Pd-catalyzed oxidative homocoupling of arylboronic acids in aqueous ethanol at room temperature

Article abstract of DOI:10.1080/15533174.2011.652280

The authors demonstrate for the first time that the Pd(OAc) ₂/N,O-ligands act as catalysts toward homocoupling of arylboronic acid in aqueous ethanol (95%) under mild conditions: at room temperature and under aerobic condition. Under the optimal reaction conditions, all reactions gave the homocoupling products in good to excellent yields.

Full text of DOI:10.1080/15533174.2011.652280

This article was downloaded by: [University of Haifa Library]
On: 29 August 2013, At: 14:27
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Synthesis and Reactivity in Inorganic, Metal-Organic,
and Nano-Metal Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lsrt20
Pd-Catalyzed Oxidative Homocoupling of Arylboronic
Acids in Aqueous Ethanol at Room Temperature
a
a
a
a
a
a
Zhonggao Zhou , Qiaosheng Hu , Ziyi Du , Jun Xue , Shiyong Zhang & Yongrong Xie
a
College of Chemistry and Chemical Engineering , Gannan Normal University , Ganzhou , P.
R. China
Accepted author version posted online: 03 May 2012.Published online: 06 Aug 2012.
To cite this article: Zhonggao Zhou , Qiaosheng Hu , Ziyi Du , Jun Xue , Shiyong Zhang & Yongrong Xie (2012) Pd-Catalyzed
Oxidative Homocoupling of Arylboronic Acids in Aqueous Ethanol at Room Temperature, Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry, 42:7, 940-943, DOI: 10.1080/15533174.2011.652280
To link to this article: http://dx.doi.org/10.1080/15533174.2011.652280
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained
in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the
Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and
should be independently verified with primary sources of information. Taylor and Francis shall not be liable for
any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever
or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of
the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any
form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://
www.tandfonline.com/page/terms-and-conditions

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 42:940–943, 2012
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.652280
Pd-Catalyzed Oxidative Homocoupling of Arylboronic
Acids in Aqueous Ethanol at Room Temperature
Zhonggao Zhou, Qiaosheng Hu, Ziyi Du, Jun Xue, Shiyong Zhang,
and Yongrong Xie
College of Chemistry and Chemical Engineering, Gannan Normal University, Ganzhou, P. R. China
ligand-free Pd(OAc)2 (5.0 mol%) catalyzed homocoupling re-
The authors demonstrate for the first time that the action in ethanol (95%), and they just got moderate yields even
Pd(OAc)
/N,O-ligands act as catalysts toward homocoupling of with prolonged the reaction time (24 h).[ Kirai reported copper-
2]
2
arylboronic acid in aqueous ethanol (95%) under mild conditions:
at room temperature and under aerobic condition. Under the opti-
mal reaction conditions, all reactions gave the homocoupling prod-
ucts in good to excellent yields.
catalyzed homocoupling of arylboronic acids accelerated by
,10-phenanthroline with a 2.0–4.0 mol% loading of the cata-
1
[27]
lyst.
Developing eco-friendly reactions for the preparation of
biaryls continues to be the subject of intense investigation
according to the green solvent selection guide.[ Wu and
coworkers reported a cyclopalladated ferrocenylimines cat-
Keywords C-C coupling, green chemistry, N-O compounds, oxida-
31]
tive homocoupling, palladium
[
8]
alyzed homocoupling reaction in n-PrOH/H2O (v/v = 1:2).
Pd(OAc)2-catalyzed (0.5–3.0 mol%) homocoupling reaction in
acetone/water (v/v = 1:1) was reported by Zhang and cowork-
ers.[ However, when using the thiophene-3-boronic acid as
substrate, they got 25% yield of the heterobiaryl even prolonged
the reaction time. Sakurai and coworker reported gold(0) nan-
oclusters, stabilized by PVP catalyzed homocoupling reaction
INTRODUCTION
Symmetrical and unsymmetrical biaryl derivatives have been
used in numbers synthetic ventures, including pharmaceuticals,
7]
[
1–10]
natural products, and polymers.
Transition metal–catalyzed
couplings such as from the Suzuki, Ullmann, Heck, Negishi,
Stille, Sonogashira, and Kumada reactions are the most impor-
[
29]
[
11–19]
of potassium aryltrifluoroborate in neat water.
tant tools to obtain substituted biaryls.
[
30,32]
Recently, Mohanty reported
N,O-ligands (Figure 1, 3
In contrast, relatively few preparations of symmetrical substi-
tuted biaryls from arylboronic acids have been reported since the
and 4) and we also found[ ligand 3 display good activity in Pd-
catalyzed Suzuki reactions at room temperature, and trace prod-
ucts from the homocoupling of arylboronic acids were found
33]
[
20]
first example in 1987 studied by Suzuki. This is another good
method to obtain symmetrical substituted biaryls from just a sin-
[
21]
[3,4,7,8,20,22] under aerobic condition. Herein, we found a similar catalyst
gle precursor. Several different metals, such as Pd,
[
23]
[24]
[25]
[9,20,26–28]
[5,6,29]
system, which is highly efficient, to prepare symmetrical aryl
catalyzed ox-
Rh,
Ni,
Cr,
Cu,
and Au
biphenyls with good-to-excellent yields in the environmentally
idative homocoupling of arylboronic acids were reported, and
they provide the corresponding biaryls in moderate to excellent
yields.
friendly solvents (95% aqueous ethanol). Herein we report read-
ily accessible moisture- and air-stable and most importantly
highly efficient N,O-ligands (Figure 1, 1 and 2) for homocou-
pling of arylboronic acids under very mild conditions. Mean-
while, in our catalyst system, the amount of Pd(OAc)2 is lower
(0.01 mol%).
Several ligand-free and phosphorus-free catalytic reactions
containing mainly nitrogen donor atoms were designed and
employed in various catalytic reactions with remarkable suc-
cess.[9,11,22,30]
For example, Smith and coworkers reported
EXPERIMENTAL
Materials
Received 20 August 2011; accepted 2 October 2011.
Financial support for this work was provided by the National
Natural Science Foundation of China (Contract Nos. 20861001 and
All chemicals employed in the synthesis were analyt-
ical grade, obtained commercially, and used as received
without further purification. Piperazine derivative 2-(N-
methylpiperazinyl-N’-methyl)-4,6-di-tert-butyl-phenol (1) and
21061001), the Key Laboratory of Jiangxi University for Functional
Materials Chemistry.
Address correspondence to Zhonggao Zhou, College of Chem-
istry and Chemical Engineering, Gannan Normal University, Ganzhou
2
-(N-methylpiperazinyl-N’-phenyl)-4,6-di-tert-butyl-phenol (2)
[
34]
3
41000, P. R. China. E-mail: zhgzhou@foxmail.com
were prepared as reported. NMR spectra were performed on
940

OXIDATIVE HOMOCOUPLING OF ARYLBORONIC ACIDS
941
a Bruker Avance III 400 MHz spectrometer (TMS as an internal
TABLE 1
standard), and coupling constants (J values) are given in hertz.
Investigation of homocoupling reaction of phenylboronic acid
a
conditions
General Procedure for the Pd(OAc)2/L Catalyzed
Homocoupling Reaction
In a two-necked round-bottom flask, the appropriate amount
of ligand, metal precursor, and 3.0 mL of solvent were placed
with a magnetic stir bar. After stirring for 5 min, arylboronic Entry (mol%)
acids (1.0 mmol) and base (2.0 mmol) were added to the re-
Pd/L
Time Yield
(h)
Base
Solvent
Water
Ethanol
Ethanol (95%)
Methanol
(%)b
1
2
3
4
5
6
7
8
9
1 : 1.0
1 : 1.0
1 : 1.0
1 : 1.0
1 : 1.0
1 : 1.0
1 : 1.0
1 : 1.0 none
1 : 1.0 KOH
1 : 1.0
1 : 1.0 NaOAc Ethanol (95%)
1 : 1.0 Cs2CO3 Ethanol (95%)
1 : 1.0 NaOBu Ethanol (95%)
1 : 1.0 NaOH
1:0
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
24
24
12
12
43
97
98
68
70
action flask. The reaction mixture was stirred under air atmo-
sphere at room temperature for the predetermined reaction time.
The course of reaction was monitored by GC-MS analysis, and
yields were calculated against biaryl derivatives. The solvent
was removed under reduced pressure after the reaction was
completed. The pure products can be obtained through a short
column chromatography using petroleum ether as eluent from
any arylboronic acid. The organic solvent was evaporated under
reduced pressure. Products were characterized by comparison
Toluene
c
Ethanol (95%)
Ethanol (95%)
Ethanol (95%)
Ethanol (95%)
Ethanol (95%)
96
d
56
trace
c
67
70
53
95
90
58
c
1
10
K3PO4
of H NMR spectroscopic data with those in the literature.
c
1
1
1
1
1
1
1
1
1
2
1
2
3
4
5
6
7
8
9
0
[
33]
c
1
2
3
4
5
6
7
8
9
. Biphenyl (T 2–1)
ꢁ
[8]
[9]
[6]
t
c
. 4,4 -dichloro-biphenyl (T 2–2)
. 4,4 -Difluorobiphenyl (T 2–3)
. 4,4 -dicyano-biphenyl (T 2–4)
. 4,4 -dinitro-biphenyl (T 2–5)
. 4,4 -Dimethylbiphenyl (T 2–6)
. 4,4 -Dimethoxybiphenyl (T 2–7)
. 9,9 -Biphenanthrene (T 2–8)
. 2,2 -Bithiophene (T 2–9)
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
[9]
c
Ethanol (95%)
Ethanol (95%)
Ethanol (95%)
Ethanol (95%)
Ethanol (95%)
Ethanol (95%)
Ethanol (95%)
c
K 2CO3
K2CO3
K2CO3
K2CO3
K 2CO3
K 2CO3
trace
[9]
c
1 : 1.5
1 : 2.0
1 : 3.0
1 : 1.0
1 : 1.0
98
97
96
c
[35]
c
[9]
e
trace
[35]
94f
[9]
1
0. 3,3 -Bipyridine (T 2–10)
aReaction conditions: phenylboronic acid (1.0 mmol), base
(
2.0 mmol), solvent (3.0 mL), catalyst (Pd:1 = 1:1.0) 0.05 mol%,
b
RESULTS AND DISCUSSION
Solvents have a key influence on the environmental impact
r.t., air. Determined by GC-MS with internal standard, average of
two runs. Catalyst 0.01 mol%. Catalyst 0.005 mol%. Under N
atmosphere; Complex 2 as ligand.
c
d
e
2
f
[13]
of chemical processes and affect the catalyst activity signifi-
cantly in metal-catalyzed chemistry. Generally, transition metal-
catalyzed homocoupling of arylboronic acids proceed in organic
With the best solvent in hand, next, the catalytic activity of
Pd(OAc)2 1 was examined toward homocoupling of phenyl-
boronic acid with different bases under aerobic conditions.
[
4]
solvent which is harmful to the environment, such as DMSO,
DMF,[
26,28]
or toluene.
[3,6]
The first step was to identify the best solvent system. Be- When used KOH and NaOH as base (Table 1, entries 8 and 13),
cause the higher solubility of the ligands in methanol, ethanol, trace phenol was observed as a byproduct. This side product
and water[
30,32]
and we wanted to find the most widely used and should result from the oxidation of the C–B bond of phenyl-
environmentally friendly solvent system. In our initial exper- boronic acid with hydrogen peroxide[ arising from boron
iments, we chose the phenylboronic acid as a model reaction peroxide and water, which would be generated in situ from
using Pd(OAc)2 as the palladium source, K2CO3 as the base. phenylboronic acid via tri(p-tolyl)boroxine production.[ Sev-
Formation of phenols was usually observed in oxidative homo- eral bases were employed in the reactions, K2CO3, Cs2CO3,
37]
4]
[
22,36]
t
coupling of arylboronic acids.
Herein, when used water and NaOBu , proving superior (Table 1, entries 6, 11, and 12),
as solvent, tri(p-tolyl)boroxine production and phenol were ob- and we chose K2CO3 in the next reactions. Next, the same ex-
served as side products also. Although pure ethanol is available, periment was repeated using 0.01 mol% then with 0.005 mol%
it is much more expensive and is used only when definitely loading of the catalyst under the identical reaction conditions
required, nearly all the ethanol used industrially is a mixture mentioned previously, and gave desired products in 96% and
[
10]
of 95% ethanol and 5% water. Interestingly, herein, ethanol 56% GC yields, respectively (Table 1, entries 6 and 7). The re-
95%) was found to be the best solvent and gave desired product action in the absence of base was carried out under the present
in 98% GC yields (Table 1, entry 3), our results provided a basis catalytic system, with only trace products detected by GC-MS
for industrial applications. (Table 1, entry 8).
(

942
Z. ZHOU ET AL.
Considering the palladium-ligand molar ratio always played
TABLE 2
important roles in metal-catalyzed organoboron chemistry.[
38–40]
a
Pd-catalyzed homocoupling of various arylboronic acids
To identify the metal ligand molar ratio effect on oxidative
homocoupling reactions, we investigated a series of reactions
by taking the model reaction in different metal ligand molar
ratios (Table 1, entries 3 and 15–18). Compared to the results
of Smith and coworkers, we observed that a 1:1.0 palladium-
ligand ratio afforded optimum reaction rate, which is higher than
the other results for different ratios (Table 1, entry 6). Increasing
the amount of ligand 1 in the system had little influence on the
reaction (Table 1, entries 16–18), which might be owing to its
Yield (%)b
Entry
1
Product
[
2]
98 (97)
2
3
4
5
6
7
99 (98)
94 (95)
96 (94)
98 (96)
95 (94)
98 (95)
difficulty to dissociate from the metal center to produce the bulk
metal.[
41,42]
Therefore, it is clear that the ligand of piperazine
derivative 1 accelerated the oxidative homocoupling reactions
(
Table 1).
All these results indicated that oxygen in air as oxidizing
agent is necessary to make the reaction proceed smoothly. Dif-
ferent from the traditional protocols during our experiments sin-
gle product was obtained by running the reaction directly in air
without heating and using additional oxygen source. However,
only trace products were detected by GC-MS under nitrogen at-
mosphere (Table 1, entry 19). In this way, we can further verify
the developed mechanism of this reaction.[7,8,10]
Under the optimized reaction conditions (ethanol [95%],
K2CO3, room temperature), the relative activity of another piper-
azine derivative for the same model reaction was studied. How-
ever, N-phenyl-substituted complex 2 as ligand is also efficient,
and showed slightly lower activity (Table 1, entry 20).
To establish the generality of the present protocol, a variety
of electronically and structurally diverse aromatic boronic acid
were examined under the optimized reaction conditions, and
the obtained results are summarized in Table 2. Once again, the
reaction proceeded smoothly and tolerated various functional
groups on the phenyl ring of aromatic boronic acids and afforded
the products in moderate to good yields under the optimum
reaction conditions.
8
9
83 (80)
75 (65)
86 (83)
1
0
aReaction conditions: arylboronic acid (1.0 mmol), K
CO (2.0
2 3
equiv), catalyst (Pd/1) 0.01 mol%, ethanol (95%) (3.0 mL), r.t., 12h,
in air. GC yields calibrated via internal standard, isolated yields were
given in parentheses; average of two runs.
b
We found aromatic boronic acids bearing electron-
withdrawing chloro, fluoro, cyano, and nitro groups gave cor-
responding homocoupling products in 98%, 95%, 94%, and
96% yields, respectively (Table 2, entries 2–5). Next, we ex-
amined the homocoupling reaction of aromatic boronic acids bearing electron-donating groups. Aromatic boronic acids bear-
ing methyl and methoxy groups gave desired products in 94%
and 95% isolated yields (Table 2, entries 6 and 7). A similar
result has been reported by Luo using copper-catalyzed homo-
coupling reaction of aromatic boronic acids.[
9]
Reaction with 2-naphthylboronic acid afforded correspond-
ing dimmer in 80% isolated yield (Table 2, entry 8). Finally,
the homocoupling of heteroarylboronic acids 3-pypridylboronic
acid and 2-thienylboronic acid gave corresponding products in
65 and 83% isolated yields, respectively, under the optimum re-
action conditions (Table 2, entries 9 and 10). It can be concluded
that oxidative homocoupling reaction of arylboronic acids may
accelerate by Pd(OAc)2 1 system at room temperature under
aerobic conditions in ethanol (95%).
FIG. 1. Some ligands of piperazine derivative.

OXIDATIVE HOMOCOUPLING OF ARYLBORONIC ACIDS
943
In summary, moisture- and air-stable N,O-ligand (1) was 16. Monnereau, L.; S e´ meril, D.; Matt, D.; Toupet, L.; Mota, A.J. Adv. Synth.
Catal. 2009, 351, 1383–1389.
7. Suzuka, T.; Nagamine, T.; Ogihara, K.; Higa, M. Catal. Lett. 2010, 139,
effective for the Pd-catalyzed homocoupling reaction of
1
arylboronic acids, good to excellent yields of symmetrical
biaryls were achieved under eco-friendly and extremely mild
conditions: in ethanol (95%) under aerobic atmosphere at room
temperature with very low catalyst loading (0.01 mol%). Our
result showed that oxygen was essential for the homocoupling
reaction also.
8
5–89.
1
1
8. Ehrentraut, A.; Zapf, A.; Beller, M. Synlett. 2000, 1589–1592.
9. Larhed, M.; Lindeberg, G.; Hallberg, A. Tetrahedron Lett. 1996, 37,
8219–8222.
0. Miyaura, N.; Suzuki, A. Main Group Met. Chem. 1987, 10, 295.
1. Partyka, D.V. Chem. Rev. 2011, 111, 1529–1595.
2
2
2
2. Yamamoto, Y.; Suzuki, R.; Hattori, K.; Nishiyama, H. Synlett. 2006, 1027,
1
030.
2
2
3. Vogler, T.; Studer, A. Adv. Synth. Catal. 2008, 350, 1963–1967.
4. Liu, G.Y.; Du, Q.L.; Xie, J.J.; Zhang, K.L.; Tao, X.C. Chin. J. Catal. 2006,
SUPPLEMENTARY MATERIALS
This article is distributed under the terms of the Creative
Commons Attribution Noncommercial License, which permits
any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
2
7, 1051–1052.
2
2
2
5. Falck, J.R.; Mohapatra, S.; Bondlela, M.; Venkataraman, S.K. Tetrahedron
Lett. 2002, 43, 8149–8151.
¨
6. Demir, A.S.; Reis, O.; Emrullahoglu, M. J. Org. Chem. 2003, 68,
0130–10134.
7. Kirai, N.; Yamamoto, Y. Eur. J. Org. Chem. 2009, 1864–1867.
1
REFERENCES
28. Kaboudin, B.; Haruki, T.; Yokomatsu, T. Synthesis 2011, 91–96.
. Moreno-Ma n˜ as, M.; P e´ rez, M.; Pleixats, R. J. Org. Chem. 1996, 61, 29. Sakurai, H.; Tsunoyama, H.; Tsukuda, T. J. Organomet. Chem. 2007, 692,
1
2
346–2351.
368–374.
2. Smith, K.A.; Campi, E.M.; Jackson, W.R.; Marcuccio, S.; Naeslund, C.G.
M.; Deacon, G.B. Synlett. 1997, 1, 131–132.
30. Mohanty, S.; Suresh, D.; Balakrishna, M.S.; Mague, J.T. Tetrahedron 2008,
64, 240–247.
3
4
. Wong, M.S.; Zhang, X.L. Tetrahedron Lett. 2001, 42, 4087–4089.
31. Capello, C.; Fischer, U.; Hungerb u¨ hler, K. Green Chem. 2007, 9, 927–934.
. Yoshida, H.; Yamaryo, Y.; Ohshita, J.; Kunai, A. Tetrahedron Lett. 2003, 32. Mohanty, S.; Suresh, D.; Balakrishna, M.S.; Mague, J.T. J. Organomet.
4, 1541–1544. Chem. 2009, 694, 2114–2121.
. Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. Lang- 33. Zhou, Z.G.; Shi, J.C.; Hu, Q.S.; Xie, Y.R.; Du, Z.Y.; Zhang, S.Y. Appl.
muir 2004, 20, 11293–11296. Organomet. Chem. 2011, 25, 616–619.
. Carrettin, S.; Guzman, J.; Corma, A. Angew. Chem. Int. Ed. 2005, 44, 34. Farwell, J.D.; Hitchcock, P.B.; Lappert, M.F.; Luinstra, G.A.; Protchenko,
4
5
6
2
242–2245.
A.V.; Wei X-H J. Organomet. Chem. 2008, 693, 1861–1869.
35. Gonzalez-Arellano, C.; Corma, A.; Iglesias, M.; Sanchez, F. J. Catal. 2006,
238, 497–501.
36. Aramend ´ı a, M.A.; Lafont, F.; Moreno-Ma n˜ as, M.; Pleixats, R.; Roglans,
A. J. Org. Chem. 1999, 64, 3592–3594.
37. Brown, H.C.; Midland, M.M.; Kabalka, G.W. J. Am. Chem. Soc. 1971, 93,
1024–1025.
38. Littke, A.F.; Fu, G.C. Angew. Chem. Int. Ed., 1998, 37, 3387–3388.
39. Grasa, G.A.; Nolan, S.P. Org. Lett. 2001, 3, 119–122.
7
8
9
. Xu, Z.; Mao, J.C.; Zhang, Y.W. Catal. Commun. 2008, 9, 97–100.
. Mu, B.; Li, T.S.; Fu, Z.H.; Wu, Y.J. Catal. Commun. 2009, 10, 1497–1501.
. Cheng, G.J.; Luo, M.M. Eur. J. Org. Chem. 2011, 2519–2523.
0. Adamo, C.; Amatore, C.; Ciofini, I.; Jutand, A.; Lakmini, H. J. Am. Chem.
Soc. 2006, 128, 6829–6836.
1. Marion, N.; Navarro, O.; Mei, J.G.; Stevens, E.D.; Scott, N.M.; Nolan, S.P.
J. Am. Chem. Soc. 2006, 128, 4101–4111.
2. McGlacken, G.P.; Bateman, L.M. Chem. Soc. Rev. 2009, 38, 2447–2464.
1
1
1
1
3. Polshettiwar, V.; Decottignies, A.; Len, C.; Fihri, A. ChemSusChem 2010, 40. Grasa, G.A.; Viciu, M.S.; Huang, J.K.; Zhang, C.M.; Trudell, M.L.; Nolan,
, 502–522. S.P. Organometallics 2002, 21, 2866–2873.
4. Lemaire, M.; Hassan, J.; S e´ vignon, M.; Gozzi, C.; Schulz, E. Chem. Rev. 41. Herrmann, W.A.; K o¨ cher, C. Angew. Chem. Int. Ed. 1997, 36, 2162–2187.
3
1
1
2
002, 102, 1359–1469.
42. Qin, C.M.; Wu, H.Y.; Chen, J.X.; Liu, M.C.; Cheng, J.; Su, W.K.; Ding,
5. Corbet, J.P.; Mignani, G. Chem. Rev. 2006, 106, 2651–2710.
J.C. Org. Lett. 2008, 10, 1537–1540.