Ring carbon functionalization of N-heterocyclic carbene ligand with ester groups. Electronic effect of ester groups on coordination properties

Article abstract of DOI:10.1246/bcsj.79.1781

Palladium complexes with an imidazol-2-ylidene ligand functionalized with ester groups at its 4,5-positions were synthesized from a readily available imidazole derivative. σ-Donation from the carbene ligand to the palladium atom is considerably weakened by functionalization with the two ester moieties.

Full text of DOI:10.1246/bcsj.79.1781

ꢀ 2006 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 79, No. 11, 1781–1786 (2006) 1781
Ring Carbon Functionalization of N-Heterocyclic Carbene
Ligand with Ester Groups. Electronic Effect of
Ester Groups on Coordination Properties
ꢀ
Kenji Hara, Yoshikazu Kanamori, and Masaya Sawamura
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810
Received May 11, 2006; E-mail: sawamura@sci.hokudai.ac.jp
Palladium complexes with an imidazol-2-ylidene ligand functionalized with ester groups at its 4,5-positions were
synthesized from a readily available imidazole derivative. ꢀ-Donation from the carbene ligand to the palladium atom is
considerably weakened by functionalization with the two ester moieties.
N-Heterocyclic carbenes (NHC) are strong ꢀ-donating li-
Ad
Ad
N
N
gands, and their use in synthetic catalytic reactions has ex-
panded.1,2 Since Herrmann et al. reported the catalytic applica-
tions of a palladium complex with a very simple NHC ligand,1
numbers of studies have been done on the modification of the
original imidazol-2-ylidene ring in order to enhance the prop-
erties of the ligand. One widespread approach is the modifica-
tion of the two nitrogen atoms. It has been demonstrated that a
highly active catalyst was prepared by the attachment of bulky
groups on the nitrogen atoms of the NHC ligands.2c,3,4 Intro-
duction of functional groups on the nitrogen atoms has also
been widely studied.5,6 These studies have produced a variety
of chiral NHC ligands, multidentate NHC ligands, or ligands
with special chemical properties or practically useful proper-
ties such as recyclability.
Mes
Mes
N
N
Cl
Cl
X
X
1
2
Ad =
;
Mes =
X = OMe, H, F
Me
Me
N
N
EtO2C
CO2Et
In contrast, only a few studies on the modification of an
imidazol-2-ylidene ring at its 4,5-positions have been report-
ed.7–9 Modification at this position would not disturb the
steric environment created by the substituents on the nitro-
gen atoms. Tuning of the electronic properties of a NHC
ligand is possible by these modifications. Investigation of
the electronic effects of NHC ligands in catalytic reactions
would afford mechanistic insights towards further devel-
3
Fig. 1. Ring carbon-functionalized imidazol-2-ylidene ligands.
al groups, which would allow the production of a diverse
array of functionalized NHC ligands.
Results and Discussion
opment of the NHC ligands. Furstner et al. synthesized a
¨
Synthesis of palladium(II) complexes with the imidazol-2-
ylidene ligand bearing ester moieties at the 4,5-positions starts
with substituted imidazole 5 as shown in Scheme 1. Imidazole
5 is readily obtained by the reported procedure from commer-
cially available imidazole-4,5-dicarboxylic acid.10 Treatment
of diethyl N-methylimidazole-4,5-dicarboxylate (5) with MeI
in refluxing CH3CN afforded the imidazolium salt 6 quantita-
tively. The reaction of palladium(II) acetate with 2.2 equiva-
lent of imidazolium salt 6 in refluxing THF afforded, after pu-
rification by flash chromatography, an isomeric mixture of two
palladium carbene complexes, cis-7 and trans-7 in 59 and 16%
yields, respectively. These complexes are both air- and mois-
ture-stable. Recrystallization of the mixture from CH2Cl2/
hexane afforded two types of crystals: yellow plate crystals
and yellow needles. They were separated from each other
and characterized separately.
ruthenium complex with 4,5-dichloroimidazol-2-ylidene 1
(Fig. 1), but there were only marginal substituent effects in
ring-closing alkene metathesis and intramolecular enyne cy-
cloisomerization reactions.8 Organ et al. prepared the precur-
sor of benzimidazol-2-ylidene 2 having electronically dif-
ferent substituents and showed that electron-donating substi-
tutents on the ligand enhanced the catalytic activity of the
complex in the Suzuki–Miyaura reaction.9 The modification
of an imidazol-2-ylidene ligand at its 4,5-positions is, how-
ever, still limited despite its potential utility. We report,
herein, the preparation and structures of palladium com-
plexes with an imidazol-2-ylidene ligand 3, which is func-
tionalized with ester groups at its 4,5-positions. The ester
groups exerted a considerable electronic influence on the co-
ordination properties of the NHC ligand. The ester groups
can potentially be transformed into a wide range of function-
X-ray crystal structure analysis was possible only for the
Published on the web November 9, 2006; doi:10.1246/bcsj.79.1781

1782 Bull. Chem. Soc. Jpn. Vol. 79, No. 11 (2006)
Functionalization of NHC with Esters
˚
Table 1. Selected Bond Lengths (A), Angles (deg) Around
Palladium Core and NMR Data (ꢂ) for Complexes cis-7
and 81
Me
I-
CO2Et
CO2Et
CO2Et
CO2Et
N+
MeI (5.0 eq)
N
N
CH3CN
reflux, 5 h
N
Me
Compound
cis-7
81
Me
Pd–C (Ring I)
Pd–C (Ring II)
Pd–I (1, trans to Ring II)
Pd–I (2, trans to Ring I)
2.009(6)
2.008(6)
2.6419(7)
2.6455(7)
5
6
1.990(3), 1.997(3)
100%
2.6479(3), 2.6572(3)
I
I
Me
Me
N
C (Ring I)–Pd–C (Ring II) 91.4(2)
90.2
93.59
Pd
N
I(1)–Pd–I(2)
94.29(2)
87.0(1)
87.3(2)
174.6
EtO2C
CO2Et
I(1)–Pd–C (Ring I)
I(2)–Pd–C (Ring II)
13C NMR (carbene-C)
1H NMR (NCH3)
N
87.32, 88.97
N
Me Me
EtO2C
CO2Et
168.2
3.92
4.11
Pd(OAc)2
6
cis-7
59%
(2.2 eq)
THF
reflux
1.5 h
+
Me
N
Me
I
Pd
I
EtO2C
CO2Et
CO2Et
O1
N
C5
1.192(8)
C6
O2
N
N
EtO2C
1.384(8)
N2
C1
1.496(9)
Pd
Me
Me
1.301(7)
O4
C2
1.357(8)
I2
Ring I
C4
C3
trans-7
1.471(9)
16%
I1
1.391(8)
N1
C9
1.193(9)
1.326(9)
O3
Scheme 1.
O
I1
I2
O
Me
Me
+
N2 N
N
OEt
OEt
OEt
Pd1
C2
Ring I
Pd
Pd
C4
C1
C12
N4
C15
N2
N
OEt
N
C3
C9
C16
Ring II
C13
Ring I
N1
N3
C3
C5
Me
C14
O3
Me
O
O3 O
O6
O8
C2
C6
O2
C9
O1
C19
C20
A
O4
C17
O5
O7
C10
C22
C21
C18
Fig. 4. ORTEP plot (30% probability thermal elipsoids) of
the molecular structure of complex cis-7 with selected
˚
bond lengths (A) and possible contributing resonance
structures.
C11
C7
C8
Fig. 2. ORTEP plot (30% probability thermal elipsoids) of
the molecular structure of complex cis-7.
˚
and 2.6419(7) A (trans to ring II) (see Table 1), the average
of which is slightly shorter than that of complex 8. These dif-
ferences are small but suggest that ꢀ-donation from the NHC
to the palladium atom is weakened by functionalization with
the ester groups at the 4,5-positions (vide infra for stronger
evidence). The torsion angles between the carbene ring and
each of the attached carbonyl groups are approximately 10
and 68ꢁ for ring I, and 34 and 38ꢁ for ring II. These angles
are suggestive of the existence of partial ꢁ-orbital overlap
between the ester groups and the NHC rings in the solid state.
From the X-ray crystal structure analysis (see Fig. 4), the
C(2)=C(3) bond is longer than those of complex 8, and the
C(3)–C(9) bond is much shorter than that of the corresponding
bond in analogous molecules, which possibly implies the con-
tribution of the resonance structure A shown in Fig. 4. Other
bond lengths of cis-7 are also consistent with such ꢁ-orbital
overlap effect. Extended ꢁ-orbital overlap from the ester group
to the carbene core would weaken the ꢀ-donating ability of
the carbene, which is consistent with elongation of the Pd–
carbene carbon bonds and the shortening of the Pd–I bonds
of ester-functionalized complex cis-7 compared to those of
I
I
Me
Me
Pd
N
N
N
N
Me
Me
8
Fig. 3. Reported N-heterocyclic carbene–palladium complex.
yellow plate crystals (cis-7). Complex cis-7 has a square-pla-
nar coordination geometry with the two carbene ligands in a
cis arrangement as shown in Fig. 2. The angles between the
coordination plane and each of the carbene rings are 78.4(2)ꢁ
(ring I) and 77.9(2)ꢁ (ring II). Pd–C bond lengths are 2.009(6)
˚
˚
A (ring I) and 2.008(6) A (ring II). The average of the Pd–C
bond lengths is longer than those of the reported values for a
native non-substituted palladium complex (8)1 (Fig. 3). Pd–I
˚
bond lengths of complex cis-7 are 2.6455(7) A (trans to ring I)

K. Hara et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 11 (2006) 1783
I2
I1
100
C20
C21
Pd1
C7
80
60
40
20
0
C19
C22
N2
C1
N1
C18
C23
C29
O3
P1
C8
C2
C17
C14
C9
O4
C24
C25
C28
C12
C3
C16
C10
C11
C27
C4
C13
C15
O2
C26
O1
C5
C6
0
1
2
3
4
5
6
7
Fig. 6. ORTEP plot (30% probability thermal elipsoids) of
the molecular structure of complex 10.
time / day
˚
Table 2. Selected Bond Lengths (A), Angles (deg) Around
Fig. 5. Thermal isomerization between ester-functionalized
NHC-palladium complexes cis-7 ( ) and trans-7 ( ) in
CDCl3 at 65 ꢁC.
Palladium Core for Complexes 10, 11,5b and 126
Compound
10
115b
126
Pd–C
Pd–P
Pd–I (1, trans to C)
Pd–I (2, trans to P)
C–Pd–P
1.989(3)
2.2827(8)
2.6515(3)
2.6516(3)
89.65(9)
94.22(1)
86.15(8)
90.18(2)
1.9974(4)
2.2812(10)
2.6519(5)
2.6370(5)
92.71(10)
93.27(1)
84.89(9)
N/AaÞ
1.990(3)
2.2788(7)
2.6482(3)
2.6467(3)
90.35(8)
92.789(10)
87.42(8)
89.49(2)
Pd(OAc)2
NaI (4.1 eq)
t-BuOK (1.2 eq)
(1.0 eq)
Me
N+
CO2Et
CO2Et
I-
N
Me
CH3CN
60°C, 21 h
I(1)–Pd–I(2)
I(2)–Pd–C
I(1)–Pd–P
6
(1.0 eq)
Me
N
a) N/A = not available.
I
EtO2C
Me
N
I
Pd
I
Pd
I
CO2Et
CO2Et
N
I
I
EtO2C
CO2Et
I
O
N
I
Me
Pd
N
N
N
Pd
Ph3P
Me
Ph3P
9
N
N
37%
Me
EtO2C
I
I
11
12
Me
N
PPh3
(2.0 eq)
Pd
Fig. 7. Reported functionalized N-heterocyclic carbene–
palladium complexes.
Ph3P
CO2Et
N
CH2Cl2
rt, 1 h
Me
complex of palladium,11 which isomerizes from cis to trans
on treatment with H2O.
CO2Et
10
99%
The reaction of palladium(II) acetate with 1.0 equivalent of
imidazolium salt 6 in the presence of NaI and KO(t-Bu) in
refluxing CH3CN for 21 h afforded an iodo-bridged dimeric
monocarbene complex 9 (Scheme 2). Purification by flash
chromatography on silica gel afforded complex 9 in 37% yield
as red brown powder. Addition of PPh3 to dimeric complex 9
in CH2Cl2 gave a mixed phosphine–carbene complex 10 as
bright yellow crystals in nearly quantitative yield.
Scheme 2.
native complex 8.
Small 13C NMR chemical shift changes were observed for
the carbene carbons of the ester-functionalized complexes
when compared with that of non-functionalized complex 8.
The carbene carbon signal of complex cis-7 was observed at
174.6 ppm, which is shifted down-field by 6.4 ppm from that
of native complex 8. The corresponding carbon signal of the
other isomer trans-7 was observed at 171.9 ppm.
From X-ray crystal structure analysis, complex 10 has a
square-planar coordination geometry with the carbene ligand
and PPh3 in a cis arrangement as shown in Fig. 6. Palla-
˚
It was found that these isomeric complexes cis-7 and trans-7
were in slow equilibrium in CDCl3 and that cis-7 was thermo-
dynamically more stable than trans-7. A CDCl3 solution of
cis-7 and trans-7 in a ratio of 70:30 sealed in a NMR tube
was converted to a mixture in a 91:9 ratio within 4 days at
65 ꢁC (Fig. 5). This isomerization from trans to cis is different
from that observed for a N,N-dimethyltriazole-derived NHC
dium–carbene carbon bond length is 1.989(3) A, and the angle
between the plane of the carbene ring and the coordination
plane is 88.79(9)ꢁ (see Table 2). The two Pd–I bond lengths
˚
are almost identical: 2.6515(3) A (trans to the carbene) and
˚
2.6516(3) A (trans to the phosphine). In the case of N,N-
bis(ethoxycarbonylmethyl)-substituted carbene complex 115b
(Fig. 7), Herrmann et al. reported that the Pd–I bond trans to

1784 Bull. Chem. Soc. Jpn. Vol. 79, No. 11 (2006)
Functionalization of NHC with Esters
and used without purification. Anhydrous CH3CN was purchased
from Kanto Chemical Co., Inc. and dried over molecular sieves in a
bottle. MeOH was purchased from Junsei Chemcal Co., Ltd. and
used without purification. MeI was distilled before use. Pd(OAc)2
was purchased from Wako Chemical Industries, Ltd. and used
without purification. NaI was purchased from Kanto Chemical Co.,
Inc. and dried at 70 ꢁC for 10 h in vacuo before use. t-BuOK was
purchased from Kishida Chemical Co., Ltd. and used without pu-
rification. PPh3 was recrystallized from EtOH. 4,5-Bis(ethoxycar-
bonyl)-1-methylimidazole10 was prepared by literature procedure.
4,5-Bis(ethoxycarbonyl)-1,3-dimethylimidazolium Iodide (6).
To a solution of 4,5-bis(ethoxycarbonyl)-1-methylimidazole (734
mg, 3.25 mmol) in CH3CN (20 mL), MeI (1.42 mL, 16.3 mmol)
was added, and the reaction mixture was heated at reflux for 5 h.
After cooling to room temperature, the solvent was removed under
a reduced pressure to give 6 (1.20 g, 100%). This compound was
used for further transformation without purification.
6: 1H NMR (300 MHz, CDCl3) ꢂ 11.02 (s, 1H, NCHN), 4.47 (q,
J ¼ 7:2 Hz, 4H, CO2CH2), 4.20 (s, 6H, NCH3), 1.41 (t, J ¼ 7:2
Hz, 6H, CO2CH2CH3); 13C NMR (75 MHz, CDCl3) ꢂ 165.53 (2C,
C=O), 141.31 (NCHN), 126.93 (2C, C=C), 63.46 (2C, OCH2-
CH3), 36.96 (2C, NCH3), 13.60 (2C, OCH2CH3); IR (neat)/cmꢂ1
2992 (m), 2959 (m), 1720 (s), 1578 (m), 1479 (w), 1408 (w), 1262
(m), 1145 (m), 1109 (m), 1016 (m), 907 (w), 871 (w), 739 (m).
Bis[4,5-bis(ethoxycarbonyl)-1,3-dimethylimidazolin-2-yli-
Pd cat.
(2.0 mol%)
NaOAc (1.2 eq)
Ph-Br
+
Ph
Ph
Ph
DMF
150 °C , 2 h
(1.5 eq)
Pd cat. yield
cis-7
55%
20% 12
8
Scheme 3.
˚
the carbene ring is 0.0149 A longer than the Pd–I bond trans to
the phosphine and confirmed that the carbene in complex 11 is
still a stronger donor than PPh3. The almost identical two Pd–I
bond lengths in complex 10 indicates that ꢀ-donation from
NHC is weakened by the vinylogous modification with the
two ester moieties, although partial contribution from crystal
packing forces cannot be excluded. N-Carbamoyl-substituted
carbene complex 12, reported by Batey et al.,6 has also
almost no difference between the two Pd–I bonds. The present
study, therefore, demonstrates that ester-modification of imida-
zol-2-ylidene at the vinylogous positions exhibits electronic
effects comparable to those obtained by the direct modification
at the nitrogen atom. Notably, vinylogous modification does
not disturb the steric environment around the metal center,
although modification of the nitrogen atom does. The ring-
functionalization method studied here will help the further
development of a new N-heterocyclic carbene ligand.
As preliminarily results, the ester-functionalized palladium
complex cis-7 exhibited catalytic activity in the Mizoroki–
Heck reaction as shown in Scheme 3. In the presence of 2.0
mol % of palladium complex cis-7, the reaction of bromoben-
zene with styrene at 150 ꢁC for 2 h afforded stilbene in 55%
yield with >98% trans selectivity, whereas the reported yield
with parent non-substituted complex 8 was 20% in the identi-
cal reaction conditions.12
dene]diiodopalladium(II) (7).
A degassed suspension of
Pd(OAc)2 (109 mg, 0.49 mmol) and imidazolium iodide 6 (394
mg, 1.07 mmol) in THF (7.5 mL) was heated for 1.5 h at reflux.
After cooling to room temperature, the solvent was removed under
a reduced pressure. The crude product was recrystallized from
CH2Cl2/hexane to give yellow plate crystals of cis-7 (242 mg,
59%) and yellow needles of trans-7 (67 mg, 16%). The two types
of crystals were separately collected. A crystal of cis-7 was used
for X-ray diffraction study (vide infra).
1
cis-7: H NMR (300 MHz, CDCl3) ꢂ 4.38 (q, J ¼ 7:2 Hz, 8H,
CO2CH2CH3), 4.11 (s, 12H, NCH3), 1.37 (t, J ¼ 7:2 Hz, 12H,
CO2CH2CH3); 13C NMR (75 MHz, CDCl3) ꢂ 174.56 (2C, car-
bene-C), 158.88 (4C, C=O), 128.39 (4C, C=C), 62.30 (4C,
OCH2CH3), 37.94 (4C, NCH3), 13.88 (4C, OCH2CH3); IR (neat)/
cmꢂ1 2982 (w), 2954 (w), 1717 (s), 1595 (m), 1454 (m), 1380 (s),
1240 (s), 1124 (m), 1084 (s), 1013 (m), 916 (m), 854 (m), 774 (w),
745 (w), 701 (m).
Conclusion
Palladium complexes coordinated by an imidazol-2-ylidene
ligand that was functionalized with ester groups at its 4,5-
positions were synthesized from a readily available imidazole
derivative. These complexes are both air- and moisture-stable.
ꢀ-Donation from the carbene ligand to the palladium atom is
weakened considerably by functionalization with the two ester
moieties. It is expected that the ester groups can potentially
be transformed into a wide range of functional groups. Such
transformations would allow the production of a diverse array
of functionalized N-heterocyclic carbene ligands. Investigation
in this line is now underway in our laboratory.
trans-7: 1H NMR (300 MHz, CDCl3) ꢂ 4.38 (q, J ¼ 7:2 Hz,
8H, CO2CH2CH3), 4.16 (s, 12H, NCH3), 1.37 (t, J ¼ 7:2 Hz,
12H, CO2CH2CH3); 13C NMR (75 MHz, CDCl3) ꢂ 171.88 (2C,
carbene-C), 158.45 (4C, C=O), 128.72 (4C, C=C), 62.72 (4C,
OCH2CH3), 38.76 (4C, NCH3), 13.97 (4C, OCH2CH3); IR (neat)/
cmꢂ1 2982 (w), 2936 (w), 1738 (s), 1717 (s), 1614 (m), 1445 (s),
1377 (s), 1280 (s), 1248 (s), 1138 (s), 1075 (s), 1012 (m), 919 (w),
854 (m), 776 (w), 742 (w), 695 (m); Found: C, 31.50; H, 3.60; N,
6.61%. Calcd for C22H32I2N4O8Pd: C, 31.43; H, 3.84; N, 6.66%.
Di-ꢀ-iodobis[4,5-bis(ethoxycarbonyl)-1,3-dimethylimidazo-
lin-2-ylidene]diiododipalladium(II) (9). A mixture of Pd(OAc)2
(361 mg, 1.61 mmol), imidazolium iodide 6 (592 mg, 1.61 mmol),
NaI (978 mg, 6.52 mmol), and t-BuOK (220 mg, 1.96 mmol) in
CH3CN (55 mL) was stirred for 21 h at 60 ꢁC. After cooling to
room temperature, the solvent was removed under a reduced pres-
sure. The residue was purified by silica gel column chromatog-
raphy (MeOH/CH2Cl2) to give 9 (357 mg, 37%) as red brown
powder.
Experimental
General. NMR spectra were obtained with a Varian GEMINI
2000 (1H 300 MHz, 13C 75 MHz, 31P 121 MHz) spectrometer and
are reported in ppm (ꢂ). Infrared spectra were obtained with a
Perkin-Elmer Spectrum One and are reported in cmꢂ1. Column
chromatographies were performed with silica gel 60 N (40–100
mm, Kanto Chemical Co.).
Materials. Anhydrous tetrahydrofuran (THF), hexane, and
dichloromethane were purchased from Kanto Chemical Co., Inc.
9: 1H NMR (300 MHz, CDCl3) ꢂ 4.39 (q, J ¼ 7:2 Hz, 8H, CO2-
CH2CH3), 4.20 (s, 12H, NCH3), 1.38 (t, J ¼ 7:2 Hz, 12H, CO2-

K. Hara et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 11 (2006) 1785
˚
Table 3. Summary of Crystal Data and Details of Data
Collection and Refinement Parameters for cis-7 and 10
0:71070 A). Cell constants and an orientation matrix for data col-
lection were obtained. All of the data were corrected for Lorentz
and polarization effects. A summary of the cell parameters, data
collection conditions, and refinement results are given in Table 3.
The structures were solved by heavy-atom Patterson methods13
for cis-7, or direct methods for complex 10, and expanded using
Fourier techniques.14 The positional parameters and thermal pa-
rameters of non-hydrogen atoms of the complexes cis-7 and 10
were refined using a full-matrix least-square method. Hydrogen
atoms were included but not refined. All calculations were per-
formed using the teXsan crystallographic software package.15
Crystallographic data have been deposited with Cambridge
Crystallographic Data Centre: Deposition number CCDC-
610576 for cis-7; CCDC-610575 for 10. Copies of the data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax: +44
1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
cis-7
10
Formula
MW
Crystal system
Space group
C22H32I2N4O8Pd
840.73
triclinic
C29H31I2N2O4PPd
862.76
monoclinic
P21=c (#14)
11.1658(9)
17.3109(8)
16.882(1)
90.00
100.299(1)
90.00
3210.5(4)
4
1.785
ꢀ
P1 (#2)
˚
a/A
11.670(3)
11.840(2)
13.690(3)
64.33(1)
85.80(1)
63.49(1)
1508.6(6)
2
˚
b/A
˚
c/A
ꢃ/deg
ꢅ/deg
ꢆ/deg
3
˚
V/A
Z
Dcalcd/g cmꢂ3
Crystal size/mm3
ꢇ(Mo Kꢃ)/cmꢂ1
2ꢈmax/deg
1.851
0:20 ꢃ 0:10 ꢃ 0:05
27.10
55.0
0:20 ꢃ 0:20 ꢃ 0:20
25.87
55.0
We thank Prof. Takanori Suzuki, Hokkaido University for
the X-ray crystal structure analysis.
No. of measd reflns 12258
26302
Unique reflns
Obsd reflns
No. of variables
R
6796 (Rint ¼ 0:020) 7340 (Rint ¼ 0:015)
3995 (I > 3:00ꢀðIÞ) 5975 (I > 2:90ꢀðIÞ)
References
334
352
0.048
0.060
1.20
0.029
0.040
1.04
1
W. A. Herrmann, M. Elison, J. Fischer, C. Kocher, G. R. J.
¨
Rw
GOF
Artus, Angew. Chem., Int. Ed. Engl. 1995, 34, 2371.
For reviews, see: a) W. A. Herrmann, C. Kocher, Angew.
2
¨
Max Shift/erorr
in final cycle
Max peak in diff
Chem., Int. Ed. Engl. 1997, 36, 2162. b) D. Bourissou, O. Guerret,
F. P. Gabbai, G. Bertrand, Chem. Rev. 2000, 100, 39. c) W. A.
Herrmann, Angew. Chem., Int. Ed. 2002, 41, 1290. d) M. C. Perry,
K. Burgess, Tetrahedron: Asymmetry 2003, 14, 951. e) V. Nair,
S. Bindu, V. Sreekumar, Angew. Chem., Int. Ed. 2004, 43, 5130.
f) C. M. Crudden, D. P. Allen, Coord. Chem. Rev. 2004, 248,
0.043
1.20
0.001
0.94
ꢂ3
˚
Fourier map/eA
Min peak in diff
ꢂ1:22
ꢂ1:10
ꢂ3
˚
Fourier map/eA
´
2247. g) V. Cesar, S. Bellemin-Laponnaz, L. H. Gade, Chem.
Soc. Rev. 2004, 33, 619.
CH2CH3); 13C NMR (75 MHz, DMSO-d6) ꢂ 157.92 (4C, C=O),
128.20 (4C, C=C), 62.68 (4C, OCH2CH3), 38.26 (4C, NCH3),
13.16 (4C, OCH2CH3), carbene carbon signal was not detected;
Found: C, 22.04; H, 2.61; N, 4.55; I, 42.19%. Calcd for C22H32-
N4I4O8Pd2: C, 22.00; H, 2.69; N, 4.67; I, 42.27%.
3
For recent selected papers, see: a) C. W. K. Gstottmayr,
¨
V. P. W. Bohm, E. Herdtweck, M. Grosche, W. A. Herrmann,
¨
Angew. Chem., Int. Ed. 2002, 41, 1363. b) Y. Ma, C. Song, W.
Jiang, Q. Wu, Y. Wang, X. Liu, M. B. Andrus, Org. Lett. 2003,
5, 3317. c) K. Arentsen, S. Caddick, F. G. N. Cloke, A. P. Herring,
P. B. Hitchcock, Tetrahedron Lett. 2004, 45, 3511. d) O. Navarro,
H. Kaur, P. Mahjoor, S. P. Nolan, J. Org. Chem. 2004, 69, 3173.
e) G. Altenhoff, R. Goddard, C. W. Lehmann, F. Glorius, J. Am.
Chem. Soc. 2004, 126, 15195. f) N. Hadei, E. A. B. Kantchev,
C. J. O’Brien, M. G. Organ, J. Org. Chem. 2005, 70, 8503.
g) C. Song, Y. Ma, Q. Chai, C. Ma, W. Jiang, M. B. Andrus,
Tetrahedron 2005, 61, 7438.
cis-[4,5-Bis(ethoxycarbonyl)-1,3-dimethylimidazolin-2-yli-
dene]diiodo(triphenylphosphino)palladium(II) (10). To a so-
lution of diiodopalladium complex 9 (305 mg, 0.25 mmol) in
CH2Cl2 (23 mL), PPh3 (133 mg, 0.51 mmol) was added at room
temperature. The reaction mixture turned orange gradually. The
mixture was stirred for 1 h at room temperature. After evaporation
of the solvent, the crude product was washed with hexane
(5 mL ꢃ 3) to give 10 (432 mg, 99%). Bright yellow crystals of
10 suitable for the X-ray diffraction study (vide infra) were ob-
tained by recrystallization from CH2Cl2/hexane.
4 For recent development of a novel NHC ligand bearing
bulky substituents on the nitrogen atoms, see: M. Yamashita,
1
10: H NMR (300 MHz, CDCl3) ꢂ 7.67–7.61 (m, 6H, phenyl),
K. Goto, T. Kawashima, J. Am. Chem. Soc. 2005, 127, 7294.
5
7.45–7.36 (m, 9H, phenyl), 4.30 (q, J ¼ 7:2 Hz, 4H, CO2CH2-
CH3), 3.74 (s, 6H, NCH3), 1.33 (t, J ¼ 7:2 Hz, 6H, CO2CH2CH3);
13C NMR (75 MHz, DMSO-d6) ꢂ 170.89 (carbene-C), 157.79 (2C,
C=O), 134.26 (br d, J ¼ 10 Hz, 6C, phenyl), 131.62 (br s, 3C,
phenyl), 130.83 (d, J ¼ 51 Hz, 3C, phenyl), 128.94 (d, J ¼ 11 Hz,
6C, phenyl), 128.28 (2C, NC=CN), 62.57 (2C, OCH2CH3), 37.51
(2C, NCH3), 13.79 (2C, OCH2CH3); 31P NMR (121 MHz, DMSO-
d6) ꢂ 24.42; Found: C, 40.39; H, 3.58; N, 3.11; I, 29.45%. Calcd
for C29H31I2N2O4PPd: C, 40.37; H, 3.62; N, 3.25; I, 29.42%.
X-ray Crystal Structure Analysis of cis-7 and 10. The data
were collected at 20 ꢁC on a Rigaku/MSC Mercury CCD diffrac-
tometer with graphite monochromated Mo Kꢃ radiation (ꢄ ¼
See for example: a) W. A. Herrmann, C. Kocher, L. J.
¨
Goossen, G. R. J. Artus, Chem. Eur. J. 1996, 2, 1627. b) W. A.
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¨
Lehmann, Chem. Commun. 2001, 2240. d) A. A. Danopoulos,
S. Winston, T. Gelbrich, M. B. Hursthouse, R. P. Tooze, Chem.
Commun. 2002, 482. e) D. Broggini, A. Togni, Helv. Chim. Acta
2002, 85, 2518. f) C. Bolm, M. Kesselgruber, G. Raabe, Organo-
metallics 2002, 21, 707. g) T. Fujihara, Y. Obora, M. Tokunaga,
H. Sato, Y. Tsuji, Chem. Commun. 2005, 4526.
6
1411.
R. A. Batey, M. Shen, A. J. Lough, Org. Lett. 2002, 4,

1786 Bull. Chem. Soc. Jpn. Vol. 79, No. 11 (2006)
Functionalization of NHC with Esters
12 V. P. W. Bohm, W. A. Herrmann, Chem. Eur. J. 2000, 6,
7 a) A. J. Arduengo, III, F. Davidson, H. V. R. Dias, J. R.
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¨
J. Organomet. Chem. 2004, 689, 2995. c) M. Bortenschlager,
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¨
Weberskirch, J. Organomet. Chem. 2005, 690, 6233.
8
A. Furstner, L. Ackermann, B. Gabor, R. Goddard, C. R.
¨
Lehmann, R. Mynott, F. Stelzer, O. R. Thiel, Chem. Eur. J.
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9 N. Hadei, E. A. B. Kantchev, C. J. O’Brien, M. G. Organ,
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10 P. M. Hay, W. R. Wilson, W. A. Denny, Tetrahedron 2000,
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¨
11 W. A. Herrmann, J. Fischer, K. Ofele, G. R. J. Artus,
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14 DIRDIF94: P. T. Beurskens, G. Admiraal, G. Beurskens,
W. P. Bosman, R. de Gelder, R. Israel, J. M. M. Smits, The
DIRDIF-94 Program System, Technical Report of the Crystal-
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1994.
15 teXsan: Crystal Structure Analysis Package, Molecular
Structure Corporation, 1985 & 1999.