Synthesis of 1,3-bis-(5-ferrocenylisoxazole-3-yl) benzene-derived palladium(II) acetate complex and its application in Mizoroki-Heck reaction in an aqueous solution

Article abstract of DOI:10.1016/j.jorganchem.2008.06.020

The first synthesis and characterization of 1,3-bis-(5-ferrocenylisoxazole-3-yl) benzene-derived palladium(II) acetate complex were described and its application in Heck coupling reactions in an aqueous solution was studied. Complex 5 had been demonstrated to be a highly stable, active and efficient catalyst for Mizoroki-Heck coupling reactions.

Full text of DOI:10.1016/j.jorganchem.2008.06.020

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Journal of Organometallic Chemistry 693 (2008) 2963–2966
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Synthesis of 1,3-bis-(5-ferrocenylisoxazole-3-yl) benzene-derived palladium(II)
acetate complex and its application in Mizoroki–Heck reaction in an aqueous
solution
*
Yong-jia Shang , Jian-wei Wu, Chen-li Fan, Jin-song Hu, Ben-ye Lu
Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 May 2008
Received in revised form 18 June 2008
Accepted 19 June 2008
Available online 25 June 2008
The first synthesis and characterization of 1,3-bis-(5-ferrocenylisoxazole-3-yl) benzene-derived palla-
dium(II) acetate complex were described and its application in Heck coupling reactions in an aqueous
solution was studied. Complex 5 had been demonstrated to be a highly stable, active and efficient catalyst
for Mizoroki–Heck coupling reactions.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Mizoroki–Heck reaction
Palladium complex
Catalyst
Isoxazole
Ferrocene derivative
1. Introduction
also as catalysts for cross-coupling reactions compared to P, P-type
ligands, due to the stronger
r-donation which favors both oxida-
Palladium catalyzed aryl or vinyl halides with alkenes, known
as the Mizoroki–Heck reaction, has been recognized as a powerful
tool in multiple organic transformations [1]. Mostly palladium
complexes are used to catalyze these couplings reactions as they
offer high product yields, high regioselectivity, high stereoselectiv-
ity and compatibility with many functional moieties. Palladium
complexes containing phosphine, phosphate, and phosphine oxi-
des are widely used as catalysts in Mizoroki–Heck coupling reac-
tions. Important examples of the use of phosphines are found in
the group of Fu [2], Beller [3], and Harttwig [4] who have made
use of sterically demanding and electron-rich tertiary phosphines
as catalyst modifiers in the Mizoroki–Heck reactions. Despite their
effectiveness in controlling reactivity and selectivity, phosphine
catalysts require air-free handling to prevent the oxidation of the
ligand and are subject to P–C activation at elevated temperatures
[5–7]. Furthermore, the difficulties involved in the removal of
these by-products from organic products and high price of phos-
phine ligands led chemists to discover phosphine-free ligands.
There has been increasing interest in the development of new
phosphorus free palladium catalysts involving the use of N-, O-,
S-donor ligands in recent years [8]. Especially N, N-type ligand
have shown excellent properties for palladium complexation and
tive addition and slow reductive elimination steps in the catalytic
cycle [9].
Our continuing interest in the synthesis and applications of iso-
xazol(in)es and ferrocene derivatives in organic synthesis [10] also
prompted us to use them as ligands [11] in metal-catalyzed reac-
tions. To the best of our knowledge, there had been no example
of any transition metal complexes of benzene-1,3-isoxazole li-
gands. Thus, benzene-1,3-bis(isoxazole) has attracted our attention
as a possible alternative for the widely used phosphine ligands in
the catalysis. In order to obtain highly stable and efficient palla-
dium catalyst, we chose to build a benzene-bis(isoxazole) on the
robust ferrocenic backbones [12] to obtain the ligand 4, in which
there were four potential co-ordination sites (two N and two O).
The multidentate ligand can offer certain advantages that an addi-
tional co-ordination site in the ligand as a stabilizing group during
the course of a Pd-mediated reaction can improve the catalytic effi-
ciency of the complex [13]. The isoxazole ring has proven to be a
suitable system for metal co-ordination to form pincer type metal
complex which is testified by our experiment.
The use of water as solvent represents one of the safest and
most advantageous alternatives economically and environmentally
to organic solvents for metal-catalyzed reactions [14]. In addition,
the use of water may have positive effects such as increasing selec-
tivity and catalytic efficiency [15]. Herein, we report the first syn-
thesis and characterization of 1,3-bis-(5-ferrocenylisoxazole-3-yl)
* Corresponding author. Fax: +86 0553 3883517.
E-mail address: shyj@mail.ahnu.edu.cn (Y.-j. Shang).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.06.020

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Y.-j. Shang et al. / Journal of Organometallic Chemistry 693 (2008) 2963–2966
benzene-derived palladium(II) acetate complex and its application
in Mizoroki–Heck coupling reactions in an aqueous solution. The
results showed that the complex was highly effective for
Mizoroki–Heck coupling reactions.
2. Results and discussion
2.1. Complex
The preparation of 1,3-bis-(5-ferrocenylisoxazole-3-yl) ben-
zene-derived palladium(II) acetate complex 5 was shown in
Scheme 1. The ligand 4 was obtained in 87% yield via 1,3-dipolar
cycloaddition reaction of ethynylferrocene 3 with the isophthald-
nitrile oxides in situ generated from isophthaldehyde. The struc-
ture of ligand
4 was unambiguously confirmed by X-ray
Fig. 1. The single-crystal X-ray structure of 1,3-bis-(5-ferrocenylisoxazole-3-yl)
benzene 4.
diffraction analysis [16], which was in accordance with 1H NMR,
13C NMR, and EIMS spectra [16] (Fig. 1).
Complex 5, which was synthesized according to the method de-
scribed by Hartshorn and Steel [17] was thermally stable and not
sensitive to air and moisture. Complex 5 was determined by spec-
troscopic analysis. The ligand 4 is bound to palladium in a triden-
tate fashion forming two five-membered chelate rings. The C@N
bands of complex 5 were shifted to lower frequency in the IR spec-
trum compared to that of the free ligand 4. Moreover, the 1H NMR
spectrum of 4 shows a singlet at 8.33 ppm for the phenyl ring pro-
low co-ordinated Pd(0) species, and it was also as the phase trans-
fer catalyst for the inorganic base/polar solvent/organic substrates/
product phases [17]. The survey of aqueous solutions revealed that
N,N-dimethyl formamide (DMF) with H2O was much better than
others. We also studied the reaction temperature in the presence
of TBAB with NaOAc as a base using 1 mol% complex 5 as a catalyst
in an aqueous solution under an argon atmosphere. We found
100 °C was the optimal reaction temperature for coupling of sty-
rene and 4-iodotoluene. Instead of 1 mol%, it was found that the
loading of complex 5 could be reduced to 0.01 mol% with similar
yields under the same reaction conditions. However, the relatively
long reaction time would be needed. We found the following opti-
mized conditions: using complex 5 as a catalyst, the reaction pro-
ceeded in 88% yield with NaOAc as a base in the presence of TBAB
in an aqueous solution (DMF:H2O = 1:1) at 100 °C for 6 h under an
argon atmosphere.
ton which is adjacent to both the isoxazole heterocycles. In the 1
H
NMR spectrum of complex 5, the signal of the phenyl ring proton at
8.33 ppm disappears and all the protons were observed to appear a
downfield shift. The signal of the acetyl proton in the 1H NMR at
2.32 ppm and the signal of carbonyl in the 13C NMR at
172.2 ppm were also observed.
2.2. Catalytic activity of the complex
These optimized reaction conditions were then applied to cross-
coupling reactions between olefins and electron-poor or electron-
rich aryl iodides or bromides or chlorides to give internal alkenes
6 (Scheme 2 and Table 1). Couplings of 4-nitrophenyl iodide with
a variety of olefins proceeded in short times and with excellent
yields in the presence of 1 mol% complex 5 (Table 1, entries 1–3).
The deactivated 4-methoxyphenyl iodide was coupled with differ-
ent olefins. These results were similar to those obtained with the
activated aryl iodides (Table 1, entries 4–7). The acrylonitrile gave
a relative low yields because the desired E-isomeric product was
obtained accompanied with Z-isomeric, according to 1H NMR anal-
ysis (Table 1, entry 7). The stereo-hindered 2-nitrophenyl iodide
gave the cross-coupled product in excellent yield in comparison
with 4-nitrophenyl iodide (Table 1, entries 2 and 9). The heterocy-
clic compound of 2-chloro-5-iodopyridine also afforded high yield
when it reacted with acrylic acid n-butyl ester (Table 1, entry 11).
We then studied the cross-coupling reaction with aryl bro-
mides, using 2 mol% palladium complex 5 to provide alkenes
6m–t (Table 1, entries 13–23) [18]. In general these coupling reac-
tions proceeded with lower rates and yields than those with aryl
iodides. Therefore we elevated the reaction temperature and the
loading of catalyst and prolonged the reaction time and added an
excess of olefin in order to increase the yields of the expected prod-
uct 6. The stereo-hindered 2-bromotoluene gave the cross-coupled
product in modest yields in comparison with 4-bromotoluene and
We next investigated this novel palladium complex as a catalyst
for Mizoroki–Heck reaction. We applied the coupling of 4-iodotol-
uene and styrene as substrates to determine the optimum reaction
conditions. Firstly, a variety of bases were essayed for the coupling
between styrene and 4-iodotoluene to give desired product in the
presence of 1 mol% complex 5 as catalyst and 1 equiv. tetra-n-
butylammonium bromide (TBAB) as the reagent for activation
and stabilization of palladium(0) species in an aqueous solution
(DMF:H2O = 1:1) at 80 °C under an argon atmosphere. Using NaO-
Ac as a base in the presence of TBAB, the reaction had proceeded in
high yield. The reaction carried out in the absence of TBAB was in a
relative low yield. TBAB probably strengthened the stabilizations of
(a)
(b)
N
N
OHC
O N
CHO
OH
HO
1
2
(c)
N O
N
N
O
Fe
O
Fe
4
OAc
(d)
N
N
O
Fe
O
Pd
Fe
complex 5
Ar
OAc
5
ArX
+
R
R
NaOAc, TBAB
DMF, H2O,100-140oC
6
Scheme 1. Synthesis of 1,3-bis-(5-ferrocenylisoxazole-3-yl) benzene-derived pal-
ladium(II) acetate complex 5. Reagents and conditions: (a) NH2OH Á HCl, Na2CO3;
(b) NCS, CH2Cl2; (c) ethynylferrocene (3), Et3N; (d) Pd (OAc)2, CH3COOH.
Scheme 2. Cross-coupling reaction between olefins and aryl halides in an aqueous
solution.

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Y.-j. Shang et al. / Journal of Organometallic Chemistry 693 (2008) 2963–2966
Table 1 (continued)
2965
Table 1
The results of cross-coupling reaction between olefins and aryl halides in an aqueous
Entry
R
Aryl halides
Products
Yieldb (%)
Trace
solutiona
Entry
1
R
Aryl halides
Products
Yieldb (%)
93
21
CO2(CH2)3CH3
6u
Cl
CO2CH2CH3
6a
I
NO2
22
CO2(CH2)3CH3
CO2(CH2)3CH3
6v
58
78
Cl
Cl
NO2
2
3
4
5
6
7
8
CO2(CH2)3CH3
Phenyl
6b
6c
6d
6e
6f
94
93
85
80
89
78c
93
I
I
I
I
I
I
I
NO2
23
6w
COCH3
a
NO2
General reaction conditions for coupling reaction between olefins and aryl
halides: olefin (1.5 mmol), aryl halides (1 mmol), NaOAc (2 mmol), TBAB (1 mmol),
complex 5 (1–5 mol%), aqueous solution (4 mL) under an argon atmosphere.
b
Phenyl
Isolated yields after column chromatography.
E/Z = 4:1.
OCH3
OCH3
OCH3
OCH3
c
CO2CH2CH3
CO2(CH2)3CH3
CN
3-bromotoluene (Table 1, entries 13–15). We unexpectedly found
that the aryl bromide with nitro group gave excellent results with
catalyst loading 2 mol% and the reactions performed in short times.
The above findings were similar to activated aryl iodides (Table 1,
entries 18 and 19). Bromonaphthalene could also perform the
cross-coupling reaction smoothly under the same reaction condi-
tions to afford the cross-coupling product in good yield (Table 1,
entry 20). As for non-activated aryl chlorides, complex 5 shows rel-
ative low activities under the similar reaction conditions. We found
that even aryl chlorides with nitro group gave modest results with
catalyst loading 5 mol% and the reaction proceeded in long times
(Table 1, entry 21–23). It is worth noting that in all the reactions
no Pd black was observed and complex 5 was easily removed in
the extraction step after the reaction because of its low solubility
in most of solvents.
In conclusion, 1,3-bis-(5-ferrocenylisoxazole-3-yl) benzene-de-
rived palladium(II) acetate complex 5 had been demonstrated to be
a highly active and efficient catalyst for Mizoroki–Heck coupling
reactions in an aqueous solution. A variety of functional groups
such as methoxy and nitro were well tolerated under the present
catalytic system. Complex 5 showed very high stability in an aque-
ous solution at reflux and no Pd black was observed. Moreover, it is
easy to recovery and re-use of the catalyst because of its low solu-
bility in most of solvents. Further work is in progress to broaden
the scope of this catalytic system for aryl chlorides and other or-
ganic transformations.
6g
6h
CO2(CH2)3CH3
I
9
CO2(CH2)3CH3
CO2(CH2)3CH3
6i
6j
92
78
O2N
10
I
I
CH3
11
CO2(CH2)3CH3
6k
90
N
Cl
Cl
CH3
12
13
CO2(CH2)3CH3
CO2(CH2)3CH3
6l
90
65
I
6m
Br
Br
14
15
CO2(CH2)3CH3
CO2(CH2)3CH3
6n
6o
67
73
3. Experimental
H3C
Br
3.1. General
CH3
All reagents and solvents were obtained from commercial sup-
pliers and used without further purification. Melting points were
measured using a WC-1 microscopic apparatus. IR spectra were re-
corded on a Perkin–Elmer 983 FT-IR spectrometer. 1H NMR spectra
16
17
18
CO2CH2CH3
6p
6q
6r
60
80
90
Br
Br
Br
Br
CH3
NO2
were recorded on
a Bruker Avance DMX 300 instrument
CO2(CH2)3CH3
CO2(CH2)3CH3
(300 MHz). Chemical shifts are reported in ppm with tetramethyl-
silane (TMS, 0.00 ppm) as internal standard. GC–MS analyses were
performed on a HP-5973 spectrometer. Elemental analyses were
carried out on an EA-1110 elemental analyzer. X-ray crystallo-
graphic data were made on a Rigaku Mercury CCDC X-ray diffrac-
tometer (3 kV, sealed tube) at 193 K by using graphite
19
20
CO2(CH2)3CH3
CO2(CH2)3CH3
6s
6t
93
89
monochromated Mo Ka (k = 0.71070 AX).
O2N
Br
3.2. Preparation of 1,3-bis-(5-ferrocenylisoxazole-3-yl) benzene 4
Hydroxylamine hydrochloride (1.2 mmol) and sodium bicar-
bonate (1.5 mmol) were added to the solution of 1,3-benzenedial-
dehyde (1.0 mmol) in 10 mL ethanol. The mixture was stirred at

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Y.-j. Shang et al. / Journal of Organometallic Chemistry 693 (2008) 2963–2966
room temperature over night. The precipitate was removed by fil-
tration and washed with water. After dried, the aldoxime was ob-
tained in 98% yield.
HRMS (ESI): 239.07 ([M]+). Anal. Calc. for C12H14ClNO4: C, 60.13;
H, 5. 89; N, 5.84. Found: C, 60.10; H, 5.86; N, 5.82%.
Aldoxime (1 mmol) and chlorosuccinimide (NCS, 1 mmol) were
stirred in flask containing dry dichloromethane (5 mL). The reac-
tion mixture was refluxed at 30 °C for 40 min. The ethynylferro-
cene (1 mmol) was added. Triethylamine (1.4 mL in 3 mL of
CH2Cl2) was added dropwise over about 1.5 h and the mixture
was stirred overnight at room temperature. The complete con-
sumption of starting materials was judged by TLC analysis. After
stirring overnight, the residue was extracted with dichlorometh-
ane (3 Â 3 mL) and washed with water for three times. The com-
bined organic layers were dried with anhydrous magnesium
sulfate. The solution was filtered and the solvents were removed
in vacuo. The residue was purified by flash column chromatogra-
phy on alumina gel to afford red crystals in 87% yield. M.p. 200–
4. Supplementary material
CCDC 292945 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Acknowledgement
This work was financially supported by the Natural Science
Foundation of Education Administration of Anhui Province
(2006kj117B and KJ2008A064).
202 °C. IR (KBr): 1612 (C@N), 1560 (C@C) cmÀ1
.
1H NMR
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derived palladium(II) acetate complex 5
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scribed by Hartshorn and Steel. A solution of 1,3-bis-(5-ferroceny-
lisoxazole-3-yl) benzene 4 (1.5 mmol) and palladium acetate
(1.5 mmol) in acetic acid (15 mL) was heated under reflux for
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4.73 (s, 4H, C5H4), 4.40 (s, 10H, C5H5), 3.09 (s, 3H, CH3CO), 2.92
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(1 mmol), olefin (1.5 mmol), NaOAc (2 mmol), tetrabutylammo-
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EtOAc (3 Â 15 mL), dried over MgSO4, concentrated in vacuo and
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COCH2CH2CH2CH3), 1.73–1.64 (m, 2H, COCH2CH2CH2CH3), 1.49–
(e) B. Liang, M. Dai, J. Chen, Z. Yang, J. Org. Chem. 70 (2005) 391.
[15] J.P. Genet, M. Savignac, J. Organomet. Chem. 576 (1999) 305.
ꢀ
[16] Crystallographic data for 4: space group P1, a = 11.8066(3) Å, b = 21.3730(18) Å,
c = 11.2852(9) Å,
T = 293(2) K, Z = 4.
a = 90.00(2)°, b = 116.79(2)°, c
= 90.00(2)°, V = 2542.2(2) Å3,
1.42
(m,
2H,
COCH2CH2CH2CH3),
0.98–0.94
(t,
3H,
[17] C.M. Hartshorn, P.J. Steel, Organometallics 17 (1998) 3487.
[18] G.A. Grase, R. Singh, E.D. Stevens, S.P. Nolan, J. Organomet. Chem. 687 (2003)
269.
COCH2CH2CH2CH3); 13C NMR (75 MHz, CDCl3) d: 166.1, 152.7,
149.5, 139.2, 136.5, 129.3, 124.5, 121.2, 64.8, 30.7, 19.1, 13.7.