Cyclopalladated complexes of tertiary arylamines as highly efficient catalysts using in the Heck reactions

Article abstract of DOI:10.1016/S0022-328X(02)01195-6

Cyclopalladated complexes of tertiary arylamines are reported in this paper as highly efficient catalysts for the Heck reactions of iodoarenes with methylacrylate. The monomeric complexes (2 and 4), which contain mixed phosphorous-nitrogen (P-N) donors, w

Full text of DOI:10.1016/S0022-328X(02)01195-6

Journal of Organometallic Chemistry 651 (2002) 146–148
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Cyclopalladated complexes of tertiary arylamines as highly efficient
catalysts using in the Heck reactions
Fan Yang, Yangming Zhang, Rui Zheng, Jie Tang *, Mingyuan He
Center for the Chemistry of Ionic Liquids, Department of Chemistry, East China Normal Uni6ersity, 3663 Zhongshanbei Road, Shanghai 200062,
China
Received 19 October 2001; received in revised form 7 January 2002; accepted 9 January 2002
Abstract
Cyclopalladated complexes of tertiary arylamines are reported in this paper as highly efficient catalysts for the Heck reactions
of iodoarenes with methylacrylate. The monomeric complexes (2 and 4), which contain mixed phosphorous–nitrogen (P–N)
donors, were found to be more active than the corresponding dimeric ones, which contain only single nitrogen donor. Palladium
mirror was observed, indicating the involvement of classic Pd(0)/Pd(II) catalytic cycle using these cyclopalladated complexes.
©
2002 Elsevier Science B.V. All rights reserved.
Keywords: Heck reaction; Cyclopalladated complex; Palladium mirror; Palladium catalyst
Olefination of aryl halides, known as the Heck reac-
tions, is an important transition-metal-catalyzed reac-
tion for organic synthesis [1]. However, these coupling
reactions used to suffer from the need of high amounts
of palladium catalyst (1–5 mol%). Recently, very effi-
cient catalysis using cyclopalladated phosphines [2],
phosphites [3] and imines [4], carbene [5], and dimethyl-
glycine [6] have been investigated, but the Heck reac-
tions catalyzed by cyclopalladated tertiary arylamines
have rarely been reported. This paper reports a study
Among complexes 1–4, the monomeric complexes (2
and 4), which contain mixed phosphorous–nitrogen
(P–N) donors, were found to be more active than the
corresponding dimeric ones (1 and 3), which contain
only single nitrogen donor. In the coupling of iodoben-
zene with methylacrylate catalyzed by complex 4 in
DMA (N,N-dimethylacetamide) at 140 °C, the TON up
to 9 600 000 and turnover frequencies (TOF) up to
8
70 000 were obtained, respectively. This is the highest
catalytic activity of all Heck reactions reported to date.
on the catalytic activity of these complexes using in the
Heck reactions of iodoarenes with methyl acrylate with
turnover numbers (TON) more than 10 in some cases.
The complexes 1–4 are thermally stable and not
sensitive to oxygen and moisture for the metal center
are stabilized by a five-member ring. These complexes
were conveniently prepared according to the literature
methods [7]. For example, the dimeric complexes are
readily prepared by treatment of Li [PdCl ] or
6
*
Corresponding author. Fax: 86-21-5251-0411.
E-mail address: tangecnu@sina.com (J. Tang).
2
4
0
022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(02)01195-6

F. Yang et al. / Journal of Organometallic Chemistry 651 (2002) 146–148
147
Table 1
The results in Table 2 showed that complexes 1–4
are more efficient for the Heck reactions with TON of
10 in some cases as compared with traditional catalysts
a
Heck reactions of iodobenzene with methylacrylate
6
Yield (%) b
Run
Solvent
Catalyst
Pd(PPh ) or PdX –PPh . Meanwhile, the monomeric
3
4
2
3
1
2
3
4
5
6
7
8
1,4-Dioxane
1,4-Dioxane
Toluene
1
2
1
2
1
2
1
2
34
55
35
58
38
66
81
100
complexes (2 and 4) were more active than the corre-
sponding dimeric ones (1 and 3) (runs 1–8 in Table 1
and runs 5–8 in Table 2). In the coupling of iodoben-
Toluene
−
5
zene with methylacrylate catalyzed by 1×10
mol%
Mesitylene
Mesitylene
DMA c
of complex 4, the highest TON of 9 600 000 and TOF
of 870 000 of all Heck reactions reported to date were
obtained (run 8 in Table 2). The best TON reported
previously for Heck reactions was 8 900 000 in the
coupling of iodobenzene with n-butyl acrylate under a
higher temperature (180 °C) [3]. In addition, complex 2,
whose structure is similar to complex 4, also showed
outstanding catalytic activity with the TON of 9 100 000
and TOF of 830 000 (run 6, Table 2). This is also
among the highest catalytic activity for Heck reactions.
In view of the simplicity and efficiency, complexes 2
and 4 are very attractive for industrial application.
Complexes 2 and 4 also exhibited fairly high activi-
ties when the iodoarenes with electron donating groups
were used as the substrates. In the cases of reactions of
p-iodotoluene and p-iodophenol with methylacrylate,
catalyzed by 0.01 mol% of complexes 2 and 4, the Heck
products were obtained in good yields (runs 9–12 in
Table 2). For these deactivated iodoarenes, the yields of
Heck products depended much on the amounts of
catalysts used. For example, when 0.001 mmol% of
complex 4 was used, the Heck products were obtained
only in 89 and 45% yield for p-iodotoluene and p-
DMA
a
All reactions were carried out with 5 mmol of PhI, 6 mmol of
methylacrylate, 7 mmol of NEt3 and 0.1 mol% catalyst in 10 ml of
solvent for 1.5 h at 140 °C.
b
Determined by GC, based on iodobenzene.
N,N-Dimethylacetamide.
c
Na [PdCl ] with the corresponding amines, and the
2
4
monomeric complexes are obtained immediately by
treatment of PPh3 with the respective dimers in
benzene.
In order to find a suitable solvent, the Heck reactions
of iodobenzene with methylacrylate catalyzed by com-
plexes 1 or 2 were examined with a number of solvents.
The results are summarized in Table 1. The reaction
can be carried out in low polar solvents (1,4-dioxane)
or non-polar solvents (mesitylene), although at rela-
tively low rates. Of all solvents tested, DMA was found
to be the best one. So the after reactions were all
carried out in DMA and the results are summarized in
Table 2.
Table 2
a
Results of Heck reactions of iodoarenes in DMA
Run
ArX
Catalyst (mol%Pd)
t (h)
T (°C)
Yield (%) b
TON/TOF
99 000/9000
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
3
3
3
3
5
5
5
5
2
2
2
2
3
3
3
3
3
3
3
3
1
2
3
4
5
6
7
8
9
PhI
PhI
PhI
PhI
PhI
PhI
PhI
PhI
p-MePhI
p-MePhI
p-HOPhI
p-HOPhI
p-MePhI
p-HOPhI
p-MePhI
p-HOPhI
PhBr
1 (1×10
2 (1×10
3 (1×10
4 (1×10
1 (1×10
2 (1×10
3 (1×10
4 (1×10
2 (1×10
4 (1×10
2 (1×10
4 (1×10
4 (1×10
4 (1×10
4 (1×10
4 (1×10
1 (1×10
2 (1×10
3 (1×10
4 (1×10
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
)
11
11
11
11
11
11
11
11
11
11
11
11
11
11
22
22
11
11
11
11
140
140
140
140
140
140
140
140
140
140
140
140
140
140
160
160
140
140
140
140
99
100
99
100
34
91
33
96
97
99
93
94
89
45
94
46
100 000/9100
99 000/9000
100 000/9100
3 400 000/310 000
9 100 000/830 000
3 000 000/300 000
9 600 000/870 000
9700/880
9900/900
9300/850
9400/850
89 000/8100
45 000/4100
94 000/4300
46 000/2100
10
11
12
13
14
15
16
17
18
19
20
B1
B1
B1
B1
–
–
–
–
PhBr
PhBr
PhBr
a
All reactions were carried out with 5 mmol of ArI–PhBr, 6 mmol of methylacrylate and 7 mmol of NEt3 in 10 ml of DMA.
Determined by GC, based on iodoarenes.
b

1
48
F. Yang et al. / Journal of Organometallic Chemistry 651 (2002) 146–148
iodophenol, respectively (runs 13 and 14, Table 2).
Longer reaction time and higher reaction temperature
turned out to be little improvement (runs 15 and 16,
Table 2). Moreover, the reaction of bromobenzene was
carries out under the same conditions (runs 17–20 in
Table 2), and the results indicate that only traces of
products were yielded.
mirror formation and the application of these com-
plexes for other substrates are currently in progress.
1. Experimental
All GC results were recorded on a SRI8610C
chromatograph.
The issue of a possible Pd(II)/Pd(IV) cycle in Heck
catalysis is currently under debate, particularly for cy-
clopalladated complexes [9]. In our cases, we think the
major pathway of the reactions is via a Pd(0)/Pd(II)
cycle, since similar catalytic activity was observed for
different catalysts (runs 5 and 7, runs 6 and 8, Table 2).
Moreover, when 0.1 mmol% of complex 4 was used in
the coupling reaction of iodobenzene with methylacry-
late at the same conditions except under a lower tem-
perature (80 °C), a beautiful palladium mirror was
observed on the wall of the flask at the end of reaction.
The observed palladium mirror assured the formation
of Pd(0) species during the reaction. As far as we know,
there is no report on palladium mirror formation in
Heck reactions catalyzed by cyclopalladated complexes.
The possible Pd(II)/Pd(IV) cycle cannot be ruled out in
our cases either; it may function as a side pathway.
However, what interests us now is the remarkable
difference in the catalytic activity shown by the
monomeric complexes and the dimeric ones. We sup-
pose that the catalytic difference resulted from the
triphenylphosphine part, which is the main difference in
their structures. In many Heck reactions, tertiary phos-
phine, usually triphenylphosphine, is required to fur-
nish the reaction [5]. It is also widely accepted that
1
.1. General procedure
In a 25 ml flask were placed 5 mmol of ArI, 6 mmol
of methylacrylate, 7 mmol of NEt , 0.1 mol% catalyst
3
and 10 ml of solvent. The reaction mixture was stirred
at 140 °C for a period of time, and determined by GC
based on iodobenzene. The product was identified by
comparison with authentic sample.
Acknowledgements
Project supported by the Foundation of Science and
Technology Development of Shanghai, China.
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