The synthesis and catalytic properties of imidazolinium salts and their palladium(II) complexes

Article abstract of DOI:10.1007/s11243-011-9573-0

Three 1,3-dialkylimidazolinium chloride salts and their palladium(II) complexes have been synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR and IR spectroscopy. The catalytic activities of the 1,3-dialkylimidazolinium chloride salts and their complexes in the Heck and Suzuki coupling reactions were investigated. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11243-011-9573-0

Transition Met Chem (2012) 37:183–188
DOI 10.1007/s11243-011-9573-0
The synthesis and catalytic properties of imidazolinium salts
and their palladium(II) complexes
˘
Beyhan Yigit
Received: 2 November 2011 / Accepted: 9 December 2011 / Published online: 21 December 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Three 1,3-dialkylimidazolinium chloride salts
and their palladium(II) complexes have been synthesized
and characterized by elemental analysis, 1H NMR, 13C
NMR and IR spectroscopy. The catalytic activities of the
1,3-dialkylimidazolinium chloride salts and their com-
plexes in the Heck and Suzuki coupling reactions were
investigated.
complexes. These reactions have been used extensively for
the synthesis of a range of useful stilbene and biaryl deriv-
atives [16–23]. Palladium(II) carbene complexes can be
easily prepared by the reaction of palladium(II) acetate
with two equivalents of a suitable azolium salt [24] or the use
of silver carbene complexes as carben transfer reagents
[25–28]. Generally, silver NHC complexes are prepared by
the treatment of silver oxide, silver carbonate or silver
acetate with the corresponding azolium salts [29–31].
Palladium complexes with 1,3-di[(R)-1-phenylethyl]
imidazolin-2-ylidene ligands were described by Herrmann,
who showed that the NHC-palladium complexes were
effective as catalysts for Heck, Suzuki and Stille reactions
[32, 33]. N,N0-dibenzhydrybenzimidazolium-palladium sys-
tems have been reported to be suitable for the Suzuki cou-
pling of aryl bromides with arylboronic acid [34]. Palladium
complexes with benzimidazolylidene ligands bearing an
(S)-a-methylbenzyl group on the N atom were reported by
Page et al. [35]. Recently, Ku¨ndig et al. [36] showed that
chiral NHC-palladium complexes are well suited as catalysts
for the asymmetric synthesis of fused indolines.
Introduction
The isolation of the first stable N-heterocyclic carbenes
(NHCs) by Arduengo and their successful use as ancillary
ligands for the preparation of various metal complexes ini-
tiated an intensive search for new N-heterocyclic carbene
ligands and their metal complexes [1–6]. The strong electron
donating ability of NHC ligands combined with their easily
varied steric demands has made them attractive ligands in a
variety of metal-catalyzed processes. NHC metal complexes
of palladium, ruthenium, rhodium, platinum and nickel have
been shown to act as effective pre-catalysts in many C–C
and C–N arylation, olefin metathesis, hydrogenation and
hydrosilylation reactions [7–15]. Among these systems, the
palladium-catalyzed Heck and Suzuki reactions are perhaps
the most widespread and successful applications of carbene
We have previously reported the synthesis of N-hetero-
cyclic carbene derivative ligands and their palladium(II)
carbene complexes and investigated their catalytic activity
in the Heck and Suzuki reactions [37–39]. Herein, we report
the synthesis of new imidazoliniums 3a–c (Scheme 1) and
their palladium(II) complexes 4a–c (Scheme 2), and their
application in Heck and Suzuki cross-coupling using aryl
halides.
Experimental
˘
B. Yigit (&)
Department of Chemistry, Faculty of Science and Art,
Adiyaman University, 02040 Adiyaman, Turkey
e-mail: byigit@adiyaman.edu.tr
All reactions for the preparation of 1,3-dialkylimidazolin-
ium salts 3a–c and their palladium complexes were carried
123

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Transition Met Chem (2012) 37:183–188
Ar
R
Ar
N C
Ar
H
N
O
N
NH2
NH2
R
R
CH R
CH(OEt)3 , NH4Cl
NaBH4
CH3OH
+) Cl-
N
R
C Ar
+
N C
Ar
R
N
H
CH
Ar
R
Ar
3a-c
1
2
OMe
N
N
N
+) Cl-
N
+) Cl-
N
+) Cl-
N
OMe
3a
3b
3c
Scheme 1 Synthesis of 1,3-dialkylimidazolinium salts
MeO
OMe
Cl
Cl
Cl
N
N
N
N
N
N
N
N
N
N
N
Pd
Cl
Pd
Cl
Pd
Cl
N
OMe
MeO
4c
4a
4b
Scheme 2 Palladium–carbene complexes reported in this work
1,3-Di(1-phenylethyl)imidazolinium chloride, (3a)
out under argon using standard Schlenk-type flasks. Heck
and Suzuki coupling reactions were carried out in air. All
1
To a solution of 1,2-bis(1-phenylethylamino)ethane (2.00 g,
7.46 mmol), CH(OEt)3 (3 mL, 18.06 mmol) and NH4Cl
(0.40 g, 7.46 mmol) were added, and reaction mixture was
heated for 12 h at 110 °C. The white precipitate was filtered
off and recrystallized from ethanol.
reagents were purchased from Aldrich (Turkey). All H
and 13C n.m.r. spectra were recorded in CDCl3 using a
Bruker AC300P FT spectrometer operating at 300.13 MHz
(1H) or 75.47 MHz (13C). Chemical shifts (d) are given
in ppm relative to TMS, coupling constants (J) in Hertz.
FTIR spectra were recorded as KBr pellets in the range
400–4,000 cm-1 on a Mattson 1000 spectrophotometer
(wavenumbers, cm-1). GC spectra were recorded on an
Agilent 6890 N GC system by GC-FID with an HP-5
column of 30 m length, 0.32 mm diameter and 0.25 lm
film thickness. Melting points were measured in open
capillary tubes with an Electrothermal-9200 melting point
apparatus and uncorrected. Elemental analyses were per-
Yield: 1.90 g, 81%, m.p. 143–145 °C. I.r., m(NCN)
:
1,644 cm-1. (Found: C, 72.4; H, 7.4; N, 8.9. C19H23N2Cl
calcd.: C, 72.5; H, 7.3; N, 8.9%). 1H-n.m.r. (CDCl3): d 1.77
(d, 6H, J = 6.6 Hz, CH(CH3)C6H5); 5.30 (quart., 2H, J =
6.6 Hz, CH(CH3)C6H5); 3.53 and 3,71 (m, 4H, NCH2CH2N);
7.28 and 7.42 (m, 10H, Ar–H); 10.65 (s, 1H, NCHN). 13C-
n.m.r. (CDCl3): d 19.1 (CH(CH3)C6H5); 45.5 (NCH2CH2N);
57.6 (CH(CH3)C6H5); 127.1; 128.8; 128.9; 129.2; 137.3 and
137.4 (Ar–C); 156.8 (NCHN).
¨ ¨
formed at Inonu University Research Center.
123

Transition Met Chem (2012) 37:183–188
185
1,3-Di[1-(4-methoxyphenyl)ethyl]imidazolinium
56.9 (CH(CH3)C6H5); 127.5; 128.0; 128,5; 139.4; 139.5;
139.6; 139.7 and 139.8 (Ar–C); 197.7 (Ccarbene).
chloride, (3b)
This compound was prepared in the same way as 3a
from 1,2-bis[1-(4-methoxyphenyl)ethylamino]ethane (1.91 g,
5.82 mmol), CH(OEt)3 (3 mL, 18.06 mmol) and NH4Cl
(0.31 g, 5.80 mmol) to give white crystals of 3b.
Bis[1,3-di(1-(4-methoxyphenyl)ethyl)imidazolin-2-
ylidene]dichloropalladium(II), (4b)
Compound 4b was prepared in a similar way to 4a, from
1,3-di[1-(4-methoxyphenyl)ethyl]imidazolinium chloride
(0.40 g, 1.08 mmol) and Pd(OAc)2 (0.12 g, 0.54 mmol).
Yield: 1.61 g, 74%, m.p. 147–150 °C. I.r., m(NCN)
:
1,639 cm-1. (Found: C, 67.4; H, 7.2; N, 7.4. C21H27N2O2Cl
1
calcd.: C, 67.3; H, 7.2; N, 7.5%). H-n.m.r. (CDCl3): 1.70
Yield: 0.32 g, 71%, m.p. 218 °C. I.r., m(NCN): 1,515 cm-1
.
(d, 6H, J = 6.9 Hz, CH(CH3)C6H4(OMe)-4); 3.49 and 3,69
(m, 4H, NCH2CH2N); 3.77 (s, 6H, CH(CH3)C6H4(OMe)-4);
5.26 (quart., 2H, J = 6.9 Hz, CH(CH3)C6H4(OMe)-4);
6.86 and 7.36 (d, 8H, J = 8.1 Hz and J = 7.5 Hz,
CH(CH3)C6H4(OMe)-4); d 10.60 (s, 1H, NCHN). 13C-
n.m.r. (CDCl3): d 19.0 (CH(CH3)C6H4(OMe)-4); 45.2
(NCH2CH2N); 55.3 CH(CH3)C6H4(OMe)-4); 57.1 (CH(CH3)
C6H4(OMe)-4); 114.0; 114.5; 128.1; 128,5; 129.3; 129.4
and 156.7 (Ar–C); 159.8 (NCHN).
(Found: C, 59.1; H, 6.0; N, 6.6. C42H52N4O4PdCl2 calcd.: C,
1
59.0; H, 6.1; N, 6.5%). H-n.m.r. (CDCl3): d1.73 and 1.81
(m, 12H, CH(CH3)C6H4(OMe)-4); 2.84 and 3,39 (m, 8H,
NCH2CH2N); 3.78 and 3.82 (m, 12H, CH(CH3)C6H4(OMe)-
4); 6.63 and 6.67 (m, 4H, CH(CH3)C6H4(OMe)-4); 6.85 and
7.59 (m, 16H, CH(CH3)C6H4(OMe)-4). 13C-n.m.r. (CDCl3):
d 16.8 (CH(CH3)C6H4(OMe)-4); 42.6 (NCH2CH2N); 55.2
CH(CH3)C6H4(OMe)-4); 56.3 (CH(CH3)C6H4(OMe)-4);
113.7; 129.1; 131,7; 131.8; 131.9 and 158.9 (Ar–C); 197.2
(Ccarbene).
1,3-Dibenzhydrylimidazolinium chloride, (3c)
Bis(1,3-dibenzhydrylimidazolin-2-
ylidene)dichloropalladium(II), (4c)
This compound was prepared in the same way as 3a from
1,2-bis(dibenzhydrylamino)ethane (2.00 g, 5.09 mmol),
CH(OEt)3 (3 mL, 18.06 mmol) and NH4Cl (0.27 g, 5.05
mmol) to give white crystals of 3c.
Compound 4c was prepared in a similar way to 4a, from 1,3-
dibenzhydrylimidazolinium chloride (0.47 g, 1.08 mmol)
and Pd(OAc)2 (0.12 g, 0.54 mmol).
Yield: 1.92 g, 86%, m.p. 197–198 °C. I.r., m(NCN)
:
Yield: 0.44 g, 84%, m.p. 306 °C. I.r., m(NCN)
:
1,631 cm-1. (Found: C, 79.4; H, 6.2; N, 6.4. C29H27N2Cl
calcd.: C, 79.4; H, 6.1; N, 6.4%). 1H-n.m.r. (CDCl3): d 4,07
(s, 4H, NCH2CH2N); 6.42 (s, 2H, CH(C6H5)2); 7.10 and
7.41 (m, 20H, Ar–H); 8.24 (s, 1H, NCHN). 13C-n.m.r.
(CDCl3): d 48.7 (NCH2CH2N); 66.3 (CH(C6H5)2); 128.4;
128,9; 129.3 and 135.6 (Ar–C); 158.6 (NCHN).
1,504 cm-1. (Found: C, 71.0; H, 5.4; N, 5.8. C58H52N4
PdCl2 calcd.: C, 70.9; H, 5.3; N, 5.7%). 1H-n.m.r. (CDCl3):
d 3,29 (s, 8H, NCH2CH2N); 7.21 and 7.30 (m, 40H, Ar–H);
7.60 (s, 4H, CH(C6H5)2). 13C-n.m.r. (CDCl3): d 45.1
(NCH2CH2N); 65.6 (CH(C6H5)2); 127.3; 128,1; 128.4;
129.0; 129.4 and 138.5 (Ar–C); 201.6 (Ccarbene).
Bis[1,3-di(1-phenylethyl)imidazolin-2-
General procedure for the Heck-type coupling reactions
ylidene]dichloropalladium(II), (4a)
Pd(OAc)2 (1.0 mmol %), salts 3a–c (2.0 mmol %) or 4a–
c (1.5 mmol %), aryl bromide (1.0 mmol), styrene
(1.5 mmol), Cs2CO3 (2.0 mmol) and dioxane (3 mL) were
placed in a Schlenk tube under argon, and mixture was
heated at 80 °C for 8 h. At the conclusion of the reaction,
the mixture was cooled, extracted with Et2O, filtered
through a pad of silica gel with copious washings, con-
centrated and purified by flash chromatography on silica
gel. Purity of compounds was checked by NMR and GC.
The yields are based on aryl bromide.
A stirred DMSO solution (10 mL) of 1,3-di(1-phenyl-
ethyl)imidazolinium chloride (0.34 g, 1.08 mmol) and
Pd(OAc)2 (0.12 g, 0.54 mmol) was heated at 60 °C for 3 h
and then at 110 °C for a further 2 h. The volatile compounds
were removed under vacuum and the precipitate was filtered
off, then washed twice with THF. Recrystallization from
CH2Cl2–Et2O was carried out. The crystals were washed
with diethyl ether (3 9 15 mL) and dried under vacuum.
Yield: 0.32 g, 80%, m.p. 282 °C. I.r., m(NCN)
:
1,510 cm-1. (Found: C, 62.2; H, 5.9; N, 7.7. C38H44N4
PdCl2 calcd.: C, 62.2; H, 6.0; N, 7.6%). 1H-n.m.r. (CDCl3):
d 1.77 and 1.85 (m, 12H, CH(CH3)C6H5); 2.84 and 3,43
(m, 8H, NCH2CH2N); 6.70 (quart., 4H, J = 6.6 Hz,
CH(CH3)C6H5); 7.24 and 7.70 (m, 20H, Ar–H). 13C-n.m.r.
(CDCl3): d 16.52 (CH(CH3)C6H5); 41.8 (NCH2CH2N);
General procedure for the Suzuki-type coupling
reactions
Pd(OAc)2 (1.0 mmol %), salts 3a–c (2.0 mmol %) or 4a–
c (1.5 mmol %), aryl chloride (1.0 mmol), phenylboronic
123

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Transition Met Chem (2012) 37:183–188
acid (1.5 mmol), Cs2CO3 (2.0 mmol) and water (3 mL)-
DMF (3 mL) were placed in a Schlenk tube under argon,
and the mixture was heated at 80 °C for 5 h. At the
conclusion of the reaction, the mixture was cooled,
extracted with Et2O, filtered through a pad of silica gel
with copious washings, concentrated and purified by flash
chromatography on silica gel. Purity of the compounds
was checked by NMR and GC. The yields are based on
aryl chloride.
carbene complexes, show that Ccarbene is substantially de-
shielded. The values of d (13Ccarbene) are 197.7, 197.2 and
201.6 ppm for 4a, 4b and 4c, respectively, and are similar
to those found in other carbene complexes. These new
complexes show typical spectroscopic signatures, which
are in line with those recently reported for trans-
[PdCl2(NHC)2] complexes [43–46].
The Pd-catalyzed Heck reaction between aryl halides
and alkenes is a very useful method to prepare styrene
derivatives. The Heck coupling reactions of styrene with
aryl bromides to form alkenes were undertaken with
Pd(OAc)2/3a–c or the complexes 4a–c as catalysts in the
presence of a base in air. The performance of such metal
catalyzed cross-coupling reactions depends on a number of
factors such as base, solvent, time, temperature, ligands
and nature of the catalyst. The choices of base and solvent
are usually important in achieving an efficient cross-cou-
pling reaction. For optimization of the reaction conditions,
the Pd(OAc)2-catalyzed cross-coupling of bromobenzene
with styrene was employed as a model reaction using
ligand 3a at 80 °C, and the commonly used bases Cs2CO3,
K2CO3, K3PO4 and t-BuOK were tested. The coupling of
aryl bromides and styrene was carried out in dioxane
(3 mL) with 1 mol % Pd(OAc)2, 2 mol % 3a–c or 1.5 mol
% 4a–c and 2 equivalents of Cs2CO3 for 8 h at 80 °C.
Reaction of p-bromoacetophenone, p-bromotoluene, p-
bromobenzaldehyde, p-bromoanisol and bromobenzene
with styrene gave high yields (77–99%) using this catalytic
system, and the coupling reaction did not occur in the
absence of salt or palladium complex. The results are
summarized in table 1.
Results and discussion
Saturated imidazolinium salts can be obtained by alkylation
of dihydroimidazole and by cyclization reactions of
N,N0-dialkyl-1,2-diaminoethane dihydro halides with tri-
ethyl orthoformate [40, 41]. The general route for the
preparation of 1,3-dialkylimidazolinium salts is shown in
Scheme 1. The condensation of 1,2-diaminoethane with
ketones gave the corresponding imines 1 in high yields.
Reduction of these imines with NaBH4 in methanol leads
to N,N0-dialkylethane-1,2-diamines 2. The symmetrical
1,3-dialkylimidazolinium salts 3 were easily synthesized in
high yields by reaction of 2 with triethyl orthoformate and
ammonium chloride. After purification, the 1,3-dialkylim-
idazolinium salts 3a–c were obtained in good yields
(74–86%). The salts are soluble in the common polar sol-
vents such as ethanol and dichloromethane and are stable
under air and in the presence of moisture. The structures of
3 were deduced from their spectroscopic data and elemental
analyses (see Experimental section). The 13C n.m.r. spectra
of 3a–c show sharp high-field signals at 156.8 ppm (3a),
159.8 ppm (3b) and 158.6 ppm (3c) for the imino carbon.
Of the five different aryl bromides, those bearing elec-
tron-withdrawing groups react with styrene to give the
coupled products in excellent yields (Table 1, entries 2, 8,
13, 21 and 26). The imidazolinium salt bearing a methoxy
group on the aromatic ring (3b) was the most effective in
catalyzing these reactions. Similarly, the palladium carbene
complex (4b) derived from (3b) showed better catalytic
activity than the other complexes. Hence, the electron
donating p-methoxy substituent increased the catalytic
activity. As expected, in situ reaction of the azolium salts
with palladium led to significantly better results than the
use of the preformed palladium complexes. Recently, the
Suzuki reactions of aryl chlorides catalyzed by palladium–
NHC complexes have been extensively studied and
these catalysts are very effective for Suzuki coupling of
aryl chlorides in dioxane at 80 °C, with Cs2CO3 as base
[45–47]. The Suzuki coupling of phenylboronic acid with
aryl chlorides to form biaryls was therefore studied with
Pd(OAc)2/3a–c or 4a–c as catalysts. Similar reaction con-
ditions were employed as for the Suzuki reactions, with
Cs2CO3 as the base. The coupling reactions of aryl chlo-
rides and phenylboronic acid were carried out in DMF/H2O
1
The H n.m.r. spectra of the imidazolinium salts further
supported the assigned structures. The C(2)-H resonances
were observed as sharp singlets at d 10.65, 10.60 and 8.24
ppm for 3a–c, respectively. The IR data for 3a–c clearly
indicate the presence of the –C=N- group with a m(C=N)
vibration at 1,644, 1,639 and 1,631 cm-1 for 3a–c,
respectively. These NMR and IR values are similar to those
reported for other 1,3-dialkylimidazolinium salts [42].
The palladium complexes 4a–c were prepared by reac-
tion of the 1,3-dialkylimidazolinium chlorides with
Pd(OAc)2 in DMSO (Scheme 2). The complexes were
characterized by 1H and 13C n.m.r. spectroscopy, FTIR
and elemental analysis. They show a characteristic t(NCN)
band at 1,510, 1,515 and 1,504 cm-1 for 4a, 4b and 4c,
1
respectively. In the H n.m.r. and 13C n.m.r. spectra, the
absence of signals for the acidic proton (2-CH) at 10.65,
10.60, 8.24 ppm and imidazolinium carbon (2-CH) at
156.8, 159.8, 158.6 ppm showed the formation of the
expected palladium complexes. The 13C n.m.r. chemical
shifts, which provide a useful diagnostic tool for metal
123

Transition Met Chem (2012) 37:183–188
187
Table 1 The Heck coupling reaction of aryl bromides with styrene
Table 2 The Suzuki coupling reaction of aryl chlorides with phen-
ylboronic acid
+
R
Br
R
R
Cl
B(OH)2
R
+
Entry
R
Catalyst
Yield (%)
Entry
R
Catalyst
Yield (%)
1
COCH3
COCH3
COCH3
COCH3
COCH3
COCH3
CH3
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
98
99
99
92
95
94
85
87
84
79
81
77
98
97
95
90
93
90
83
85
86
80
84
83
93
97
96
89
93
90
1
COCH3
COCH3
COCH3
COCH3
COCH3
COCH3
CH3
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
3a
3b
3c
4a
4b
4c
98
97
95
96
95
91
81
84
80
75
79
77
94
96
94
90
93
91
83
85
80
74
79
76
87
91
90
83
86
82
2
2
3
3
4
4
5
5
6
6
7
7
8
CH3
8
CH3
9
CH3
9
CH3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
CH3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
CH3
CH3
CH3
CH3
CH3
CHO
CHO
CHO
CHO
CHO
CHO
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
H
CHO
CHO
CHO
CHO
CHO
CHO
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
H
H
H
H
H
H
H
H
H
H
H
1.0 mmol of p-R-C6H4Br, 1.5 mmol of styrene, 2 mmol Cs2CO3,
1 mmol % Pd(OAc)2, 2 mmol % 3a–c or 1.5 mmol % 4a–c, dioxane
(3 mL), 80 °C, 8 h; purity of compounds checked by n.m.r., and
yields are based on aryl halide; All reactions were monitored by TLC
1.0 mmol of p-R-C6H4Cl, 1.5 mmol of phenylboronic acid, 2 mmol
Cs2CO3, 1 mmol % Pd(OAc)2, 2 mmol % 3a–c, or 1.5 mmol % 4a–c,
DMF/H2O (3/3 mL), 80 °C, 5 h; purity of compounds checked by
n.m.r. and yields are based on aryl chloride; all reactions were
monitored by TLC
(3:3 mL) with 1.0 mol % Pd(OAc)2, 2.0 mol % 3a–c or
1,5 mol % 4a–c and 2 equivalents of Cs2CO3 for 5 h at
80 °C. We chose the coupling of 4-chloroacetophenone
with phenylboronic acid for these experiments. The results
are summarized in Table 2.
Conclusion
In summary, three 1,3-dialkylimidazolinium chloride salts
3a–c and their palladium complexes 4a–c were prepared and
their catalytic activities in the Heck and Suzuki coupling
reactions were investigated. The in situ catalytic system
proved to be more effective than the preformed palladium
complex in both the Heck reactions of aryl bromides with
styrene and the Suzuki reactions of aryl chlorides with
phenylboronic acid. More detailed investigations focusing
These salts proved to be effective catalyst precursors for
the coupling of inactivated, activated and deactivated
chlorides. These catalysts give similar activities to those of
other Pd-NHC complexes [34, 47] or in situ prepared
Pd(OAc)2/NHC systems [42, 48, 49] and are in agreement
with other reports [33, 50].
123

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on imidazolidin-2-ylidene and benzimidazolin-2-ylidene
substituent effects, functional group tolerance and catalytic
activities in other coupling reactions are ongoing.
Acknowledgment This research was supported by the Adiyaman
University Research Fund (FEFBAP 2009-7).
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