Iodine-catalyzed suzuki-miyaura coupling performed in air

Article abstract of DOI:10.1002/adsc.200800624

Suzuki-type coupling reactions can be performed directly in air using a catalytic system composed of iodine, potassium carbonate, and polyethylene glycol 400 (I₂/K₂CO₃/PEG-400). Iodine was found to be an effective additive for accelerating these couplings. The methodology is also suitable for the coupling of (E)-β-bromostyene with phenylbor-onic acid, with retention of the double bond geometry.

Full text of DOI:10.1002/adsc.200800624

FULL PAPERS
DOI: 10.1002/adsc.200800624
Iodine-Catalyzed Suzuki–Miyaura Coupling Performed in Air
Jincheng Mao,a,* Qiongqiong Hua,b Guanlei Xie,a Jun Guo,a Zhigang Yao,c
Daqing Shi,a and Shunjun Jia
a
College of Chemistry, Chemical Engineering and Materials Science, Suzhou University, Suzhou 215123, Jiangsu, Peopleꢀs
Republic of China
Fax : (+86)-512-6588-0089; phone: (+86)-512-6588-0403; e-mail: jcmao@suda.edu.cn
Department of Chemistry, Xuzhou Normal University, Xuzhou 221116, Jiangsu, Peopleꢀs Republic of China
Analytical & Testing Center, Suzhou University, Suzhou 215123, Jiangsu, Peopleꢀs Republic of China
b
c
Received: October 12, 2008; Revised: January 15, 2009; Published online: March 4, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800624.
Abstract: Suzuki-type coupling reactions can be per- onic acid, with retention of the double bond geome-
formed directly in air using a catalytic system com- try.
posed of iodine, potassium carbonate, and polyethy-
lene glycol 400 (I2/K2CO3/PEG-400). Iodine was
found to be an effective additive for accelerating Keywords: accelerating effect; arylboronic acids;
these couplings. The methodology is also suitable for cross-coupling reactions; iodine; polyethylene glycol
the coupling of (E)-b-bromostyene with phenylbor- 400 (PEG-400)
Introduction
copper-based nanocolloids are good catalysts for
these C C coupling reactions. However, only the cou-
À
Since its discovery by Suzuki and Miyaura in 1979, plings of aryl iodides were investigated with copper
the cross-coupling of aryl halides with arylboronic alone as catalyst and the additional metals (Pd and
acids has been considered one of the most powerful, Ru) could show better efficiency.[11] Although the
versatile and popular tools for the selective construc- study of Goossen and co-workers represents a differ-
tion of carbon-carbon bonds.[1] The impact of this re- ent way to prepare biaryls by decarboxylative cou-
action in organic synthesis is largely attributed to the pling,[12] their catalytic system still includes a Pd com-
fact that it provides biaryl compounds which are im- plex together with CuCO3 or CuI as the co-catalyst.
portant structural moieties in numerous polymers, In addition, the ligand and a molecular sieve are also
agrochemicals, natural products and pharmaceutical required. Recently, Fagnou reported the catalytic
intermediates.[2–5] In the past two decades, advances cross-coupling of unactivated arenes with moderate to
have been made to develop even more efficient cata- good yields. But their catalytic system also needs a Pd
lytic systems for the Suzuki–Miyaura coupling reac- complex, copper catalyst and ligand.[13] In view of
tion. Among these improvements, palladium-phos- these findings, we turned back to reconsider the tradi-
phine complexes are now amongst the most common- tional Suzuki–Miyaura coupling reaction, wondering
ly employed catalysts of choice.[6–7] Furthermore, if it could be directly catalyzed by more accessible
chemists have developed an acceptable catalytic and less expensive catalysts, even without using syn-
cycle.[1]
However, the drawbacks of Pd catalyst systems,
such as their high cost, toxicity and the potential con-
thetic ligands.
tamination of the products, limit massive applications Results and Discussion
in large-scale synthesis, particularly in the pharma-
ceutical industry, where close monitoring of the metal PolyACHTUNTRGNEUNG(ethylene)glycol (PEG) is widely considered as a
contamination of products is required.[8] The use of a good recyclable solvent with a low environmental
less expensive metal than Pd may be advantageous, as impact. Thus, it has been used for biomedical applica-
developed and previously reported by our group and tions, especially in the field of drug discovery. It is
others.[9,10] A recent study showed that copper and noteworthy that PEG may afford amazingly good re-
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Jincheng Mao et al.
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sults compared with the common solvents.[14] This is new stirring bars and new caps. We also ensured that
perhaps attributable to its special role in catalysis rel- all of reagents including aryl halides, arylboronic
ative to classical phase-transfer catalysts.
acids, K2CO3 and PEG-400 had the low levels of Pd
We carried out a control catalytic coupling reaction (below 10 ppb) as measured by ICP-MS.[17,18] Further-
between iodobenzene and 4-methylphenylboronic more, to make sure of the validity of these results,
acid without using a metal catalyst. The catalytic con- various research workers in our laboratory independ-
ditions were: 0.3 mmol of aryl halides, 0.4 mmol of ar- ently carried out these experiments, in some cases
ylboronic acids, 0.6 mmolK2CO3, 2 mL of PEG-400. using different reagents from various known chemical
As expected, there was no product when the catalytic corporations: the results obtained were similar in all
reaction was conducted in an argon atmosphere. cases. Scrutinizing the reaction phenomenon, we ob-
However, to our surprise, we found that the colour of served that the colour of the mixture changed with
the mixture became gradually yellow when the cata- time as shown in Figure 2. At the beginning of the re-
lytic reaction was performed in air. After the catalytic action (picture A, Figure 2), idodine was gradually
reaction had proceeded at 1108C for 12 h, it was
quenched by adding an equivalent of a water-ethyl
acetate mixture. The catalytic reactions performed in
air or under an argon atmosphere were visibly differ-
ent (Figure 1). After a series of work-ups, we ob-
Figure 2. The different colours of the reaction system within
Figure 1. The different colours between the catalytic reac-
44 h: (A) starting state before heating; (B) 1 min after heat-
tion performed in air or argon atmosphere (the reaction was
ing at 1108C; (C) at 2 min; (D) at 5 min; (E) at 5 h; (F) at
quenched with 2 mL of water and 2 mL of ethyl acetate).
8 h; (G) at 22 h; (H) at 44 h.
tained the desired coupling product with 14% yield
from the reaction in air; however, no reaction oc- dispersed in the system and then the mixture became
curred when the catalytic reaction was performed in transiently dark, transparent and homogeneous upon
argon. Therefore, we conclude tentatively that the the catalytic reaction being heated at 1108C for about
oxygen from the air played an important role in cata- 1 min (picture B). Then the colour of the system
lyzing the Suzuki–Miyaura coupling reactions under became clearer after another minute (picture C) and
these conditions.[15]
to our surprise, it turned to colorless after 5 min (pic-
To improve the yield of biaryl products, we tried ture D). From then on, the colour of the system con-
various additives. We had previously found accidently tinued to become gradually yellower, eventually turn-
that a stoichiometric amount of iodine promotes the ing to brown at 44 h. This raised the question: at what
homocoupling of arylboronic acids with good time is the highest conversion of the catalytic reaction
yields.[16] Thus, we wondered whether iodine, which is obtained?
inexpensive and readily available, could also acceler-
To address this time vs. yield issue, we carried out a
ate the Suzuki–Miyaura coupling between iodoben- series of experiments with different reaction times.
zene and 4-methylphenylboronic acid. To our delight, For example, the reaction after about 44 h was
the desired product was obtained with 77% yield quenched and the product was obtained with only
when the catalytic reaction was carried out in air with 42% yield after flash column chromatography. In this
iodine as the additive (20 mol%) at 1108C for 12 h. In case, the colour of the mixture did not correlate with
order to control for issuses of potential metal contam- the efficiency of the catalytic system. As shown in
ination, we carried out experiments in new flasks with Figure 3, the I2/K2CO3/PEG-400 system afforded the
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Iodine-Catalyzed Suzuki–Miyaura Coupling Performed in Air
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Subsequently, a further series of optimized experi-
ments were carried out to explore different solvents
and bases. First, we found that no product was ob-
tained in the absence of base. Second, various
common solvents (such as DMF, DMSO, toluene, di-
oxane, H2O, CH3CN, i-PrOH) and various bases (such
as Cs2CO3, Na2CO3 and K3PO4·H2O) all gave worse
results than our original system. Further, other bases
such as t-BuOK, KF, KOH, NaOH, NaOAc gave
almost no product. Thus, we conclude that the combi-
nation of PEG-400 with K2CO3 was good for catalysis.
Other PEGs with different molecular weights were
also tested as solvent and similar results were ac-
quired. However, very low yields of desired product
were obtained when simple glycols with different
structures (such as glycol and propanediol) were em-
ployed. This suggested that PEGs acted not only as
the solvent but could accelerate the coupling reac-
tions.[14] Looking through the published papers, it can
Figure 3. The relationship between the reaction time and
the isolated yield of the coupling product.
highest conversion at 12 h (77% yield). To our sur- be seen that Tu[19] has reported the rapid and efficient
prise, the yields of the catalytic reaction decreased microwave-assisted amination of electron-rich aryl
after 12 h. It seems that the coupling reaction acceler- halides without a transition-metal catalyst and Lead-
ated by iodine reached an equilibrium at this point, beater[20] also disclosed an example of the transition
beyond which it was not favourable for the coupling metal-free Sonogashira-type coupling. Therefore, in
reaction. At this moment, some by-products were also this paper we have shown that Suzuki–Miyaura cou-
generated which was indicated by TLC. Prolongation pling reaction could be carried out without using a
of the reaction time did not result in higher conver- transition metal catalyst.
sions.
Table 1. Substrate scope of the Suzuki–Miyaura coupling reaction promoted by iodine.[a]
Entry
Aryl halide
Product
Time [h]
Yield [%][b]
1
2
3
4
5
1a
1b
1c
1d
1e
12
18
36
24
36
80
75
89
>98
37
6
7
1f
1g
36
24
24
>98
8
1h
12
88
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Table 1. (Continued)
Entry
Aryl halide
Product
Time [h]
Yield [%][b]
9
1i
36
12
36
36
24
65
84
55
74
47
10
1j
1d
1k
1l
11
12
13
14
15
16
1m
1n
1o
12
12
24
90
90
>98
17
18
19
20
21
22
23
1b
1d
1j
28
28
36
36
36
40
40
40
73
66
66
74
86
90
28
29
1d
1p
1l
1b
1q
24
[a]
Reaction conditions: 0.06 mmol of I2, 0.3 mmol of aryl halides, 0.4 mmol of arylboronic acids, 0.6 mmolK2CO3, 2 mL of
PEG-400, 1408C, 12–40 h, in air.
Isolated yield.
[b]
Elevated temperature (1408C) gave a higher con-
Furthermore, we also applied our protocol to the
version of the catalytic reaction when comparing the coupling of (E)-b-bromostyrene with phenylboronic
same time-points (12 h) (Table 1, entry 1). High yields acid (Scheme 1) with the desired product being ob-
were obtained for the couplings of various aryl hal- tained with high yield (90%). In this case, it is re-
ides and arylboronic acids with different reaction markable that the double bond geometry of the vinyl
times at 1408C (summarized in Table 1). As to the bromide was retained under these reaction conditions.
aryl halides, the highest yields of the corresponding However, the same catalytic reaction in the presence
coupling products were also obtained (entries 4, 7, of 10 mol% of TEMPO (2,2,6,6-tetramethyl-1-piperi-
14–16). Notably, I2/K2CO3/PEG-400 could not cata- din-1-yloxy) gave no corresponding coupling product,
lyze the couplings of aryl iodides and heteroarylbor- as corroborated by GC-MS. Thus, we began to consid-
onic acids. Subsequently, various aryl bromides and er this type of coupling as being a possible radical re-
chlorides were investigated as the substrates. The cou- action.[21]
pling of these difficult substrates could also be carried
The coupling between the phenyl tosylate and 4-
out sucessfully using iodine as the additive (en- methylphenylboronic acid was conducted with our
tries 17–24).
protocol as shown in Scheme 2. Although the conver-
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Iodine-Catalyzed Suzuki–Miyaura Coupling Performed in Air
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Scheme 1. Coupling reaction of vinyl bromide with phenylboronic acid using the I2/K2CO3/PEG-400 system.
Scheme 2. Coupling reaction of phenyl tosylate with 4-methylphenylboronic acid using I2/K2CO3/PEG-400 system.
sion of the catalytic reaction was not high (45%), it is cessible, which will open up new opportunities for the
noteworthy that, besides the desired coupling product, industrial synthesis of high-value pharmaceutical in-
small amounts of iodobenzene and 4-iodotoluene termediates.[23] Further work is in progress with the
were obtained as by-products which were proved by aim of understanding the reaction mechanisms. Fur-
GC-MS. Thus, we believe that these aryl halides are thermore, we are seeking to expand potential applica-
probably generated from the reactants will also sug- tions of similar catalytic protocols to other coupling
gests the possible radical mechanism.
processes of similar industrial interest.
In addition, it is well known that iodine acts as elec-
tron acceptor, due to the presence of vacant antibond-
ing orbitals. It is possible, therefore, that iodine can Experimental Section
act as a catalyst because of its Lewis acidity. We
speculated that the transition from the deep colour to General Experimental Details
a transparent solution that is obtained during the re-
All reactions were directly carried out in air. All arylboronic
action after several times (pictures A–D in Figure 2),
may be due to the coordination of iodine with the ar-
omatic compound/boronic acid. Moreover, iodine also
acts as an oxidant. In the presence of some oxidants,
aryl radicals will be generated from arylboronic
acids[22] and obviously, in this case the oxygen in the
air is an important factor for the success of the reac-
tion. Our further work will focus on exploring the pu-
tative radical mechanism in this new type of coupling
reactions.
acids or esters and other chemical reagents were purchased
from Aldrich or Alfa. Flash column chromatography was
performed using silica gel (300–400 mesh). Analytical thin-
layer chromatography was carried out using glass plates pre-
coated with 200–400 mesh silica gel impregnated with a fluo-
rescent indicator (254 nm). NMR spectra were measured in
CDCl3 on a Varian Inova-400 NMR spectrometer (400 MHz
or 300 MHz) with TMS as an internal reference. Products
1
were characterized by comparison of H and 13C NMR data
with values in the literatures.
General Procedure for Iodine-Mediated Suzuki–
Miyaura Coupling Reactions of Arylboronic Acid and
Aryl Halides
Conclusions
In summary, it is of great interest to find that the tra-
ditional Suzuki–Miyaura coupling could be performed A Schlenk tube (10 mL) was charged with potassium car-
bonate (2.0 equiv.), iodine (0.2 equiv.), arylboronic acid
(1.0 equiv.) and any remaining solids (aryl halides) in the
solvent of PEG-400 (2 mL), aryl halide (1.3 equiv.) was
added directly. The tube was sealed under air, and the mix-
ture was heated to 1408C and stirred for 12–40 h. After
cooling to room temperature, the mixture was diluted with
water, and the combined aqueous phases were extracted
three times with ethyl acetate. The organic layers were com-
bined, dried over Na2SO4, and concentrated to yield the
crude product, which was further purified by silica gel chro-
in air with an I2/K2CO3/PEG-400 catalytic system
without using a transition metal catalyst. The iodine
was an effective and indispensable additive for accel-
erating the couplings. This methodology is also suita-
ble for the coupling of (E)-b-bromostyene with phe-
nylboronic acid, which could maintain the double
bond geometry. All in all, it provides a fascinating al-
ternative to the traditional transition metal-catalyzed
reaction conditions because the novel coupling reac-
tion is more economic, easily operable and more ac- matography.
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1
4-Methylbiphenyl (1a): H NMR (CDCl3, 300 MHz): d=
2.37 (s, 3H, CH3), 7.22 (d, J=7.5 Hz, 2H, ArH), 7.30 (t, J=
7.2 Hz, 1H, ArH), 7.40 (t, J=7.5 Hz, 2H, ArH), 7.48 (d, J=
7.8 Hz, 2H, ArH), 7.56 (d, J=7.5 Hz, 2H, ArH); 13C NMR
(75 MHz, CDCl3): d=141.6, 138.8, 137.4, 129.9, 129.2, 127.4,
127.4, 21.6.
7.50–7.52 (m, 2H); 13C NMR (100 MHz, CDCl3): d=160.5,
141.9, 140.0, 133.9, 130.4, 129.3, 128.9, 119.9, 113.3, 113.3,
55.8.
2-Methoxylbiphenyl (1n): 1H NMR (300 MHz, CDCl3):
d=3.70 (s, 3H, CH3O), 6.87–6.96 (m, 2H, ArH), 7.21 (t, J=
6.3 Hz, 3H, ArH), 7.31 (t, J=7.2 Hz, 2H, ArH), 7.44 (d, J=
7.5 Hz, 2H, ArH); 13C NMR (100 MHz, CDCl3): d=156.8,
138.9, 131.3, 129.9, 129.0, 128.4, 127.3, 121.2, 111.6, 55.9.
4-Nitro-3’-methoxylbiphenyl (1o): 1H NMR (400 MHz,
CDCl3): d=6.97–7.00 (m, 1H), 7.13 (t, J=2.0 Hz, 1H), 7.41
(t, J=8.0 Hz, 1H), 7.71 (d, J=8.4 Hz, 2H), 8.27 (d, J=
8.8 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=160.6, 147.9,
140.6, 130.7, 128.3, 124.5, 120.3, 114.6, 113.7, 55.9.
Biphenyl (1b): 1H NMR (CDCl3, 400 MHz): d=7.36 (t,
J=7.2 Hz, 2H, ArH), 7.45 (d, J=7.2 Hz, 4H, ArH), 7.60 (d,
J=7.2 Hz, 4H, ArH); 13C NMR (100 MHz, CDCl3): d=
141.7, 129.2, 127.7, 127.6.
4-Fluorobiphenyl (1c): 1H NMR (CDCl3, 300 MHz): d=
7.12 (t, J=8.7 Hz, 2H, ArH), 7.34 (t, J=7.1 Hz, 1H, ArH),
7.43 (t, J=7.2 Hz, 2H, ArH), 7.51–7.56 (m, 4H, ArH);
13C NMR (100 MHz, CDCl3): d=164.1, 161.7, 140.7, 137.8,
137.8, 129.3, 129.2, 129.1, 127.7, 127.5, 116.2, 116.0.
4-Chloro-4’-fluorobiphenyl (1p): 1H NMR (400 MHz,
CDCl3): d=7.12 (t, J=8.8 Hz, 2H, ArH), 7.38–7.51 (m, 6H,
ArH); 13C NMR (100 MHz, CDCl3): d=164.3, 161.8, 139.1,
136.6, 136.5, 133.8, 129.4, 128.7, 116.3, 116.1.
1
4-Chlorobiphenyl (1d): H NMR (CDCl3, 400 MHz): d=
7.33–7.45 (m, 5H, ArH), 7.49–7.55 (m, 4H, ArH); 13C NMR
(75 MHz, CDCl3): d=140.4, 140.1, 133.8, 129.5, 129.4, 129.4,
129.3, 128.8, 128.1, 127.4, 110.2.
4-Trifluoromethylbiphenyl (1q): mp 69–708C; 1H NMR
(400 MHz, CDCl3): d=7.35 (t, J=7.6 Hz, 1H, ArH), 7.40–
7.49 (m, 3H, ArH), 7.60 (d, J=7.2 Hz, 3H, ArH), 7.69 (s,
2H, ArH); 13C NMR (100 MHz, CDCl3): d=129.5, 129.2,
128.7, 127.9, 127.7, 127.7, 126.2.
Biphenyl-4-carbaldehyde (1e): 1H NMR (400 MHz,
CDCl3): d=7.42–7.51 (m, 2H, ArH), 7.64 (d, J=7.6 Hz, 2H,
ArH), 7.76 (d, J=8.0 Hz, 2H, ArH), 7.96 (d, J=8.0 Hz, 2H,
ArH), 10.06 (s, 1H, CHO); 13C NMR (100 MHz, CDCl3):
d=192.4, 147.6, 140.2, 135.6, 130.7, 129.5, 128.9, 128.1, 127.8.
Biphenyl-3-carbaldehyde (1f): 1H NMR (400 MHz,
CDCl3): d=7.40 (s, 1H, ArH), 7.41–7.49 (m, 3H, ArH), 7.62
(t, J=6.8 Hz, 3H, ArH), 7.86 (d, J=7.2 Hz, 2H, ArH), 8.10
(s, 1H, ArH), 10.09 (s, 1H, CHO); 13C NMR (100 MHz,
CDCl3): d=192.9, 142.7, 140.2, 137.4, 133.6, 130.0, 129.5,
129.1, 128.7, 128.5, 127.6.
1
(E)-1,2-diphenylethene (1r): H NMR (400 MHz, CDCl3):
d=7.13 (s, 2H), 7.24–7.30 (m, 2H), 7.38 (t, J=7.6 Hz, 4H),
7.54 (d, J=7.2 Hz, 4H); 13C NMR (100 MHz, CDCl3): d=
137.8, 129.2, 128.1, 127.0.
Acknowledgements
3-Methoxylbiphenyl (1g): 1H NMR (300 MHz, CDCl3):
d=3.76 (s, 3H, CH3O), 6.80 (d, J=7.8 Hz, 1H, ArH), 7.08
(t, J=9.3 Hz, 6H, ArH), 7.23–7.36 (m, 2H, ArH), 7.50 (d,
J=7.5 Hz, 2H, ArH); 13C NMR (100 MHz, CDCl3): d=
160.4, 143.2, 141.5, 130.2, 129.2, 127.9, 127.6, 120.1, 113.3,
113.1, 55.7.
Financial support from the National Natural Science Founda-
tion of China (No. 20802046) and the Key Laboratory of Or-
ganic Synthesis of Jiangsu Province is gratefully acknowl-
edged. We thank Prof. Dr. Chaozhong Li at Shanghai Insti-
tute of Organic Chemistry, Chinese Academy of Sciences for
helpful discussions. We also thank Dr. Marc Creus from the
University of Basel, Switzerland for English revision.
1-Phenylnaphthalene (1h): 1H NMR (300 MHz, CDCl3):
d=7.45–7.49 (m, 6H, ArH), 7.82–7.84 (m, 6H, ArH);
13C NMR (75 MHz, CDCl3): d=133.82, 130.46, 129.86,
128.64, 128.26, 126.20.
4-Methoxylbiphenyl-4’-carbaldehyde
(1i):
1H NMR
(300 MHz, CDCl3): d=3.87 (s, 3H, CH3O), 7.01 (d, J=
6.3 Hz, 2H, ArH), 7.60 (d, J=6.6 Hz, 2H, ArH) ,7.71 (d, J=
6.0 Hz, 2H, ArH), 7.93 (d, J=6.0 Hz, 2H, ArH), 10.04 (s,
1H, CHO); 13C NMR (75 MHz, CDCl3): d=192.4, 130.8,
129.0, 127.6, 119.4, 114.9, 55.9.
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[2] F. Diederich, P. J. Stang, Metal-Catalyzed Cross-Cou-
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[3] N. Miyaura, Cross-Coupling Reaction, Springer, Berlin,
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[4] a) E. Negishi, Handbook of Organopalladium Chemis-
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4-Methoxylbiphenyl (1j): 1H NMR (300 MHz, CDCl3):
d=3.86 (s, 3H, CH3O), 6.96–7.00 (m, 2H, ArH), 7.26 (s,
1H, ArH), 7.40–7.44 (m, 2H, ArH), 7.52–7.57 (m, 4H,
ArH); 13C NMR (100 MHz, CDCl3): d=129.2, 128.6, 127.2,
127.1, 114.7, 55.8.
4-Chloro-4’-methylbiphenyl (1k): 1H NMR (400 MHz,
CDCl3): d=2.38 (s, 3H), 7.24 (t, J=8.0 Hz, 2H), 7.36–7.63
(m, 6H); 13C NMR (100 MHz, CDCl3): d=140.0, 137.9,
137.5, 133.5, 130.1, 129.3, 128.6, 127.3, 21.6.
1
4, 4’-Dichlorobiphenyl (1l): H NMR (300 MHz, CDCl3):
d=7.33 (d, J=8.4 Hz, 4H, ArH), 7.40 (d, J=8.1 Hz, 4H,
ArH); 13C NMR (75 MHz, CDCl3): d=138.9, 134.0, 129.5,
128.7.
4-Chloro-3’-methoxylbiphenyl (1m): 1H NMR (400 MHz,
CDCl3): d=3.86 (s, 3H), 6.90–6.93 (m, 1H), 7.09 (t, J=
2.0 Hz, 1H), 7.14 (d, J=7.6 Hz, 1H), 7.34–7.41 (m, 3H),
[6] a) G. W. Parshall, S. Ittel, Homogeneous Catalysis, John
Wiley and Sons: New York, 1992; b) L. H. Pignolet,
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ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 635 – 641

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Adv. Synth. Catal. 2009, 351, 635 – 641
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