Monodentate phosphorus-coordinated palladium(II) complexes as new catalyst for Mizoroki-Heck reaction of aryl halides with electron-deficient olefins

Article abstract of DOI:10.1016/j.cclet.2017.08.004

Four novel palladium(II) complexes coordinated by phosphorus atoms from both 3,5-disubstituted-1H-1,2,4-diazaphospholes in monodentate fashion were developed as efficient catalyst for the Mizoroki-Heck reaction of aryl halides with electron-deficient olefins. The coupling reaction of aryl halide bearig different functional groups with olefin derivatives took place and the corresponding products were isolated in good to excellent yields under optimal conditions. The procedure exhibits good functional group tolerance and wide substrate scope. This Mizoroki-Heck reaction was further achieved using Pd(OAc)₂ and 3,5-di-isopropyl-1H-1,2,4-diazaphospholes as combined catalyst, which provide the convenient and alternative method in organic synthesis

Full text of DOI:10.1016/j.cclet.2017.08.004

Accepted Manuscript
Title: Monodentate phosphorus-coordinated palladium(II)
complexes as new catalyst for Mizoroki-Heck reaction of aryl
halides with electron-deficient olefins
Authors: Fang Zhao, Li Xin, Yingying Zhang, Xuefeng Jia
PII:
S1001-8417(17)30280-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2017.08.004
CCLET 4153
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Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
8-5-2017
11-7-2017
2-8-2017
Please cite this article as: Fang Zhao, Li Xin, Yingying Zhang, Xuefeng Jia,
Monodentate phosphorus-coordinated palladium(II) complexes as new catalyst for
Mizoroki-Heck reaction of aryl halides with electron-deficient olefins, Chinese
Chemical Lettershttp://dx.doi.org/10.1016/j.cclet.2017.08.004
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Communication
Monodentate phosphorus-coordinated palladium(II) complexes as new catalyst for
Mizoroki-Heck reaction of aryl halides with electron-deficient olefins
Fang Zhaoa,§, Li Xina,§, Yingying Zhangb, Xuefeng Jiaa,b,

aSchool of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China
bDepartment of Chemistry, Modern College of Arts and Sciences, Shanxi Normal University, Linfen 041000, China
Graphical abstract
Four novel monodentate phosphorus-coordinated palladium(II) complexes derived from 3,5-disubstituted-1H-1,2,4-
diazaphospholes were developed as efficient catalyst for Mizoroki-Heck reactions of aryl halides with electron-deficient olefins.
The role of these monophosphine ligands in catalysis was illustrated by control experiments using Pd salt and ligands as
combined catalyst.
ABSTRACT
Four novel palladium(II) complexes coordinated by phosphorus atoms from both 3,5-disubstituted-1H-1,2,4-diazaphospholes in monodentate fashion were
developed as efficient catalyst for the Mizoroki-Heck reaction of aryl halides with electron-deficient olefins. The coupling reaction of aryl halide bearig different
functional groups with olefin derivatives took place and the corresponding products were isolated in good to excellent yields under optimal conditions. The
procedure exhibits good functional group tolerance and wide substrate scope. This Mizoroki-Heck reaction was further achieved using Pd(OAc)2 and 3,5-di-
isopropyl-1H-1,2,4-diazaphospholes as combined catalyst, which provide the convenient and alternative method in organic synthesis
Keywords:
Palladium(II) complexes
Monodentate phosphine ligands
Mizoroki-Heck reaction
Aryl halides
Olefins
Since 1971, the Mizoroki-Heck reaction [1-3] between aryl halides and olefins is one of the most efficient methods for the C-C bond
formation and it has been widely used in synthesis of pharmaceuticals [4], agrochemicals [5], and natural products [6]. In the past few
years, the Mizoroki-Heck reactions were successfully achieved with different metal catalysts such as Pd [7-9], Rh [10-12], Cu [13,14],
Ni [15,16] and so on. Initially, the Mizoroki-Heck cross-coupling reaction using palladium complexes with various phosphorous
———
 Corresponding author.
E-mail address: jxfliu@163.com (X. F. Jia)
§ These two authors contributed equally to this work.

ligands [17-21] as catalyst was investigated. Bulky electron-rich phosphine-based and phosphorus-containing palladacycles exhibited
good catalytic activity for the Mizoroki-Heck reaction [22-23]. In 2010, Diebold and co-worker reported palladium complexes based
on polystyrene-supported phosphine ligands as reusable catalysts for the Mizoroki-Heck reaction [24]. In 2015, palladium complexes
bearing meta-terarylphosphine ligands was synthesized and used as catalyst for the Mizoroki-Heck reaction of (hetero)aryl bromides
and olefins by Tay’s groups [25]. However, most phosphine-based palladium complexes have some drawbacks of expensive,
poisonous, air- and moisture-sensitive which limited their application in many fields [26-28]. Therefore more attention was paid on
palladium complexes with nitrogen-based ligands [29-31]. For example, N-heterocyclic carbene-based palladium complexes [32-35]
and pincer palladium(II) complexes [36-38] showed good catalytic performance for the Mizoroki-Heck reaction. Beyond that,
dinuclear palladium complexes with rotaxane were employed in ring-closure Mizoroki-Heck reaction by Suzaki’s groups in 2011 [39].
At the same year, palladium complexes with anilido-oxazolinate [40] or N-[1-alkylpyridin-4(1H)-ylidene]amides ligands [41] were
developed as efficient catalyst for the Mizoroki-Heck reaction. In 2012, Keske and co-worker documented the synthesis of 1,2,3-
triazol-5-ylidene mesoionic carbine-based PEPPSI palladium complexes and their catalytic activity in Mizoroki-Heck reaction was
investigated [42]. Although great progress had been made in this field, the design of palladium complexes containing novel ligands and
their catalytic application in the Mizoroki-Heck reaction is therefore a crucial target.
In view of the application value of palladium(II) complexes in organic synthesis, we try to develop new heterocycles as efficient
ligands for preparation of palladium complexes. We found that the synthesis of 1H-1,2,4-diazaphospholes containing low-coordinated
phosphorous and nitrogen atoms was early reported in 1984 [43]. However, the catalytic application of metal complexes with 1H-
1,2,4-diazaphospholes is rarely studied in spite of its good coordinating properties to metal. Recently we have accidentally synthesized
a series of phosphorus-coordinated palladium(II) complexes (Fig. 1) derived from 3,5-disubstituted-1H-1,2,4-diazaphospholes {H[3,5-
R2dp] (R = H, i-Pr, Ph, t-Bu)} as unique heteroatom-containing monophosphine ligands and explored their catalytic activity in Suzuki-
Miyaura cross-coupling reactions [44]. The structure of the catalyst was confirmed by X-ray single-crystal diffraction of palladium(II)
complexes (IV) [44]. In consideration of our continuing interest on catalytic application of these new palladium(II) complexes, we
herein report catalytic performance of novel phosphorus-coordinated palladium(II) complexes for Mizoroki-Heck reaction of aryl
halide with electron-deficient olefins.
Firstly, the catalytic activities of palladium complexes I-IV were examined by using 4-bromoacetophenone (1b) and methyl acrylate
(2a) as the standard substrates (detail experimental procedure and characterization data of the products 3, please see Supporting
information). When we used palladium complex (5 mol%) as catalyst, K2CO3 as base, DMF as solvent to perform the model reaction at
110 oC for 24 h, the desired product 3ba was obtained in moderate to good yield (Table 1, entries 1-4). These results demonstrated that
four novel phosphorus-coordinated palladium complexes could catalyze Mizoroki-Heck reaction of 1b with 2a. Among them, complex
II was obviously the best catalyst for this reaction. Next, we studied the effect of different bases on the model reaction. When K2CO3
was switched to Cs2CO3, Na2CO3 or KOH, the products 3ba was isolated in lower yields (Table 1, entries 5-7). We also tried to use
Et3N as organic base to perform the reaction and the 86% yield was afforded (Table 1, entry 8). So K2CO3 was the best choice. When
the reaction of 1b with 2a performed in DMSO or dioxane, the yield of 3ba obviously decreased (Table 1, entries 9 and 10). This
indicated that DMF should be the best solvent. Then the effect of temperature on the reaction was also investigated. Changing the
reaction temperature to 100 °C or 130 °C, the yield of 3ba had a slight decrease (Table 1, entries 11 and 12). Finally, the Mizoroki-
Heck coupling reaction of 1b with 2a was carried out in the presence of 3 mol% and 1 mol% palladium catalyst, providing the product
3ba in 80% and 71% yields respectively (Table 1, entries 13 and 14). However, no improved yield was observed when 10 mol%
palladium complex II was used as catalyst (Table 1, entry 15). Thus the optimized conditions were established for exploring substrate
scope of this Mizoroki-Heck reaction: palladium complex II (5mol%) as catalyst, K2CO3 (2 equiv.) as base, and 1b/2a = 1:1.5 in DMF
(1 mL) at 110 °C for 24 h.

With optimal reaction conditions in hand, the reactions of different aryl bromide with methyl acrylate (2a) were firstly tested to
evaluate the substrate scope of this Mizoroki-Heck reaction. As shown in Table 2, the substrates with electron-withdrawing groups
such as 4-formyl (1a) and 4-acetyl (1b) were tolerated and the desired products were afforded in 97% and 89% yields (Table 2, entries
1 and 2). Comparatively, the substrates with no substituent (1c) or electron-donating groups such as 4-methoxy (1d) reacted with
methyl acrylate (2a) to furnish the products in 81% and 79% yields, showing a lower reactivity (Table 2, entries 3 and 4). Then we
explored the reactivity of other acrylate with respect to the ester group. When ethyl acrylate (2b) was used as substrate to react with
different aryl bromide (1a-d), the corresponding products were isolated in 71%-74% yields (Table 2, entries 5-8). The results illustrate
that the activity of ethyl acrylate (2b) was lower than methyl acrylate (2a). However, tert-butyl acrylate (2c) was further chosen as an
olefin source and reacted with 1a-d, leading to the desired products in higher yields (Table 2, entries 9-12). Subsequently, styrene (2d)
was selected as coupling partner and provided the corresponding products in 69%-73% yields (Table 2, entries 13-16). These
experimental results demonstrated that the substituents on olefin substrates affected this Mizoroki-Heck reaction activity, which may
be attributed to the electron density change of carbon-carbon double bonds and the steric hindrance of ester group on acrylate. Finally,
the Mizoroki-Heck reaction of 2-bromothiophene (1e) with methyl acrylate (2a) was carried out and the product (3ea) was isolated in
80% yield, indicating that the heteroaryl bromide could be tolerated as good coupling partner (Table 2, entry 17).
In addition, the activity of aryl chloride was also tested. To our delight, We found that the cross-coupling reaction of 4-
chloroacetophenone (1e) with olefins derivatives such as 2a, 2b, 2c, 2d proceeded well, giving the desired product in moderate to good
yields (Table 2, entries 18-21). Then several different aryl chlorides such as 4-chlorobenzaldehyde (1g), chlorobenzene (1h) and 4-
chloroanisole (1i) reacted with 2a to furnish the desired products in good yields (Table 2, entries 22-24), which indicated that aryl
chlorides were also suitable substrates in the reaction.
In order to illustrate the role of these monophosphine ligands in catalysis, control experiments using Pd salt as the catalyst under the
standard conditions were performed. We used different monophosphine ligands in combination with PdCl2 or Pd(OAc)2 for the
Mizoroki-Heck coupling reaction between 4-bromoacetophenone (1b) and methyl acrylate (2a). As shown in Table 3, the
substituents on the ligands obviously affected the yields of the product 3ba (Table 3, entries 1-4 and 6-9).
Among them, H[3,5-i-Pr2dp] exhibited higher efficiency than other three ligands, which might be attributed to electronic effects and
steric hindrance of substituents. Compared with H[3,5-R2dp] (R = H, Ph, t-Bu), H[3,5-i-Pr2dp] was more suitable for this coupling
reaction. Furthermore, we found that the combination of H[3,5-i-Pr2dp] with Pd(OAc)2 was better catalytic system than those with
PdCl2 (Table 3, entry 7 vs. entry 2). When no ligands were used, the yields of 3ba significantly decreased (Table 3, entries
5 and 10). These results demonstrated that the ligands were crucial for this transformation.
Thus we employed Pd(OAc)2 (5 mol%) and H[3,5-i-Pr2dp] (10 mol%) as combined catalyst for the Mizoroki-Heck reactions. As
summarized in Table 4, the reaction of several aryl bromide such as 1a, 1b, 1c and 1d with methyl acrylate (2a) performed well and
offered the desired products in moderate to good yields (Table 4, entries 1-4). When the coupling reaction between 4-bromoanisole
(1d) and ethyl acrylate (2b) was carried out under combined catalyst system, the product (3db) was isolated in 67% yield (Table 4,
entry 5). The coupling of bromobenzene (1c) with tert-butyl acrylate (2c) also took place and led to the product (3cc) in 70% yield
(Table 4, entry 6). We also observed that 4-bromobenzaldehyde (1a) reacted with styrene (2d) to provide the product (3ad) in 63%
yield (Table 4, entry 7). These data showed that the different aryl bromide and olefin substrates could be tolerated under this catalytic
system. More importantly, the reaction of aryl chlorides such as 1f, 1g, 1h, 1i and 1j with methyl acrylate (2a) also furnished the
corresponding product in 54-86% yield (Table 4, entries 8-11). Therefore we believe that this alternative catalytic system might be
more attractive and convenient for application of Mizoroki-Heck reaction in organic synthesis.
In conclusion, we have developed Mizoroki-Heck cross-coupling reaction of aryl halides and electron-deficient olefins using novel
phosphorus-coordinated palladium (II) complexes derived from 3,5-disubstituted-1H-1,2,4-diazaphospholes as efficient catalyst. The
experiment results demonstrated that the reaction of aryl halide bearing different groups (-CHO, -COCH3, -OCH3, 2-thienyl) with
electron-deficient olefin derivatives performed well and high yields( up to 98%) were obtained under optimal conditions. The role of

monophosphine ligands in catalysis was manifested by control experiments. Furthermore, the Mizoroki-Heck reaction of different
substrates were achieved under Pd(OAc)2 and H[3,5-i-Pr2dp] combined systems, providing the direct and convenient methods for C-C
bond formation in organic synthesis.
Acknowledgments
Financial support from Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 2015157),
Teaching Reform Project of Shanxi Normal University (No. SD2014JGKT-52) and Undergraduate Training Programs for Innovation
and Entrepreneurship of Modern College of Arts and Sciences, Shanxi Normal University (No. WL2015CXCY-YJ-26) are gratefully
acknowledged.

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Fig. 1 Palladium complexes I-IV.

Table 1
Optimization of reaction conditions. a
Entry
1
Catalyst
I
Base
Solvent
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
Dioxane
DMF
DMF
DMF
DMF
DMF
T (°C)
110
110
110
110
110
110
110
110
110
110
100
130
110
110
110
Yield (%)b
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
KOH
61
89
35
42
56
37
5
2
II
3
III
IV
II
4
5
6
II
7
II
8
II
NEt3
86
42
0
9
II
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
10
11
12
13c
14d
15e
II
II
83
76
80
71
88
II
II
II
II
a Reaction conditions: 1b (0.2 mmol), 2a (0.3 mmol, 1.5 equiv.), catalyst (5 mol%), base (0.4 mmol, 2 equiv.) in solvent (1.0 mL) at indicated temperature for 24
h.
b Isolated yield.
c II (3 mol%) was used.
d II (1 mol%) was used.
e II (10 mol%) was used.

Table 2
Substrate scope for Mizoroki-Heck reactions.a
Entry
1
1 (R)
X
2 (R')
3
Yield (%)b
98
1a (4-CHO)
1b (4-COCH3)
1c (H)
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
2a (COOMe)
2a (COOMe)
2a (COOMe)
2a (COOMe)
2b (COOEt)
2b (COOEt)
2b (COOEt)
2b (COOEt)
2c (COOBut)
2c (COOBut)
2c (COOBut)
2c (COOBut)
2d (Ph)
3aa
3ba
3ca
3da
3ab
3bb
3cb
3db
3ac
3bc
3cc
3dc
3ad
3bd
3cd
3dd
2
89
3
81
4
1d (4-OCH3)
1a (4-CHO)
1b (4-COCH3)
1c (H)
79
5
74
6
72
7
71
8
1d (4-OCH3)
1a (4-CHO)
1b(4-COCH3)
1c (H)
71
9
87
10
11
12
13
14
15
16
80
79
1d (4-OCH3)
1a (4-CHO)
1b (4-COCH3)
1c (H)
71
73
2d (Ph)
72
2d (Ph)
70
1d (4-OCH3)
2d (Ph)
69
17
1e (2-bromothiophene)
2a (COOMe)
3ea
80
18
19
20
21
1f (4-COCH3)
1f (4-COCH3)
1f (4-COCH3)
1f (4-COCH3)
Cl
Cl
Cl
Cl
2a (COOMe)
2b (COOEt)
2c (COOBut)
2d (Ph)
3fa
3fb
3fc
3fd
66
67
77
63

22
23
24
1g (4-CHO)
1h (H)
Cl
Cl
Cl
2a (COOMe)
2a (COOMe)
2a (COOMe)
3ga
3ha
3ia
74
59
62
1i (4-OCH3)
a Reaction conditions: Aryl halide 1 (0.2 mmol), olefins 2 (0.3 mmol, 1.5 equiv.), II (5 mol%), K2CO3 (0.4 mmol, 2 equiv.) in DMF (1.0 mL) at 110 °C for 24 h.
b Isolated yield.

Table 3
Control experiments for the role of monophosphine ligands.a
Yield
of
Entry
Pd salt
Ligands {H[3,5-R2dp]}
3ba (%)b
27
1
PdCl2
R= H
R = i-Pr
R = Ph
R = t-Bu
/
2
PdCl2
80
3
PdCl2
48
4
PdCl2
52
5
PdCl2
50
6
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
R= H
R = i-Pr
R = Ph
R = t-Bu
/
40
7
86
8
70
9
64
10
39
a Reaction conditions: 1b (0.2 mmol), 2a (0.3 mmol, 1.5 equiv.), Pd salt (5 mol%), ligand(10 mol%), K2CO3 (0.4 mmol, 2 equiv.) in DMF (1.0 mL) at 110°C for
24 h.
b Isolated yield.

Table 4
The Mizoroki-Heck reaction of several aryl halides and olefins catalyzed by Pd(OAc)2 and H[3,5-i-Pr2dp].a
Entry
1 (R)
X
2 (R')
3
Yield (%) b
1
1a (4-CHO)
1b (4-COCH3)
1c (H)
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
2a (COOMe)
2a (COOMe)
2a (COOMe)
2a (COOMe)
2b (COOEt)
2c (COOBut)
2d (Ph)
3aa
3ba
3ca
3da
3db
3cc
3ad
3fa
3ga
3ha
3ia
79
86
59
81
67
70
63
86
68
54
73
2
3
4
1d (4-OCH3)
1d (4-OCH3)
1c (H)
5
6
7
1a (4-CHO)
1f (4-COCH3)
1g (4-CHO)
1h (H)
8
2a (COOMe)
2a (COOMe)
2a (COOMe)
2a (COOMe)
9
10
11
1i (4-OCH3)
a
Reaction conditions: aryl halide 1 (0.2 mmol), olefin 2 (0.3 mmol, 1.5 equiv), Pd(OAc)2(5 mol %), H[3,5-i-Pr2dp](10 mol%), K2CO3 (0.4 mmol, 2 equiv) in
DMF (1.0 mL) at 110°C for 24h.
b Isolated yield.