Suzuki-Miyaura reaction of unactivated aryl chlorides using benzimidazol-2-ylidene ligands

Article abstract of DOI:10.1081/SCC-200036594

Four functionalized bis(benzimidazolium) salts (2a-d) have been prepared and characterized by conventional spectroscopic methods and elemental analyses. A highly effective, easy to handle, and environmentally bengin process for palladium-mediated Suzuki cross-coupling was developed. The in situ prepared three-component system Pd(OAc)₂/bis(benzimidazolium) bromides (2a-d) and Cs₂CO₃ catalyzes quantitatively the Suzuki cross-coupling of deactivated aryl chloride in aqueous media.

Full text of DOI:10.1081/SCC-200036594

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Suzuki–Miyaura Reaction of
Unactivated Aryl Chlorides
Using Benzimidazol‐2‐Ylidene
Ligands
İsmail Özdemir a , Yetkin Gök a , Nevin Gürbüz a ,
Engin Çetinkaya b & Bekir Çetinkaya b
a Department of Chemistry , Inönü University ,
44069, Malatya, Türkiye
b Department of Chemistry , Ege University ,
Bornova‐İzmir, Türkiye
Published online: 10 Jan 2011.
To cite this article: İsmail Özdemir , Yetkin Gök , Nevin Gürbüz , Engin Çetinkaya
& Bekir Çetinkaya (2004) Suzuki–Miyaura Reaction of Unactivated Aryl Chlorides
Using Benzimidazol‐2‐Ylidene Ligands, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 34:22, 4135-4144,
DOI: 10.1081/SCC-200036594
To link to this article: http://dx.doi.org/10.1081/SCC-200036594
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 22, pp. 4135–4144, 2004
Suzuki–Miyaura Reaction of
Unactivated Aryl Chlorides Using
Benzimidazol-2-Ylidene Ligands
1,
Ismail Ozdemir, Yetkin Gok, Nevin Gurbuz,
1
1
¨
˙
*
¨
¨
¨
Engin C¸ etinkaya,2 and Bekir C¸ etinkaya2
1
¨ ¨
¨
Department of Chemistry, Inonu University, Malatya, Turkiye
2
˙
Department of Chemistry, Ege University, Bornova-Izmir, Tu¨rkiye
ABSTRACT
Four functionalized bis(benzimidazolium) salts (2a–d) have been prepared
and characterized by conventional spectroscopic methods and elemental
analyses. A highly effective, easy to handle, and environmentally bengin
process for palladium-mediated Suzuki cross-coupling was developed.
The in situ prepared three-component system Pd(OAc)2/bis(benzimidazo-
lium) bromides (2a–d) and Cs2CO3 catalyzes quantitatively the Suzuki
cross-coupling of deactivated aryl chloride in aqueous media.
Key Words: Suzuki; Carbene; Palladium; Benzimidazol-2-ylidene;
Aryl chlorides; Phenylboronic acid.
˙
¨
¨ ¨
Correspondence: Ismail Ozdemir, Department of Chemistry, Inonu University, 44069
*
Malatya, Tu¨rkiye; E-mail: iozdemir@inonu.edu.tr.
4135
DOI: 10.1081/SCC-200036594
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

¨
Ozdemir et al.
4136
INTRODUCTION
Palladium-catalyzed cross-coupling reaction of aryl halides or triflates to
form unsymmetrical biaryls are one of the most useful tools in synthetic
organic chemistry. Several of the most common methods for performing these
transformations are the Stille (organostannane), Suzuki–Miyaura (organoboron),
and Hiyama (organosilicon) reactions (Scheme 1).[1–5]
Suzuki cross-coupling reaction is one of the most important methods of
forming sp2-sp2 carbon–carbon bonds in synthetic chemistry, as well as in
industrial applications.[6] In the past few years, many attempts have been
made to develop effective palladium complexes, which can act as highly
active catalysts for this reaction.[7] These studies revealed the crucial role
played by the ancillary ligands in the efficiency of this reactions. Sterically
hindered, electron-rich alkyl phosphines[8] and carbene[9] ligands have received
increasing interest in recent years. However, the development of the new
ligands or the application of existing ligands in this reaction, particularly
those involving aryl chlorides as substrates, is still of considerable importance.
Since the discovery of stable imidazoline-2-ylidenes,[10] much interest has been
generated in the chemistry of both free heteroatom carbenes and metal
complexes of these ligands. Most recently, the synthesis and application of
1,3-dialkylimidazolium salts have been reviewed.[11] The late transition metal
N-heterocyclic carbene complexes have been used as catalysts for the formation
Scheme 1.

Suzuki–Miyaura Reaction of Unactivated Aryl Chlorides
4137
of furans,[12] cyclopropanation,[13] olefin metathesis,[14] cycloisomerization,[15]
and Heck and Suzuki coupling reactions.[16]
Recently, a major study on Suzuki reactions focused on increasing the
activity of the catalysts and decreasing the catalyst loading; the study included
the use of additives, the modification of the catalyst, and changing the
solvents. A major advance achieved by increasing the catalytic activity is
the extension of the Suzuki reaction to unactivated aryl chlorides, as noted
by the research groups of Buchwald et al.,[8] Littke and Fu,[17] and Herrmann
and Bohm,[18] as well as several other groups. The use of water as a solvent for
¨
chemical reactions clearly has both economical and environmental advantages
because it is inexpensive, abundant, nontoxic, nonflammable, and readily
separable from organic compounds.[19] There have been a number of reports
of the palladium-mediated Suzuki reaction being performed by using water
as a solvents[20] which relate to the coupling of the aryl boronic acids with
aryl iodides or activated bromide and aryl chlorides but involve the use of
an oxime-carbapalladacycle as the catalyst.[21] Recently, we developed
improved procedures Heck and Suzuki reactions of aryl chlorides making
use of novel ligands 1,3-bis(dialkyl)imidazolium salts, 1-alkylimidazoline,
a-bis(imine).[22]
Although the nature of the NHC ligand on complexes has a tremendous
influence on the rate of catalyzed reactions, the use of benzimidazol-2-
ylidene ligands in coupling reactions is a neglected area. To find more efficient
palladium catalysts we have prepared a series of new functionalized bis
(benzimidazolium) bromides LHX, 2a–d, (Scheme 2), containing a 2-metho-
xyethyl moiety and we report here in situ Pd-carbene based catalytic system
for the Suzuki coupling reaction in aqueous media.
Scheme 2.

¨
Ozdemir et al.
4138
EXPERIMENTAL
1H NMR and 13C NMR spectra were recorded by using a Bruker
AC300P FT spectrometer operating at 300.13 MHz (1H), 75.47 MHz (13C).
Chemical shifts (d) are given in ppm relative to TMS, coupling constants
(J) in Hz. FT-IR spectra were recorded on a Mattson 1000 spectropho-
tometer, wave numbers in cm21. Melting points were measured in open
capillary tubes with an Electrothermal-9200 melting point apparatus and
uncorrected. Elemental analyses were performed by TUBITAK (Ankara,
Turkey) Microlab.
Synthesis of 3,30-Di(2-methoxyehyl)-1,
10-methylenedi(benzimidazolium) dibromide (2a)
To a solution of 1-(2-methoxyethyl)benzimidazole (1.11 g; 6.30 mmol) in
DMF (1 mL) 1,1-dibromoethane (0.22 L; 3.13 mmol) was added, the resulting
solution was stirred for 2 hr at room temperature and heated for 18 hr at 808C.
Et2O (10 mL) was added the reaction mixture. A white solid was precipitated
in this period. The precipitate was then crystallized from EtOH/Et2O (1 : 2).
1
Yield: 1.21 g, 74%, mp: 211–2128C. IR, n: 1576 cm21 (C55N). H NMR
(CDCl3) d: 3.28 (s, 6H, CH2CH2OCH3), 3.64 (s, 2H, NCH2N), 3.82
(t, J4.8 Hz, 4H, CH2CH2OCH3), 4.80 (t, J4.8 Hz, 4H, CH2CH2OCH3),
7.70–8.43 (m, 8H, Ar–H), 10.59 (s, 2H, 2-CH). 13C NMR (CDCl3) d: 47.8,
59.0, 69.5 (CH2CH2OCH3), 56.2 (NCH2N), 114.5, 115.1, 127.9, 128.1,
130.8, 132.0 (Ar-C), 145.0 (2-CH). Found: C, 47.56; H, 4.67; N, 11.02%.
Calcd. for C21H26N4O2Br2: C, 47.92; H, 4.98; N, 10.65%.
Synthesis of 3,30-Di(2-methoxyethyl)-1,
10-ethylenedi(benzimidazolium) dibromide (2b)
This compound was prepared from 1-(2-methoxyethyl)benzimidazole
(1.21 g; 6.87 mmol) and 1,2-dibromoethane (0.29 mL; 3.43 mmol) in DMF
(1 mL). Yield: 1.47 g, 79%, mp: 271–2728C. IR, n: 1571 cm21 (C55N).
1H NMR (CDCl3) d: 3.12 (s, 6H, CH2CH2OCH3), 3.68 (t, J4.8 Hz, 4H,
CH2CH2OCH3), 4.61 (t, J4.8 Hz, 4H, CH2CH2OCH3), 5.13 (s, 4H,
NCH2CH2N), 7.49–8.10 (m, 8H, Ar-H), 9.88 (s, 2H, 2-CH). 13C NMR
(CDCl3) d: 45.7, 46.6, 58.1 (CH2CH2OCH3), 68.9 (NCH2CH2N), 112.8,
113.9, 126.6, 126.7, 130.7, 131.0 (Ar-C), 143.1 (2-CH). Found: C, 48.56;
H, 4.98; N, 10.45%. Calcd. for C22H28N4O2Br2: C, 48.91; H, 5.22; N, 10.37%.

Suzuki–Miyaura Reaction of Unactivated Aryl Chlorides
4139
Synthesis of 3,30-Di(2-methoxyethyl)-1,
10-propylenedi(benzimidazolium) dibromide (2c)
This compound was prepared from 1-(2-methoxyethyl)benzimida-
zole (1.0 g; 5.68 mmol) and 1,2-dibromopropane (0.29 mL; 2.84 mmol)
in DMF (1 mL). Yield: 1.29 g, 82%, mp: 207–2088C. IR, n: 1572 cm21
(C55N). 1H NMR (CDCl3) d: 2.61 (p, J7.2 Hz, 2H, NCH2CH2CH2N),
3.25 (s, 6H, CH2CH2OCH3), 3.79 (t, J4.8 Hz, 4H, CH2CH2OCH3), 4.71
(t, J4.6 Hz, 4H, CH2CH2OCH3), 4.76 (t, J7.2 Hz, 4H, NCH2CH2CH2N),
7.68–8.16 (m, 8H, Ar-H), 9.99 (s, 2H, 2-CH). 13C NMR (CDCl3) d:
45.6, 47.3, 58.2 (CH2CH2OCH3), 42.7, 60.1 (NCH2CH2CH2N), 112.5,
113.3, 126.5, 126.0, 130.7, 132.4 (Ar-C), 143.2 (2-CH). Found: C,
50.21; H, 5.83; N, 10.47%. Calcd. for C23H30N4O2Br2: C, 49.84; H,
5.45; N, 10.11%.
Synthesis of 3,30-Di(2-methoxyethyl)-1,
10-butylenedi(benzimidazolium) dibromide (2d)
This compound was prepared from 1-(2-methoxyethyl)benzimidazole
(1.01 g; 5.74 mmol) and 1,2-dibromobuthane (0.34 mL; 2.87 mmol) in
DMF (1 mL). Yield: 1.31 g, 80%, mp: 247–2488C. IR, n: 1576 cm21
(C55N). 1H NMR (CDCl3) d: 1.97 (m, 4H, NCH2CH2CH2CH2N), 3.21
(s, 6H, CH2CH2OCH3), 3.76 (t, J4.6 Hz, 4H, CH2CH2OCH3), 4.59 (m, 4H,
NCH2CH2CH2CH2N), 4.68 (t, J4.6 Hz, 4H, CH2CH2OCH3), 7.67–8.09
(m, 8H, Ar-H), 9.99 (s, 2H, 2-CH). 13C NMR (CDCl3) d: 45.9, 46.8, 58.1
(CH2CH2OCH3), 35.1, 56.4 (NCH2CH2CH2CH2N), 112.3, 112.9, 126.0,
126.8, 130.3, 132.5 (Ar-C), 143.2 (2-CH). Found: C, 50.28; H, 6.04; N,
10.13%. Calcd. for C24H32N4O2Br2: C, 50.72; H, 5.67; N, 9.86%.
General Procedure for the Suzuki–type Coupling Reactions
Pd(OAc)2 (1.5% mmol), bis(benzimidazolium) bromides, 2a–d
(1.5% mmol), aryl chloride (1.0 mmol), phenylboronic acid (1.2 mmol), Cs2-
CO3(2 mmol) water (3 mL)-DMF (3 mL) were added in a small Schlenk
tube under argon, and the mixture was heated at 808C for 6 hr. At the con-
clusion of the reaction, the mixture was cooled, extracted with Et2O, filtered
through a pad of silicagel with copious washings, concentrated, and purified
by flash chromatography on silicagel. Purity of compounds is checked by
NMR, and yields are based on arylchloride.

¨
Ozdemir et al.
4140
RESULTS AND DISCUSSION
Bis(benzimidazolium) bromides (2a–d) are conventional NHC precursors.
The functionalized benzimidazolium salts 2a–dwere synthesized by consecu-
tive alkylation of 1-alkylbenzimidazole (Scheme 2). According to Scheme 2, the
salts (2a–d) were obtained in almost quantitative yield by quarternazition
of 1-alkylbenzimidazole (1) in DMF with Br(CH2)nBr (n ¼ 1–4).[23,24] The
salts are air and moisture stable both in the solid state and in solution. The
structures of 2a–d were determined by their characteristic spectroscopic
data and elemental analyses. 13C NMR chemical shifts were consistent with
the proposed structure, and the imino carbon appeared as a typical singlet in
1
the H-decoupled mode in the 145.0, 143.1, 143.2, and 143.2 ppm, respect-
ively, for benzimidazolium chlorides 2a–d. The 1H NMR spectra of the ben-
zimidazolium salts further supported the assigned structures; the resonances
for C(2)-H were observed as sharp singlets in the 10.59, 9.88, 9.99, and
9.99 ppm, respectively for 2a–d. The IR data for benzimidazolium salts
2a–d clearly indicate the presence of the 2C55N- group with a n(C55N)
vibration at 1576, 1571, 1572, and 1576 cm21, respectively, for 2a–d. The
NMR values are similar to those found for other 1,3-dialkylbenzimidazolium
salts.[24]
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 used with
varying degrees of success according to the substrates.
To find optimum conditions, a series of experiments was performed with
4-chloroanisole and phenylboronic acid as model compounds. As a base,
Cs2CO3 was the best choice in water/DMF systems. In addition, the reactions
were performed in air and without degassing the water prior to use. After
having established the optimized coupling reaction conditions, the scope of
the reaction and efficiencies of the salts were evaluated by investigating
the coupling of C6H5B(OH)2 with various p-substituted aryl chlorides. The
results were shown in Table 1.
Under those conditions, p-chloroanisole, p-chlorobenzaldehyde, p-chloro-
toluene, p-chloroacetophenone, and chlorobenzene react very cleanly with
phenylboronic acid in goods yields (Table 1, entry 2, 6, 10, 14, and 18).
Table 1 summarizes our results from the screening of four benzimidazo-
lium salts for Suzuki cross-coupling reaction. Several trends are readily
apparent. First, the procedure is simple and does not require induction
periods. Second, the scope of this reaction is broad and includes aryl chlorides
that are activated or deactivated. Third, all complexes led to good conver-
sions at low catalyst concentration (1.5 mol%). It is evident that the NHC
precursors that contain electron donating methoxyethyl, substituent (2a–d)

Suzuki–Miyaura Reaction of Unactivated Aryl Chlorides
4141
Table 1. The Suzuki coupling of aryl chloride with phenylboronic acid.
Entry
R
LHX
Yielda,,b,,c,,d (%)
1
2
OCH3
OCH3
OCH3
OCH3
CHO
CHO
CHO
CHO
CH3
2a
2b
2c
2d
2a
2b
2c
2d
2a
2b
2c
2d
2a
2b
2c
2d
2a
2b
2c
2d
75
91
77
59
70
97
77
63
68
83
65
61
84
97
86
69
60
88
59
54
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
CH3
CH3
CH3
COCH3
COCH3
COCH3
COCH3
H
H
H
H
aReactions conditions: 1.0 mmol of R-C6H4Cl-p, 1.5 mmol of phenylboronic acid,
2 mmol Cs2CO3, 1.5 mol% Pd(OAc)2, 1.5 mol% 2water (3 mL)-DMF (3 mL).
bPurity of compounds is checked by NMR and yields are based on arylchloride.
cAll reactions were monitored by TLC.
d808C, 6 hr.
are the most effective of the salts examined. The coordinating ability of the
alkoxy group may be an important contributor to the increase in reactivity,
as has been demonstrated by previous examples.[25]
CONCLUSION
From readily available starting four bis(benzimidazolium), salts (2a–d)
have been prepared and characterized. A convenient and highly user-friendly

¨
Ozdemir et al.
4142
method for Suzuki cross-coupling reaction is presented, which uses a catalyst
formed in situ from Pd(OAc)2, the readily accessibly and fully air-stable
benzimidazolium salts. The new ligand family allows highly efficient coupling
reactions of electron rich as well as electron poor aryl chlorides with phenyl-
boronic acid under mild conditions. This concept for making catalysts in situ
opens the way for the discovery of many new catalysts via the interaction of
commercially available metal complexes and suitable electron releasing
ligands. Due to the increase in chain length in 2a–2d, the coordination of
the second carbene unit was obscured to the palladium; therefore, the catalytic
activity of carbene complexes toward Suzuki coupling reaction lowered.
Detailed investigations, focusing on benzimidazolin-2-ylidene substituent
effects, functional group tolerance, and catalytic activity in this and other
coupling reactions are ongoing.
ACKNOWLEDGMENTS
We thank Technological and Scientific Research Council of Tu¨rkiye
¨
¨ ˙
¨ ¨
TUBITAK (TUBITAK COST D17) and Inonu University Research Fund
for financial support of this work.
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Received in the UK May 11, 2004