Resorcinarene-mono-benzimidazolium salts as NHC ligands for Suzuki-Miyaura cross-couplings catalysts

Article abstract of DOI:10.3906/kim-1506-50

Two mono-benzimidazolium salts of resorcinarene have been prepared and used as ligands in Suzuki-Miyaura cross-coupling reactions. They have been fully characterized by ¹H and ¹³C NMR, MALDI, and FT-IR spectroscopic methods, and their structures were confirmed by X-ray diffraction analysis. These two new resorcinarene-based mono-benzimidazolium salts showed good catalytic activity for coupling reactions in DMF. The highest conversion was achieved for arylation of 4-bromotoluene using the resorcinarenyl mono-dimethylbenzimidazolium salt.

Full text of DOI:10.3906/kim-1506-50

Turk J Chem
Turkish Journal of Chemistry
(2015) 39: 1300 – 1309
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1506-50
Research Article
Resorcinarene-mono-benzimidazolium salts as NHC ligands for Suzuki–Miyaura
cross-couplings catalysts
1,∗
2
Umit IS¸CI , Muhittin AYGUN , Resul SEVINCEK ,
2
¨
˙
˙
¨
˙
1
Yunus ZORLU , Fabienne DUMOULIN
1
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1Department of Chemistry, Gebze Technical University, Gebze, Kocaeli, Turkey
2Department of Chemistry, Faculty of Sciences, Dokuz Eylu¨l University, Tınaztepe Campus,
˙
Buca, Izmir, Turkey
Received: 18.06.2015
•
Accepted/Published Online: 30.09.2015
•
Printed: 25.12.2015
Abstract: Two mono-benzimidazolium salts of resorcinarene have been prepared and used as ligands in Suzuki–Miyaura
cross-coupling reactions. They have been fully characterized by 1 H and 13 C NMR, MALDI, and FT-IR spectroscopic
methods, and their structures were confirmed by X-ray diffraction analysis. These two new resorcinarene-based mono-
benzimidazolium salts showed good catalytic activity for coupling reactions in DMF. The highest conversion was achieved
for arylation of 4-bromotoluene using the resorcinarenyl mono-dimethylbenzimidazolium salt.
Key words: Suzuki–Miyaura reaction, resorcinarene, cavitand, N-heterocyclic carbene, benzimidazolium salt, catalyst
1. Introduction
Palladium-catalyzed Suzuki–Miyaura cross-couplings are among the most important and studied cross-coupling
reactions.1−4 Phosphine ligands are often used in palladium-catalyzed reactions, but most of these ligands are
not air-stable.5 The development of air-stable ligands is therefore crucial and N-heterocyclic carbenes (NHCs)
have proved to be suitable ligands for Suzuki–Miyaura cross-coupling reactions. NHCs show a powerful σ-
donating and weak π-accepting character, allowing for the generation of a stronger bond to the metal in
transition metal catalysis. Metal–NHC complexes are air-stable, easy to handle, and highly active in several
catalytic transformations.6−10
Transition metal complexes of resorcinarenes and their functionalized derivatives such as cavitands
and capsules are widely investigated owing to their interesting properties in catalysis.11−17 The bridging C2
carbon atoms of resorcinarene cavitands are easily functionalized to prepare ligands bearing phosphines and
imidazolium salts for several cross-coupling reactions.18−22 Functionalized structures such as calixarenes,23−26
cyclodextrins,29−39 and resorcinarene cavitands40 have therefore been successfully employed in homogeneous
catalysis. All these supramolecular ligands exhibited significant catalytic properties thanks to their hosting
abilities toward the reaction substrates. In particular, resorcinarene cavitands have additional advantages
thanks to their tailorable structural versatility.
A limited number of cavitands containing mono-imidazolium salts are described.22,26 These precursors
of NHC ligands are known to improve palladium-catalyzed cross-coupling between arylboronic acids and aryl
∗Correspondence: u.isci@gtu.edu.tr
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halides.22 As it is known that fine structural modifications in a ligand can affect the activity of a catalytic com-
plex, we designed resorcinarenyl mono-benzimidazolium salts that have never been evoked as monodentate proli-
gands in organometallic catalysis. Herein, we describe the synthesis of the resorcinarenyl mono-benzimidazolium
salts and their use in Suzuki–Miyaura cross-coupling reactions between 3,5-dimethoxyphenylboronic acid and
several aryl bromides and chlorides. To fully investigate the cavity effect of these new catalytic systems, analo-
gous ligands without cavities have been prepared as well.
2. Results and discussion
2.1. Synthesis and characterization of resorcinarenyl mono-benzimidazolium salts
Mono-benzimidazolium salts 6 and 7 were prepared, both in high yields, according to Scheme 1. Their successful
preparation was evidenced by 1 H and 13 C NMR, MALDI, and FT-IR methods. The synthesis of 6 and 7 began
with corresponding benzyl benzimidazoles, which were obtained in 90% yield for 3 and 92% yield for 4. The
benzimidazolium salts 6 and 7 were obtained in quantitative yields by a quaternization reaction of cavitand
541 with N -substituted benzimidazoles 3 and 4 in chloroform under refluxing conditions.
Scheme 1. Synthesis of resorcinarenyl mono-benzimidazolium salts 6 and 7.
FT-IR data for mono-benzimidazolium salts 6 and 7 clearly showed the presence of the -C=N- group with
a ν (C=N) vibration at 1466 and 1471 cm−1 , respectively. The 1 H NMR spectra of mono-benzimidazolium
salts 6 and 7 are accordance with Cs -symmetrical cavitands, each of these cavitands displaying the presence
of two distinct AB patterns for the OCH2 O protons and two methine triplets. The 1 H NMR spectra clearly
exhibit a singlet at 11.32 ppm for mono-benzimidazolium salt 6 and at 11.14 ppm for 7 typical of the N=CH−N
fragment. The 1 H NMR spectra of mono-benzimidazolium salts 6 and 7 show singlets at 5.73 ppm for 6 and
at 5.64 ppm for 7, characteristic for the benzyl protons (-CH2 Ph). 13 C NMR chemical shifts were consistent
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with the proposed structure: the imino carbon appeared as a typical singlet at 143.5 ppm for 6 and at 142.3
ppm for 7, respectively.
In order to evidence the supramolecular effect of the cavity likely to act as a receptor for the reaction
substrate, it was necessary to have analogous NHC ligands without the cavity. This is why benzimidazolium
salts 8 and 9 (Figure 1) were prepared according to a reported protocols.42,43 These two compounds were used
as references in Suzuki–Miyaura cross-coupling reactions to fully investigate the cavity effect of the resorcinarene
mono-benzimidazolium ligands.
Figure 1. Reference benzimidazolium salts 8 and 9.
The solid-state structures of 6 and 7 were further confirmed by X-ray diffraction analysis. Suitable
crystals of 6 and 7 were grown by slow evaporation in EtOH at room temperature. The crystallographic
analysis revealed that both compounds crystallize in the monoclinic space group P 21 /c. As expected, the
resorcinarene moiety kept its conical shape (Figures 2 and 3). When compared with cavitand 6, the cone
conformation of cavitand 7 is distorted, as the dihedral angles between pairs of opposite aromatic rings (A/B
and C/D) are 58.29◦ and 59.36◦ in 6 and 61.30◦ and 56.35◦ in 7. The top rim diameters are 7.935 A (C16–C42)
˚
˚
and 8.060 A (C27–C52) in 6 and 7.829 A (C1–C15) and 8.124 A (C9–C25) in 7.
˚
˚
Figure 2. Side view (left) and top view (right) of the crystal structure of 6. The H atoms and bromine ion were omitted
for clarity.
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Figure 3. Side view (left) and top view (right) of the crystal structure of 7. The H atoms, bromine ion, and solvent
molecule (EtOH) were omitted for clarity.
2.2. Suzuki–Miyaura cross-couplings
Mono-benzimidazolium salts were used in Suzuki-Miyaura cross-coupling reactions between 3,5-dimethoxyphenylboronic
acid and aryl halides in the presence of a base (Scheme 2). The catalysts were prepared in situ from mono-
benzimidazolium (5 × 10−5 mmol, 1 equiv./Pd) salts and [Pd(OAc)2 ] (5 × 10−5 mmol, 1 × 10−2 mol%).
MeO
MeO
Pd(OAc) 2, benzimidazolium salt, base
B(OH)2
Br
+
R
R
MeO
MeO
Scheme 2. Suzuki–Miyaura cross-coupling reaction.
A series of experiments were performed to find optimum reaction conditions using 3,5-dimethoxyphenylboronic
acid and 4-bromoanisole in dimethylformamide (DMF) or dioxane in the presence of NaH or Cs2 CO3 as a base
(Table 1). Pd(OAc)2 salt alone was tested in the employed conditions and showed negligible activity in similar
conditions.
Table 1. The search for optimal catalytic conditions of Suzuki–Miyaura cross-coupling of 4-bromoanisole with 3,5-
dimethoxyphenylboronic acid using 6.[a]
◦
Entry Solvent
Base
Dioxane NaH
Dioxane Cs2CO3 100
T/ C Conversion (%)
1
2
3
4
100
40.1
29.6
63.4
49.2
DMF
DMF
NaH 130
Cs2CO3 130
[a] Conditions: [Pd(OAc)2 ] (5 × 10−5 mmol, 1 × 10−2 mol%), benzimidazolium salt 6 (5 × 10−5 mmol, 1 equiv./Pd),
ArBr (0.5 mmol), (CH3 O)2 C6 H3 B(OH)2 (0.091 g, 0.75 mmol), NaH (60% dispersion in mineral oil) (0.030 g, 0.75
mmol), DMF (1.5 mL), 130 ◦ C, 1 h. The conversions were determined by GC-MS. Averaged over two runs.
The highest conversion was observed in the presence of NaH in DMF at 130 ◦ C (Table 1, entry 3). After
the reaction conditions were optimized, the scope of the reaction with different aryl halides was investigated.
The results are given in Table 2.
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Table 2. Suzuki–Miyaura cross-coupling of aryl halides catalyzed by [Pd(OAc)2 ]/benzimidazolium salts.[a]
Conversion (%)
Entry
1
ArBr
6
7
Br
63.4
57.2
MeO
OMe
Br
2
40.6
49.8
Br
68.5
59.1
61.6
54.0
3
4
MeO
Br
Br
5
6
70.5
40.2
78.7
44.0
O
Cl
Me
[a] Conditions: [Pd(OAc)2 ] (5 × 10−5 mmol, 1 × 10−2 mol%), benzimidazolium salt (5 × 10−5 mmol), ArBr (0.5
mmol), (CH3 O)2 C6 H3 B(OH)2 (0.091 g, 0.75 mmol), NaH (60% dispersion in mineral oil) (0.030 g, 0.75 mmol), DMF
(1.5 mL), 130 ◦ C, 1 h. The conversions were determined by GC-MS and averaged over two runs.
As shown in Table 2, mono-benzimidazolium salts 6 and 7 exhibited good activities with aryl bromides
(entries 1–5). This shows that the activity of the catalytic systems depends on the bulkiness of the substrates.
The conversion of 4-bromotoluene, which has a linear shape, was higher than those of the more bulky 2-
bromotoluene, independently of the mono-benzimidazolium used (Table 2, entries 4 and 5 for 6, and Table 2,
entries 4 and 5 for 7). Additionally, the benzimidazolium salts 6 and 7 were efficient ligands for arylation of
bulky substrate like 2-bromo-6-methoxynaphthalene (Table 2, entry 3). The benzimidazolium salts 6 and 7
exhibited good activities in the coupling reactions with 4-chloroacetophenone (Table 2, entry 6). It is evident
that there are not enormous changes of the efficiencies of ligands in Suzuki–Miyaura cross-coupling reactions.
Even if the investigated substrates are not identical to those employed during previous studies using
ligands based on related imidazolium salts, the catalytic activities reported here are in the same range of
catalytic systems considered as efficient.
Resorcinarene-free benzimidazolium salts 8 and 9 (Figure 1) were prepared and used in the coupling
reactions to investigate any potential resorcinarene cavity effect. Comparisons were made for the arylation of
4-bromotoluene, 2-bromo-6-methoxynaphthalene, and 4-bromoanisole (Table 3). For each of these reactions,
conversions are in the same order respectively for the cavitand ligands 6 and 7 and for resorcinarene-free benzim-
idazolium salts 8 and 9. Significantly lower conversions are consistently observed when using resorcinarene-free
benzimidazolium salts 8 and 9 compared to the similar use of cavitand ligands. All of these results confirm that
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the superior activity of resorcinarene mono-benzimidazolium salts 6 and 7 in the arylation reactions is due to
the cavity effect of the resorcinarene cavitand, acting as a receptor for the reaction substrates. Another factor
likely to have an effect on these results is the greater bulkiness of the ligands of 6 and 7 compared to 8 and 9.
Table 3. Comparison of imidazolium salts in Suzuki–Miyaura cross-coupling of aryl bromides.[a]
Conversion (%)
Entry
1
ArBr
6
7
8
9
Br
70.5
78.7
37.8
34.3
Br
2
3
68.5
63.4
61.6
57.2
8.2
6.6
10.6
8.9
MeO
Br
MeO
[a] Conditions: [Pd(OAc)2 ] (5 × 10−5 mmol, 1 × 10−2 mol%), benzimidazolium salt (5 × 10−5 mmol), ArBr (0.5
mmol), (CH3 O)2 C6 H3 B(OH)2 (0.091 g, 0.75 mmol), NaH (60% dispersion in mineral oil) (0.030 g, 0.75 mmol), DMF
(1.5 mL), 130 ◦ C, 1 h. The conversions were determined by GC-MS and averaged over two runs.
In conclusion, we have prepared two novel resorcinarene based mono-benzimidazolium salts designed
for Suzuki–Miyaura cross-coupling reactions. These two new resorcinarene-based mono-benzimidazolium salts
exhibited good catalytic activity for coupling reactions in DMF. The importance of the cavity effect in which
the cavitand acts as a receptor was demonstrated by comparing these activities with those of resorcinarene-free
analogous ligands. On the other hand, the fact that 6 and 7 are bulkier than 8 and 9 is likely to play a role in the
better catalytic activities observed. These new and efficient catalytic systems are currently being investigated
in other cross-coupling reactions such as Negishi and Kumada–Tamao–Corriu.
3. Experimental
3.1. Materials
Mass spectra were recorded on a MALDI BRUKER Microflex LT using 2,5-dihydroxybenzoic acid as the matrix.
NMR spectra were recorded in CDCl3 solutions on a Varian 500 MHz spectrometer. IR spectra were recorded
between 4000 and 600 cm−1 using a PerkinElmer Spectrum 100 FT-IR spectrometer with an attenuated total
reflection accessory featuring a zinc selenide (ZnSe) crystal. The catalytic solutions were analyzed by using an
Agilent 5975C GC/MSD.
3.2. X-ray data collection and structure refinement
Data for compounds 6 and 7 were obtained with a Bruker APEX II QUAZAR three-circle diffractometer
and Agilent XCalibur diffractometer (Eos CCD detector). For 6, on the Bruker diffractometer, indexing
was performed using APEX2.44 Data integration and reduction were carried out with SAINT.45 Absorption
correction was performed by multiscan method implemented in SADABS.46 Space groups were determined
using XPREP implemented in APEX2. For 7, the CrysAlisPro software program was used for data collection
and cell refinement and data reduction.47 Absorption correction was performed by analytical method based
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on expressions derived by Clark and Reid.47 The structures were solved using SIR-2004.48 The least-square
refinement on F2 was achieved with CRYSTALS software.49 All nonhydrogen atoms were refined anisotropically.
The hydrogen atoms were all located in a difference map, but those attached to carbon atoms were repositioned
geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize
˚
their geometry (C-H in the range of 0.93–0.98 A) and Uiso (H) (in the range of 1.2–1.5 times Ueq of the parent
atom), after which the positions were refined with riding constraints. In refinement of 6, Br1 and Br2 were
refined with occupancy factor 0.5:0.5. The crystals available for X-ray structural analysis were of quite poor
quality and weak scatterers at high resolution, thus resulting in comparatively high R values. Crystallographic
data and refinement details of the data collection for 6 and 7 are given in Table S1 (on the journal’s website).
The final geometrical calculations and the molecular drawings were carried out with the Platon50 and Mercury51
programs.
Crystallographic data for these structures have been deposited with the Cambridge Crystallographic Data
Centre under deposition numbers 1027354 for 6 and 1027355 for 7.
3.3. Synthesis
The bromomethylated resorcinarene cavitand 541 was prepared following reported procedures. All reaction
solvents were dried and purified as described by Perrin and Armarego.52
3.4. Preparation of 1-benzyl benzimidazoles 3 and 4
1-Benzylbenzimidazoles 3 and 4 were prepared according to a slightly modified procedure.53 Potassium hydrox-
ide (1 mmol) was added to a solution of corresponding benzimidazole (1 mmol) in ethanol (20 mL), the mixture
was stirred for 1 h at room temperature, and benzyl chloride was added dropwise. This reaction mixture was
then heated for 8 h at 80 ◦ C. The mixture was diluted with water (50 mL) and extracted with dichloromethane
(3 × 25 mL), and the combined extracts were washed with water and dried over Na2 SO4 . The benzyl benz-
imidazole was isolated by chromatography on silica gel using CH2 Cl2 . The NMR spectra of the products were
identical to those reported previously for 3 and 4.
3.5. Preparation of mono benzyl benzimidazole resorcinarene cavitand 6
A chloroform solution of the bromomethylated resorcinarene cavitand 5 (0.5 g, 0.525 mmol) and benzimidazole
3 (0.109 g, 0.525 mmol) was refluxed for 5 days. The reaction mixture was evaporated to dryness and the solid
residue was washed with Et2 O (2 × 25 mL) to afford the benzimidazolium-resorcinarene cavitand 6 as a white
solid. Yield: 98%. 1 H NMR (500 MHz, CDCl3): δ = 11.32 (s, 1H, NCHN), 7.81 (d, 1H, arom. CH), 7.52–7.35
(m, 7H, arom. CH), 7.16 (s, 1H, arom. CH of resorcinarene), 6.96 (s, 3H, arom. CH of resorcinarene), 6.28 and
4.53 (AB spin system, 4H, OCH2 O, 2 J = 7.4Hz), 5.97 (s, 2H, ArCH2 N), 5.88 and 4.31 (AB spin system, 4H,
OCH2 O, 2 J = 7.3 Hz), 5.73 (s, 2H, ArCH2), 4.77 (t, 2H, CHCH2 , 3 J = 7.9 Hz), 4.69 (t, 2H, CHCH2 , 3 J =
7.7 Hz), 2.27–2.16 (m, 8H, CHCH2), 1.42–1.20 (m, 24H, CH2 CH2 CH2 CH3), 0.90 (t, 6H, CH2 CH3 , 3 J = 6.2
Hz), 0.85 (t, 6H, CH2 CH3 , 3 J = 6.2 Hz); 13 C NMR (125 MHz, CDCl3): δ = 153.73, 153.32, 153.20 (arom.
Cquat), 143.57 (s, NCHN), 139.06, 138.35, 137.69, 136.45, 132.25, 131.52, 130.76, 129.50, 129.41, 127.98, 127.11,
126.84, 124.19, 123.90, 122.57, 118.75, 117.17, 117.04, 114.12, 113.09, 100.15, 98.59, 51.47, 42.26, 36.97, 36.83,
32.05, 31.86, 30.14, 29.85, 27.63, 27.45, 22.66, 22.59, 14.07, 14.02, 10.37, 10.35.
MALDI-TOF: m/z = 1081.74 [M + H]+ .
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3.6. Preparation of mono benzyl benzimidazole resorcinarene cavitand 7
A chloroform solution of the bromomethylated resorcinarene cavitand 5 (0.5 g, 0.525 mmol) and benzimidazole
4 (0.124 g, 0.525 mmol) was refluxed for 5 days. The reaction mixture was evaporated to dryness and the
solid residue was washed with Et2 O (2 × 25 mL) to afford the benzimidazolium-resorcinarene cavitand 7 as
a white solid. Yield: 97%. 1 H NMR (500 MHz, CDCl3): δ = 11.14 (s, 1H, NCHN), 7.44 (s, 1H, arom. CH),
7.42–7.35 (m, 4H, arom. CH), 7.19 (s, 1H, arom. CH of resorcinarene), 7.15 (s, 1H, arom. CH of resorcinarene),
6.95 (s, 3H, arom. CH of resorcinarene), 6.24 and 4.51 (AB spin system, 4H, OCH2 O, 2 J = 7.4 Hz), 5.95
(s, 2H, ArCH2 N), 5.88 and 4.32 (AB spin system, 4H, OCH2 O, 2 J = 7.3 Hz), 5.64 (s, 2H, ArCH2), 4.77 (t,
2H, CHCH2 , 3 J = 7.9 Hz), 4.69 (t, 2H, CHCH2 , 3 J = 7.7 Hz), 2.33 (s, 3H, ArCH3), 2.31 (s, 3H, ArCH3),
2.27–2.16 (m, 8H, CHCH2), 1.44–1.20 (m, 24H, CH2 CH2 CH2 CH3), 0.90 (t, 6H, CH2 CH3 , 3 J = 6.2 Hz),
0.85 (t, 6H, CH2 CH3 , 3 J = 6.2 Hz); 13 C NMR (125 MHz, CDCl3): δ = 153.70, 153.41, 153.31, 153,15 (arom.
Cquat), 142.31 (s, NCHN), 138.87, 138.33, 137.66, 137.17, 136.93, 136.57, 132.51, 130.01, 129.46, 129.32, 129.27,
127.70, 124.19, 123.91, 122.34, 118.95, 117.16, 117.04, 113.70, 112.65, 100.15, 98.60, 51.15, 42.11, 36.97, 36.76,
32.05, 31.82, 30.14, 29.82, 27.63, 27.44, 22.66, 22.59, 20.64, 20.63, 15.22, 14.07, 13.99, 10.37, 10.34, 0.98, 0.04.
MALDI-TOF: m/z = 1109.26 [M + H]+ .
3.7. General procedure for palladium-catalyzed Suzuki–Miyaura cross-coupling reactions and GC
analyses
The solution of Pd(OAc)2 in DMF, the solution of the ligand in DMF, aryl bromide (0.5 mmol), 3,5-
dimethoxyphenylboronic acid (0.091 g, 0.75 mmol), NaH (0.030 g, 0.75 mmol), and an additional amount
of DMF (to have a final reaction volume of 1.5 mL) were put into a 10 mL reaction flask. The reaction mixture
was then heated for 1 h at 130 ◦ C. After cooling to room temperature, 1 mL of the reaction mixture was passed
through a Millipore filter and analyzed by GC-MS.
The runs were carried out using an ArBr/Pd ratio of 10,000. The calibration was based on diethylene
glycol monobuthyl ether. Conversions given in Tables 1–3 were averaged over two runs and have been calculated
relatively to the aryl bromide.
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1309

Table S1. Crystal data and refinement parameters for 6 and 7.
Crystal parameters
CCDC
6
7
1027354
1027355
Empirical formula
Formula weight (g mol–1)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C70H83BrN2O8
1160.34
C74H93BrN2O9
1234.46
120(2)
120(2)
0.71073
0.71073
Monoclinic
P21/c
Monoclinic
P21/c
a (Å)
27.039(5)
12.042(5)
20.321(5)
90
25.1170(15)
11.4920(4)
24.9090(14)
90
b (Å)
c (Å)
α (°)
β (°)
97.206(5)
90
110.942(7)
90
γ (°)
Crystal size (mm)
V (Å3)
0.135 × 0.176 × 0.207
6564(3)
0.309 × 0.558 × 0.699
6714.9(7)
4
Z
4
ρcalcd (g cm–3)
µ (mm–1)
1.174
1.221
0.680
0.669
F(000)
2464
2632
θ range for data collection (°)
h/k/l
1.52–25.68
–32/32, 0/14, 0/24
49,016
3.12–26.38
–31/29, 0/14, 0/31
29,757
Measured reflections
Independent reflections (Rint)
Absorption correction
Data / restraints / parameters
Goodness-of-fit on F2 (S)
12,440 [R(int) = 0.0710]
Multiscan
12440 / 65 / 739
1.078
13,724 [R(int) = 0.0300]
Analytical
13724 / 90 / 775
0.926
R1 = 0.1163,
wR2 = 0.2759
R1 = 0.1903,
wR2 = 0.3146
1.48 and –0.97
R1 = 0.0820,
wR2 = 0.1869
R1 = 0.1362,
wR2 = 0.2167
0.680 and –0.780
Final R indices [I > 2σ (I)]
R indices (all data)
Largest diff. peak and hole (e Å–3)
1