Oxidative Biaryl Coupling of N -Aryl Anilines by Using a Hypervalent Iodine(III) Reagent

Article abstract of DOI:10.1055/s-0036-1590875

Biaryl diamines are important building blocks in organic synthesis. Consequently, it is desirable to develop a general and mild synthetic approach to diverse biaryl diamines. Oxidative coupling is an efficient and promising strategy for the synthesis of these targets. We have now developed a direct formation of biaryl diamines by oxidative coupling using a hypervalent iodine(III) reagent.

Full text of DOI:10.1055/s-0036-1590875

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© Georg Thieme Verlag Stuttgart · New York
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K. Morimoto et al.
Letter
Synlett
Oxidative Biaryl Coupling of N-Aryl Anilines by Using a
Hypervalent Iodine(III) Reagent
Koji Morimotoa,b
Oxidative Biaryl Coupling Reaction of Aromatic Amine Derivatives
NR2Ar
Doichi Kosekia
Toshifumi Dohia
Yasuyuki Kita*b
R1
hypervalent
NR2Ar
iodine(III) reagent
R1
a Ritsumeikan University College of Pharmaceutical Sciences
1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
b Ritsumeikan University, Research Organization of Science
and Technology, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577,
Japan
R1
Yield up to 94%
hypervalent iodine(III) as the oxidant
NR2Ar
kita@ph.ritsumei.ac.jp
Dedicated to Victor Snieckus in honor of his 80th birthday
Received: 16.06.2017
Accepted after revision: 20.07.2017
Published online: 26.10.2017
Oxidative Biaryl Coupling Reaction of Anilines
NR2Ar
R1
hypervalent
NR2Ar
DOI: 10.1055/s-0036-1590875; Art ID: st-2017-r0483-l
iodine(III) reagent
R1
Abstract Biaryl diamines are important building blocks in organic
synthesis. Consequently, it is desirable to develop a general and mild
synthetic approach to diverse biaryl diamines. Oxidative coupling is an
efficient and promising strategy for the synthesis of these targets. We
have now developed a direct formation of biaryl diamines by oxidative
coupling using a hypervalent iodine(III) reagent.
rt
R1
NR2Ar
Scheme 1 The metal-free oxidative coupling of aromatic amines by
using a hypervalent iodine(III) reagent
Key words biaryl diamines, hypervalent iodine, oxidative coupling
The use of hypervalent iodine reagents in oxidative
transformations has received significant attention, with
consideration of their low toxicity and mild reaction condi-
tions.11 We recently reported the direct formation of N-al-
kylsulfonyl anilide biaryl compounds by oxidative cross-
coupling with a variety of aromatic compounds, especially
nonactivated aromatic hydrocarbons.12
The biaryl diamine core is an important motif and a key
structural attribute of many bioactive natural products, chi-
ral ligands, and pharmaceuticals.1 Therefore, the develop-
ment of efficient synthetic methods for providing biaryl di-
amines has attracted the attention of researchers in organic
chemistry. Among the methods developed, oxidative cou-
pling2 of anilines is one of the most efficient methods for
synthesizing biaryl diamines.3 One of the advantages of the
oxidative strategy is that nonfunctional aniline derivatives
are used as green and economic coupling substrates, in-
stead of aryl halides or organometallic derivatives. Recently,
various coupling reactions of anilines involving the use of
TiCl4,4 cerium(IV) ammonium nitrate,5 Cu,6,7 Fe,8 or Rh oxi-
dants,9 or organic 1,8-bis(diphenylmethylium)naphtha-
lenediyl dications10 have been developed. However, these
reactions have problems, such as the need for large excesses
of the starting aniline, low-yielding processes, or limited re-
action scopes. This is because, when simple biaryl amines
are employed in the oxidative couplings, numerous byprod-
ucts are often formed alongside the desired products. Here,
we report the oxidative coupling of biaryl diamines at am-
bient temperature by using an environmentally benign
hypervalent iodine(III) reagent (Scheme 1).
Initially, we used the reaction of N-phenylnaphthalen-
1-amine (1a) to probe the expected oxidative coupling re-
action.13 We found that when the reaction was performed
in the presence of PhI(OAc)2 (PIDA) in CH2Cl2 for one hour at
room temperature, the coupling product 2a was obtained
through C−C bond formation at the naphthalene ring, albeit
in only 13% yield (Table 1, entry 1). Encouraged by this re-
sult, we investigated a series of hypervalent iodine(III) re-
agents. The yield was markedly improved by using
PhI(OCOCF3)2 (PIFA) as the oxidant (entry 2). The use of var-
ious solvents [(F3C)2CHOH (HFIP), CF3CH2OH (TFE), and
DCE] was then examined (entries 4, 5, and 8), and DCE
proved to be effective for this coupling reaction (entry 8).
Moreover, the presence of acids such as AcOH14 or
BF3·Et2O15 was unsuitable for the reaction (entries 9 and
10). Thus, the optimized reaction conditions were deter-
mined as follows: PIFA as the oxidant in DCE at room tem-
perature for three hours.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
K. Morimoto et al.
Letter
Synlett
Table 1 Optimization of the Reaction Conditionsa
R
N
R
N
Ar
Ar
PIFA (0.75 equiv)
DCE, rt
HN
(R = H, Ar)
HN
Hypervalent
Ar
N
R
Iodine reagent
naphthylamines
solvent
OMe
F
HN
HN
HN
HN
2a
Entry
Iodine reagent (equiv)
Solvent
Yield (%)
1
2
PIDA (1)
CH2Cl2
CH2Cl2
CH2Cl2
TFE
13
64
49
34
25
58
71
75
71
N.D.
HN
HN
HN
PIFA (1)
2b
70%
2c
56%
OMe
2d
67%
F
3
PhI(OH)OTs (1)
PIFA (1)
4
Br
5
PIFA (1)
HFIP
N
N
6
PIFA (1)
CH2Cl2
CH2Cl2
DCE
7
PIFA (1.5)
PIFA (1.5)
PIFA (1.5)
PIFA (1.5)
NPh2
NPh2
8
9b
10c
DCE
DCE
N
a Reaction conditions: 1a (2 equiv.), iodine reagent (1 equiv.), solvent, rt, 3
N
2g
h.
28%
b In the presence of AcOH (2 equiv).
c In the presence of BF3·Et2O (2 equiv).
Br
2e
94%
2f
88%
Scheme 2 Scope of substrates
With the optimized reaction conditions in hand (Table 1,
entry 8), we next evaluated the scope of various naphtha-
len-1-amine and -2-amine derivatives (Scheme 2). N-Aryl-
naphthalen-1-amines were smoothly converted into the
desired coupling products 2b–d in moderate to high yields.
Substituents in the para-position of the N-aryl ring, wheth-
er the electron-donating (2b or 2c) or electron-withdraw-
ing (2d), had no influence for the coupling reaction. The re-
action using an N,N-diphenyl derivative also proceeded to
afford dimer 2e in 94% yield. A carbazole-substituted naph-
thalen-1-amine gave the corresponding product 2f in 88%
yield. N,N-Diphenylnaphthalen-1-amine gave the coupling
product 2g in moderate yield. However, naphthalen-1-
amine, with a free amino group, afforded a low yield of the
dimer product.
To further explore its applicability, we next performed
the coupling reaction with various aniline derivatives.
However, no coupling products were produced from the an-
ilines, which decomposed under our reaction conditions.
Despite this, we optimized the reaction conditions and we
were pleased to find that N-(3-tolyl)aniline (3a) was con-
verted into the desired coupling product 4a when HFIP was
used as a solvent (Scheme 3). The reaction of (3-bromophe-
nyl)diphenylamine (3b) similarly afforded the coupling
product 4b in moderate yield (Scheme 3).
A suggested reaction mechanism for the present iodine-
induced oxidative coupling of anilines is shown in Scheme
4. First, the aniline nitrogen coordinates with the iodine(III)
reagent to generate the iodine(III) complex A;16,17 this un-
dergoes one-electron oxidation to produce the intermediate
radical cation of the aniline A. This then reacts with the
neutral aniline 1 at the aryl ring to give the coupled product
2 after further one-electron oxidation and deprotonation.
After our synthetic studies, the next necessary task was
to improve the oxidative coupling involving a stoichiomet-
ric amount of the iodine(III) reagent to a catalytic process.
We have already reported a catalytic use of iodine(III) re-
agents.18 On the basis of our previous report, we examined
the coupling reaction of N-phenylnaphthalen-1-amine (1a)
in the presence of 10 mol % of PhI and one equivalent of
mCPBA in 1:1 DCE/HFIP at room temperature (Scheme 5).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
K. Morimoto et al.
Letter
Synlett
HN
HN
HN
PhI (10 mol%)
mCPBA (0.75 equiv)
TFA (1.0 equiv)
HN
PIFA (0.75 equiv)
Me
Me
HFIP, rt
52%
Me
DCE/HFIP (1:1)
71%
3a
1a
HN
HN
2a
4a
Scheme 5 Hypervalent iodine(III) catalyzed oxidative coupling of N-
phenylnaphthalen-1-amine (1a)
N
N
Funding Information
PIFA (0.75 equiv)
Br
Br
HFIP, rt
68%
This work was partially supported by Grants-in-Aid for Scientific Re-
search (A) from the Japan Society for the Promotion of Science (JSPS),
a Grant-in-Aid for Scientific Research on Innovative Areas ‘Advanced
Molecular Transformation by Organocatalysts’ from The Ministry of
Education, Culture, Sports, Science and Technology (MEXT), and the
Ritsumeikan Global Innovation Research Organization (R-GIRO) proj-
ect. T.D. acknowledges a Grant-in-Aid for Scientific Research (C) from
JSPS and the Asahi Glass Foundation. K.M. also acknowledges support
Br
3b
N
4b
Scheme 3 Scope of aniline derivatives
from a Grant-in-Aid for Scientific Research (C) from the JSPS
)(
Amine 1a was found to undergo an iodine(III)-catalyzed ox-
idative homocoupling to give the desired dimer 2a in good
yield (71%).
References and Notes
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R
R
N
I
F3COCO
N
PIFA
R
N
1
R
N
R
R
N
N
– e–
– 2H+
N
R
A
2
Scheme 4 A suggested reaction mechanism
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In summary, we have demonstrated the oxidative biaryl
coupling of aniline derivatives by using a hypervalent(III)
reagent.20 The reactions occurred under mild conditions
with complete regioselectivity and in good yields. The ob-
tained biaryl diamine derivatives are valuable building
blocks, as demonstrated by a series of follow-up reactions.
Further studies on the applications of this oxidative cou-
pling protocol are currently under investigation in our labo-
ratory.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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PIFA (0.75 equiv.) was added to a stirred solution of the appro-
priate naphthalenamine 1 (0.30 mmol, 1 equiv) in DCE (3 mL) at
r.t., and the mixture was stirred for 30 min. When the reaction
was complete, sat. aq NaHCO3 was added to the mixture, and
the aqueous phase was extracted with CH2Cl2. The extracts
were dried (Na2SO4) and evaporated to dryness, and the crude
residue was purified by column chromatography (silica gel,
hexane–EtOAc).
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Chem. Soc. 2013, 135, 14078.
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N,N′-Diphenyl-(1,1′-binaphthyl)-4,4′-diamine (2a)8c
1H NMR (400 MHz, CDCl3): δ = 6.04 (s, 2 H), 6.95 (t, J = 7.6 Hz,
2 H), 7.11 (dd, J = 1.2, 8.8 Hz, 4 H), 7.29–7.35 (m, 6 H), 7.41 (d,
J = 7.6 Hz, 2 H), 7.46–7.51 (m, 6 H), 8.14 (d, J = 8.4 Hz, 2 H). 13
C
NMR (100 MHz, CDCl3): δ = 115.1, 117.6, 120.6, 121.8, 125.5,
126.1, 127.4, 127.5, 128.3, 129.4, 133.3, 134.1, 138.5, 144.6.
N,N′-bis(4-Tolyl)-1,1′-binaphthalene-4,4′-diamine (2b)8c
1H NMR (400 MHz, CDCl3): δ = 2.33 (s, 6 H), 6.00 (s, 2 H), 7.05 (d,
J = 8.4 Hz, 4 H), 7.13 (d, J = 8.4 Hz, 4 H), 7.30 (t, J = 7.6 Hz, 2 H),
7.36–7.37 (m, 4 H), 7.44–7.50 (m, 4 H), 8.11 (d, J = 8.8 Hz, 2 H).
13C NMR (100 MHz, CDCl3): δ = 20.7, 113.4, 118.7, 121.5, 125.4,
126.0, 126.9, 127.4, 128.3, 129.9, 130.6, 132.5, 134.1, 139.3,
141.6.
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Kita, Y. Angew. Chem. Int. Ed. 2016, 55, 3652.
N,N′-Bis(4-bromophenyl)-N,N′-diphenyl-1,1′-binaphthalene-
4,4′-diamine (2e)
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2008, 47, 1301.
IR (KBr): 3064, 3037, 1582, 1485, 1272, 910, 736 cm–1. 1H NMR
(CDCl3): δ = 6.97 (d, J = 8.8 Hz, 4 H), 7.02 (t, J = 7.2 Hz, 2 H), 7.15
(d, J = 7.6 Hz, 4 H), 7.26–7.34 (m, 10 H), 7.38 (t, J = 6.8 Hz, 2 H),
7.42 (d, J = 7.6 Hz, 2 H), 7.49 (d, J = 8.0 Hz, 2 H), 7.51 (d, J = 7.6
Hz, 2 H), 8.02 (d, J = 8.0 Hz, 2 H). 13C NMR (CDCl3): δ = 113.8,
122.50, 122.53, 123.0, 124.3, 126.4, 126.5, 126.6, 127.2, 128.5,
129.3, 131.0, 132.1, 134.5, 136.8, 142.9, 147.7, 147.9. MS
(MALDI-TOF): m/z = 744.12 [M+].
N,N,N′,N′-Tetraphenyl-1,1′-binaphthalene-2,2′-diamine
(2g)19
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Yamashita, D.; Miyazaki, K.; Kita, Y. Heterocycles 2014, 88, 245.
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Chem. 2002, 67, 7424. (b) Antonchick, A. P.; Samanta, R.;
Kulikov, K.; Lategahn, J. Angew. Chem. Int. Ed. 2011, 50, 8605.
(c) Samanta, R.; Lategahn, J.; Antonchick, A. P. Chem. Commun.
2012, 48, 3194. (d) Kikugawa, Y. Heterocycles 2009, 78, 571.
1H NMR (CDCl3): δ = 6.44 (d, J = 8.1 Hz, 2 H), 6.45–6.58 (m,
12 H), 6.61–6.77 (m, 10 H), 7.12 (td, J = 1.2, 7.6 Hz, 2 H), 7.62 (m,
4 H), 7.78 (d, J = 8.6 Hz, 2 H). 13C NMR (CDCl3): δ = 121.7, 123.2,
124.4, 125.2, 126.6, 126.7, 127.1, 128.3, 128.7, 131.2, 131.7,
134.0, 144.4, 147.2.
Catalytic Oxidative Coupling of N-Phenylnaphthalen-1-
amine (1a)
PhI (10 mol%) and TFA (0.2 equiv) were added to a stirred solu-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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K. Morimoto et al.
Letter
Synlett
tion of amine 1a (1 equiv) in 1:1 DCE/HFIP (0.67 M) at r.t.
mCPBA (0.75 equiv) was then gradually added, and the mixture
was stirred for 3 h under at r.t. When the reaction was complete
(TLC), sat. aq NaHCO3 was added to the mixture, and the
aqueous phase was extracted with CH2Cl2. The extracts were
dried (Na2SO4) and then evaporated to dryness. The crude
residue was purified by column chromatography (silica gel, hex-
ane/EtOAc) to give pure 2a; yield: 71%.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E