Anti-Markovnikov hydroaminations of styrene catalyzed by palladium(II) N-heterocyclic carbene complexes under conventional and microwave heating

Article abstract of DOI:10.1007/s11243-018-0245-1

Six palladium(II) complexes with benzimidazole-based N-heterocyclic carbene ligands were synthesized by transmetallation reactions between silver(I) N-heterocyclic carbene complexes and PdCl₂(PhCN)₂. The complexes were characterized by physicochemical and spectroscopic methods. The palladium complexes were tested as catalysts for intermolecular hydroamination reactions of styrene with various anilines in ionic liquids under both conventional and microwave heating. All of these complexes proved to be catalytically active in these reactions. The anti-Markovnikov addition products were selectively obtained by using 1?mol% of the palladium complex.

Full text of DOI:10.1007/s11243-018-0245-1

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0245-1
Anti‑Markovnikov hydroaminations of styrene catalyzed
by palladium(II) N‑heterocyclic carbene complexes under conventional
and microwave heating
Yetkin Gök1 · Beyhan Yiğit2 · Özlem Özeroğlu Çelikal1 · Murat Yiğit2
Received: 23 March 2018 / Accepted: 8 May 2018
© Springer International Publishing AG, part of Springer Nature 2018
Abstract
Six palladium(II) complexes with benzimidazole-based N-heterocyclic carbene ligands were synthesized by transmetal-
lation reactions between silver(I) N-heterocyclic carbene complexes and PdCl2(PhCN)2. The complexes were character-
ized by physicochemical and spectroscopic methods. The palladium complexes were tested as catalysts for intermolecular
hydroamination reactions of styrene with various anilines in ionic liquids under both conventional and microwave heating.
All of these complexes proved to be catalytically active in these reactions. The anti-Markovnikov addition products were
selectively obtained by using 1 mol% of the palladium complex.
Introduction
transition metals have more functional group tolerance and
are less sensitive to air and moisture. Numerous catalytic
systems including transition metal catalysts have been devel-
oped for the hydroamination of alkenes [12–15]. Generally,
palladium-catalyzed intermolecular hydroamination of sty-
renes with amines gives the Markovnikov addition products
[16–18]. Hartwig and co-workers have reported the addi-
tion of anilines to vinylarenes using palladium complexes
in the presence of an acid cocatalyst such as TfOH. The
Markovnikov addition products were obtained in 67–99%
yields [19]. Unlike other palladium complexes, some
palladium(II) N-heterocyclic carbene complexes catalyze
anti-Markovnikov addition of amines to alkenes [20, 21].
For example, the anti-Markovnikov addition of secondary
amines to activated olefns with good yields using palladium
and nickel precatalysts with 1,2,4-triazole based N-heter-
ocyclic carbene ligands has been reported by Ghosh [20].
While N-heterocyclic carbene (NHC) complexes have often
been used as catalysts in the intramolecular hydroamination
of alkenes and alkynes [22–24], their use in intermolecular
hydroamination reactions is more limited [25, 26]. NHCs are
a very important class of ligands which have been widely
used in organometallic chemistry and catalysis in the last
few decades, due to their versatile and tunable steric and
electronic properties [27]. NHCs are strongly σ-donating
and weakly π-accepting ligands, which form stable NHC
complexes with a range of transition metals [28]. A variety
of NHC complexes have been used as catalysts for various
transformations [29, 30].
The direct addition of N–H bonds across unactivated C–C
multiple bonds, commonly known as hydroamination, is a
simple and atom-economical route for constructing C–N
bonds [1]. Hydroamination reactions can be applied to the
synthesis of a broad range of nitrogen-containing molecules
[2]. Nitrogen-containing organic compounds are widely
found in nature and display a wide range of biological and
pharmacological activities. They are also very important
precursors in the synthesis of pharmaceuticals, agrochemi-
cals, and various bulk and fne chemicals [3]. Therefore,
much efort has been devoted to the synthesis and derivati-
zation of amine-functionalized molecules [4]. Hydroamina-
tion is an attractive synthetic approach for efcient amine
syntheses. Hydroamination reactions can be catalyzed by
a variety of diferent metal-based catalysts, including com-
plexes of main group metals, lanthanides, and both early
and late transition metals [5–11]. Although the early transi-
tion metal as well as the lanthanide catalysts displays high
activity for hydroamination reactions, catalysts based on late
* Murat Yiğit
myigit@adiyaman.edu.tr
1
Department of Chemistry, Faculty of Science and Art, Inönü
University, 44260 Malatya, Turkey
2
Department of Chemistry, Faculty of Science and Art,
Adıyaman University, 02040 Adıyaman, Turkey
Vol.:(0123456789)
1 3

Transition Metal Chemistry
Recently, ionic liquids have attracted much attention
due to their valuable properties such as chemical and ther-
mal stability, versatile structures and properties, negligible
vapor pressure, non-fammability, good dissolving abil-
ity and recyclability. Moreover, ionic liquids have been
used as solvents and/or catalysts in various chemical reac-
tions. Reactions performed in ionic liquids often show rate
enhancement, selectivity and higher yields with respect to
traditional solvents [31]. In recent years, use of microwave
heating in organic synthesis has attracted considerable
attention because it often reduces reaction times, enhances
reaction rates and improves product yields and purity [32].
The use of ionic liquids in microwave-assisted synthesis
has great potential, and many research groups have begun
to explore this approach in synthesis [33]. In this paper,
we report a series of palladium(II) complexes of benzimi-
dazole-based N-heterocyclic carbene ligands as efective
catalysts for the intermolecular hydroamination of styrene
with anilines.
General procedure for preparation of the Ag(I)–NHC
complexes 1
A mixture of the appropriate benzimidazolium chloride
(1.0 mmol), Ag2O (0.5 mmol) and activated 4 Å molecular
sieves in dichloromethane (20 mL) was stirred for 24 h at
room temperature in the dark. The mixture was then fltered
through Celite, and the solvent was removed under reduced
pressure. The crude product was recrystallized from dichlo-
romethane/hexane (1:2) at room temperature. The resulting
white solid was isolated by fltration and dried in vacuum.
Synthesis of Pd(II)–NHC complexes 2
A solution of the required silver(I) NHC complex
(0.62 mmol) and PdCl2(PhCN)2 (0.31 mmol) in dichlo-
romethane (20 mL) was stirred for 24 h at room tempera-
ture in the dark, whereupon an of-white AgCl precipitate
was observed. The resulting mixture was fltered through
Celite, and the solvent was removed under reduced pressure.
The crude product was recrystallized from dichloromethane/
diethyl ether (1:2) at room temperature. The white crystals
were fltered of, washed with diethyl ether (3×10 mL) and
dried under vacuum.
Experimental
All preparative reactions for the palladium(II)–NHC
complexes (2a–f) were carried out under argon in flame-
dried glassware using standard Schlenk techniques. The
solvents were purified by distillation over the drying
agents indicated, and transferred under Ar; THF, Et2O
(Na/K alloy), CH2Cl2 (P4O10), hexane, toluene (Na). All
reagents were purchased from Sigma-Aldrich, Merck or
Fluka. Microwave-assisted reactions were carried out in
a microwave oven system manufactured by Milestone
(Milestone Start S Microwave Labstation for Synthesis)
Bis[1‑benzyl‑3‑(naphthalen‑1‑ylmethyl)benzimidazol‑2
‑ylidene]‑dichloropalladium(II) 2a Yield: 0.20 g, 77%,
m.p.: 204–205 °C, IR: ν(NCN) 1607 cm−1. Anal. Calc. for
C50H40N4PdCl2: C, 68.69; H, 4.61; N: 6.41. Found: C,
1
68.68; H, 4.62; N, 6.42%. H NMR (CDCl3) δ: 5.75 and
5.84 (s, 4H, CH2C6H5), 6.10 and 6.21 (s, 4H, CH2C10H7),
6.37–8.45 (m, 32H, Ar–H). 13C NMR (CDCl3) δ: 49.6 and
51.4 (CH2C6H5), 52.4 and 53.2 (CH2C10H7), 111.1, 111.2,
113.5, 113.8, 122.5, 122.8, 123.0, 125.2, 125.3, 125.5,
125.8, 126.3, 126.4, 126.5, 127.2, 127.5, 127. 7, 127.8,
127.9, 128.1, 128.3, 128.4, 128.7, 128.8, 128.9, 129.2,
130.0, 130.3, 130.4, 130.8, 131.2, 131.3, 131.5, 132.8,
133.2, 133.5, 133.8, 134.3, 134.9, 135.3, 135.8 and 144.4
(Ar–C), 182.7 and 182.8 (C–Pd).
1
under aerobic conditions. All H and 13C NMR spectra
were recorded in CDCI3 using a Bruker AC300P FT
spectrometer operating at 300.13 (1H) or 75.47 MHz
(13C). Chemical shifts (δ) are given in ppm relative to
TMS, coupling constants (J) in Hertz. FTIR spectra were
recorded as KBr pellets in the range 400–4000 cm−1 on
a Mattson 1000 spectrophotometer. Gas chromatography
was carried out by GC-FID on an Agilent 6890 N gas
chromatograph equipped with an HP-5 column of 30 m
length, 0.32 mm diameter and 0.25 μm film thickness.
Melting points were measured in open capillary tubes
with an Electrothermal-9200 melting point apparatus and
are uncorrected. Elemental analyses were obtained with a
LECO CHNS-932 elemental analyzer.
Bis[1‑(4‑methylbenzyl)‑3‑(naphthalen‑1‑ylmethyl)ben‑
zimidazol‑2‑ylidene]‑dichloropalladium(II) 2b Yield:
0.21 g, 75%, m.p.: 260–261 °C, IR: ν(NCN) 1607 cm−1
.
Anal. Calc. for C52H44N4PdCl2: C, 69.22; H, 4.92; N, 6.21.
Found: C, 69.24; H, 4.93; N, 6.20%. 1H NMR (CDCl3) δ:
2.08 and 2.24 (s, 6H, CH2C6H4CH3-4), 5.85 and 6.12 (s,
4H, CH2C6H4CH3-4), 6.36 and 6.61 (s, 4H, CH2C10H7);
1 3

Transition Metal Chemistry
6.66–8.44 (m, 28H, Ar–H). 13C NMR (CDCl3) δ: 21.1 and
21.2 (CH2C6H4CH3-4), 49.6 and 49.7 (CH2C6H4CH3-4),
51.1 and 51.3 (CH2C10H7), 111.9, 112.1, 123.3, 123.7,
125.3, 125.4, 125.6, 125.8, 126.3, 126.6, 126.9, 127.9,
128.4, 128.6, 128.9, 129.1, 129.5, 130.5, 130.8, 131.3,
131.8, 133.1, 133.3, 133.5, 133.6, 133.9, 134.0, 134.4,
134.5, 136.9 and 137.5 (Ar–C), 182.3 (C–Pd).
CH2C6H2(OCH3)3-3,4,5), 5.70 and 6.06 (s, 4H, CH2C6H2
(OCH3)3-3,4,6), 6.23 and 6.54 (s, 4H, CH2C10H7), 6.63
and 6.86 (s, 4H, CH2C6H2(OCH3)3-3,4,5), 6.90–8.22 (m,
26H, Ar–H). 13C NMR (CDCl3) δ: 56.3, 56.4, 60.6 and 60.8
(CH2C6H2(OCH3)3-3,4,5), 49.6 (CH2C6H2(OCH3)3-3,4,5),
52.6 (CH2C10H7), 104.0, 104.9, 111.0, 111.2, 111.4, 122.3,
122.4, 123.1, 123.3, 125.1, 125.3, 125.4, 125.5, 125.6,
126.3, 126.5, 128.3, 128.8, 130.3, 130.4, 130.5, 130.9,
131.3, 133.2, 133.4, 134. 1, 134.9, 135.1, 137.1, 153.1 and
153.5 (Ar–C), 182.4 and 182.9 (C–Pd).
Bis[1‑(2‑methylbenzyl)‑3‑(naphthalen‑1‑ylmethyl)benzi‑
midazol‑2‑ylidene]‑dichloropalladium(II) 2c Yield: 0.18 g,
78%, m.p.: 253–254 °C, IR: ν(NCN) 1607 cm−1. Anal. Calc.
for C52H44N4PdCl2: C, 69.22; H, 4.92; N, 6.21. Found:
Bis[1‑(2‑methoxylethyl)‑3‑(naphthalen‑1‑ylmethyl)benzi‑
midazol‑2‑ylidene]‑dichloropalladium(II) 2f This known
compound was synthesized and identifed by comparison of
its melting point and spectroscopic data with those reported
in the literature [34].
1
C, 69.22; H, 4.91; N, 6.22%. H NMR (CDCl3) δ: 1.85
and 2.18 (s, 6H, CH2C6H4CH3-2), 5.86 and 5.88 (s, 4H,
CH2C6H4CH3-2), 6.35 and 6.38 (s, 4H, CH2C10H7), 6.42–
8.35 (m, 28H, Ar–H). 13C NMR (CDCl3) δ: 19.1 and 19.4
(CH2C6H4CH3-2), 49.6 and 49.8 (CH2C6H4CH3-2), 50.3
(CH2C10H7), 111.9, 112.0, 123.6, 123.8, 125.4, 125.6, 125.9,
126.1, 126.2, 126.4, 126.6, 126.7, 126.9, 127.3, 127.5,
128.5, 128.6, 128.9, 130.1, 130.4, 130.7, 130.9, 131.5,
131.8, 133.4, 133.6, 134.0, 134.1, 134.2, 134.3, 134.4,
134.5, 135.1 and 135.5 (Ar–C), 183.3 and 183.4 (C–Pd).
General procedure for hydroamination of styrene
by conventional heating
The NHC–Pd(II) catalyst 2 (1.0 mol%), t-BuOK
(1.10 mmol), styrene (1.10 mmol), aromatic amine
(1.00 mmol) and N-butylpyridinium hexafuorophosphate
(1 mL) were added to a small Schlenk tube, and the mix-
ture was heated at 160 °C for 1 h. At the end of reaction,
the mixture was cooled to room temperature, and water
(5 mL) was added. The mixture was extracted with EtOAc.
The organic phase was dried over anhydrous Na2SO4 and
fltered through a short silica column. The fltrate was
concentrated under reduced pressure and purifed by fash
chromatography on silica gel (EtOAc/hexane: 1:5). The
yields were calculated by GC analysis based on the aro-
matic amine.
Bis[1‑(2,4,6‑trimethylbenzyl)‑3‑(naphthalen‑1‑ylmethyl)
benzimidazol‑2‑ylidene]‑dichloropalladium(II) 2d Yield:
0.28 g, 76%, m.p.: 270–271 °C, IR: ν(NCN) 1607 cm−1. Anal.
Calc. for C56H52N4PdCl2: C, 70.18; H, 5.47; N, 5.85. Found:
1
C, 70.19; H, 5.49; N, 5.85%. H NMR (CDCl3) δ: 2.06,
2.27, 2.28 and 2.2 (s, 18H, CH2C6H2(CH3)3-2,4,6), 5.56
and 6.19 (s, 4H, CH2C6H2(CH3)3-2,4,6), 6.30 and 6.64 (s,
4H, CH2C10H7), 6.73 (s, 4H, CH2C6H2(CH3)3-2,4,6), 6.83-
8.35 (m, 26H, Ar–H). 13C NMR (CDCl3) δ: 20.8, 20.9, 21.0
and 21.1 (CH2C6H2(CH3)3-2,4,6), 49.2 and 49.4 (CH2C6H
2(CH3)3-2,4,6), 49.5 and 65.8 (CH2C10H7), 110.5, 110.9,
111.5, 111.7, 122.3, 122.6, 122.8, 125.1, 125.2, 125.3,
125.5, 125.6, 126.1, 126.5, 127.8, 127.9, 128.1, 138.3,
128.5, 129.3, 129.7, 130.1, 130.4, 130.5, 131.1, 133.1,
133.4, 134.2, 134.4, 134.9, 135.2, 137.6, 138.1 and 138.4
(Ar–C), 182.6 and 183.0 (C–Pd).
General procedure for microwave‑assisted
hydroamination of styrene
The NHC–Pd(II) catalyst 2 (1.0 mol%), t-BuOK
(1.10 mmol), styrene (1.10 mmol), aromatic amine
(1.00 mmol) and N-butylpyridinium hexafuorophosphate
(1 mL) were placed within the microwave apparatus, and
the mixture was heated at 160 °C (300 W) for 5 min. At the
end of the reaction, the mixture was cooled to room tempera-
ture, and water (5 mL) was added. The mixture was extracted
with EtOAc, and the organic phase was processed as for the
conventional heating experiments.
Bis[1‑(3,4,5‑trimethoxylbenzyl)‑3‑(naphthalen‑1‑yl‑
methyl)benzimidazol‑2‑ylidene]‑dichloropalladium(II)
2e Yield: 0.19 g, 83%, m.p.: 265–266 °C, IR: ν(NCN)
1600 cm−1. Anal. Calc. for C56H52N4O6PdCl2: C, 63.79;
H, 4.97; N, 5.31. Found: C, 63.79; H, 4.96; N, 5.31%.
1H NMR (CDCl3) δ: 3.72, 3.79, 3.83 and 3.84 (s, 18H,
1 3

Transition Metal Chemistry
with previously reported values for [PdCl2(NHC)2] com-
plexes [37]. The elemental analyses of complexes 2a–e are
in agreement with the theoretical values, while the NMR
data showed that all of these Pd-NHC complexes except 2b
were cis/trans-isomer mixtures.
Results and discussion
Synthesis of the complexes
The silver complexes were prepared according to the litera-
ture method from the 1,3-dialkylbenzimidazolium chlorides,
which were obtained by two-step N-alkylation of benzimi-
dazole [35]. The palladium(II)–NHC complexes 2a–f were
prepared by treatment of the silver(I)–NHC complexes with
PdCl2(PhCN)2 in dichloromethane at room temperature, in
good yields (Scheme 1). The rapid precipitation of AgCl
indicated successful carbene transfer onto palladium, giving
the palladium(II)–NHC complexes 2a–f. This useful syn-
thetic method has been used to synthesize a variety of metal
complexes [36]. The present complexes were obtained in
high yields and proved to be stable both in solution and in
the solid state against air, light and moisture. Single crys-
tals of these complexes suitable for X-ray difraction stud-
ies could not be obtained. Therefore, all of the Pd(II)–NHC
Catalytic studies
The intermolecular hydroamination of styrene is a con-
venient method for the synthesis of phenethylamines [38],
and many transition metal complexes have been used for
this reaction. Based on our previous results, in this study,
potassium tert-butoxide was selected as the base. Accord-
ing to previous studies, imidazolium-based ionic liquids
which contain an acidic hydrogen are easily converted to
N-heterocyclic carbenes by use of strong bases. Therefore,
the N-butylpyridinium hexafuorophosphate was used as
the ionic liquid. The catalytic reactions were performed at
160 °C by conventional heating for 1 h or microwave heat-
ing (300 W) for 5 min in the presence of complexes 2a–f. In
the absence of any Pd(II) complex, the reactions of styrene
with aniline, p-chloroaniline, 2-aminopyridine or p-meth-
oxyaniline resulted in very low yields under these reac-
tion conditions. Using these systems, palladium-catalyzed
hydroaminations of styrene with various aromatic amines
bearing electron-withdrawing or donating groups at the para
position were examined. In all cases, exclusively anti-Mark-
ovnikov products were formed. The reaction products were
characterized by NMR. The conversions were screened by
GC analysis, with the results presented in Table 1.
complexes were characterized by H NMR, 13C NMR, IR
1
spectroscopy and elemental analysis, which all supported
the proposed structures. NMR analyses of the complexes
showed that the N-heterocyclic carbene ligands had been
transferred from the silver complexes to palladium. The
Pd(II)–NHC complexes all showed a characteristic υ(NCN)
band at 1600–1607 cm−1 in their IR spectra. The 13C NMR
spectrum of all of the complexes except 2b showed two
signals for the C2 carbons at 182.7 and 182.8 ppm (2a),
182.3 ppm (2b), 183.3 and 183.4 ppm (2c), 182.6 and
183.0 ppm (2d), 182.4 and 182.9 ppm (2e). This is consistent
d, R=
e, R=
f, R=
a, R=
b, R=
c, R=
OMe
OMe
OMe
CH2OMe
N
N
N
N
[PdCl2(PhCN)2], CH2Cl2
RT, 24h
AgCl
PdCl2
2
R
R
1
2 a-f
Scheme 1 Synthesis of the palladium(II) NHC complexes
1 3

Transition Metal Chemistry
Table 1 Intermolecular hydroamination of styrene with anilines in
Initially, the reaction of styrene (3) with aniline (4)
(Scheme 2) was examined in the presence of complexes 2a–f
as catalysts. From all of these reactions, only the anti-Mark-
ovnikov hydroamination product, N-(2-phenylethyl)aniline
(8) was obtained, in high yields for all six catalysts (Table 1,
entries 1–6). The best yields were achieved with catalysts
2a and 2e. No formation of the Markovnikov product was
detected. Next, p-chloroaniline (5), p-methoxyaniline (6)
and 2-aminopyridine (7) were reacted with styrene in the
presence of complexes 2a–f under the same reaction condi-
tions (Table 1, entries 7–24). The treatment of styrene with
p-chloroaniline (5) and 2-aminopyridine (7) (Table 1, entries
7–12, 19–24) gave the corresponding secondary amines with
anti-Markovnikov selectivity in high yields. However, the
reaction with p-methoxyaniline (6) gave slightly lower yields
of the desired product (10) using both conventional and
microwave-assisted heating (Table 1, entries 13–18). These
results show that both electron-withdrawing (4-chloro) and
electron-donating (4-methoxy) substituents on aniline gave
slightly lower yields when compared to aniline itself. Similar
observations have been documented by Zhou and Hartwig
[39]. Both conventional and microwave-assisted heating pro-
vided similar results for the same substrates. However, reac-
tion times were greatly reduced by employing microwave
heating instead of conventional heating. Overall, complex
(2e) with methoxy substituents on the phenyl group exhib-
ited higher catalytic activities compared to the other fve
complexes.
ionic liquid
Entry
ArNH2
Catalyst
Yield (%)a
Microwave
heatingb
Thermal heatingc
1
2
3
4
5
6
7
8
4
4
4
4
4
4
5
5
5
5
5
5
6
6
6
6
6
6
7
7
7
7
7
7
4
5
6
7
2a
2b
2c
2d
2e
2f
2a
2b
2c
2d
2e
2f
2a
2b
2c
2d
2e
2f
2a
2b
2c
2d
2e
2f
None
None
None
None
96
85
91
95
99
96
88
90
93
85
92
89
83
80
89
90
95
94
86
91
87
85
94
92
9d
99
87
89
79
96
95
89
84
90
81
92
88
85
83
89
77
98
93
97
92
90
90
96
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
91
7d
Conclusion
6d
5d
6d
Traced
Traced
The palladium(II)–NHC complexes 2a–f have been readily
prepared by reactions of the corresponding silver(I)–NHC
complexes with PdCl2(PhCN)2 in dichloromethane at room
temperature. These complexes all catalyze the hydroami-
nation of styrene with aniline, p-chloroaniline, 2-ami-
nopyridine and p-methoxyaniline, in good to excellent
yields. Only the anti-Markovnikov addition products were
obtained.
4d
Reaction conditions: Pd-NHC (0.01 mmol), styrene (1.10 mmol), aro-
matic amine (1.00 mmol), t-BuOK (1.10 mmol), IL (1 mL)
aGC yields using dodecane as internal standard in an average of two
runs. Products were characterized by NMR
b160 °C (300 W), 5 min
c160 °C, 1 h
dNo catalyst
R
E
2 a-f (1 mmol%)
KOBut, IL
N
H
+
H2N
R
E
E : CH or N
3
4-7
8-11
Scheme 2 Anti-Markovnikov hydroamination of styrene with aromatic amines
1 3

Transition Metal Chemistry
19. Kawatsuro M, Hartwig JF (2000) J Am Chem Soc 122:9547
20. Dash C, Shaikh MM, Butcher RJ, Ghosh P (2010) Dalton Trans
39:2515
Acknowledgements We thank Turkish Research Council (TUBITAK)
(Project Number: 107T419) for fnancial support of this work.
21. Yiğit B, Yiğit M, Gök Y, Özeroğlu Çelikal Ö (2016) Heterocycles
92:1994
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