Imidazole-coordinated monodentate NHC-Pd(II) complex derived from proline and its application to the coupling reaction of arylboronic acids with carboxylic acid anhydrides in water at room temperature

Article abstract of DOI:10.1002/aoc.1792

Cleavage of a C-N bond of imidazolium salt derived from N-phenyl-substituted proline was observed in this laboratory. A novel imidazole-coordinated monodentate NHC-Pd(II) complex 5 was obtained as the sole product in good yield in the reaction of imidazolium salt 4 with Pd(OAc) ₂ in refluxing THF. The structure of complex 5 was determined unambiguously by an X-ray diffraction. The complex was found to be a good catalyst in the cross-coupling reaction of arylboronic acids with carboxylic acid anhydrides in water at room temperature. Copyright

Full text of DOI:10.1002/aoc.1792

Full Paper
Received: 21 January 2011
Revised: 12 February 2011
Accepted: 15 February 2011
Published online in Wiley Online Library: 26 April 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1792
Imidazole-coordinated monodentate
NHC–Pd(II) complex derived from proline
and its application to the coupling reaction
of arylboronic acids with carboxylic acid
anhydrides in water at room temperature
Xiao-Bao Shen, Ting-Ting Gao, Jian-Mei Lu and Li-Xiong Shao∗
Cleavage of a C–N bond of imidazolium salt derived from N-phenyl-substituted proline was observed in this laboratory. A novel
imidazole-coordinated monodentate NHC–Pd(II) complex 5 was obtained as the sole product in good yield in the reaction of
imidazolium salt 4 with Pd(OAc)2 in refluxing THF. The structure of complex 5 was determined unambiguously by an X-ray
diffraction. The complex was found to be a good catalyst in the cross-coupling reaction of arylboronic acids with carboxylic acid
c
anhydrides in water at room temperature. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: N-heterocyclic carbene; proline; palladium; coupling reaction; synthetic methods
Introduction
in the presence of K2CO3 in N, N-dimethylformamide (DMF) at
100 ◦C for 24 h, followed by the addition of Me2SO4 at 0 ◦C, gave
the methyl N-phenyl prolinate 1 in 76% yield.[4] Reduction of 1
with LiAlH4 gave the corresponding N-phenyl prolinol 2 in 95%
yield,[5] which was further transformed into the bromide 3 in 85%
yield by treatment with PPh3 and CBr4.[6] It should be noted here
that compound 3 is prone to decompose and was used for the next
step as soon as possible. Finally, formation of the imidazolium salt
4 was accomplished through the reaction of the bromide 3 with
1-methylimidazole in 81% yield.
Since the renaissance of the chemistry of N-heterocyclic carbene
(NHC)–metal complexes after the isolation of the first stable
carbene by Arduengo and co-workers in 1991,[1] great progress
has been made in the field and some excellent reviews have been
reported.[2] Undoubtedly, the most important investigations on
NHCs involve the synthesis of their metal complexes and their
applications in carbon–carbon and carbon–heteroatom bond
formation reactions. Recently, we have successfully synthesized
some NHC–Pd(II) complexes derived from proline. It was found
that substituents on the N-atom of the pyrrolidine skeleton affect
the coordination pattern of the palladium center. That is, when
an electron-rich group such as benzyl group was attached to the
N-atom, both the N-atom and NHC were coordinated to the Pd(II)
center, while when an electron-poor group such as Ts group was
attached, a dimeric mono-coordinated NHC–Pd(II) was obtained
exclusively (Scheme 1).[3]
Encouraged by this development, we then turned our attention
to the synthesis of NHC–palladium complexes derived from other
substituted prolines. Herein, we wish to report the elimination of
an alkyl group from the corresponding precursor (imidazolium
salt) and the preparation of imidazole-coordinated monodentate
NHC–Pd(II) complex derived from N-phenyl substituted proline
and its catalytic activity towards the coupling reaction of
arylboronic acids with carboxylic acid anhydrides in water at room
temperature.
Synthesis of NHC–Pd(II) complex 5
Based on our previously reported results,[3] the reaction of
imidazolium salt 4 (1.0 mmol) with Pd(OAc)2 (0.5 mmol) was
carried out in refluxing tetrahydrofuran (THF) for 18 h. To our
pleasure, a new NHC–Pd(II) complex 5 was obtained as the
sole product in 82% yield after careful silica-gel chromatography
(Scheme 3). The structure of complex 5 was fully determined by
X-ray single crystal diffraction (Fig. 1) and its CIF data are presented
in the Supporting Information. It can be seen from Fig. 1 that one
molecule of 1-methylimidazole was formed with the C–N bond
cleavage from the imidazolium salt 4, which coordinated with
the metal center through the nitrogen atom, combined with the
∗
Correspondence to: Li-Xiong Shao, College of Chemistry and Materials Engi-
neering, Wenzhou University, Chashan University Town, Wenzhou, Zhejiang
Province 325035, People’s Republic of China. E-mail: shaolix@wzu.edu.cn
Results and Discussion
Synthesis of imidazolium salt 4
College of Chemistry and Materials Engineering, Wenzhou University, Chashan
University Town, Wenzhou, Zhejiang Province 325035, People’s Republic of
China
The route for the synthesis of the imidazolium salt 4 is shown in
Scheme 2. CuI-catalyzed coupling of proline with bromobenzene
c
Appl. Organometal. Chem. 2011, 25, 497–501
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

X.-B. Shen et al.
Me
N
N
I
I
N
R
N
N
Pd
.
.
Ts
N
Me
N
N
Pd
Ts
Br
Ph
N
Br
Scheme 1. Two NHC–Pd(II) complexes derived from substituted prolines
with different coordination patterns.
(a)
(b)
OH
COOH
COOMe
N
H
N
N
Figure 1. ORTEP drawing of complex 5 with thermal ellipsoids at the
30% probability level. Selected bond distances (Å) and angles (deg):
Pd1 –C14 = 1.956(3) Å, Pd1 –N5 = 2.102(3) Å, Pd1 –Br1 = 2.4450(4) Å,
Ph
2
Ph
1
Pd1 –Br2 = 2.4260(5) Å, C14 –Pd1 –Br1 = 87.55 (10)◦, C14 –Pd1 –Br2
86.47(10)◦, N5 –Pd1 –Br1 = 94.25 (8)◦ and N5 –Pd1 –Br2 = 91.94 (8)◦.
=
Me
N
(c)
Br (d)
N
N
N
Br
are good choices to serve as the carbon nucleophiles in these
transformations.[11] Development of organic transformations in
water has attracted many groups’ attention because it is cheap,
clean and safe, and matches the basic principles of green
chemistry.[12,13] With the imidazole-coordinated monodentate
NHC–Pd(II) complex derived from N-phenyl substituted proline
in hand, we then turned our attention to its application in
the coupling reaction of arylboronic acids with carboxylic acid
anhydrides in water.
Ph
3
Ph
4
Scheme 2. Synthesis of imidazolium salt, 4. Reagents and conditions: (a)
CuI, PhBr, DMF, 100 ◦C, 24 h; then Me2SO4, room temperature, 12 h, 76%.
(b) LiAlH4, THF, reflux, 95%. (c) PPh3, CBr4, CH3CN, room temperature, 85%.
(d) 1-Methylimidazole, CH3CN, 50 ◦C, 81%.
Me
N
Initial experiments were carried out using benzoic anhydride,
6 (1.5 mmol), and phenylboronic acid, 7a (0.5 mmol), as the
substrates in the presence of NHC–Pd(II) complex, 5 (3.0 mol%),
in water (1.5 ml) at room temperature to find the best base.
The results are shown in Table 1. It was found that the reaction
gave the corresponding coupling product, 8a, in 63% yield within
24 h with Na2CO3 as the base (Table 1, entry 1). For other bases
such as K2CO3, Cs2CO3, NaHCO3, KHCO3, KOAc, HCO2Na, NaOH,
N
.
.
Me
N
N
Pd(OAc)2
Pd
Br
N
Br
Ph
THF, reflux
N
Br
N
4
Ph
N
Me
5
.
Li2CO3 and K3PO4 3H2O, all reactions proceeded smoothly to give
Scheme 3. Synthesis of NHC–Pd(II) complex, 5. Reagents and conditions:
4 (1.0 mmol), Pd(OAc)2 (0.5 mmol), THF (1.5 ml), reflux, 18 h, 82%.
product 8a in low to good yield within 24 h (Table 1, entries
2–10). The best result was obtained using NaHCO3 as the base
and 8a was achieved in 81% yield (Table 1, entry 4). When the
reaction time was prolonged to 36 h with NaHCO3 as the base, the
yield of 8a could be further increased to 94% (Table 1, entry 11),
which did not increase even if the reaction time was prolonged to
48 h (Table 1, entry 12). Therefore the optimal reaction conditions
were established as using NHC–Pd(II) complex 5 (3.0 mol%) as the
catalyst and NaHCO3 as the base at room temperature for 36 h.
With the optimal reaction conditions in hand, we then
investigated the reactions of benzoic anhydride, 6 (1.5 mmol),
with various arylboronic acids, 7 (0.5 mmol). The results are shown
in Table 2. It was found that the corresponding ketones can be
obtained in moderate to high yields in most cases. Substituents
on the aryl ring of the arylboronic acids 7 affect the reactions. For
example, for the reaction of electron-donating groups substituted
arylboronic acids such as 7b (4-Me) and 7f (4-MeO), only moderate
yields of the products 8b and 8f were obtained (Table 2, entries
1 and 5). Substrate 7i with an ortho-Me group also gave inferior
yield (Table 2, entry 8).
carbene ligand to form complex 5.[7] The structure of complex 5
showed that the Pd(II) center has almost perfectly normal square
planar coordination with the angles around 90◦, which suggests
that the complex has little rigidity. Representative data of bond
distances and angles are also shown in Fig. 1.
NHC–Pd(II) Complex 5-catalyzed Coupling Reaction
of Arylboronic Acids with Benzoic Anhydride
Palladium-catalyzed coupling reactions are versatile methods
for the formation of carbon–carbon and carbon–heteroatom
bonds.[8] Among them, the palladium-catalyzed acylation of
carbon nucleophiles with carboxylic acid derivatives is an efficient
reaction for the synthesis of complex organic molecules.[9]
Generally, organotin, -zinc, -cadmiun or boron reagents, are
used as the nucleophiles.[10] Owing to their easy availability,
low toxicity and air- and moisture-stability, organoboronic acids
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 497–501

Imidazole-coordinated monodentate NHC–Pd(II) complex
Table 1. Optimization for the NHC–Pd(II) complex 5-catalyzed
reaction of benzoic anhydride 6 with phenylboronic acid 7a
Table 2.
NHC–Pd(II) complex 5-catalyzed reactions of benzoic
anhydride 6 with arylboronic acids 7
B(OH)2
O
B(OH)2
O
O
O
O
O
complex 5
NaHCO3
complex 5
base, H2O,
rt
O
+
+
O
H2O, rt ,
36 h
R
R
7a
8
8a
6
7
6
Entrya
Base
Yields (%)b
Entrya
7(R)
Yields (%)b
1
Na2CO3
K2CO3
63
55
45
81
62
20
9
1
2
3
4
5
6
7
8
9
7b (4-Me)
7c (4-F)
8b, 68
8c, 84
8d, 86
8e, 98
8f, 67
8g, 84
8h, 82
8i, 66
8j, 91
2
3
Cs2CO3
NaHCO3
KHCO3
KOAc
7d (3,5-Me2)
7e (4-Cl)
4
5
7f (4-OMe)
7g (3-NO2)
7h (3-Me)
7i (2-Me)
6
7
HCO2Na
NaOH
8
36
64
35
94
92
9
Li2CO3
B(OH)2
.
10
11c
12d
K3PO4 3H2O
NaHCO3
NaHCO3
7j
a Unless Otherwise specified, all reactions were carried out using 6 (1.5
mmol), 7a (0.5 mmol), base (2.4 mmol) in the presence of 5 (3.0 mol%)
in H2O (1.5 ml) at room temperature for 24 h.
b Isolated yields.
a All reactions were carried out using 6 (1.5 mmol), 7 (0.5 mmol),
NaHCO3 (2.4 mmol) in the presence of 5 (3.0 mol%) in H2O (1.5 ml) at
room temperature for 36 h.
b Isolated yields.
c The reaction time is 36 h.
d The reaction time is 48 h.
into ice-water (400 g, 1 : 1). The organic phase was separated, and
the aqueous phase was extracted with EtOAc (2 × 50 ml). The
combined organic phases were washed with 2 M HCl (2 × 60 ml)
and brine, and dried over anhydrous Na2SO4. After filtration and
evaporation in vacuo, the residual solvents were removed by oil
pump (100 ◦C) to give a light brown oil (76%). 1H NMR (500 MHz,
CDCl3, TMS): δ 7.23–7.20 (m, 2H, Ar), 6.71 (t, 2H, J = 7.0 Hz, Ar),
6.54 (d, 2H, J = 8.0 Hz, Ar), 4.24 (d, 1H, J = 8.5 Hz, NCH), 3.71
(s, 3H, OCH3), 3.59–3.56 (m, 1H, one of NCH2), 3.38–3.33 (m, 1H,
one of NCH2), 2.28–2.04 (m, 4H, NCHCH2CH2). 13C NMR (125 MHz,
CDCl3) δ 175.0 (O C–O), 146.7 (Ar), 129.2 (Ar), 116.6 (Ar), 111.9
(Ar), 60.8 (OCH3), 52.1 (NCH), 48.2 (NCH2), 30.9 (NCHCH2), 23.8
(NCHCH2CH2).
Conclusions
In conclusion, a novel NHC–Pd(II) complex 5 derived from
N-phenyl substituted proline was reported, which is believed to be
formed from C–N bond cleavage of the starting imidazolium salt.
The structure of the complex was unambiguously determined by
X-ray single crystal diffraction. Based on our previously reported
studies, substituents on the N-atom of the pyrrolidine skeleton
dramatically affect the coordination pattern of the palladium
center. Moreover, the complex was found to be a good catalyst in
the coupling reaction of benzoic anhydride with arylboronic acids
in water at room temperature.
Synthesis of Compound 2
Experimental Section
Into 90 ml of THF was added LiAlH4 (3.0 g, 80 mmol) in small
portions with caution. A solution of compound 1 (10.3 g, 50 mmol)
in THF (17 ml) was added dropwise over 10 min to the above
suspension with stirring. The reaction was heated under reflux
for 3 h. After cooling to 0 ◦C, 2 M NaOH (25 ml) was added
to the mixture dropwise over 20 min, and then refluxed for
another 1 h. After being cooled to room temperature, the organic
phase was separated, and the aqueous phase was extracted with
EtOAc (3 × 30 ml). The combined organic phases were dried over
anhydrous Na2SO4. After filtration and evaporation in vacuo, the
residual solvents were removed by oil pump (100 ◦C) to give a
light yellow oil (95%). 1H NMR (500 MHz, CDCl3, TMS) δ 7.24–7.21
(m, 2H, Ar), 6.72–6.68 (m, 3H, Ar), 3.85–3.84 (m, 1H, NCH), 3.64 (s,
2H, CH2OH), 3.51–3.48 (m, 1H, one of NCH2), 3.16–3.13 (m, 1H,
one of NCH2), 2.08–1.95 (m, 4H, NCHCH2CH2), 1.73 (s, 1H, OH).
General Remarks
1H- and 13C-NMR spectra were recorded on a Bruker Avance-
500 MHzspectrometerforsolutioninCDCl3 withtetramethylsilane
(TMS) as the internal standard; J-values are in Hz. Commercially
obtained reagents were used without further purification. Flash
columnchromatographywascarriedoutusingHuanghai300–400
mesh silica gel at increased pressure.
Synthesis of Compound 1
Under an N2 atmosphere, CuI (1.9 g, 0.01 mol) and K2CO3 (40.0 g,
0.28 mol) were added to a solution of L-proline (11.5 g, 0.1 mol)
and bromobenzene (12 ml, 0.12 mol) in DMF (100 ml). The mixture
was stirred at 100 ◦C for 24 h. After cooling to 0 ◦C and diluting
with DMF (40 ml), 15 ml (0.15 mol) of Me2SO4 was added. The
mixture was stirred at 0 ◦C for another 12 h and then was poured
13C NMR (125 MHz, CDCl3) δ 148.0 (Ar), 129.3 (Ar), 116.4 (Ar), 112.4
(Ar), 63.8 (OCH3), 60.2 (NCH), 49.5 (NCH2), 28.8 (NCHCH2), 23.8
(NCHCH2CH2).
c
Appl. Organometal. Chem. 2011, 25, 497–501
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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X.-B. Shen et al.
Synthesis of Compound 3
Table 3. Crystallographic data of NHC-Pd(II) complex 5
PPh3 (15.7 g, 60 mmol) and compound 2 (7.09 g, 40 mmol) were
added to anhydrous THF (150 ml) at 0 ◦C with stirring. Then CBr4
(19.9 g, 60 mmol) was added slowly into the above solution over
5 min. After being stirred at room temperature for 3 h, the reaction
was quenched with saturated solution of NaHCO3 dropwise until
the pH of the solution reach 7. The mixed solution was diluted with
CH2Cl2 and the organic phase was dried over anhydrous Na2SO4,
filtered, concentrated and purified by flash chromatography on
silica gel to give a pale brown oil (85%). This compound is prone
to decompose and was used for the next step as soon as possible.
IR (neat) ν 2806, 2667, 2351, 2116, 1597, 1503, 1360, 1340, 1236,
1181, 1033, 992, 864, 746, 691 cm−1. MS (EI): m/z (%) 241 (20), 239
(M+, 20), 160 (26), 146 (100), 77 (24).
Formula
C19H25Br2N5Pd
589.66
Formula weight
Crystal
Colorless prism
0.56 × 0.54 × 0.49 mm
Monoclinic
Dimensions
Crystal system
Space group
P2(1)
Absolute structure parameter
0.035(6)
Lattice parameters
a
10.1343(4) Å
8.8883(4) Å
b
c
12.3762(5) Å
90 deg
α
β
107.4960(10) deg
90 deg
γ
Synthesis of Compound 4
Z
2
Absorption coefficient µ
Diffractometer
Measuring range θ
Data completeness
Reflections collected
Absorption correction
Max./min. transmission
Refined parameters
4.645 mm−1
Bruker Smart APEX CCD
1.73–25.01 deg
99.6%
1-Methylimidazole (0.98 g, 12 mmol) and compound 3 (2.40 g,
10 mmol) were added into CH3CN (10 ml). The reaction was stirred
at 50 ◦C for 18 h. After cooling to room temperature, the solution
was concentrated in vacuo and purified by flash chromatography
on silica gel to give a thick brown liquid (81%). IR (neat) ν 2967,
2901, 2837, 1737, 1597, 1505, 1383, 1362, 1220, 1182, 1161, 1034,
991, 861, 746, 693 cm−1; MS (EI) m/z (%) 321 (M+, 1), 138 (48), 110
(100).
11 882
Semi-empirical from equivalents
0.2093/0.1807
244
wR2/R1[1 > 2σ(1)]
Max./min. residual electron density
0.0509/0.0195
0.518/−0.413 e.Å−3
Synthesis of Compound 5
Under an N2 atmosphere, to a solution of compound 4
(322.2 mg, 1.0 mmol) in anhydrous THF (15 ml) was added
Pd(OAc)2 (112.2 mg, 0.5 mmol), and the mixture was refluxed for
18 h. Then the mixture was filtered through Celite. The solvent was
removed under reduced pressure and the residue was purified by
a flash chromatography on silica gel to give Pd(II)–NHC complex
5 (82%) as a yellow solid. M.p. 176–178 ◦C. 1H NMR (500 MHz,
CDCl3, TMS) δ 8.18 (s, 1H, N CH–N in the imidazole moiety),
7.67 (s, 1H, CH in the imidazole moiety), 7.10–7.02 (m, 4H, Ar),
6.87 (d, 1H, J = 2.0 Hz, CH in the carbene moiety), 6.86 (d,
1H, J = 2.0 Hz, CH in the carbene moiety), 6.83 (s, 1H, CH
in the imidazole moiety), 6.65 (t, J = 7.5 Hz, 1H, Ar), 4.85 (dd,
J = 13.5, 4.5 Hz, 1H, one of NCH2), 4.81–4.77 (m, 1H, one of
NCH2), 4.17 (dd, J = 13.5, 8.5 Hz, 1H, NCH), 4.09 (s, 3H, NCH3 in
the imidazole moiety), 3.69 (s, 3H, NCH3 in the carbene moiety),
3.56–3.48 (m, 1H, one of NCH2), 3.23–3.18 (m, 1H, one of NCH2),
2.20–2.16 (m, 1H, one of NCHCH2), 2.06–1.94 (m, 3H, three of
NCHCH2CH2). 13C NMR (125 MHz, CDCl3) δ 149.9 (carbene atom),
147.1 ( CH in the imidazole moiety), 140.4 ( CH in the carbene
moiety), 130.4 ( CH in the imidazole moiety), 129.1 ( CH in the
carbene moiety), 122.9 (Ar), 119.8 (Ar), 116.3 (Ar), 112.7 (Ar), 58.0
(NCH2CHN), 51.9 (NCH2CHN), 49.0 (NCH2CH2), 38.5 (NCHCH2CH2),
34.2 (NCHCH2CH2), 28.8 (CH3 in imidazole moiety), 23.3 (CH3 in
carbene moiety). MS (ESI) 591 (M + H+). IR (neat) ν 3127, 2956,
2918, 2854, 1735, 1608, 1537, 1486, 1409, 1377, 1339, 1283, 1232,
1112, 1093, 1037, 952, 928, 839, 731, 706 cm−1. Anal. calcd for
C19H25Br2N5Pd requires: C, 38.70%; H, 4.27%, N, 11.88%; found: C,
38.72%; H, 4.13%, N, 11.87%.
appropriate size were sealed under argon in Lindemann capillaries
and the data collections were carried out at 173(2) K.
General Procedure for the Pd(II)–NHC Complex 5-catalyzed
Benzoic Anhydride, 6, with Arylboronic Acids, 7
Under an N2 atmosphere, benzoic anhydride, 6 (1.5 mmol),
phenylboronic acid, 7a (0.5 mmol), NaHCO3 (2.4 mmol) and water
(1.5 ml) were added successively into a Schlenk reaction tube.
The mixture was stirred at room temperature for 5 min. Then
complex 5 (3.0 mol%) was added. The mixture was stirred at room
temperature for another 36 h and then was diluted with EtOAc,
washed with saturated brine and dried over Na2SO4. The organic
phase was filtered and concentrated under reduced pressure and
the residue was purified by flash column chromatography to give
the pure product. Compounds 8a,[15] 8b,[16] 8c,[16] 8d,[16] 8e,[17]
8f,[16] 8g,[18] 8h,[15] 8i[15] and 8j[16] are all known and were fully
determined according to the previously reported data.
Acknowledgments
Financial support from the National Natural Science Foundation
of China (no. 21002072), the Department of Education of Zhejiang
Province (no. Y200907072) and the Start-up Fund of Wenzhou
University is acknowledged.
Supporting Information
Supporting information can be found in the online version of
this article. The crystal data of compound 5 can be obtained free
of charge from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ [Fax: +44-1223-336-033; email:
deposit@ccdc.cam.ac.uk], reference number CCDC-782429.
Crystal Structure Determination of NHC–Pd(II) Complex 5
Structure solution and refinement was carried out with program
package SHELXTL-PLUS, version 5.1.[14] Details pertinent to the
crystal structure determinations are listed in Table 3. Crystals of
c
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Imidazole-coordinated monodentate NHC–Pd(II) complex
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