Potential N-heterocyclic carbene precursors in the palladium-catalyzed heck reaction

Article abstract of DOI:10.1002/hc.21065

A novel 1-(cyclobutylmethyl)-substi-tuted imidazolidinium/benzimidazolium salts as N-heterocyclic carbene (NHC) precursors were successfully synthesized and characterized by ¹H NMR, ¹³C NMR, IR, and elemental analysis techniques. The

Full text of DOI:10.1002/hc.21065

Heteroatom Chemistry
Volume 24, Number 1, 2013
Potential N-Heterocyclic Carbene Precursors
in the Palladium-Catalyzed Heck Reaction
˙
¨
Serpil Demir, Rukiye Zengin, and Ismail Ozdemir
Department of Chemistry, Faculty of Science and Art, Ino¨nu¨ University, 44280 Malatya, Turkey
Received 18 July 2012; revised 22 October 2012
Suzuki. Of all the work undertaken, the area that
ABSTRACT: A novel 1-(cyclobutylmethyl)-substi-
has perhaps received most research effort is the de-
velopment of new catalysts and ligands for Heck
tuted imidazolidinium/benzimidazolium salts as N-
heterocyclic carbene (NHC) precursors were success-
coupling reactions and the use of these catalyst sys-
fully synthesized and characterized by H NMR, 13C
tems in, for example, natural product synthesis. As
NMR, IR, and elemental analysis techniques. These
the choice of ligand plays a major role in the ef-
compounds were easily prepared from the reaction
ficiency of the catalyst, phosphine ligands are the
of N-alkyl imidazoline/N-alkyl benzimidazole with
first choice to catalyze the Heck reaction more ac-
bromomethylcyclobutane in high yields. The in situ
tively and efficiently [5–9]. However, they are often
formed catalytic system derived from the NHC pre-
toxic, air-sensitive, or quite expensive. In the past
cursor and Pd(OAc)2 was used in the Heck reaction
15 years, the employment of N-heterocyclic carbene
between aryl halides and styrene with potassium hy-
(NHC) ligands has gained an enormous popularity in
droxide in water. The corresponding Heck products
catalysis due to their potential advantages over ter-
1
ꢀC
were obtained in good yields. 2012 Wiley Periodi-
tiary phosphines specifically, easy accessibility, bet-
ter σ donor ability, and improved thermal stability of
the complexes [10–13]. An exceptionally large num-
ber of NHC complexes have emerged and have been
used successfully in many catalytic transformations,
notably the C C and C N cross-coupling reactions
[14–21] as well as the extremely useful metathesis
reaction [22–26]. While a variety of media, such as
ionic liquids [27], fluorous solvents [28], and su-
percritical carbon dioxide [29–31] have been pro-
moted as replacements to organic solvents in metal-
catalyzed reactions, the use of water as the solvent is
considered as a key characteristic from environmen-
tal, nontoxicity, industrial, and economical points of
view [32–34]. The use of water in Pd-catalyzed cross-
coupling reactions goes back to the early develop-
ment of the Suzuki coupling with the first example
being reported by Calabrese and co-workers in 1990
[35, 36]. Recently, we have shown the influence of
imidazodinium/benzimidazolium salts as NHC pre-
cursors with Pd(OAc)2 for the C C coupling reac-
tions [37–39,41,42]. Now, we report synthesis, struc-
ture, and the use of 1,3-dialkyl imidazolidinium or
cals, Inc. Heteroatom Chem. 24:77–83, 2013; View
this article online at wileyonlinelibrary.com. DOI
10.1002/hc.21065
INTRODUCTION
The palladium-catalyzed Mizoroki—Heck reaction,
or more often the Heck reaction, arylation of olefins
with aryl halides was independently discovered by
Heck [1] and Mizoroki [2] ca. 40 years ago. It has
since become one of the most celebrated tools for
constructing sp2 C C bonds in synthetic chemistry
[3, 4]. Its significance has been highlighted by the
2010 Nobel Prize in chemistry to Professor Richard
Heck along with Professors Ei-ichi Negishi and Akira
˙
¨
Correspondence to: Ismail Ozdemir; e-mail: ismail.ozdemir@
inonu.edu.tr.
˙
˙ ¨
Contract grant sponsor: Ino¨nu¨ University Research Fund.
Contract grant number: IUBAP 2011/142.
2012 Wiley Periodicals, Inc.
ꢀC
77

¨
78 Demir, Zengin, and Ozdemir
SCHEME 1 Synthesis of imidazolidinium salts (path A) and benzimidazolium salts (path B). 1-alkylimidazoline or 1-
alkylbenzimidazole (5 mmol), bromomethylcyclobutane (5 mmol) in DMF (10 mL) at 60◦C for 10 h.
benzimidazolium salts that consist of the cyclobutyl-
metyl moiety as ligands for Pd(II)-catalyzed Heck
coupling reactions of aryl halides in water.
mental analyses, and their melting points were deter-
mined. They can dissolve in water, dichloromethane,
chloroform, methanol, and acetonitrile. Although
they have not water-soluble side groups such as
sugar, sulfonate, and amino, they are perfectly solv-
able in water.
RESULTS AND DISCUSSION
A fine study of the chemical shifts of the acidic
C(2) protons shows that they are located in the range
9.15–11.68 ppm, which indicates a significant differ-
ence in the acidity of these protons (Table 1). Com-
parison of the chemical shifts in the two series where
the aromatic substituents are identical as in (1a, 2a),
(1b, 2b), and (1c, 2c) reveals the order δ(1) < δ(2),
which is an indication of the influence of the core
structure of the properties of these salts. The 13C
NMR spectra of 1a–1c exhibit the NCHN resonances
between δ 157.7 and 156.9 ppm, which are also typ-
ical values previously reported for imidazolidinium
The new NHC precursors (1a–1c and 2a–2c) were
readily prepared upon treatment of the 1-alkyl im-
idazoline or benzimidazole with bromomethylcy-
clobutane in DMF (Scheme 1).
We chose the cyclobutyl ring as a substituent.
Because we thought that this group can exhibit dif-
ferent catalytic activities due to the cyclic structure.
All compounds are air and moisture-stable in the
solid state as well as in solution, and they were iso-
lated as solids in very good yields and fully character-
ized by 1H NMR, 13C NMR, and IR spectroscopy, ele-
TABLE 1 1H NMR and 13C NMR Spectroscopic Data for the C(2)H Proton and Carbon (δ, ppm); ν(CN) (cm−1) of the NHC
Salts
NHC Salt
1H NMR
13C NMR
ν(CN)
NHC Salt
1H NMR
13C NMR
ν(CN)
1a
1b
1c
9.92
9.53
9.15
157.7
157.4
156.9
1658
1649
1643
2a
2b
2c
11.68
11.31
10.70
142.5
142.8
142.2
1552
1550
1552
Heteroatom Chemistry DOI 10.1002/hc

Potential N-Heterocyclic Carbene Precursors in Palladium-Catalyzed Heck Reaction 79
salts [37]. The NCHN δ [13C{1H}] signal of the ben-
zimidazolium salts is usually around 142 4 ppm.
For the benzimidazolium salts 2a–c, it was found to
be 142.5, 142.8, and 142.2 ppm, respectively. These
values are in good agreement with the previously re-
ported results [39–42].
The compounds (1a–c and 2a–c) show IR ab-
sorption bands at wavelengths and that vary from
1550 to 1658 cm−1, which are assigned to ν(CN).
These IR absorption values are in good agree-
ment with previously reported values for NHC salts
[37–42].
vents, water is safe, nontoxic, nonflammable, and
inexpensive. Therefore, development of organic re-
actions in water is still attractive enough [54–61].
The in situ formed catalytic systems derived
from the NHC salts and Pd(OAc)2 have been used
successfully for the Heck reaction in water. Initially,
the reaction conditions were optimized starting from
4-bromoacetophenone and styrene catalyzed by the
NHC salts/Pd(OAc)2 system in water at 80◦C with
various bases as shown in Table 2. No good yield was
observed when either potassium carbonate or ce-
sium carbonate was used as the base (Table 2, entries
1 and 5), whereas a good yield was observed with
potassium tert-butoxide, potassium hydroxide, and
sodium hydroxide (Table 2, entries 2, 3, and 4). We
observed almost same results with potassium tert-
butoxide and potassium hydroxide and preferred
to potassium hydroxide as a base because it was
cheaper than others. It was noted that in the ab-
sence of the NHC precursor, there was no detectable
amount of the desired coupled product formed as
judged by the GC analysis (Table 2, entry 8).
The effect of temperature and reaction time on
the reaction of 4-bromoacetophenone and styrene
in water using the 1a/(Pd(OAc)2 and KOH as a base
(Table 2, entries 6 and 7) was also studied. Having
optimized conditions, we applied these conditions
to the coupling reactions of varieties of aryl halides
with styrene (Table 3).
The coupling of aryl iodides and bromides was
superior and afforded the desired products in ex-
cellent yields. Reactions involving deactivated aryl
bromides bearing a methyl or methoxy group with
styrene gave high yields of the coupled product.
The reaction between 1-bromo-4-nitrobenzene and
styrene also work well to give product for 2 h.
4-iodoanisole and 4-iodoacetophenone were com-
pletely converted into the coupled products after
Pd(OAc)2/NHC Precursor Catalyzed the Heck
Reaction Performed in Water
The Heck reaction of aryl halides with alkenes
is promoted by palladium catalysts and mean-
while one of the most important methods for C C
coupling in organic synthesis [43], in the synthesis
of intermediates for pharmaceuticals [44], and also
for the preparation of conducting polymers [45].
NHCs are potential ligands for formation of com-
plexes with transition metals that can be used as
catalysts in organic synthesis. The ligand-free sys-
tems consist of Pd(OAc)2 or Pd(O2CCF3)2; base and
solvents have also been used as a catalyst system
in Heck coupling reactions [46–48]. However, us-
ing the Pd(OAc)2-catalytic systems containing lig-
ands has been shown more effective catalytic ac-
tivity than ligand-free palladium-containing systems
[49,51]. Pd(OAc)2-benzimidazole or imidazoline lig-
ands could be very effective catalytic systems, par-
ticularly in cross-coupling reactions [37–42]. Heck
reactions are most frequently performed in dipolar
aprotic organic solvents such as DMF [52], DMA
[53], and dioxane [37]. Compared to organic sol-
TABLE 2 Effect of Different Bases on Product Yield in the Heck Coupling of 4-Bromoacetophenone and Styrene in Water
Entry
NHC Salt
Base
Temperature (◦C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
–
Cs2CO3
KOBut
KOH
NaOH
K2CO3
KOH
80
80
80
80
80
60
80
80
3
3
3
3
3
3
1
15
40
93
95
87
35
55
75
10
KOH
KOH
Reaction conditions: All reactions were carried out using styrene (1.5 mmol), 4-bromoacetophenone (1.0 mmol), KOH (2.0 mmol), 1a (2.0
mmol%), Pd(OAc)2 (1.0 mmol%), water (5 mL). All reactions were monitored by TLC and GC.
Heteroatom Chemistry DOI 10.1002/hc

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80 Demir, Zengin, and Ozdemir
TABLE 3 Heck Reaction between Aryl Halides and Styrene in the Presence of LHX/Pd(OAc)2 and KOH in Water
R/X
1a
1b
1c
2a
2b
2c
COCH3/Br
CHO/Br
OCH3/Br
95 (3 h)
92 (5 h)
85 (10 h)
95 (10 h)a
63 (10 h)
91 (10 h)a
87 (2 h)
93 (3 h)
81 (5 h)
92 (3 h)
89 (5 h)
55 (3 h)
51 (5 h)
47 (3 h)
37 (5 h)
68 (3 h)
35 (5 h)
92 (10 h)a
72 (10 h)a
53 (10 h)a
55 (10 h)a
42 (10 h)a
CH3/Br
84 (10 h)a
83 (2 h)
89 (10 h)a
81 (2 h)
83 (10 h)a
78 (2 h)
81 (10 h)a
76 (2 h)
51 (10 h)a
72 (2 h)
NO2/Br
H/Br
47 (5 h)
69 (5 h)a
97 (1/2 h)a
93 (1/2 h)
No (24 h)
No (24 h)b
OCH3/I
COCH3/I
COCH3/Cl
Reaction conditions: All reactions were carried out using styrene (1.5 mmol), aryl halide (1.0 mmol), KOH (2.0 mmol), LHX (2.0 mmol%),
Pd(OAc)2 (1.0 mmol%), water (5 mL), 80◦C. All reactions were monitored by TLC and GC.
a100◦C.
b120◦C.
30 min. The ideal substrates for coupling reactions
are aryl chlorides, since they tend to be cheaper and
more widely available than their bromide or iodide
counterparts. Unfortunately, the high C Cl bond
strength compared with C Br and C I bonds disfa-
vor oxidative addition, the first step in catalytic cou-
pling reactions. We also investigated the scope of this
method on aryl chlorides, but found that the cou-
pling of aryl chloride with styrene did not work; even
on heating the reaction mixture vigorously stirred at
120◦C in oil bath and extending the reaction time.
We found that all these Pd(OAc)2/LHX catalytic sys-
tems are very active for aryl bromide and iodide
in the Heck reaction. Like other type of palladium-
catalyst coupling reactions, the Mizoroki–Heck re-
action’s activity is also highly dependent on the
nature of the ligand, for example, its electronic
and steric effect [62]. The activity of the catalysts
(1a–1c and 2a–2c) was also tested in the Heck reac-
tion. The NHC ligands with a imidazolidinium group
(1a–1c) are much more active than those NHC lig-
ands with benzimidazolium groups (2a–2c). Thus,
we reported that the electronic properties of the NHC
ligands with the cyclobutylmethyl substituent have a
significant effect on the reactivity. When compared
to literature results [37,63], new NHC precursors are
a very active catalyst for the Heck coupling reaction.
Gu¨lcemal and co-workers reported the benzimida-
zolium bromides bearing oligoether side chains
(CH2CH2O)n (n = 1, 2, or 3) as a catalyst for the
Heck reaction of bromoacetophenone with styrene
and Cs2CO3 at 100◦C and 4 h in the presence of wa-
ter. A great advantage of our catalysts is to generate
the styrene derivatives in aqueous media with eco-
nomic KOH at short time and lower temperature
as indicated in the literature. We believe that the
cyclobutylmethyl group has a significant effect on
this reaction. The carbene ligand with the 4-methyl–
substituted imidazolidinium group is much more
active than those carbene ligands with other sub-
stituted groups.
CONCLUSIONS
In conclusion, we prepared cyclobutylmethyl func-
tionalized imidazolidinium/benzimidazolium salts
(1–2) whose structures were confirmed by 1H NMR,
13C NMR, IR, and elemental analysis. The in situ
catalytic activities of all salts, in combination with
Pd(OAc)2, were tested for the Heck reaction in water.
The advantages offered by this method are simple
procedure, mild conditions, and good yields of prod-
uct. Research in our laboratory is currently on going
to extend the coordination chemistry of function-
alized NHCs to transition metals and explore their
potential applications in catalysis.
EXPERIMENTAL
All procedures were carried out under an in-
ert atmosphere using standard Schlenk line tech-
niques. Chemicals and solvents were purchased from
Sigma Aldrich (Dorset, UK). 1-Alkylbenzimidazole
and alkylimidazoline were synthesized in our
Heteroatom Chemistry DOI 10.1002/hc

Potential N-Heterocyclic Carbene Precursors in Palladium-Catalyzed Heck Reaction 81
laboratory. Solvents were dried with standard meth-
18.2 (CH2, CH2-cyclobutane), 21.2 (CH2C6H4(CH3)-
4), 25.9 (CH2, CH2-cyclobutane), 32.9 (CH, CH2-
cyclobutane), 47.7 and 48.9 (NCH2CH2N), 51.7
(CH2C6H4(CH3)-4), 53.5 (CH2-cyclobutane), 128.9,
129.6, 129.8, and 138.9 (CH2C6H4(CH3)-4), 157.7
(NCHN), Anal. Calcd for C16H23N2Br: C: 59.45; H:
7.17; N: 8.67. Found C: 59.48; H: 7.15; N: 8.69.
ods and freshly distilled prior to use. Elemental anal-
yses were performed by a LECO CHNS-932 elemen-
tal analyzer. Melting points were measured in open
capillary tubes with an Electrothermal-9200 melting
point apparatus and are uncorrected. FT-IR spec-
tra were recorded as KBr pellets in the range of
400–4000 cm−1 on a Perkin Elmer spectrum 100.
1H NMR and 13C NMR spectra were recorded us-
ing a Varian As 400 Merkur spectrometer operat-
ing at 400 MHz (1H), 100 MHz (13C) in CDCl3 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 in-
ternal standard. Coupling constants (J values) are
given in Hertz. NMR multiplicities are abbreviated
as fallows: s = singlet, d = doublet, t = triplet,
m = multiplet, hept = heptet signal. All catalytic re-
actions were monitored on an Agilent 6890N GC sys-
tem by GC-FID with a HP-5 column of 30 m length,
0.32 mm diameter, and 0.25-μm film thickness. Col-
umn chromatography was performed using silica gel
60 (70–230 mesh). Solvent ratios are given as v/v.
[1-(Cyclobutylmethyl)-3-(2,4,6-trimethylbenzyl)]
imidazolidinium Bromide, 1b
Yield: 3.27 g (93%), mp: 163–164◦C, IR ν(CN)
= 1649 cm−1
.
1H NMR (399.9 MHz, CDCl3) δ
(ppm) = 1.81–1.72 (m, 2H, CH2, CH2-cyclobutane),
1.99–1.88 (m, 2H, CH2, CH2-cyclobutane), 2.17–
2.05 (m, 2H, CH2, CH2-cyclobutane), 2.34 and
2.25 (s, 9H, CH2C6H2(CH3)3-2,4,6), 2.68 (hept.,
1H, J = 7.5 Hz, CH, CH2-cyclobutane), 3.61 (d,
2H, J = 7.8 Hz, CH2-cyclobutane), 3.98–3.74 (m,
4H, NCH2CH2N), 4.90 (s, 2H, CH2C6H2(CH3)3-
2,4,6), 6.86 (s, 2H, CH2C6H2(CH3)3-2,4,6), 9.53
(s, 1H, NCHN). 13C NMR (100.5 MHz, CDCl3)
δ
(ppm) = 18.1 (CH2, CH2-cyclobutane), 20.2
and 20.9 (CH2C6H2(CH3)3-2,4,6), 25.8 (CH2, CH2-
cyclobutane), 32.9 (CH, CH2-cyclobutane), 47.7 and
46.2 (NCH2CH2N), 48.8 (CH2C6H2(CH3)3-2,4,6),
53.2 (CH2-cyclobutane), 125.4, 129.8, 137.6, and
139.0 (CH2C6H2(CH3)3-2,4,6), 157.4 (NCHN), Anal.
Calcd for C18H27N2Br: C: 61.54; H: 7.75; N: 7.97.
Found C: 61.55; H: 7.78; N: 7.94.
General Procedure for the Preparation of the
NHC Precursors
To
a
solution of 1-alkylimidazoline or 1-
alkylbenzimidazole (5 mmol) in DMF (10 mL),
bromomethylcyclobutane (5 mmol) at 25◦C
was added slowly and the resulting mixture
was stirred at 60◦C for 10 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 vac-
uum. The crude product was recrystallized from
EtOH/Et2O.
[1-(Cyclobutylmethyl)-3-(2,3,4,5,6-pentamethyl-
benzyl)]imidazolidinium Bromide, 1c
Yield: 3.30 g (87%), mp: 242–243◦C, IR ν(CN)
=
1
1643 cm−1. H NMR (399.9 MHz, CDCl3) δ (ppm)
= 1.81–1.72 (m, 2H, CH2, CH2-cyclobutane), 2.01–
1.89 (m, 2H, CH2, CH2-cyclobutane), 2.16–2.05 (m,
2H, CH2, CH2-cyclobutane), 2.30, 2.24, and 2.22 (s,
15H, CH2C6(CH3)5-2,3,4,5,6), 2.67 (hept., 1H, J = 7.8
Hz, CH, CH2-cyclobutane), 3.61 (d, 2H, J = 7.5 Hz,
CH2-cyclobutane), 4.01–3.82 (m, 4H, NCH2CH2N),
4.94 (s, 2H, CH2C6(CH3)5-2,3,4,5,6), 9.15 (s, 1H,
NCHN). 13C NMR (100.5 MHz, CDCl3) δ (ppm)
= 16.9, 17.1, and 17.2 (CH2C6(CH3)5-2,3,4,5,6),
18.2 (CH2, CH2-cyclobutane), 25.8 (CH2, CH2-
cyclobutane), 32.9 (CH, CH2-cyclobutane), 47.3 and
48.1 (NCH2CH2N), 48.9 (CH2C6(CH3)5-2,3,4,5,6),
53.6 (CH2-cyclobutane), 125.6, 133.4, 133.6, and
136.5 (CH2C6(CH3)5-2,3,4,5,6), 156.9 (NCHN), Anal.
Calcd for C20H31N2Br: C: 63.32; H: 8.24; N: 7.38.
Found C: 63.29; H: 8.25; N: 7.41.
[1-(Cyclobutylmethyl)-3-(4-methylbenzyl)]
imidazolidinium Bromide, 1a
Yield: 2.65 g (82%), mp: 109–110◦C, IR ν(CN) = 1658
cm−1
.
1H NMR (399.9 MHz, CDCl3) δ (ppm) =
1.80–1.74 (m, 2H, CH2, CH2-cyclobutane), 1.98–1.81
(m, 2H, CH2, CH2-cyclobutane), 2.16–2.05 (m, 2H,
CH2, CH2-cyclobutane), 2.31 (s, 3H, CH2C6H4(CH3)-
4), 2.67 (hept., 1H, J = 7.8 Hz, CH, CH2-cyclobutane),
3.59 (d, 2H, J = 7.68 Hz, CH2-cyclobutane), 3.89–3.79
(m, 4H, NCH2CH2N), 4.82 (s, 2H, CH2C6H4(CH3)-
4), 7.28 (d, 2H, J = 8.1 Hz, CH2C6H4(CH3)-4), 7.14
(d, 2H, J = 7.8 Hz, CH2C6H4(CH3)-4), 9.92 (s, 1H,
NCHN). 13C NMR (100.5 MHz, CDCl3) δ (ppm) =
Heteroatom Chemistry DOI 10.1002/hc

¨
82 Demir, Zengin, and Ozdemir
2H, J = 7.5 Hz, CH2-cyclobutane), 5.86 (s, 2H,
CH2C6(CH3)5-2,3,4,5,6), 7.47 (d, 1H, J = 7.8 Hz,
NC6H4N), 7.60 (t, 1H, J = 7.5 Hz, NC6H4N),
7.71 (t, 1H, J = 7.8 Hz, NC6H4N), 7.94 (d,
1H, J = 7.2 Hz, NC6H4N), 10.7 (s, 1H, NCHN).
13C NMR (100.5 MHz, CDCl3) δ (ppm) = 16.6,
17.1, and 17.2 (CH2C6(CH3)5-2,3,4,5,6), 17.9 (CH2,
CH2-cyclobutane), 25.7 (CH2, CH2-cyclobutane),
34.6 (CH, CH2-cyclobutane), 48.3 (CH2C6(CH3)5-
2,3,4,5,6), 52.2 (CH2-cyclobutane), 113.1, 113.8,
127.0, 127.2, 131.4, and 131.7 (NC6H4N), 124.9,
133.4, 133.9, and 137.3 (CH2C6(CH3)5-2,3,4,5,6),
142.2 (NCHN), Anal. Calcd for C27H31N2Br: C:
69.97; H: 6.74; N: 6.04. Found C: 69.95; H: 6.71;
N: 6.05.
[1-(Cyclobutylmethyl)-3-(4-methylbenzyl)]
benzimidazolium Bromide, 2a
Yield: 2.93 g (79%), mp: 210–211◦C, IR ν(CN)
=
1
1552 cm−1. H NMR (399.9 MHz, CDCl3) δ (ppm)
= 2.09–1.87 (m, 4H, CH2, CH2-cyclobutane), 2.20–
2.12 (m, 2H, CH2, CH2-cyclobutane), 2.31 (s, 3H,
CH2C6H4(CH3)-4), 3.07 (hept., 1H, J = 7.8 Hz,
CH, CH2-cyclobutane), 4.62 (d, 2H, J = 7.8 Hz,
CH2-cyclobutane), 5.88 (s, 2H, CH2C6H4(CH3)-4),
7.41 (d, 2H, J = 8.1 Hz, CH2C6H4(CH3)-4), 7.15
(d, 2H, J = 7.5 Hz, CH2C6H4(CH3)-4), 7.72–7.52
(m, 4H, NC6H4N), 11.68 (s, 1H, NCHN). 13C NMR
(100.5 MHz, CDCl3) δ (ppm) = 17.9 (CH2, CH2-
cyclobutane), 21.2 (CH2C6H4(CH3)-4), 25.8 (CH2,
CH2-cyclobutane), 34.5 (CH, CH2-cyclobutane), 51.2
(CH2C6H4(CH3)-4), 52.3 (CH2-cyclobutane), 113.0,
113.9, 127.1, 127.2, 131.1, and 131.5 (NC6H4N),
128.3, 129.8, 129.9, and 139.2 (CH2C6H4(CH3)-4),
142.5 (NCHN), Anal. Calcd for C20H23N2Br: C: 64.69;
H: 6.24; N: 7.54. Found C: 64.71; H: 6.21; N: 7.55.
General Procedure for the Heck Reaction
Aryl halide (1.0 mmol), styrene (1.5 mmol), KOH (2.0
mmol), NHC precursor (2.0 mmol%), and Pd(OAc)2
(1.0 mmol%) were taken in a 25-mL Schlenk tube
in which 5 mL of water was added. The reaction
mixture was stirred for 1/2–24 h at 80–120◦C. The
progress of the reaction was monitored by TLC. Af-
ter completion of the reaction, the reaction mixture
was filtered and the filtrate extracted with ethyl ac-
etate (10 mL × 2). The combined organic layer was
dried over anhydrous MgSO4 and evaporated under
reduced pressure. The crude material was purified by
column chromatography with EtOAc/hexane (1/5)
over silica gel to afford the corresponding product
in high purity.
[1-(Cyclobutylmethyl)-3-(2,4,6-trimethylbenzyl)]
benzimidazolium Bromide, 2b
Yield: 3.39 g (85%), mp: 219–220◦C, IR ν(CN)
=
1
1550 cm−1. H NMR (399.9 MHz, CDCl3) δ (ppm)
= 2.02–1.94 (m, 4H, CH2, CH2-cyclobutane), 2.15–
2.05 (m, 2H, CH2, CH2-cyclobutane), 2.31 and 2.29
(s, 9H, CH2C6H2(CH3)3-2,4,6), 3.04 (hept., 1H, J =
7.5 Hz, CH, CH2-cyclobutane), 4.64 (d, 2H, J = 7.8
Hz, CH2-cyclobutane), 5.92 (s, 2H, CH2C6H2(CH3)3-
2,4,6), 6.93 (s, 2H, CH2C6H2(CH3)3-2,4,6), 7.18 (d,
1H, J = 8.7 Hz, NC6H4N), 7.45 (t, 1H, J = 6.6 Hz,
NC6H4N), 7.58 (t, 1H, J = 6.6 Hz, NC6H4N), 7.71
(d, 1H, J = 8.4 Hz, NC6H4N), 11.31 (s, 1H, NCHN).
13C NMR (100.5 MHz, CDCl3) δ (ppm) = 17.9 (CH2,
CH2-cyclobutane), 21.1 and 20.3 (CH2C6H2(CH3)3-
2,4,6), 25.7 (CH2, CH2-cyclobutane), 34.5 (CH,
CH2-cyclobutane), 47.5 (CH2C6H2(CH3)3-2,4,6), 52.2
(CH2-cyclobutane), 113.0, 113.9, 127.0, 127.2, 131.3,
and 131.6 (NC6H4N), 125.1, 130.2, 137.9, and 139.7
(CH2C6H2(CH3)3-2,4,6), 142.8 (NCHN), Anal. Calcd
for C22H27N2Br: C: 66.16; H: 6.81; N: 7.01. Found C:
66.15; H: 6.78; N: 7.04.
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