Application of a dimeric P,C-palladacycle complex as a catalyst in Suzuki and Heck cross-coupling reactions

Article abstract of DOI:10.1016/j.tetlet.2012.12.071

In this Letter the dimeric palladacycle [Pd(μ-C1)(P (OPh) ₂(OC₆H₄)]₂ containing a phosphorus donor atom is prepared and its catalytic activity was tested in the Suzuki reaction of phenylboronic acid at room temperature, and the Heck reaction of styrene at 130 °C with aryl halides of varying electron density. All reactions were monitored by GC and generally gave high yields of coupled products. Copyright

Full text of DOI:10.1016/j.tetlet.2012.12.071

Tetrahedron Letters 54 (2013) 1352–1355
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Application of a dimeric P,C-palladacycle complex as a catalyst in Suzuki
and Heck cross-coupling reactions
⇑
Kazem Karami , Mahdiyeh Ghasemi, Nasrin Haghighat Naeini
Department of Chemistry, Isfahan University of Technology, Isfahan 84156/83111, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this Letter the dimeric palladacycle [Pd(l-C1)(P (OPh)2(OC6H4)]2 containing a phosphorus donor atom
Received 1 November 2012
Revised 5 December 2012
Accepted 18 December 2012
Available online 28 December 2012
is prepared and its catalytic activity was tested in the Suzuki reaction of phenylboronic acid at room tem-
perature, and the Heck reaction of styrene at 130 °C with aryl halides of varying electron density. All reac-
tions were monitored by GC and generally gave high yields of coupled products.
Published by Elsevier Ltd.
Keywords:
Palladacycle
Palladium catalyst
Suzuki reaction
Heck reaction
Cyclopalladated complexes are very important materials in
organometallic chemistry.1,2 Palladacycles have gained significant
interest due to their applications in organic synthesis,3–5 materials
science,6 photochemistry,7 and in the pharmaceutical industry.8,9
These thermally and air-stable complexes with low toxicity are
used as catalysts in Suzuki,10,11 Sonogashira,12 Heck,13,14 and Stille
reactions.15,16 Suzuki and Heck reactions are important methods
for carbon–carbon bond formation, that involve a Pd(0)–Pd(II) cat-
alytic cycle.17–19 Among existing catalysts for these reactions, the
preferred classes of catalyst are usually palladacycles containing
a metalated carbon and a coordinating donor atom (P, N, S).20–22
By changing the ligands on the transition metal atom, the catalyst
properties can be altered. Bulky electron-rich phosphines tend to
increase the reaction performance and they also act as catalysts
themselves.23–25
Our initial investigation started with the cross-coupling reac-
tion of phenylboronic acid and bromobenzene as a Suzuki model
system. There are various related heterocycles that have been used
for room temperature Suzuki reactions.31–34 The reaction was car-
ried out in the presence of 0.5 mol % of the catalyst in THF/water
(2:1) at room temperature. The effect of various bases on the cou-
pling reaction was studied (Table 1, entries 1–5) and Et3N was
found to be the most effective. Using Et3N as the base, other sub-
strates were examined (Table 1, entries 6–16). Solvents, such as
acetonitrile (CH3CN), N,N-dimethylformamide (DMF), acetone,
and toluene were not effective (Table 1, entries 8, 9, 10, and 12).
The conversion of bromobenzene was low when THF was used as
the solvent (Table 1, entry 6). Also, the conversion of bromoben-
zene was reduced with a THF/water ratio of 1:1 rather than a ratio
of 2:1 (Table 1, entry 7).
Various aryl halides can be used in these reactions. Since the
bond strengths of C–Cl and C–Br are stronger than C–I (bond disso-
ciation energies for phenyl halides: Cl: 96 kcal/mol; Br: 81 kcal/
mol; I: 65 kcal/mol), their reactivity decreases in the order ArI >
ArBr > ArCl.26,19,27,28 As aryl chlorides are cheaper, more readily
available and practical, they are considered as the best substrates
for coupling reactions in comparison with their bromide or iodide
analogs.29
The conversion was found to decrease on increasing the reac-
tion temperature; the reasons for this are not clear (Table 1, entries
13 and 14). The highest yield was obtained at room temperature.
The color of the suspension turned black within a few minutes
due to the conversion of Pd(II) into the more active Pd(0)
OPh
OPh
O
The palladacycle [Pd(l-C1)(P(OPh)2(OC6H4)]2 was reported by
Albinati et al.30 This dimeric complex contains mixed phospho-
rus–carbon (P–C) donors and is shown in Scheme 1. In this Letter,
the catalytic activity of this complex was investigated in homoge-
neous Heck and Suzuki reactions.
P
Cl
Cl
O
Pd
Pd
P
OPh
PhO
⇑
Corresponding author. Tel.: +98 3113912351; fax: +98 3113912350.
E-mail address: karami@cc.iut.ac.ir (K. Karami).
Scheme 1. The dimeric palladacycle used in this work.
0040-4039/$ - see front matter Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2012.12.071

K. Karami et al. / Tetrahedron Letters 54 (2013) 1352–1355
1353
Table 1
Table 2 (continued)
Optimization of the reaction conditions for the Suzuki reaction of bromobenzene with
Entry
Ar-X
Product
Yieldb (%)
phenylboronic acida
Br
H3C
Entry
Solvent
Base
Temp (°C)
Time (h)
Yield (%)
CH3
1
2
3
4
5
6
7
8
THF:H2O (2:1)
THF:H2O (2:1)
THF:H2O (2:1)
THF:H2O (2:1)
THF:H2O (2:1)
THF
THF:H2O (1:1)
CH3CN
DMF
Acetone
MeOH
Toluene
THF:H2O (2:1)
THF:H2O (2:1)
THF:H2O (2:1)
THF:H2O (2:1)
K3PO4Á3H2O
Na2CO3
Et3N
K2CO3
KOH
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
50
70
rt
rt
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3.5
1
29
73
85
81
83
17
59
>15
<10
10
66
<10
77
62
60
22
9
39
H3C
CH3
Br
10
11
84
90
9
I
I
10
11
12
13
14
15
16
CH3
12
13
90
CH3
a
Reaction conditions: bromobenzene (0.5 mmol), phenylboronic acid
(0.55 mmol), base (1 mmol), solvent (4 ml), [Pd(
(0.005 mmol).
I
l-C1)(P(OPh)2(OC6H4)]2
NO2
100
NO2
Reaction conditions: derivatives of aryl halides (0.5 mmol), phenylboronic acid
Table 2
a
Suzuki reactions of various aryl halides with phenylboronic acid using 0.5 mol % of
(0.55 mmol), Et3N (1 mmol), THF:H2O (2:1), (4 ml), Pd catalyst (0.005 mmol), rt, 2 h.
the Pd catalysta
b
Determined by GC.
Table 3
Entry
1
Ar-X
Product
Yieldb (%)
47
Optimization of the reaction conditions for the Heck reaction of bromobenzene with
styrene
a
Cl
Entry
Solvent
Base
Temp (°C)
Time (h)
Yield (%)
1
2
3
4
5
THF:H2O (2:1)
THF:H2O (2:1)
DMF
DMF
DMF
Et3N
rt
rt
rt
130
130
2
2
5
3
4
Trace
Trace
Trace
86
K2CO3
K2CO3
K2CO3
K2CO3
Cl
NO2
92
2
97
a
Reaction conditions: bromobenzene (0.5 mmol), phenylboronic acid
NO2
Cl
(0.55 mmol), base (1 mmol), solvent (4 ml), [Pd(
(0.01 mmol).
l-C1)(P(OPh)2(OC6H4)]2
3
4
5
71
85
97
catalyst.35 As the catalyst is thermally stable and not sensitive to
oxygen or moisture, the reactions were carried out under air.
Increasing the reaction time did not lead to a higher conversion
of bromobenzene (Table 1, entry 15). The best conversion of 85%
for this reaction was attained at room temperature over 2 h
(Table 1, entry 3). We applied the optimized conditions to the
Suzuki cross-coupling reactions of different aryl halides with phen-
ylboronic acid. The results are summarized in Table 2.
The catalytic activity of the palladacycle was also studied in the
Heck reaction of bromobenzene with styrene using the optimized
Suzuki reaction conditions (Table 3, entry 1). As satisfactory yields
were not obtained, we tried to improve the conditions. Bases are
used for the regeneration of the catalyst by removing HX and leav-
ing the active Pd complex in the final stage of the catalytic cycle in
the Heck reaction, according to the well-established mecha-
nism.36,37 The yield of the Heck reaction of bromobenzene with
styrene at room temperature was negligible, and only a trace
amount of the product was obtained (Table 3, entries 1–3).
Moreover, a high temperature appears to be essential for the Heck
reaction to take place (Table 3, entries 4 and 5). The best results for
the Heck reaction of bromobenzene with styrene were obtained by
using 0.01 mmol of the Pd catalyst in DMF as the solvent and K2CO3
as the base at 130 °C for up to 3.5 h.
Br
Br
O
H
H
O
Br
Br
H
O
6
98
H
O
OCH3
7
8
60
60
OCH3
Br
CH3
CH3

1354
K. Karami et al. / Tetrahedron Letters 54 (2013) 1352–1355
Table 4
Heck reactions of various aryl halides with phenylboronic acid using 1 mol % of the Pd catalysta
Entry
1
Ar-X
Product
Yieldb (%)
52
Cl
Cl
NO2
CH3
2
3
4
95
44
85
NO2
Cl
CH3
Cl
O
H
H
O
Br
Br
5
6
92
75
OCH3
OCH3
Br
O
H
7
100
H
O
S
N
Br
8
9
89
25
S
Br
N
Br
H
O
10
95
30
H
O
O
Br
O
11c
a
Reaction conditions: aryl halide (0.5 mmol), styrene (0.6 mmol), K2CO3 (1 mmol), DMF (4 ml), Pd catalyst (0.01 mmol),
130 °C, 4 h.
b
Determined by GC.
Reaction was carried out with methyl acrylate instead of styrene.
c
The optimized conditions were applied to Heck reactions be-
tween styrene and various aryl halides (Table 4). As expected, aryl
halides with electron-withdrawing substituents in para positions
reacted smoothly; aryl halides with electron-donating substituents
were less reactive. Deactivated electron-rich aryl chlorides also
participated in these reactions but were poor substrates. The
activated electron-deficient substrate, 4-chlorobenzaldehyde and
4-chloronitrobenzene reacted to give relatively high yields of the
expected products (Table 2, entry 2 and Table 4, entries 2 and 4).
In conclusion, we have successfully employed
a dimeric
palladacycle complex as an efficient catalyst for the Suzuki and
Heck cross-coupling reactions of various aryl halides. In the Suzuki
reactions the corresponding products were obtained in yields in
the range 39–100%, and those of the Heck reactions were

K. Karami et al. / Tetrahedron Letters 54 (2013) 1352–1355
1355
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ring of the aryl halide. The advantage of these methods is that
the Suzuki reactions of aryl halides with phenylboronic acid were
performed at room temperature and that the Heck reaction worked
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This investigation was supported by the Department of
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