Efficient palladium(II) precatalysts bearing 4,5-dicyanoimidazol-2-ylidene for the Mizoroki-Heck reaction

Article abstract of DOI:10.1002/ejic.201402040

The new N-heterocyclic carbene (NHC) complex [PdCl₂{(CN) ₂IMes}(PPh₃)] (2) ({(CN)₂IMes}: 4,5-dicyano-1,3-dimesitylimidazol-2-ylidene) and the NHC palladacycle [PdCl(dmba){(CN)₂IMes}] (3) (dmba: N,N-di

Full text of DOI:10.1002/ejic.201402040

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
FULL PAPER
DOI:10.1002/ejic.201402040
Efficient Palladium(II) Precatalysts Bearing
4,5-Dicyanoimidazol-2-ylidene for the Mizoroki–Heck
Reaction
Heiko Baier,[a] Philipp Metzner,[b] Thomas Körzdörfer,[b]
Alexandra Kelling,[a] and Hans-Jürgen Holdt*[a]
Keywords: Homogeneous catalysis / Palladium / Cross coupling / Carbene ligands
The new N-heterocyclic carbene (NHC) complex
[PdCl2{(CN)2IMes}(PPh3)] (2) ({(CN)2IMes}: 4,5-dicyano-1,3-
dimesitylimidazol-2-ylidene) and the NHC palladacycle
[PdCl(dmba){(CN)2IMes}] (3) (dmba: N,N-dimethylbenzyl-
amine) have been synthesized by thermolysis of 4,5-dicyano-
1,3-dimesityl-2-(pentafluorophenyl)imidazoline (1) in the
presence of suitable palladium(II) precursors. The acyclic
complex 2 was formed by ligand exchange using the mono-
nuclear precursor [PdCl2(PPh3)2] and the palladacycle 3 was
formed by cleavage of the dinuclear chloro-bridged precur-
sor [Pd(μ-Cl)(dmba)]2. The new NHC precursor 1-benzyl-4,5-
dicyano-2-(pentafluorophenyl)-3-picolylimidazoline (5) was
formed by condensation of pentafluorobenzaldehyde with N-
thermolysis of 5 in the presence of [PdCl2(PhCN)2]. The three
palladium(II) complexes were characterized by NMR and IR
spectroscopy, mass spectrometry and elemental analysis. In
addition, the molecular structures of 2 and 3 were deter-
mined by X-ray diffraction. The π-acidity of (CN)2IBzPic was
compared with (CN)2IMes and perviously reported π-acidic
imidazol-2-ylidenes by NBO analysis. The Mizoroki–Heck
(MH) reactions of various aryl halides with n-butyl acrylate
were performed in the presence of complexes 2, 3 and 6. The
new precatalysts showed high activity in the MH reactions
giving good-to-excellent product yields with 0.1 mol-% pre-
catalyst. The nature of the catalytically active species of 2, 3
and 6 was investigated by poisoning experiments with mer-
benzyl-NЈ-picolyldiaminomaleonitrile (4). The NHC pallada- cury and transmission electron microscopy. It was found that
cycle [PdCl2{(CN)2IBzPic}] (6) ({(CN)2IBzPic}: 1-benzyl-4,5-
dicyano-3-picolylimidazol-2-ylidene) was prepared by in situ
palladium nanoparticles formed from the precatalysts were
involved in the catalytic process.
complexes, leading to a variety of NHC ligands with a
broad range of steric and electronic properties.[5] This vari-
ety has allowed the synthesis of customized NHC ligands
for specific applications. In particular, strong σ-donating li-
gands with negligible π-accepting ability have been used to
create efficient precatalysts. On the other hand, there are
only a few reports of transition-metal precatalysts bearing
π-acceptor NHCs.[6] The few contributions concerning
transition-metal catalysis by complexes bearing π-acceptor
carbenes showed that the MH reaction could be performed
efficiently with these complexes.[6d] In 2005, Herrmann and
co-workers presented the NHC palladacycle [PdCl-
(dmba)(NHC)] (dmba: N,N-dimethylbenzylamine) (A)
bearing a 1,2,4-triazole-based NHC ligand (Scheme 1),[7a]
Introduction
For the last 40 years, the Mizoroki–Heck reaction (MH
reaction) has been one of the most useful preparative tools
for the synthesis of pharmaceuticals, natural products and
polymer building blocks,[1] leading to the Nobel Prize in
chemistry in 2010. Since the early 1970s, complexes bearing
tertiary phosphine ligands have been used as efficient pre-
catalysts for the MH reaction.[2] However, their use has
been limited due their expense and the intolerance of the
phosphine ligands towards air and moisture. N-Heterocy-
clic carbenes (NHCs) offer an efficient alternative to terti-
ary phosphines. Since the report of the first isolated NHC
in 1991, NHCs have become one of the most intensively
investigated ligand families in the field of transition-metal
catalysis.[3] Since the initial presentation of NHC-bearing
palladium precatalysts for the Mizoroki–Heck reaction by
Herrmann et al. in 1995,[4] tremendous effort has been de-
voted to the development of new NHCs and their metal
[a] Department of Inorganic Chemistry, Universität Potsdam,
Karl-Liebknecht-Straße 24–25, 14476 Golm, Germany
E-mail: Holdt@uni-potsdam.de
http://www.chem.uni-potsdam.de/groups/anorganik/
[b] Computational Chemistry, Universität Potsdam,
Karl-Liebknecht-Straße 24–25, 14476 Golm, Germany
Scheme 1. Palladacycle precatalysts bearing NHCs.
1 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
and in 2008, Ying and co-workers synthesized the efficient cess, should be enhanced, we prepared 1-benzyl-4,5-dicy-
precatalyst [PdCl(dmba)(IMes)] (B) bearing 1,3-dimes- ano-2-(pentafluorophenyl)-3-picolylimidazoline (5), which
itylimidazol-2-ylidene (IMes).[7b] Both palladacycles is the precursor for the chelating NHC ligand (CN)2IBzPic
showed good catalytic activity in the coupling of aryl brom- ({(CN)2IBzPic}: 1-benzyl-4,5-dicyano-3-picolylimidazol-2-
ides with terminal olefins. Under optimized conditions, ylidene; see Scheme 3). The complex [PdCl2{(CN)2IBzPic}]
even rather unreactive aryl chlorides could be used for the (6) was prepared by thermolysis of 5 in the presence of
MH reaction.
[PdCl2(PhCN)2]. The three complexes 2, 3 and 6 were used
Another example of a catalytically active complex con- as precatalysts in the MH reactions of various aryl halides
tains a NHC bearing a chelating unit. In 2000, Hursthouse with n-butyl acrylate. They showed good catalytic activity,
and co-workers presented the catalytically active palladi- with the precatalysts 3 and 6 showing superior catalytic ac-
um(II) complex C bearing a picolyl-substituted NHC tivity than their analogues B and C, respectively.[7b,10]
(Scheme 1).[10] In the palladacycle, the carbenic carbon
atom and the nitrogen atom of the picolyl substituent act
as coordinating atoms. Unfortunately, these complexes only
showed high reaction yields in the MH reactions of iodo-
Results and Discussion
benzene or strongly activated aryl bromides with methyl
and n-butyl acrylate.
Syntheses
In general, palladacycles form catalytically active palla-
The NHC palladium(II) complexes 2 and 3 were pre-
dium(0) under MH conditions (e.g., high temperature).[8] pared by heating the NHC precursor 1, recently presented
The zero-valent palladium can be stabilized by a bulky li- by our group,[11] in the presence of [PdCl2(PPh3)2] or [Pd(μ-
gand, thus forming the catalytically active [Pd0(NHC)] spe- Cl)(dmba)]2, respectively, in toluene at reflux for 16 h
cies.[9] Otherwise palladium black formation and precipi- (Scheme 2). After work-up, including chromatographic pu-
tation will occur and terminate the MH reaction.
rification on silica with dichloromethane as eluent, com-
It was our goal to prepare stable palladium(II) precata- plexes 2 and 3 were obtained in good yields (2: 64%; 3:
lysts for the MH reaction with π-acceptor carbenes. By 72%).
using our new precatalysts, stable [Pd0(NHC)] particles
The new NHC precursor 1-benzyl-4,5-dicyano-2-(penta-
should be formed during the catalytic process by reductive fluorophenyl)-3-picolylimidazoline (5) was prepared in a
elimination, and it is these particles that are expected to be facile five-step synthesis. We started with the formation of
the catalytically active species.
the known N-benzyl-NЈ-picolyldiaminomaleonitrile (4)[12]
In this contribution we present the acyclic palladium(II) and the synthetic procedure was concluded with the con-
complex [PdCl2{(CN)2IMes}(PPh3)] (2) ({(CN)2IMes}: 4,5- densation of 4 with pentafluorobenzaldehyde in glacial
dicyano-1,3-dimesitylimidazol-2-ylidene) and the pallada- acetic acid to obtain 5 in a yield of 67% (Scheme 3). Ther-
cycle [PdCl(dmba){(CN)2IMes}] (3) (see Scheme 2). 4,5-Di- mogravimetric analysis (TGA-MS) and differential thermo-
cyano-1,3-dimesitylimidazol-2-ylidene is a strong π-ac- analysis (DTA) were used to investigate the thermal decom-
ceptor, as indicated by the high Tolman electronic param- position and carbene formation of imidazoline 5 (Figure 1).
eter (TEP) of 2060 cm–1.[11] The NHC (CN)2IMes was A loss of mass of 34% was observed between 132 and
formed in situ by thermolysis of imidazoline 1 and reacted 191 °C, which correlates with the theoretical loss of penta-
with suitable palladium(II) sources to give the palladium fluorobenzene (36 wt.-%) and the formation of the NHC
precatalysts
2 and 3. Because the stability of the (CN)2IBzPic. An exothermic peak in the DTA at an onset
[Pd0(NHC)] species, which is formed in the catalytic pro- temperature of 132 °C, accompanied by a mass peak at m/z
Scheme 2. Preparation of NHC palladium(II) complexes 2 and 3.
Eur. J. Inorg. Chem. 0000, 0–0
2
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
Scheme 3. Preparation of imidazoline 5 and NHC palladium(II) complex 6.
= 168 for pentafluorobenzene with a maximum at 160 °C,
The Pd–Ccarbene distance of 2 is 2.0351(19) Å and the
was observed by TGA-MS.
Pd–P distance is 2.2964(5) Å. The latter is in the range of
Figure 1. Thermogravimetric analysis (black line) and differential
thermoanalysis (grey line) of 1-benzyl-4,5-dicyano-2-(pentafluoro-
phenyl)-3-picolylimidazoline (5).
Figure 2. ORTEP structure of 2. Ellipsoids are drawn at the 50%
probability level. Selected bond lengths [Å] and angles [°]: C1–N1
1.358(2), C1–N2 1.360(2), C2–N1 1.386(2), C3–N2 1.388(3), C1–
Pd1 2.0351(19), P1–Pd1 2.2964(5), Cl1–Pd1 2.2918(6), Cl2–Pd1
2.3051(5), C1–Pd1–P1 177.20(5), C1–Pd1–Cl1 90.81(5), C1–Pd1–
Cl2 90.47(5), Cl1–Pd1–Cl2 178.56(2), N1–C1–N2 105.43(16).
Hydrogen atoms have been omitted for clarity.
The new palladium(II) complex [PdCl2{(CN)2IBzPic}]
(6) was prepared by slow dropwise addition of a chloroben-
zene solution containing [PdCl2(PhCN)2] to a solution of
5 in chlorobenzene at an oil-bath temperature of 135 °C.
Filtration of the reaction mixture after cooling to room
temperature gave crude 6, which was washed with chloro-
form, dissolved in dimethyl sulfoxide, precipitated with
water and finally washed with chloroform and diethyl ether
to obtain 6 in 27% yield. Higher reaction temperatures did
not lead to the desired product due to the decomposition
of [PdCl2(PhCN)2] (Scheme 3).
Compounds 2, 3, 5 and 6 are stable towards air and
moisture, and were satisfactorily characterized by NMR
and IR spectroscopy as well as by mass spectrometry and
elemental analysis (C, H, N). The melting points of the
compounds could not be determined due to the slow de-
composition of the compounds while heating.
Figure 3. ORTEP structure of 3. Ellipsoids are drawn at the 50%
probability level. Selected bond lengths [Å] and angles [°]: C1–N1
1.371(3), C1–N2 1.373(3), C2–N1 1.374(3), C3–N2 1.397(3), C1–
Pd1 1.963(3), N5–Pd1 2.124(2), Cl1–Pd1 2.3853(7), C24–Pd1
2.007(2), C1–Pd1–N5 172.24(8), C1–Pd1–Cl1 92.50(6), C1–Pd1–
C24 93.67(9), Cl1–Pd1–C24 172.83(7), C24–Pd1–N5 82.44(9), N5–
Pd1–Cl1 91.00(6), N1–C1–N2 103.9(2), C24–C25–C26–N5
–27.9(3). Hydrogen atoms have been omitted for clarity.
X-ray Analysis
Single crystals of the palladium(II) complexes 2 and 3
suitable for X-ray analysis were obtained by slow diffusion
of pentane steam into saturated chloroform (2) or dichloro-
methane (3) solutions of the complexes at room tempera-
ture (Figures 2 and 3).
Eur. J. Inorg. Chem. 0000, 0–0
3
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
the Pd–P distance in the precursor [PdCl2(PPh3)2].[13] The IBzPic to show a similar π-accepting behaviour to (CN)2-
square-planar coordination geometry is slightly distorted IMes.
with a Cl–Pd–Cl angle of 178.56(2)° and a Ccarbene–Pd–P
Table 1. NBO energies of the σ-donor (Eσ) and π-acceptor (Eπ)
angle of 177.20(5)°. The Pd–Ccarbene distance in 3 is
orbitals of (CN)2IMes, (CN)2IBzPic and previously reported imid-
azol-2-ylidenes.
1.963(3) Å, which is slightly shorter than the Pd–Ccarbene
distance of 1.980(2) Å found in the analogous complex
B.[7b] The phenylidene moiety of the chelating co-ligand
dmba in palladacycle 3 is coordinated in the cis position
and the amino unit in the trans position with respect to the
NHC ligand. This is the same coordination pattern as
found in the analogous complex B.[7b]
NHC
Eσ [eV]
Eπ [eV]
(CN)2IMes
(CN)2IBzPic
IPr[a]
–8.37
–8.37
–7.64
–8.30
–8.98
–2.10
–2.01
–1.38
–1.86
–2.57
IMeDNP[a]
IDNP[a]
[a] See ref.[14]
.
The four angles C1–Pd1–Cl1 [92.50(6)°], C1–Pd1–C24
[93.67(9)°], C24–Pd1–N5 [82.44(9)°] and Cl1–Pd1–N5
[91.00(6)°] around the palladium centre of 3 are in the range
of those of the analogous complex B.[7b] The torsion angle
between the phenylidene unit and the methyleneamino
group (C24–C25–C26–N5) is –27.9(3)°, which is higher
than the same torsion angle in B [26.3(3)/27.0(3)°]. The Pd–
Catalytic Studies
Initial experiments showed that the MH reaction of
bromobenzene with n-butyl acrylate gave high yields in the
presence of the three palladium(II) precatalysts 2, 3 and 6
(Scheme 4). The first goal was to improve the reaction con-
ditions by varying the solvent, base, reaction time and tem-
perature. To conclude the optimization experiments, the in-
fluence of the additive tetra-n-butylammonium bromide
(TBAB) was tested.
C
carbene distance in complex 3 [1.963(3) Å] is slightly shorter
than that in 2 [2.0351(19) Å] due to the higher π-acceptor
strength of the trans-coordinating tertiary triphenylphos-
phine in complex 2 compared with the coordinated amino
unit in complex 3.
DFT Calculations
The Tolman electronic parameter, as a measure of the π-
acceptor strength, of (CN)2IMes has already been deter-
mined by our group.[11] As it was not possible to determine
the Tolman electronic parameter of the ylidene (CN)2-
IBzPic, we performed a natural bond orbital (NBO) analy-
sis on (CN)2IBzPic and (CN)2IMes and calculated the ener-
gies of the σ-donor (Eσ) and π-acceptor (Eπ) orbitals as
comparative values. The σ-donor is a lone electron pair lo-
cated in the sp2-hybridized orbital and the π-acceptor is the
empty p-orbital of the Ccarbene atom.
Scheme 4. Reaction scheme for MH reactions of monocyclic aryl
bromides.
The initial precatalyst loading of 0.1 mol-% (relative to
the aryl bromide) was deployed for all further experiments.
The best results for the MH reaction of bromobenzene with
n-butyl acrylate were achieved with N,N-dimethylform-
amide (dmf) as solvent; especially in the reaction with pre-
catalyst 3, the yield of the desired n-butyl cinnamate
dropped massively when another solvent was deployed
(Table 2, entries 1 and 2). After 8 h, almost 90% yield was
achieved with all three precatalysts (Table 2, entry 3). After
16 h, the reactions were complete with almost quantitative
yields of the desired product (entry 6). A decrease in the
Therefore, by comparison of the calculated values of Eσ
and Eπ, we were able to compare the π-acceptor strengths of
(CN)2IBzPic with (CN)2IMes and the previously reported
imidazole-based NHCs 1,3-bis(diisopropylphenyl)imidazol-
2-ylidene (IPr), 1,3-bis(2,4-dinitrophenyl)imidazol-2-ylidene
(IDNP) and 1-(2,4-dinitrophenyl)-3-methylimidazol-2-ylid-
ene (IMeDNP; Table 1).[14] The Eσ and Eπ values of the two
4,5-dicyanoimidazol-2-ylidenes (CN)2IMes (Eσ: –8.37 eV;
Eπ: –2.10 eV) and (CN)2IBzPic (Eσ: –8.37 eV; Eπ: –2.01 eV)
are lower in energy than those of IPr (Eσ: –7.64 eV; Eπ:
–1.38 eV)[14] and IMeDNP (Eσ: –8.30 eV; Eπ: –1.86 eV),[14]
which implies a decrease in the σ-donor strength of the 4,5-
dicyanoimidazol-2-ylidenes compared with IPr and
IMeDNP. The decrease in Eπ results in an increase in the
π-acceptor strength of the dicyanocarbenes (CN)2IMes and
(CN)2IBzPic compared with IPr and IMeDNP. IDNP (Eσ:
–8.89 eV; Eπ: –2.57 eV)[14] is the weakest σ-donor, but the
strongest π-acceptor of the five NHCs. These data correlate
with the TEP ranking of IPr (2052 cm–1),[15] IMeDNP
(2058 cm–1),[14] (CN)2IMes (2060 cm–1)[11] and IDNP
Table 2. Results for the solvent optimization of the conditions for
the MH reaction of bromobenzene with n-butyl acrylate.[a]
Entry Solvent
t [h]
T [°C]
Yield [%][b]
2
3
6
1
2
3
4
5
6
nmp
dma
dmf
16
16
8
16
16
16
140
140
140
100
140
140
96
84
88
8
89
100
13
6
89
9
90
100
76
81
88
14
90
98
dmf
dmf[c]
dmf
[a] Reagents and conditions: bromobenzene (1.0 equiv.), n-butyl
acrylate (2.0 equiv.), sodium carbonate (1.4 equiv.), TBAB
(0.1 equiv.), 1,3,5-trimethoxybenzene (0.1 equiv.) and precatalyst
(0.1 mol-%), solvent (0.5 mL), 16 h, 140 °C (oil bath). [b] GC yield:
average yield of two independent runs, 1,3,5-trimethoxybenzene as
(2063 cm–1).[14] Based on these results, we expect (CN)2- internal standard. [c] Without TBAB.
Eur. J. Inorg. Chem. 0000, 0–0
4
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
temperature to 100 °C resulted in a massive decrease in the reactions catalysed by B were significantly lower than those
product yield (Table 2, entry 4). The use of TBAB as co- catalysed by 3 when electronically deactivated substrates
catalyst seems a prerequisite for the precatalysts reported were used, especially phenols, which reacted in good-to-ex-
herein; the average product yield dropped by 10% when no cellent yields with 3 but did not undergo any conversion
co-catalyst was used (entry 5).
with B (Table 4, entries 19–21). Chlorobenzene (Table 4, en-
Sodium carbonate proved to be the most suitable base try 1) and chloroanisole (Table 4, entry 2) gave only trace
for the MH reaction using precatalysts 2, 3 and 6 (Table 3). amounts of the cinnamates, whereas chloroacetophenone
Low product yields were obtained with organic bases due reacted well with all the tested precatalysts to give moderate
to their low boiling points, whereas the stronger caesium yields of the product (Table 4, entry 3).
carbonate led to unidentified decomposition products,
Table 4. Substrate scope of the Mizoroki–Heck reaction of n-butyl
which could be seen as additional peaks in the gas chroma-
acrylate vwith arious aryl halides using precatalysts 2, 3, 6 and B.[a]
tograms. Therefore we decided to use a reaction time of
Entry Aryl bromide
Yield [%][b]
16 h and a temperature of 140 °C for further catalytic ex-
periments (Table 2, entry 6) with sodium carbonate as base
and TBAB as additive in the reaction mixture.
2
3
6
B
1
3
chlorobenzene
4-chloroanisole
0
0
2
1
0
0
1
1
3
4
5
6
7
8
9
4-chloroacetophenone
bromobenzene
4-bromoacetophenone
4-bromoanisole
3-bromoanisole
2-bromoanisole
55 56 57 61
100 100 98 99
98 95 89 96
94 98 80 86
76 77 40 43
54 83 55 49
99 100 98 99
81 98 73 65
97 89 91 88
78 82 85 39
96 94 99 61
99 99 89 100
91 99 63 90
87 84 84 80
92 100 87 100
64 84 75 44
Table 3. Optimization of the base for the MH reaction of bromo-
benzene with n-butyl acrylate.[a]
Entry Base
Yield [%][b]
2
3
6
1
2
3
4
5
6
sodium carbonate
sodium acetate
caesium carbonate
calcium hydroxide
piperidine
100
33
40
n.d.
16
30
100
45
34
trace
18
32
98
27
19
n.d.
12
25
4-bromobenzaldehyde
4-bromotoluene
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
4-bromofluorobenzene
2-bromofluorobenzene
4-bromo-N,N-diethylaniline
4-bromonitrobenzene
2-bromo-5-nitroanisole
methyl 4-bromobenzoate
4-bromobenzonitrile
4-bromoaniline
4-bromophenol
3-bromophenol
2-bromophenol
bromomesitylene
1,4-dibromobenzene
2-bromobiphenyl
3-bromopyridine
1-bromo-2,4,6-triisopropylbenzene
1-bromo-3,4,5-trimethoxybenzene
1-bromonaphthalene
9-bromoanthracene
diisopropylethylamine
[a] Reagents and conditions: bromobenzene (1.0 equiv.), n-butyl ac-
rylate (2.0 equiv.), base (1.4 equiv.), TBAB (0.1 equiv.), 1,3,5-trime-
thoxybenzene (0.1 equiv.), precatalyst (0.1 mol-%), N,N-dimethyl-
formamide (0.5 mL), 16 h, 140 °C (oil bath). [b] GC yield: average
yield of two independent runs, using 1,3,5-trimethoxybenzene as
internal standard; n.d.: not determined.
95 100 67
52 72 48
44 57 33
0
0
0
17 30 15 15
82 79 62 70
92 91 87 76
73 58 49 45
The MH reactions of various aryl halides with n-butyl
acrylate were performed under the optimized reaction con-
ditions to prepare n-butyl cinnamate and a variety of its
derivatives (Table 4). The three precatalysts 2, 3 and 6 are
11 28
5
12
suitable for use in MH reactions with electronically deacti- 27
46 66 58 53
98 92 72 70
64 88 70 66
28
vated aryl bromides such as 4-bromoanisole (Table 4, en-
try 6), resulting in excellent yields. Although the precata-
29
[a] Reagents and conditions: aryl halide (1.0 equiv.), n-butyl acryl-
ate (2.0 equiv.), sodium carbonate (1.4 equiv.), TBAB (0.1 equiv.),
1,3,5-trimethoxybenzene (0.1 equiv.), precatalyst (0.1 mol-%), N,N-
dimethylformamide (0.5 mL), 16 h, 140 °C (oil bath); reaction
scale: 126 μmol aryl bromide. [b] GC yield: average yield of two
independent runs, using 1,3,5-trimethoxybenzene as internal stan-
dard.
lysts 2 and 6 achieved good yields only with sterically less
demanding aryl bromides, precatalyst 3 showed excellent
overall catalytic activity, giving moderate yields of 30 and
28% in the reactions of the electronically deactivated and
sterically hindered bromomesitylene and 1-bromo-2,4,6-tri-
isopropylbenzene, respectively (Table 4, entries 22 and 26).
With precatalyst 6, the yields of ortho-substituted butyl cin-
namates were especially low, which indicates that the cis
coordination of the chelating (CN)2IBzPic is not favoured
in the reactions of bulky aryl bromides. The three precata-
lysts showed tolerance against functional groups such as
amino (Table 4, entry 18) or hydroxy groups (Table 4, en-
tries 19–21), and also the N-heterocycle 3-bromopyridine
(Table 4, entry 25) reacted to give the desired product in
moderate-to-good yields. As precatalyst 3 proved to be the
most efficient of the three precatalysts presented herein, we
prepared the analogous complex B[7b] and deployed it as
precatalyst to compare their precatalyst activities under the
optimized conditions. Most of the reactions were catalysed
The long-term stabilities of the precatalysts 2, 3 and 6
were investigated by performing the MH reaction of bro-
mobenzene with n-butyl acrylate using decreased precata-
lyst loadings of 0.01, 0.001 and 0.0001 mol-% over a reac-
tion time of 64 h at 140 °C. The experiments showed that
the MH reaction occurred even with reduced precatalyst
loadings. Complex 3 proved to be more efficient than 2 and
6. A maximum turnover number (TON) of 150000 was ob-
tained by using 0.0001 mol-% of complex 3 (Table 5).
Mechanistic Investigations
The formation of palladium black was not observed dur-
efficiently by both precatalysts. However, the yields of the ing the reaction when the palladacycles 3 and 6 were used.
Eur. J. Inorg. Chem. 0000, 0–0
5
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
Table 5. Long-term stability experiments performed by using re- poisoning experiments could be confirmed. Presumably the
duced catalyst loadings.[a]
palladium nanoparticles formed during the catalytic reac-
Entry Precatalyst loading
[mol-%]
Yield [%][b]
tion are the leading actors in the MH reactions performed
with the precatalysts 2, 3 and 6.[16]
2
3
6
1
2
3
0.01
0.001
0.0001
85
22
7
91
33
15
81
33
12
Conclusions
[a] Reagents and conditions: bromobenzene (1.0 equiv.), n-butyl
acrylate (2.0 equiv.), sodium carbonate (1.4 equiv.), TBAB
(0.1 equiv.), 1,3,5-trimethoxybenzene (0.1 equiv.), precatalyst
(0.1 mol-%), N,N-dimethylformamide (0.5 mL), 64 h, 140 °C (oil
bath). [b] GC yield: average yield of two independent runs, using
1,3,5-trimethoxybenzene as internal standard.
The new acyclic NHC palladium(II) complex
[PdCl2{(CN)2IMes}(PPh3)] (2) and the palladacycle
[PdCl(dmba){(CN)2IMes}] (3) have been synthesized in sat-
isfactory yields by using the NHC precursor 4,5-dicyano-
1,3-dimesityl-2-(pentafluorophenyl)imidazoline (1) and
their crystal structures determined. The new NHC precur-
sor 1-benzyl-4,5-dicyano-2-(pentafluorophenyl)-3-picolyl-
imidazoline (5) was synthesized as precursor of the chelat-
ing NHC (CN)2IBzPic and used for the formation of pre-
catalyst [PdCl2{(CN)2IBzPic}] (6). The π-acceptor strength
of the new (CN)2IBzPic were compared with (CN)2IMes
and other π-acidic NHCs by NBO analysis. The three com-
plexes 2, 3 and 6 and the previously reported analogous
precatalyst B were tested as precatalysts for the Mizoroki–
Heck reactions between various aryl bromides and n-butyl
acrylate at low catalyst loadings. Precatalyst 3 showed the
best overall catalytic performance and facilitated the MH
reaction even with sterically hindered and electronically de-
activated substrates to give moderate yields of the products.
In addition, complex 3 gave higher yields than the analo-
gous complex B. Poisoning experiments with mercury
showed that palladium nanoparticles play a leading role in
the catalytic process in all three cases. The formation of
palladium nanoparticles was confirmed by TEM analysis
of the MH reaction mixtures.
Hydrolysis with 1 n hydrochloric acid in the work-up pro-
cedure led to palladium black formation, which could be
observed in the separation funnel. In the case of acyclic
complex 2, the formation of palladium black could be ob-
served in the reaction vessel. For further insights into the
nature of the catalytically active species, poisoning experi-
ments were performed. In the Mizoroki–Heck reaction of
bromobenzene with n-butyl acrylate under the optimized
conditions, a large excess of elemental mercury (1000 equiv.,
based on the precatalyst) was added at the start of the reac-
tions. In all cases, the reaction was quenched by the mer-
cury and only traces of n-butyl cinnamate could be detected
by gas chromatography. In another experiment using pre-
catalyst 3, mercury was added 4 h after the start of the reac-
tion. At the same time, a sample of the reaction mixture
was removed and the yield of n-butyl cinnamate was deter-
mined to be 44% by GC-FID. Further samples were taken
1, 2 and 3 h after the addition of mercury, but no increase
in yield could be determined. This indicates the formation
of nanoparticles during the catalytic process and their par-
ticipation in the catalytic reaction. To confirm the forma-
tion of palladium nanoparticles, dynamic light scattering
(DLS) measurements were performed on freshly prepared
reaction mixtures three times for each of the precatalysts,
but the presence of nanoparticles could not be confirmed.
As the DLS experiments did not prove the presence of
nanoparticles, transmission electronic microscopy (TEM)
was conducted on the same samples. Figure 4 shows a rep-
resentative TEM image taken from a reaction mixture con-
taining 3. In all cases, nanoparticles as well as larger ag-
Experimental Section
General: All synthetic manipulations were performed under argon
or nitrogen using standard Schlenk techniques. For all manipula-
tions, solvents were dried according to literature procedures.[17]
Compounds 1,[11] 4,[12] [PdCl2(PPh3)2],[18a] [Pd(μ-Cl)(dmba)]2
[18b]
and [PdCl2(PhCN)2][18c] were synthesized as described in the litera-
ture. Other chemicals were purchased from commercial sources and
used in the reactions as received. TGA-MS and DTA analyses were
performed with a Linseis STA PT-1600 thermoscale coupled with
glomerations of palladium are visible and the results of the a Pfeiffer MS Thermostar mass spectrometer. Elemental analyses
(C, H, N) were performed with an Elementar Vario EL elemental
analyser. NMR spectra were recorded with a Bruker Avance 300
or 500 spectrometer. Resonances for NMR spectra are reported
relative to Me4Si (δ = 0.0 ppm) and calibrated based on the solvent
1
signal for H and 13C.[19] Spectra are reported as follows: chemical
shift (δ), multiplicity, coupling constant (J) and integration. Mass
spectra were recorded with a Micromass Q-TOFmicro (ESI) or
Thermo Quest SSQ 710 (70 eV, EI) spectrometer. IR spectra were
recorded with a Thermo Nicolet NEXUS FTIR spectrometer in a
KBr disk between 400 and 4000 cm–1 using a resolution of 4 cm–1.
A background measurement was performed before recording spec-
tra of the samples using a bare KBr disc. The DLS measurements
were performed with a Malvern Zetasizer Nano ZS instrument at
25 °C using freshly prepared reaction mixtures. Each measurement
consisted of a minimum of 50 runs. TEM images were recorded
Figure 4. TEM image of a colloidal solution containing palladium
particles created from 3 (ϫ130k, 100 nm scale bar is shown).
Eur. J. Inorg. Chem. 0000, 0–0
6
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
with a Philips CM100 electron microscope at 80 kV using freshly
prepared reaction mixtures on a 400 nm copper grid coated with
carbon. Single-crystal X-ray diffraction data were collected with a
2248, 2236, 1605, 1480, 1436, 1383, 1338, 1300, 1207, 1187, 1161,
1099, 1069, 1029, 1001, 867, 856, 749, 741, 713, 691, 561, 530, 514,
495, 459 cm–1. HRMS: calcd. for C41H37Cl2N4Pd 794.0682 [M]+;
STOE IPDS 2 diffractometer at 293 K using graphite-monochro- found 757.1519 [M – Cl]+. C41H37Cl2N4PPd (794.07): calcd. C
mated Mo-Kα radiation (λ = 0.71073 Å). The crystal structures
were solved by direct methods and refined by full-matrix least-
squares methods on F2 by using SHELX-97.[20] Non-hydrogen
atoms were refined by using anisotropic temperature factors. The
deposited atom data (CIF) as well as the data in Table 6 reflect
only the known cell content.
62.02, H 4.70, N 7.06; found C 61.86, H 4.57, N 6.91.
[PdCl{(CN)2IMes}(dmba)] (3): [Pd(μ-Cl)(dmba)]2
(100 mg,
181.1 mmol) and 1 (189 mg, 362.2 mmol; 2.0 equiv.) were added to
toluene (20 mL) under argon in a 50 mL round-bottomed flask fit-
ted with a reflux condenser and magnetic stirrer, . The mixture was
heated at reflux for 16 h under vigorous stirring and then cooled
to room temperature. The solvent was removed in a rotary evapora-
tor and the crude product was isolated by column chromatography
on silica (height: 30 cm; diameter 3 cm) with dichloromethane as
eluent. The product was obtained as a colourless crystalline solid
Table 6. Crystallographic data for NHC palladium(II) complexes 2
and 3.
2
3
1
Formula
M [gmol–1]
Crystal size [mm3]
T [K]
C41H37Cl2N4PPd
794.02
1.5ϫ0.93ϫ0.42
210(2)
C32H34ClN5Pd
630.49
0.36ϫ0.22ϫ0.13
210(2)
in 72% yield (164 mg). H NMR (300 MHz, CD2Cl2): δ = 7.08 (s,
2 H), 6.92 (s, 2 H), 6.87 (dd, J = 7.1, 1.2 Hz, 1 H), 6.82 (dd, J =
7.3, 1.5, 1 H), 6.78–6.72 (m, 1 H), 6.51 (dd, J = 7.5, 0.9 Hz, 1 H)
3.54 (s, 2 H), 2.41 (s, 12 H), 2.34 (s, 6 H), 2.23 (s, 6 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 189.6, 149.1, 148.1, 141.3, 138.2,
137.7, 134.7, 133.0, 130.4, 128.8, 124.8, 124.3, 122.2, 117.2, 107.0,
Radiation
Crystal system
Space group
a [Å]
b [Å]
c [Å]
Mo-Kα
monoclinic
P21/n
12.3413(5)
21.8296(7)
14.2030(6)
90.00
99.703(4)
90.00
3771.6(3)
4
1.398
0.710
24078
Mo-Kα
triclinic
¯
P1
72.9, 50.5, 21.3, 20.2, 20.0 ppm. FTIR (KBr): ν = 3050, 2995, 2963,
˜
9.3510(12)
13.5202(17)
13.5656(15)
74.508(9)
72.639(9)
69.875(10)
1510.9(3)
2
1.386
0.731
9804
4993
2914, 2885, 2854, 2238, 1606, 1582, 1477, 1452, 1371, 1318, 1295,
1285, 1070, 1025, 992, 972, 856, 742, 713 cm–1. HRMS: calcd. for
C32H34ClN5Pd 629.1532 [M]+;
found
629.1521
[M]+.
α [°]
β [°]
C32H34ClN5Pd (629.15): calcd. C 60.96, H 5.44, N 11.11; found C
60.89, H 5.52, N 11.22.
γ [°]
V [Å3]
Z
1-Benzyl-4,5-dicyano-3-picolyl-2-(pentafluorophenyl)imidazoline (5):
Compound 4 (700 mg, 2.42 mmol) was suspended in glacial acetic
acid (0.4 mL) in a 2 mL Schlenk flask fitted with a magnetic stirrer,
and the Schlenk flask was then sealed with a septum and flushed
with argon. Pentafluorobenzaldehyde (712 mg, 448 μL, 3.63 mmol;
1.5 equiv.) was added in one portion through a syringe and the
mixture was stirred for 48 h in the dark. After the reaction, crude 5
was filtered and washed with water (20 mL) and ice-cold methanol
(5 mL) and dried in vacuo to afford 754 mg of 5 as a pale-yellow
ρcalcd. [gcm–3]
μ [mm–1]
Reflns. measured
Unique reflections
6621
Unique reflns. with I0 Ͼ 2σ(I) 6317
4512
Rint
0.0746
0.0314
2θmax
50.00
50.00
R1, wR2 [I0 Ͼ 2σ(I)]
R1, wR2 (all data)
Goodness of fit
0.0258, 0.0682
0.0271, 0.0691
1.086
0.0278, 0.0742
0.0312, 0.0756
1.034
1
solid (67% yield). H NMR (500 MHz, CDCl3): δ = 8.46 (d, J =
Difference density min./max. –0.471/0.607
–0.590/0.555
4.2 Hz, 1 H), 7.60 (td, J = 7.7, 1.7 Hz, 1 H), 7.24–7.18 (m, 4 H),
7.10–7.06 (m, 3 H), 6.16 (s, 1 H), 4.21 (d, J = 15.0 Hz, 1 H), 4.15
(d, J = 15.0 Hz, 1 H), 4.03 (t, J = 15.0 Hz, 2 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 154.2, 149.8, 137.0, 133.6, 128.9, 128.8,
128.7, 123.3, 123.0, 114.3 (CN), 113.8 (CN), 110.8 (CCN), 110.7
CCDC-985751 (for 2) and -985752 (for 3) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
DFT calculations were performed at the M06[21]/cc-pVTZ[22] level
of theory using the Gaussian 09 package of programs.[23] NBO
analysis was carried out by using the NBO version 3.1 program.[24]
(CCN), 78.0, 54.4, 53.8 ppm. FTIR (KBr): ν = 3085, 3065, 3032,
˜
3010, 2935, 2873, 2215, 1734, 1717, 1700, 1684, 1655, 1594, 1523,
1511, 1454, 1439, 1393, 1371, 1305, 1297, 1262, 1204, 1130, 1082,
1050, 960, 911, 837, 818, 750, 741, 702, 678, 655, 643, 586, 575,
560, 508, 482 cm–1. HRMS: calcd. for C24H14F5N5 467.1250 [M]+;
found 468.1242 [M + H]+.
Synthetic Procedures
[PdCl2{(CN)2IMes}(PPh3)]
(2):
[PdCl2(PPh3)2]
(200 mg,
[PdCl2{(CN)2IBzPic}] (6): Imidazoline 5 (200 mg, 427.9 μmol) was
dissolved in chlorobenzene (60 mL) in a 250 mL two-necked flask
fitted with a magnetic stirrer, dropping funnel and reflux condenser,
and the dropping funnel was filled with a solution of [PdCl2-
(PhCN)2] (156 mg, 427.9 μmol; 1.0 equiv.) in chlorobenzene
(60 mL). The apparatus was evacuated and subsequently flushed
with argon three times and the flask heated in an oil bath to 135 °C.
After stirring at this temperature for 5 min, the solution of
[PdCl2(PhCN)2] was added dropwise over a period of 120 min to
284.9 mmol) and 1 (149 mg, 284.9 mmol; 1.0 equiv.) were added to
toluene (20 mL) under argon in a 50 mL round-bottomed flask fit-
ted with a reflux condenser and magnetic stirrer. The mixture was
heated at reflux for 16 h under vigorous stirring and then cooled
to room temperature. The solvent was removed in a rotary evapora-
tor and the product was isolated by column chromatography on
silica (height: 30 cm; diameter 3 cm) with dichloromethane as elu-
ent. The product was obtained as a yellow crystalline solid in 64%
1
yield (145 mg). H NMR (300 MHz, CD2Cl2): δ = 7.44–7.36 (m, 3 the imidazoline solution. The mixture was stirred at 135 °C for an
H), 7.32–7.23 (m, 12 H), 7.19 (s, 4 H, HMes-Aryl), 2.50 (s, 6 H,
additional 120 min and the mixture cooled to 0 °C. The resulting
HMes-p-Methyl), 2.32 (s, 12 H, HMes-o-methyl) ppm. 13C NMR bright solid was filtered through a glass frit and washed with
(75 MHz, CDCl3): δ = 186.0, 141.8, 136.7, 135.4, 135.2, 132.2, chloroform (3ϫ 20 mL), dissolved in dimethyl sulfoxide (2 mL)
130.8, 130.7, 130.2, 129.5, 128.2, 128.1, 117.4 (CN), 106.6 (CCN),
and precipitated by the addition of water (20 mL). The solid was
filtered through a glass frit and washed three times with chloroform
21.6, 19.1 ppm. FTIR (KBr): ν = 3060, 2974, 2954, 2916, 2857,
˜
Eur. J. Inorg. Chem. 0000, 0–0
7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
(3ϫ 10 mL) and three times with diethyl ether (3ϫ 10 mL) to ob-
tain 53 mg of 6 (27% yield) as a colourless solid. 1H NMR
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 175.4, 158.7, 137.9,
130.2, 129.4, 123.2, 116.2, 109.1.3, 106.6, 65.1, 56.6, 31.1, 19.6,
(300 MHz, [D6]DMSO): δ = 8.99 (d, J = 5.2 Hz, 2 H), 8.15 (dt, J 14.09 ppm. HRMS: calcd. for C14H17NO5 279.1107 [M]+; found
= 7.6, 1.5 Hz, 1 H), 8.05 (d, J = 7.0 Hz, 1 H), 7.65–7.58 (m, 1 H),
7.49–7.34 (m, 5 H), 6.22 (d, J = 15.6 Hz, 1 H), 6.05 (d, J = 15.7 Hz,
1 H), 5.81 (d, J = 15.5 Hz, 1 H), 5.68 (d, J = 15.8 Hz, 1 H) ppm.
13C NMR (75 MHz, [D6]DMSO): δ = 162.2, 154.2, 151.2, 140.4,
134.3, 128.7, 128.6, 127.9, 126.2, 125.3, 117.0 (CN), 114.3 (CN),
279.1102 [M]+.
The analytical data for literature-known, but not further described
n-butyl (E)-3-(3Ј-Hydroxyphenyl)acrylate (Table 5, entry 20)[26a]
and n-butyl (E)-3-(9Ј-anthryl)acrylate (Table 5, entry 29),[25e,26b] are
also presented.
106.8 (CCN), 106.7 (CCN), 55.4, 53.7 ppm. FTIR (KBr): ν = 3367,
˜
3322, 3014, 2980, 2944, 2922, 2861, 2735, 2206 (CN), 1742, 1602,
1484, 1457, 1448, 1437, 1352, 1227, 1158, 1038, 1010, 891, 857, 822,
777, 706, 564, 502, 419 cm–1. HRMS: calcd. for C18H13Cl2N5Pd
474.9583 [M]+; found 439.9992 [M – Cl]+. C18H13Cl2N5Pd (474.96):
calcd. C 45.36, H 2.75, N 14.69; found C 45.07, H 2.87, N 14.85.
n-Butyl (E)-3-(3Ј-Hydroxyphenyl)acrylate: 1H NMR (300 MHz,
CD2Cl2): δ = 7.66 (d, J = 15.9 Hz, 1 H), 7.25 (t, J = 7.8 Hz, 1 H),
7.18 (s, 1 H), 7.10–7.06 (m, 2 H), 7.07–7.03 (m, 1 H), 6.43 (d, J =
16.2 Hz, 1 H), 4.25 (t, J = 6.6 Hz, 2 H), 1.76–1.66 (m, 2 H), 1.51–
1.38 (m, 2 H), 0.97 (t, J = 7.2 Hz, 3 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 168.1, 156.5, 145.2, 135.7, 130.0, 120.6, 118.1, 117.8,
114.7, 65.0, 30.7, 19.2, 13.7 ppm. HRMS: calcd. for C13H16O3
220.1099 [M]+; found 220.1095 [M]+.
Representative Procedure for the Mizoroki–Heck Reaction: Sodium
carbonate (18.7 mg, 176 μmol; 1.4 equiv.) and TBAB (4.1 mg,
12.6 μmol; 0.1 equiv.) were added to a 2 mL Schlenk tube fitted
with a magnetic stirrer. A solution of precatalyst 2 (100 μg,
126 nmol; 0.001 equiv.) and 1,3,5-trimethoxybenzene (2.11 mg,
12.6 μmol; 0.1 equiv.) in N,N-dimethylformamide (0.5 mL) was
added and the mixture was stirred. Bromobenzene (19.8 mg,
13.2 μL; 126 μmol) and n-butyl acrylate (32.3 mg, 35.9 μL,
252 μmol; 2.0 equiv.) were added through a syringe and the tube
was sealed and then secured by evacuating three times and sub-
sequently flushing with argon. The flask was then placed in a pre-
heated oil bath at 140 °C and stirred for 16 h. After completion of
the reaction, the flask was immediately cooled to 0 °C in an ice
bath. The cold mixture was hydrolysed with 1 n hydrochloric acid
(2 mL) and chloroform (2 mL) was then added. The mixture was
poured into water (20 mL) and the aqueous phase extracted three
times with chloroform (2 mL).
1
n-Butyl (E)-3-(9Ј-Anthryl)acrylate: H NMR (300 MHz, CDCl3): δ
= 8.65 (d, J = 16.2 Hz, 1 H), 8.45 (s, 1 H), 8.30–8.21 (m, 2 H),
8.06–7.97 (m, 2 H), 7.55–7.44 (m, 4 H), 6.45 (d, J = 16.2 Hz, 1 H),
4.32 (t, J = 6.6 Hz, 2 H), 1.83–1.73 (m, 2 H), 1.57–1.44 (m, 2 H),
1.02 (t, J = 7.2 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
166.6, 141.9, 131.3, 129.3, 128.8, 128.2, 127.3, 126.3, 125.4, 125.2,
64.8, 30.8, 19.3, 13.8 ppm. HRMS: calcd. for C21H20O2 303.1463
[M]+; found 303.1460 [M]+.
[1] For reviews of the MH reaction, see: a) G. T. Crisp, Chem. Soc.
Rev. 1998, 27, 427–436; b) I. P. Beletskaya, A. V. Cheprakov,
Chem. Rev. 2000, 100, 3009–3066; c) N. T. S. Phan, M.
Van Der Sluys, C. W. Jones, Adv. Synth. Catal. 2006, 348, 609–
679; d) J. P. Knowles, A. Whiting, Org. Biomol. Chem. 2007, 5,
31–44; e) M. Oestreich, The Mizoroki–Heck Reaction, Wiley,
Chichester, UK, 2009; f) N. Selander, K. J. Szabo, Chem. Rev.
2011, 111, 2048–2076; g) A. Molnar, Chem. Rev. 2011, 111,
2251–2320; h) M. Yus, I. M. Pastor, Chem. Lett. 2013, 42, 94–
108.
For the catalytic screening, the crude product was dissolved in
chloroform (1.5 mL; GC grade from Merck) and the GC yield de-
termined by using a Perkin–Elmer Clarus 580 instrument equipped
with a Perkin–Elmer Elite 5 MS column (length: 30 m, diameter:
0.25 mm). Signals were detected by using an FID detector. 1,3,5-
Trimethoxybenzene was used as internal standard to determine GC
yields.
[2] R. F. Heck, J. P. Nolley Jr., J. Org. Chem. 1972, 37, 2320–2322.
[3] A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
1991, 113, 361–363.
To calibrate the GC, the MH products were isolated by flash
chromatography on silica (height: 460 mm, diameter: 15 mm) with
hexane/ethyl acetate mixtures as eluent and afterwards analysed by
1H and 13C NMR spectroscopy and mass spectrometry. The spec-
troscopic data of already known products were in agreement with
literature data.[25]
[4] W. A. Herrmann, M. Elison, J. Fischer, C. Kocher, G. R. J.
Artus, Angew. Chem. Int. Ed. Engl. 1995, 34, 2371–2374; An-
gew. Chem. 1995, 107, 2602.
[5] For recent reviews on the development of NHCs, see: a) F. E.
Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122–
3172; Angew. Chem. 2008, 120, 3166; b) P. de Fremont, N. Mar-
ion, S. P. Nolan, Coord. Chem. Rev. 2009, 253, 862–892; c) M.
Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed.
2010, 49, 8810–8849; Angew. Chem. 2010, 122, 8992; d) T.
Dröge, F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 6940–6952;
Angew. Chem. 2010, 122, 7094; e) L. Benhamou, E. Chardon,
G. Lavigne, S. Bellemin-Laponnaz, V. Cesar, Chem. Rev. 2011,
111, 2705–2733; for recent contributions involving electron-de-
ficient NHC ligands, see: f) A. G. Tennyson, E. L. Rosen, M. S.
Collins, V. M. Lynch, C. W. Bielawski, Inorg. Chem. 2009, 48,
6924–6933; g) T. W. Hudnall, A. G. Tennyson, C. W. Bielawski,
Organometallics 2010, 29, 4569–4578; h) T. W. Hudnall, J. P.
Moerdyk, C. W. Bielawski, Chem. Commun. 2010, 46, 4288–
4290; i) A. G. Tennyson, R. J. Ono, T. W. Hudnall, D. M.
Khramov, J. A. V. Er, J. W. Kamplain, V. M. Lynch, J. L.
Sessler, C. W. Bielawski, Chem. Eur. J. 2010, 16, 304–315; j) F.-
G. Sun, X.-L. Huang, S. Ye, J. Org. Chem. 2010, 75, 273–276.
[6] For examples of the MH reaction, see: a) Q. Yao, M. Zabawa,
J. Woo, C. Zheng, J. Am. Chem. Soc. 2007, 129, 3088–3089; b)
S. G. Fiddy, J. Evans, T. Neisius, M. A. Newton, N. Tsoureas,
A. A. D. Tulloch, A. A. Danopoulos, Chem. Eur. J. 2007, 13,
3652–3659; c) M. T. Lee, H. M. Lee, C. H. Hu, Organometallics
n-Butyl (E)-3-(4Ј-Diethylaminophenyl)acrylate (Table 5, entry 13)
and n-butyl (E)-3-(2Ј-Methoxy-4Ј-nitrophenyl)acrylate (Table 5, en-
try 15) were prepared for the first time.
n-Butyl
(E)-3-(4Ј-Diethylaminophenyl)acrylate:
1H
NMR
(300 MHz, CDCl3): δ = 7.61 (d, J = 15.9 Hz, 1 H), 7.39 (d, J =
9.0 Hz, 2 H), 6.62 (d, J = 9.0 Hz, 2 H), 6.19 (d, J = 15.9 Hz, 1 H),
4.18 (t, J = 12.0 Hz, 2 H), 3.39 (q, J = 7.2 Hz, 4 H), 1.73–1.63 (m,
2 H), 1.50–1.37 (m, 2 H), 1.18 (t, J = 7.2 Hz, 6 H), 0.96 (t, J =
7.2 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 168.5, 149.7,
145.5, 130.4, 121.9, 112.4, 111.6, 64.3, 44.8, 31.3, 19.4, 14.2,
13.0 ppm. HRMS: calcd. for C17H25NO2 275.1885 [M]+; found
275.1892 [M]+.
n-Butyl (E)-3-(2Ј-Methoxy-4Ј-nitrophenyl)acrylate: 1H NMR
(300 MHz, CDCl3): δ = 7.96 (d, J = 16.2 Hz, 1 H), 7.86 (dd, J =
8.4, 2.1 Hz, 1 H), 7.78 (d, J = 2.1 Hz, 2 H), 7.66 (d, J = 8.4 Hz, 1
H), 6.65 (d, J = 16.2 Hz, 1 H), 4.25 (t, J = 6.9 Hz, 2 H), 4.02 (s, 3
H), 1.77–1.67 (m, 2 H), 1.52–1.40 (m, 2 H), 0.99 (t, J = 7.2 Hz, 3
Eur. J. Inorg. Chem. 0000, 0–0
8
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
2007, 26, 1317–1324; d) K. Hara, Y. Kanamori, M. Sawamura,
Bull. Chem. Soc. Jpn. 2006, 79, 1781–1786.
a) G. D. Frey, J. Schütz, E. Herdtweck, W. A. Herrmann, Orga-
nometallics 2005, 24, 4416–4426; b) E. Assen, B. Kantchev, G.-
R. Peh, C. Zhang, J. Y. Ying, Org. Lett. 2008, 10, 3949–3952.
G. M. Sheldrick, SHELXL-97, Program for the Refinement of
Crystal Structures, University of Göttingen, Germany, 1997.
[21] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215–241.
[22] R. A. Kendall, T. H. Dunning, R. J. Harrison, J. Chem. Phys.
1992, 96, 6796–6806.
[7]
[8]
[23] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hase-
gawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M.
Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Starov-
erov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Ad-
amo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Mar-
tin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, re-
vision C.01, Gaussian Inc., Wallingford, CT, 2009.
a) I. P. Beletskaya, A. V. Cheprakov, J. Organomet. Chem. 2004,
689, 4055–4082; b) J. Dupont, C. S. Consorti, J. Spencer, Chem.
Rev. 2005, 105, 2527–2571.
U. Christmann, R. Vilar, Angew. Chem. Int. Ed. 2005, 44, 366–
374; Angew. Chem. 2005, 117, 370.
A. A. D. Tulloch, A. A. Danopoulos, R. P. Tooze, S. M. Caffer-
key, S. Kleinhenz, M. B. Hursthouse, Chem. Commun. 2000,
1247–1248.
H. Baier, A. Kelling, R. Jackstell, H.-J. Holdt, Z. Anorg. Allg.
Chem. 2013, 1731–1739.
T. Schwarze, A. Kelling, H. Müller, M. Trautmann, T. Klam-
roth, O. Baumann, P. Strauch, H.-J. Holdt, Chem. Eur. J. 2012,
18, 10506–10510.
G. Ferguson, R. McGrindle, A. J. McAlees, M. Parvez, Acta
Crystallogr., Sect. B 1982, 38, 2679–2681.
[9]
[10]
[11]
[12]
[13]
[14] T. Sato, Y. Hirose, D. Yoshioka, S. Oi, Organometallics 2012,
31, 6995–7003.
[24] E. D. Glendening, A. E. Reed, J. E. Carpenter, F. Weinhold,
NBO, version 3.1, Gaussian Inc., Wallingford, CT, 2009 .
[15] R. Dorta, E. D. Stevens, N. M. Scott, C. Costabile, L. Cavallo,
C. P. Hoff, S. P. Nolan, J. Am. Chem. Soc. 2005, 127, 2485– [25] a) J. Zou, K. S. Iyer, S. G. Stewart, C. L. Raston, New J. Chem.
2495.
2011, 35, 854–860; b) M. M. S. Andappan, P. Nilsson, M.
Larhed, Mol. Diversity 2003, 7, 97–106; c) R. Martinez, F. Vo-
ica, J.-P. Genet, S. Darses, Org. Lett. 2007, 9, 3213; d) J.-Y. Lee,
P.-Y. Cheng, Y.-H. Tsai, G.-R. Lin, S.-P. Liu, M.-H. Sie, H. M.
Lee, Organometallics 2010, 29, 3901–3911; e) M. Feuerstein, H.
Doucet, M. Santelli, Synlett 2001, 1980–1982; f) T. Mino, Y.
Shirae, Y. Sasai, M. Sakamoto, T. Fujita, J. Org. Chem. 2006,
71, 6834–6839; g) D. Yang, Y.-C. Chen, N.-Y. Zhu, Org. Lett.
2004, 6, 1577–1580; h) X. Cui, Z. Li, C.-Z. Tao, Y. Xu, J. Li,
L. Liu, Q.-X. Guo, Org. Lett. 2006, 8, 2467–2470; i) Y. Shen,
G. Liu, Z. Zhou, X. Lu, Org. Lett. 2013, 15, 3366–3369; j) T.
Sato, W. D. Jones, Organometallics 2001, 20, 2916–2919.
[26] a) E. Jung, S. Kim, Y. King, S. H. Seo, S. S. Lee, M. S. Han, S.
Lee, Angew. Chem. Int. Ed. 2011, 50, 4386–4389; Angew. Chem.
2011, 123, 4478; b) Y.-M. Liu, Y.-C. Lin, W.-C. Chen, J.-H.
Cheng, Y.-L. Chen, G. P. A. Yap, S. S. Sun, T.-G. Ong, Dalton
Trans. 2012, 41, 7382–7389.
[16] a) D. R. Anton, R. H. Crabtree, Organometallics 1983, 2, 855–
859; b) G. M. Whitesides, M. Hackett, R. L. Bainard,
J. P. P. M. Lavalleye, A. F. Sowinski, A. N. Izumi, S. S. Moore,
D. W. Brown, E. M. Staudt, Organometallics 1985, 4, 1819–
1830; c) J. A. Widegren, R. G. Finke, J. Mol. Catal. A 2003,
198, 317–341; d) R. H. Crabtree, Chem. Rev. 2012, 112, 1536–
1554; Y.-C. Lin, H.-H. Hsueh, S. Kanne, L.-K. Chang, F.-C.
Liu, I. J. B Lin, Organometallics 2013, 32, 3859–3869.
[17] W. L. F. Armarego, C. L. L. Chai, Purification of Laboratory
Chemicals, 6th ed., Elsevier, Oxford, UK, 2009.
[18] a) N. Miyaura, A. Suzuki, Org. Synth. 1990, 68, 130–137; b)
A. C. Cope, E. C. Friedrich, J. Am. Chem. Soc. 1968, 90, 909–
913; c) G.-Z. Fan, T. Li, G. X. Li, Appl. Organomet. Chem.
2006, 20, 656–662.
[19] H. E. Gottlieb, V. Kotlyar, A. Nudelman, J. Org. Chem. 1997,
62, 7512–7515.
[20] a) G. M. Sheldrick, SHELXS-97, Program for the Solution of
Crystal Structures, University of Göttingen, Germany, 1997; b)
Received: February 11, 2014
Published Online: ᭿
Eur. J. Inorg. Chem. 0000, 0–0
9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42040
/KAP1
Date: 19-05-14 18:41:43
Pages: 10
www.eurjic.org
FULL PAPER
Mizoroki–Heck Reaction
H. Baier, P. Metzner, T. Körzdörfer,
A. Kelling, H.-J. Holdt* .................. 1–10
Efficient Palladium(II) Precatalysts Bear-
ing 4,5-Dicyanoimidazol-2-ylidene for the
Mizoroki–Heck Reaction
Keywords: Homogeneous catalysis / Pal-
ladium / Cross coupling / Carbene ligands
The new precatalysts [PdCl2{(CN)2IMes}],
[PdCl(dmba){(CN)2IMes}] and [PdCl2-
{(CN)2IBzPic}] have been prepared as can-
didates for the Mizoroki–Heck reaction
and used in catalytic screening for reactions
between n-butyl acrylate and 27 aryl brom-
ides.
Eur. J. Inorg. Chem. 0000, 0–0
10
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim