoic acid, TFA=trifluoroacetic acid,
TFE=2,2,2-trifluoroethanol.
liquid crystals. The direct arylation through C H bond
activation has become one of the most attractive synthetic
strategies to produce symmetrical and nonsymmetrical biar-
yls, because the reactants do not have to be prefunctionalized,
and because of the atom and step economy.[3,4] However, only
few examples of direct arylation using heterogeneous cata-
lysts have been reported. In 2013, Glorius and co-workers
reported the first heterogeneously catalyzed direct arylation
with aryl chlorides and aryliodonium salts.[5] There still
remains ample room to develop heterogeneous metal-cata-
catalysts in combination with mCPBA (Scheme 1a).[8e] Wald-
vogel and co-workers have developed electrochemical oxida-
tive phenol–aniline cross-coupling with high selectivity
(Scheme 1b).[9e] Despite these advances, there remains no
general method for aniline–aniline cross-coupling.[10] Further-
more, replacement of the stoichiometric oxidant with molec-
ular oxygen represents an important advance and thus direct
catalytic cross-coupling using molecular oxygen as the only
oxidant is highly desirable.[11] Herein, we demonstrate the first
heterogeneously catalyzed aerobic oxidative dehydrogena-
tive cross-coupling of aryl amines (Scheme 1c). Our method-
ology enables a concise and convenient preparation of
nonsymmetrical biaryls using air or oxygen at room temper-
ature.
Recently, we found that rhodium on carbon functions as
an excellent catalyst for the oxidative homocoupling of the
aryl amine 1 under mild reaction conditions to provide the
dehydrodimer 2 in a high yield (Scheme 2).[12] Notably, this
reaction can be carried out with low catalyst loading using air
as a terminal oxidant, which is especially advantageous
À
lyzed C H bond activation strategies.
À
À
Among direct arylation methods, oxidative C H/C H
cross-coupling between two distinct arenes, also known as
cross-dehydrogenative coupling (CDC), is an efficient and
promising strategy to synthesize a variety of biaryls.[6] How-
ever, the oxidative cross-coupling between two arenes with
similar chemical and physical properties, such as phenol–
phenol or aniline–aniline coupling, are still difficult because
of the concomitant formation of homocoupling products and
thus only limited success has been reported until recently.[7–9]
In particular, oxidative cross-coupling of aryl amines remains
largely unexplored because aryl amines are easily oxidized,
and thus generate many side products. Recently Kita and co-
workers have reported the metal-free oxidative cross-cou-
pling of N-Ms-protected aryl amines using organoiodine
[*] Prof. M. Shindo
Institute for Materials Chemistry and Engineering, Kyushu University
6-1, Kasuga-koen, Kasuga, 816–8580 (Japan)
E-mail: shindo@cm.kyushu-u.ac.jp
Dr. K. Matsumoto, Prof. M. Yoshida
Faculty of Pharmaceutical Sciences, Tokushima Bunri University
180 Nishihama-Boji, Yamashiro-cho, Tokushima 770–8514 (Japan)
Scheme 2. Heterogeneous Rh/C-catalyzed homocoupling of aryl
amines.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600400.
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Table 1: Optimization for dehydrogenative cross-coupling of the aryl
amine 3a.
because previous methods usually require a stoichiometric
amount of metal salts or oxidants.[13] Thus, we envisioned
extending the homocoupling to a heterogeneously catalyzed
CDC reaction. Based on our previous results, the aryl amines
3 can be oxidized by a rhodium catalyst to afford the radical
cation 4, which reacts with 3 to give the dehydrodimer 5
(Scheme 3). Thus, we hypothesized that the homocoupling
could be suppressed if 3 had sterically hindered substituents
on the amino group, thus the resulting radical cations 4 would
preferentially react with sterically less hindered arenes (6) to
provide the cross-coupled biaryls 7.
Entry Catalyst
Solvent air/
O2
t
Yield
7aa/dehydro
dimer of 3a
[h] [%][a]
1
2
3
4
5
6
7
8
–
TFA
TFA
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
air
air
air
air
air
80
16
42
46
42
42
17
12
20
80
40
33
5
n.r.
85
9
23
4
–
5% Rh/C
10.9:1
2.0:1
10.0:1
2.6:1
6.3:1
1.4:1
>20:1
6.3:1
2.6:1
16.7:1
14.3:1
18.7:1
16.7:1
0.5:1
–
5% Rh/Al2O3 TFA
5% Ru/C TFA
5% Ru/Al2O3 TFA
10% Pd/C TFA
5% Pd/Al2O3 TFA
5% Pt/C
5% Pt/Al2O3
PtO2
3% Cu/C
5% Rh/C
5% Rh/C
5% Rh/C
5% Rh/C
5% Rh/C
6
36
90
55
9
12
59
84
78
<10
n.r.
TFA
TFA
TFA
TFA
TFA
TFA
TFA
AcOH
CH2Cl2
9
10
11
12
13[b]
14[c]
15[d]
16[d]
26
24
24
Scheme 3. Working hypothesis for cross-coupling of aryl amines.
[a] Yield of isolated product. [b] 608C. [c] 5.0 equiv of 6a was used.
[d] 1.5 equiv of 6a was used. n.r. =no reaction.
If the nucleophilicity of 3 toward radical cations can be
controlled by electronic and steric effects, the cross-coupling
of 3 should be enabled. Thus, we selected N,N-dimethyl-
amino-2-naphthalene (3a) as a substrate with a bulky amino
group and examined the effects of various catalysts in the
cross-coupling of 3a with 3 equivalents of 6a in trifluoroacetic
acid (TFA) under oxygen (Table 1). In the presence of 5%
Rh/C (5 mol % of rhodium),[14] the desired cross-coupled
product 7aa was obtained in 85% along with a small amount
of the dehydrodimer of 3a (entry 2). No reaction was
observed in the absence of catalysts (entry 1). With Rh/
Al2O3, the yield of 7aa was dramatically decreased because of
the slow conversion (entry 3). While other heterogeneous
catalysts resulted in low yields and poor selectivities
Table 2: Dehydrogenative cross-coupling of various aryl amines.[a]
3a
3b
3c
3d
3e
3 f
NR2
NMe2
NEt2
NiPr2
7
7aa
85
7ba
84
7ca
93 (92)[b]
>20:1
22
7da
79
11.2:1
50
7 fa
19
1:1.3
4
Yield [%]
cross/homo 10.9:1 >20:1
t [h] 16 43
trace
–
45
(entries 4–11), Pt/C afforded
a superior yield of 7aa
(entry 8).[15] Under aerobic conditions, cross-coupling
between 3a and 6a resulted in a decreased yield of 7aa
because of the slow conversion (entry 12). When the reaction
temperature was elevated to 608C (entry 13) or 5 equivalents
of 6a were employed (entry 14), the cross-coupling proceeded
more efficiently to give 7aa in 84% and 78%, respectively.
We also assessed several solvents (entries 15 and 16) and
found that TFAwas the most effective, probably because of its
ability to stabilize radical cation intermediates.[16]
With the optimized reaction conditions in hand, we
investigated the cross-coupling using various 2-naphthyl-
amines (Table 2). When the N,N-diethylamino analogue 3b
and piperidino analogue 3c were employed, their homocou-
pling products were not observed and the desired products
7ba (84%) and 7ca (93%), respectively, were obtained.
Furthermore, even using a small amount of 6a, the reaction of
[a] Reaction conditions: 3, 6a (3.0 equiv), 5% Rh/C (5 mol%), TFA, RT,
O2. Yield is that of the isolated product. [b] The yield given within
parentheses was obtained using 1.5 equiv of 6a at 508C.
3c provided an excellent yield of 7ca. The morpholino
analogue 3d also afforded 7da in good yield but the
selectivity was not better than that with 3c. In contrast, the
reaction of the N,N-diisopropylamino analogue 3e was very
sluggish owing to the steric hindrance of the bulky amino
group and the pyrrolidino analogue 3 f resulted in a sharp
decrease in the yield of 7 fa. Probably, since pyrrolidine is less
sterically hindered than diethylamino and piperidino groups,
the homocoupling of 3 f proceeded faster.[17] These results
revealed the effect of amino substituents on the selectivity of
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Table 3: The cross-coupling of aryl amines with aromatic nucleophiles.[a]
Table 4: Scope of dehydrogenative cross-coupling of aryl amines.[a]
NMe2 (3a)
Piperidino (3a)
6
7
yield[c] cross/
7
yield[c] cross/
[%]
homo
[%]
homo
7ab
97
>20:1
7cb
89
>20:1
7ac
7ad
42
26
2.1:1
7cc
7cd
95
92
>20:1
>20:1
0.78:1
[a] Reaction conditions: 3, 6 (3.0 equiv), 5% Rh/C (5 mol%), TFA, 608C,
air. [b] Conducted at RT under O2. [c] Yield of isolated product.
[a] Reaction conditions: 3, Ar-H (3.0 equiv), 5% Rh/C (5 mol%), TFA,
608C. Yield is that of the isolated product. [b] 5% Rh/C (0.27 mol% of
rhodium).
the reaction. Consequently, 3b and 3c afforded the most
efficient and selective cross-coupling.
Next, we examined the cross-coupling of 3a and 3c with
several electron-rich arenes (6; Table 3). Phenols were also
good coupling partners. The coupling between 3a and 3c with
phenol 6b afforded 7ab and 7cb, respectively, in excellent
yields. However, the coupling between 3a and either N-
phenylmorpholine (6c) or 2-methylanisole (6d) were less
selective and resulted in decreased yields of 7ac and 7ad,
respectively. In contrast, the cross-coupling of 3c with either
6c or 6d proceeded selectively to give 7cc (95%) and 7cd
(92%), respectively. Probably, since 6c and 6d were less
nucleophilic than 6a and 6b, the difference in nucleophilicity
between 3a and 6c (or 6d) became smaller and thus the
relative amount of homocoupling of 3a increased.[18,19] Since
3c is less nucleophilic than 3a as a result of the large
piperidino group, even 6c and 6d function as more powerful
nucleophiles toward 3c and the high selectivity is obtained
when using 3c. These insights support our hypothesis shown
in Scheme 1.[20] The steric hindrance of the aryl amines and
the nucleophilicity of coupling partners are important to
obtain the cross-coupled product rather than the homocou-
pling product.
was comparable to that on a small scale. Furthermore, even
using 0.27 mol % of 5% Rh/C, 7ga was obtained in good yield
and the turnover number (TON) reached up to 280.[21]
To demonstrate the potential applications of the present
cross-coupling, the preparation of versatile 1,1’-binaphthyl-
based ligands was examined (Scheme 4). Cross-coupling of 3a
with an excess of 2-naphthol proceeded to give the NOBIN
analogue 8 in 62%.[22] The N,N-dibenzyl group of 7ga was
easily removed by Pd/C-catalyzed hydrogenolysis to give 9 in
97%. Since the resulting primary amino group can be used for
Since excellent selectivity was observed with 3c, we
investigated the scope of the CDC (Table 4). Various
substituted anilines and phenols were reacted with 3c to
give the cross-coupled biaryls 7ce, 7cf, 7cg, and 7ch in
excellent yields and selectivities. Several anisoles also
afforded good yields of 7ci, 7cj, and 7ck. N,N-Dibenzyl-
amino-2-naphthalene (3g) and 3b reacted with anilines to
give the corresponding 7ga and 7bc efficiently. The reaction
of 3g on a large scale also proceeded in excellent yield, and
Scheme 4. Synthetic utility of the developed cross-coupling reactions.
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In conclusion, we developed the first heterogeneous,
catalytic, aerobic cross dehydrogenative coupling of amino-
naphthalenes with electron-rich arenes to provide nonsym-
metrical biaryls in high yields and selectivities. This reaction
provides a mild and operationally simple approach for the
synthesis of biaryl amines. Further studies regarding the
synthetic applications and mechanistic details are underway
in our laboratory.
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Acknowledgments
We thank Kozue Kodama (Kyushu University) for assistance
with spectral data measurements. This work was partially
supported by a Grant-in-Aid for Exploratory Research
(No.22659023 and No.26670003), for Scientific Research (B)
(No.26293004) from MEXT, Science and Technology
Research Promotion Program for Agriculture, forestry, fish-
eries and food industry (M.S.), and the Asahi Glass Founda-
tion (K.M.).
Keywords: biaryls · cross-coupling · green chemistry ·
heterogeneous catalysis · rhodium
ˇ
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5272–5276
Angew. Chem. 2016, 128, 5358–5362
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oxygen (1 atm) provided 93% of 7ga. This result revealed that
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Received: January 15, 2016
Revised: February 28, 2016
Published online: March 21, 2016
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