Solvent-controlled selective synthesis of a trans-configured benzimidazoline-2-ylidene palladium(II) complex and investigations of its Heck-type catalytic activity

Article abstract of DOI:10.1016/j.jorganchem.2005.04.053

Reaction of N,N′-dimethylbenzimidazolyl iodide (A) with Pd(OAc) ₂ in DMSO gives selectively trans-bis(N,N′- dimethylbenzimidazoline-2-ylidene) palladium(II) diiodide (trans-2) in 77% yield. The selective formation of the trans-coordination isom

Full text of DOI:10.1016/j.jorganchem.2005.04.053

Journal of Organometallic Chemistry 690 (2005) 3854–3860
www.elsevier.com/locate/jorganchem
Solvent-controlled selective synthesis of a trans-configured
benzimidazoline-2-ylidene palladium(II) complex
and investigations of its Heck-type catalytic activity
*
Han Vinh Huynh , Joanne Hui Hui Ho, Tiong Cheng Neo, Lip Lin Koh
Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore, Singapore
Received 21 January 2005; received in revised form 26 April 2005; accepted 26 April 2005
Available online 27 June 2005
Abstract
0
0
Reaction of N,N -dimethylbenzimidazolyl iodide (A) with Pd(OAc) in DMSO gives selectively trans-bis(N,N -dimethylbenzim-
2
idazoline-2-ylidene) palladium(II) diiodide (trans-2) in 77% yield. The selective formation of the trans-coordination isomer and thus
1
3
the cis–trans rearrangement is driven by the insolubility of trans-2 in DMSO. X-ray single-crystal diffraction analysis and C NMR
spectroscopy confirm the trans-geometry of the square planar Pd(II) complex. Catalytic studies show that cis-1 and trans-2 are
highly efficient in the Mizoroki–Heck coupling reaction of aryl bromides and activated aryl chlorides both in DMF and [N(n-
C
4
H
9
)
4
]Br as ionic liquid. The catalytic activities of Pd(II) complexes with N-heterocyclic carbene ligands derived from benzimid-
azole are comparable to their imidazole-derived analogues.
2005 Elsevier B.V. All rights reserved.
ꢀ
Keywords: Palladium; N-Heterocyclic carbene; Homogeneous catalysis; Heck coupling reaction
1
. Introduction
precatalysts contain NHC as ancillary ligands that are
derived from imidazolium salts. It has been reported
that carbenes derived from benzimidazolium precursors
exhibit the topology of an unsaturated N-heterocyclic
carbene, but show spectroscopic and structural proper-
ties and the reactivity of carbenes with a saturated
N-heterocyclic ring [3]. Previously, several examples of
palladium(II) carbene complexes derived from benzimi-
dazolium precursors with achiral [4] and chiral [5] alkyl
groups or even the ferrocene moiety [6] adjacent to the
nitrogen atoms have been reported. However, their po-
tential application as phosphine free precatalysts re-
mains relatively unexplored [7]. We herein present a
solvent-controlled selective synthesis and structural
characterization of a trans-configured palladium(II)
bis(benzimidazoline-2-ylidene) complex and investiga-
tions of its catalytic activity in the Mizoroki–Heck
coupling reaction.
Nucleophilic N-heterocyclic carbenes (NHC) and
their transition metal complexes have been the focus
of intense research in organometallic chemistry and
homogeneous catalysis since the past decade [1]. In par-
ticular, palladium(II) carbene complexes were success-
fully developed as highly active precatalysts for C–C
coupling reactions such as Mizoroki–Heck and Suzuki–
Miyaura couplings as well as CO-olefin co-polymeriza-
tion. It has been shown that such complexes offer the
distinctive advantage of greater stability over the classi-
cal Pd/phosphine systems as the latter suffer from sensi-
tivity to air and moisture [2]. The majority of these
*
Corresponding author. Tel.: +65 6874 2670; fax: +65 6779 1691.
E-mail address: chmhhv@nus.edu.sg (H.V. Huynh).
0
022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.04.053

H.V. Huynh et al. / Journal of Organometallic Chemistry 690 (2005) 3854–3860
3855
1
2
. Results and discussion
rearrangement by H NMR spectroscopy failed due
to the different solubilities of the two isomers. It is
noteworthy that a trans–cis rearrangement of a similar
benzimidazoline-2-ylidene palladium(II) complex has
recently been reported by Wang and Lin [10] based on
2
.1. Synthesis and characterization
In connection with our research on carbene carboxyl-
ate complexes [8], we were interested in the complex
cis-bis(N,N -dimethylbenzimidazoline-2-ylidene) palla-
dium(II) diiodide (cis-1) as a precursor. cis-1 can be
1
H NMR data.
0
2.2. Molecular structures of salt A and complex trans-2
0
easily prepared by reacting two equivalents of N,N -
dimethylbenzimidazolyl iodide (A) with Pd(OAc)2 in
THF at ambient temperature as described by Hahn
and Foth [4a]. However, we found that this procedure
not only afforded cis-1 but also the novel trans-isomer
trans-2 in substantial amount (40%) explaining the mod-
erate yield of cis-1 reported by the authors (Scheme 1).
The remarkably different solubility of the two coordina-
tion isomers in organic solvents allows an easy separa-
tion step. The trans form, e.g., is readily soluble in
halogenated solvents and insoluble in more polar sol-
vents such as DMSO and DMF, while the cis-form on
the contrary dissolves only in the two latter solvents
Single crystals of trans-2 suitable for X-ray diffraction
studies were obtained by evaporation of a saturated
CH Cl solution at ambient temperature. For the pur-
pose of a comparison, we have also carried out the X-
ray crystal structure analysis of the benzimidazolium
salt A. Crystallographic data are listed in Table 1. The
molecular structures of the salt A and trans-2 are de-
picted in Fig. 1 and selected bond lengths and angles
are listed in Table 2.
2
2
Compound trans-2 crystallizes as a mononuclear
complex with half a molecule in the asymmetric unit.
The palladium center is coordinated by two carbene
and two iodo ligands in a square-planar fashion. As
and CH CN. The configuration of trans-2 was con-
3
1
3
13
firmed by C NMR spectroscopy, in which the carbene
carbon resonances at 181.0 ppm and as expected down-
field from the reported value of 172.1 ppm (175.1 ppm in
this work) for cis-1 [9]. Upon prolonged standing of a
CH Cl solution trans-2 slowly converts into cis-1,
found by C NMR spectroscopy in solution, the two
carbene ligands are arranged trans to each other with
an ideal angle of 180ꢁ due to symmetry. Both carbene
ring planes are oriented almost perpendicular to the
PdC I plane with a torsion angle of 88.09(21)ꢁ. Signif-
2
2
2 2
which precipitates as a fine yellow powder. However,
evaporation of a concentrated CH Cl solution at ambi-
icantly smaller torsion angles were found in the cis iso-
mer [4a] with values of 83.06(10)ꢁ and 79.84(13)ꢁ.
Compared to its benzimidazolium salt precursor A, the
Ccarbene–N1/2 bonds of the heterocyclic ligand in both
2
2
ent temperature afforded yellow cubic crystals of trans-2
suitable for X-ray diffraction studies (see Section 2.2).
On the other hand, the trans-form precipitates from a
DMSO solution of the cis-isomer upon standing, pre-
sumably helping to drive the equilibrium towards the
trans-isomer. Thus this isomerization process is strongly
influenced by the solubility of the two isomers in differ-
ent solvents. Based on this observation we could in-
crease the yield of trans-2 up to 77% by employing
DMSO as the reaction media at moderate temperatures
of 80 ꢁC. Unfortunately, our attempts to monitor this
˚
isomers have become elongated by Dd = 0.03 A. This
bond elongation is furthermore accompanied by a de-
crease of the N–C–N angle from 110.6(3)ꢁ in A to about
106ꢁ in both isomers. Other structural parameters re-
main largely unchanged indicating that the coordination
to the Pd-center is only affecting the carbene carbon and
the two neighboring nitrogen atoms.
More importantly, a comparison of the mean Pd–
˚
Ccarbene bond lengths in both isomers (2.010(2) A for
N
I
N
N
I
Pd(OAC)2
i) or ii)
N
N
N
N
N
N
I
I
H
Pd
N
+
Pd
I
-
2 HOAc
A
trans-2
cis-1
cis-1, 54% vs. trans-2, 40%
i) THF, RT:
ii) DMSO, 80˚C: cis-1, 20% vs. trans-2, 77%
Scheme 1. Preparation of cis-1 and trans-2.

3856
H.V. Huynh et al. / Journal of Organometallic Chemistry 690 (2005) 3854–3860
Table 1
Selected crystal data, data collection and refinement parameters for salt A Æ H
2
O and complex trans-2
A Æ H
11IN
292.11
2
O
trans-2
Formula
C
9
H
2
Æ H
2
O
18 20 2 4
C H I N Pd
Formula weight
Color, habit
Crystal size (mm)
T (K)
652.58
Colorless, block
0.36 · 0.14 · 0.14
223(2)
Yellow, block
0.44 · 0.24 · 0.18
223(2)
Crystal system
Space group
Monoclinic
/c
Monoclinic
P2
1
1
P2 /c
˚
a (A)
8.8666(10)
7.0570(8)
17.5218(19)
90
8.2797(5)
16.2675(10)
7.6549(5)
90
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
101.107(2)
90
99.6000(10)
90
˚
3
V (A )
1075.8(2)
4
1.804
1016.60(11)
2
2.132
Z
ꢀ3
D
calc (g cm
)
Radiation used
l (mm
Mo Ka
2.942
Mo Ka
3.958
ꢀ
1
)
h Range (ꢁ)
2.37–24.99
5870
0.6835 and 0.4173
2.49–27.50
6897
0.5360 and 0.2748
Number of unique reflections measured
Maximum and minimum transmission
Final R indices [I > 2r(I)]
R indices (all data)
R
R
1
= 0.0234, wR
= 0.0250, wR
2
= 0.0652
= 0.0664
R
R
1
= 0.0206, wR
= 0.0223, wR
2
= 0.0508
= 0.0516
1
2
1
2
2
Goodness-of-fit on F
Large difference in peak and hole (eA
1.093
0.908 and ꢀ0.411
1.108
0.727 and ꢀ0.312
˚
ꢀ3
)
Table 2
˚
Selected bond lengths (A) and bond angles (ꢁ) for the salt A Æ H
2
O and
complex trans-2
A Æ H
2
O
trans-2
2.5969(2)
Pd1–I1
Pd1–C1
N1–C1
N1–C7
N1–C8
N2–C1
N2–C2
N2–C9
C2–C7
–
–
2.010(2)
1.353(3)
1.397(3)
1.459(3)
1.353(3)
1.390(3)
1.452(3)
1.396(3)
1.320(4)
1.384(4)
1.471(4)
1.320(4)
1.394(4)
1.463(4)
1.388(4)
C1–Pd1–I1
–
–
–
–
89.11(11)
90.89(7)
180.0(2)
C1–Pd1–I1A
C1–Pd1–C1A
I1–Pd1–I1A
N1–C1–N2
180.000(6)
106.4(2)
110.6(3)
of the benzimidazoline-2-ylidene ligand. These findings
strongly support the view that a cis arrangement is
generally more favorable for N-heterocyclic carbene
complexes [10].
Fig. 1. Molecular structure of benzimidazolium salt A (upper, the
disordered water molecule is not depicted) and complex trans-2
(lower).
2
.3. Catalysis
˚
trans-2 vs. 1.988(4) A for cis-1) reveals that the carbene
ligands are more strongly bound to the Pd-center in the
cis-form. In addition, the longer Pd–I bond lengths in
Palladium complexes of NHC derived from imidazo-
lium and imidazolinium salts are highly active catalysts
for a wide range of C–C coupling reactions [1]. Com-
plexes derived from benzimidazolium precursors on
the other hand have scarcely been investigated for their
˚
cis-1 (2.6371(5) and 2.6805(5) A) as compared to those
˚
in trans-2 (2.5969(2) A) reflects the strong trans-effect

H.V. Huynh et al. / Journal of Organometallic Chemistry 690 (2005) 3854–3860
3857
catalytic activities [7]. This prompted us to start a preli-
minary study on the catalytic activities of the benzimi-
dazoline-2-ylidene complexes cis-1 and trans-2 for the
Mizoroki–Heck coupling reaction of aryl halides with
t-butyl acrylate to give the corresponding cinnamates.
Since the coupling of aryl iodides is in general common
and facile, we focused on the coupling of the less reac-
tive aryl bromides and chlorides. Our results, summa-
rized in Table 3, indicate that both isomers are highly
efficient in the coupling of aryl bromides. For example,
both cis-1 and trans-2 (1 mol%) can couple 4-bromo-
benzaldehyde (entry 1) and bromobenzene (entry 2) with
the acrylate in quantitative yield at 120 ꢁC.
A more detailed kinetic study on the coupling of the
electron-withdrawing 4-bromobenzaldehyde employing
1 mol% catalyst revealed that both isomers showed dif-
ferent catalytic behavior. In the case of trans-2, the
conversion is already complete within 90 min (Fig.
2). Furthermore, the concentration/time diagram indi-
cates an induction period for this reaction of about
Table 3
Mizoroki–Heck coupling reactions catalyzed by cis-1 and trans-2
a
O
O
[Pd]
R
X
+
O
DMF, NaOAc
O
-HX
R
X = Br, Cl
R = CHO, H, CH CO, CH O
3
3
Entry
Catalyst
Catalyst load (mol%)
Aryl halide
t (h)
T (ꢁC)
Yield (%)
b
1
2
3
4
5
6
cis-1, trans-2
cis-1, trans-2
trans-2
1
4-Bromobenzaldehyde
Bromobenzene
Bromobenzene
2
20
20
65
65
18
20
24
20
4
120
120
120
120
140
120
140
140
140
120
120
120
100
100
b
1
0.1
b
100
72
c
trans-2
trans-2
0.02
0.002
Bromobenzene
Bromobenzene
4-Bromoanisole
c
59
b
trans-2
trans-2
1
1
1
1
1
1
1
90
d
b
7
4-Chlorobenzaldehyde
4-Chloroacetophenone
Chlorobenzene
86
d
b
8
d
trans-2
trans-2
63
20
c
9
e
10
11
12
trans-2
trans-2
trans-2
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
0
b
e
18
2
100
100
e,f
b
a
b
c
d
e
f
Reaction conditions generally not optimized.
1
Yields were determined by H NMR spectroscopy.
Isolated yields.
With addition of 1.5 equivalents of [N(n-C
Coupling reaction was carried out in neat [N(n-C
With addition of NaO CH as reducing agent.
4 9 4
H ) ]Br.
4 9 4
H ) ]Br as ionic liquid.
2
100
8
6
4
2
0
0
0
0
x [%]
0
0
20
40
60
80
100
120
t [min]
Fig. 2. Concentration/time diagram (amount of substance x (%), time t (min)) for the Mizoroki–Heck olefination of 4-bromobenzaldehyde with
t-butyl acrylate to form t-butyl (E)-4-formylcinnamate catalyzed by cis-1 (ꢁ) and trans-2 (¤).

3858
H.V. Huynh et al. / Journal of Organometallic Chemistry 690 (2005) 3854–3860
O
elevated reaction temperature of 140 ꢁC electron-poor
aryl chlorides like 4-chlorobenzaldehyde (entry 7) and
4-chloroacetophenone (entry 8) couple with t-butyl
acrylate affording good to high yields. This positive
salt-effect has been described by other researchers and
it has been suggested that quaternary salts like [N(n-
C H ) ]Br can react as reducing agent to generate active
HCO2-
- I-
L
I
I
L
L
O
I
H
L
L
H
I
PdII
PdII
PdII
L
-
CO2
b)
(
a)
(
-
HI
c)
(
4
9 4
"L Pd0 L"
Pd(0) species [2,9]. To test this statement, we investi-
gated the coupling of 4-bromobenzaldehyde in neat
catalytic cycle
Scheme 2. Feasible formation pathway of the active Pd(0) species.
[
N(n-C H ) ]Br as ionic liquid at 120 ꢁC. Surprisingly,
4
9 4
no conversion was observed even after 4 h (entry 10)
indicating that no active Pd(0) species were formed dur-
ing this time in neat [N(n-C H ) ]Br. However, exten-
4
0 min, after which the concentration of the product
increases exponentially. Similar induction periods were
observed for palladium carbene complexes derived
from imidazolium salts and it was proposed that dur-
ing this period catalytically active palladium(0) species
are formed [2].
4
9 4
sion of the reaction time to 18 h yielded complete
conversion (entry 11) corroborating that the ionic liquid
can indeed react as a reducing agent. Nevertheless, the
formation of active Pd(0) species with [N(n-C H ) ]Br
4
9 4
The coupling with cis-1, however, is remarkably fas-
ter and the reaction profile does not show an induction
period in the observed time intervals (Fig. 2). This sur-
prising result is worthy of comment and suggests that
reduction of Pd(II) to Pd(0) and thus formation of
the catalytically active species occurs faster in the cis-
isomer. This reductive process can already be accom-
plished by traces of formic acid in DMF [11]. Scheme
is a very slow process that requires prolong reaction
times. On addition of stronger reducing agents like for-
mic acid or sodium formate to the ionic liquid, however,
we could achieve quantitative conversions in 2 h (entry
12). Based on these results, we believe that the reducing
properties of [N(n-C H ) ]Br play only a minor role in
4
9 4
promoting the Mizoroki–Heck reaction. It is more likely
that the addition of such salts helps in stabilizing inter-
mediates by coordination or formation of ion-pairs in
the catalytic cycle [11]. However, further investigations
are needed to gain better understanding of the cause
for the promoting effects of quaternary ammonium salts
in Heck-type reactions. Finally, we note that the more
difficult coupling of inactivated chlorobenzene afforded
only a low isolated yield of 20% (entry 9) even in the
presence of [N(n-C H ) ]Br.
2
stitution (a), CO elimination (b) and reductive elimina-
depicts a feasible pathway which includes ligand sub-
2
tion (c) affording the active Pd(0) species, which
subsequently enters the catalytic cycle. A facile dissoci-
ation of the iodo ligand as a possible introductory step
is favored in the cis-isomer, in which the strong trans-
effect of the carbene ligand weakens the Pd–I bond.
On the other hand, such a Pd–I bond cleavage is less
favored in trans-2 and may account for the observed
induction period, in which a trans–cis isomerization
might occur to facilitate Pd–I bond cleavage prior to
the reduction step.
4
9 4
The results of this preliminary catalytic study clearly
demonstrates that benzannulated N-heterocyclic carbene
ligands derived from benzimidazole are as versatile and
excellent ligands for catalysis as their well-investigated
imidazole-derived analogues. Work is currently under-
way to expand the scope of benzimidazoline-2-ylidene
complexes in catalysis.
To investigate the effect of the catalyst loading on the
conversion rate, we examined the coupling of bromo-
benzene with the acrylate. On lowering the amount of
catalyst from 0.1 mol% (entry 3) to 0.02 mol% (entry
4
) and further to 0.002 mol% (entry 5), we could still ob-
serve high to moderate isolated yields of 72% (TON
600) and 59% (TON 29,500), respectively. Not surpris-
3. Experimental
3
ingly, prolonged reaction times and higher temperatures
were required to achieve a high TON of 29,500. It is ex-
pected, that much higher TONs can be achieved for the
coupling of activated aryl iodides and electron-poor aryl
bromides. Electron-donating substituents on the aryl ha-
lide are deactivating and thus electron-rich aryl bro-
mides like 4-bromoanisole are usually more difficult to
couple. However, the conversion of 4-bromoanisole pro-
ceeded smoothly with 1 mol% of trans-2 giving an excel-
lent yield of 90% (entry 6). The coupling of aryl
chlorides was far more difficult and yielded only poor re-
sults. However, with addition of [N(n-C H ) ]Br and an
3.1. General procedures
Unless otherwise noted all manipulations were per-
formed in air. All solvents were used as received.
0
N,N -Dimethylbenzimidazolyl iodide (A) was prepared
according to the literature procedures [12]. Pd(OAc)
2
ꢂ
was purchased from Avocado and used as received.
1
13
H and C NMR spectra were recorded on a Bruker
ACF 300 spectrometer using Me Si as internal standard.
4
Elemental analyses were performed on a Perkin–Elmer
PE 2400 elemental analyzer at the Department of Chem-
istry, National University of Singapore.
4
9 4

H.V. Huynh et al. / Journal of Organometallic Chemistry 690 (2005) 3854–3860
3859
0
3.2. Synthesis of cis and trans-bis(N,N -
dimethylbenzimidazoline-2-ylidene)palladium(II)
diiodide (cis-1 and trans-2)
tion mixture and the organic layer was repeatedly
washed with water (5 · 10 mL) and dried over MgSO .
4
The solvent and any volatiles were removed completely
under high vacuum to give a crude product which was
either subjected to column chromatography or analyzed
0
A mixture of N,N -dimethylbenzimidazolyl iodide (A)
274 mg, 1.00 mmol) and Pd(OAc)2 (112.0 mg, 0.5
1
(
by H NMR spectroscopy.
mmol) was suspended in THF (15 mL) and stirred over-
night at ambient temperature. The orange suspension
first turned dark brown and lightened up to pale brown
after a few hours. The volatiles were removed in vacuo
and the residue was washed with CH Cl several times.
3.5. Structure determinations
Single crystals of A and trans-2 were obtained by slow
evaporation of CH Cl solutions of the corresponding
2
2
2
2
Slow evaporation of the combined CH Cl solutions
2
compounds. Suitable crystals were mounted on quartz
fibers and X-ray data collected on a Bruker AXS APEX
diffractometer, equipped with a CCD detector, using
2
afforded yellow cubic crystals of trans-2 (130 mg, 0.2
mmol, 40%) suitable for X-ray diffraction studies. The
residue, insoluble in CH Cl , was recrystallized from
˚
Mo Ka radiation (k = 0.71073 A). The data were cor-
2
2
CH CN to yield an orange powder of cis-1 (176 mg,
3
rected for Lorentz and polarization effects with the
SMART suite programs [13] and for absorption effects
with SADABS [14]. Structure solution and refinement were
carried out with the SHELXTL suite of programs [15]. The
structures were solved by direct methods to locate the
heavy atoms, followed by difference maps for the light
non-hydrogen atoms. The data collection and process-
ing parameters are given in Table 3.
0
.27 mmol, 54%). Anal. Calc. for C H N I Pd: C,
18 20 4 2
3
3.13; H, 3.09; N, 8.59. Found: C, 33.19; H, 3.12; N,
1
8
.54%. H NMR (300 MHz, DMSO-d ): d 7.64 (dd,
6
3
4
J(H,H) = 6 Hz, J(H,H) = 3.3 Hz, 4 H, Ar–H), 7.35
3
dd, J(H,H) = 6 Hz, J(H,H) = 2.7 Hz, 4 H, Ar–H),
4
(
1
3
1
.19 (s, 12 H, CH3). C{ H} NMR (75.48 MHz,
4
DMSO-d ): 175.1 (s, N–C–N), 134.5, 123.2, 110.9 (s,
Ar–C), 35.8 (s, CH3).
6
0
3.3. Improved synthesis of trans-bis(N,N -dimethylbenz-
imidazoline-2-ylidene) palladium(II) diiodide (trans-2)
4. Supplementary material
CCDC 260776 (complex trans-2) and CCDC 260777
(compound A) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif
or by emailing data_request@ccdc.cam.ac.uk or by con-
tacting the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
0
A sample of N,N -dimethylbenzimidazolyl iodide (A)
1.117 g, 4.08 mmol) and Pd(OAc) (456 mg, 2.03 mmol)
2
(
was dissolved in wet DMSO (10 mL) and stirred over-
night at 80 ꢁC. The yellow precipitate obtained was fil-
tered off and washed with small portions of DMSO
and diethyl ether. It was then dried to give the product
as a yellow powder of trans-2 (927 mg, 1.42 mmol,
7
0%). Upon standing of the DMSO-filtrate a second
crop of 7% can be obtained giving a total yield of
7%. Anal. Calc. for C H N I Pd: C, 33.13; H, 3.09;
7
1
8
20 4 2
Acknowledgements
1
N, 8.59. Found: C, 33.15; H, 3.15; N, 8.62%.
3
H
NMR (300 MHz, CDCl ): d 7.39 (dd, J(H,H) = 6 Hz,
3
We acknowledge the National University of Singapore
for financial support (R 143-000-195-101) and technical
assistance from our department. H.V.H. is grateful to
the Alexander von Humboldt Foundation for a Feodor
Lynen Research Fellowship.
4
3
J(H,H) = 3.5 Hz, 4 H, Ar–H), 7.30 (dd, J(H,H) = 6
4
Hz, J(H,H) = 3.2 Hz, 4 H, Ar–H), 4.20 (s, 12 H,
1
CH3). C{ H} NMR (75.48 MHz, CDCl ): 181.0 (s,
1
3
3
N–C–N), 135.4, 122.7, 109.9 (s, Ar–C), 35.1 (s, CH3).
3
.4. General procedure for the Mizoroki–Heck coupling
References
In a typical run, a Schlenk-tube was charged with a
mixture of aryl halide (1.0 mmol), anhydrous sodium
acetate (1.5 mmol), t-butyl acrylate (1.4 mmol). 3 mL
DMF was added and the mixture degassed under vac-
uum and filled with nitrogen. The reaction mixture
was vigorously stirred at the appropriate temperature
before a catalyst solution in DMF was injected. After
the desired reaction time, the solution was allowed to
cool. 5 mL of dichloromethane was added to the reac-
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