Triflic salt-catalyzed coupling: Iron triflate-catalyzed homocoupling of aryl bromides in the presence of metallic magnesium

Article abstract of DOI:10.1080/00397910903289230

In the presence of metallic magnesium, the homocoupling reaction of aryl bromides catalyzed by iron triflate was carried out readily in one pot. The catalyst was used successfully in this coupling reaction without preparation of Grignard reagent in advance. Meanwhile, the catalyst was recovered easily and reused smoothly with only a little loss of its activity. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903289230

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Triflic Salt–Catalyzed Coupling: Iron
Triflate–Catalyzed Homocoupling of Aryl
Bromides in the Presence of Metallic
Magnesium
Yan-Yan Zhang a , Jing-Du Lin a , Xiao-Liang Xu b & Jing-Hua Li a
a College of Pharmaceutical Sciences, Zhejiang University of
Technology , Hangzhou, China
b College of Chemical Engineering and Material Sciences, Zhejiang
University of Technology , Hangzhou, China
Published online: 05 Aug 2010.
To cite this article: Yan-Yan Zhang , Jing-Du Lin , Xiao-Liang Xu & Jing-Hua Li (2010) Triflic
Salt–Catalyzed Coupling: Iron Triflate–Catalyzed Homocoupling of Aryl Bromides in the Presence of
Metallic Magnesium, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 40:17, 2556-2563, DOI: 10.1080/00397910903289230
To link to this article: http://dx.doi.org/10.1080/00397910903289230
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Synthetic Communications1, 40: 2556–2563, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903289230
TRIFLIC SALT–CATALYZED COUPLING: IRON
TRIFLATE–CATALYZED HOMOCOUPLING OF ARYL
BROMIDES IN THE PRESENCE OF METALLIC
MAGNESIUM
Yan-Yan Zhang,1 Jing-Du Lin,1 Xiao-Liang Xu,2
and Jing-Hua Li1
1College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou, China
2College of Chemical Engineering and Material Sciences, Zhejiang University
of Technology, Hangzhou, China
In the presence of metallic magnesium, the homocoupling reaction of aryl bromides
catalyzed by iron triflate was carried out readily in one pot. The catalyst was used
successfully in this coupling reaction without preparation of Grignard reagent in advance.
Meanwhile, the catalyst was recovered easily and reused smoothly with only a little loss
of its activity.
Keywords: Aromatic bromide; homocoupling; iron triflate
INTRODUCTION
Transition metal–catalyzed coupling reaction of halogen compounds is one of
the most important methods in organic synthesis[1–12] and is widely applied in phar-
maceutical chemistry[13] where palladium and nickel complexes are more often used
as the catalysts.[14,15]
However, many auxiliary reagents, which were used in the reaction to improve
catalytic effect, are generally toxic, not environmentally friendly, expensive, or sensi-
tive to oxygen and moisture. In 1975, Kochi et al.[16–21] reported that coupling
reaction of halogen compounds with Grignard reagents could be catalyzed smoothly
by iron salt, a cheap and ecofriendly compound. Since then, a large number of
studies were reported about iron salt–catalyzed coupling reactions.[22–38]
Although the methods are better than their earlier efforts for the synthesis of
symmetrical biaryls, there were obvious drawbacks: (1) Grignard reagents had to
be prepared before the coupling reaction, (2) the toxic 1,2-dihalogen ethane was used
as the oxidant, and (3) some iron salts such as Fe(acac)3 and Fe(dbm)3 are apt to
Received October 26, 2007.
Address correspondence to Jing-Hua Li, College of Pharmaceutical Sciences, Zhejiang University
of Technology, Hangzhou 310032, China. E-mail: lijh@zjut.edu.cn
2556

HOMOCOUPLING REACTION OF ARYL BROMIDE
2557
dissolve in organic solvents, which made the workup procedure of the reaction
mixtures more complex and tedious.
Therefore, the developing of a more efficient synthetic methodology for
carbon–carbon bond formation is still in demand.
Metal triflates, which were confirmed to be ecofriendly and water-incompatible
catalysts, could be recovered and reused with only a little loss of their activity in
various kinds of reactions.[39–42] These encouraged us to try the aryl–aryl coupling
reactions using triflate as a catalyst.
RESULTS AND DISCUSSION
The homocoupling reaction of aryl bromide was investigated in the presence of
magnesium strips and a catalytic amount of transition-metal salt. Thus, the reaction
of 1 mmol bromobenzene with 2 mmol magnesium strips in presence of 5 mol% cata-
lytic transition metal was carried out in anhydrous solvent. The results are shown in
Table 1.
As shown in Table 1, compared with other iron compounds such as FeCl3,
Fe(acac)3, and Fe(dbm)3 (entries 1–4), Fe(OTf)3 gives a better yield (entry 5). It
seems that the catalytic effect of the coupling reaction was improved by triflate,
which induced us to investigate the catalytic effect of a series of transition-metal
triflates on this reaction system. However, the results were not satisfying (entries
6–11). Meanwhile, Fe(OTf)3 is a comparatively excellent catalyst with many advan-
tages, such as environmental friendliness and water incompatibility, and it also can
be recovered and reused with only a little loss of its activity after several runs.
The effect of different solvents on this coupling reaction was studied (entries 5
and 12–14), in which tetrahydrofuran (THF) was found to be the best solvent in
this reaction.
Table 1. Optimization of catalyzed homocoupling reaction of bromobenzenea
Entry
Catalyst
Solvent
Time (h)
Yieldb (%)
1
2
FeCl3
THF
THF
THF
4
4
2
2
2
4
2
4
2
2
4
2
4
4
76
81
82c
80c
88
51
53
64
78
71
45
73
51
50
FeCl3=ClCH2CH2Cl
Fe(dbm)3
Fe(acac)3
Fe(OTf)3
3
4
THF
THF
5
6
Yb(OTf)3
VO(OTf)2
Mn(OTf)2
Ni(OTf)2
THF
THF
7
8
THF
THF
9
10
11
12
13
14
Cu(OTf)2
Sc(OTf)3
Fe(OTf)3
THF
THF
Ether
Benzene
Toluene
Fe(OTf)3
Fe(OTf)3
aReaction condition: 5 mol% catalyst, 1 mmol PhBr, 2 mmol Mg, 3 mL solvent,
room temperature; dbm, dibenzoyl methane; acac, acetylacetonate.
bIsolated yield.
cQuoted from Ref. 38.

2558
Y.-Y. ZHANG ET AL.
Figure 1. Effect of amount of catalysts on the homocoupling of bromobenzene.
Table 2. Catalyst recycling
Run
Recovery rate of catalyst (%)
Yield (%)
1
2
3
92
91
92
84
81
80
Notes. Reaction condition: 5 mol% Fe(OTf)3, 1 mmol PhBr, 2 mmol Mg,
3 mL THF, room temperature, and reaction time: 4 h.
Table 3. Fe(OTf)3-catalyzed homocoupling of aryl bromidesa
Entry
Aryl bromide 1
Product
Yieldb (%)
1
2
C6H5Br
2a
2b
2c
2d
2e
2f
88
90
92
71
93
82
70
63c
66c
81
78
4-MeC6H4Br
3-MeC6H4Br
2,3-Me2C6H3Br
4-MeOC6H4Br
4-ClC6H4Br
3
4
5
6
7
3-NO2C6H4Br
4-CF3C6H4Br
3,5-(CF3)2C6H3Br
3-Thienyl Br
3-Pyridinyl Br
2g
2h
2i
8
9
10
11
2j
2k
aReaction conditions: 5 mol% Fe(OTf)3, 1 mmol aryl bromides, 2 mmol Mg,
3 mL anhydrous THF, room temperature, and standard reaction time: 4 h.
bIsolated yield.
cReaction time: 6 h.

HOMOCOUPLING REACTION OF ARYL BROMIDE
2559
The effect of the amount of catalyst and reaction time on the homocoupling
reaction was also investigated using 1 mmol PhBr, 2 mmol Mg, and 3 mL anhydrous
THF.
As shown in Fig. 1, with the same reaction time, the more catalyst was
employed, the better the yield, and the reaction yield reached the maximum value
of 88% when 5 mol% of catalyst was employed.
Meanwhile, the catalyst was recovered from the aqueous layer during the
workup after the reaction and was reused in subsequent reactions for three times.
The activity was still maintained at a desirable level as shown in Table 2.
A diverse array of biaryls from aryl bromides were efficiently synthesized under
the standard reaction condition (Table 3).
As shown in Table 3, all of the aryl bromides gave their corresponding pro-
ducts in moderate to good yields under the standard conditions. Aromatic bromide
substituted with electron-donating groups (methyl, methoxy, etc.) at the m- or
p-position gave good yields. It is noteworthy that the present reaction system is tol-
erant of strong electron-withdrawing groups such as trifluoro-methyl and nitro, and
the desired products were obtained in moderate yields. Heteroaryl bromides such as
2-bromothiophene and 3-bromopyridine also work well in this system.
CONCLUSION
In summary, the homocoupling reaction of aryl bromides catalyzed by an eco-
friendly catalyst, Fe(OTf)3, was carried out readily in one pot, in which it is not
necessary to prepare Grignard reagent in advance.
EXPERIMENTAL
General
All reactions were carried out under an atmosphere of nitrogen in oven-dried
glassware with magnetic stirring. Tetrahydrofuran (THF) was distilled from a ben-
1
zophenone–sodium adduct and stored over sodium wires. H NMR spectra were
measured on a Varian Plus-400 (400-MHZ) apparatus (CDCl3 solution); MS spectra
were found on an AEI MS-902 apparatus. Melting points were determined on X-4
micromelting-point apparatus.
Homocoupling Reaction Catalyzed by FeCl3
The general synthetic process is as follows: 1 mmol PhBr, 3 mL anhydrous
THF, and 1 mmol Mg strips were added to a 50-ml, three-necked, round-bottom
flask equipped with dropping funnel and condenser, the top of which was connected
to a calcium chloride drying tube, and then stirred at reflux temperature until the Mg
strips disappeared. Then 1.2 mmol 1,2-dihaloethane and 5 mol% FeCl3 were added.
The mixture was then stirred at reflux temperature for 1 h and quenched with
1 mol=L HCl. The solvent was evaporated, and the residue was extracted with
Et2O (3 ꢀ 10 mL). The organic extracts were combined, dried with MgSO4, concen-
trated, and then purified by column chromatography on silica gel (300–400 mesh)
with hexane as eluant to give the pure homocoupling product.

2560
Y.-Y. ZHANG ET AL.
Homocoupling Reaction Catalyzed by Fe(OTf)3 and
Recovery of Fe(OTf)3
The general synthetic process is as follows: 1 mmol PhBr, 3 mL anhydrous
THF, 5 mol% Fe(OTf)3, and 2 mmol Mg strips were added to a 50-ml, three-necked,
round-bottom flask equipped with dropping funnel and condenser, the top of which
was connected to a calcium chloride drying tube. The whole reaction mixture was
stirred for 2–6 h at room temperature and quenched with 1 mol=L HCl. The solvent
was evaporated, and the residue was treated with Et2O (10 ml) and water (15 ml).
The organic phase was separated, and the aqueous layer was washed with Et2O
(2 ꢀ 10 ml). Concentration of the combined organic layer, dried over MgSO4 in
advance, afforded the crude product, which was further purified by column chroma-
tography on silica gel (300–400 mesh) with hexane as eluant to give the pure
homocoupling product.
The aqueous layer containing Fe(OTf)3 in the workup procedure was evapo-
rated under reduced pressure, and the reside was extracted with ethyl acetate
(2 ꢀ 10 ml). The organic phases were combined, evaporated under reduced pressure,
and dried at 70 ꢁC for 2 h to give pure Fe(OTf)3 in 92% recovery. The recovered
catalyst was reused for the next run in the same way.
Selected Data
1
Biphenyl 2a. Mp 68–71 ꢁC (lit.[38] 68–71 ꢁC); H NMR d: 7.35–7.47 (6H, m),
7.58–7.62 (4H, m); MS (EI): m=z (%): 154 (Mþ, 100), 153 (35), 152 (20), 115 (3), 76
(13), 51 (4).
1
4,4’-Dimethylbiphenyl 2b. Mp 119–120 ꢁC (lit.[38] 119–120 ꢁC); H NMR d:
2.56 (6H, s), 7.41 (4H, d, J ¼ 7.6 Hz), 7.66 (4H, d, J ¼ 7.6 Hz); MS (EI): m=z (%): 182
(Mþ, 100), 167 (55), 152 (12), 89 (11), 76 (3).
3,3’-Dimethylbiphenyl 2c. Oil; 1H NMR d: 2.42 (6H, s), 7.16 (2H, d,
J ¼ 6.0 Hz), 7.32–7.40 (6H, m); MS (EI): m=z (%): 182 (Mþ, 100), 167 (33), 152
(12), 89 (8), 76 (4).
1
2,3,2’,3’-Tetramethylbiphenyl 2d. Mp 114–115 ꢁC (lit.[43] 114–115 ꢁC); H
NMR d: 1.95 (6H, s), 2.33 (6H, s), 6.96 (2H, d, J ¼ 6.8 Hz), 7.16–7.10 (4H, m);
MS (EI): m=z (%): 210 (Mþ, 93), 195 (100), 180 (25), 165 (19), 89 (11), 76 (3).
1
4,4’-Dimethoxydiphenyl 2e. Mp 177–179 ꢁC (lit.[38] 177–179 ꢁC); H NMR
d: 3.84 (6H, s), 6.95 (4H, d, J ¼ 8.8 Hz), 7.47 (4H, d, J ¼ 8.8 Hz); MS (EI): m=z
(%): 182 (Mþ, 100), 167 (55), 152 (12), 89 (11), 76 (3).
1
4,4’-Dichlorobiphenyl 2f. Mp 149–150 ꢁC (lit.[44] 146–148 ꢁC); H NMR d:
7.40 (4H, d, J ¼ 8.0 Hz), 7.48 (4H, d, J ¼ 8.0 Hz); MS (EI): m=z (%): 222 (Mþ,
100), 186 (15), 152 (51), 111 (5), 93 (7), 75 (9).
1
3,3’-Dinitrobiphenyl 2g. Mp 204–205 ꢁC (lit.[45] 205 ꢁC); H NMR d: 7.69–
7.72 (2H, m), 7.96–7.98 (2H, m), 8.29–8.32 (2H, m), 8.50–8.51 (2H, m); MS (EI):
m=z (%): 244 (Mþ, 100), 202 (21), 152 (15), 96 (7), 76 (15), 62 (5).

HOMOCOUPLING REACTION OF ARYL BROMIDE
2561
4,4’-Bis(trifluoromethyl)biphenyl 2h. Mp 89–91 ꢁC (lit.[46] 83–84 ꢁC); 1H
NMR d: 7.38 (4H, d, J ¼ 8.0 Hz), 7.48 (4H, d, J ¼ 8.0 Hz); MS (EI): m=z (%): 290
(Mþ, 100), 271 (25), 240 (10), 219 (6), 201 (20), 152 (14), 145 (5).
3,5,3’,5’-Tetrakis(trifluoromethyl)biphenyl 2i. Mp 68–70 ꢁC (lit.þ[38] 68–
1
71 ꢁC); H NMR d: 7.96 (2H, s), 8.04 (4H, s); MS (EI): m=z (%): 426 (M , 100),
407 (42), 357 (15), 337 (15), 287 (10), 213 (8).
1
3,3’-Bithienyl 2j. Mp 130–131 ꢁC (lit.[45] 132–133 ꢁC); H NMR d: 7.33–7.35
(4H, m), 7.37–7.38 (2H, m); MS (EI): m=z (%): 166 (Mþ, 100), 83 (21).
1
3,3’-Bipyridinyl 2k. Mp 68–69 ꢁC (lit.[45] 67 ꢁC); H NMR d: 7.34–7.35 (2H,
m), 7.80–7.82 (2H, m), 8.56–8.57 (2H, m), 8.76 (2H, s); MS (EI): m=z (%): 156
(Mþ, 100), 152 (11), 72 (5), 66 (21).
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