Direct C-H bond arylation of arenes with aryltin reagents catalysed by palladium complexes

Article abstract of DOI:10.1039/b717251f

Direct C-H bond arylation of simple arenes with aryltin reagents has been successfully catalysed by PdCl₂ in the presence of CuCl₂. CuCl₂ proved to be an activator for a palladium intermediate as well as an oxidant. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717251f

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Direct C–H bond arylation of arenes with aryltin reagents catalysed by
palladium complexesw
Hiroshi Kawai, Yasuhiro Kobayashi, Shuichi Oi* and Yoshio Inoue*
Received (in Cambridge, UK) 7th November 2007, Accepted 19th December 2007
First published as an Advance Article on the web 29th January 2008
DOI: 10.1039/b717251f
Direct C–H bond arylation of simple arenes with aryltin
reagents has been successfully catalysed by PdCl2 in the pre-
sence of CuCl2. CuCl2 proved to be an activator for a palladium
intermediate as well as an oxidant.
2–4). In the case of phenyltintrichloride (2a, entry 4), the total
yield of the cross-coupling products increased to 60% (40%
yield of 1 : 1 coupling product 3aa and 20% yield of 1 : 2
coupling product 4aa), whereas formation of by-product 5
decreased to 12%. Addition of ligands, such as phosphines or
amines, for the palladium catalyst decreased the yield of the
cross-coupling products (20–40% yield). In the absence of
CuCl2 (entry 5), the reaction did not result in any products,
proving that CuCl2 is essential. Other oxidants, such as
Cu(OAc)2, PhI(OAc)2, and oxone (entries 6–8 respectively),
are not effective for the present reaction. The predominant
formation of the a-coupling product strongly suggests that the
reaction pathway involves the electrophilic substitution pro-
cess on the naphthalene ring.
The direct C–H bond arylation of aromatic compounds in the
presence of transition metal catalysts has recently gained
significant attention as an efficient and useful method of biaryl
synthesis.1 In recent years, significant progress has been made
toward the development of direct arylation of a wide variety of
substrates using Pd,2,3 Rh,4 Ru,5 and Ir6 catalysts. For these
reactions, the critical step is the C–H bond cleavage and
metalation of the aromatic ring with the transition metal
complex. Among the several mechanisms that can be proposed
for this step, the electrophilic aromatic substitution with a
transition metal intermediate can be considered as one of the
very effective processes. Accordingly, electrophilic metalations
with several transition metal complexes have been proposed in
the direct coupling reaction of aromatic compounds.7 In these
reactions, however, the substrates are limited to electron-rich
heteroarenes or arenes that possess directing groups in many
cases. For substrates that do not possess such features, harsher
reaction conditions and/or loading of a large excess of arene
substrate are required to obtain satisfactory results. One
possible solution to these limitations is to enhance the electro-
philicity of the transition metal intermediate. Herein, we
describe the direct C–H bond arylation of simple arenes with
aryltin reagents in the presence of a palladium catalyst and
CuCl2 as the oxidant. Stoichiometric experiments on the
reaction mechanism suggested that certain electrophilic active
species were generated from PdCl2 and CuCl2.
Using the optimized reaction conditions (Table 1, entry 4),
phenanthrene (1b) was reacted with aryltintrichlorides that
possess both electron-donating and -withdrawing groups
(2a–e) to afford the corresponding 9-arylphenanthrenes
3ba–be in good yields (Scheme 2). In the case of 1b, the
regioisomer and 1 : 2 coupling products were not observed.
Next, the direct arylations of various arenes with phenyltin-
trichloride (2a) were examined (Table 2). Phenylation of
fluoranthene 1c successfully gave the 1 : 1 (56% yield) and
1 : 2 (38% yield) coupling products as a mixture of regio-
isomers (entry 1). For the alkyl benzenes, the yields were lower
than those of the polycyclic arenes (1a–c). Using the same
reaction conditions as entry 1, the reaction between tetraline
(1d) and 2a afforded the 1 : 1 coupling product 3da with a yield
of merely 20% (entry 2), which can be attributed to the lower
electronic density of the aromatic ring. Modifying the reaction
conditions (doubling the amounts of 2a and CuCl2, and
dropwise addition of 2a over 11 h) improved the formations
of 3da (47%) and 1 : 2 coupling product 4da (14%, entry 3).
The reaction of 1,2,3-trimethylbenzene (1e) with 2a gave
coupling product 3ea in a moderate yield of 44% (entry 4).
Among the xylene isomers (entries 5–7), meta-isomer 1g gave
product 3ga in the highest yield as a single regioisomer.
Subsequently, several experiments were carried out in at-
tempt to gain insight into the reaction mechanism. As stated
Initially, using naphthalene (1a) as the arene substrate,
reactions were carried out to determine the optimal combina-
tion of catalyst precursor, aryltin reagent, and oxidant
(Scheme 1). Using a combination of PdCl2, tetraphenylstan-
nane, and CuCl2 (Table 1, entry 1), a small amount of the
desired a-phenylnaphthalene (3aa) was obtained together with
a large amount of biphenyl (5). Interestingly, substitution of
the phenyl groups of tetraphenylstannane with chlorides im-
proved the yields and the cross-coupling selectivities (entries
Department of Biomolecular Engineering, Graduate School of
Engineering, Tohoku University, Sendai 980-8579, Japan. E-mail:
oishu@aporg.che.tohoku.ac.jp; Fax: +81 (0)22 795 5876;
Tel: +81 (0)22 795 5876
w Electronic supplementary information (ESI) available: Experimental
part, including analytical and spectroscopic data of all products. See
DOI: 10.1039/b717251f
Scheme 1
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Table 1 Optimization of reaction conditionsa
Yield (%)b
Table 2 Direct C–H phenylation of arenes with phenyltintrichlorides
(2a)a
Entry
1
Product, yield (%)b
3aa (a : b)
4aa
5c
Entry
PhmSnXn 2
Oxidant (mmol)
Arene (1)
Mono (3)
Di (4)
1
2
3
4
5
6
7
8
a
Ph4Sn
Ph3SnCl
Ph2SnCl2
CuCl2 (2.0)
CuCl2 (2.0)
CuCl2 (2.0)
4 (100 : 0)
5 (100 : 0)
37 (78 : 22) 13
40 (78 : 22) 20
0
0
106
64
74
12
0
(1c)
(1d)
56 (3ca)d
38 (4ca)e
PhSnCl3 (2a) CuCl2 (2.0)
PhSnCl3
PhSnCl3
PhSnCl3
PhSnCl3
None
0
0
0
0
0
Cu(OAc)2 (2.0)
PhI(OAc)2 (1.0)
Oxone (1.0)
6 (100 : 0)
1 (100 : 0)
0
56
8
2
Conditions: 1a (0.5 mmol), 2 (1.0 mmol), PdCl2 (0.025 mmol) in
b
2
20 (3da)f
0
1,2-dichloroethane (1.0 ml) at 80 1C for 16 h. Determined by GC
using internal standard. Based on Sn.
c
3c
4c
47 (3da)g
44 (3ea)
14 (4da)
above, in the absence of CuCl2, the reaction between 1a and 2a
did not result in any coupling products, even when a stoichio-
metric amount of PdCl2 was employed, which indicates that
PdII/Pd0 mechanisms including the divalent phenylpalladium
and naphthylpalladium complexes shown in Scheme 3 can be
ruled out. Secondly, the stoichiometric reaction between
PhPdCl(PPh3)2 and 1a in the presence of CuCl2 did not afford
any coupling product, and therefore the interaction and gen-
eration of an active species from the divalent phenylpalladium
complex with CuCl2 is unlikely (Scheme 4).8 Third, addition of
TEMPO (20 mol%) as a radical scavenger to the catalytic
reaction of 1a with 2a did not affect the yield, indicating that
the participation of radical species in the catalytic cycle is
unlikely. Finally, because the reactions between 1a and 2a in
the presence of a stoichiometric amount of palladium(0) com-
plexes, such as Pd(PPh3)4 and Pd(dba)2, did not yield any
coupling products, the contribution of PhPd(SnCl3)L2 formed
via oxidative addition of 2a to a palladium(0) complex9 is also
unlikely. From these experimental results, it can be reasoned
that PdCl2 reacts with CuCl2 prior to the reaction with 1a or
2a. Unfortunately, we do not have clear information about the
active species at present, however, chloride-bridged Pd–Cu
heterometallic clusters10 or higher oxidized palladium(IV)
species11,12 can be assumed as possible intermediates. Scheme 5
shows a possible reaction mechanism, in which a palladium(IV)
species is assumed as the active species and indicated as PdCl4
for clarity. The resulting active species would be highly
electrophilic, and should react with 2a to give an arylpalla-
dium intermediate 6. Subsequent electrophilic substitution of
1a with 6 would form diaryl intermediate 7, which would then
undergo reductive elimination to afford the coupling product
and PdCl2. The reaction pathway in the opposite direction, in
(1e)
(1f)
(1g)
(1h)
—
—
—
—
0
5c
6c
26 (3fa)h
38 (3ga)
14 (3ha)
0
0
0
7c
a
Conditions: arenes (0.5 mmol), PhSnCl3 (2a, 1.0 mmol), PdCl2 (0.025
mmol), CuCl2 (2.0 mmol) in 1,2-dichloroethane (1.0 ml) at 80 1C for
16 h. Isolated yield. Modified conditions: 2a (2.0 mmol,
b
c
d
dropwise addition over 11 h), CuCl2 (4.0 mmol). 2- : 3-Ph =
e
17 : 83. 2,5- : 3,4- : 3,5-Ph = 8 : 44 : 48. 5- : 6-Ph = 100 : 0. 5- : 6-Ph
f
g
h
= 86 : 14. 3- : 4-Ph = 76 : 24.
Scheme 3
which electrophilic substitution of 1a with the palladium(IV)
species takes place first and then transmetalation with 2a,
cannot be ruled out. However, homo-coupling products of
arene substrates 1, such as binaphthalene, was not observed in
all cases but a small amount of biphenyl was always observed
as the by-product, suggesting that the reaction pathway would
not involve monoarylpalladium species corresponded to arene
substrates.
It is known that arylpalladium(IV) species can be formed via
reaction of palladium(II) complexes with diaryliodonium
salts.13 Accordingly, the reaction of [Ph2I]PF6 with 1a in the
presence of a catalytic amount of PdCl2 gave the coupling
products with a ratio of regioisomers that is comparable to
that of the present reaction (Scheme 6). These results suggest
the contribution of the arylpalladium(IV) intermediate to the
reaction pathway.
Scheme 2 Arylation of phenanthrene with aryltintrichlorides.
Scheme 4
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Scheme 6
In conclusion, we have demonstrated the direct C–H bond
arylation of simple arenes with aryltin reagents catalysed by a
palladium complex. Although direct evidence for the reaction
mechanism has not been obtained, we proposed the contribu-
tion of an electrophilic arylpalladium species formed by the
combination of aryltintrichlorides, CuCl2 and PdCl2.
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas ‘‘Advanced Molecular Transfor-
mations of Carbon Resources’’ from MEXT, Japan.
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