Activation of CX (=Cl, Br) bond in aryl halides toward the palladium-catalyzed Heck reaction using 2,6-bis(diphenylphosphino)pyridine

Article abstract of DOI:10.1016/j.molcata.2012.08.025

The 2,6-bis(diphenylphosphino)pyridine/palladium catalytic system successfully catalyzes the Heck coupling reaction of less reactive aryl chlorides as well as aryl bromides with styrene to give the corresponding olefins in reasonable yields. TBAB (tetrabu

Full text of DOI:10.1016/j.molcata.2012.08.025

Journal of Molecular Catalysis A: Chemical 366 (2013) 30–35
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Activation of C X (=Cl, Br) bond in aryl halides toward the palladium-catalyzed
Heck reaction using 2,6-bis(diphenylphosphino)pyridine
Ali Ataeia, Shirin Nadria, Ezzat Rafieea, Sirous Jamalib, Mohammad Joshaghania,∗
a Faculty of Chemistry, Razi University, Kermanshah 67149, Iran
b Chemistry Department, Sharif University of Technology, P.O. Box 11155-3615, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2012
Received in revised form 26 August 2012
Accepted 27 August 2012
Available online 3 September 2012
The 2,6-bis(diphenylphosphino)pyridine/palladium catalytic system successfully catalyzes the Heck cou-
pling reaction of less reactive aryl chlorides as well as aryl bromides with styrene to give the corresponding
olefins in reasonable yields. TBAB (tetrabutylammoniumbromide) as an additive was found to be essen-
tial for these reactions. The results of Heck reaction exhibited a high selectivity (>99/1) favoring the trans
product.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
C
C coupling
Heck reaction
2,6-Bis(diphenylphosphino)pyridine
Palladium catalyst
1. Introduction
active species during the catalytic process, which manipulate the
activity of the catalyst. Also, increasing the rigidity of the suppor-
Palladium-catalyzed Heck coupling has been recognized as one
of the most important tools for building up of carbon carbon bond
and represents the best technology for the synthesis of the arylated
olefins [1–5]. In recent years, numerous papers have been reported
concerning improvements to the Heck reaction, but it still is an
attractive synthetic goal in academia as well as in industry [6–9].
In Heck reaction, depending on the reaction conditions includ-
ing the properties of the aryl substrate, olefin, solvent and catalytic
system a mixture of arylated products, trans/cis isomers (␤-
arylation), and gem isomer (␣-arylation) can be formed (Scheme 1).
On the other hand, the readily accessible and cheaper aryl chlo-
rides have not been employed much in cross-coupling reactions,
due to their reluctance to oxidatively add to Pd(0) center. There-
fore, coupling aryl chlorides as industrially important compounds
by Heck reaction has become another equally important topic of
research [10–12].
ting ligand decreases the number of the possible conformations of
the metal–ligand assembly, thus leading to improved selectivity
[13,14].
The presence of both soft and hard donor sites (e.g. P and
N) in phosphorus–nitrogen containing ligands forms an ideal
acceptor–donor pair for catalytic uses [15–17]. Indeed, the ␲-
acceptor ability of the phosphorous atom, together with ␴-donor
nature of nitrogen, synergistically induce an alteration of the cata-
lyst reactivity, though the stabilization of the low-valent transition
metals (by phosphine), and promote the oxidative addition of the
aryl halide on metal center (by nitrogen).
The juxtaposition of a pyridine and two phosphine moieties,
presenting a P–N–P framework, in a 2,6-relationship of type
shown in Fig. 1, has found considerable attention in the field of
organometallic chemistry [18–22].
Of
special
interest
in
this
context
is
2,6-
On the basis of these considerations, it is important to develop
rewarding catalytic system that meets the goals of high selectiv-
ity and high activity to aryl chlorides. It is clear that the nature
of the ligand plays an important role in determining the catalytic
activity. The use of multifunctional ligands gives advantage as
high capacity chelating molecules for the stabilization of catalytic
bis(diphenylphosphino)pyridine, (Ph2P)2py (1), first synthesized
by Newkome and Hager [23], owing to its rigid backbone. Absence
of the methylene groups connecting the phosphorus atoms and
the pyridine ring make (Ph2P)2py (1), more rigid than other
phosphine–pyridine ligands presented in Fig. 1. The coordina-
tion chemistry of (Ph2P)2py has been well studied [24–28], but
relatively scarce attention has been given to its application as a
supporting ligand in catalysis.
Because of rigidity and multi-donor nature of (Ph2P)2py, and
also based on our experience in C C coupling reaction [29–35],
herein, we thought to use this ligand in Heck reaction of aryl
∗
Corresponding author. Tel.: +98 831 4274559; fax: +98 831 4274559.
E-mail addresses: mjoshaghani@razi.ac.ir,
joshaghanim@yahoo.com (M. Joshaghani).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.025

A. Ataei et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 30–35
31
Pd complex
Base
R'
+
+
+
X
R"
+
R"
Base-HX
R'
R'
R"
R"
R'
trans
gem
cis
R' = aryl, vinyl
R" = aryl, CO R, OR
2
_
+
X= halide, OSO R, N X , COCl
2
2
Scheme 1. The Heck coupling reaction, principle and possible isomers.
P(But)2
a dry nitrogen atmosphere. A solution of (Ph2P)2py (0.05 mol%
in 2 mL of DMAc) and prepared solution of palladium acetate
(0.025 mol% in 2 mL) was added through a rubber septum, and
the resulting mixture was refluxed at 135 ◦C for appropriate
time. Upon completion of the reaction, the reaction mixture was
then cooled to room temperature. After extraction with CH2Cl2
(3 × 20 mL), the combined organic layer was dried over MgSO4.
The solvent was evaporated and the crude product was char-
acterized by 1H NMR spectroscopy. Isolated yield of coupling
products of chlorobenzene, and 4-bromotoluene were 82% and 86%,
respectively.
PPh2
PPh2
PPh2
N
P(Cy)2
N
N
N
P(But)2
PPh2
P(Cy)2
1
4
2
3
Fig. 1. Selected examples of 2,6-disubstituted phosphine-pyridine ligands.
chlorides as well as aryl bromides. To the best of our knowledge, this
is the first report for the use of (Ph2P)2py for palladium-catalyzed
C
C coupling reactions with high efficiency.
3. Results and discussion
2. Experimental
3.1. Initial optimization
2.1. General remarks
By employing a reaction of chlorobenzene and styrene as a
probe, the reaction conditions were optimized. We were pleased
to find that the reaction proceeded smoothly with only 0.05 mol%
Pd(OAc)2/0.1 mol% (Ph2P)2py when K2CO3 and DMAc were used
as the base and solvent respectively. Thus, after 24 h at 135 ◦C,
the product trans-stilbene was obtained in 75% yield, correspond-
ing to a turnover number (TON) of 1500 (Table 1, entry 1). It
should be noted that in the absence of (Ph2P)2py (ligand free
condition) no amount of the desired product was formed (not
shown in the table). When the palladium loading was reduced
to 0.025 mol% the process was less efficient (Table 1, entry 2).
Significantly lower yield (20%, 24 h) was obtained in the pres-
ence of triethylamine as a representative organic base (Table 1,
entry 3). One possible interpretation of this result is blocking
of free coordination sites on the palladium center by amine
[36,37].
All reactions were performed under an atmosphere of
dry nitrogen. All chemicals purchased from Fluka and/or
Merck companies were used without further purification. 2,6-
bis(diphenylphosphino)pyridine was commercially available.
1H (200 MHz), 13C (100 MHz) NMR spectra were recorded on a
Bruker Avance Spectrometer. Elemental analysis was performed
using CHN Herause rapid model. Thin layer chromatography
on precoated silica gel Fluorescent 254 nm (0.2 mm) on alu-
minum plates were used for monitoring the reactions. The
cross coupling products were characterized by their 1H NMR
spectra.
2.2. General procedure for Heck coupling of styrene with aryl
halides
A reaction tube was charged with chlorobenzene (4 mmol),
styrene (4 mmol), TBAB (3 mmol), and K2CO3 (4 mmol) under
The effect of tetraalkylammonium salts, the so-called Jef-
fery condition [38], was also considered. Addition of 0.5 mmol
Table 1
Optimizations of reaction condition.a
Pd(OAc) / (Ph P) py
2
2
2
Cl
+
Ph
base, solvent, 135 °C
Ph
Entry
Pd(OAc)2 (mol%)
Base
Solvent
TBAB (mmol)
Yield (%)b
25 (75)
TON
500 (1500)
480 (2200)
0 (800)
1200 (2200)
2120 (3600)
2280 (3600)
2280 (3600)
100 (3000)
0 (0)
1
2
3
0.05
K2CO3
K2CO3
NEt3
K2CO3
K2CO3
K2CO3
K2CO3
–
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMSO
Toluene
DMF
–
–
–
0.5
1.5
3
5
5
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
0.025
12 (55)
0 (20)
4
30 (55)
53 (90)
57 (90) [82]
58 (92)
25 (75)
0 (0)
5
6c
7
8
9
10
11
K2CO3
K2CO3
K2CO3
3
3
3
0 (15)
48 (67)
0 (600)
1920 (2680)
a
Reaction conditions: chlorobenzene (4 mmol), styrene (4 mmol), K2CO3 (4 mmol), Pd/(Ph2P)2py and DMAc (4 mL), 135 ◦C, 5 h. Only trans isomer was observed in the 1
H
NMR spectrum.
b
NMR yield. The numbers in parentheses are from the reaction time 24 h.
The number in the bracket is isolated yield after 24 h.
c

32
A. Ataei et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 30–35
Table 2
Heck reaction of aryl chlorides with styrene.a
Ph
Ph
R
Pd(OAc) / (Ph P) py
R
2
2
2
Ph
R
R
Cl
+
+
+
Ph
°
K CO , DMAc, TBAB, 135 C
2
3
trans
cis
gem
c
d
Entry
1e
Aryl chloride
Trans (%)b
Cis (%)b
KSI
gem (%)b
KRI
TON
Cl
57 (90) [82]
0 (0)
>99/1
0 (0)
>99/1
2280 (3600)
Cl
2
3
75 (93)
62 (72)
0 (0)
0 (0)
>99/1
>99/1
0 (0)
0 (0)
>99/1
>99/1
3000 (3720)
3000 (3720)
H2N
Cl
MeOC
Cl
4
5
6
32 (46)
28 (81)
20 (52)
0 (0)
0 (0)
0 (0)
>99/1
>99/1
>99/1
0 (0)
0 (0)
0 (0)
>99/1
>99/1
>99/1
1280 (1840)
1120 (3240)
800 (2080)
MeOC
Cl
OHC
MeO
Cl
Cl
7
8
52 (82)
62 (78)
0 (0)
0 (0)
>99/1
>99/1
0 (0)
0 (0)
>99/1
>99/1
2080 (3280)
2480 (3120)
PhOC
HO
Cl
Cl
9
48 (65)
67 (80)
0 (0)
0 (0)
>99/1
>99/1
0 (0)
0 (0)
>99/1
>99/1
1920 (2600)
2680 (3200)
N
O2N
Cl
10
a
Reaction conditions: aryl chloride (4 mmol), styrene (4 mmol), K2CO3 (4 mmol), Pd(OAc)2 (0.025 mol%), (Ph2P)2py (0.05 mol%), TBAB (3 mmol) and DMAc (4 mL), 135 ◦C,
5 h.
b
NMR yield. The numbers in parentheses are from the reaction time 24 h.
c
d
e
Stereoisomer ratio KSI = trans/cis. The cis isomer was not observed in the 1H NMR spectrum.
Regioisomer ratio KRI = ˇ/˛, ˇ = sum of trans and cis isomers, ˛ = geminal isomer (gem). The gem isomer was not observed in the 1H NMR spectrum.
The number in the bracket is isolated yield after 24 h.
of tetra-n-butylammonium bromide (TBAB) had just a moderate
DMF gave acceptable conversion (93%) but a small amount of cis-
isomer was also formed (Table 1, entry 11). Thus, among the tested
solvents, DMAc (dimethylacetamide) in regard to both yield and
selectivity was chosen as the preferred solvent for the reactions.
Thus, on comparing results of Table 1, it was seen that the com-
bination of Pd/(Ph2P)2py and DMAc as the solvent, K2CO3 as the
base and 3 mmol TBAB as additive gives the best result.
effect on the reaction rate (Table 1, entry 4), while it increased
markedly in the presence of 1.5 or 3 mmol of TBAB (Table 1,
entries 5 and 6). Adding more TBAB (5 mmol) could not fur-
ther improve the yield (Table 1, entry 7). TBAB is proposed to
increase the solubility of the inorganic base K2CO3 in the organic
reaction medium and strengthened the stabilizations of Pd(0)
species [39,40].
In the next step several solvent systems have been examined.
The reaction could not proceed in DMSO and no conversion was
observed even when reaction lifted for 24 h (Table 1, entry 9). With
toluene as non-polar solvent, the reaction was extremely slow
and afforded stilbene in 15% after 24 h (Table 1, entry 10). Using
3.2. Heck coupling reactions of various aryl chlorides
To survey the generality of Pd/(Ph2P)2py catalysis system,
we next investigated the reactions of a variety of aryl chlorides
with styrene under the identical conditions. Results are listed

A. Ataei et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 30–35
33
Table 3
Comparison of Heck coupling of chlorobenzene and styrene using Pd/(Ph2P)2py and other catalytic systems.
Entry
Catalytic system
Pd (mol%)
Reaction conditions
Yield (%)a
Ref.
–
[41]
[42]
[43]
[44]
1
Pd/(Ph2P)2py
[Pd(C3H5)Cl]2
Pd(OAc)2[dpme]
PdCl2/wool
Pd/mesoporous NiFe2O4
0.025
–
0.1
5.5
0.4
K2CO3, DMAc, TBAB, 135 ◦C, 24 h
Na2CO3, DMAc, TBAB, 120 ◦C, 24 h
KOH, DMF, 80 ◦C, 14 h
90 (NMR)
2
Trace (isolated)
4 (isolated)
20 (isolated)
9 (isolated)
3b
4
NaOAc, aqueous media, PEG-400, 80 ◦C, 24 h
NEt3, DMF, 140 ◦C, 24 h
5
O
O
N
N
O
6
7
0.1–0.5
NEt3, DMAc, 120 ◦C, 8 h
0 (GC, isolated)
[45]
Pd
O
CH
3
CH
3
N
P
0.5
K2CO3, DMF, 100 ◦C, 24 h
12 (GC and GCMS)
[46]
N
Pd
AcO
OAc
8
9c
Pd2(dba)3/P(t-Bu)3
Pd2(dba)3/P(t-Bu)3
[Pd]/[(t-Bu)3PH]BF4
3
3
5
Cs2CO3, dioxane, 120 ◦C, 21 h
83 (isolated)
78 (isolated)
65 (isolated)
[47]
[48]
[49]
Cy2NM, dioxane, r.t.
10d
Cy2NMe, bmimPF6-dioxane, MWꢀ, 180 ◦C, 60 min
H2
N
O
O
N
N
Cl
Cl
Pd
11
0.1 mmol
NEt3, r.t., 8 h
83 (HPLC)
[50]
N
N
N
O
O
H2
Cl
N
N
N
Pd
Cl
12
1
K2CO3, DMF/H2O, CuI, 343 K, 36 h
6 (GCMS)
[51]
N
CH2Ph
PhH2C
a
Trans-isomer exclusively.
Methylacrylate has been used as olefin substrate.
4-Chloroacetophenone has been used as aryl halide substrate.
Butylacrylate has been used as olefin substrate.
b
c
d
in Table 2. Various types of electronically different aryl chlo-
rides were converted into the corresponding coupling products
in good to excellent yields with very low palladium loading (only
0.025 mol%). However, in the cases of 3-chloroacetophenone and 4-
chlorobenzaldehyde the yield of corresponding coupled products
decreased dramatically to 46% and 52% yield respectively due to
dehalogenation reaction (Table 2, entries 4 and 6).
addition, some catalyst systems create practical problems because
of high ligand sensitivity toward air and moisture and multistep
synthesis procedures. Table 3 illustrates that the Pd/(Ph2P)2py gives
higher yield in very lower catalyst loading in comparison with other
systems.
The Heck reaction of sterically hindered 1-chloronaphthalene
afforded a good yield of the coupled product (Table 2, entry 2).
Also, as reported in Table 2, the heteroaryl chloride, including the
2-chloro-5-nitro-pyridine, gave high yield (80%) (Table 2, entry 10).
As shown in Table 2, the corresponding trans-stilbenes
were obtained with complete regioselectivity at the ␤-position
(KRI > 99/1). In addition, no cis olefin products were observed
(KSI > 99/1).
We used the reaction of chlorobenzene with styrene to com-
pare the efficacy of the Pd/(Ph2P)2py with other reported catalysis
systems. Several goals have to be achieved for the utilization of
aryl chlorides in Heck reaction, such as the use of stable and
inexpensive starting materials and ligands, achievement of high
turnover numbers (TONs), use of water and ionic liquids as reaction
medium, and easy separation of catalyst. However, the high palla-
dium loadings and long reaction times in some reported systems
are not beneficial to the industrial and synthetic applications. In
3.3. Heck coupling reactions of various aryl bromides
We also examined the Heck arylation of styrene with vari-
ous bromides with different electronic and steric effects using
Pd/(Ph2P)2py catalysis system (Table 4). Full conversion of aryl bro-
mides was obtained in all cases when 0.025 mol% Pd was applied.
As shown in Table 3, even for the aryl bromides bearing a electron-
donor functional groups, such as OMe the reaction was finished
within 5 h at a 0.025 mol% Pd loading (Table 4, entry 5). Ary-
lation of styrene with 4-bromobiphenyl was completed within
2 h bringing about a turnover number of 4000 (Table 4, entry
8). It is noteworthy that a high selectivity (>99/1) favoring the
trans products (KRI and KSI ꢀ 99/1) was also observed in the Heck
reaction of aryl bromides with (Ph2P)2py as same as aryl chlo-
rides.
As aforementioned, rigidity of ligand could improve selectivity
of reaction. In agreement with this theory, our experiments showed

34
A. Ataei et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 30–35
Table 4
Heck reaction of aryl bromides with styrene.a
Ph
Ph
R
Pd(OAc) / (Ph P) py
R
2
2
2
Ph
R
R
Br
+
+
+
Ph
°
K CO , DMAc, TBAB, 135 C
2
3
trans
cis
gem
c
d
Entry
1e
Aryl bromide
Trans (%)b
Cis (%)b
KSI
gem (%)b
KRI
TON
H3C
Br
72 (99, 6 h) [86]
78 (99, 3 h)
0 (0)
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
0 (0)
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
>99/1
2880 (4000)
OHC
Br
2
3
4
5
6
7
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
3120 (4000)
3440 (4000)
2000 (4000)
2640 (4000)
1680 (4000)
2040 (4000)
Br
86 (99, 3 h)
MeOC
MeO
H2N
Br
50 (98, 10 h)
66 (99, 5 h)
Br
Br
42 (98, 10 h)
51 (99, 10 h)
O2N
Br
Br
8
92 (99, 2 h)
68 (98, 5 h)
0 (0)
0 (0)
>99/1
>99/1
0 (0)
0 (0)
>99/1
>99/1
3680 (4000)
2720 (4000)
Br
9
a
Reaction conditions: aryl bromide (4 mmol), styrene (4 mmol), K2CO3 (4 mmol), Pd(OAc)2 (0.025 mol%), (Ph2P)2py(0.05 mol%), TBAB (3 mmol) and DMAc (4 mL), 135 ◦C,
1 h.
b
NMR yield.
c
d
e
Stereoisomer ratio KSI = trans/cis. The cis isomer was not observed in the 1H NMR spectrum.
Regioisomer ratio KRI = ˇ/˛, ˇ = sum of trans and cis isomers, ˛ = geminal isomer (gem). The gem isomer was not observed in the 1H NMR spectrum.
The number in the bracket is isolated yield after 6 h.
that adding (Ph2P)2py dramatically increased the selectivity of the
Acknowledgment
Heck reactions toward the trans-isomers.
Rate acceleration were observed in the reactions of aryl
bromides with styrene compared to aryl chlorides, primarily
because the bond strength of C Cl is stronger than C Br, thus
the rate of oxidative addition C Cl to Pd(0) species became
slow.
The authors thank the Razi University Research Council for sup-
port of this work.
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