Synthesis of novel 1-alkylimidazoline and 1-alkylbenzimidazole palladium(II) complexes as efficient catalysts for Heck and Suzuki reactions involving arylchlorides

Article abstract of DOI:10.1016/j.molcata.2003.07.010

Six palladium(II) complexes of the type [PdCl₂(PR ₃)L], {PR₃=PPh₃ or PPhMe₂; L=1-alkyl-2-imidazoline (1) or 1-alkylbenzimidazole(2)} have been prepared by reactions of [PdCl₂(PR₃

Full text of DOI:10.1016/j.molcata.2003.07.010

Journal of Molecular Catalysis A: Chemical 208 (2004) 109–114
Synthesis of novel 1-alkylimidazoline and 1-alkylbenzimidazole
palladium(II) complexes as efficient catalysts for Heck
and Suzuki reactions involving arylchlorides
Ismail Özdemira,∗, Bekir Çetinkayab, Serpil Demira
˙
a
Department of Chemistry, Inönü University, 44069 Malatya, Turkey
b
˙
Department of Chemistry, Ege University, 35100 Bornova-Izmir, Turkey
Received 21 March 2003; accepted 15 July 2003
Abstract
Six palladium(II) complexes of the type [PdCl2(PR3)L], {PR3 = PPh3 or PPhMe2; L = 1-alkyl-2-imidazoline (1) or 1-alkylbenzimidazole-
(2)} have been prepared by reactions of [PdCl2(PR3)]2. The complexes were characterized by conventional spectroscopic methods and
elemental analyses. The incorporation of N-coordinated imidazoline and benzimidazole complexes of palladium(II) gave high catalytic
activity in the Suzuki coupling and Heck reaction of aryl bromides and deactivated aryl chloride substrates.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Palladium; Imidazolidine; Benzimidazole; Suzuki; Heck
1. Introduction
Furthermore, tertiary phosphines require air-free handling
to prevent their oxidation and are susceptible to P–C bond
cleavage at elevated temperatures [7].
The olefination of aryl halides (via Heck reaction
Scheme 1a), or the reaction of aryl halides with arylboronic
acids (via Suzuki reaction, Scheme 1b) is one of the most
general C–C coupling reactions in organic synthesis, is cat-
alyzed by palladium complexes in homogeneous solution
[1,2].
The reactivity of the aryl halide component decreases
drastically in the order X = I > Br > Cl and electron with-
drawing substituents R are required for the chlorides to react
[1–3]. The low reactivity of aryl chlorides in cross-coupling
reactions is generally ascribed to their reluctance to oxida-
tively add to Pd(0) [3]. Current interest focuses on the use
of aryl chlorides since they are cheaper and more readily
accessible than bromides and iodides [4]. Significant ad-
vances have been recently achieved by use of palladacycles
and especially, by use of bulky and electron-rich tertiary
phosphines as catalyst modifiers systems [5,6]. However,
the major drawback of these is that the phosphine ligands
are comparatively difficult to make or rather expensive.
On the other hand, transition metal complexes with lig-
ands containing nitrogen-donor atoms have been shown their
potential to successfully promote the catalytic transforma-
tion of organic compounds [8,9]. These discoveries motivate
the search for the new metal complexes with N-coordinated
ligands and the evaluation of their catalytic properties.
Our contribution to this field has started with synthe-
ses of 2-imidazoline and benzazole complexes of Pt(II),
Rh(I) and Ru(II) which are capable of catalyzing the cy-
clopropanation of styrene with ethyl diazoacetate and in-
tramolecular cyclization of (Z)-3-methylpent-2-en-4-yn-1-ol
into 2,3-dimethylfuran in good yields [10–14]. The imida-
zoline and benzimidazole complexes are cheap to prepare,
insensitive to air and moisture and are thermally stable in
both the solid state and in solution.
However, the development of new ligands or the applica-
tion of existing ligands in Suzuki and Heck reactions, par-
ticularly those involving aryl chlorides as substrates, is still
of considerable importance.
We report here the straightforward preparation of a series
phosphinepalladium complexes of N-coordinated 1-alkyl-2-
imidazoline and benzimidazole derivatives. These com-
∗
Corresponding author. Fax: +90-4223410037.
˙
E-mail address: iozdemir@inonu.edu.tr (I. Özdemir).
1381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.07.010

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I. Özdemir et al. / Journal of Molecular Catalysis A: Chemical 208 (2004) 109–114
Scheme 1. Heck and Suzuki reactions.
plexes were shown to be effective catalysts for Heck and
Suzuki reactions especially with arylchlorides.
crystals of 1b in 270 mg, 78% yield, mp = 168–169 ◦C,
ν(CN) = 1605 cm−1
.
Anal. Cal. for C14H23N2OPPdCl2; C: 37.89, H: 5.19, N:
6.31; found C: 37.94, H: 5.22, N: 6.29.
1H-NMR (␦, CDCl3): 3.53 (t, J 9.80, CH2CH2OCH3);
4.1 (t, J 9.80, CH2CH2OCH3), 3.19 (s, CH2CH2OCH3);
3.23 [m, 4H, NCH2CH2NPd]; 1.67 and 1.70 [s, 6H,
P(C6H5)(CH3)2]; 7.30 and 7.68 [m, P(C6H5)(CH3)2];
7.25 [s, 1H, NCHN]; 13C{H}-NMR (␦, CDCl3): 57.26
(CH2CH2OCH3); 70.35 (CH2CH2OCH3); 58.81 (CH2CH2O
CH3)]; 47.10 [NCH2CH2NPd]; 49.04 [NCH2CH2NPd];
14.05 and 14.44 [P(C6H5)(CH3)2]; 129.31, 130.63, 131.94
and 138.27 [P(C6H5)(CH3)2]; 161.22 [NCHN].
2. Experimental
All reactions were performed using Schlenk-type flask
under argon and standard high vacuum-line techniques. Sol-
vents were analytical grade and distilled under argon from
sodium benzophenone (Et2O, dioxane, toluene, n-hexane).
NMR spectra were recorded at 297 K on a Bruker AC300P
FT spectrometer operating at 300.13 MHz (1H), 75.47 MHz
(13C). FT-IR spectra were recorded on a Mattson 1000 spec-
trophotometer. Samples were prepared as KBr discs. El-
emental analyses were performed by TUBITAK Microlab
(Ankara).
2.3. Preparation of 1-(2,4,6-trimethylbenzyl)benzimidazole
dichloro(dimethylphenylphosphine)palladium(II), 2a
Compound 2a was prepared in the same way as 1a
from 1-(2,4,6-trimethylbenzyl)benzimidazole (150 mg,
0.599 mmol) and [PdCl2(PPhMe2)]2 (189 mg, 0.299 mmol)
to give orange crystals of 2a in 291 mg, 86% yield, mp =
2.1. Preparation of 1-(2,4,6-trimethylbenzyl)-2-imidazoline
dichloro(dimethylphenylphosphine)palladium(II), 1a
A
solution of 1-(2,4,6-trimethylbenzyl)imidazoline
205–206 ◦C, ν(CN) = 1609 cm−1
.
(150 mg, 0.742 mmol) in toluene (15 ml) and [PdCl2(PPh-
Me2)]2 (234 mg, 0.371 mmol) was heated for 2 h under
reflux. n-Hexane (5 ml) was added to the warm solution.
Upon cooling to room temperature orange crystals of com-
plex 1a were filtered off, washed with hexane (2 × 5 ml)
and dried in vacuum, mp = 156–157 ◦C, and the yield was
Anal. Cal. for C25H29N2PPdCl2; C: 53.06, H: 5.13, N:
4.95; found C: 52.96, H: 5.08, N: 4.87.
1H-NMR (␦, CDCl3): 2.17 and 2.26 [s, 9H, 2,4,6-(CH3)3
C6H2CH2]; 5.17 [s, 2H, 2,4,6-(CH3)3C6H2CH2]; 6.89
[s, 2H, 2,4,6-(CH3)3C6H2CH2]; 1.80 and 1.83 [s, 6H,
P(C6H5)(CH3)2]; 7.40 and 7.82 [m, P(C6H5)(CH3)2];
7.39 and 8.29 [m, 4H, NC6H4N]; 7.66 [s, 1H, NCHN];
13C{H}-NMR (␦, CDCl3): 20.19 and 21.65 [2,4,6-(CH3)3
C6H2CH2]; 44.47 [2,4,6-(CH3)3C6H2CH2]; 129.84, 129.95,
131.09, 139.24 [2,4,6-(CH3)3C6H2CH2]; 14.24 and 14.63
[P(C6H5)(CH3)2]; 132.04, 132.07, 132.26 and 132.36
[P(C6H5)(CH3)2]; 100.61, 111.06, 122.14, 124.65, 125.13
and 125.72 [NC6H4N]; 144.15 [NCHN].
311 mg, 81%, ν(CN) = 1611 cm−1
.
Anal. Cal. for C21H29N2 PPdCl2; C: 48.70, H: 5.60, N:
5.41; found C: 48.75, H: 4.58, N: 5.37.
1H-NMR (␦, CDCl3): 2.18 and 2.21 [s, 9H, 2,4,6-(CH3)3
C6H2CH2]; 4.21 [s, 2H, 2,4,6-(CH3)3C6H2CH2]; 6.79
[s, 2H, 2,4,6-(CH3)3C6H2CH2]; 1.71 and 1.74 [s, 6H,
P(C6H5)(CH3)2]; 7.37 and 7.73 [m, P(C6H5)(CH3)2]; 3.21
[t, J = 10.3 Hz, NCH2CH2NPd]; 3.85 [t, J = 10.3 Hz,
NCH2CH2N-Pd]; 7.30 [s, 1H, NCHN]; 13C{H}-NMR (␦,
CDCl3): 21.21 and 21.48 [2,4,6-(CH3)3C6H2CH2]; 52.95
[2,4,6-(CH3)3C6H2CH2]; 128.97, 129.73, 129.84, 138.58
[2,4,6-(CH3)3C6H2CH2]; 14.25 and 14.64 [P(C6H5)(CH3)2];
130.41, 131.82, 132.24 and 139.40 [(C6H5)(CH3)2];
46.03 [NCH2CH2NPd]; 48.93 [NCH2CH2NPd]; 160.23
[NCHN].
2.4. Preparation of 1-(2,4,6-trimethylbenzyl)benzimidazole
dichloro(triphenylphosphine)palladium(II), 2b
Compound 2b was prepared in the same way as 1a
from 1-(2,4,6-trimethylbenzyl)benzimidazole (150 mg,
0.599 mmol) and [PdCl2(PPh3)]2 (263 mg, 0.299 mmol) to
give orange crystals of 2b in 342 mg, 83% yield, mp =
2.2. Preparation of 1-methoxyethyl-2-
imidazolinedichloro(dimethylphenylphosphine)-
palladium(II), 1b
267–268 ◦C, ν(CN) = 1610 cm−1
.
Anal. Cal. for C35H33N2PPdCl2; C: 60.92, H: 4.78, N:
4.06; found C: 61.18, H: 4.73, N: 4.12.
1H-NMR (␦, CDCl3): 2.22 and 2.31 [s, 9H, 2,4,6-(CH3)3
C6H2CH2]; 5.22 [s, 2H, 2,4,6-(CH3)3C6H2CH2]; 6.94 [s,
2H, 2,4,6-(CH3)3C6H2CH2]; 7.44 [m, 15H, P(C6H5)3]; 7.81
[m, 4H, NC6H4N]; 8.46 [s, 1H, NCHN]; 13C{H}-NMR
Compound 1b was prepared in the same way as 1a
from 1-methoxyethylimidazoline (100 mg, 0.781 mmol) and
[PdCl2(PPhMe2)]2 (246 mg, 0.390 mmol) to give orange

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111
(␦, CDCl3): 20.09 and 21.52 [2,4,6-(CH3)3C6H2CH2];
44.29 [2,4,6-(CH3)3C6H2CH2]; 128.61, 131.32, 135.28,
135.51 [2,4,6-(CH3)3C6H2CH2]; 129.22, 130.34, 131.67
and 139.69 [P(C6H5)3]; 110.42, 121.59, 123.96, 124.40,
126.25 and 128.37 [NC6H4N]; 143.49 [NCHN].
crystals of 2d in 311 mg, 89% yield, mp = 110–111 ◦C,
ν(CN) = 1597 cm−1
.
C18H23N2PPdCl2; C: 45.43, H: 4.84, N: 5.89; found C:
45.39, H: 4.76, N: 5.83.
1H-NMR (␦, CDCl3): 4.61 [sept, 1H, J = 6.72 Hz,
CH(CH3)2]; 1.59 [d, 6H, J = 6.72 Hz, CH(CH3)2]; 1.89
and 1.94 [s, 6H, P(C6H5)(CH3)2]; 7.48 and 7.94 [m,
P(C6H5)(CH3)2]; 7.30 [m, 4H, NC6H4N]; 8.32 [s, 1H,
NCHN]; 13C{H}-NMR (␦, CDCl3): 48.68 [CH(CH3)2];
22.44 [CH(CH3)2]; 13.70 and 14.22 [P(C6H5)(CH3)2];
130.84, 130.87, 131.03 and 131.16 [P(C6H5)(CH3)2];
110.44, 121.00, 123.19, 123.69, 128.61 and 128.74
[NC6H4N]; 141.22 [NCHN].
2.5. Preparation of
1-methoxyethylbenzimidazoledichloro(dimethylphenyl-
phosphine)-palladium(II), 2c
Compound 2c was prepared in the same way as 1a from
1-methoxyethylbenzimidazole (120 mg, 0.681 mmol) and
[PdCl2(PPhMe2)]2 (214 mg, 0.340 mmol) to give orange
crystals of 2c in 291 mg, 87% yield, mp = 102–103 ◦C,
ν(CN) = 1613 cm−1
.
Anal. Cal. for C18H23N2OPPdCl2; C: 43.96, H: 4.68, N:
5.69; found C: 43.89, H: 4.61, N: 5.71.
3. Results and discussion
1H-NMR (␦, CDCl3): 3.65 [t, 2H, J
=
4.96 Hz,
The N-coordinated palladium complexes were synthe-
sized according to the steps illustrated in Scheme 2. The re-
action of the 1-alkyl-2-imidazoline or 1-alkylbenzimidazole
with binuclear [PdCl2(PPhMe2)]2 and [PdCl2(PPh3)]2
proceeds smoothly, in refluxing toluene, to give the or-
ange complexes 1b (81%), 1b (78%), 2a (86%), 2b
(83%) and 2c (89%) and 2d (87%). The complexes 1
and 2 which are very stable in the solid state, have
been characterized by analytical and spectroscopic data.
The spectroscopic properties indicating that they all are
N(3)-bonded.
CH2CH2OCH3]; 4.25 [t, 2H, J = 4.96 Hz, CH2CH2OCH3];
3.28 [s, 3H, CH2CH2OCH3]; 1.89 and 1.94 [s, 6H,
P(C6H5)(CH3)2]; 7.36 and 7.48 [m, P(C6H5)(CH3)2];
7.29 and 8.31 [m, 4H, NC6H4N]; 8.32 [s, 1H, NCHN];
13C{H}-NMR (␦, CDCl3): 45.65 [CH2CH2OCH3]; 70.30
[CH2CH2OCH3]; 59.04 [CH2CH2OCH3]; 13.73 and 14.25
[P(C6H5)(CH3)2]; 130.84, 131.06, 131.85 and 133.01
[P(C6H5)(CH3)2]; 110.10, 120.84, 123.17, 123.89, 128.63
and 128.77 [NC6H4N]; 144.39 [NCHN].
2.6. Preparation of 1-
isopropylbenzimidazoledichloro(dimethylphenyl-
phosphine)-palladium(II), 2d
The nature of the bonding in the imidazoline and benzim-
idazole complexes 1 and 2 was readily shown by NMR spec-
troscopy (experimental part). 13C NMR spectroscopy was
the most useful tool for structure elucidation. Thus, from our
previous experience [15] we expected that C-coordination
would result in a shift of the carbene carbon nucleus signal
toward low field i.e. 190–212 ppm, while N-coordination
Compound 2d was prepared in the same way as 1a
from 1-isopropylbenzimidazole (118 mg, 0.737 mmol) and
[PdCl2(PPhMe2)]2 (232 mg, 0.368 mmol) to give orange
Scheme 2. Synthesis of Pd(II)-imidazoline and benzimidazole complexes.

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Table 1
The Suzuki coupling reaction of aryl halides with phenylboronic acid
Entry
R
X
Catalysts
Time (h)
Yield (%)a,b,c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
COCH3
COCH3
COCH3
COCH3
COCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
CH3
CH3
CH3
CH3
CH3
CH3
CHO
CHO
CHO
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1a
1b
2a
2b
2d
1a
1b
2a
2b
2c
2d
1a
1b
2a
2b
2c
2d
1a
1b
2a
2b
2c
2d
2.0
2.0
2.0
2.0
2.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
95
94
95
87
94
89
87
97
92
88
90
84
83
90
91
93
92
90
90
92
88
93
95
CHO
CHO
CHO
a
Reaction conditions: 1.0 mmol of p-R-C6H4X, 1.5 mmol of phenylboronic acid, 1.5 mmol Cs2CO3, 1.5 mol% Pd catalyst, dioxane (3 ml).
Isolated yield (purity of yield checked by NMR).
All reactions were monitored by TLC.
b
c
Table 2
The olefination of aryl halides with styrene
Entry
R
X
Catalysts
Time (h)
Yield (%)a,b,c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
COCH3
COCH3
COCH3
COCH3
COCH3
COCH3
CHO
CHO
CHO
CHO
CHO
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1a
1b
2a
2b
2c
2d
1a
1b
2a
2b
2c
2d
1a
2a
2b
2c
2d
1a
2a
2d
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
93
91
91
96
92
90
86
82
91
90
93
95
90
92
95
90
94
65
60
63
CHO
OCH3
OCH3
OCH3
OCH3
OCH3
CH3
CH3
CH3
a
Reaction conditions: 1.0 mmol of p-R-C6H4X, 1.5 mmol of styrene, 1.5 mmol Cs2CO3, 1.5 mol% Pd catalyst, dioxane (3 ml).
Isolated yield (purity of yield checked by NMR).
All reactions were monitored by TLC.
b
c

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113
would result in a higher field shift towards 160 ppm. 13C
NMR chemical shifts were consistent with the proposed
structure, the imino carbon appeared as a typical singlet
2 show by far the highest activity yet reported in the Heck
and Suzuki coupling of aryl halides, regardless of whether
the substrates are electron rich or poor. The procedure is
simple and efficient towards various aryl halides and does
not require induction periods.
1
in the H-decoupled mode in the 160.23, 160.75, 144.15,
143.49, 144.39 and 141.22 ppm, respectively for complexes
1a and b and 2a–d. The 1H NMR spectra of the complexes
further supported the assigned structures; the resonances for
C(2)–H were observed as sharp singlets in the 7.30, 7.15,
7.66, 8.46, 8.32, and 8.32 ppm, respectively for complexes
1a and b and 2a–d. The IR data for complexes 1 and 2 clearly
Acknowledgements
We thank the Technological and Scientific Research
˙
==
==
Council of Türkiye TÜBITAK (TÜBITAK COST D17)
indicate the presence of the –C N– group with a ν(C N)
vibration at 1611, 1605, 1609, 1610, 1613 and 1597 cm−1
,
and Inönü University Research Fund (BAP 2002/23) for
financial support of this work.
respectively for complexes 1a and b and 2a–d. The NMR
and IR values are similar to those found for N-coordinated
metal complexes [10–14].
The palladium-catalyzed cross-coupling of arylboronic
acids with aryl halides has been shown to proceed under a
variety of conditions: A wide range of bases and solvents, as
well as catalysts, have been employed with varying degrees
of success according to the substrates [1]. In a first test of the
activity of complexes 1 and 2 in the Suzuki type coupling
of para-substituted aryl halides with phenyl boronic acid,
classical conditions for solvent and base were chosen (diox-
ane as solvent, Cs2CO3 as base, and reaction temperature
of 80 ◦C). The coupling of activated and deactivated aryl
halogenides and phenylboronic acid proceeds in high yields
and quite rapidly even with a low catalyst loading. The re-
sults were summarized in Table 1. Under those conditions,
4-bromoacetophenone, p-chloroanisole, p-chlorotoluene
and p-chlorobenzaldehyde react very cleanly with phenyl-
boronic acid in goods yields (Table 1, entries 3, 8, 16
and 23).
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robust for high temperature Heck olefination of aryl chlo-
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