Facile and efficient reductive homocoupling of benzyl and aryl halides catalyzed by ionic liquid [C12mim][CuCl2] in the presence of metallic zinc and copper

Article abstract of DOI:10.1007/s10562-010-0535-5

A facile and efficient synthesis of bibenzyl and biaryl derivatives by reductive homocoupling reaction is described. Treatment of benzyl and aryl halides with metallic zinc and copper powder in the presence of a catalytic amount of [C₁₂mim][CuCl₂] under ligand- and base-free conditions gives the corresponding bibenzyls and biaryls in good to high yields. The product can be isolated by a simple extraction with organic solvent, and the catalytic system can be recycled or reused without any significant loss of catalytic activity.

Full text of DOI:10.1007/s10562-010-0535-5

Catal Lett (2011) 141:467–473
DOI 10.1007/s10562-010-0535-5
Facile and Efficient Reductive Homocoupling of Benzyl
and Aryl Halides Catalyzed by Ionic Liquid [C mim][CuCl ]
in the Presence of Metallic Zinc and Copper
12
2
Yu-Lin Hu
Ming Lu
•
Fu Li
•
Guo-Liang Gu
•
Received: 15 August 2010 / Accepted: 5 December 2010 / Published online: 4 January 2011
Ó Springer Science+Business Media, LLC 2010
Abstract A facile and efficient synthesis of bibenzyl and
biaryl derivatives by reductive homocoupling reaction is
described. Treatment of benzyl and aryl halides with
metallic zinc and copper powder in the presence of a cat-
alytic amount of [C mim][CuCl ] under ligand- and base-
as low yields, long reaction times, high reaction tempera-
tures, high catalyst loading, tedious workup procedures,
costly metal catalysts, difficulty of catalyst recovery, and
environmental toxicity. Reductive coupling reactions using
transition metal complexes as catalysts are another
important methods for the formation of carbon–carbon
bonds, and a variety of reagents, such as palladium
[25–29], nickel [30–32], iron [33, 34], vanadium [35, 36],
lanthanum [37], zinc [38, 39], and other complexes
[40–44], have been developed for this conversion. How-
ever, most of the procedures still suffered from the use of
expensive reagents, difficulties in work up, environmental
hazards, and difficulties in recycling of the catalyst.
Sometimes, complicated coordinating and expensive
ligands are often required to stabilize the active metal
intermediates during the reactions. Consequently, search
for new and environmentally benign synthetic methodolo-
gies for carbon–carbon bond formation that address these
drawbacks remains to be of value and interest.
1
2
2
free conditions gives the corresponding bibenzyls and
biaryls in good to high yields. The product can be isolated
by a simple extraction with organic solvent, and the cata-
lytic system can be recycled or reused without any sig-
nificant loss of catalytic activity.
Keywords Reductive homocoupling ꢀ Bibenzyls/biaryls ꢀ
Zn/Cu ꢀ Ionic liquids
1
Introduction
The carbon–carbon bond formation reaction is one of the
most fundamental transformations due to the large number
of applications of the resulting compounds in organic,
pharmaceutical, and polymer chemistry [1–3]. The usual
synthetic methods for such a conversion include the
Ullmann coupling [4–6], Suzuki coupling [7–12], Scholl
reaction [13, 14], Gomberg–Bachmann reaction [15, 16],
Stille reaction [17–20], and other couplings [21–24].
However, these above-mentioned protocols, in general, are
invariably associated with one or more disadvantages, such
Ionic liquids (ILs) have attracted considerable attention
during the past few years due, in some cases, to their
favourable properties such as very low vapor pressure,
wide liquid temperature range, good ionic conductivity,
excellent electrochemical properties, and strong ability to
dissolve many chemicals [45, 46]. Therefore, ILs have got
broad attention of scholars from various fields such as
chemical synthesis [47–49], biocatalytic transformations
[
50], electrochemistry [51], and analytical and separation
science [52]. In continuation of our interest in exploring
green cross-coupling reactions in ionic liquids, we report
herein a new, efficient and environmentally friendly pro-
tocol for the reductive homocoupling of benzyl and
aryl halides to bibenzyls and biaryls with the ionic
liquid 1-dodecyl-3-methylimidazolium cuprous chloride
([C mim][CuCl ]) as catalyst and reaction medium in the
Y.-L. Hu ꢀ G.-L. Gu ꢀ M. Lu (&)
College of Chemical Engineering, Nanjing University of Science
and Technology, Nanjing 210094, People’s Republic of China
e-mail: luming1963@163.com
F. Li
College of Pharmacy, China Pharmaceutical University,
Nanjing 210009, People’s Republic of China
1
2
2
123

4
68
Y.-L. Hu et al.
(2 9 10 mL), and dried with anhydrous sodium sulfate,
filtered and concentrated in vacuo. The residue was purified
by recrystallization from ethanol and dimethylbenzene to
give the product 1,2-diphenylethane. The next run was
performed under identical reaction conditions. The product
1
was characterized by H NMR, Elemental analysis or
compared with its authentic sample. Spectroscopic data of
the selected products are as follows.
2.3 Spectral Data of Selected Products
2.3.1 1,2-Diphenylethane (Table 2, entry 1)
Scheme 1 Reductive homocoupling of benzyl and aryl halides
1
White solid: m.p. = 51–53 °C; H NMR (400 MHz,
CDCl ): d 2.87 (s, CH , 4H), 7.23–7.32 (m, Ar–H, 10H);
3
2
presence of metallic zinc and copper powder (Scheme 1).
Furthermore, we demonstrate that the catalyst can be
recycled and reused without any significant loss of catalytic
activity.
Anal. Calcd. for C H (182): C, 92.21; H, 7.76. Found: C,
14 14
92.26; H, 7.74.
2.3.2 1,2-Bis(3,4-dimethylphenyl)ethane (Table 2, entry 3)
1
White solid: m.p. = 88–89 °C; H NMR (400 MHz,
2
Experimental
CDCl ): d 2.56 (s, CH , 12H), 2.84 (s, CH , 4H), 6.95–7.09
3
3
2
(m, Ar–H, 6H); Anal. Calcd. for C H (238): C, 90.65; H,
18 22
2
.1 Apparatus and Reagents
9.31. Found: C, 90.70; H, 9.30.
All the chemicals were from commercial sources without
any pretreatment. All reagents were of analytical grade.
The ionic liquids were synthesized according to the liter-
2.3.3 1,2-Bis(4-methoxyphenyl)ethane (Table 2, entry 4)
1
White solid: m.p. = 78–81 °C; H NMR (400 MHz,
1
ature procedure [46]. H NMR spectra were recorded on a
CDCl ): d 3.79 (s, CH , 6H), 2.86 (s, CH , 4H), 7.02–7.11
3
3
2
Bruker 400-MHz spectrometer using CDCl as the solvent
3
(m, Ar–H, 8H); Anal. Calcd. for C H O (242): C, 79.27;
16 28 2
with tetramethylsilane (TMS) as an internal standard. High
performance liquid chromatography (HPLC) experiments
were performed on a liquid chromatograph (Dionex Sof-
tron GmbH, America), consisting of a pump (P680)
and ultraviolet–visible light detector (UVD) system
H, 7.49; O, 13.22. Found: C, 79.31; H, 7.49; O, 13.21.
2.3.4 1,2-Bis(4-isopropylphenyl)ethane (Table 2, entry 6)
1
White solid: m.p. = 96–98 °C; H NMR (400 MHz,
(
170U). Elemental analysis were performed on a Vario
CDCl ): d 1.22 (d, J = 7.6 Hz, CH , 12H), 2.83–2.91 (m,
3
3
EL III instrument (Elmentar Analysen Systeme GmbH,
Germany).
CH and CH , 6H), 7.04–7.19 (m, Ar–H, 8H); Anal. Calcd.
2
for C H (266): C, 90.13; H, 9.86. Found: C, 90.16; H,
20 26
9.84.
2
.2 Typical Procedure for Homocoupling Reaction
Table 2, entry 1)
(
2.3.5 1,1,2,2-Tetraphenylethane (Table 2, entry 7)
1
The homocoupling reaction was carried out in a glass batch
reactor. To a stirred solution of benzyl chloride (10 mmol),
Cu (0.1 mmol), Zn (5 mmol) was added [C mim][CuCl ]
White solid: m.p. = 113–116 °C; H NMR (400 MHz,
CDCl ): d 4.67 (s, CH, 2H), 7.18–7.33 (m, Ar–H, 20H);
Anal. Calcd. for C H (334): C, 93.32; H, 6.65. Found: C,
26 22
3
1
2
2
(
0.1 mmol) and then stirring was continued at 70 °C for
h in an oil bath. To study the progress of the reaction the
93.37; H, 6.63.
2
products were collected at different time intervals and
identified and quantified by HPLC. For isolation of prod-
ucts at the end of the catalytic homocoupling, the reaction
mixture was first extracted with dichloromethane
2.3.6 1,2-Bis(4-chlorophenyl)ethane (Table 2, entry 8)
1
White solid: m.p. = 99–101 °C; H NMR (400 MHz,
CDCl ): d 2.83 (s, CH , 4H), 7.09–7.20 (m, Ar–H, 8H);
3
2
(
3 9 10 mL) and then filtered. The organic layers
Anal. Calcd. for C H Cl (251): C, 66.91; H, 4.83; Cl,
14 12 2
thus collected were combined and washed with water
28.25. Found: C, 66.95; H, 4.82; Cl, 28.23.
1
23

Reductive Homocoupling of Benzyl and Aryl Halides
469
2
.3.7 1,2-Bis(4-nitrophenyl)ethane (Table 2, entry 9)
after 8 h (Table 1, entry 1). When the reaction was per-
formed in the presence of the ionic liquid [C mim][CuCl ],
1
2
2
1
Yellow solid: m.p. = 182–184 °C; H NMR (400 MHz,
CDCl ): d 3.05 (s, CH , 4H), 7.28 (m, Ar–H, 4H), 8.13 (m,
Ar–H, 4H); Anal. Calcd. for C H N O (272): C, 61.72;
the yield increased to 61% in a shorter time (5 h) when only
0.002 equivalents were used (Table 1, entry 2). The yield
improved with increasing amount of [C mim][CuCl ]
3
2
1
4
12
2
4
12
2
H, 4.46; N, 10.27; O, 23.50. Found: C, 61.76; H, 4.44; N,
(Table 1, entries 2–5), and reached maximum with 0.01
equivalents of [C mim][CuCl ] (Table 1, entry 5). How-
1
0.29; O, 23.51.
1
2
2
ever, further addition of [C mim][CuCl ], under the same
2
1
2
2
.3.8 1,2-Bis(2,4,6-trinitrophenyl)ethane
conditions, did not enhance significantly the yield (Table 1,
entry 6). In addition, the metallic zinc and copper are crucial
for this homocoupling, and the lack of any component leads
to a much lower yield (Table 1, entries 7 and 8). Further-
more, the catalytic system could be typically recovered and
reused with no appreciable decrease in yields and reaction
rates (Table 1, entries 9 and 10). Besides [C mim][CuCl ],
(Table 2, entry 10)
1
Yellow solid: m.p. = 221–224 °C; H NMR (400 MHz,
CDCl ): d 3.32 (s, CH , 4H), 9.01 (s, Ar–H, 4H); Anal.
Calcd. for C H N O (452): C, 37.14; H, 1.79; N, 18.57;
3
2
1
4 8 6 12
O, 42.43. Found: C, 37.18; H, 1.78; N, 18.58; O, 42.45.
1
2
2
catalysts of other type of ionic liquids were tested in this
model reaction (Table 1, entries 11–16), and the results
showed that [C mim][CuCl ] demonstrated the best per-
0
2
.3.9 4,4 -Dimethylbiphenyl (Table 3, entry 3)
1
2
2
1
White solid: m.p. = 118–120 °C; H NMR (400 MHz,
CDCl ): d 2.54 (s, CH , 6H), 7.46–7.59 (m, Ar–H, 8H);
Anal. Calcd. for C H (182): C, 92.23; H, 7.75. Found: C,
formance. Therefore, the combination of [C mim][CuCl ]
12 2
3
3
and metallic zinc and copper powder was chosen as the
optimal conditions for further exploration.
1
4 14
9
2.26; H, 7.74.
With these results in hand, we subjected other benzyl
halides to the homocoupling reactions, and the results are
0
2
.3.10 4,4 -Dimethoxybiphenyl (Table 3, entry 4)
1
Table 1 Optimization studies for the catalytic homocoupling reac-
tion of benzyl chloride
White solid: m.p. = 178–180 °C; H NMR (400 MHz,
CDCl ): d 3.85 (s, CH , 6H), 7.01 (m, Ar–H, 4H), 7.52 (m,
Ar–H, 4H); Anal. Calcd. for C H O (214): C, 78.45; H,
3
3
Entry
Ionic liquid
Ionic liquid
equiv.)
Time
(h)
Yield
a
(%)
1
4 14 2
(
6
.59; O, 14.94. Found: C, 78.48; H, 6.59; O, 14.93.
1
2
3
4
5
6
7
8
9
–
0
8
5
3
2
2
2
2
2
2
2
4
4
2
2
2
5
13
61
80
92
96
96
0
2
.3.11 4,4 -Bis(trifluoromethyl)biphenyl (Table 3, entry 9)
[C12mim][CuCl
[C12mim][CuCl
[C12mim][CuCl
[C12mim][CuCl
[C12mim][CuCl
[C12mim][CuCl
[C12mim][CuCl
[C12mim][CuCl
[C12mim][CuCl
2
2
2
2
2
2
2
2
]
]
]
]
]
]
]
]
]
0.002
0.005
0.008
0.01
0.012
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
1
White solid: m.p. = 86–88 °C; H NMR (400 MHz,
CDCl ): d 7.35 (m, Ar–H, 4H), 7.53 (m, Ar–H, 4H); Anal.
3
Calcd. for C H F (290): C, 57.89; H, 2.79; F, 39.31.
14 8 6
Found: C, 57.94; H, 2.78; F, 39.28.
b
67
c
88
0
2
.3.12 4,4 -Diacetylbiphenyl (Table 3, entry 10)
d
95
e
95
10
11
12
13
14
15
16
2
1
White solid: m.p. = 189–192 °C; H NMR (400 MHz,
CDCl ): d 2.59 (s, CH , 6H), 7.71 (m, Ar–H, 4H), 8.06 (m,
Ar–H, 4H); Anal. Calcd. for C H O (238): C, 80.62; H,
[C
6
mim][CuCl
mim][CuCl
2
2
]
83
89
92
78
86
52
3
3
[C
8
]
1
6 14 2
[C10mim][CuCl
[C12mim][CuCl
2
]
]
5
.94; O, 13.43. Found: C, 80.65; H, 5.92; O, 13.43.
3
[C12mim][NiCl
[C12mim][FeCl
3
]
4
]
3
Results and Discussion
Unless otherwise noted, the reactions were carried out with benzyl
chloride (10 mmol), Cu (0.1 mmol), Zn (5 mmol), in IL at 70 °C
It was determined during a preliminary survey of the
reaction conditions that we should use benzyl chloride as
the model substrate in the presence of metallic zinc and
copper powder (Table 1). Initial reaction screening led to
disappointing results in the absence of a ionic liquid, the
reaction proceeded very slowly, and the yield was only 13%
a
Isolated yield
b
No Zn powder was added
c
No Cu powder was added
d
The first run
e
The second run
123

4
70
Y.-L. Hu et al.
2
Table 2 Reductive homocoupling of benzyl halides with zinc and copper powder in [C12mim][CuCl ]
a
Entry
1
Substrate
Product
Time (h)
2
Yield (%)
Cl
96
96
Br
Cl
2
3
2
1.5
97
98
Br
O
4
5
1.5
1.5
O
O
Cl
O
97
O
O
6
7
1.5
96
95
Cl
2
8
9
2
93
88
91
2.5
2.5
Cl
O N
NO2
2
10
NO2
The reactions were carried out with benzyl halide (10 mmol), Cu (0.1 mmol), Zn (5 mmol), in [C12mim][CuCl
a
Isolated yield
2
] (0.1 mmol) at 70 °C
1
23

Reductive Homocoupling of Benzyl and Aryl Halides
471
2
Table 3 Reductive homocoupling of aryl halides with zinc and copper powder in [C12mim][CuCl ]
a
Entry
Organic halide
Product
Time (h)
Yield (%)
1
4
89
Br
Cl
2
3
4
4
90
93
Cl
O
4
5
6
4
4
6
95
94
85
Cl
O
Cl
7
8
9
6
6
6
85
88
82
CN
I
CN
Br
10
6
86
O
The reactions were carried out with aryl halide (10 mmol), Cu (0.1 mmol), Zn (5 mmol), in [C12mim][CuCl
2
] (0.1 mmol) at 90 °C
a
Isolated yield
123

4
72
Y.-L. Hu et al.
listed in Table 2. It is clear that various types of benzyl
halides, both primary and secondary, can be efficiently
converted to the corresponding bibenzyls in good to high
yields (Table 2, entries 1–10). Various functionalities such
as alkyl, alkoxy, chloro and nitro groups can tolerate the
reaction. It was also observed that the electronic nature of
the substituents on the aromatic ring has some impact on
the reaction rate and the yield. Substrates with electron-
withdrawing groups (Table 2, entries 9 and 10) are less
reactive than those with electron-donating groups (Table 2,
entries 3–6), and a decrease in the isolated yield or reaction
rate was obtained.
The another portion of this work involved the applica-
tion of our catalytic protocol to prepare biaryls by reductive
homocoupling of aryl halides with metallic zinc and copper
powder in ionic liquid [C mim][CuCl ]. The optimal
1
2
2
Scheme 2 Possible mechanism for homocoupling of benzyl chloride
homocoupling conditions were found to be the similar as
those in the case of homocoupling of benzyl halides, and
the desired biaryls products were obtained in excellent
yields (Table 3). Results revealed that our protocol can
facilitate efficiently the reductive homocoupling of aryl
halides. Various aryl halides were efficiently converted to
the corresponding biaryls in good to excellent isolated
yield using the catalytic protocol (Table 3, entries 1–10).
The aryl halides such as chlorobenzene, bromobenzene,
looks like that the formation of II between benzyl chloride
and cuprous ion is the rate-determining step, and the step to
afford Cu species likely initiates the homocoupling
reaction.
4
Conclusion
1
1
-chloro-4-methylbenzene, 1-chloro-4-methoxybenzene, and
-chloro-2-methoxybenzene gave products in higher yields
In conclusion, we have developed an efficient and experi-
mentally simple procedure for the reductive homocoupling
of benzyl and aryl halides to bibenzyls and biaryls
employing a combination of the catalytic [C mim][CuCl ]/
under milder reaction conditions (Table 3, entries 1–5)
than those of 1-chloro-4-nitrobenzene, 4-chlorobenzoni-
trile, 4-iodobenzonitrile, 1-chloro-4-(trifluoromethyl)ben-
zene, and 1-(4-bromophenyl)ethanone (Table 3, entries
1
2
2
Cu/Zn system. A wide range of benzyl and aryl halides were
found to be applicable to the catalytic system. Advantages
of our procedure include simplicity of operation, low cost,
high yields, and excellent recyclability of the catalyst.
Efforts to definite of the homocoupling mechanism and
expand the utility of the catalytic system to other organic
synthesis are currently under study in our laboratory.
6
–10), which was attributed to the electron-donating effect,
that may be explains why they showed more active for the
reactions. On the basis of the rates and yields of reactions,
obviously, our protocol was found to be more effective in
reductive homocoupling of benzyl halides than that of aryl
halides.
The excellent results of the catalytic system suggest the
homocoupling has a particular reaction mechanism, and it
possibly consists of five steps. Take the reductive homo-
coupling of benzyl chloride for an example, a possible
mechanism for the homocoupling, similar to that of Pal-
ladium-catalyzed homocoupling of aromatic electrophiles
Acknowledgments We thank the National Basic Research Program
(973) of China (No. 613740101) and Natural Science Foundation of
Jiangsu Province for support of this research.
References
[
26], is shown in Scheme 2. In the reaction, catalyst of the
1
. Miyaura N, Suzuki A (1995) Chem Rev 95:2457
2. Zhu SS, Swager TM (1996) Adv Mater 8:497
. Papillon J, Schulz E, Gelinas S, Lessard J, Lemaire M (1998)
Synth Met 96:155
. Fanta PE (1974) Synthesis 1974:9
. Hassan J, Sevignon M, Gozzi C, Schulz E, Lemaire M (2002)
Chem Rev 102:1359
. Kuroboshi M, Waki Y, Tanaka H (2003) J Org Chem 68:3938
ionic liquid [C mim][CuCl ] can provide a source of
1
2
2
?
cuprous ion (Cu ), which reacts with benzyl chloride to
3
?
form transition state (II) and then, II eliminated Cu to
4
5
yield benzyl free radical (III), then III reacts with each
other through radical homocoupling to from the desired
2
product (IV). The copper ion (Cu ) produced during the
?
6
?
first step can be converted into Cu via the oxidation–
7. Nun P, Martinez J, Lamaty F (2009) Synlett 2009:1761
8
. Kirchhoff JH, Netherton MR, Hill ID, Fu GC (2002) J Am Chem
Soc 124:13662
9. Fujihara T, Yoshida S, Terao J, Tsuji Y (2009) Org Lett 11:2121
reduction reaction by Cu powder, which can be continu-
2
ously obtained by the reduction of Cu by Zn powder. It
?
1
23

Reductive Homocoupling of Benzyl and Aryl Halides
473
1
1
0. Silva AC, Senra JD, Aguiar LCS, Simas ABC, de Souza ALF,
Malta LFB, Antunes OAC (2010) Tetrahedron Lett 51:3883
1. So CM, Yeung CC, Lau CP, Kwong FY (2008) J Org Chem
33. Kazuhiko O, Junzo Y, Harukichi H (1970) Bull Chem Soc Jpn
439:836
34. Khurana JM, Sushma C, Maikap GC (2003) Org Biomol Chem
1:1337
7
3:7803
1
1
2. Li HL, Wu ZS, Yang M, Qi YX (2010) Catal Lett 137:69
3. Simpson CD, Mattersteig G, Martin K, Gherghel L, Bauer RE,
Rader HJ, Mullen K (2004) J Am Chem Soc 126:3139
35. Sustmann R, Kopp C (1988) J Organomet Chem 347:313
36. Eisch JJ, Fregene PO (2008) Eur J Org Chem 2008:4482
37. Nishino T, Watanabe T, Okada M, Nishiyama Y, Sonoda N
(2002) J Org Chem 67:966
38. Mukhopadhyay S, Rothenberg G, Gitis D, Baidossi M, Ponde
DE, Sasson Y (2000) J Chem Soc Perkin Trans 2:1809
39. Mukhopadhyay S, Rothenberg G, Gitis D, Sasson Y (2000) Org
Lett 2:211
40. Qian YL, Li GS, Huang YZ (1990) J Organomet Chem 381:29
41. Moncomble A, Floch PL, Gosmini C (2009) Chem Eur J 15:4770
42. Barrero AF, Herrador MM, Moral JFQ, Arteaga P, Akssira M,
Hanbali FE, Arteaga JF, Dieguez HR, Sanchez EM (2007) J Org
Chem 72:2251
43. Barrero AF, Herrador MM, Moral JFQ, Arteaga P, Arteaga JF,
Dieguez HR, Sanchez EM (2007) J Org Chem 72:2988
44. Dahlen A, Prasad E, Flowers RA II, Hilmersson G (2005) Chem
Eur J 11:3279
45. Rogers RD, Seddon KR (2005) Ionic liquids: fundamentals, pro-
gress, challenges, and opportunities. American Chemical Society,
Washington
46. Zhang SJ, Lu XM (2006) Ionic liquids: from fundamental
research to industrial applications. Science Press, Beijing
47. Wasserschein P, Welton T (2008) Ionic liquids in synthesis.
Wiley-VCH, Weinheim
48. Blaszczyk I, Trzeciak AM, Ziolkowski JJ (2009) Catal Lett
133:262
49. Wang W, Cheng W, Shao L, Yang J (2008) Catal Lett 121:77
50. Roberts NJ, Lye GJ (2002) Application of room temperature
ionic liquids in biocatalysis: opportunities and challenges.
American Chemical Society, Washington
1
1
1
1
1
1
4. King BT, Kroul ´ı k J, Robertson CR, Rempala P, Hilton CL,
Korinek JD, Gortari LM (2007) J Org Chem 72:2279
5. Patrick TB, Willaredt RP, de Gonia DJ (1985) J Org Chem
5
0:2232
6. Smith MB, March J (1992) In: Carey FA, Sundberg RJ (eds)
Advanced organic chemistry, 4th ed. Wiley, New York, p 715
7. Santos LS, Rosso GB, Pilli RA, Eberlin MN (2007) J Org Chem
7
2:5809
8. Coelho AV, de Souza ALF, de Lima PG, Wardell JL, Antunes
OAC (2007) Tetrahedron Lett 48:7671
9. Imperato G, Vasold R, Konig B (2006) Adv Synth Catal
3
48:2243
2
2
0. Mee SPH, Lee V, Baldwin JE (2005) Chem Eur J 11:3294
1. Becht JM, Gissot A, Wagner A, Mioskowski C (2004) Tetrahe-
dron Lett 45:9331
2
2. Johnson DK, Ciavarri JP, Ishmael FT, Schillinger KJ, van Geel
TAP, Stratton SM (1995) Tetrahedron Lett 36:8565
3. Becht JM, Catala C, Drian CL, Wagner A (2007) Org Lett 9:1781
4. Mukhopadhyay S, Rothenberg G, Gitis D, Sasson Y (2000) J Org
Chem 65:3107
2
2
2
2
5. Goossen LJ, Rodriguez N, Melzer B, Linder C, Deng G, Levy
LM (2007) J Am Chem Soc 129:4824
6. Robinson MK, Kochurina VS Jr, Hanna JM (2007) Tetrahedron
Lett 48:7687
7. Jutand A (2008) Chem Rev 108:2300
2
2
8. Adamo C, Amatore C, Ciofini I, Jutand A, Lakmini H (2006) J
Am Chem Soc 128:6829
2
3
3
9. Hashim J, Kappe CO (2007) Adv Synth Catal 349:2353
0. Inaba S, Matsumoto H, Rieke RD (1984) J Org Chem 49:2093
1. Raynal F, Barhdadi R, Perichon J, Savall A, Troupel M (2002)
Adv Synth Catal 344:45
51. Mac Farlane DR, Forsyth M, Howlett PC, Pringle JM, Sun JZ,
Annat G, Neil W, Izgorodina EI (2007) Acc Chem Res 40:1165
52. Meng Y, Pino V, Anderson JL (2009) Anal Chem 81:7107
3
2. Everson DA, Shrestha R, Weix DJ (2010) J Am Chem Soc
1
32:920
123