A novel pyridine-bridged bis-benzimidazolylidene pincer palladium complex: Synthesis and catalytic properties

Article abstract of DOI:10.1002/adsc.200800244

A novel pyridine-bridged bis-benzimidazolylidene CNC pincer complex 1 was synthesized from cheap, commercially available precursors under microwave assistance in moderate yield. It catalyzes cross-coupling reactions of aryl halides with alkyl acrylates (Heck reaction) and phenylboronic acid (Suzuki reaction) under aerobic conditions with extremely high turn-over numbers (frequencies) indicating that a planar extension of the π-system by benzannelation significantly increases the catalytic activity even with trace amounts of catalyst loading.

Full text of DOI:10.1002/adsc.200800244

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DOI: 10.1002/adsc.200800244
A Novel Pyridine-Bridged Bis-benzimidazolylidene Pincer
PalladiumComplex: Synthesis and Catalytic Properties
Tao Tu,a,* Jagadeesh Malineni,a and Karl Heinz Dçtza,*
a
KekulØ Institute of Organic Chemistryand Biochemistry, Universityof Bonn, Gerhard-Domagk-Strasse 1, 53121, Bonn,
Germany
Fax : (+49)-228-73-5813; e-mail: tao.tu@uni-bonn.dedoetz@uni-bonn.de
Received: April 25, 2008; Revised: June 9, 2008; Published online: July10, 2008
Dedicated to Professor R. W. Hoffmann on the occasion of his 75th birthday.
Abstract: A novel pyridine-bridged bis-benzimida-
zolylidene CNC pincer complex 1 was synthesized
from cheap, commerciallyavailable precursors
under microwave assistance in moderate yield. It
catalyzes cross-coupling reactions of aryl halides
with alkyl acrylates (Heck reaction) and phenylbor-
onic acid (Suzuki reaction) under aerobic condi-
tions with extremelyhigh turn-over numbers (fre-
quencies) indicating that a planar extension of the
p-system by benzannelation significantly increases
the catalytic activity even with trace amounts of cat-
alyst loading.
higher stabilityof pincer carbene complexes against
oxygen, moisture and heat.[3]
Pyridine- and lutidine-bridged CNC pincer carbene
complexes
bearing
two
fused
metallacycles
(Scheme 1) have been developed to efficient catalysts
À
À
for homogenous C C and C N coupling reactions
and olefin metathesis.[2–4] Beside palladium complexes,
related iron,[5] nickel,[6] cobalt,[5c,7] ruthenium[8] and
chromium[5c,9] CNC pincer complexes have been stud-
ied. Recently, we reported that bis-imidazolylidene
palladium pincer complexes efficientlygelate a varie-
tyof organic solvents even in verylow concentration,
and this type of complexes has been applied to the
catalytic Michael addition in the gel state.[10a] Com-
pared to imidazolylidenes, benzimidazolylidenes
behave quite differently;[11] we speculated that their
extended p-system and increased s-donor properties
mayfacilitate the synthesis of novel CNC pincer com-
plexes revealing higher catalytic activities and specific
Keywords: benzimidazoles; carbenes; Heck reac-
tion; palladium; Suzuki coupling
Currently, organometallic pincer complexes attract material properties. For example, lutidine-bridged bis-
much attention because of their widespread applica- benzimidazolylidene pincer complexes have been suc-
tions in catalysis and material sciences.[1] Compared cessfullyapplied to Heck reactions even with less
with phosphine ligands, N-heterocyclic carbene active bromobenzenes.[3c] Following our recent inter-
(NHC) ligands act as stronger s-donors and weaker est in the development of novel organometallics and
p-acceptors[2] which increases electronic densityat the
their application in catalysis and soft matter,[10,12] we
metal center and improves the catalytic activity. designed pyridine-bridged bis-benzimidazolylidene
Moreover, the stable carbon-metal bonds impose a pincer complexes.
Scheme 1. Lutidine- and pyridine-bridged CNC pincer palladium carbene complexes.
Adv. Synth. Catal. 2008, 350, 1791 – 1795
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Palladium pincer complex 1 is easilyaccessible in
the resistance of carbene complex 1 against air, mois-
moderate yield by microwave-assisted palladation of ture and heat (Table 1). A 0.2 mol% catalyst loading
the corresponding pyridine-bridged bis-benzimidazoli- turned out to be sufficient for the coupling of iodo-
um bromide 2 with PdACHTRE(UNG OAc)2 (Scheme 2). Bis-benzi- benzene with butyl acrylate within 8 h in good to
midazolium bromide 2 was synthesized by amination quantitative yields using a selection of organic and in-
of 2,6-difluoropyridine with 2 equivalents of benzimi- organic bases (Table 1, entries 1–4). Similarly, the cou-
dazole followed by N-alkylation with butyl bromide pling of the less reactive bromobenzene resulted in an
in a sealed tube. In contrast to its imidazolylidene almost quantitative yield under these conditions
bromo analogues,[4c] the bis-benzimidazolylidene (Table 1, entry5); chlorobenzene, however, afforded
pincer palladium complex 1 is readilysoluble in
onlytrace amounts of 3a even after the reaction time
CH2Cl2, even though its extended planar p-system is had been extended to 24 h.
expected to favour intermolecular stacking.
In order to investigate the influences of electronic
and steric properties of the substrates, a varietyof
As part of our effort to demonstrate the utilityof
novel metal carbenes in catalysis and material scien- aryl halides were studied in model reactions using
ces,[10] we explored the catalytic properties of bis- butyl acrylate and K2CO3 under our standard condi-
NHC pincer palladium complex 1 in the Heck reac- tions in which the catalyst loading was decreased to
tion of aromatic halides and alkyl acrylates. The reac- 10À4 mol% Pd. Under these conditions, iodobenzene
tions were performed under aerobic conditions in 1- still afforded a 94% isolated yield of 3a after 24 h
methyl-2-piperidone (NMP) at 1408C demonstrating (Table 1, entry6) which, however, decreased to 27%
Scheme 2. Synthesis of pyridine-bridged bis-benzimidazolylidene pincer palladium complex 1.
Table 1. Heck reaction of aryl halides and alkyl acrylates catalyzed by CNC pincer palladium complex 1.[a]
[b]
EntryArX
R
Castatl[ymol%]
Base
Time [h]
8
Product
3a
Yield [%]
TON
1
Iodobenzene
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Bu
Me
Me
0.2
0.2
0.2
0.2
TEA
98
99
80
495
497
403
499
2
Iodobenzene
i-Pr2NEt
NaHCO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
8
8
8
8
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
3f
3g
3h
3c
3i
3
Iodobenzene
4
Iodobenzene
>99
5
6
Bromobenzene
Iodobenzene
0.2
99
94
27
85
98
65
85
71
8
96
65
88
13
498
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.00001
0.00001
0.0000001
24
24
24
24
24
24
24
24
24
24
24
24
9.38”105
2.70”105
8.49”105
9.84”105
6.52”105
8.59”105
7.11”105
8.1”104
9.63”105
6.53”106
8.84”106
1.32”108
7
8
9
Bromobenzene
2-Iodotoluene
3-Iodotoluene
4- Iodotoluene
4-Iodoanisole
4-Iodoacetophenone
4-Iodo-trifluorobenzene
1-Iodonaphthalene
3-Iodotoluene
3-Iodotoluene
3-Iodotoluene
10
11
12
13
14
15
16
17
3i
[a]
[b]
All reactions were carried out under air.
Isolated yield.
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A Novel Pyridine-Bridged Bis-benzimidazolylidene Pincer Palladium Complex
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when bromobenzene was applied (Table 1, entry7)
ed in 67% yield of arylated styrenes (TON: 3.36”
quantifying the difference in reactivity between aryl 105). A comparison with the highest TON (7.5”104)
iodides and bromides hidden in the preceeding series obtained with the pyridine-bridged[4c] and lutidine-
with a catalyst loading of 0.2 mol%. The yield strong- bridged[4g] imidazolylidene palladium analogues ap-
lydepends on mesomeric effects in the aryl iodides:
plied for the same reaction (even with 20 mol% n-
The coupling of ortho-, meta- and para-iodotoluenes Bu4NBr as an additive) characterizes the pyridine-
in the presence of 1 ppm of pincer palladium complex bridged CNC pincer bis-benzimidazolylidene complex
1 gave 3b, 3c and 3d in isolated yields of 85, 98 and 1 as distinctlymore efficient, reflecting its better s-
65%, respectively(Table 1, entries 8–10) correspond-
donor properties (electronic effect). In addition, steric
ing to a turnover number (TON) of 9.84”105 for the factors mayalso account for the catalytic properties
meta-isomer. Good TONs were also observed for the of complex 1 as the planar extended metal-ligand
coupling of butyl acrylate with para-iodoanisole hybrid p-system ring present in complex 1 mayen-
(8.59”105) and para-iodoacetophenone (7.11”105) hance the catalytic activity owing to reduced bond
(Table 1, entries 11 and 12). Acceptor substituents in angles in the twisted conformation at the metal center
the aryl halide reduced the yield dramatically as dem- in comparison with its lutidine-bridged (benz)imida-
onstrated for the CF3 derivative (Table 1, entry13,
8%). No significant steric effect was observed as
zolylidene analogues.[3c,4g]
Under the same conditions palladium pincer com-
shown for 1-iodonaphthalene which afforded a 96% plex 1 also efficientlycatalyzes the Suzuki coupling as
yield (9.63”105 TON) (Table 1, entry14).
demonstrated for aryl halides and phenylboronic acid;
a catalyst loading of 1 ppm resulted in 62–97% isolat-
The efficiencyof the catalyst was demonstrated in
the coupling of meta-iodotoluene with butyl or methyl ed yields for the phenylation of iodobenzene and a
acrylate using a steadily reduced catalyst loading. A series of para-halobenzenes (Table 2). As observed
catalyst loading of 0.1 ppm of palladium pincer car- for the phenylation of 4-bromoacetophenone a com-
bene complex 1 applied to butyl acrylate under stan- parison of pincer complex 1 and its imidazolylidene
dard conditions (Table 1, entry15) gave a 65% yield
analogue which gave a 88% yield with 1 mol% cata-
(TON: 6.53”106) while methyl acrylate afforded an lyst loading and a 70% yield with 0.2 mol% catalyst
88% isolated yield (TON: 8.84”106, Table 1, loading in DMA in 30 h[4c] underlined that the catalyt-
entry16). The more reactive methyl acrylate even tol-
ic activityis enhanced bythe planar extention of the
erated 1 ppb as a minimum loading of 1 to result in a p-system. At 808C with 1 ppm catalyst loading, a 62%
still 13% isolated yield of 3i (TON: 1.32”108; TOF: yield of 4d was obtained (Table 2, entry4), whereas
5.50”106 h or 1.53”103/sec, Table 1, entry17).
an elevated temperature of 1108C[4c] and a replace-
The Heck reaction of 4-bromoanisole and styrene ment of DMA for NMP as a solvent increased the
(K2CO3, refluxing dimethylacetamide (DMA), 13 h) yield to 78% (Table 2, entry 5). Even at a low loading
applying a 2 ppm catalyst loading of complex 1 result- the pincer complex catalyst 1 tolerates a varietyof
Table 2. Suzuki coupling of aryl halides and phenylboronic acid catalyzed by CNC pincer palladium complex 1.[a]
EntryArX
Catsatl[ymol%]
Solvent
Temperature [
8C]
Product
Yield [%][b]
TON
1
2
3
4
5
6
7
8
Iodobenzene
4-Iodotoluene
4-Iodoanisole
4-Bromoacetophenone
4-Bromoacetophenone
4-Bromoacetophenone
4-Iodoacetophenone
4-Iodo-trifluorobenzene
1-Iodonaphthalene
4-Iodoanisole
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0001
0.0000001
0.0000001
NMP
140
140
140
80
110
140
140
140
140
140
140
4a
4b
4c
4d
4d
4d
4d
4e
4f
4c
4e
90
72
9.05”105
7.18”105
9.69”105
6.23”105
7.84”105
8.43”105
8.32”105
9.03”105
7.99”105
2.48”108
1.49”108
NMP
NMP
NMP
DMA
NMP
NMP
NMP
NMP
NMP
NMP
97
62[c]
78
84
83[d]
90
9
10
11
80
25
15
4-Iodo-trifluorobenzene
[a]
[b]
[c]
[d]
All reactions were carried out under air.
Isolated yield.
In order to compare with literature data (reaction time: 30 h).
Full conversion resulted in formation of a homo-coupling by-product (12% isolated yield).
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13C NMR (CDCl3, 75 MHz, 298 K): d=149.52, 144.64,
142.08, 141.14, 131.93, 124.82, 123.93, 120.94, 112.99, 111.36;
HR-MS (MALDI): m/z=310.1096, calcd. for [MÀ1]+:
310.1171.
substituents differing in their electronic and steric
properties and allows for good yields of biphenyl de-
rivatives 4a–e (Table 2, entries 2–9). Nearlyidentical
yields of 4d have been obtained under the same con-
ditions from 4-bromoacetophenone and 4-iodoaceto-
phenone which, however, also gave 12% of homo-
coupling product after full conversion (Table 2, en-
tries 6 and 7). The coupling of 1-iodonaphthalene is
similarlyeffective (Table 2, entry9, 80%) indicating
that steric bulk does not hamper severelythe cross-
coupling even with a low catalyst loading of 1 ppm.
Unlike what is observed in the Heck reactions, the
Suzuki protocol tolerates a strong electron-withdraw-
ing substituent such as in 4-iodotrifluorobenzene
(Table 2, entry8, 90%). A further decrease of the cat-
alyst loading to 1 ppb still resulted in a 25% yield
(Table 2, entry10; TON: 2.48”10 8; TOF: 1.03”107 h
or 2.87”103/sec) for coupling with 4-iodoanisole and
a 15% isolated yield (Table 2, entry 11; TON: 1.49”
108; TOF: 6.21”106 h or 1.72”103/sec) for reaction
with 4-iodotrifluorobenzene.
A mixture of 2,6-bis(benzimidazol-1-yl)pyridine (0.622 g,
2 mmol) and n-BuBr (0.43 mL, 4 mmol) in benzonitrile
(5 mL) was stirred at 1608C for 30 h in a sealed tube. After
cooling, the mixture was dissolved in CHCl3 (50 mL), and
then Et2O (250 mL) was added. The crude product was puri-
fied byreprecipitation from CHCl 3/Et2O to give a white
NMR-pure solid in almost quantitative yield. 1H NMR
(DMSO-d6, 400 MHz, 298 K): d=9.98 (s, 2H), 7.90 (t, J=
8 Hz, 1H), 7.60 (d, J=8.1 Hz, 2H), 7.51 (d, J=8.1 Hz, 2H),
7.43 (d, J=8 Hz, 2H), 6.92–6.99 (m, 2H), 6.85–6.91 (m,
2H), 3.83 (t, J=7.3 Hz, 4H), 1.14–1.23 (m, 4H), 0.61 (tq,
J=7.5 and 7.5 Hz, 4H), 0.12 (t, J=7.3 Hz, 6H); 13C NMR
(DMSO-d6, 100 MHz): d=146.8, 144.9, 143.3, 132.1, 130.0,
128.3, 127.8, 118.4, 116.2, 114.7, 47.7, 30.9, 19.6, 13.9; MS
(ESI): m/z=504.2 [MÀBr]+, 456.3, 424.3 [MÀ2Br]+, 368.2
[MÀ2BrÀBu]+, 312.1 [MÀ2BrÀ2Bu]+, 212.6, 184.6, 156.6;
HR-MS (ESI): m/z=504.1757, calcd. for
anal. calcd. for C27H31Br2N5·0.5H2O: C 54.56, H 5.34, N
11.78; found: C 54.66, H 5.35, N 11.62.
A
In conclusion, the pyridine-bridged bis-benzimida-
zolylidene CNC pincer palladium complex 1 which is
readilyaccessible from cheap, commercial precursors
is an efficient catalyst for Heck and Suzuki cross-cou-
pling reactions even with catalyst loadings as low as
0.1 to 1 ppm. It is stable against air, moisture and
heat, and tolerates a varietyof functional groups in
the arene varying in their electronic and steric proper-
ties which suggests that the catalytic activity of this
type of pincer complexes is significantly enhanced by
the planar extended metal ligand hybrid p-system.
Synthesis of Pyridine-Bridged Bis(benzimidazol-2-
ylidene)palladiumComplex 1
A suspension of pyridine-bridged bis-benzimidazolium di-
bromide
2 (0.585 g, 1 mmol) and PdACHTRE(UNG OAc)2 (0.224 g,
1 mmol) was stirred in DMSO (8 mL) for 1 h at room tem-
perature under vacuum. After refilling the argon, the mix-
ture was heated under stirring in the open vessel model at
1608C for 25 min (at 40 W with a CEM Discover microwave
instrument). DMSO was removed under vacuum with heat-
ing. After cooling to room temperature, the resulting resi-
due was dissolved in 10 mL of CHCl3; then Et2O was added
(100 mL) and the precipitate was collected byfiltration and
dried under vacuum. Another two re-precipitation opera-
tions afforded a yellowish solid; yield: 528 mg (75%).
1H NMR (DMSO-d6, 500 MHz, 298 K): d=7.64 (t, J=
8.3 Hz, 1H), 7.55 (d, J=7 Hz, 2H), 7.52 (d, J=8.3 Hz, 2H),
7.14 (dd, J=7 and 2.2 Hz, 2H), 6.75–6.82 (m, 4H), 3.96 (t,
J=7.5 Hz, 4H), 0.94 (q, J=7.5 Hz, 4H), 0.56 (tq, J=7.5 and
7.5 Hz, 4H), 0.05 (t, J=7.5 Hz, 6H); 13C NMR (DMSO-d6,
125 MHz, 298 K): d=175.45, 151.43, 147.46, 134.24, 130.13,
127.78, 127.12, 114.45, 114.05, 110.61, 48.13, 32.79, 20.08,
14.54; MS (ESI): m/z=690.0 [M+2]+, 610.1 [MÀBr+2]+,
564.1, 484.4, 454.3, 368.2 [MÀ2BrÀBuÀPd]+, 298.2, 268.1,
212.6; HR-MS (ESI): m/z=608.0640, calcd. for [MÀBr]+:
608.0641; anal. calcd. for C27H29Br2N5Pd·H2O: C 45.82, H
4.41, N 9.89; found: C 45.44, H 4.32, N .9.55.
Experimental Section
General Remarks
All commercial reagents and solvents were used directly
without further purification. All reactions were carried on
1
air unless otherwise noted. H and 13C NMR spectra were
recorded on Bruker 500 DRX/400 DPX/300 DPX. ESI-mass
spectra were recorded on a micrOTOF-Q from Bruker Dal-
tonik. GC-MS: HP 5890 Series II. CEM Discover micro-
wave instrument was applied for the palladation.
Synthesis of Pyidine-Bridged Bis-benzimidazolium
Dibromide 2
General Procedure for the Heck Reactions
A mixture of 2,6-difluoropyridine (2.99 g, 26 mmol) and
benzimidazole (15.34 g, 130 mmol) was heated to 1858C
under stirring in a sealed tube for 2 h and then at 1608C for
a further 30 h. The mixture was cooled, and water was
added. The precipitate was washed byhot EtOAc. The resi-
due was dissolved in CHCl3 and then reprecipitated upon
addition of Et2O to give 2,6-bis(imidazol-1-yl)pyridine as a
white powder; yield: 7.6 g (94%). 1H NMR (CDCl3,
300 MHz, 298 K): d=8.70 (s, 2H), 8.10–8.17 (m, 3H), 7.89–
7.95 (m, 2H), 7.60 (d, J=8 Hz, 2H), 7.38–7.45 (m, 4H);
To a suspension of K2CO3 (1.68 g, 12 mmol) in 10 mL NMP
aryl halide (10 mmol), alkyl acrylate (11 mmol) and the
CNC pincer palladium carbene complex 1 (solution in
NMP) were added. The reaction mixture was heated at
1408C (monitored byGC-MS) and then allowed to cool to
room temperature. Then the reaction mixture was diluted
with water, and the product was extracted with ether (3”
20 mL). The combined extracts were dried over MgSO4, the
organic phase was concentrated under vacuum and the
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Adv. Synth. Catal. 2008, 350, 1791 – 1795

A Novel Pyridine-Bridged Bis-benzimidazolylidene Pincer Palladium Complex
COMMUNICATIONS
crude product was purified byflash column chromatography
(hexane/EtOAc=100/1).
Commun. 2001, 201; e) J. C. C. Chen, I. J. B. Lin,
Dalton Trans. 2000, 839; for lutidine-bridged palladium
carbene complexes, see: f) A. A. D. Tulloch, A. A. Da-
nopoulos, G. J. Tizzard, S. J. Coles, M. B. Hursthouse,
R. S. Hay-Motherwell, W. B. Motherwell, Chem.
Commun. 2001, 1270; g) S. Grꢃndemann, M. Albrecht,
J. A. Loch, J. W. Faller, R. H. Crabtree, Organometal-
lics 2001, 20, 5485.
General Procedure for the Suzuki Reactions
A mixture of catalyst, K2CO3 (1.68 g, 12 mmol), aryl halide
(10 mmol) and phenyl boronic acid (1.29 g, 11 mmol) in
10 mL NMP was heated at 1408C for 24 h. The work-up of
the reaction was performed as described above for the Heck
reaction.
[5] a) A. A. Danopoulos, J. A. Wright, W. B. Motherwell,
Chem. Commun. 2005, 784; b) A. A. Danopoulos, N.
Tsoureas, J. A. Wright, M. E. Light, Organometallics
2004, 23, 166; c) D. S. McGuinness, V. C. Gibson, J. W.
Steed, Organometallics 2004, 23, 6288.
[6] a) K. Inamoto, J. Kuroda, T. Sakamoto, K. Hiroya, Syn-
thesis 2007, 2853; b) K. Inamoto, J. Kuroda, K. Hiroya,
Y. Noda, M. Watanabe, T. Sakamoto, Organometallics
2006, 25, 3095.
Acknowledgements
T.T. thanks the Alexander-von-Humboldt-Foundation for a
research fellowship; J. M. thanks the Konrad-Adenauer
Foundation for a PhD grant. Financial support from the
DFG (SFB 624) is gratefully acknowledged. We thank Prof.
S. Hçger for technical assistance (CEM Discover microwave
instrument).
[7] A. A. Danopoulos, J. A. Wright, W. B. Motherwell, S.
Ellwood, Organometallics 2004, 23, 4807.
[8] a) J. A. Wright, A. A. Danopoulos, W. B. Motherwell,
R. J. Carroll, S. Ellwood, J. Organomet. Chem. 2006,
691, 5204; b) E. Masllorens, M. Rodrꢁguez, I. Romero,
A. Roglans, T. Parella, J. Benet-Buchholz, M. Poyatos,
A. Llobet, J. Am. Chem. Soc. 2006, 128, 5306; c) S. U.
Son, K. H. Park, Y. Lee, B. Y. Kim, C. H. Choi, M. S.
Lah, Y. H. Jang, D. Jang, Y. K. Chung, Inorg. Chem.
2004, 43, 6896; d) M. Poyatos, J. A. Mata, E. Falomir,
R. H. Crabtree, E. Peris, Organometallics 2003, 22,
1110; e) A. A. Danopoulos, S. Winston, W. B. Mother-
well, Chem. Commun. 2002, 1376.
[9] a) D. Pugh, J. A. Wright, S. Freeman, A. A. Danopou-
los, Dalton Trans. 2006, 775; b) D. S. McGuinness, V. C.
Gibson, D. F. Wass, J. W. Steed, J. Am. Chem. Soc.
2003, 125, 12716.
[10] a) T. Tu, W. Assenmacher, H. Peterlik, R. Weisbarth,
M. Nieger, K. H. Dçtz, Angew. Chem. 2007, 119, 6486;
Angew. Chem. Int. Ed. 2007, 46, 6368; b) T. Klawonn,
A. Gansꢅuer, I. Winkler, T. Lauterbach, D. Franke,
R. J. M. Nolte, M. C. Feiters, H. Bçrner, J. Hentschel,
K. H. Dçtz, Chem. Commun. 2007, 1894; c) G. Bꢃhler,
M. C. Feiters, R. J. M. Nolte, K. H. Dçtz, Angew.
Chem. 2003, 115, 2599; Angew. Chem. Int. Ed. 2003, 42,
2494.
References
[1] a) J. Dupont, C. S. Consorti, J. Spencer, Chem. Rev.
2005, 105, 2527; b) M. E. van der Boom, D. Milstein,
Chem. Rev. 2003, 103, 1759; c) M. Albrecht, G. van Ko-
ten, Angew. Chem. 2001, 113, 3866; Angew. Chem. Int.
Ed. 2001, 40, 3750.
[2] For recent reviews, see: a) F. E. Hahn, M. C. Jahnke,
Angew. Chem. 2008, 120, 3166; Angew. Chem. Int. Ed.
2008, 47, 3122; b) Recent developments in the organo-
metallic chemistry of N-heterocyclic carbenes, (Ed.:
R. H. Crabtree), Coord. Chem. Rev. 2007, 251, 596–
895; c) N-Heterocyclic Carbenes in Transition Metal
Catalysis, (Ed.: F. Glorius), Top. Organomet. Chem.
2007, 21, 1–218; d) N. Marion, S. Dꢁez-Gonzꢂlez, S. P.
Nolan, Angew. Chem. 2007, 119, 3046; Angew. Chem.
Int. Ed. 2007, 46, 2988; e) O. Kꢃhl, Chem. Soc. Rev.
2007, 36, 592; f) F. E. Hahn, Angew. Chem. 2006, 118,
1374; Angew. Chem. Int. Ed. 2006, 45, 1348.
[3] a) F. E. Hahn, M. C. Jahnke, T. Pape, Organometallics
2007, 26, 150; b) F. E. Hahn, M. C. Jahnke, T. Pape, Or-
ganometallics 2006, 25, 5927; c) F. E. Hahn, M. C.
Jahnke, V. Gomez-Benitez, D. Morales-Morales, T.
Pape, Organometallics 2005, 24, 6458.
[4] For pyridine-bridged metal carbene complexes, see:
a) V. Lillo, E. Mas-Marzꢂ, A. M. Segarra, J. J. Carbꢄ, C.
Bo, E. Peris, E. Fernandez, Chem. Commun. 2007,
3380; b) F. Churruca, R. SanMartin, B. InØs, I. Tellitu,
E. Domꢁnguez, Adv. Synth. Catal. 2006, 348, 1836;
c) J. A. Loch, M. Albrecht, E. Peris, J. Mata, J. W.
Faller, R. H. Crabtree, Organometallics 2002, 21, 700;
d) E. Peris, J. Mata, J. A. Loch, R. H. Crabtree, Chem.
[11] a) F. E. Hahn, L. Wittenbecher, D. Le Van, R. Frçhlich,
Angew. Chem. 2000, 112, 551; Angew. Chem. Int. Ed.
2000, 39, 541; b) V. P. W. Bçhm, W. A. Herrmann,
Angew. Chem. 2000, 112, 4200; Angew. Chem. Int. Ed.
2000, 39, 4036; c) F. E. Hahn, L. Wittenbecher, R.
Boese, D. Blꢅser, Chem. Eur. J. 1999, 5, 1931; d) Y. Liu,
P. E. Lindner, D. M. Lemal, J. Am. Chem. Soc. 1999,
121, 10626.
[12] F. Fages, Angew. Chem. 2006, 118, 1710; Angew. Chem.
Int. Ed. 2006, 45, 1680.
Adv. Synth. Catal. 2008, 350, 1791 – 1795
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