Synthesis of symmetrical biaryls through palladium-catalyzed ligand-free homocoupling of aryliodine(iii) diacetates

Article abstract of DOI:10.1055/s-0034-1380320

An efficient process for the synthesis of symmetrical biaryls from readily available and bench-stable aryliodine(III) diacetates has been developed with high regional selectivity. The process exhibited high selectivities and good functional group toleranc

Full text of DOI:10.1055/s-0034-1380320

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© Georg Thieme Verlag Stuttgart · New York
2015, 26, 975–979
letter
975
Q. Xiong et al.
Letter
Synlett
Synthesis of Symmetrical Biaryls through Palladium-Catalyzed
Ligand-Free Homocoupling of Aryliodine(III) Diacetates
Qiheng Xiong
Zhengjiang Fu*
Zhaojie Li
Pd(OAc)2
K2CO3
R
R
R
I(OAc)2
DMF, 110 °C
air
Hu Cai*
College of Chemistry, Nanchang University, Nanchang,
Jiangxi 330031, P. R. of China
fuzhengjiang@ncu.edu.cn
caihu@ncu.edu.cn
Received: 16.12.2014
Accepted after revision: 19.01.2015
Published online: 19.02.2015
C7H15
NC
C8H17
O
DOI: 10.1055/s-0034-1380320; Art ID: st-2014-w1035-l
liquid-crystalline NCB807
Abstract An efficient process for the synthesis of symmetrical biaryls
from readily available and bench-stable aryliodine(III) diacetates has
been developed with high regional selectivity. The process exhibited
high selectivities and good functional group tolerance with respect to
methyl, bromo, chloro and cyano groups. Mechanistic studies revealed
that the transformation involved in situ generation of aryl iodide from
heating-promoted degradation of aryliodine(III) diacetate, followed by
Ullmann-type homocoupling.
OH
HO
HO
OH
O
mastigophorene A
n-Bu
N
CO2H
Key words biaryls, palladium, homogeneous catalysis, coupling, che-
OMe
moselectivity
MeO
PCy2
N
N
i-Pr
i-Pr
NH
As important structural motifs, biaryls are widely found
in many functional materials, natural products and phar-
maceuticals.1 Additionally, biaryl fragments are also usually
employed as useful building blocks in organic catalysts and
ligands in organic synthesis to construct complex mole-
cules (Figure 1).2 Therefore, their fundamental importance
in synthetic organic chemistry has led to intensive studies
to develop a number of transformations to create various
biaryl compounds, and great progress has been achieved by
transition-metal-catalyzed cross-coupling and homocou-
pling reactions in the synthesis of symmetrical and unsym-
metrical biaryls.3 Among them, transition-metal-catalyzed
homocoupling reactions are the most straightforward and
concise route for preparing symmetrical biaryl compounds
from only one substrate. In this regard, the Ullmann cou-
pling reaction has gained much attention as an efficient
process, although it has been known for over a century.3d,e
Over the past decade, major advances have been made in
transition-metal-mediated oxidative coupling reactions be-
tween two nucleophilic partners to construct various
chemical bonds, including Csp2–Csp3, Csp2–Csp2, and Csp–Csp3
N
valsartan
i-Pr
brettphos
Figure 1 Four examples illustrating the importance of compounds
containing a biaryl fragment
bonds.4 Thus, oxidative dimerization of sp2 arylmetal re-
agents has emerged as a method of choice to synthesize
symmetrical biaryls.5 For example, Yamamoto,5a Luo5b and
other groups5c,d reported transition-metal-promoted ho-
mocoupling reactions of boronic acids, Lei5e disclosed the
Pd-catalyzed Negishi-type oxidative homocoupling of
arylzinc reagent by utilizing desyl chloride as the oxidant,
Hayashi5f and Severin5g independently revealed Fe-mediat-
ed oxidative homocoupling of aryl Grignard reagents by us-
ing 1,2-dichloroethane and nitrous oxide as the oxidant.
Unfortunately, many of the arylmetal reagents are sensitive,
expensive, and tedious to prepare. To address these issues,
transition-metal-catalyzed oxidative homocoupling of sp2
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 975–979

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arene C–H bond has been employed as a powerful tool for
direct synthesis of valuable biaryl products.6 In contrast to
oxidative dimerization of sp2 arylmetal reagents, oxidative
homocoupling of sp2 arene C–H bond is the most widely ac-
cepted green chemical processes to form symmetrical biar-
yls. However, this method often lacks selectivity and/or
usually requires directing group, to some extent, limiting
the widespread application of this protocol in organic syn-
thesis. Recently, aromatic carboxylic acids, sodium arylsul-
finates and N′-tosyl arylhydrazines have gained much at-
tention owing to their broad availability, high stability and
low cost, thus, employing them as the single aryl precursor
in transition-metal-catalyzed homocoupling provides an
attractive alternative for the synthesis of symmetrical bi-
aryl compounds. Su disclosed for the synthesis of biaryls a
Pd/Ag-mediated decarboxylative coupling reaction be-
tween two different (hetero)aryl carboxylic acids.7a Larro-
sa,7b Tan and Deng7c independently reported Pd/Ag-mediat-
ed decarboxylative homocoupling of (hetero)aromatic car-
boxylic acids with good and moderate yields. Luo7d and
Forgione,7e each developed a Pd-catalyzed desulfitative ho-
mocoupling of sodium arylsulfinates under aerobic condi-
tions for the synthesis of symmetrical biaryls. Lu illustrated
the preparation of biaryl compounds via Pd-catalyzed ho-
mocoupling of N′-tosyl arylhydrazines.7f Therefore, the fur-
ther exploration of efficient procedure for the preparation
of symmetrical biaryls from transition-metal-catalyzed ho-
mocoupling is currently of great interest in organic synthe-
sis. Herein, we exhibit an attempt to use bench-stable
aryliodine(III) diacetate ArI(OAc)2 as single aryl precursor in
Pd-catalyzed ligand-free homocoupling transformation to
generate symmetrical biaryl derivatives.
Aryliodine(III) diacetate is one of the most useful hyper-
valent iodine reagents with the characteristics of easy avail-
ability (benzene + I2),8 high stability and low cost. There-
fore, it is widely used in organic synthesis and represents
rich chemistry, for example, it is not only used as a surro-
gate for toxic oxidants in oxidative transformations,9 but is
also employed in radical reactions to generate oxygen- and
carbon-centered radicals.10 Furthermore, aryliodine(III) di-
acetate can serve as acetoxy and aryl sources in oxidative
transformations.11,12 On this point, Suna11a and others11b,c
reported Pd-catalyzed acetoxylation of (hetero)arenes or
alkenes with the employment of aryliodine(III) diacetate.
Mao,12a Magedov,12b Cheng,12c and Fairlamb12d revealed Pd-
catalyzed arylation of olefins, and benzoxazole C–H bonds
with aryliodine(III) diacetate. Recently, we have reported an
efficient Pd-catalyzed direct arylation of electron-deficient
polyfluoroarenes with aryliodine(III) diacetates to afford
the corresponding polyfluorobiaryls with moderate to good
yields.13 Among these transformations, to the best of our
knowledge, the synthesis of symmetrical biaryls via the ho-
mocoupling of aryliodine(III) diacetates as single aryl pre-
cursors has not been developed.
A variety of reaction parameters were screened for the
homocoupling of phenyliodine(III) diacetate (1a) as the
model reaction under an air atmosphere. Selected results
that illustrated the effect of catalyst, base and other factors
on the reaction outcome are listed in Table 1. Initially, we
observed that the homocoupling reaction of phenylio-
dine(III) diacetate (1a) in DMF in the presence of 10 mol% of
Pd(OAc)2 as catalyst, 4 equivalents of K2CO3 as base and one
equivalent of ammonium phosphomolybdate hydrate
(PMA) as additive at 110 °C smoothly furnished the desired
product biphenyl (2a) in 71% yield (Table 1, entry 1). Unfor-
tunately, either drastically reduced yield (20%) was ob-
tained when the amount of K2CO3 was decreased to 2
equivalents, or trace amounts of product 2a was observed
when K3PO4, Cs2CO3 and t-BuOK were employed instead of
K2CO3 under otherwise identical conditions, respectively
(Table 1, entries 2–5). Thus, K2CO3 appeared to be the best
choice for this transformation to provide a satisfactory
yield. The effect of additive was then evaluated, and studies
showed that moderate yields were obtained when ammoni-
um phosphomolybdate hydrate was replaced with Cu(OAc)2
and Cu(OTf)2 (Table 1, entries 6 and 7). To our delight, a
comparable yield (73%) was delivered when additive was
absent (Table 1, entry 8), and the result indicated that the
phenyliodine(III) diacetate might act both as the aryl pre-
cursor and as an oxidant in this transformation. However, a
nitrogen atmosphere was detrimental to the transforma-
tion (Table 1, entry 9). The reaction temperature played a
key role in the reaction conversion, and lowering reaction
temperature to 80 °C did not lead to the product (Table 1,
entry 10). Then, different reaction times were screened un-
der otherwise equivalent conditions. It was found that a
short reaction time (2 h) could gave a comparable yield
(66%), whereas much shorter time (1 h) furnished a de-
creased yield (55%; Table 1, entries 12 and 13). The influ-
ence of solvent on the catalytic process was also investigat-
ed, and results showed that inferior yields were obtained in
the screened solvents than that in DMF (Table 1, entries 14–
16). Decreasing the loading of Pd(OAc)2 led to an unsatisfac-
tory yield (26%; entry 17). Finally, replacing Pd(OAc)2 with
CuI in the reaction furnished none of the desired product
(Table 1, entry 18), which demonstrated that the Pd catalyst
played an essential role in the catalytic reaction.
With the optimized conditions in hand, the scope of this
reaction with respect to easily available aryliodine(III) diac-
etate substrate was explored under the standard conditions
of entry 12 in Table 1. As shown in Table 2, beside phenylio-
dine(III) diacetate (1a), this Pd-catalyzed ligand-free homo-
coupling reaction was shown to be effective for a wide array
of aryliodine(III) diacetates with different function groups
to furnish the corresponding symmetrical biaryls in moder-
ate to good yields. Electron-rich and -deficient substituents
such as methyl, bromo, chloro and cyano groups all could
be quite well tolerated. It was also observed that electronic
properties of the aromatic moiety in this homocoupling
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 975–979

977
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Table 1 Selected Results of Screening for the Optimal Conditionsa
catalyst
base, additive
I(OAc)2
solvent, time
air, 110 °C
1a
2a
Solvent
Entry
Catalyst (mol%)b
Base (equiv)
Additive (equiv)
Time (h)
Isolated yield (%)
1
2
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (5)
CuI (10)
K2CO3 (4)
K2CO3 (2)
K3PO4 (2)
Cs2CO3 (2)
t-BuOK (2)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
K2CO3 (4)
PMA (1)
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
toluene
dioxane
MeCN
DMF
DMF
20
20
20
20
20
20
20
20
20
20
10
2
71
PMA (1)
20
3
PMA (1)
trace
trace
trace
52
4
PMA (1)
5
PMA (1)
6
Cu(OAc)2 (1.5)
7
Cu(OTf)2 (1.5)
48
8
–
–
–
–
–
–
–
–
–
–
–
73
9c
10d
11
12
13
14
15
16
17
18
67
trace
68
66
1
55
2
55
2
10
2
8
2
26
2
0
a Reaction conditions: 1a (0.2 mmol), solvent (2 mL), 110 °C, air atmosphere.
b Relative to 1a.
c Reaction was conducted under an N2 atmosphere.
d Reaction was conducted at 80 °C.
transformation did not play a significant role, with good
yields recorded for the homocoupling of 1a,b and 1e (albeit
at a slightly elevated temperature; Table 2, entries 1, 2 and
5), whereas moderate yields were obtained for the homo-
coupling of 1c,d and 1f,g (Table 2, entries 3, 4 and 6, 7). It is
worth noting that 4-chloro(diacetoxyiodo)benzene (1e)
and 4-bromo(diacetoxyiodo)benzene (1f) formed the de-
sired products (2e and 2f) with the halogen moieties (Cl, Br)
remaining intact (Table 2, entries 5 and 6), which could be
used for further functionalization of the C–Hal bond, exhib-
iting the potential application of this transformation. Inter-
estingly, aryliodine(III) compound bonding with a different
anion was also a suitable substrate for this protocol. For ex-
ample, homocoupling of [bis(trifluoroacetoxy)iodo]ben-
zene (1h) generated the hoped-for product 2a (Table 2, en-
try 8), albeit in a moderate yield (40%) under a slightly ele-
vated temperature.
in DMF at 110 °C for 0.5 hour generated iodobenzene in 40%
isolated yield. This observation is consistent with the fact
that phenyliodine(III) diacetate degrades to iodobenzene
under heating conditions with a similar base (Cs2CO3 and
Ag2CO3).12d,13 Next, reacting this freshly isolated iodoben-
zene with 10 mol% of Pd(OAc)2 and 4 equivalents of K2CO3
under an air atmosphere for ten hours led to the biphenyl
product 2a in 58% isolated yield, as detected by GC–MS and
NMR. Based on above observations, we can conclude that
this transformation involves an in situ generation of aryl io-
dide from the thermal degradation of aryliodine(III) diace-
tate with a base, followed by an Ullmann-type homocou-
pling.
In summary, we have established a new Pd-catalyzed li-
gand-free homocoupling method for the direct synthesis of
symmetrical biaryl compounds.14 This ligand-free transfor-
mation displays attractive features including using cheap
and readily available aryliodine(III) diacetates as the single
aryl precursors, a range of functional group tolerance, good
chemoselectivity and moderate to good yields. With these
features, the present protocol provides a broad utility in
Some experiments were performed to gain a prelimi-
nary insight into the reaction mechanism. At first, heating
phenyliodine(III) diacetate (1a) with 4 equivalents of K2CO3
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 975–979

978
Q. Xiong et al.
Letter
Synlett
Table 2 Palladium-Catalyzed Ligand-Free Homocoupling of Arylio-
Acknowledgment
dine(III) Diacetatesa
Financial support from the National Natural Science Foundation of
China (NSFC 21301088 and 21162015), the National Key Basic Re-
search Program of MOST of China (2012CBA01204), and the Natural
Science Foundation of Jiangxi Province (20142BAB213001) is greatly
appreciated.
Pd(OAc)2
K2CO3
R
R
R
I(OAc)2
DMF, 110 °C
air, 2 h
1a
2a
Supporting Information
Entry Substrate
Product
Isolated
yield (%)b
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1380320.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
I(OAc)2
1
70
64
1a
2a
2b
References and Notes
I(OAc)2
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a Reaction conditions: aryliodine(III) diacetate 1 (0.2 mmol), Pd(OAc)2 (10
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(14) General Procedure (Preparation of 2): A mixture of arylio-
dine(III) diacetate 1 (0.2 mmol), K2CO3 (110.6 mg, 0.8 mmol, 4
equiv), Pd(OAc)2 (4.5 mg, 0.02 mmol, 10 mol%) and DMF (2 mL)
was stirred at 110 °C for 2 h. After cooling to r.t., the reaction
mixtures were diluted with H2O (10 mL) and filtered through a
pad of silica gel that was then washed with Et2O (3 × 10 mL).
The combined organic phase was washed with brine (2 × 20
mL), dried over Na2SO4, filtered and concentrated in vacuo. The
residue was then purified by flash chromatography on silica gel
to provide the corresponding product. The product was charac-
terized by GC–MS and NMR spectroscopy.
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