Palladium N-heterocyclic-carbene-catalyzed ortho-arylation of benzaldehyde derivatives

Article abstract of DOI:10.1002/hc.20479

New, sterically demanding 1,3-dialkylhenzimidazolium salts (2a-c) as N-heterocyclic-carbene precursors have been synthesized and characterized. The ortho position of aromatic aldehydes was directly and selectively arylated with aryl chlorides in the presence of a catalytic system prepared in situ from Pd(OAc)₂, 1,3-dialkylbenzimidazolium chlorides (2s-c), and Cs ₂CO₃.

Full text of DOI:10.1002/hc.20479

Heteroatom Chemistry
Volume 19, Number 6, 2008
Palladium N-Heterocyclic-Carbene-Catalyzed
ortho-Arylation of Benzaldehyde Derivatives
1
1
1
¨
˙
¨
Oznur Dog˘an, Nevin Gu¨rbu¨z, Ismail Ozdemir,
and Bekir C¸ etinkaya2
1Department of Chemistry, Faculty of Science and Art, Ino¨nu¨ University, 44280 Malatya, Turkey
2
˙
Department of Chemistry, Faculty of Science, Ege University, 35100 Bornova-Izmir, Turkey
Received 8 February 2008
catalysts. Typically, these synthetic operations are
ABSTRACT: New, sterically demanding 1,3-
carried out with the aid of functional groups such
as boronic acid/esters or triorganotin fragments, re-
spectively [2,3].
dialkylbenzimidazolium
salts
(2a–c)
as
N-
heterocyclic-carbene precursors have been syn-
thesized and characterized. The ortho position of
aromatic aldehydes was directly and selectively
arylated with aryl chlorides in the presence of a
catalytic system prepared in situ from Pd(OAc)2,
1,3-dialkylbenzimidazolium chlorides (2a–c), and
Therefore, the ability to couple an aryl halide di-
rectly at the unreactive C H position of an arene
without the need for a sacrificial electrophilic boron
or tin fragment would be highly desirable [4]. The se-
lective functionalization of C H bonds has attracted
a substantial interest owing to potential shortening
of synthetic sequences [5]. At the present, develop-
ment of the methods for sp2 C H bond function-
alization in directing group containing arenas and
electron-rich heterocycles has received the most at-
tention. For a number of directing group containing
substances, the conversion of aromatic ortho-C H
bonds to C C bonds has been demonstrated. Com-
pounds containing amide-, pyridine-, oxazoline-,
imine-, ketone-, and phenol-directing groups have
been ortho-arylated or alkylated under palladium,
ruthenium, or rhodium catalysis [6]. However, aryl
chlorides were rarely used, despite the fact that
chlorinated arenes are cheaper to manufacture and
therefore, play a vital role as intermediates in the
chemical industry. Presumably, this is because the
chlorides were generally found to be unreactive un-
der the conditions employed to couple bromides, io-
dides, and triflates.
ꢀ
C
Cs2CO3. 2008 Wiley Periodicals, Inc. Heteroatom Chem
19:569–574, 2008; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.20479
INTRODUCTION
Biaryls are important structural units frequently
found in pharmaceuticals, polymers, liquid crystals,
new materials, and ligands for homogeneous transi-
tion metal catalysts [1]. Among the most commonly
employed catalytic methods in biaryl synthesis are
Suzuki or Stille cross-coupling reactions, which in-
variably proceed using either nickel or palladium
˙
¨
Correspondence to: Ismail Ozdemir; e-mail: iozdemir@
inonu.edu.tr.
Contract grant sponsor: Technological and Scientific Research
¨
˙
Council of Turkey (TUBITAK).
Contract grant number: 106T106.
¨
˙
Recently, it has been shown that palladium com-
plexes of N-heterocyclic-carbene (NHC) ligands of-
fer distinct advantages as possible alternatives for
Pd/phosphine systems in C C coupling reactions [7].
Contract grant sponsor: TUBITAK-CNRS (France).
Contract grant number: TBAG-U/181 (106T716).
Contract grant sponsor: Ino¨nu¨ University Research Fund.
˙ ¨
Contract grant number: I.U. B.A.P: 2008/39.
c
ꢀ
2008 Wiley Periodicals, Inc.
569

570 Dogˇan et al.
CHO
Rn
CHO
+
X
R
Rn
R
SCHEME 1
Thus, some highly active palladium systems with
monodentate carbene ligands have been developed
for the activation of aryl chlorides [8].
of the benzimidazolium salts further supported the
assigned structures; the resonances for C(2)–H were
observed as sharp singlets in the 9.43, 10.16, and
9.95 ppm, respectively for 2a–c. The IR data for ben-
zimidazolium salts 2a–c clearly indicate the pres-
ence of the C N group with a ν(C N) vibration
at 1543, 1544, and 1557 cm−1, respectively for 2a–c.
The NMR values are similar to those found for other
1,3-dialkylbenzimidazolium salts [12].
We have previously reported the use of
in situ formed imidazolidin-2-ylidene, tetrahydro-
pyrimidin-2-ylidene, tetrahydrodiazepin-2-ylidene,
and benzimidazol-2-ylidene palladium(II) systems
that exhibit high activity for various coupling re-
actions of aryl bromides and aryl chlorides [9].
Recently, we report that the in situ generation
of catalysts, from [RuCl2(p-cymene)]2 and pyrim-
idinium or benzimidazolium salt in the presence
of Cs2CO3, selectively promote the diarylation of 2-
pyridylbenzene with aryl bromides [10].
The nature of the NHC ligand has a tremen-
dous influence on the rate of catalyzed reactions.
To find more efficient palladium catalysts, we have
prepared a series of new bulky or functional 1,3-
dialkylbenzimidazolium chlorides (LHCl = 2a–c),
containing a benzimidazole ring and herein, we re-
port a mild, practical use of the in situ generated
catalytic system composed of commercially avail-
able and stable reagents, the Pd(OAc)2 as palladium
source, 1,3-dialkylbenzimidazolium chloride (2a–c)
as a carbene precursor, and Cs2CO3 as a base for
Miaura coupling of aryl chlorides (Scheme 1).
It is worth noting that in situ formation of the
NHC complex by deprotonation of the azol(in)ium
salt led to significantly better results than the use
of the preformed complex. The success of these
processes as well as the recent reports by Miura
and coworkers [13–16] prompted us to examine
whether in situ generated NHC complexes could
be used for the direct arylation of the arene rings
of aromatic aldehydes. Formyl groups are syn-
thetically very useful because they can be con-
verted to many other functional groups. Herein,
we report a mild, practical, Pd-catalyzed aryla-
tion of benzaldehydes using air-stable Pd(OAc)2 as
the catalyst, 1,3-dialkylbenzimidazolium chlorides
(LHCl, 2a–c, Scheme 2) as the NHC ligand pre-
cursors, Cs2CO3 as the base, and DMF as the sol-
vent. Our initial exploration of the reaction con-
ditions for the Pd-catalyzed arylation of aldehydes
focused on the coupling of benzaldehyde and 4-
chloroacetophenone (Table 1, entries 1–3). The best
results for mono-ortho-arylation of benzaldehydes
using 4-chloroacetophenone were obtained at 100◦C
in DMF using Cs2CO3 as base, and a catalyst system
generated in situ from 1% mmol of Pd(OAc)2 and 2%
mmol of LHCl (2a–c).
Table 1 summarizes representative results from
screening of the three benzimidazolium salts (LHCl),
for a variety of substrates that undergo ortho-
arylation. Several trends are readily apparent: The
use of NHC ligand precursors 2a–c allowed lower
reaction temperatures (100◦C) and shorter reaction
times. The procedure is simple and does not re-
quire induction periods. All complexes led to good
conversions (78–97%) at low-catalyst concentration
(1.0 mmol %). Although not dramatic, consistent
differences in yields were observed in the reac-
tions according to the ligand precursors 2a–c. Pre-
sumably, the bulkier ligands derived from 2a–c are
RESULTS AND DISCUSSION
Dialkylbenzimidazolium salts, (2a–c) are con-
ventional NHC precursors. The functionalized
or bulky benzimidazolium salts, 2a–c, were
synthesized by consecutive alkylation of 1-
benzhydrylbenzimidazole (1) with alkyl halides
(Scheme 2).
According to Scheme 2, the salts (2a–c) were
obtained in almost quantitative yield by quarternazi-
tion of 1-benzhydrylbenzimidazole (1) in DMF with
alkyl halides [11, 12]. The salts are air- and moisture
stable both in the solid state and in the solution. The
structures of 2a–c were determined by their charac-
teristic spectroscopic data and elemental analyses.
13C NMR chemical shifts were consistent with the
proposed structure, the imino carbon appeared as
1
a typical singlet in the H-decoupled mode in the
142.4, 142.7, and 142.6 ppm, respectively for benz-
1
imidazolium chlorides 2a–c. The H NMR spectra
Heteroatom Chemistry DOI 10.1002/hc

Palladium N-heterocyclic-Carbene-Catalyzed ortho-Arylation of Benzaldehyde Derivatives 571
N
+
-
)
Cl
Cl
N
2a
N
Cl
N
1
OCH3
Cl
N
+
-
)
Cl
N
C2H5O
OC2H5
2c
N
+
-
)
Cl
N
OCH3
2b
SCHEME 2 Synthesis of 1,3-dialkylbenzimidazolium chlorides (LHCl).
more effective in stabilizing the palladium complex.
This new method was compatible with the presence
of both electron-withdrawing and electron-donating
groups in the para position of the halobenzene. Table
1 also shows that a diverse group of aromatic alde-
hydes can be coupled. Control experiments showed
that in the absence of either Pd(OAc)2 or LHCl, no
reaction was observed. It is worth noting that, in con-
trast to our findings, arylation of benzaldehyde with
4-bromoanisole in the presence of Ni(dppe)Br2/Zn
has been reported to give diaryl carbinols [17].
The palladium-catalyzed arylation of carbonyl
compounds or phenols, reported by Miura, is consid-
ered to proceed via coordination between the pheno-
late or enolate oxygen of the substrates and the aryl-
palladium intermediate [13]. Consequently, one may
expect that oxygen from the aldehyde may function
as phenolate oxygen.
Pd(OAc)2, afford a highly active catalyst for the
ortho-arylation of benzaldehyde derivatives. The lig-
and precursors 2a–c are particularly effective, and
with 1.0 mmol % Pd, a variety of aryl halides and ben-
zaldehydes react efficiently and with high selectivity.
Detailed investigations, focusing on benzimidazolin-
2-ylidene substituent effects, functional group toler-
ance, and catalytic activity in this and other coupling
reactions are in progress.
EXPERIMENTAL
All reactions for the preparation of 1,3-
dialkylbenzimidazolium salts (2a–c) were carried
out under argon using standard Schkenk-type flasks.
All reagents were purchased from Aldrich Chemical
Co. (Ankara, Turkey). The solvents, Et2O over Na,
DMF over BaO, EtOH over Mg, were distilled prior to
use. All 1H and 13C-NMR were performed in DMSO-
1
d6. H NMR and 13C NMR spectra were recorded
CONCLUSION
using a Bruker AC300P FT spectrometer operating at
300.13 MHz (1H) and 75.47 MHz (13C), respectively.
Chemical shifts (δ) are given in ppm relative to TMS,
Bulky, electron-rich NHC ligand precursors with
a benzimidazoline backbone, when combined with
Heteroatom Chemistry DOI 10.1002/hc

572 Dogˇan et al.
TABLE 1 Arylation of Benzaldehyde Derivatives by Ru–NHC Complexes
Entry
LHCl
R
Aromatic Aldehyde
Product
Yielda,b (%)
1
2
3
2a
2b
2c
COCH3
COCH3
COCH3
84
86
83
CHO
O
CH3
CHO
C
4
5
6
2a
2b
2c
OCH3
OCH3
OCH3
92
96
81
CHO
OCH3
CHO
7
8
9
2a
2b
2c
COCH3
COCH3
COCH3
86
95
83
O
CH3
CHO
C
10
11
12
2a
2b
2c
OCH3
OCH3
OCH3
80
83
78
CHO
OCH3
CHO
13
14
15
2a
2b
2c
COCH3
COCH3
COCH3
87
91
84
H3CH2C
O
C
H3CH2C
CHO
CH3
16
17
18
2a
2b
2c
OCH3
OCH3
OCH3
96
97
85
H3CH2C
CHO
OCH3
19
20
21
2a
2b
2c
COCH3
COCH3
COCH3
86
89
85
CHO
O
CHO
C
CH3
22
23
24
2a
2b
2c
OCH3
OCH3
OCH3
83
82
79
CHO
OCH3
aReactions conditions: 1.0 mmol of R-C6H4Cl-p, 1.0 mmol of aldehyde, 2 mmol Cs2CO3, 1.0 mmol % Pd(OAc)2, 2 mmol % 1,3-dialkyl
benzimidazolium salt, DMF (3 mL), 100◦C, 15 h.
bYield determined by NMR, GC, purity of compounds was checked by NMR and yields are based on the aldehyde.
Heteroatom Chemistry DOI 10.1002/hc

Palladium N-heterocyclic-Carbene-Catalyzed ortho-Arylation of Benzaldehyde Derivatives 573
coupling constants (J) in hertz. Melting points
(t, 3H, J = 7.2 Hz, OCH2CH ), 3.39 (q, 2H, J = 6.8
3
were measured in open capillary tubes with an
Electrothermal-9200 melting point apparatus and
uncorrected. Elemental analyses were performed by
TUBITAK Microlab (Ankara, Turkey).
Hz, OCH CH3), 3.85 (t, 2H,J = 4.4 Hz, NCH2CH O),
2
2
4.90 (t, 2H, J = 4.8 Hz, CH CH2O), 7.39 (s, 1H,
2
CH(C6H5)2, 7.33–7.95 (m, 14H, Ar), 9.95 (1H,
2-CH). 13C NMR (CDCl3) δ: 15.1 (OCH2CH3), 58.3
(OCH2CH3), 66.6 (NCH2CH2O), 67.9 (NCH2CH2O),
131.2 (CH(C6H5)2), 114.6, 114.7, 127.2, 127.3, 128.7,
129.8, 129.7, 132.6, 135.4 (Ar), 142.6 (2-CH). Found:
C, 73.55; H, 6.81; N, 7.09%. Calcd for C24H25N2OCl:
C, 73.36; H, 6.41; N, 7.13%.
1-(Benzhydryl)-3-(2,4,6-trimethylbenzyl)-
benzimidazolium Chloride (2a)
To
a solution of 1-(benzhydryl)benzimidazole
(2.0 g, 7.04 mmol) in DMF (3 mL), 2,4,6-
trimethylbenzylchloride (1.30 g, 7.70 mmol) was
added; the resulting solution was stirred for 1 h
at room temperature and heated for 12 h at 80◦C.
Et2O (10 mL) was added to the reaction mixture.
A white solid was precipitated in this period. The
precipitate was then crystallized from EtOH/Et2O
(1:2). Yield: 3.07 g, 93%, mp 204–205◦C. IR, ν:
General Procedure for Arylation of Benzaldehyde
Derivatives
A dried Schlenk flask equipped with a magnetic stir-
ring bar was charged with the aldehyde (1.0 mmol),
aryl chloride (1.0 mmol), Pd(OAc)2(0.01 mmol),
benzimidazolium chloride (0.02 mmol), Cs2CO3
(2.0 mmol), and DMF (3 mL). After stirring at 100◦C
for 15 h, the mixture was cooled to room tempera-
ture and then quenched by addition of aqueous 1 N
HCl and extracted with diethyl ether. The isolated
organic layer was dried over MgSO4, filtered, concen-
trated in vacuo, and purified by column chromatog-
raphy on silica gel eluting with ethyl acetate/hexane
(1:5). Analysis of the reaction product was carried
out by NMR and GC-MS.
1
1543 cm−1 (C N). H NMR (DMSO) δ: 2.25 (s, 3H,
CH2C6H2(CH )3-4), 2.27 (s, 6H, CH2C6H2(CH )3-2,6),
3
3
5.81 (s, 2H, CH -(C6H2)-(CH3)3), 6.97 (s, 2H, CH2-
2
(C6 H )-(CH3)3), 7.54 (s, 1H, CH(C6H5)2), 7.38–7.73
2
(m, 14H, Ar), 9.43 (s,1H,2-CH). 13C NMR (DMSO) δ:
19.9 (CH2C6H2(CH3)3-4), 21.1 (CH2C6H2(CH3)3-2,6),
64.7 (CH2-(C6H2)-(CH3)3-2,4,6), 131.2 (CH(C6H5)2),
114.5, 115.2, 126.4, 127.3, 127.5, 128.8, 128.9, 129.6,
129.7, 130.1, 132.2, 136.5, 138.5, 139.1 (Ar), 142.4 (2-
CH). Found: C, 79.41; H, 4.57; N, 6.21%. Calcd for
C30H29N2Cl: C, 79.54; H, 4.65; N, 6.18%.
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was
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from
1-
(benzhydryl)benzimidazole (2 g, 7.04 mmol) and
2-methoxyethyl chloride (0.73 g, 7.74 mmol) in DMF
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1
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Heteroatom Chemistry DOI 10.1002/hc