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Synthesis and catalytic activity of novel xylyl-linked benzimidazolium salts.

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Full Paper
Received: 5 April 2009
Revised: 5 September 2009
Accepted: 7 September 2009
Published online in Wiley Interscience
(www.interscience.com) DOI 10.1002/aoc.1558
Synthesis and catalytic activity of novel
xylyl-linked benzimidazolium salts
Serpil Demira∗ , Ismail Özdemira and Bekir Çetinkayab
The reaction of 1-alkylbenzimidazole derivatives with o-/p-di(chloromethyl)benzene results in the formation of the new o-/pxylyl-linked bis(benzimidazolium) salts, 1 and 2, respectively. The salts were characterized by NMR spectroscopy and elemental
analysis. The in situ prepared complexes derived from Pd(OAc)2 –1 and 2 exhibit catalytic activity (61–98%), to give the Heck
c 2009 John Wiley & Sons, Ltd.
coupling products of aryl bromides and styrene. Copyright Keywords: bisbenzimidazolium chlorides; carbene; palladium catalysis; Heck coupling
Introduction
520
The formation of carbon–carbon bonds is a fundamental reaction
in organic synthesis that has interested organic chemists for a
long time. In this respect, the Pd-catalyzed Heck reaction is one of
the most useful methods of forming natural products, drug design
and organic synthesis.[1 – 5] Monodentate, bulky, electron-donating
tertiary phosphines are generally employed as ancillary ligands in
coupling systems.[6,7] Although tertiary phosphine ligands are
useful in controlling reactivity and selectivity in organometallic
chemistry and homogeneous catalysis, they usually require air
free handling to prevent ligand oxidation. In addition, they are
subject to P–C bond degradation at elevated temperatures, and as
a consequence, higher phosphine concentrations are required.[8]
Therefore, with their phosphine mimic ligating properties,[9] Nheterocyclic carbenes (NHCs) have attracted the attention of a large
number of research groups.[10 – 16] The highly stable metal–carbon
bond leads to a high stability of carbene complexes against
heat and moisture, which also causes stabilization of the metal
center in catalytic C–C coupling reactions.[17] The preparation of
chelating ligands of NHCs in order to impart higher air and moisture
stability to palladium centers is receiving much attention.[18 – 23]
In the same context, the use of polydentate NHC ligands has
allowed the preparation of new compounds whose stability
is entropically improved by the chelate effect, extending this
promising area.[4,24] Most of these poly(carbenes) reported so far
are bidentate and pincer(tridentate-mer) biscarbene ligands that
were first coordinated to Pd and later on to other transition metals
such as Rh, Ru and Ir.[24 – 26]
Metal catalyzed C–C coupling reactions between aryl halides
and olefins are usually carried out homogenously in the presence
of a base under inert atmosphere.[27] It was noted that in some
cases the in situ formation of the catalytically active species
led to significantly better results than the use of preformed
complex.[28] We have previously reported the use of in situ formed
imidazolidine–pyrimidine–benzimidazole–bisbenzimidazole2-ylidene–palladium(II) systems which have displayed high
activity in various coupling reactions of aryl bromides and
aryl chlorides.[29 – 34] In a more recent study on Pd-catalyzed
Heck reaction, we found that bisbenzimidazolium precursors
connected by an m-xylyl bridge as a spacer showed better activity
in comparison to p-xylyl linked bisbenzimidazolium salts. As a
Appl. Organometal. Chem. 2009, 23, 520–523
continuation of this report, we decided to focus on the variation
of bridging xylyl groups and the alkyl substituent on the other
N atom.[29] We studied the Pd-catalyzed Heck-type coupling
reaction of p-substituted bromobenzene and styrene using
the new o- and p-xylyl-linked bis(benzimidazolium) salts (LHX)
1–2 and palladium acetate in the ratio of 0.7 mmol% of LHX,
0.7 mmol% of Pd(OAc)2 and 2.0 mmol of KOBut in DMF–water at
80 ◦ C for 1 h.
Experimental
Materials
All preparations were carried out in an atmosphere of purified
argon using standard Schlenk techniques. Solvents were dried
with standard methods and freshly distilled prior to use.
Starting materials and reagents used in reactions were obtained
commercially from Aldrich Chemical Co. Test reactions for the
catalytic activity of catalysts were carried out in air. Elemental
analyses were performed by Turkish Resarch Council (Ankara,
Turkey) Microlab.
Melting Point Determination
Melting points were measured in open capillary tubes with an
Electrothermal-9200 melting point apparatus and are uncorrected.
IR Spectroscopy
FT-IR spectra were recorded as KBr pellets in the range
400–4000 cm−1 on a ATI Unicam 1000 spectrometer.
∗
Correspondence to: Serpil Demir, Inonu University, Faculty of Science and Art,
Department of Chemistry, 44280 Malatya, Turkey. E-mail: sdemir@inonu.edu.tr
a Inönü University, Faculty of Science and Art, Department of Chemistry, 44280
Malatya, Turkey
b Ege University, Faculty of Science, Department of Chemistry, 35100 Bornovaİzmir, Turkey
c 2009 John Wiley & Sons, Ltd.
Copyright Novel xylyl-linked benzimidazolium salts
NMR Spectroscopy
o-Xylylbis[N-2-(methoxyethyl)benzimidazolium] dichloride, 1b
1 H NMR and 13 C NMR spectra were recorded using a Varian As
400 Merkur spectrometer operating at 400 MHz (1 H), 100 MHz
(13 C) in CDCl3 and DMSO-d6 with tetramethylsilane as an internal
reference. The NMR studies were carried out in high-quality 5 mm
NMR tubes. Signals are quoted in parts per million as δ downfield
from tetramethylsilane (δ 0.00) as an internal standard. Coupling
constants (J values) are given in hertz. NMR multiplicities are
abbreviated as fallows: s = singlet, d = doublet, t = triplet, m =
multiplet signal.
Gas Chromatography
Yield: 2.16 g (82%); m.p. 237–238 ◦ C; ν(CN) = 1570 cm−1 . Anal.
calcd for C28 H32 N4 O2 Cl2 : C, 63.76; H, 6.11; N, 10.62. Found: C, 63.81;
H, 6.10; N, 10.70%. 1 H NMR (399.9 MHz, DMSO-d6 ) δ (ppm) = 10.22
(s, 2H, NCHN), 8.15 and 8.05 (d, 4H, J = 8.4 Hz, NC6 H4 N), 7.68 and
7.63 (t, 4H, J = 7.6 Hz, NC6 H4 N), 7.35 and 7.10 (m, 4H, CH2 C6 H4 CH2 ),
6.28 (s, 4H, CH2 C6 H4 CH2 ), 4.77 (t, 4H, J = 5.2 Hz, CH2 CH2 OCH3 ),
3.81 (t, 4H, J = 5.2 Hz, CH2 CH2 OCH3 ), 3.25 (s, 6H, CH2 CH2 OCH3 ).
13 C NMR (100.5 MHz, DMSO-d ) δ (ppm) = 144.1 (NCHN), 133.1,
6
131.7 and 128.9 (CH2 C6 H4 CH2 ), 132.5, 129.5, 127.7, 127.5, 115.4
and 113.5 (NC6 H4 N), 69.8 (CH2 CH2 OCH3 ), 56.7 (CH2 CH2 OCH3 ), 48.3
(CH2 C6 H4 CH2 ), 47.5 (CH2 CH2 OCH3 ).
All reactions were monitored on a Agilent 6890N GC system with
GC-FID with an HP-5 column of 30 m length, 0.32 mm diameter
and 0.25 µm film thickness.
o-Xylylbis(N-2-(diisopropylaminoethyl)benzimidazolium) dichloride,
1c
Column Chromatography
Column chromatography was performed using silica gel 60
(70–230 mesh). Solvent ratios are given as v/v.
Synthesis of xylyl-linked bisbenzimidazolium salts (1–2)
To a solution of 1-alkylbenzimidazole (10 mmol) in DMF (10 ml)
was added slowly 1,2- or 1,4-di(chloromethyl)benzene (5 mmol) at
25 ◦ C and the resulting mixture was stirred at 50 ◦ C for 6 h. Diethyl
ether (15 ml) was added to obtain a white crystalline solid which
was filtered off. The solid was washed with diethyl ether (3×15 ml),
dried under vacuum. The crude product was recrystallized from
EtOH–Et2 O and melting points were measured in open capillary
tubes.
Yield: 3.06 g (92%); m.p. 247–248 ◦ C; ν(CN) = 1564 cm−1 . Anal.
calcd for C38 H54 N6 Cl2 : C, 68.55; H, 8.18; N, 12.62. Found: C, 68.61; H,
8.22; N, 12.60%. 1 H NMR (399.9 MHz, DMSO-d6 ) δ (ppm) = 10.15 (s,
2H, NCHN), 8.16 and 8.07 (d, 4H, J = 8.4 Hz, NC6 H4 N), 7.71 and 7.65
(t, 4H, J = 7.2 Hz, NC6 H4 N), 7.32 and 7.06 (m, 4H, CH2 C6 H4 CH2 ), 6.28
(s, 4H, CH2 C6 H4 CH2 ), 4.56 {t, 4H, J = 5.4 Hz, CH2 CH2 N[CH(CH3 )2 ]2 },
2.97 {hept, 4H, J = 6.4 Hz, CH2 CH2 N[CH(CH3 )2 ]2 }, 2.85 {t, 4H,
J = 5.4 Hz, CH2 CH2 N[CH(CH3 )2 ]2 }, 0.71 {d, 24H, J = 6.8 Hz,
CH2 CH2 N[CH(CH3 )2 ]2 }. 13 C NMR (100.5 MHz, DMSO-d6 ) δ (ppm)
= 143.8 (NCHN), 132.9, 131.7 and 128.8 (CH2 C6 H4 CH2 ), 132.0,
129.8, 127.5, 127.4, 114.8 and 114.7 (NC6 H4 N), 47.9 (CH2 C6 H4 CH2 ),
47.5 {CH2 CH2 N[CH(CH3 )2 ]2 }, 47.2 {CH2 CH2 N[CH(CH3 )2 ]2 }, 43.8
{CH2 CH2 N[CH(CH3 )2 ]2 }, 21.1 {CH2 CH2 N[CH(CH3 )2 ]2 }.
p-Xylylbis(N-2,4,6-trimethylbenzylbenzimidazolium) dichloride, 2a
General Method for Heck Coupling Reactions
Palladium acetate (0.7 mmol%), azolium salt 1–2 (0.7 mmol%),
KOBut (2.00 mmol), aryl bromide (1 mmol) and styrene (1.5 mmol)
in DMF–water (3 ml/3 ml) were added under air and then
stirred at 80 ◦ C for 1 h. The reaction was monitored by thin-layer
chromatography. The mixture was extracted with ethyl acetate
(3 × 5 ml). The organic phases were dried over anhydrous MgSO4 .
The solvent was removed and the residue was chromatographed in
silica gel column (ethyl acetate–n-hexane mixture 1 : 5 ratio). The
resulting solution was analyzed by gas chromatography. Coupling
product yields were calculated from GC data relative to the residual
aryl halide.
o-Xylylbis(N-2,4,6-trimethylbenzylbenzimidazolium) dichloride, 1a
Appl. Organometal. Chem. 2009, 23, 520–523
p-Xylylbis(N-2-(methoxyethyl)benzimidazolium) dichloride, 2b
Yield: 2.29 g (87%); m.p. 234–235 ◦ C; ν(CN) = 1563 cm−1 . Anal.
calcd for C28 H32 N4 O2 Cl2 : C, 63.76; H, 6.11; N, 10.62. Found: C,
63.84; H, 6.17; N, 10.60%. 1 H NMR (399.9 MHz, DMSO-d6 ) δ (ppm)
= 10.41 (s, 2H, NCHN), 8.12 and 8.00 (d, 4H, J = 7.5 Hz, NC6 H4 N),
7.67–7.54 (m, 4H, NC6 H4 N), 7.61 (s, 4H, CH2 C6 H4 CH2 ), 5.89 (s,
4H, CH2 C6 H4 CH2 ), 4.76 (t, 4H, J = 4.8 Hz, CH2 CH2 OCH3 ), 3.81
(t, 4H, J = 4.8 Hz, CH2 CH2 OCH3 ), 3.26 (s, 6H, CH2 CH2 OCH3 ).
13 C NMR (100.5 MHz, DMSO-d ) δ (ppm) = 143.5 (NCHN), 135.1
6
and 131.0 (CH2 C6 H4 CH2 ), 131.9, 129.3, 129.2, 127.1, 114.6 and
114.3 (NC6 H4 N), 69.4 (CH2 CH2 OCH3 ), 58.6 (CH2 CH2 OCH3 ), 49.7
(CH2 C6 H4 CH2 ), 47.1 (CH2 CH2 OCH3 ).
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
521
Yield: 3.00 g (89%); m.p. 173–174 ◦ C; ν(CN) = 1562 cm−1 . Anal.
calcd for C42 H44 N4 Cl2 : C, 74.65; H, 6.56; N, 8.29. Found: C, 74.73; H,
6.40; N, 8.36%. 1 H NMR (399.9 MHz, CDCl3 ) δ (ppm) = 10.74 (s, 2H,
NCHN), 8.15 and 7.24 (d, 4H, J = 8.4 Hz, NC6 H4 N), 7.49 and 7.40 (t,
4H, J = 7.6 Hz, NC6 H4 N), 7.20 and 6.82 (m, 4H, CH2 C6 H4 CH2 ), 6.89
[s, 4H, CH2 C6 H2 (CH3 )3 -2,4,6], 6.60 (s, 4H, CH2 C6 H4 CH2 ), 5.77 [s, 4H,
CH2 C6 H2 (CH3 )3 -2,4,6], 2.28 [s, 12H, CH2 C6 H2 (CH3 )3 -2,6], 2.24 [s, 6H,
CH2 C6 H2 (CH3 )3 -4]. 13 C NMR (100.5 MHz, CDCl3 ) δ (ppm) = 143.2
(NCHN), 132.7, 131.6 and 128.1 (CH2 C6 H4 CH2 ), 139.9, 138.2, 130.4
and 125.3 [CH2 C6 H2 (CH3 )3 -2,4,6], 132.5, 129.5, 127.7, 127.5, 115.4
and 113.5 (NC6 H4 N), 49.3 (CH2 C6 H4 CH2 ), 47.6 [CH2 C6 H2 (CH3 )3 2,4,6], 21.2 [CH2 C6 H2 (CH3 )3 -4], 20.4 [CH2 C6 H2 (CH3 )3 -2,6].
Yield: 3.07 g (91%); m.p. 246–247 ◦ C; ν(CN) = 1554 cm−1 . Anal.
calcd for C42 H44 N4 Cl2 : C, 74.65; H, 6.56; N, 8.29. Found: C, 74.58;
H, 6.62; N, 8.35%. 1 H NMR (399.9 MHz, DMSO-d6 ) δ (ppm) = 9.63
(s, 2H, NCHN), 8.06 and 7.87 (d, 4H, J = 8.4 Hz, NC6 H4 N), 7.66
and 7.59 (t, 4H, J = 7.8 Hz, NC6 H4 N), 7.44 (s, 4H, CH2 C6 H4 CH2 ),
6.99 [s, 4H, CH2 C6 H2 (CH3 )3 -2,4,6], 5.73 (s, 4H, CH2 C6 H4 CH2 ), 5.68
[s, 4H, CH2 C6 H2 (CH3 )3 -2,4,6], 2.27 [s, 6H, CH2 C6 H2 (CH3 )3 -4], 2.24 [s,
12H, CH2 C6 H2 (CH3 )3 -2,6]. 13 C NMR (100.5 MHz, DMSO-d6 ) δ (ppm)
= 142.3 (NCHN), 135.4 and 131.7 (CH2 C6 H4 CH2 ), 139.5, 139.0, 130.2
and 126.5 (CH2 C6 H2 (CH3 )3 -2,4,6), 132.4, 129.1, 128.9, 127.5, 127.4
and 114.6 (NC6 H4 N), 50.1 (CH2 C6 H4 CH2 ), 45.9 [CH2 C6 H2 (CH3 )3 2,4,6], 21.4 [CH2 C6 H2 (CH3 )3 -4], 20.0 [CH2 C6 H2 (CH3 )3 -2,6].
S. Demir, I. Özdemir and B. Çetinkaya
Scheme 1. Synthesis of xylyl-linked bisbenzimidazolium salts (LHX), 1 and 2.
p-Xylylbis(N-2-(diisopropylaminoethyl)benzimidazolium) dichloride,
2c
Yield: 3.16 g (95%); m.p. 266–267 ◦ C; ν(CN) = 1560 cm−1 . Anal.
calcd for C38 H54 N6 Cl2 : C, 68.55; H, 8.18; N, 12.62. Found: C,
68.64; H, 8.25; N, 12.65%. 1 H NMR (399.9 MHz, DMSO-d6 ) δ (ppm)
= 10.17 (s, 2H, NCHN), 8.09 and 8.04 (d, 4H, J = 8.0 Hz, NC6 H4 N),
7.63–7.56 (m, 4H, C6 H4 N), 7.58 (s, 4H, CH2 C6 H4 CH2 ), 5.83 (s, 4H,
CH2 C6 H4 CH2 ), 4.48 {t, 4H, J = 4.8 Hz, CH2 CH2 N[CH(CH3 )2 ]2 },
2.82 {hept, 4H, J = 6.4 Hz, CH2 CH2 N[CH(CH3 )2 ]2 }, 2.75 {t, 4H,
J = 4.8 Hz, CH2 CH2 N[CH(CH3 )2 ]2 }, 0.56 {d, 24H, J = 6.4 Hz,
CH2 CH2 N[CH(CH3 )2 ]2 }. 13 C NMR (100.5 MHz, DMSO-d6 ) δ (ppm)
= 143.4 (NCHN), 135.5 and 131.2 (CH2 C6 H4 CH2 ), 131.8, 129.8,
127.3, 127.2, 114.6 and 114.5 (NC6 H4 N), 49.8 (CH2 C6 H4 CH2 ),
47.4 {CH2 CH2 N[CH(CH3 )2 ]2 }, 47.1 {CH2 CH2 N[CH(CH3 )2 ]2 }, 43.6
{CH2 CH2 N[CH(CH3 )2 ]2 }, 21.0 {CH2 CH2 N[CH(CH3 )2 ]2 }.
Results and Discussion
Preparation of Xylyl-linked Bisbenzimidazolium Salts and
Spectroscopic Characterization
522
The xylyl-linked bis(benzimidazolium) salts were readily prepared
by treatment of 1,2- or 1,4-di(chloromethyl)benzene with 1alkyl benzimidazole derivatives in heating DMF (Scheme 1). The
resulting benzimidazolium salts were obtained in good yields
of 82–95%. The salts are air- and moisture-stable both in
the solid state and in solution. The 1 H NMR spectra of the
bis(benzimidazolium) salts 1–2 exhibit the signal for the NCHN
proton in the range of δ 9.63–10.74 ppm. These values are
typical for NCHN protons of benzimidazolium salts.[35 – 37] The
13
C NMR spectra of 1–2 exhibit the NCN resonances between
δ 142.3 and 144.1 ppm, which are also typical values previously
reported for imidazolinium and benzimidazolium salts.[25,26,35 – 37]
www.interscience.wiley.com/journal/aoc
The resonances for the methylene protons of the xylene bridge
were observed as broad singlets between δ 5.73 and 6.60 ppm.
The IR data for benzimidazolium salts clearly indicate the presence
of the –C N-group with a υ(C N) vibration between 1554 and
1570 cm−1 .
The Heck C–C Coupling Reaction
Bis(benzimidazolium) salts (1–2)–Pd(OAc)2 catalytic systems have
been evaluated as catalysts in Heck coupling reactions of
activated and nonactivated aryl bromides with styrene under
aerobic conditions. A variety of parameters such as solvent and
catalyst loading temperature can influence the reactivity of Heck
coupling. To find optimum conditions a series of experiments were
performed with 4-bromoacetophenone and styrene as model
compounds. As a base, KOBut was the best choice and as a solvent
DMF–water was found to be better than other solvents. 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 styrene with various psubstituted aryl bromides. Table 1 summarizes our results from the
screening of new bisbenzimidazolium salts to afford the coupling
products.The procedure is simple and does not require induction
periods and all compounds led to good conversions at low catalyst
concentration (0.7 mmol%). The influence of the N-alkyl group of
the NHC precursor is more important than the xylyl bridges.
Methoxyethyl and diisopropylaminoethyl substituents are more
efficient than trimethylbenzyls. Generally speaking, o-xylyl spacer
seems to be better than the p-xylyl spacer. It should be noted
that in all cases only the trans products were selectively obtained
as confirmed by 1 H NMR. The formation of the trans product is
favored with sterically demanding N-substituents at the carbene
precursor.
c 2009 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2009, 23, 520–523
Novel xylyl-linked benzimidazolium salts
Table 1. The Heck coupling reaction of aryl bromides with styrene
R
Br
Acknowledgments
This work was financially supported by the Technological and
Scientific Research Council of Turkey TUBYTAK-CNRS (France)
[TBAG-U/181 (106T716)] and Inönü University Research Fund.
+
Pd(OAc)2, 1-2
KOBüt, DMF/H2O
80 οC,1h
Entry
LHX
R
1
2
3
4
5
6
1a
1b
1c
2a
2b
2c
COCH3
COCH3
COCH3
COCH3
COCH3
COCH3
7
8
9
10
11
12
1a
1b
1c
2a
2b
2c
CHO
CHO
CHO
CHO
CHO
CHO
13
14
15
16
17
18
1a
1b
1c
2a
2b
2c
H
H
H
H
H
H
19
20
21
22
23
24
1a
1b
1c
2a
2b
2c
CH3
CH3
CH3
CH3
CH3
CH3
25
26
27
28
29
30
1a
1b
1c
2a
2b
2c
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
R
References
Product
Yield (%)
COCH3
CHO
71
85
96
67
80
95
68
84
71
70
86
75
69
92
66
61
86
68
CH3
OCH3
82
98
95
79
97
93
91
80
87
92
85
81
a
Reaction conditions: 1.0 mmol of R-C6 H4 Br-p, 1.5 mmol of styrene,
2 mmol KOBut , 0.7 mmol% of Pd(OAc)2 , 0.7 mmol% of LHX, DMF–water
(3 ml/3 ml). b Yields are based on arylbromide. c All reactions were
monitered by TLC and GC. d Temperature 80 ◦ C.
Conclusion
We prepared six xylyl-linked bis(benzimidazolium) salts (1
and 2) from 1,2- or 1,4-di(chloromethyl)benzene and 1alkylbenzimidazole. It was shown that these carbene precursors
(LHX) in combination with Pd(OAc)2 form an effective catalytic system for Heck reactions of aryl bomides and styrene in presence air
at 80 ◦ C. The comparison of the results indicates that the influence
of N-alkyl substituent is more important than the xylyl spacers.
Further study is underway to optimize the reactivity of these Nheterocyclic carbene precursors for C–C and C–N coupling with
Pd(OAc)2 and transition metal complexes of Pd, Ru, Rh and Ir to
explore their catalytic activity.
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c 2009 John Wiley & Sons, Ltd.
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salt, synthesis, catalytic, activity, novem, xylyl, benzimidazole, linked
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