A novel ditopic ring-expanded N-heterocyclic carbene ligand-assisted Suzuki-Miyaura coupling reaction in aqueous media

Article abstract of DOI:10.1016/j.tetlet.2017.07.096

A series of seven-membered ditopic ring-expanded N-heterocyclic carbene (dre-NHC) precursors, bearing sterically demanding and electron-rich aryl groups, were synthesised in moderate yields via the reaction of 1,2,4,5-tetrakis(bromomethyl)benzene with the corresponding N,N′-diarylformamidines in the presence of K₂CO₃ in acetonitrile under an air atmosphere. All new compounds were characterised by HRMS, NMR spectroscopy, and microanalysis, as well as X-ray crystallography for compound 1c. The development of an efficient catalytic system for the Suzuki-Miyaura coupling reaction of aryl chlorides with various boronic acids was also investigated using the in situ generated dre-NHC ligands.

Full text of DOI:10.1016/j.tetlet.2017.07.096

Accepted Manuscript
A novel ditopic ring-expanded N-heterocyclic carbene ligand-assisted Suzuki-
Miyaura coupling reaction in aqueous media
Emine Özge Karaca, Mitat Akkoç, Muhammad Nawaz Tahir, Cengiz Arıcı,
Fatma İmik, Nevin Gürbüz, Sedat Yaşar, İsmail Özdemir
PII:
S0040-4039(17)30965-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.07.096
TETL 49178
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 June 2017
18 July 2017
28 July 2017
Please cite this article as: Karaca, E.O., Akkoç, M., Nawaz Tahir, M., Arıcı, C., İmik, F., Gürbüz, N., Yaşar, S.,
Özdemir, I., A novel ditopic ring-expanded N-heterocyclic carbene ligand-assisted Suzuki-Miyaura coupling
reaction in aqueous media, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.07.096
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Tetrahedron Letters
journal hom epage: www.els evier.com
A novel ditopic ring-expanded N-heterocyclic carbene ligand-assisted Suzuki-
Miyaura coupling reaction in aqueous media
Emine Özge Karacaa, Mitat Akkoçb, Muhammad Nawaz Tahirc, Cengiz Arıcıd, Fatma İmikb, Nevin
Gürbüza,b, Sedat Yaşara,b,*, İsmail Özdemira,b
The authors dedicate this work to Prof. Christian Bruneau for his first 30 years of contributions to catalysis.
a İnönü University, Catalysis, Research and Application Center, 44280 Malatya, Turkey
b İnönü University, Faculty of Science and Arts, Department of Chemistry, 44280 Malatya, Turkey
c Department of Physics, University of Sargodha, Sargodha, Pakistan
d The Scientific and Technological, Research and Council of Turkey, Pk, 80 06100, Ankara, Turkey
*Corresponding Author: Tel:+904223773735, Fax:+904223410212, e-mail:syasar44@gmail.com
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A series of seven-membered ditopic ring-expanded N-heterocyclic carbene (dre-NHC)
precursors, bearing sterically demanding and electron-rich aryl groups, were synthesised in
moderate yields via the reaction of 1,2,4,5-tetrakis(bromomethyl)benzene with the
corresponding N,N’-diarylformamidines in the presence of K2CO3 in acetonitrile under an air
atmosphere. All new compounds were characterised by HRMS, NMR spectroscopy, and
microanalysis, as well as X-ray crystallography for compound 1c. The development of an
efficient catalytic system for the Suzuki-Miyaura coupling reaction of aryl chlorides with
various boronic acids was also investigated using the in situ generated dre-NHC ligands.
Available online
Keywords:
Ditopic ring-expanded N-heterocyclic carbene
Suzuki-Miyaura coupling
C-C bond formation
2009 Elsevier Ltd. All rights reserved
.
Palladium
coupling reaction20 consists of three distinct parts: (i) oxidative
addition of aryl halides to the active catalyst species, (ii)
transmetalation of phenylboronic acid derivatives to the metal
halides, and (iii) reductive elimination of the product from the
active catalyst. For the oxidative addition step, electron-rich
ligands improve insertion into the strong carbon-halogen bond of
the aryl halide. Meanwhile, the steric bulk of the ligand is
believed to increase the rate of reductive elimination to
regenerate the active catalyst. Despite the bulky ligand increasing
the reductive elimination rate, the rate of the oxidative addition
step can be dramatically affected by the increasing steric effect of
the ligand. According to these points, a balance between the
steric and electronic properties of the ligand is an essential
requirement.20
Introduction
The palladium-catalysed Suzuki-Miyaura cross-coupling reaction
between organoboron compounds and aryl halides is one of the
most powerful methods1 for the synthesis of biaryl compounds,
which are often important chemicals in pharmaceutics, organic
chemistry and fine materials.2,3 The reputation of the Suzuki
reaction stems from its use of low-cost and non-toxic chemicals,
wide substrate tolerance, mild reaction conditions, and easy
separation of products from the reaction media.4,5 The use of non-
flammable, inexpensive and abundant green solvents, such as
water or water solutions, is an important area of research for the
Suzuki reaction.6–8 However, low substrate solubility or catalyst
stability in water are potential issues. To overcome these
problems, several additives have been successfully utilised.9–12
The employment of hydrophilic ligands, such as N-heterocyclic
carbenes (NHCs) or phosphine ligands, is another method of
improving the efficiency of Suzuki catalysis in water. However,
despite the wide applications of phosphine ligands, they have
limitations for use in the Suzuki reaction due to several
disadvantages, including toxicity, sensitivity to air and moisture,
and difficult separation from the products. As a result, NHCs
have emerged as an alternative to phosphine ligands.13–19 The
unique steric and electronic properties of NHCs as well as their
specific coordination chemistry (strong σ-donor and weak π-
acceptor properties) contribute to the generation of highly active
and stable metal centres in the key catalytic steps of the Suzuki
reaction. The accepted mechanism for the Pd-catalysed cross-
So far, studies have focused on the synthesis and catalytic
applications of classic NHC ligands. Recently, ring expanded N-
heterocyclic carbenes (re-NHCs) have attracted attention due to
their tuneable sterics and strong σ-donating nature, compared to
classic NHCs, which make them desirable ligands for a range of
transition metals and catalytic transformations.21–26 Despite these
important features, re-NHCs have not been sufficiently
investigated with regards to catalytic applications and complex
syntheses. Preliminary investigations by the groups of Stahll and
Cavell reported the syntheses of sterically and electronically
unique six, seven and eight-membered re-NHCs, as well as their
complexes and applications in catalysis.22–27 According to their
results, large rings and N-C -N bond angles offer steric and

2
Tetrahedron
electronic advantages in some applications when compared to
Several catalytic systems have been reported for the aqueous
Suzuki-Miyaura coupling reactions of aryl chlorides. However,
most of these require at least 1 mol% of the Pd precursor and
high reaction temperatures.34–35 Our group has previously
reported a series of aqueous coupling protocols for aryl
chlorides.36–39 In the present study, the activity of the in-situ
generated dre-NHC/Pd catalyst system was investigated.41
conventional five-membered derivatives. The key facets of these
ligands are the highly basic carbon and steric pressure related to
the expanded ring and metal centre.28 re-NHCs have
demonstrated excellent
activity in C-C coupling,29
hydrosilylation,30 hydrogenation,31 and transfer hydrogenation
reactions.32 In order to better understand the behaviour of these
ligands, herein, we report more sterically demanding dre-NHC
ligands and in-situ generation of the active catalyst for the
Suzuki-Miyaura coupling reaction.
For this purpose, several inexpensive and readily water soluble
alkaline metal bases, e.g. Na2CO3, K2CO3 and Cs2CO3, were
examined. K2CO3 showed excellent catalytic activity in the
DMF/H2O solvent mixture (Table 1, entries 9–11). The use of a
single solvent medium, such as H2O, i-PrOH dioxane or DMF,
afforded lower yields (Table 1, entries 1–8). The solvents ratio
was also examined (Table 1, entries 9-11); the best result was
obtained using DMF/H2O (1:1) (Table 1, entry 10).
Results and Discussion
According to Scheme 1, compounds 1a-c were synthesised in
moderate
tetrakis(bromomethyl)benzene with the corresponding N,N’-
diarylformamidines in the presence of K2CO3 in acetonitrile
under an air atmosphere.33 Well-known seven-membered NHCs
have been reported by Cavell and co-workers.23–26,28–32 Upon
formation of the seven membered dre-NHC precursors 1a-c, H
NMR spectroscopy revealed the appearance of the characteristic
formamidinium salt NCHN proton at 8.37, 8.29, and 8.42 ppm,
yields
via
the
reaction
of
1,2,4,5-
Table 1. Effect of solvent and base on the model Suzuki coupling
reaction of p-chloroacetophenone with phenylboronic acid, catalysed
by the Pd(OAc)2/1a catalyst system.
1
1
respectively. The H NMR shifts of 1a-c were consistent with
previously reported seven-membered NHC salts.23 The methylene
protons (NCH2) of salts 1a-c were visible as very broad singlets
(between 4.0-6.5 ppm) at ambient temperature in CDCl3 and
DMSO-d6. The imino carbons of 1a-c (NCHN) were detected as
Entry
Solvent
Base
Na2CO3
Conversion [%]a
1
2
Dioxane
Dioxane
Dioxane
64
75
35
60
62
63
79
37
84
>99
88
80
65
K2CO3
Cs2CO3
Na2CO3
K2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
1
3
typical singlets in the H-decoupled mode at 158.5, 157.4, and
156.7 ppm, respectively. All dre-NHC precursors were soluble
in water and exhibited high stability in air and aqueous solutions.
4
i-PrOH
5
i-PrOH
6
i-PrOH
DMF
7
8
H2O
9
DMF/H2O (2:1)
DMF/H2O (1:1)
DMF/H2O (1:2)
DMF/H2O (1:1)
DMF/H2O (1:1)
10
11
12
13
aReagents and conditions: Pd(OAc)2 (0.5 mol%), 1a (0.5 mol%), 4-
chloroacetophenone (1 mmol), PhB(OH)2 (1.5 mmol), base (1.5 eq.), solvent
(6 mL) 80 oC,
(undecane);
1 h. GC yield calibrated against an internal standard
Scheme 1. Synthesis of dre-NHC compounds
Scheme 1. Synthesis of dre-NHC precursors 1a-c.
The reaction of various phenylboronic acids with aryl chlorides
was examined (Table 2; see ESI for the full version of Table 2).
High yields were obtained with low catalyst loadings in the case
of p-activated substrates, such as p-chloroacetophenone and p-
chloroaldehyde (Table 2, entries 1-8, 26-28). Additionally, we
examined the effect of temperature and reaction time (Table 2,
entries 3-14, 26-28). Unactivated aryl chlorides required higher
temperatures and longer reaction times to obtain the cross-
coupled products in high yield. Conversely, activated aryl
chlorides gave high yields with short reaction times at room
temperature (Table 2, entries 3-8, 26-28). When compared with
relevant literature studies,42–46 salts 1a-c showed high catalytic
activity with short reaction times, even for reactions performed at
room temperature. Similar yields were obtained when this
catalytic system was examined using Jeffery conditions (Table 2,
entries 18-20).47,48 Trace amount of the coupled products were
observed when Pd(OAc)2 was solely used (Table 2, entries 21-
25). From the obtained results, it was clear that p-phenoxyphenyl
boronic acid gave better results than phenylboronic acid (Table 2,
entries 26-34, 38-40, 44-46). This difference in catalytic activity
was attributed to the electron withdrawing properties of the
phenoxy group in the para-position. 2-Methylphenyl chloride
was also converted to the coupled products in good to high yields
(Table 2, entries 41-46). These results can be related to steric
effects. In light of these results, it should be noted that ligands
1a-c exhibit similar catalytic performance. This can be attributed
Figure 1. ORTEP view of 1c. Thermal ellipsoids are drawn at
the 40% probability level and H-atoms are shown as small circles
of arbitrary radii. The labeling of symmetry related atoms is
omitted for clarity and symmetry related Br-ions and diethyl
ether are not shown.

to the well-balanced electronic and steric properties of the NHC
Acknowledgements
precursors 1a-c.
This work was financially supported by The Scientific and
Technological Research Council of Turkey (TÜBİTAK Project
No:113Z285).
Table 2. Suzuki coupling reactions with activated or unactivated aryl
chlorides
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Entry
R1/R2
dre-NHC
Time
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1a
1b
1a
1b
1c
1a
1b
1c
1a
1b
1c
1a
1b
1c
1a
1b
1c
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100
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98c
2
3
4-COCH3/H
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1 h
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100c
91c
5
6
77c
7
4-COH/H
4-OCH3/H
4-CH3/H
4-H/H
80c
8
78c
9
85 (24c)
89 (11c)
64 (13c)
91 (18c)
95 (24c)
89 (16c)
99
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
86
77
93d
4-COCH3/H
4-CH3/H
4-H/H
1a
94d
96d
4-COCH3/H
4-COH/H
4-OCH3/H
4-CH3/H
4-H/H
-
-
3
2
-
1 h
1
-
1
-
5
99c
1a
15
27
4-COCH3/OPh
4-OCH3/OPh
4-CH3/OPh
1b
97c
min
28
29
1c
1a
97c
90
15
30
1b
88
min
31
32
1c
1a
80
77
15
33
1b
73
min
34
35
36
37
38
39
40
41
42
43
44
45
46
1c
1a
1b
1c
1a
1b
1c
1a
1b
1c
1a
1b
1c
69
79
70
73
96
96
94
69
66
59
92
88
89
4-tert-Bu/H
4-tert-Bu/OPh
2-CH3/H
1h
1 h
1 h
1 h
2-CH3/OPh
aReagents and conditions: R-C6H4Cl-p (1 mmol), phenylboronic acid (1.5
mmol), K2CO3 (1.5 mmol), dre-NHC (0.5 mol%), Pd(OAc)2 (0.5 mol%), H2O
(3 mL)/DMF(3 mL), 80 oC, 1 h, Reactions were performed in duplicate. GC
yield calibrated against an internal standard (undecane); bH2O as solvent; rt;
c
dEt4NBr (2 mmol) was used as an additive under the general reaction
conditions.
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their Pd complexes and catalytic activity.

4
Tetrahedron
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Highlights
* A series of seven-membered ditopic ring-
expanded N-heterocyclic carbene (dre-NHC)
precursors were synthesized.
* The dre-NHC precursor/Pd(OAc)2 catalytic
system was used in the Suzuki-Miyaura coupling
reaction.
* High catalytic activities were exhibited for - the
coupling of aryl chloro substrates.

6
Tetrahedron
Graphical Abstract
Leave this area blank for abstract info.
A novel ditopic ring-expanded N-heterocyclic carbene ligand-assisted Suzuki-
Miyaura coupling reaction in aqueous media
Emine Özge Karaca, Mitat Akkoç, Muhammad Nawaz Tahir, Cengiz Arıcı,
Fatma İmik, Nevin Gürbüz, Sedat Yaşar*, İsmail Özdemir