Suzuki-Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported N-heterocyclic carbene-palladium complex catalyst

Article abstract of DOI:10.3762/bjoc.6.70

The Suzuki-Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported Pd-NHC complex catalyst has been realized for the first time. The polymer-supported catalyst can be re-used several times still retaining high activity for this transformation. Various aryltriazenes were investigated as electrophilic substrates at room temperature to give biaryls in good to excellent yields and showed good chemoselectivity over aryl halides in the reactions.

Full text of DOI:10.3762/bjoc.6.70

Suzuki–Miyaura cross-coupling reaction of 1-aryl-
triazenes with arylboronic acids catalyzed by a
recyclable polymer-supported N-heterocyclic
carbene–palladium complex catalyst
Guangming Nan1,2, Fang Ren1 and Meiming Luo*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, No. 70. doi:10.3762/bjoc.6.70
1Key Laboratory of Green Chemistry & Technology of Ministry of
Education, College of Chemistry, Sichuan University, Chengdu
610064, P. R. China and 2Department of Chemistry, Ili Teachers
College, Yining 835000, Xinjiang, P. R. China
Received: 28 April 2010
Accepted: 04 June 2010
Published: 28 June 2010
Email:
Associate Editor: I. Marek
Meiming Luo* - luomm@scu.edu.cn
© 2010 Nan et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
1-aryltriazenes; N-heterocyclic carbene–palladium complex;
polymer-supported catalyst; recyclable catalyst; Suzuki–Miyaura
reaction
Abstract
The Suzuki–Miyaura cross-coupling reaction of 1-aryltriazenes with arylboronic acids catalyzed by a recyclable polymer-supported
Pd–NHC complex catalyst has been realized for the first time. The polymer-supported catalyst can be re-used several times still
retaining high activity for this transformation. Various aryltriazenes were investigated as electrophilic substrates at room tempera-
ture to give biaryls in good to excellent yields and showed good chemoselectivity over aryl halides in the reactions.
Introduction
The unsymmetrical biaryls feature in a diverse range of organic sulfonates and carboxylates, as the electrophilic component
compounds, such as natural products, advanced materials, liquid [5-18]. As an additional candidate for the electrophilic coupling
crystals, ligands and molecules of medicinal interest [1-4]. The partner, arenediazonium salts have also been used in place of
palladium-catalyzed Suzuki–Miyaura cross-coupling reaction aryl halides in the Suzuki–Miyaura cross-coupling reaction, and
has evolved as a powerful synthetic tool for the synthesis of show higher activity than the corresponding aryl halides [19-
unsymmetrical biaryls in both academic laboratories and 28]. However, arenediazonium salts are prone to decompose
industry [5-8]. Most of the reported Suzuki–Miyaura reactions upon storage, which restricts their practical use. Recently,
are based on the use of aryl halides and triflates, and recently 1-aryltriazenes, which are stable and can be easily prepared
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Beilstein J. Org. Chem. 2010, 6, No. 70.
Scheme 1: Synthesis of the polymer-supported NHC–Pd catalyst 1.
from the corresponding arylamines, have been employed by The Suzuki–Miyaura cross-coupling of 1-aryltriazenes and
Tamao et al. as arenediazonium salt surrogates in the boronic acids catalyzed by the polymer-supported Pd–NHC
Suzuki–Miyaura cross-coupling reaction [29]. Pd2(dba)3 and catalyst 1 was investigated in detail with the coupling of 1-(3-
P(tBu)3 were used as catalyst in this reaction where, as in most nitrophenyl)-2-(pyrrolidin-1-yl)diazene and phenylboronic acid
of homogeneous catalytic systems, the difficulties of catalyst as a model reaction (Scheme 2). As described for the homogen-
recovery and recycling constitute major problems. One possible eous catalytic conditions [29], a Lewis acid was essential for the
solution to these problems is the ‘heterogenizing’ of a homo- formation of the biphenyl products. Without BF3·OEt2, no
geneous catalyst by anchoring the catalyst onto a support. This diaryl product was observed whatsoever.
offers many advantages for industrial application due to their
versatile processing capabilities and ease of product/catalyst Various solvents were explored for the cross-coupling reaction.
separation. In view of the potential industrial applications, the As shown in Table 1, solvent significantly affected the product
development of a recyclable catalytic system for the yields. Among the solvents investigated, 1,4-dioxane proved to
Suzuki–Miyaura coupling reactions of 1-aryltriazenes is highly be the most effective (Table 1, entry 1). DME also led to good
desirable. Although several types of heterogenous catalytic
systems have been described for the Suzuki–Miyaura reactions
Table 1: Effect of solvent on the cross-coupling reactiona.
of aryl halides [30], to the best of our knowledge, there has been
no general study on the Suzuki–Miyaura reactions of 1-aryl-
triazenes under heterogeneous catalysis described to date. Previ-
ously, we reported an active and recyclable polystyrene-
supported Pd–NHC (N-heterocyclic carbene) catalyst 1
(Scheme 1) for the Suzuki–Miyaura cross-coupling reactions of
aryl bromides, arylsulfonyl chlorides and arenediazonium salts
with arylboronic acids that gave biaryls in good to excellent
yields [28,31,32]. As part of our ongoing investigations aimed
at the development of transition metal catalysis, we report here
that the Suzuki–Miyaura cross-couplings of 1-aryltriazenes with
arylboronic acids can be readily effected with our polystyrene-
supported Pd–NHC catalyst, which shows high efficiency and
can be easily recovered and reused several times still retaining
high activity.
Entry Solvent
Yield (%)b
1 dioxane
2 DME
92
84
42
46
0
3 THF
4 CH3CN
5 DMF
6 DMSO
0
Results and Discussion
aReaction conditions: 1-(3-nitrophenyl)-2-(pyrrolidin-1-yl)diazene (0.5
mmol), BF3·OEt2 (0.5 mmol) and phenylboronic acid (1.0 mmol), cata-
lyst 1 (100 mg, 10 μmol Pd), solvent (5 mL), 8 h, rt.
bIsolated by silica-gel column chromatography and based on 1-(3-
nitrophenyl)-2-(pyrrolidin-1-yl)diazene.
The polystyrene-supported Pd–NHC catalyst 1 was prepared
according to our reported procedure [31] (Scheme 1). The Pd
loading was determined to be 0.1 mmol/g by inductively
coupled plasma-atomic emission spectrometry (ICP-AES).
Scheme 2: Reaction of 1-(3-nitrophenyl)-2-(pyrrolidin-1-yl)diazene and phenylboronic acid.
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Beilstein J. Org. Chem. 2010, 6, No. 70.
yields of the desired product (Table 1, entry 2) whereas THF
and CH3CN gave lower product yields. No product was formed
in either DMF or DMSO.
Table 2: Effect of the catalyst loading on the cross-coupling reactiona.
The effect of catalyst loading on the cross-coupling reaction is
shown in Table 2. The amount of the polymer-supported
Pd–NHC catalyst employed in the reaction is of importance. No
product was observed in the absence of catalyst 1 (Table 2,
entry 1). The yield of the corresponding biaryl product
increased with the catalyst loading (Table 2, entries 2, 3, 4). A
high yield of 92% was obtained when 2.0 mol % Pd was
employed (Table 2, entry 5) whilst Pd loadings of greater than
2.0 mol % Pd did not lead to improved yields (Table 2, entry 6).
Entry
Pd (mol%)
Yield (%)b
1
2
3
4
5
6
0
0
0.5
1.0
1.5
2.0
2.5
79
83
89
92
92
Table 3 shows the effect of the molar ratio of phenylboronic
acid to 1-(3-nitrophenyl)-2-(pyrrolidin-1-yl)diazene on the reac-
tion yield. A yield of 77% was obtained when the molar ratio
was 1.25 (Table 3, entry 1). The yield of the corresponding
biaryl product increased with increasing substrate ratio. The
highest yield (92%) was obtained when the ratio was increased
to 2.0 (Table 3, entry 4) whilst higher substrate ratios did not
produce any yield improvement (Table 3, entries 5, 6). Only
different reaction rates were observed when the coupling reac-
tion was conducted at different temperatures, which suggested
that the reaction temperature did not apparently influence the
final reaction yield (Table 3, entry 7).
aReaction conditions: 1-(3-nitrophenyl)-2-(pyrrolidin-1-yl)diazene (0.5
mmol), phenylboronic acid (1 mmol), BF3·OEt2 (0.5 mmol), catalyst,
dioxane (5 mL), rt, 8 h.
bIsolated by silica-gel column chromatography and based on 1-(3-
nitrophenyl)-2-(pyrrolidin-1-yl)diazene.
Table 3: Effect of the molar ratio of substrates on the cross-coupling
reactionsa.
After optimizing the amount of catalyst, solvent, substrate ratio
and temperature, the recyclability of the polymer-supported
Pd–NHC catalyst for the Suzuki–Miyaura cross-coupling reac-
tions of 1-(3-nitrophenyl)-2-(pyrrolidin-1-yl)diazene and
phenylboronic acid was investigated (Table 4). The catalyst
could be re-used eight times and still retained high activity after
separation, washing and drying under vacuum, under the same
reaction conditions. To demonstrate the general recyclability of
the catalyst, the recovered catalysts from the first, fifth and
eighth runs of the reaction of 1-(3-nitrophenyl)-2-(pyrrolidin-1-
yl)diazene and phenylboronic acid were also used to catalyze
the reaction of 1-phenyl-2-(pyrrolidin-1-yl)diazene and 4-meth-
oxyphenylboronic acid. The yields were 85%, 80% and 72%,
respectively. Contamination with the previous product was not
observed in the new product. Analysis of the reaction mixture,
following separation and washing of the resin, by ICP-MS indi-
cated that after the initial run (0.4% leaching) there were low
levels of Pd leaching (about 90 ppm for the second run and
45 ppm for the fifth run) from the resin. In order to confirm that
the reactive catalyst was a solid-supported catalyst rather than a
boomerang system, we undertook experiments in which the
solid catalyst was filtered off after 2 hours (about 40% conver-
sion), and no further reaction was observed despite stirring for
additional 12 hours.
Entry
Molar ratiob
Yield (%)c
1
2
3
4
5
6
7
1.25
1.5
77
87
90
92
92
91
91d
1.75
2.0
2.25
2.5
2.0
aReaction conditions: 1-(3-nitrophenyl)-2-(pyrrolidin-1-yl)diazene (0.5
mmol), BF3·OEt2 (0.5 mmol), catalyst 1 (100 mg, 10 μmol Pd), dioxane
(5 mL), rt, 8 h.
bThe ratio of phenylboronic acid to1-(3-nitrophenyl)-2-(pyrrolidin-1-
yl)diazene.
cIsolated by silica-gel column chromatography and based on 1-(3-
nitrophenyl)-2-(pyrrolidin-1-yl)diazene.
dAt 80 °C for 5 h.
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Beilstein J. Org. Chem. 2010, 6, No. 70.
cross-coupling reactions afforded biaryl products in good to
excellent yields. The electronic nature of substituents and steric
factors of both substrates affected the yields of the cross-coup-
ling products. Electron-withdrawing substituents on the 1-aryl-
triazenes and electron-donating groups on the arylboronic acids
gave better yields of biaryl products. Electron-donating groups
on the 1-aryltriazenes and electron-withdrawing substituents on
the arylboronic acids lead to reduced yields. Steric hindrance of
ortho substituents slightly reduced the product yields (Table 5,
entries 4, 7). Halogens on 1-aryltriazenes gave lower yields
(Table 5, entries 5, 6). Notably, good reactivity and chemo-
selectivity were achieved with 1-(4-bromophenyl)- and 1-(4-
iodophenyl)-2-(pyrrolidin-1-yl)diazene (Table 5, entries 5, 6),
showing that triazenes were more active than the corresponding
aryl bromides and iodides under these reaction conditions,
while the homogeneous Pd-phosphine catalyst system gave very
low product yields with these substrates [29]. The good reactiv-
ity and chemoselectivity may allow haloaryltriazenes to be used
as substrates for differential cross-coupling reactions in the
Table 4: Recycling of the polymer-supported NHC–Pd catalyst 1a.
Run
1
2
3
4
5
6
7
8
Yield (%)b
92
92
89
88
85
82
82
78
aReaction conditions: 1-(3-nitrophenyl)-2-(pyrrolidin-1-yl)diazene (0.5
mmol), phenylboronic acid (1.0 mmol), BF3·OEt2 (0.5 mmol), catalyst 1
(100 mg, 10 μmol Pd), dioxane (5 mL), rt, 8 h.
bIsolated by silica-gel column chromatography and based on 1-(3-
nitrophenyl)-2-(pyrrolidin-1-yl)diazene.
The scope of the cross-coupling reactions with variety of 1-aryl- same way as haloarenediazonium salts [33].
triazenes and arylboronic acids was then explored under the
optimized reaction conditions. As shown in Table 5, most of the Conclusion
In conclusion, we have disclosed that the Suzuki–Miyaura
cross-coupling of aryltriazenes and arylboronic acids can be
catalyzed by the recyclable polystyrene-supported Pd–NHC
catalyst 1 to produce biaryls in good to excellent yields. The
supported Pd–NHC catalyst can be re-used several times and
still retains high activity, which reflects its high stability and re-
cyclability. Advantages of this method also include good
substrate generality, good chemoselectivity over arylhalides and
mild reaction conditions.
Table 5: Cross-coupling of 1-aryltriazenes and arylboronic acids cata-
lyzed by the polystyrene-supported NHC–Pd catalysta.
Time
(h)
Yield
(%)b
Entry Ar1
Ar2
Experimental
General
1 4-NO2C6H4
C6H5
C6H5
C6H5
8
10
8
92
87
92
88
66
71
81
77
96
84
82
96
94
93
74
2 2-NO2C6H4
3 3-NO2C6H4
All chemicals were obtained from commercial sources and
were, in general, used without further purification. Melting
points were determined with XRC-1 melting point apparatus
and were uncorrected. 1H NMR spectra were recorded on
Varian INOVA 400 MHz or Bruker Avance 600 MHz spectro-
meter. GC-MS were recorded on an Agilent 5973 N. Palladium
content was determined by ICP-AES on IRIS Adv. Pd-leaching
was determined by ICP-MS on VG PQ Exceu.
4 2-Me-4-NO2C6H3 C6H5
10
12
12
12
12
8
5 4-IC6H4
C6H5
6 4-BrC6H4
7 1-Naphthyl
8 4-MeOC6H4
9 3-NO2C6H4
10 C6H5
C6H5
C6H5
C6H5
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeC6H4
4-MeC6H4
3-NO2C6H4
10
12
8
11 4-MeC6H4
12 4-NO2C6H4
13 3-NO2C6H4
14 4-NO2C6H4
15 C6H5
General procedure for the preparation of
1-aryltriazenes
8
8
1-Aryltriazenes were prepared by a modification of the litera-
ture procedure [34]. A solution of arylamine (10 mmol) in
concentrated HCl (2 mL) was cooled in an ice bath while a
solution of NaNO2 (10 mmol) in cold water (1 mL) was added
dropwise. The resulting solution of the diazonium salt was
stirred in the cold for 10 min and then added all at once to a
12
aReaction conditions: 1-aryltriazene (0.5 mmol), arylboronic acid (1.0
mmol), BF3·OEt2 (0.5 mmol), catalyst 1 (100 mg, 10 μmol Pd), dioxane
(5 mL), rt.
bIsolated by silica-gel column chromatography and based on 1-aryl-
triazene.
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Beilstein J. Org. Chem. 2010, 6, No. 70.
Inductively coupled plasma mass spectro-
chilled solution of pyrrolidine (11 mmol) in 1 M KOH (10 mL).
The reaction mixture was stirred for 30 min with cooling and metry (ICP-MS)
the resulting precipitate isolated by filtration. The damp solid When the reaction was completed, the catalyst was filtered and
was recrystallized from EtOH and dried under reduced pressure. washed with ether (3 × 5 mL). The combined organic phase was
evaporated under reduced pressure. The residue was heated in a
General procedure for the Suzuki–Miyaura
crucible at 600 °C and ignition continued until constant weight
cross-couplings of 1-aryltriazenes and aryl-
boronic acids
and the residue treated with a mixture (5 mL) of hydrochloric
acid and nitric acid (3:1, v:v) at 100 °C for 4 h. The resulting
solution was diluted to 50 mL with distilled water and analyzed
by ICP-MS.
Polystyrene-supported Pd–NHC catalyst 1 (100 mg, 10 μmol
Pd), 1-aryltriazene (0.5 mmol), arylboronic acid (1 mmol) were
mixed in dioxane (5 mL). The mixture was stirred and
BF3·OEt2 (65 μL, 0.50 mmol) added dropwise at room tempera-
ture under an argon atmosphere. When the reaction was
complete, the catalyst was filtered, washed with ether
(5 mL × 3), and then dried under vacuum for the next run. After
evaporation of the solvent from the filtrate under reduced pres-
sure, the product was purified by silica gel column chromatog-
raphy.
Acknowledgements
This work was supported by the Natural Science Foundation of
China (20962022), and Chengdu Key Technologies R & D
Program. We are also grateful to the Analytical & Testing
Center of Sichuan University for assistance in NMR and MS
analyses.
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