New bisbenzimidazolin-2-ylidene salts as N-heterocyclic dicarbene precursors: Synthesis, characterization, and involvement in palladium-catalyzed Suzuki reactions

Article abstract of DOI:10.1002/hc.21148

A new series of bis-N-heterocyclic carbene precursors, LHX (3a-d), have been synthesized and characterized by appropriate spectroscopic techniques and microanalyses. In situ prepared Pd(OAc)₂/bis-N-heterocyclic carbene precursors catalysts have catalyzed unactivated aryl chlorides on the Suzuki cross-coupling reaction under mild reaction conditions in aqueous media.

Full text of DOI:10.1002/hc.21148

Heteroatom Chemistry
Volume 25, Number 3, 2014
New Bisbenzimidazolin-2-Ylidene Salts as
N-Heterocyclic Dicarbene Precursors:
Synthesis, Characterization, and Involvement
in Palladium-Catalyzed Suzuki Reactions
1
2
Sedat Yas¸ar,1,2 Suzan C¸ ekirdek, and Ismail Ozdemir
˙
¨
1Department of Chemistry, Faculty of Science and Art, Gaziosmanpas¸a University, 60150 Tokat,
Turkey
2Department of Chemistry, Catalysis, Research and Applied Center, Ino¨nu¨ University, 44280
Malatya, Turkey
Received 4 November 2013; revised 17 February 2014
course of the last few decades because of its several
ABSTRACT: A new series of bis-N-heterocyclic
useful advantages compared to other available meth-
carbene precursors, LHX (3a–d), have been syn-
ods. One of the advantages of the Suzuki reaction is
thesized and characterized by appropriate spectro-
the use of nontoxic, thermally, air- and moisture-
scopic techniques and microanalyses. In situ prepared
stable phenyl boronic acid and its derivatives.
Pd(OAc)2/bis-N-heterocyclic carbene precursors cata-
Research have focused on developing a new cat-
lysts have catalyzed unactivated aryl chlorides on the
alyst precursor that can catalyze less reactive and
Suzuki cross-coupling reaction under mild reaction
cheaper aryl chlorides with low catalyst loading
ꢀC
conditions in aqueous media.
2014 Wiley Peri-
[8–10]. At the same time, the use of additives, design
of catalyst, and changing of solvents are a couple of
efforts for creating the best catalysis system. The use
of water as a solvent for reactions has economical
and environmental advantages as it is inexpensive,
abundant, nontoxic, nonflammable, and easily sepa-
rable from organic compounds. A number of studies
of the Suzuki reaction have been performed in water
as a solvent [11–14].
Lately, many bulky and electron-rich phos-
phines have been synthesized and promoted the
Suzuki cross-coupling reaction [15, 16]. Substi-
tution of commonly used triarylphosphines and
phosphines ligands with sterically and electroni-
cally superior N-heterocyclic carbene (NHC) ligands
generates more active catalyst for the Suzuki re-
action of aryl chlorides and unactivated substrates
[10]. Buchwald [17], Fu [18], Herrmann [19], and
other groups have carried out pioneer studies with
unactivated aryl chlorides.
odicals, Inc. Heteroatom Chem. 25:157–162, 2014;
View this article online at wileyonlinelibrary.com.
DOI 10.1002/hc.21148
INTRODUCTION
The Suzuki coupling reaction is one of the most com-
mon and preferred reaction for C C bond formation
of aryl and alkyl halogens in the presence of aryl
boronic acid and has attracted much interest [1–7].
Although there are numerous existing methods for
the synthesis of biaryls via cross-coupling reactions,
the Suzuki reaction has been the most used over the
Correspondence to: Sedat Yas¸ar; e-mail: syasar44@gmail.com,
sedat.yasar@gop.edu.tr.
Contract grant sponsor: Gaziosmanpasa University Research
Fund (GOP. B.A.P).
NHCs have become the most preferable ligand
for numerous transition metal–catalyzed reactions
Contract grant numbers: 2010/84, 2011/51, and 2011/105.
ꢀC
2014 Wiley Periodicals, Inc.
157

¨
158 Yas¸ar, C¸ekirdek, and Ozdemir
since the first stable free NHC was reported by Ar-
duengo et al. [20, 21]. NHCs complexes are gener-
ally highly stable complexes. They have strong σ-
donor [22] and weak π-acceptor [23] or sometimes
weak π-donor [24] character and have an unrivalled
form that may be used to generate sterically demand-
ing ligands. The basicity of NHCs can be related to
their σ-donor abilities [25]. σ-Donor abilities and
ternazition of 1-alkyl-benzimidazoline [35–37] in
DMF with alkyl halides (Fig. 1). The structures of
3a–d were determined by their characteristic spec-
troscopic data and elemental analyses (see the Ex-
perimental section). 13C NMR chemical shifts were
consistent with the proposed structures; the imino
carbon appeared as a typical singlet in the 1H-
decoupled mode in the 144.0, 143.5, 143.8, and 145.6
ppm, respectively, for imidazolinium salts 3a–d. The
1H NMR spectra of the imidazolinium salts further
supported the assigned structures; the resonances
for C(2) protons were observed as a sharp singlet
in the 11.39, 11.13, 10.09, and 10.05 ppm, respec-
tively, for imidazolinium salts 3a–d. The IR data
for imidazolinium salts 3a–d clearly indicate the
presence of the C N group with a ν(C N) vibra-
tion at 1615, 1645, 1632, and 1636 cm−1, respec-
tively, for 3a–d. The NMR values are similar to those
found for other 1,3-dialkylbenzimidazolinium salts
[12].
basicity of NHCs increase with expanding N
C N
angles. Recently, ring-expanded NHCs have been
synthesized, and catalytic features have been inves-
tigated by Cavell et al. [26–29]. Electronic properties
of catalysts are more important for an oxidative ad-
dition step of the Suzuki reaction. Numerous NHC
ligands, which have different electronic properties
and similar steric demand, have been synthesized
and used as a ligand for the Suzuki cross-coupling
reaction of aryl chlorides. As expected, the highest
yield was obtained by a palladium catalyst bear-
ing electron-reach ligands [30]. However, electron-
drawing group substitute ligands showed close re-
activity to electron-rich group substitute ligands for
aryl chlorides with arylboronic acid. Thus, reach-
ing to a level of electron richness required for ox-
idative addition, a sterical modification of ligands
becomes more important than an electrical modifi-
cation [31]. Bulky NHC ligands can provide several
advantages for a sterically demanding substrate for
the Suzuki coupling reaction. Sterically big NHCs
can simplify formation of catalytically active species,
as well as make the reductive elimination step easy.
Besides, oxidative addition or transmetalation steps
might slow down because of the steric effect of NHCs
[32,33].
It has been found that in situ formation of the
active catalyst by deprotonation of the imidazolin-
ium chlorides with appropriate base leads to signif-
icantly better results than the use of the preformed
palladium carbene complexes [34]. Encouraged by
these results and to find a more efficient Suzuki
catalysis system, we have synthesized a series of new
bis-1,3-dialkylbenzimidazolium halides that have
different steric properties (3a–d). We report a mild,
practical in situ generated catalytic system com-
posed of using Pd(OAc)2 as the palladium source,
bis-1,3-dialkylbenzimidazolium chlorides (3a–d) as
carbene precursors, KOtBu as a base, and water–
N,N-dimethylformamide (DMF) as a solvent for
Suzuki cross-coupling of aryl chlorides.
In this study, a series of related ligands with sim-
ilar electronic character and different steric demand
have been used for a systematic optimization of the
Suzuki reaction.
Before finding optimum conditions, we in-
vestigated the role of NHCs ligand precursors
with a couple of test reactions (Table 1, entries
1, 6, 11, 16, 21). The results obtained from test
reactions were as expected. Then to find optimum
conditions, a series of experiments were carried
out with 4-chloroacetophenone and phenylboronic
acid as role model compounds. Different bases
such as Cs2CO3, K2CO3, NaOH, and KOtBu were
tested, and KOtBu was found as the best base in
water/DMF systems. In addition, the reactions were
performed in air and without degassing the water
with N2 gas prior to use. After determining the
optimum cross-coupling reaction condition, the
scope of the reaction and efficiency of the NHCs
were evaluated by investigating the coupling of
phenylboronic acid with p-substituted aryl chlo-
rides. The results are shown in Table 1. Both the
electron-rich and the electron-poor unactivated aryl
chlorides (p-chloroacetophenone, p-chlorotoluene,
p-chlorobenzaldehyde,
p-chloroanisole,
and
chlorobenzene) were coupled successfully with
boronic acid in good yields to give desirable
products by Pd(OAc)2/3a–d on a mild reaction
condition. But still, we observed that electron
attracting aryl chlorides are more appropriate
substrate for this catalyzed system as suggested
by our results (Table 1, entries 1–10). However,
NHC precursors (3c–d), which contain an aromatic
ring on a bridge position, are the more active
ligands than others. In all entries, on average
RESULTS AND DISCUSSION
Bisdialkylbenzimidazolinium salts, LHX (3a–d) are
classic bis-NHC precursors. The salts, 3a–c, were
obtained in almost quantitative yield by quar-
Heteroatom Chemistry DOI 10.1002/hc

New Bisbenzimidazolin-2-Ylidene-Salts as N-Heterocyclic Dicarbene Precursors 159
FIGURE 1 Preparation of bis-1,3-dialkylbenzimidazolium halogens, LHX (3a–d).
10–15% homocoupling product of the boronic acid
(biphenyl), probably originating from the reduction
of the Pd(II) precatalyst, was acquired. When 0.5
mol% Pd(OAc)2/0.75 mol% LHX was used for p-
chloroacetophenone and p-chlorobenzaldehyde, the
quantity of homocoupling products (diphenyl and
o-terphenyl) increased to 40–45% and quantity of
desired product decreased (Table 1, entries 26–29)
. As a result, these bisbenzimidazolium salts were
found to be useful NHC ligand precursors for the
difficult formation of biaryls from unactivated aryl
chlorides.
All the catalytic systems are successful for unac-
tivated chlorine substrates, and most of them lead to
the formation of products in good yields. However,
chelating carbene ligands do not bring catalytically
significant improvement with respect to monoden-
tate ligands. This study has supported the fact that
aromatic ring bridged sterically demanding NHCs
showed more catalytic activity than alkyl-bridged
sterically NHCs. We have developed a highly active,
easy to producible, and environmentally friendly lig-
ands and catalysis process for palladium-mediated
Suzuki cross-coupling in aqueous media using bis-
1,3-dialkylbenzimidazolidin-2-ylidene ligands. The
catalysis system is simple and efficient for various
aryl chlorides without preactivated.
CONCLUSIONS
New bisbenzimidazolium salts have been prepared
and fully characterized. After deprotonation with
KOtBu, the resulting species are associated with a
Pd(OAc)2 center to generate catalyst precursors for
the Suzuki reactions of unactivated aryl chloride
derivatives with phenylboronic acid.
EXPERIMENTAL
1H NMR and 13C NMR spectra were recorded us-
ing a Bruker AC400P FT spectrometer operating at
400 MHz (1H), 100 MHz (13C). Chemical shifts (δ)
Heteroatom Chemistry DOI 10.1002/hc

¨
160 Yas¸ar, C¸ekirdek, and Ozdemir
TABLE 1 The Suzuki Coupling Reaction of Aryl Chlorides with Phenylboronic Acid Catalyzed by Pd(OAc)2/LHX (3a–d)
Entry
R
LHX
Yielda,b,c (%)
1
2
3
4
5
6
7
8
COCH3
COCH3
COCH3
COCH3
COCH3
CHO
CHO
CHO
CHO
CHO
CH3O
CH3O
CH3O
CH3O
CH3O
CH3
CH3
CH3
CH3
CH3
H
H
H
H
–
3a
3b
3c
3d
–
3a
3b
3c
3d
–
3a
3b
3c
3d
–
3a
3b
3c
3d
–
25d
100
99
98
95
30d
100
100
99
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
99
20d
80
75
100
65
15d
80
75
90
65
15d
95
3a
3b
3c
3d
3a
3b
3a
3b
90
90
100
65e
59e
73e
70e
H
COCH3
COCH3
CHO
CHO
aReaction conditions: 1.0 mmol of p-R–C6H4Cl, 1.5 mmol of phenylboronic acid, 2 mmol KOtBu, 1 mmol% Pd(OAc)2, 1.5 mmol% 3a–d, 50°C,
3 h, water (3 mL)–DMF (3 mL).
bPurity of compounds was checked by NMR, and yields are based on arylchloride.
cAll reactions were monitored by thin-layer chromatography (TLC).
d No LHX (3a–d).
ePd(OAc)2 (0.5 mol%), LHX (0.75 mol%).
are given in parts per million relative to tetram-
ethylsilane (TMS), coupling constants (J) in hertz.
FT-IR spectra were recorded on a Mattson 1000
spectrophotometer, wave numbers in cm−1. Melting
points were measured in open capillary tubes with
an Electrothermal-9200 melting point apparatus and
uncorrected. Elemental analyses were performed at
TUBITAK (Ankara, Turkey) Microlab.
added to obtain red sticky oil. The red sticky oil was
washed with diethyl ether (3 × 10 mL), dried under
vacuo, and the crude product was recrystallized from
ethanol/diethyl ether (1:3) to obtain hygroscopic yel-
low crystals.
Yield: 4.48 g; 70%. ν(CN) = 1615 cm−1
.
1H
NMR (δ, CDCl3): 2.59 (m, 8H, N(CH2CH2)(N(CH2)2
(CH2)2O), 3.65 (m, 8H, N(CH2CH2)(N(CH2)2
(CH2)2O), 3.77 (m, 4H, N(CH2CH2)(N(CH2)2
(CH2)2O), 4.12 (m, 4H, N(CH2CH2O)), 4.71 (m, 4H,
N(CH2CH2O)), 4.89 (m, 4H, N(CH2CH2)(N(CH2)2
(CH2)2O)), 7.89–7.61 (m, 8H, aromatic protons),
11.39 (s, 1H, NCHN). 13C NMR (δ, CDCl3): 47.6
(N(CH2CH2)(N(CH2)2(CH2)2O)), 53.5 (N(CH2CH2)
(N(CH2)2(CH2)2O)), 56.5 (N(CH2CH2)(N(CH2)2
(CH2)2O)), 66.9 (N(CH2CH2O)), 69.2 (N(CH2
2,2ꢁ -Bis-(N-2-morpholinethylbenzimidazolium)-
ethylether Dichloride, 3a
To a solution of 1-(2-morpholinethyl)benzimid-
azoline (1; 2.02 g, 10 mmol) in DMF (5 mL), 2-
chloroethylether (1.5 g, 10.07 mmol) was added
slowly at 25οC, and the resulting mixture was stirred
at 100οC for 3 days. Diethyl ether (30 mL) was
CH2O)),
71.2
(N(CH2CH2)(N(CH2)2(CH2)2O)),
Heteroatom Chemistry DOI 10.1002/hc

New Bisbenzimidazolin-2-Ylidene-Salts as N-Heterocyclic Dicarbene Precursors 161
132.1; 131.1; 126.9; 126.8; 114.3; 112.5 (C6H4,
10 mmol) were dissolved in 5 mL DMF at room
temperature and heated at 100°C for 2 days. White
solid was filtrated and washed with ether (3 ×
10 mL). Dried under vacuo and white solid was
crystallized with methanol–ether (1:2).
aromatic carbons), 144.0 (NCHN). Microanalyses
found: C, 59.65; H, 7.25; N, 14.20. Calcd. for
C30H42N6Cl2O3: C, 59.50; H, 6.99; N, 13.88.
2,2ꢁ -Bis-(N-2-methoxyethylbenzimidazolium)-
ethylether Dichloride, 3b
mp: 224–226°C. Yield: 4 g; 60%. ν(CN)
=
1536 cm−1.1H NMR (δ, DMSO-d6): 3.34 (s,
6H, N(CH2CH2OCH3)), 3.91 (t, J = 4 Hz, 4H,
1-(2-Methoxyethyl)-benzimidazole (3.52 g, 20 mmol)
and 2-chloroethyl ether (1.43 g, 10 mmol) were dis-
solved in 5 mL and heated at 100°C for 3 days. Thirty
milliliters of dried diethyl ether was added to a dark
brown solution and stirred. Sticky oily brown gel
was obtained and dried under vacuo.
N(CH2CH2OCH3)), 4.94 (t,
J
=
4
Hz, 4H,
N(CH2CH2OCH3)), 6.69 (s, 4H, C6H4N2C2(CH2)2),
8.27–7.70 (m, 12H, aromatic protons), 10.05
(s, 2H, NCHN). 13C NMR (δ, DMSO-d6): 48.6
(N(CH2CH2OCH3) 57.3 (N(CH2CH2OCH3), 59.4
(N(CH2CH2OCH3), 71.5 (C6H4(CH2)2), 136.5; 133.2;
132.2; 129.9; 128.9; 128.3; 127.8; 115.9; 115.6 (aro-
matic carbons) and 145.6 (NCHN). Microanaly-
ses found: C, 54.01; H, 5.10; N, 12.76. Calcd. for
C30H33N6Br2O2: C, 53.83; H, 4.97; N, 12.55.
1
Yield: 3.5 g; 70%. ν(CN) = 1645 cm−1. H NMR
(δ, CDCl3): 3.31 (s, 6H, N(CH2CH2OCH3)), 3.93
(t, J = 4 Hz, 4H, N(CH2CH2OCH3)), 4.09 (t, J =
4 Hz, 4H, N(CH2CH2O)), 4.96 (m, 4H, N(CH2CH2O)),
4.96 (m, 4H, N(CH2CH2OCH3)), 7.96–7.58 (m,
8H, aromatic protons), 11.13 (s, 2H, NCHN),
13C NMR (δ, CDCl3): 47.2 (N(CH2CH2OCH3),
47.8 (NCH2CH2O), 58.1(N(CH2CH2OCH3), 59.1
(NCH2CH2O), 70.8 (N(CH2CH2OCH3), 132.9; 131.2;
127.1; 126.9;114.0;113.2; (aromatic carbons), 143.5
(NCHN). Microanalyses found: C, 58.32; H, 6.75; N,
11.40. Calcd. for C24H32N4Cl2O3: C, 58.18; H, 6.51;
N, 11.31.
General Procedure for the Suzuki
Cross-Coupling Reactions
Pd(OAc)2 (1.0 mmol%), 1,3-dialkylimidazolinium-
salts, 3a–c (3 mmol%), aryl chloride (1.0 mmol),
phenylboronic acid (1.5 mmol), KOtBu (2 mmol)
and water (3 mL)–DMF (3 mL) were added to a small
round-bottomed flask in air, and the mixture was
heated at 50°C for 3 h. At the conclusion of the re-
action, the mixture was cooled, extracted with Et2O,
filtered through a pad of silica with copious wash-
ings, and concentrated and purified by flash chro-
matography on silica. The purity of the compounds
was checked by NMR, and yields are based on aryl
chloride.
1,2-Di-(N-2-methoxyethylbenzimidazolium-
methyl)-benzene Dichloride, 3c
1-(2-Methoxyethyl)-benzimidazole (3.52 g, 20 mmol)
and 1,2-bis(chloromethyl) (2.1 g, 12 mmol) were dis-
solved in 5 mL DMF and heated at 100°C for 2 days.
Thirty milliliters of dried diethyl ether was added to
a dark brown solution and stirred. Sticky oily brown
gel was obtained and dried under vacuo.
REFERENCES
Yield: 2.5 g; 50%. ν(CN) = 1632 cm−1
.
1H
NMR (δ, DMSO-d6): 3.27 (s, 6H, N(CH2CH2OCH3),
3.82 (t, J = 4 Hz, 4H, N(CH2CH2OCH3)), 4.78
(t, J = 4 Hz, 4H, N(CH2CH2OCH3)), 6.21 (s, 4H,
(CH2)2C6H4), 8.18–7.10 (m, 12H, aromatic pro-
tons), 10.09 (s, 2H, NCHN). 13C NMR (δ, DMSO-
d6): 48.0 (N(CH2CH2OCH3), 56.5 (N(CH2CH2OCH3),
58.7 (N(CH2CH2OCH3), 69.5 (C6H4(CH2)2), 132.7;
132.0; 131.5; 129.7; 128.6; 127.3;127.2; 114.7; 114.6
(aromatic carbons), 143.8 (NCHN). Microanalyses
found: C, 64.02; H, 6.80; N, 11.32. Calcd. for
C16H24N3ClO2: C, 63.76; H, 6.11; N, 10.62.
[1] Herrmann, W. A. In Applied Homogeneous Catalysis
with Organometallic Compounds; VCH: Weinheim,
Germany, 1996; p. 712.
[2] Diederich, F.; Stang, P. J. (Eds.). Metal-Catalyzed
Cross-Coupling Reactions; VCH: Weinheim, Ger-
many, 1996.
[3] Miyaura, N.; Suzuki, A. Chem Rev 1995, 95, 2457–
2483.
[4] Suzuki, A. J Organomet Chem 1999, 576, 147–168.
[5] Stanforth, S. P. Tetrahedron 1998, 54, 263–266.
[6] Whitcombe, N. J.; Hii, K. K.; Gibson, S. E. Tetrahe-
dron 2001, 57, 7449–7476.
[7] Crisp, G. T. Chem Soc Rev 1998, 27, 427–436.
[8] Li, C. J.; Chan, T. H. Organic Reactions in Aqueous
Media; Wiley: New York, 1997.
[9] (a) Littke, A. F.; Fu, G. C. Angew Chem, Int Ed Engl
2002, 41, 4176–4211; (b) Suzuki, A. J Organomet
Chem 2002, 653, 83–90; (c) Xu, L.; Chen, W.; Xiao, J.
J Mol Catal A 2002, 187, 189–193.
1,1ꢁ-[2,3-Quinoxalinyldiylbis(methylene)]bis[3-
(2-methoxyethyl)-1H-benzimidazolium]
dibromide, 3d
1-(2-Methoxyethyl)-benzimidazole (3.52 g, 20 mmol)
and 2,3-bis(bromomethyl)quinoxalin (3.16 g,
[10] (a) Walker, S. D.; Barder, T. E.; Martinelli, J. R.;
Buchwald, S. L. Angew Chem, Int Ed Engl 2004,
Heteroatom Chemistry DOI 10.1002/hc

¨
162 Yas¸ar, C¸ekirdek, and Ozdemir
43, 1871–1876; (b) Littke, A. F.; Dai, C.; Fu, G. C.
J Am Chem Soc 2000, 122, 4020–4028; (c) Rosa, G.
R.; Ebeling, G.; Dupont, J.; Monteiro, A. L. Synthesis
2003, 2894–2897; (d) Bourissou, D.; Guerret, O.; Gab-
bai, F. P.; Bertrand, G. Chem Rev 2000, 100, 39–92;
(e) Herrmann, W. A.; Wescamp, T.; Bohm, V. P.W.
Adv Organomet Chem 2001, 48, 169.
[24] Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.;
Cavallo, L.; Nolan, S. P. J Am Chem Soc 2005, 127,
3516–3526.
[25] Magill, A. M.; Cavell, K. J.; Yates, B. F. J Am Chem
Soc 2004, 126, 8717–8724.
[26] Abeer, B.; Iglesias, M.; Beetstra, D.; Dervisi, A.; Fallis,
I.; Cavell, K. J. Eur J Inorg Chem 2010, 5426–5431.
[27] Lu, Y. W.; Cavell, K. J.; Wixey, J. S.; Kariuki, B. M.
Organometallics 2011, 30, 5649–5655.
[28] Dunsford, J. J.; Cavell, K. J.; Kariuki, B. M.
Organometallics 2012, 31, 4118–4121.
[11] (a) Sakurai, H.; Sukuda, T.; Hirao, T. J Org Chem
2002, 67, 2721–2722; (b) Parisot, S.; Kolodziuk, R.;
Henry, C. G.; Iourtchenko, A.; Sinou, D. Tetrahedron
Lett 2002, 43, 7397–7400.
¨
[12] Gu¨rbu¨z, N.; Ozdemir, I.; Demir, S.; Cetinkaya, B. J
[29] Binobaid, A.; Iglesias, M.; Beetstra, D. J.; Kariuki, B.;
Dervisi, A.; Fallis, I. A.; Cavell, K. J. Dalton Trans
2009, 7099.
Mol Catal A 2004, 209, 23–28.
¨
[13] Ozdemir, I.; Yasar, S.; Demir, S.; C¸ etinkaya, B. Het-
eroatom Chem 2005, 16, 557–561.
[14] Go¨k, Y.; Gu¨rbu¨z, N.; Ozdemir, I.; C¸ etinkaya, B. Appl
[30] Gstottmayr, C. W. K.; Bohm, V. P. W.; Herdtweck, E.;
Grosche, M.; Herrmann, W. A. Angew Chem, Int Ed
2002, 41, 1363–1365.
[31] O’Brien, C. J.; Kantchev, E. A. B.; Chass, G. A.;
Hadei, N.; Hopkinson, A. C.; Organ, M. G.; Se-
tiadi, D. H.; Tang, T.-H. Tetrahedron 2005, 61,
9723–9735.
[32] (a) Christman, U.; Vilar, R. Angew Chem, Int Ed 2005,
44, 366–374; (b) Galardon, E.; Ramdeehul, S.; Brown,
J. M.; Cowley, A.; Hii, K. K. (M.) Angew Chem, Int
Ed 2002, 41, 1760.
[33] Wolfe, J. P.; Singer, R. A.; Yang, B. H.; Buchwald, S.
L. J Am Chem Soc 1999, 121, 9550–9561.
[34] Hillier, A. C.; Grasa, G. A.; Viciu, M. S.; Lee, H. M.;
Yang, C.; Nolan, S. P. J Organomet Chem 2002, 653,
69–82.
¨
Organomet Chem 2005, 19, 870–874.
[15] Grushin, V. V.; Alper, H. Organomet Chem 1999, 3,
193–197.
[16] Miura, M. Angew Chem, Int Ed 2004, 43, 2201–2203.
[17] (a) Wolfe, J. P.; Singer, R. A.; Yang, B. H.; Buchwald,
S. L. J Am Chem Soc 1999, 121, 9550–9561; (b) Wolfe,
J. P.; Buchwald, S. L. Angew Chem, Int Ed 1999, 38,
2413–2416.
[18] Littke, A. F.; Fu, G. C. Angew Chem, Int Ed Engl 1998,
37, 3387–3388.
[19] Herrmann, W. A.; Bohm, V. P. W. J Organomet Chem
1999, 576, 23–41.
[20] Arduengo, A. J.; Harlow, R. L.; Kline, M. J Am Chem
Soc 1991, 113, 361–363.
[21] Arduengo, A. J. Acc Chem Res 1999, 32, 913–921.
[22] Leuthhauber, S.; Schwarz, D.; Plenio, H. Chem Eur
J 2007, 13, 7195–7203.
[35] Demirhan, F.; Yildirim, O.; Cetinkaya, B. Transition
Met Chem 2003, 28, 558–562.
[36] Gurbuz, N.; Ozdemir, I.; Demir, S.; Cetinkaya, B. J
Mol Catal A 2004, 209, 23–28.
[23] Sanderson, M. D.; Kamplain, J. W.; Bielawski,
¨
˙
C. W.
16515.
J
Am Chem Soc 2006, 128, 16514–
[37] Demir, S.; Ozdemir, I.; C¸ etinkaya, B. Appl Organomet
Chem 2009, 23, 520–523.
Heteroatom Chemistry DOI 10.1002/hc