A novel and efficient coupling reaction of sodium tetraphenylborate with hypervalent iodonium salts

Article abstract of DOI:10.1055/s-2006-926355

The novel coupling reaction of sodium tetraphenylborate with hypervalent iodonium salts was achieved in acidic water, providing an efficient and rapid method for the formation of carbon-carbon bonds in excellent yield. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-926355

SHORT PAPER
943
A Novel and Efficient Coupling Reaction of Sodium Tetraphenylborate with
Hypervalent Iodonium Salts
A
Novel
a
i
nd Effi
e
cient Coupling Re
Y
action an,*a Weixiao Hu,b Guowu Raob
a
College of Chemical Engineering and Materials Sciences, Zhejiang University of Technology, Hangzhou 310032, Zhejiang,
P. R. of China
Fax +86(571)88320238; E-mail: jieyan87@zjut.edu.cn
College of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou 310032, Zhejiang, P. R. of China
b
Received 20 September 2005
ability, and their nontoxic properties. They have been
widely used in the Suzuki reaction to replace aryl halides
or triflates affording products in excellent yield under
mild reaction conditions.4 As part of a program to investi-
gate the catalyst-free Suzuki reaction, we explored the
coupling reaction of sodium tetraphenylborate with iodo-
nium salts in acidic water. In this paper, we would like to
report a novel and efficient coupling reaction of sodium
tetraphenylborate with iodonium salts in acidic water.
This method was successfully applied to the synthesis of
biaryls, and to the best of our knowledge, the metal-cata-
lyst-free Suzuki-type coupling reaction of sodium tet-
raphenylborate with iodonium salts in acidic water has not
been reported.
Abstract: The novel coupling reaction of sodium tetraphenylborate
with hypervalent iodonium salts was achieved in acidic water, pro-
viding an efficient and rapid method for the formation of carbon–
carbon bonds in excellent yield.
Key words: coupling reaction, sodium tetraphenylborate, iodoni-
um salt
The Suzuki reaction is one of the most versatile and uti-
lized reactions for the selective construction of carbon–
carbon bonds, in particular for the formation of biaryls.1
These coupling reactions are utilized extensively in the
synthesis of pharmaceuticals, herbicides, and natural
products as well as conducting polymers and liquid crys-
talline materials because they use nontoxic readily avail-
able organoboranes, yield easily removable nontoxic
byproducts, and tolerate many functional groups. Howev-
er, the use of heavy metals as catalysts leads to the gener-
ation of waste, which can have a number of problems
associated with it, thus the eradication of the catalyst from
the Suzuki reaction offers significant advantages. Recent-
ly, Leadbeater and Marco reported the first catalyst-free
Suzuki coupling reaction under microwave irradiation.2
They investigated the coupling of aryl halides with aryl-
boronic acids, which afforded biaryls in excellent yields
when aryl bromides were reacted with arylboronic acids
in water under microwave heating at 150 °C for five min-
utes. However, due to the development of pressure and the
need for specialized sealed vessels this method is not suit-
able for organic reactions carried out at atmospheric pres-
sure and it is also difficult to apply on an industrial scale.
In order to extend the scope of the catalyst-free Suzuki re-
action the development of mild, efficient, and environ-
mentally more benign metal-catalyst-free conditions are
required.
Initially, we investigated the catalyst-free coupling reac-
tion of phenylboronic acid with diphenyliodonium chlo-
ride in water by traditional heating methods. We found
that the coupling was difficult to effect and the desired bi-
phenyl was not obtained either in the presence or absence
of a base (Na2CO3), even prolonged heating of the reac-
tion mixture was unsuccessful. We then used a stronger
nucleophilic reagent – sodium tetraphenylborate – in
place of phenylboronic acid, however, the same result was
obtained. Next, we explored the coupling reaction in acid-
ic conditions. It was found that diphenyliodonium chlo-
ride decomposed quickly when p-TsOH was added to a
mixture of phenylboronic acid and diphenyliodonium
chloride. Fortunately, when sodium tetraphenylborate
was used in place of phenylboronic acid, the product bi-
phenyl was obtained, in good yield, when the mixture was
stirred for 30 minutes at 50 °C. Prompted by this exciting
result a series of experiments were performed on the cou-
pling of sodium tetraphenylborate with diphenyliodonium
chloride in the presence of p-TsOH in order to determine
suitable reaction conditions. The reaction of four equiva-
lents of sodium tetraphenylborate, two equivalents of p-
TsOH, and one equivalent of diphenyliodonium chloride
in water at 50 °C for 30 minutes afforded biphenyl in 95%
yield. Then we extended this protocol to other iodonium
salts, both symmetrical and unsymmetrical biaryls were
readily synthesized in high yields under the optimum con-
ditions (Scheme 1, Table 1).
During the past few years the chemistry of hypervalent io-
dine compounds has experienced an unprecedented
growth.3 As powerful electrophilic reagents, hypervalent
iodine compounds, in particular, hypervalent iodonium
salts have found synthetic application due to their high re-
activity toward various nucleophiles, their ready avail-
With the exception of iodonium salt 2c (Table 1, entry 3),
all iodonium salts afforded the desired products in excel-
lent yields. Electron-poor iodonium salt 2f gave a slightly
SYNTHESIS 2006, No. 6, pp 0943–0945
Advanced online publication: 27.02.2006
DOI: 10.1055/s-2006-926355; Art ID: P14805SS
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x
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x
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© Georg Thieme Verlag Stuttgart · New York

944
J. Yan et al.
SHORT PAPER
Table 1 Coupling Reaction of Sodium Tetraphenylborate with Iodonium Saltsa
Entry
1
Iodonium salt
Product
Time (h)
0.5
Yield (%)b
95
Ph2I+Cl–
PhPh
2a
3a
2
3
4
5
6
7
(p-MeC6H4)2I+Br–
2b
p-MeC6H4Ph
3b
0.5
1
92
61
94
92
80
81
(p-BrC6H4)2I+Br–
2c
p-BrC6H4Ph
3c
p-MeC6H4I+PhBr–
2d
3b
1
p-MeOC6H4I+PhBr–
2e
p-MeOC6H4Ph
3d
1
(m-NO2C6H4)2I+Br–
2f
m-NO2C6H4Ph
3e
1
2
PhOTs–
I+
Ph
S
S
2g
3f
–
8
9
(p-MeC6H4)2 I+BF4
2h
3b
1
2
78c
72c
(p-MeOC6H4)2 I+BF4
3d
–
2i
a Reaction conditions: Ph4BNa (4 equiv), iodonium salt (1 equiv), p-TsOH (2 equiv), and H2O (5 mL).
b Isolated yields.
c The amount of p-TsOH used was increased to 6 equivalents.
Table 2 Coupling Reaction in Aqueous HCla
p-TsOH
+
Ar1I+Ar2X–
Ph4BNa
Ph Ar
H2O, 50 °C
1
2
3
Entry
Iodonium salt Product
Time (min) Yield (%)
Scheme 1
1
2
3
2a
2e
2h
3a
3d
3b
15
15
50
92
92
65
lower yield than electron-rich phenyl iodonium salts 2b,
2d, and 2e. We also observed that the anions of iodonium
salts affected the yield greatly: with 2b the reaction was
complete after only 30 minutes affording 3b in 92% yield;
when 2h was used in place of 2b under the same reaction
conditions, even after a prolonged reaction time of 28
hours, 3b was obtained in a poor 29% yield. To try to im-
prove the yield, we increased the amount of acid to six
equivalents and found that the coupling reaction was com-
plete after one hour, giving the desired product in a much-
improved 78% yield. Similarly, 2i was reacted under these
modified reaction conditions giving the desired product in
72% yield, compared to a yield of 20% after a reaction
time of 35 hours under the standard conditions. The pro-
longed reaction time for tosylate iodonium salt 2g and the
somewhat reduced yield, compared to 2a, can be attribut-
ed to the influence of the anion (Table 1, entry 7).
a Concentration of HCl was 0.1 N.
ploys acidic conditions. The mechanism is thought to
involve an anion exchange of the iodonium salt with
Ph4B–, promoted by the formation of HX, while an in-
tramolecular coupling reaction follows to furnish the
product.
In summary, a novel and efficient method for the forma-
tion of carbon–carbon bonds in acidic water has been re-
ported. This method has advantages over other procedures
in that it is simple, mild, high-yielding, and more environ-
mentally benign. Furthermore, the scope of the metal-cat-
alyst-free Suzuki-type coupling reaction could be
extended to other substrates.
We also used HCl in place of p-TsOH to examine the ef-
fect on the reaction. The reactions occurred easily in a
shorter time, however, lower isolated yields were ob-
tained in dilute aqueous HCl (0.1 N; Table 2).
Mps were determined on a digital mp apparatus and were not cor-
rected. IR spectra were recorded on a FT-170 SX instrument, H
NMR spectra were measured on a Bruker AM-400 FT-NMR spec-
trometer, and mass spectra were determined on HP5989A mass
spectrometer. Iodonium salts were prepared according to literature
procedures.6–8 Sodium tetraphenylborate is commercially available.
1
To date, iodonium salts had been reacted with a series of
nucleophiles to form carbon–carbon bonds, generally in
basic conditions,5 while our new coupling reaction em-
Synthesis 2006, No. 6, 943–945 © Thieme Stuttgart · New York

SHORT PAPER
A Novel and Efficient Coupling Reaction
945
Reaction of Sodium Tetraphenylborate with Iodonium Salts;
General Procedure
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Acknowledgment
Financial support from the Zhejiang Province Natural Science
Foundation of China (Project Y404016) and Zhejiang Province
Education Foundation of China (Project 20040569) is greatly ap-
preciated.
Synthesis 2006, No. 6, 943–945 © Thieme Stuttgart · New York