Heck-type reaction of aryldiazonium silica sulfates

Article abstract of DOI:10.1007/s00706-011-0632-1

Aryldiazonium silica sulfates were employed as new and efficient reagents for Heck-type arylation reactions with styrene and methyl acrylate. The reactions were carried out in an organic solvent, in the presence of Pd(OAc)₂ as precatalyst without use of base and free ligand. The trans isomers of desired products, methyl cinnamates and stilbenes, were obtained with full conversion. Use of a catalytic amount of Pd(OAc)₂ produced the corresponding products in short reaction times and excellent yields. Springer-Verlag 2011.

Full text of DOI:10.1007/s00706-011-0632-1

Monatsh Chem (2012) 143:791–795
DOI 10.1007/s00706-011-0632-1
ORIGINAL PAPER
Heck-type reaction of aryldiazonium silica sulfates
Azadeh Pirisedigh
•
•
Amin Zarei
Leila Khazdooz Abdol R. Hajipour
•
Hojjat Seyedjamali
•
Received: 22 June 2011 / Accepted: 19 August 2011 / Published online: 23 September 2011
Ó Springer-Verlag 2011
Abstract Aryldiazonium silica sulfates were employed as
new and efficient reagents for Heck-type arylation reactions
with styrene and methyl acrylate. The reactions were car-
ried out in an organic solvent, in the presence of Pd(OAc)2
as precatalyst without use of base and free ligand. The trans
isomers of desired products, methyl cinnamates and stilb-
enes, were obtained with full conversion. Use of a catalytic
preparation of natural products [8–11], fine chemicals, and
pharmaceuticals [12, 13].
The application of expensive electrophiles such as aryl
iodides, bromides, or triflates is a drawback for this sig-
nificant reaction. The interesting use of diazonium salts
instead of halides or triflates has been preferred in the last
decade [14–19] owing to the higher reactivity of diazoni-
ums which produce a better leaving group (nitrogen) than
halides or triflates [20, 21]. Moreover, the use of the more
readily available aniline derivatives as the arenediazonium
salt precursors has more economic benefits in comparison
with their halide or triflate alternatives [22–26]. Also, these
reactions can usually be carried out under milder condi-
tions, shorter reaction times, and with higher yields than
conventional Heck reactions without using a base and
additional ligands [27, 28].
amount of Pd(OAc) produced the corresponding products
2
in short reaction times and excellent yields.
Keywords Catalyst Á Arylation Á
Transition metals compounds Á Silicates
Introduction
The palladium-catalyzed Heck coupling reaction of aryl
halides and alkenes [1, 2] was first reported in the early
Because of the instability and explosive nature of dia-
zonium salts, their reactions must be carried out in the
same medium in which they are produced [29]. Therefore,
extended investigations have been aimed at the synthesis
of arenediazonium salts which are stable under dry con-
ditions [30, 31]. The application of arenediazonium salts
in Heck-type reactions is highly dependent on the nature
of the diazonium counter-anions [32]. These properties
are distinctive to aryl diazonium tetrafluoroborates
1
970s and is still one of the most important C–C bond
formation reactions in synthetic chemistry [3–7] and the
A. Pirisedigh (&) Á H. Seyedjamali
Department of Chemistry, Islamic Azad University,
Kazerun Branch, Kazerun, Iran
e-mail: a.pirisedigh@yahoo.com; azadeh.pirisedigh@kau.ac.ir
[
33], hexafluorophosphates [34–37], and arenediazonium
A. Zarei
Department of Science, Islamic Azad University, Fasa Branch,
P.O. Box 364, 7461713591 Fasa, Fars, Iran
o-benezenedisulfonimides [37, 38].
In following up our research on C–C coupling reactions
using ortho-palladated complexes as precatalysts [39–41],
we have employed various arenediazonium salts synthe-
sized previously [42–44] as electrophiles with styrene and
methyl acrylate as olefins in Heck-type coupling reactions.
L. Khazdooz
Department of Science, Islamic Azad University,
Khorasgan Branch, 81595158 Isfahan, Iran
?
–
The stability of aryldiazonium salts, ArN2 OSO SiO ,
3
2
A. R. Hajipour
Pharmaceutical Research Laboratory, College of Chemistry,
Isfahan University of Technology, 84156 Isfahan, Iran
was investigated at room temperature (RT) and the prod-
1 13
ucts were characterized by H NMR, C NMR, and IR.
123

7
92
A. Pirisedigh et al.
Scheme 1
O
O
OCH3
Ar-N2 OSO -SiO2
3
Ar
OCH3
Ar
Pd(OAc)2
EtOH, RT
Pd(OAc)2
EtOH, RT
5
4
3
Ar = Ph, 4-CN-Ph, 4-CH CO-Ph, 2-
Ar = Ph, 4-CN-Ph, 4-CH CO-
3
3
NO -Ph, 2-PhCO-Ph, 4-CH -Ph, 2-
Ph, 2-NO -Ph, 2-PhCO-Ph,
2
2
3
4
-CH -Ph, 2-CH -Ph, 4-Br-
CH -Ph, 4-Br-Ph, 4-NO -Ph, 4-Cl-
3 3
3
2
Ph, 4-CO H-Ph, 2-CO H-Ph
Ph, 1-Naph
2
2
Results and discussion
Table 1 Optimization of catalyst concentration
O
O
Synthesis of arenediazonium salts
OCH3
N2 OSO SiO
OCH3
3
2
Pd(OAc)2
EtOH
The instability of diazonium salts has often limited their
applications as intermediates in organic synthesis. Usually,
these compounds are synthesized at around 10 °C and
handled below 0 °C to avoid their decomposition. In pre-
vious work [42–44] different kinds of aromatic amines
were rapidly converted to the corresponding diazonium
salts under solvent-free conditions at RT using amine
Catalyst/mol%
Time/min
Temperature
Conversion/%
None
30
10
10
10
30
RT
RT
RT
RT
RT
0
100
100
100
50
8
4
(
(
1 mmol), silica sulfuric acid (0.7 g), and sodium nitrite
2 mmol) which were ground in with a pestle a mortar for a
2
1
.5
few minutes to obtain an homogeneous mixture. After that,
a few drops of water were slowly added and the reaction
mixture was ground for 10–15 min until the complete
evolution of nitrous acid gas (generated via the reaction of
excess nitrite under acidic conditions). The corresponding
arenediazonium salts were prepared quantitatively.
The aforementioned conditions were applied in the
Heck-type reaction of a variety of aryldiazonium salts 3
with methyl acrylate. Both electron-withdrawing and
electron-donating groups (independent of their position on
the aromatic ring, whether ortho or para) led to the pro-
duction of the desired coupling compounds in excellent
yields (Table 2, entries 1–10).
Heck-type arylation of aryldiazonium silica sulfates
The application of arenediazonium salts on the coupling
reactions with two different olefins in the presence of a
All of the methyl cinnamate derivatives were obtained
in trans configuration. Entries 9 and 10 of Table 2 confirm
that the reactions produce high chemoselectivity, because
in the presence of diazonium groups, bromo or chloro
moieties remain unchanged.
catalytic amount of Pd(OAc) without using a base or
2
additional ligands at RT has been investigated (Scheme 1).
The synthesis of methyl cinnamate derivatives 4 was
investigated because of their importance in the synthesis of
a vast variety of agrochemicals, as antioxidants in plastics,
and their use as starting materials in the synthesis of a great
set of pharmaceuticals [45, 46].
Besides the Table 2 entries, 1-naphthyldiazonium silica
sulfate was synthesized and treated by methyl acrylate
under the same reaction conditions and the related coupling
product was obtained in 65% yield after 1 h.
In order to evaluate the solvent effects the Pd(OAc)2-
catalyzed coupling reactions of phenyldiazoium silica sul-
fate with methyl acrylate at RT were carried out in several
organic solvents such as ethanol, tetrahydrofuran (THF),
CH Cl , CH CN, and toluene. Accordingly ethanol turned
To further explore the scope of the reaction, the pro-
duction of several stilbene derivatives 5 through Heck-type
reactions of various arenediazonium salts and styrene was
examined as well (Table 3, entries 1–10). These beneficial
compounds are used as the key monomers for the synthesis
of conjugated aromatic oligomers and polymers [47].
Interestingly, trans-stilbene, which is the preferred isomer
for the fabrication of the conjugated polymers, was
obtained via the described procedure.
2
2
3
out to be the best solvent. A variety of catalyst concentrations
were tested and 2.5 mol% gave the best results (Table 1).
The optimized reaction conditions were the use of eth-
anol as reaction medium and 2.5 mol% of Pd(OAc) at RT.
2
It is interesting to mention that the reaction under the stated
conditions proceeded successfully in the absence of
ligands, bases, and nitrogen atmosphere.
The molar ratio of diazonium salts to styrene was 1:1.5
and the same molar amount of Pd(OAc) was used as
2
catalyst. The reactions were carried out in ethanol at RT
1
23

Heck-type reaction of aryldiazonium silica sulfates
793
Table 2 Heck-type reaction of aryldiazonium salts with methyl
Table 4 shows the results obtained from the Heck-type
2
acrylate using Pd(OAc) as catalyst
reactions using arenediazonium o-benzenedisulfonimides 1
[
48], tetrafluoroborates 2 [20, 49, 50], and the reported
aryldiazonium silica sulfates 3. The comparison of theses
data confirms that the presented study has provided a rel-
atively rapid manner to perform Heck-type reactions in
high yields. Furthermore, the difficulties of the substrates
containing nitro substitutions [51] are overcome com-
pletely (Table 2, entries 8, 9 and Table 3, entry 4).
a
Entry
Product
X
Y
Time/min
Yield/%
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
H
H
10
15
20
15
15
45
15
15
15
15
88
84
82
85
84
82
92
88
91
87
CH
H
3
3
H
Conclusion
CH
H
3
CN
CH
H
We have used the previously synthesized aryldiazonium
silica sulfates as efficient substrates for the C–C bond
formation with methyl acrylate and styrene to form the
corresponding trans-cinnamates and trans-stilbenes,
respectively, using palladium catalyst at RT. The stated
products were obtained in excellent yields using either
electron-withdrawing or electron-donating substituents
independent of their positions on the aromatic rings.
Additionally, it is to be noted that arenediazonium salts
involving bromide or chloride substituents on the aromatic
ring showed high chemoselectivity in that essentially only
the diazonium group reacted. Further investigations of the
applications of these aryldiazonium silica sulfates as stable
diazonium salts are ongoing in our laboratories.
CO
H
COPh
H
4g
4h
4i
NO
H
2
NO
H
2
Br
Cl
1
0
4j
H
Reaction conditions: aryldiazonium silica sulfate, 1 mmol; methyl
2
acrylate, 1.5 mmol; catalyst Pd(OAc) , 0.025 mmol; RT
a
Isolated yield
Table 3 Heck-type reaction of aryldiazonium salts with styrene
using Pd(OAc) as catalyst
2
Experimental
FT-IR spectra were recorded on a Jasco-680 spectropho-
tometer (Japan). NMR spectra were recorded in CDCl3
solutions at RT on a Bruker 400 MHz spectrometer (TMS
was used as an internal standard).
a
Entry
Product
X
Y
Time/min
Yield/%
1
2
3
4
5
6
7
8
9
5a
5b
5c
5d
5e
5f
H
H
30
60
70
30
75
40
50
75
80
90
87
86
86
85
84
83
87
93
85
90
All reagents were purchased from Merck and Aldrich
and used without further purification. Aryldiazonium silica
sulfates were prepared according to the previous work [42–
44]. The products were characterized by comparison with
CN
CH
H
H
3
CO
H
NO
2
1
H
COPh
H
authentic samples and by spectroscopic data (FT-IR, H
1
NMR, C NMR).
3
COOH
H
5g
5h
5i
COOH
H
CH
H
3
Heck-type reactions of aryldiazonium silica sulfate
with methyl acrylate and styrene
CH
H
3
1
0
5j
Br
To
a solution of Pd(OAc)2 (2.5 mol%, 0.0056 g,
Reaction conditions: aryldiazonium silica sulfate, 1 mmol; methyl
0
.025 mmol) and methyl acrylate or styrene (1.5 mmol) in
2
acrylate, 1.5 mmol; catalyst Pd(OAc) , 0.025 mmol; RT
3
a
10 cm EtOH, aryldiazonium silica sulfate (1 mmol) was
added with stirring in one portion and the reaction mixture
was stirred at RT. A plentiful evolution of nitrogen took
place. The reaction progress was monitored by TLC
(hexane/EtOAc, 75:25). After completion of the reaction,
Isolated yield
and the products with trans geometry were prepared. The
conversions were 100% except for entry 8 which had a
conversion of 90%.
3
the mixture was diluted with 10 cm EtOAc and filtered
123

7
94
A. Pirisedigh et al.
Table 4 Comparison of aryldiazonium silica sulfate (3) in reaction of styrene or methyl acrylate at RT with other aryldiazonium salts (1, 2)
?
2
Ar-N
Ar
Olefin
Catalyst/mol%
Solvent
Time/min
Yield/%
a
1
3
2
2
3
3
2
2
3
3
2
2
3
3
a
[48]
4-Me-Ph
4-Me-Ph
Ph
Styrene
1
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
BmimPF
BmimPF
EtOH
EtOH
THF
840
60
93
97
38
87
85
76
82
88
91
75
91
85
90
Styrene
2.5
10
5
75
255
240
30
[20]
[20]
Styrene
2-Me-Ph
Ph
Styrene
Styrene
2.5
2.5
2
2-Me-Ph
Ph
Styrene
80
[49]
[49]
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Styrene
6
6
180
180
10
4-Br-Ph
Ph
2
2.5
2.5
2
4-Br-Ph
2-Me-Ph
4-Br-Ph
2-Me-Ph
4-Br-Ph
15
[50]
[50]
300
240
80
Styrene
2
THF
Styrene
2.5
2.5
EtOH
EtOH
Styrene
90
At 70 °C
after vigorous stirring. The residue was washed with
3
EtOAc (3 9 10 cm ) and the combined organic layer was
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