Rapid microwave-promoted catalyst- and solvent-free suzuki-type coupling reaction

Article abstract of DOI:10.1080/00397910600636477

The catalyst- and solvent-free Suzuki coupling reaction of sodium tetraphenylborate with hypervalent iodonium salts was achieved under microwave irradiation in the absence of a base. A convenient method for formation of carbon-carbon bonds afforded good yields in a short time. Copyright Taylor & Francis Group, LLC.

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Rapid Microwave‐Promoted Catalyst‐ and
Solvent‐Free Suzuki‐Type Coupling Reaction
Jie Yan a , Weixiao Hu b & Wei Zhou b
a College of Chemical Engineering and Materials Sciences, Zhejiang
University of Technology, Hangzhou, Zhejiang, China
b College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou, Zhejiang, China
Version of record first published: 16 Feb 2007.
To cite this article: Jie Yan , Weixiao Hu & Wei Zhou (2006): Rapid Microwave‐Promoted Catalyst‐ and
Solvent‐Free Suzuki‐Type Coupling Reaction, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 36:15, 2097-2102
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Synthetic Communicationsw, 36: 2097–2102, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600636477
Rapid Microwave-Promoted Catalyst- and
Solvent-Free Suzuki-Type Coupling
Reaction
Jie Yan
College of Chemical Engineering and Materials Sciences, Zhejiang
University of Technology, Hangzhou, Zhejiang, China
Weixiao Hu and Wei Zhou
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou, Zhejiang, China
Abstract: The catalyst- and solvent-free Suzuki coupling reaction of sodium tetraphe-
nylborate with hypervalent iodonium salts was achieved under microwave irradiation
in the absence of a base. A convenient method for formation of carbon–carbon bonds
afforded good yields in a short time.
Keywords: Iodonium salt, microwave irradiation, sodium tetraphenylborate, Suzuki
reaction
The Suzuki reaction is one of the most versatile and utilized reactions for the
selective construction of carbon–carbon bonds, in particular for the formation
of biaryls.[1] For development of improved conditions for the Suzuki reaction,
a wide range of metal complexes have been used as catalysts in these coupling
reactions; attention has particularly been focused on palladium. Recently,
N. E. Leadbeater and M. Marco reported a catalyst-free Suzuki coupling
reaction,[2] in which aryl halides reacted with arylboronic acids under
microwave irradiation without a catalyst and afforded biaryls in good yields
Received in Japan December 12, 2005
Address correspondence to Jie Yan, College of Chemical Engineering and Materials
Sciences, Zhejiang University of Technology, Hangzhou 310032, Zhejiang, China.
Fax: þ86 (571) 88320238; E-mail: jieyan87@hotmail.com
2097

2098
J. Yan, W. Hu, and W. Zhou
(Fig. 1). It was also reported that sodium tetraphenylborate could be used in
place of phenylboronic acid as a phenylating agent in the reaction.
However, the yields were somewhat low.
The report was very interesting, and we were inspired to find other
catalyst-free Suzuki-type coupling reactions to extend the scope of this
reaction. In our attempts to find an alternative to halides, we focused our
attention on hypervalent iodonium compounds because hypervalent
iodonium compounds (in particular, iodonium salts) have found synthetic
application as powerful electrophilic reagents in the reactions with various
nucleophiles;[3] they have also been used to replace aryl halides and triflates
in Suzuki reaction under mild conditions, and excellent yields have been
afforded.[4] As part of a program to investigate the catalyst-free Suzuki
reaction, we have explored the catalyst-free coupling of sodium tetraphenyl-
borate with hypervalent iodonium salts under microwave irradiation. Now
we report here a rapid and efficient catalyst- and solvent-free microwave-
assisted Suzuki coupling reaction of sodium tetraphenylborate with
hypervalent iodonium salts.
We first used diphenyliodonium chloride to couple with phenylboronic
acid under microwave irradiation, and we found that the coupling reaction
did not occur regardless of the presence of a base. However, when sodium
tetraphenylborate was used in place of phenylboronic acid to react with diphe-
nyliodonium chloride, the coupling was easy to carry out under solvent-free
conditions, and only a product of biphenyl was obtained after the mixture
was heated 5 min with microwave irradiation. The result was very exciting,
and to determine suitable reaction conditions, a series of experiments was
performed on the coupling of sodium tetraphenylborate with diphenyliodo-
nium chloride. It was found that when the molar ratio of sodium tetraphenyl-
borate to diphenyliodonium chloride reached 3 : 1, a 92% yield biphenyl was
afforded under microwave irradiation for 5 min. The result prompted us to
investigate the reactions of sodium tetraphenylborate (1) with a series of
iodonium salts (2) under the optimized conditions to afford products (3).
They are shown in Scheme 1, and the results are summarized in Table 1.
The products were characterized by 1H NMR, IR, and MS spectra and
melting points, which were consistent with the literature data.
It was obvious from Table 1 that most reactions could be completed in
5 min and gave products in excellent yields (entries 1–6). When an iodonium
salt with electron-withdrawing substituent in the phenyl 2g was used under
Figure 1. The catalyst-free Suzuki coupling reaction in water under microwave
irradiation.

Microwave-Assisted Suzuki-Type Coupling Reaction
2099
Scheme 1.
microwave heating for 5 min, the reaction gave only 37% of product 3f
regardless of whether we extended the reaction time (entry 7). It was quite
different from the result reported by Leadbeater and Marco that using aryl
bromides with electron-withdrawing substituents usually obtained higher
yields. When 2h was used in place of 2b, it was difficult to carry out the
same coupling reaction after longer reaction time, and a lower yield was
obtained (entry 8). To verify the result, we examined another iodonium salt
(p-MeO-C6H4)IþBF24 , and the reaction yielded only nearly 20% product
after microwave heating for 15min. It was shown that the anion of iodo-
nium salt could affect the reaction greatly; when it was Cl2 or Br2, the
reaction usually afforded high yield products in shorter time, but when it was
BF24 , the reaction was difficult to carry out and the yield was low after a
longer time.
Iodonium salts have been widely used in Suzuki reactions. However,
to the best of our knowledge, there have no reports about catalyst- and
Table 1. Results of the Suzuki reaction of sodium tetraphenylborate with
iodonium salts
Time
(min)
Yield
(%)a
Entry
1
Iodonium salt
Product
Ph2IþCl2
2a
Ph-Ph
3a
5
3
5
2
5
92
95
94
82
71
2
3
4
5
(p-Me-C6H4)2 IþBr2
2b
p-Me-C6H4-Ph
3b
(p-Br-C6H4)2 IþBr2
2c
p-Br-C6H4-Ph
3c
3b
p-Me-C6H4-Iþ-PhBr2
2d
p-MeO-C6H4-Iþ-PhBr2
2e
p-MeO-C6H4-Ph
3d
6
3
83
7
8
(m-O2N-C6H4)2IþBr2
m-O2N- C6H4-Ph
5
37
42
2 g
(p-Me-C6H4)2 IþBF42
2 h
3f
3b
15
aIsolated yield.

2100
J. Yan, W. Hu, and W. Zhou
solvent-free coupling reactions of sodium tetraphenylborate with hypervalent
iodonium salts in the absence of base. The microwave-promoted catalyst-
and solvent-free coupling reaction without base provided a fast and effi-
cient method for preparation of biaryls; it had some advantages such as
mild reaction conditions, simple procedure, and good yields. Furthermore,
the scope of catalyst-free Suzuki coupling reactions could be extended.
Further investigation of catalyst-free Suzuki reactions will be reported in
due course.
EXPERIMENTAL
Melting points were determined on a digital mp apparatus and are not
1
corrected. IR spectra were recorded on a FT-170 SX instrument, H NMR
spectra were measured on a Bruker AM-400 FT-NMR spectrometer, and
mass spectra were determined on HP5989A mass spectrometer. Microwave
irradiation was carried out with an LWMC-201 microwave synthesizer at
full power (650 W). Iodonium salts were prepared according to the literature
procedures.[5–7] Sodium tetraphenylborate was commercially available.
Reaction of Sodium Tetraphenylborate with Iodonium Salts,
General Procedure
Sodium tetraphenylborate 1 (513 mg, 1.5 mmol) and iodonium salt 2
(0.5 mmol) were mixed in 10-mL glass tube. The mixture tube was placed
inside in an alumina bath and irradiated for several minutes (shown in
Table 1) in a microwave synthesizer (LWMC-201) at full power (650 W).
After cooling to room temperature, the mixture was extracted with diethyl
ether (20 mL ꢀ 3). The organic layer was dried over anhydrous MgSO4 and
evaporated in vacuo. The crude product was separated by a silica-gel plate
using hexane as developer and afforded the pure product 3 in good yields.
Data
Biphenyl (3a)
Mp 68–698C (lit.[8] 69–728C). 1H NMR (CDCl3): d ¼ 7.35 (m, 2H), 7.44
(m, 4H), 7.60 (m, 4H); IR (KBr): n ¼ 3035, 1569, 1481, 730 cm21; MS
(70 eV, EI) m/z (%): 154 (Mþ, 100).
p-Methylbiphenyl (3b)
Mp 43–468C (lit.[9] 44–478C). 1H NMR (CDCl3): d ¼ 2.39 (s, 3H), 7.25
(m, 2H), 7.32 (m, 1H), 7.42 (m, 2H), 7.49 (m, 2H), 7.57 (m, 2H); IR (KBr):

Microwave-Assisted Suzuki-Type Coupling Reaction
2101
n ¼ 3067, 3033, 1488, 1007, 823, 766, 690 cm21; MS (70eV, EI) m/z (%):
168 (Mþ, 100).
p-Bromobiphenyl (3c)
Mp 83–858C (lit.[9] 85–878C). 1H NMR (CDCl3): d ¼ 7.35 (m, 1H), 7.43
(m, 4H), 7.54 (m, 4H); IR (KBr): n ¼ 3064, 3031, 1477, 1393, 1080, 830, 767,
691 cm21; MS (75eV, EI) m/z (%): 234 (M þ 1), 233 (Mþ, 13.3), 232 (100).
p-Methoxylbiphenyl (3d)
1
Mp 86–878C (lit.[9] 888C). H NMR (CDCl3): d ¼ 3.83 (s, 3H), 6.97
(m, 2H), 7.29 (m, 1H), 7.41 (m, 2H), 7.53 (m, 4H); IR (KBr) n ¼ 3068,
3033, 1262, 1035, 835, 761 cm21; MS (70eV, EI) m/z (%): 184 (Mþ, 100).
2-Phenylthiophene (3e)
Oil.[4a] 1H NMR (CDCl3): d ¼ 7.10 (m, 1H), 7.30–7.34 (m, 2H), 7.39
(m, 1H), 7.47 (m, 2H), 7.63 (m, 2H); IR (KBr) n ¼ 3103, 3062, 3036, 1496,
1451, 747 cm21; MS (70eV, EI) m/z (%): 160 (Mþ, 100).
m-Nitrobiphenyl (3f)
Mp 56–588C (lit.[9] 58–608C). 1H NMR (CDCl3): d ¼ 7.44–7.58
(m, 6H), 7.88 (m, 1H), 8.17 (m, 1H), 8.42 (m, 1H); IR (KBr): n ¼ 3064,
3036, 1536, 1362, 877, 772, 733 cm21; MS (75eV, EI) m/z (%): 199 (Mþ,
100).
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 appreciated.
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