FeCl3/PPh3-catalyzed Sonogashira coupling reaction of aryl iodides with terminal alkynes

Article abstract of DOI:10.1016/j.tetlet.2010.03.063

Conditions for a FeCl₃/PPh₃-catalyzed and palladium-, copper-, amine free-Sonogashira coupling reaction of aryl halides with terminal alkynes are reported. The protocol was applicable to a wide variety of substituted aryl iodides and alkynes with different steric and electronic properties and gave excellent yields of the desired coupling products.

Full text of DOI:10.1016/j.tetlet.2010.03.063

Tetrahedron Letters 51 (2010) 2758–2761
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
FeCl /PPh -catalyzed Sonogashira coupling reaction of aryl iodides
3
3
with terminal alkynes
Dinesh N. Sawant, Pawan J. Tambade, Yogesh S. Wagh, Bhalchandra M. Bhanage *
Department of Chemistry, Institute of Chemical Technology (Autonomous), N. M. Parekh Marg, Matunga, Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Conditions for a FeCl /PPh -catalyzed and palladium-, copper-, amine free-Sonogashira coupling reaction
3
3
Received 5 February 2010
Revised 16 March 2010
Accepted 17 March 2010
Available online 20 March 2010
of aryl halides with terminal alkynes are reported. The protocol was applicable to a wide variety of substi-
tuted aryl iodides and alkynes with different steric and electronic properties and gave excellent yields of
the desired coupling products.
Ó 2010 Elsevier Ltd. All rights reserved.
Sonogashira coupling of terminal acetylenes with aryl halides is
a widely used tool for the synthesis of a variety of intermediates in
pharmaceuticals, agrochemicals and natural products.1 After the
first report on this reaction by Sonogashira and Hagihara in 1975
using a palladium catalyst in combination with copper,2 several
other modified protocols based on palladium systems were devel-
The cross-coupling reaction of phenyl acetylene (1) with iodo-
benzene (2) was chosen as a model reaction to optimize different
reaction parameters. Initially, the reaction was conducted for
120 h, giving a 98% yield of the desired diphenylacetylene 3a (Ta-
ble 1, entry 1). The reaction was studied at different time intervals,
and it was observed that the product 3a was obtained in 94% yield
after 48 h (Table 1, entry 3). The utility of the different phosphine
ligands shown in Figure 1 was investigated (Table 1, entries 3 and
5–9). Most of these ligands were found to be ineffective, with only
L1 and L3 giving good yields (94% and 65%, respectively) (Table 1,
entries 3 and 6).
3
oped. Palladium-catalyzed protocols have limitations due to the
use of expensive Pd metal, which is also toxic and detrimental to
4
the environment. There is a rapidly growing demand for new cat-
alytic systems which are effective, yet economical and environ-
mentally friendly. Iron is one of the better potential candidates
5
as it is cheaper, nontoxic and abundant. In the last few years there
The effects of different organic and inorganic bases were then
screened (Table 1, entries 10–17) and inorganic bases were found
has been growing awareness of iron-catalyzed cross-coupling reac-
6
tions. Recently Bolm and co-workers reported various iron-based
to be superior to organic bases. It was observed that K
PO showed similar results and gave good yields of the desired
product (Table 1, entries 3 and 17). However, K PO was preferred
over K CO for further study due to the ease of handling. The sol-
2 3
CO and
catalytic systems for C–C,7 C–N,8 C–O9 and C–S bond formation
10
K
3
4
reactions, and they were the first to introduce the iron-catalyzed
3
4
7
Sonogashira reaction. There are also a few reports on Fe–Cu co-
2
3
catalyzed Sonogashira coupling reactions.11
vents acetonitrile, toluene N,N-dimethylformamide (DMF) and N-
methyl-2-pyrrolidone (NMP) were screened (Table 1, entries 18–
20). It was observed that the reaction gave better results with
the use of acetonitrile or toluene as a solvent. However, due to cost
considerations and environmental aspects, toluene was our pre-
ferred solvent for further study (Table 1, entry 17 vs 18). The reac-
tion temperature was also optimized to 135 °C. The reaction at
lower temperature gave lower yield of 3a (Table 2).
While there has been significant growth in the area of iron-cat-
alyzed cross-coupling methodologies.12 Buchwald and Bolm re-
cently, demonstrated that the catalytic activity in some iron-
catalyzed reactions is due to trace amounts of copper present as
an impurity in iron salts. They arrived at this conclusion by demon-
13
3
strating that yield decreases with higher-purity FeCl and this is re-
versed by introducing trace amount of Cu O. These findings
2
suggest that there is still much to learn about the role of iron in
the catalysis of Sonogashira coupling reactions.14
Decreasing the catalyst loading to 5% led to lower yield of 3a.
However, it should be noted that increasing catalyst loading above
15% did not affect the yield (Table 3).
In this work, we report an efficient and facile protocol for Sono-
gashira cross-coupling reactions catalyzed by FeCl
3
/PPh
3
. The reac-
In order to probe the role of iron and possible foreign metal con-
tamination in reagents, test experiments were carried out in the
tion works as well with high-purity FeCl as it does with lower
3
grades, suggesting that iron is indeed the metal responsible for
the observed catalytic activity.
absence of FeCl
try 1). PPh and K
reaction (Table 4, entry 2), demonstrating that FeCl
PO were all necessary. This indirectly confirms the absence of
active metals in PPh , K PO and toluene.
3
and PPh
3
giving no reaction products (Table 4, en-
were not able to catalyze the
, PPh and
3
3
PO without FeCl
4
3
3
3
K
3
4
*
Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614.
E-mail address: bhalchandra_bhanage@yahoo.com (B.M. Bhanage).
3
3
4
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.063

D. N. Sawant et al. / Tetrahedron Letters 51 (2010) 2758–2761
2759
Table 1
Table 2
Reagent screening for the iron-catalyzed coupling of phenyl acetylene (1) with
Influence of reaction temperature on iron-catalyzed Sonogashira coupling of phen-
ylacetylene (1) with phenyl iodide (2)
iodobenzene (2)a
a
FeCl3, ligand
No.
Temp (°C)
Yield of 3ab (%)
+
I
base, solvent,
1
2
3
4
5
6
7
8
145
135
125
115
105
95
98
98
70
32
22
21
18
12
o
1
35 C, time
1
2
3a
Yield of 3ab (%)
Entry
Ligand
Base
Solvent
t (h)
Effect of time
75
rt
1
2
3
4
L1
L1
L1
L1
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
3
PhMe
PhMe
PhMe
PhMe
120
72
48
98
96
94
27
a
Reaction conditions:
(0.30 mol %), K
1
(2 mmol),
2
(1 mmol), FeCl
3
(0.15 mol %), PPh
3
24
3
PO
4
(2.0 mmol), toluene (3 mL/mmol), 48 h under nitrogen.
b
GC yield.
Effect of ligand
5
6
7
8
9
L2
L3
L4
L5
L6
K
K
K
K
K
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
PhMe
PhMe
PhMe
PhMe
PhMe
48
48
48
48
48
Traces
65
Traces
12
Table 3
Influence of catalyst loading on iron-catalyzed Sonogashira coupling of phenylacet-
ylene (1) with phenyl iodide (2)
0
a
Effecy of base
Yield of 3ab (%)
No.
FeCl
3
(mol %)
3
PPh (mol %)
À
10
11
12
13
14
15
16
L1
L1
L1
L1
L1
L1
L1
[BMIM]OH
Morpholine
Et
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
48
48
48
48
48
48
48
0
0
1
2
3
4
5
10
15
20
10
20
30
40
38
72
98
98
3
N
Traces
7
Traces
10
DBU
t
K BuO
KOH
a
Reaction conditions: 1 (2 mmol), 2 (1 mmol), K
mmol), 135 °C, 48 h under nitrogen.
3
PO
4
(2.0 mmol), toluene (3 mL/
Cs
2
CO
3
70
b
Effect of solvent
GC yield.
17
18
19
20
L1
L1
L1
L1
K
K
K
K
3
PO
3
PO
3
PO
3
PO
4
4
4
4
PhMe
ACN
DMF
NMP
48
48
48
48
96
96
60
85
Table 4
Test experiment for the iron-catalyzed Sonogashira coupling of phenylacetylene (1)
with phenyl iodide (2)
a
Reaction conditions: 1 (2 mmol), 2 (1 mmol), commercial grade >95% FeCl
0.15 mol %), ligand (0.30 mol %), base (2 mmol), solvent (3 mL/mmol), 135 °C under
nitrogen.
3
a
(
No.
FeCl
3
PPh
3
K
3
PO
4
1
2
Yield of 3ab (%)
b
GC yield.
1
2
3
4
5
—
—
U
U
U
—
U
U
U
U
—
U
U
U
U
U
U
U
U
U
U
0
Trace
Trace
0
—
U
U
U
96
a
P
P
Reaction conditions:
(0.30 mol %), K PO
Yield of isolated product after column chromatography.
1
(2 mmol),
2
(1 mmol), FeCl
3
(0.15 mol %), PPh
3
O
PPh2
P
3
4
(2.0 mmol), toluene (3 mL/mmol), 135 °C, 48 h under nitrogen.
PPh2
b
L1
L2
L3
Table 5
Sonogashira coupling of phenyl acetylene (1) with iodobenzene (2) using different
a
commercial sources of FeCl
3
P
P
P
Yield of 3ab (%)
P
Entry
FeCl
3
P
P
1
2
3
>95% (S.D fine chemicals)
>98% (Merck)
>99.99+% (Aldrich)
96
96
99
L4
L5
Figure 1. Various phosphine ligands.
L6
a
Reaction conditions:
0.30 mol %), K PO
Yield of isolated product after column chromatography.
1
(2 mmol),
2
(1 mmol), FeCl
3
(0.15 mol %), PPh
3
(
3
4
(2.0 mmol), toluene (3 mL/mmol), 135 °C, 48 h under nitrogen.
b
Finally, to gain insight into whether the reaction was catalyzed
by trace impurities in FeCl , we carried out the same experiment
with different sources of FeCl (from >95% to 99.99+% purity) (Ta-
ble 5, entries 1–3). Using higher purity FeCl did not have a nega-
tive impact on the yield of 3a (Table 5, entry 3), so the lowest grade
of FeCl (from S.D fine chemicals, purity >95%) was used for the
3
aryl halides (Table 6). There was no reaction of phenyl acetylene
with bromobenzene (entry 1) or with ortho-substituted aryl io-
dides (entries 8–10). Electron-donating substituents gave good to
excellent yields (entries 3, 4 and 6), and bulky 1-iodonaphthalene
also gave 85% yield (entry 5). Electron-deficient 4-iodonitroben-
zene, on the other hand, gave no desired product (entry 7). The
reaction also worked well with heterocyclic aryl iodides 2-iodopyr-
idine, 3-iodopyridine and 2-iodothiophene, giving good to excel-
lent yields (entries 11–13).
3
3
3
subsequent substrate study.
The optimized reaction conditions are phenylacetylene (1,
2
3 3
mmol), iodobenzene (2, 1 mmol), FeCl (0.15 mol %), PPh
(
0.30 mol %) and K PO (2 mmol) in toluene (3 mL/mmol of ido-
3
4
15
benzene) at 135 °C for 48 h. These conditions were applied to
The generality of the protocol was explored using reactions of
various aliphatic, heteroaromatic and aromatic substituted alkynes
the coupling of phenyl acetylene with a range of aryl- and hetero-

2
760
D. N. Sawant et al. / Tetrahedron Letters 51 (2010) 2758–2761
Table 6
Table 7
a
Iron-catalyzed Sonogashira coupling of terminal alkynes with aryl iodidesa
Iron-catalyzed Sonogashira coupling of phenyl acetylene (1) with aryl halides
FeCl3, PPh3
Entry Alkyne (1)
Aryl iodide (2)
Product
(3)
Yieldb
(%)
+
X
Ar
Ar
K3PO4, toluene,
o
n-C H
H
H
H
1
35 C, 48h
6
13
13
45c
1
I
I
3m
3n
3
1
2
n-C H
6
62c
66c
36c
Entry Aryl halide (2)
Product (3)
Yieldb
%)
2
3
4
(
n-C H
6
13
9
I
OCH3 3o
1
2
3
4
Br
I
Ph
3a Trace
3a 99
n-C H
H
4
I
3p
3q
Ph
Ph
Ph
Ph
n-C H
H
H
4
9
9
47c
58c
5
6
I
I
I
H3CO
H3C
I
OCH3 3b 98
n-C H
4
OCH3 3r
I
CH3
3c 99
3d 85
H
7
8
3s
3t
0
0
N
N
5
6
H
I
I
I
N
H3C
Ph
OCH3
H
H
3e 78
9
0
3u
0
H3CO
O2N
I
CH3
NO2
H3CO
7
8
I
Ph
Ph
Ph
3f
0
I
1
3v
0
CH3
H3C
H2N
I
H2N
H2N
3g Trace
3h Trace
I
I
85b
H
H
1
1
2
I
I
3w
NH2
N
92b
1
3x
9
0
a
Reaction conditions:
1
(2 mmol),
2
(1 mmol), FeCl
3
(0.15 mol %), PPh
3
I
(
0.30 mol %), K PO (2.0 mmol), toluene (3 mL/mmol), 135 °C, 48 h under nitrogen.
3
4
b
1
3i
3j
0
Yield of isolated product after column chromatography.
GC Yield.
I
I
Ph
Ph
c
1
1
1
1
2
3
69
N
N
S
mized with respect to various parameters and enabled coupling
reactions of various aryl iodides with different alkynes. Further
investigations on the application of this catalytic system in other
coupling reactions are in progress.
I
Ph
Ph
3k 92
N
N
I
3l
96
S
Acknowledgments
a
Reaction conditions:
0.30 mol %), K PO
Yield of isolated product after column chromatography.
1
3 3
(2 mmol), 2 (1 mmol), FeCl (0.15 mol %), PPh
4
(2 mmol), toluene (3 mL/mmol), 135 °C, 48 h under nitrogen.
(
3
b
We are thankful to University Grant Commission, India for pro-
viding financial support under UGC-SAP Green Technology Center,
collaborative program of Institute of Chemical Technology and
University of Mumbai.
with aryl iodides (Table 7). Aliphatic alkynes were found to react
smoothly with a series of aryl iodides in moderate to good yields
(
entries 1–6), with more electron-rich aryl halides giving better
Supplementary data
yields. Heteroaromatic alkynes were non-reactive towards the
coupling reaction under optimized reaction conditions (entries 7
and 8). Surprisingly, alkynes substituted with electron-donating
groups in their meta position (entries 9 and 10) were also non-
reactive. On the other hand, the electron-rich alkynes 4-ethynyl
aniline and 4-ethynyl-N,N-dimethylaniline gave 85% and 92% yield
of the desired products (entries 11 and 12). These observations
suggest that using aryl iodides or alkynes containing electron-
donating groups in the para position leads to an increase in yield,
while electron-withdrawing groups tend to decrease yield.
During all these studies no alkyne homo coupling products
were observed in any reaction.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.03.063.
References and notes
1. For application of Sonogashira reaction: (a) Chinchilla, R.; Nájera, C. Chem. Rev.
2007, 107, 874–922; (b) Yin, L.; Liebscher, J. Chem. Rev. 2007, 107, 133–173; (c)
Doucet, H.; Hierso, J. C. Angew. Chem. 2007, 119, 850–888. Angew. Chem., Int. Ed.
2007, 46, 834–871; (d) Odlo, K.; Klaveness, J.; Rongved, P.; Hansen, T. V.
Tetrahedron Lett. 2006, 47, 1101–1103; (e) Nicolaou, K. C.; Bulger, P. G.; Sarlah,
D. Angew. Chem. 2005, 117, 4516–4563. Angew. Chem., Int. Ed. 2005, 44, 4442–
4489; (f) Pedersen, J. M.; Bowman, W. R.; Elsegood, M. R. J.; Fletcher, A. J.;
Lovell, P. J. J. Org. Chem. 2005, 70, 10615–10618; (g) Sonogashira, K.. In Metal-
Catalyzed Cross-Coupling Reactions; Diederich, F., de Meijere, A., Eds.; Wiley-
VCH: Weinheim, 2004; Vol. 1, pp 319–345; (h) Hiroya, K.; Matsumoto, S.;
3 3
In summary, FeCl /PPh has been shown to be an effective cat-
alytic system for Sonogashira couplings. The reaction was opti-

D. N. Sawant et al. / Tetrahedron Letters 51 (2010) 2758–2761
2761
Sakamoto, T. Org. Lett. 2004, 6, 2953–2956; (i) Li, C. C.; Xie, Z. X.; Zhang, Y. D.;
Chen, J. H.; Yang, Z. J. Org. Chem. 2003, 68, 8500–8504; (j) Brandsma, L.;
Vasilevsky, S. F.; Verkruijsse, H. D. Application of Transition Metal Catalysts in
Organic Synthesis; Springer: Berlin, 1998. Chapter 10.
Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467–4470.
For examples palladium-catalyzed Sonogashira crosscoupling reactions see: (a)
Nandurkar, N. S.; Bhanage, B. M. Tetrahedron 2008, 64, 3655–3660; (b) Silva, S.;
Sylla, B.; Suzenet, F.; Tatibouët, A.; Rauter, A. P.; Rollin, P. Org. Lett. 2008, 10,
9. (a) Bistri, O.; Correa, A.; Bolm, C. Angew. Chem. 2008, 120, 596–598. Angew.
Chem., Int. Ed. 2008, 47, 586–588; (b) Bonnamour, J.; Bolm, C. Org. Lett. 2008, 10,
2665–2667.
10. Correa, A.; Carril, M.; Bolm, C. Angew. Chem. 2008, 120, 2922–2925. Angew.
Chem., Int. Ed. 2008, 47, 2880–2883.
2
3
.
.
11. (a) Huang, H.; Jiang, H.; Chen, K.; Liu, H. J. Org. Chem. 2008, 73, 9061–9064; (b)
Mao, J.; Xie, G.; Wu, M.; Guo, J.; Ji, S. Adv. Synth. Catal. 2008, 350, 2477–2482;
(c) Rao Volla, C. M.; Vogel, P. Tetrahedron Lett. 2008, 49, 5961–5964.
12. (a) Ren, Y.; Cheng, L.; Tian, X.; Zhao, S.; Wang, J.; Hou, C. Tetrahedron Lett. 2010,
51, 43–45; (b) Wu, A.; Darcel, C. Eur. J. Org. Chem. 2009, 4753–4756; (c) Zhang,
X.; Ye, D.; Sun, H.; Guo, D.; Wang, J.; Huang, H.; Zhang, X.; Jiang, H.; Liu, H.
Green Chem. 2009, 11, 1881–1888; (d) Lee, H. W. et al Tetrahedron Lett. 2009,
50, 5868–5871; (e) Wu, J. R.; Lin, C. H.; Lee, C. F. Chem. Commun. 2009, 4450–
4452; (f) Zou, T.; Pi, S. S.; Li, J. H. Org. Lett. 2009, 11, 453–456; (g) Liu, Y.; Yang,
J.; Bao, W. Eur. J. Org. Chem. 2009, 5317–5320; (h) Yao, B.; Liang, Z.; Niu, T.;
Zhang, Y. J. Org. Chem. 2009, 74, 4630–4633; (i) Mao, J.; Xie, G.; Zhan, J.; Hua, Q.;
Shi, D. Adv. Synth. Catal. 2009, 351, 1268–1272; (j) Xie, X.; Xu, X.; Li, H.; Xu, X.;
Yang, J.; Lia, Y. Adv. Synth. Catal. 2009, 351, 1263–1267; (k) Wen, J.; Zhang, J.;
Chen, S-Y.; Li, J.; Yu, X. Q. Angew. Chem., Int. Ed. 2008, 47, 8897–8900.
13. Buchwald, S. L.; Bolm, C. Angew. Chem. 2009, 121, 5694–5695; Buchwald, S. L.;
Bolm, C. Angew. Chem., Int. Ed. 2009, 48, 5586–5587.
8
53–856; (c) ander Heiden, M. R.; Plenio, H. S.; Immel, E.; Burello, G.;
Rothenberg, H. C.; Hoefsloot, J. Chem. Eur. J. 2008, 14, 2857–2866; (d) Favero, A.;
Nogueira, C. W.; Menezes, P. H.; Zeni, G. Tetrahedron Lett. 2006, 47, 2179–2182;
(
e) Li, P. H.; Wang, L. Adv. Synth. Catal. 2006, 348, 681–685; (f) Köllhofer, A.;
Plenio, H. Adv. Synth. Catal. 2005, 347, 1295–1300; (g) Liang, B.; Dai, M.; Chen,
J.; Yang, Z. J. Org. Chem. 2005, 70, 391–393; (h) Gil-Moltó, J.; Karlström, S.;
Nájera, C. Tetrahedron 2005, 61, 12168–12176; (i) Genin, E.; Amengual, R.;
Michelet, V.; Savignac, M.; Jutand, A.; Neuville, L. J.; Genét, P. Adv. Synth. Catal.
2
6
004, 346, 1733–1741; (j) Gelman, D.; Buchwald, S. L. Angew. Chem. 2003, 115,
175–6178. Angew. Chem., Int. Ed. 2003, 42, 5993–5996.
4
5
.
.
King, A.; Yasuda, N.. In Palladium-Catalyzed Cross-Coupling Reactions in the
Synthesis of Pharmaceuticals; Springer: Heidelberg, Berlin, 2004; Vol. 6. pp 205–
2
45.
For details of iron catalyzed coupling see: (a) Sarhan, A. O.; Bolm, C. Chem. Soc.
Rev. 2009, 38, 2730–2744; (b) Correa, A.; Mancheno, O. G.; Bolm, C. Chem. Soc.
Rev. 2008, 37, 1108–1117; (c) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem.
14. Larsson, P. F.; Correa, A.; Carril, M.; Norrby, P. O.; Bolm, C. Angew. Chem., Int. Ed.
2009, 48, 5691–5693.
15. General procedure for the iron-catalyzed Sonogashira coupling: toluene (3 ml/
2
008, 120, 3363–3367. Angew. Chem., Int. Ed. 2008, 47, 3317–3321; (d) Sherry,
3 3 3 4
mmol), FeCl (0.15 mol %), PPh (0.30 mol %), K PO (2.0 mmol), alkynes (1,
B. D.; Fürstner, A. Acc. Chem. Res. 2008, 41, 1500–1522; (e) Chen, M. S.; White,
M. C. Science 2007, 318, 783–787; (f) Fürstner, A.; Martin, R. Chem. Lett. 2005,
2.0 mmol) and aryl iodide (2, 1 mmol) were added in the reaction vial under
nitrogen atmosphere. All the reactions were performed in closed vials with a
Teflon-coated screw cap, and the reaction mixture was heated at 135 °C with
stirring for 48 h in oil bath. After completion of the reaction, the reaction
mixture was cooled to room temperature and diluted with diethyl ether. The
resulting heterogeneous solution was filtered through a pad of silica and
concentrated to afford the crude product, which was purified by column
chromatography using petroleum ether as eluent or petroleum ether/ethyl
acetate (98:2) eluent mixture to give the expected product. The identity and
purity of the products were confirmed by 1H NMR, C NMR and GC–MS.
34, 624–629; (g) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104,
6
217–6254; (h) Shinokubo, H.; Oshima, K. Eur. J. Org. Chem. 2004, 2081–2091.
6
7
.
.
Bolm, C. Nat. Chem. 2009, 1, 420.
Carril, M.; Correa, A.; Bolm, C. Angew. Chem. 2008, 120, 4940–4943. Angew.
Chem., Int. Ed. 2008, 47, 4862–4865.
(a) Correa, A.; Bolm, C. Adv. Synth. Catal. 2008, 350, 391–394; (b) Correa, A.;
Carril, M.; Bolm, C. Chem. Eur. J. 2008, 14, 10919–10922; (c) Correa, A.; Elmore,
S.; Bolm, C. Chem. Eur. J. 2008, 14, 3527–3529; (d) Correa, A.; Bolm, C. Angew.
Chem. 2007, 119, 9018–9021. Angew. Chem., Int. Ed. 2007, 46, 8862–8865.
8
.
13